THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA PRESENTED BY PROF. CHARLES A. KOFOID AND MRS. PRUDENCE W. KOFOID THE GEOLOGICAL OBSERVER. THE G'EO LOGICAL OBSERVER, BY SIR HENRY T. DE LA BECHE, C.B., F.R.S., &c DIKECTOR-GENERAL OF THE GEOLOGICAL SUKVEY OF THE UNITED KINGDOM. SECOND EDITION, REVISED. LONDON : LONGMAN, BROWN, GREEN, AND LONGMANS. 1853. LONDON : PRINTED BY W. CLOWES AND SONS, STAMFORD STREET AND CHARING CROSS. PREFACE. IT has been well remarked by Humboldt * that to behold is not necessarily to observe, that is, to compare and combine. The history of Geology, like that of all sciences depending for their effective advance on experiment or correct observation, amply proves the truth of this statement. We are not required to look far back to be fully aware of the many brilliant hypotheses which have given way before the advance of correct research. It was not that these brilliant hypotheses were intended as substitutes for sound geological knowledge, based on correct data, or that those who formed them were not as capable as any who may in after- times succeed in still farther systematically embodying the accu- mulated data of such times, but merely that correct observations were not then sufficiently abundant, and that powerful, and, some- times, impatient minds supplied their place with conceptions more captivating than well founded. It is obvious that with a hundred well-established facts more can be accomplished than with ten, the deductions from which, however apparently correct, may even be fallacious as respects those derived from the consideration of the greater number. Let it not, nevertheless, be hastily concluded that the views which have passed away have not materially advanced geology, as those of a similar character have aided the progress of other sciences. Without them, though a few may have been impediments for the time, many a subject would have longer remained disregarded by its zealous investigator. Even the con- troversies which have from time to time appeared, many from differences of opinion arising .the more readily as the subject was * Kosmos. M37Z528 vi PREFACE. less perfectly understood, gave a certain impulse to progress which the commencement of many inquiries so often demands. The following work was undertaken in the hope that the expe- rience of many years might assist, and, perhaps, abridge the labours of those who may be desirous of entering upon the study of geology, and especially in the field. Its object is, to afford a general view of the chief points of that science, such as existing observations would lead us to infer were established ; to show how the correct- ness of such observations may be tested ; and to sketch the directions in which they may apparently be extended. Having been, to a certain extent, founded upon a little treatise, entitled " How to Observe in Geology," long since out of print, a somewhat similar name has been retained for the present volume. H. T. DE LA BECHE. CONTENTS. I'age INTRODUCTION - 4 --------___ X v CHAPTER I. Decomposition of rocks Formation of soils Decomposition of granitic rocks Decomposition of sandstones and limestones Influence of structure and organic remains on decomposition Decomposition of rocks containing iron -------------- i_n CHAPTER II. Removal of soluble parts of rocks by water Travertine and calcareous breccia Chloride of sodium in spring water Silica in water Hot springs Springs on the outcrop of beds Springs from faults Causes of land- slips ------- 12-22 CHAPTER III. Substances mechanically suspended in water Transport of detritus by rivers Deposit of detritus in valleys Action of rivers on their beds Removal of lakes by river action Formation and discharge of lakes Lacustrine deposits -------------- 23-45 CHAPTER IV. Action of the sea on coasts Difference in tidal and tideless seas Unequal abrasion of coasts Shingle beaches Chesil bank Coast sand-hills - 46-62 CHAPTER V. Distribution and deposit of sediment in tideless seas Deposits of the Nile Of the Po and Rhone Contemporaneous deposits of gravel, sand, and mud Deposit of volcanic ashes and lapilli Deposits in the Black Sea and the Baltic Gulf of Mexico and Mississippi ------- 63-76 CHAPTER VI. Distribution and deposit of sediment in tidal seas Bars at river mouths- Rise and influence of the tides Deposits in estuaries Delta of the Gauges Of the Quorra Deposits on the coast near Swansea Influence of waves Form of the sea-bed round the British Islands Influence of currents- Specific gravity of sea-water Distribution of sediment over the floor of the ocean --------- 77-101 viii CONTENTS. Page CHAPTER VII. Chemical deposits in seas Deposits in the Caspian and inland seas Calca- reous deposits Formation of oolitic rocks Salts in sea-water Chemical deposits not necessarily horizontal -------- 102-111 CHAPTER VIII. Preservation of remains of existing life amid mineral matter Of plants and vegetable matter Bogs Dismal Swamp Rafts in the Mississippi Ani- mal remains on the land Vertebrata Ossiferous caverns and lake de- posits Insects Land molluscs Effects of showers of volcanic ashes Estuary deposits Footprints on mud __-_-__ 112-130 CHAPTER IX. Organic remains in marine deposits Modification of conditions on coasts of America Of Pacific Ocean Of the Indian Ocean Of coasts of Africa and Europe Of Arctic Sea Distribution of marine life Modifications from temperature and pressure From light and supply of air Researches of Professor E Forbes in the JEgean Sea Zones of depth Professor Lbven on the molluscs of Norway Zones of depth in the British Seas Organic remains deposited in the deep ocean On coasts ----- 131-164 CHAPTER X. Coral reefs and islands Distribution of coral animals Chemical composition of coral Keeling atoll Form of coral islands Barrier reefs Lagoon islands Isle of Bourbon --- ______ 165-180 CHAPTER XI. Great barrier reef of Australia Coral reefs of the Red Sea Conditions for the occurrence of coral reefs Influence of volcanic action on coral reefs Composition of coral reef accumulations Influence of changes in the level of sea and land Reefs near Bermuda - - - - - - - 181-205 CHAPTER XII. Transportal of mineral matter by ice Height of snow line Glaciers Cause of the movement of glaciers Glacier moraines Motion of glaciers Grooving of rocks by glaciers Advance and retreat of glaciers Glaciers of the Himalaya - --________ 206-224 CHAPTER XIII. Arctic glaciers reaching the sea Northern icebergs and their effects Ant- arctic glaciers and ice barrier Geological effects of Antarctic icebergs Glaciers of South Georgia Glaciers of South America Transport of detritus by river ice Geological effects of coast ice Effects of grounded icebergs General geological effects of ice - - - - - 225-250 CONTENTS. ix CHAPTER XIV. Influence of a general increase of cold Modifications of temperature from changes in the distribution of sea and land Erratic blocks Effects of gradual rise of the sea-bottom strewed with ice-transported detritus Effects of a supposed depression of the British Islands Increase of Alpine glaciers Transport of erratic blocks by glaciers Former existence of glaciers in Britain Elevation of boulders by coast ice during submergence of land Erratic blocks of the Alps Erratic blocks of Northern Europe Erratic blocks of America - - - - - - -._ _ _ 251-278 CHAPTER XV. Mollusc remains in superficial detritus Arctic shells found in British deposits Evidence of a colder climate in British Islands Extinct Siberian elephant Changes of land and sea in Northern Europe Extinction of the great Northern mammals Range of the mammoth Frozen soil of Siberia - 279-291 * CHAPTER XVI. Ossiferous caves and osseous breccia Former connection of Britain with the Continent Mammoth remains found in British seas Kirkdale Cave Mud in ossifewis caves General state of these caverns Human remains in Paviland Cave Caves formerly dens of extinct carnivora Human re- mains in ossiferous caverns Complicated accumulations in bone caves Pebbles in ossiferous caves Deposits in subterranean river channels Osseous breccia in fissures Changes in the entrances to caves Occurrence of Mastodon remains Association with those of the mammoth Extinct mammals of Central France -___-_--- 292-316 CHAPTER XVII. Volcanos and their products Height above the sea Craters of elevation and eruption Fossiliferous volcanic tuff beds Several craters on one fissure Volcanic vapours and gases Volcanic sublimations Molten volcanic pro- ducts Flow of lava streams Vesicular lavas Volcanic cones Cotopaxi Volcanos of Hawaii Effects of lava on trees ----- 317-340 CHAPTER XVIII. Volcanos and their products, continued Vesuvius Etna Iceland Strom- boii in constant activity Intervals of long repose Sudden formation of Jorullo Sudden formation of Monte Nuovo Falling in of the volcano Papandayang in Java Subterranean lakes with fish Discharge of acid waters Inundations from volcanos Chemical character of volcanic products Trachyte and dolerite Composition of the felspathic minerals Composi- tion of lavas Composition of obsidians Composition of olivine and.leucite Diffusion of minerals in volcanic rocks Fusibility of minerals in volcanic rocks Sinking of minerals in fused lava Fusion of rocks broken off in volcanic vents -- - - - - - - -.- - ' 341-363 x CONTENTS. Tage CHAPTER XIX. Volcanos and their products, continued Lamination of streams of lava Laminae of spherules in obsidian Composition of volcanic ashes Modified composition of volcanic ashes Volcanic tuff Composition of palagonite tuff of Iceland Modification of volcanic tuffs by gases and vapours Solu- tion of palogonite tuff in acids Solfataras The Geysers, and their mode of action, Iceland Siliceous deposits from the Geysers Sulphurous waters of Iceland Gypsum deposits of Iceland Fusibility of volcanic products Fis- sures in volcanos filled by molten lava Lava ejected through fissures Direction of fissures in volcanos Effects of sea on volcanic gases Soften- ing and raising of tuffs and lavas - - - 364-382 CHAPTER XX. Volcanos and their products, continued The Caldera, Island of Palma Sections of Etna and Vesuvius Form and structure of Etna Origin of the Val del Bove, Etna Fossiliferous volcanic tuff of Monte Somma, Vesuvius Mixed molten volcanic rocks and conglomerates Modification of submarine volcanic deposits Peak of Teneriffe Santorin group Raised fossiliferous, volcanic tuff of Santorin group Quiet deposits inside the Santorin group Island of St. Paul, Indian Ocean Distribution of volcanos in the ocean Distribution of volcanos on continents, and amid inland seas Variable proximity of volcanos to water Extinct volcanos Mineral and chemical composition, and structure of basalt ------- 383-407 CHAPTER XXI. Salses or mud volcanos Gaseous emanations unconnected with volcanos Results of decomposed iron pyrites amid bituminous shale Mud volcanos of the Baku district Of the neighbourhood of Taman and Kertch Of Macu- laba, Gergenti Boracic acid lagunes of Tuscany - 408-414 CHAPTER XXII. Earthquakes Connection of volcanos and earthquakes Extent of earthquakes Movement of the earth- wave during earthquakes Sea-waves produced during earthquakes Transmission of earthquake-waves complicated by different mineral structures Unequal transmission of earthquakes Locally extended range of earthquakes Earthquakes traversing mountain chains Fissures produced during earthquakes Settlement of unconsolidated beds adjoining hard rocks during earthquakes Breaking of great sea- wave, of earthquakes, on coasts Effects of earthquakes on lakes and rivers Flame and vapours from earthquake fissures Sounds accompanying earthquakes Elevation and depression of land during earthquakes Coast of Chili raised during earthquakes Effects of earthquakes in the Runn of Cutch - 41 5-434 CHAPTER XXIII. Quiet rise and subsidence of land Rise and depression of coast in the Bay of Naples Elevation and depression of land from increase and decrease of heat Gradual rise of land in Sweden and Norway Raised coasts in Scan- dinavia Gradual depression of land in Greenland Movements of coasts in the Mediterranean Unstable state of the earth's surface - 435-444 CONTENTS. xi CHAPTER XXIV. Sunk (submarine) forests and raised beaches Sunk forests of Western Europe Sunk forests beneath the sea in roadsteads Mode of occurrence of sunk forests Localities of sunk forests Remains of mammals and insects in sunk forests of Western England Footprints of deer and oxen amid the rooted trees of sunk forests in South Wales Sunk forests viewed with regard to the varied heights of tide on tidal coasts Raised beaches concealed by detritus Raised beaches of Plymouth, Falmouth, and New Quay Raised dunes of blown sand, Perran Bay, Cornwall Distribution of detritus in the English Channel, and in sea south of Ireland Care required in tracing raised coast lines Raised coast lines, Scandinavia - - 445-462 CHAPTER XXV. Temperature of the earth Temperature of different depths in Siberia- Temperatures found in Artesian wells Heat of waters rising through faults, and other fissures Variable temperature from unequal percolation of water through rocks Temperature of waters in limestone districts 463-470 CHAPTER XXVI. Mode of accumulation of detrital and fossiliferous rocks Detrital rocks chiefly old sea-bottoms Mixture of beds with and without fossils Variable mode of occurrence of organic remains among detrital and fossiliferous rocks Beaches on the shores of land at the time of the Silurian deposits and old red sandstones Beaches of the new red sandstone period in the Mendip Hills, and near Bristol Beaches at the time of the lias Lias resting on disturbed beds of carboniferous limestone Varied modes of occurrence of the lias Boring molluscs of the inferior oolite period Overlap of the inferior oolite, Mendip Hills Lias conglomerate, pierced by boring mol- luscs Land of the time of the lias Evidence of land from fresh-water deposits Effects produced on coasts, rivers, and lakes, by continued ele- vation of land above sea Elevation of land over a wide area Effects of closing the Straits of Gibraltar Unequal elevation of land Lakes on the outskirts of mountains Mixture of organic remains of different periods from submergence of land Variable effects of submergence of present dry land - - 471-499 CHAPTER XXVII. Mode of accumulation of detrital and fossiliferous rocks, continued Evi- dence afforded by the coal-measures Stigmaria beds in the coal-measures Stems of plants in their positions of growth in the coal-measures Filling up of hollow vertical stems, and mixture of prostrate plants with them Growth of terrestrial plants in successive planes in the coal-measures Thickness of South Wales coal-measures False bedding in coal-measure sandstones Surfaces of coal-measure sandstones Drifts of matted plants in the coal-measures Extent of coal-beds Partial removal of coal-beds Channels eroded in coal-beds, Forest of Dean Lapse of time during deposit of coal-measures Pebbles of coal in coal accumulations Marine remains in part of the coal-measures Gradual subsidence of delta lands Fossil xii CONTENTS. Page trees and ancient soils, Isle of Portland Conditions under which the ancient soils and growth of plants were produced at Portland Wealden deposits, South-eastern England Raised sea-bottom round British Islands Overlap of cretaceous rocks in England ------ 500-521 CHAPTER XXVIII. Mode of accumulation of detrital and fossiliferous rocks, continued Cracked surfaces of deposits Footprints of air-breathing animals on surfaces of rocks Rain-marks on surfaces of rocks Elevation or depression of the bottom of the ocean Character of the surfaces of rocks Friction-marks on rock surfaces Ridged and furrowed surfaces of rocks Effects of earth- quakes on sea-bottoms Diagonal arrangement of the minor parts of beds among detrital rocks Beds formed by unequal drifts Mode of occurrence of organic remains Distribution of organic remains Effect of the rise and fall of land on the distribution of organic remains Distribution of organic remains with respect to different kinds of sea-bottoms Infusorial remains Chemical composition of organic remains Caution as to forms of life supposed characteristic of different geological 'times - 522-550 CHAPTER XXIX. Igneous products of earlier date than those of modern volcanos Simple sub- stances forming igneous rocks Volcanic products amid the older rocks Fossils amid old igneous products Volcanic tuff and conglomerates among the Silurian deposits of Wales and Ireland Old volcanic products inter- mingled with the Devonian rocks of South-western England Igneous rocks associated with the carboniferous limestone of Derbyshire Relative geolo- gical date of the Wicklow and Wexford granites Date of the granites of Devon and Cornwall Uncertain dates of some igneous dykes Granitic or porphyry dykes, known as el vans in Cornwall and Devon Igneous rocks in the lower portion of the new red sandstone series in Devonshire Dates of the Cornish and Devonian el vans Elvans of Wicklow and Wexford Chemical composition of igneous rocks Effect of silicate of lime in igneous rocks ------' -____'_. 551-572 CHAPTER XXX. Mode of occurrence of granites in South-western England and South-eastern Ireland Outline of the granite range of Wicklow Granite veins Chemical composition of granitic rocks Schorlaceous granites of Cornwall and Devon Slight covering of granite by the older rocks in Wicklow, Wexford, and Cornwall Serpentine and diallage rock of Cornwall Serpentine of Caer- * narvonshire Serpentine of Anglesea Chemical composition of serpentine Composition of serpentine and olivine compared Composition of green- stone and syenite Resemblances and differences between the ordinary granitic and hornblendic rocks Granitic, felspathic, hornblendic, and ser- pentinous rocks ejected at various times Relative fusibility of igneous, rocks Modification of the matter of igneous rocks Additional minerals entering into the composition of ordinary igneous rocks ::: - :: General character of igneous rocks - - - - - - -,-__ ___ 573-593 CONTENTS. xiii CHAPTER XXXI. Consolidation and adjustment of the component parts of rocks Adjustment of the component parts of calcareous and argillaceous deposits Arrange- ment of similar matter in nodules Central fractures in septaria Nodules of phosphate of lime in rocks Spheroidal concretions in the Silurian rocks Crystals of iron pyrites in clays and shales Mode of occurrence of sul- phate of lime Modification in the structure of rocks from exposure to changes of temperature Chloride of sodium disseminated amid rocks Variable deposits of detrital matter Importance of silica and the silicates in the consolidation of the detrital rocks Alteration of rocks, on minor scale, by heat Formation of crystals in altered rocks Crystalline modifi- cation of rocks Alteration of rocks near granitic masses Readjustment of parts of igneous rocks Production of certain minerals in altered rocks Mineral matter introduced into altered rocks Mica slate and gneiss 594-613 CHAPTER XXXII. Cleavage of rocks Cleavage in sandstones and shales Cleavage in shales and limestones Minor interruptions to cleavage action Cleavage on the large scale Range of cleavage through contorted rocks Double cleavage Relative dates of cleavage Elongation and distortion of organic remains by cleavage action Different directions of cleavage in the same or juxta- posed districts Joints in rocks Directions and range of joints Joints in granite Joints in sedimentary rocks Joints among conglomerates Joints in limestone Movement of rocks after jointing Complication of bedding, jointing, and cleavage -------___ 614-631 CHAPTER XXXIII. Bending, contortion, and fracture of bedded rocks Earth's radius compared with mountain heights The volume of the earth compared with mountain ranges Effects of a gradually-cooling globe Mountain ranges viewed' on the large scale Directions of mountain chains Conditions effecting the obliteration or preservation of mountain chains Lateral pressure on beds of rock amid mountains Bending and folding of deposits rn the Appalachian zone, North America Flexures and plications of rocks in the Alps, and district of older deposits of the Rhine Igneous rocks amid contorted beds Contorted coal-measures of South Wales Contortion of the component parts of beds _-_-_-__--_- 632-648 CHAPTER XXXIV. Faults Production and^direction of fissures through rocks Evidence of the relative dates of fissures Fallacious appearance from a single movement .shifting various fissures Fissures split at their ends Lines of least resist- ance to fissures Range of mineral veins and common faults in South- western England Range of faults near Swansea Inclination of faults- Parts of deposits';preserved by faults Complicated faults - 649-664 xiv CONTENTS. Page CHAPTER XXXV. Filling of fissures and other cavities with mineral matter Sulphurets of lead, copper, &c., replacing shells Filling of minor fissures Solubility and deposit of mineral matter in fissures Solubility of sulphate of baryta Deposits from solutions in fissures Effects produced in heated fissures beneath seas Many similar substances found in mineral springs and veins Frequent occurrence of sulphur, arsenic, &c., with certain metals in mineral veins Action and reaction of substances upon each other in fissures and cavities Character of metalliferous veins amid associated dissimilar rocks Condition of mineral veins traversing elvan dykes in Cornwall Influence of the different rocks traversed on the mineral contents of fissures Mode of occurrence of lead ores amid the limestones and igneous rocks of Derbyshire ' Flats ' of lead ore in limestone districts Metalliferous deposits in the joints of rocks Relative different dates of ' mineral veins - - 665-687 CHAPTER XXXVI. Modification of the contents of mineral veins in their depth and range Modi- fications of the upper part of mineral veins from atmospheric influences Sulphurets of lead and zinc converted into carbonates in mineral veins Replacement of mineral matter of one kind by another in veins Coating of the walls of fissures by layers of mineral matter Fissures coated by dis- similar substances Several successive movements through the same fissure Sliding of the sides of fissures on mineral matter accumulated in them at intervals Fractures through the mineral contents of fissures Modification of the contents of fissures at the crossing of veins Effects on the contents of fissures meeting or crossing at small angles - - 688-704 CHAPTER XXXVII. Partial removal or denudation of rocks Great denudation arising from the action of breakers Ancient exposure of the coasts of the area of the British Islands to the Atlantic Care required respecting the amount of denudation of overlapping rocks Island masses of deposits left by denudation Dis- tricts of bent and plicated beds worn down by denudation New slices of land now being cut away by breaker action Amount of matter removed by denudation Denudation in parts of England and Wales Needful attention to the greater geological problems - ______ 705-716 APPENDIX. Geological maps and sections Great Salt Lake of North America Prof. Burisen's Researches on the volcanic rocks of Iceland - 717-726 INTRODUCTION. OBSERVATIONS have now been sufficiently extended and multiplied to show that, during a long lapse of time, the surface of our planet has been undergoing modifications and changes. Of these the most marked have been produced by the uprise of mineral matter in a molten state from beneath that surface ; by the wearing away and removal to other localities of this matter, either in its first state, after cooling, or in some secondary condition, by atmospheric influences and waters variously distributed for the time being; by the preservation of the remains of animal and vegetable life during at least a portion of this lapse of time amid deposits accumulated, for the most part, in horizontal layers beneath waters, and by the unquiet state of the earth's surface itself, from which, while consi- derable areas have been at different times Raised slowly above, and depressed beneath the level of the ocean, whole masses of mineral matter of various kinds have occasionally been squeezed, bent, and plicated, sometimes ridged up into ranges of mountains. To enable the geologist systematically to proceed with his researches, it became as needful for him as for other cultivators of science to have the power of classifying his observations. Of the .various classifications proposed or modified at different times to satisfy the amount of knowledge of those times, it Vould be out of place here to make mention, further than to remark that at present a more mixed classification is often employed than seems desirable. For example, it is not unusual for the term tertiary r , or tertiaries, to be applied to all accumulations posterior to the chalk of western Europe, while the other terms of secondary and primary or primitive, to which it has reference, are scarcely or seldom mentioned. We have, again, a mixed nomenclature for the groups of deposits, or the deposits themselves, for which it has been thought desirable to xvi INTRODUCTION. find distinctive names. While some groups are referred to localities, such as Cambrian, Silurian, Jurassic, and the like; others are named after some circumstance supposed characteristic, such as carboniferous, from containing the great coal deposits of Europe and North America ; or oolitic, from many of the limestones in it being oolitic, that is, resembling the roe of a fish, being composed i A J &<3f 'numerous small rounded grains. It has been often considered that names derived from localities, where certain deposits have been taken as types, are preferable to those pointing to any mineral structure, inasmuch, as not only can the geologist readily make himself familiar with the kind of accu- mulations intended to be represented by the names, by visiting and studying the localities whence they are taken, but as also particular mineral structures having been repeated as often as the conditions for them arose, they form no guide for determining the relative age of rocks, whatever may have been the impression when names of that kind were given, and geological science less advanced than at present. The two structural names mentioned are thus liable to objection, carboniferous deposits extending from an earlier period than that supposed to be represented by the term, and up to the higher accumulations above the cretaceous series inclusive, and the oolitic character reaching from limestones amid the earlier fossili- ferous rocks to the present day.* The mixed character of the present geological nomenclature arises, no doubt, from the manner in which, from time to time, various geologists have directed atten- tion to different rocks or accumulations of them, those names having generally remained which have been found convenient and suffi- cient, up to the present time, for the purposes for which they have been employed. The igneous products being those from which the chief part, if not the whole, of the detrital, and even chemical deposits have been directly or indirectly derived, it would appear desirable to consider them in the first place. Whatever the views entertained of the fluid condition of our planet, whence its form has resulted, such fluid condition produced by heat sufficient to keep all its com- * One of the limestones of the lower Silurian series in North Wales, the Rhiwlas, near Bala, is oolitic. INTRODUCTION. xvii ponent parts in that state, the present condition of the earth's surface in dispersed localities shows an abundance of points through which igneous products are now ejected, and the more extended the observation, the more certain does the inference appear correct, that the like has happened from the earliest times ; at least since the seas were tenanted by life. It has also been ascertained that molten matter has risen from beneath in more massive forms, and in a manner with which we are not familiar, as now occurring, though such molten masses may, indeed, be formed at depths in the earth's crust, whence only future geological changes could bring them above the level of the sea. At all events, this massive form of intrusion is found amid comparatively recent geological accumulations, as well as among those of the most ancient date. The mode of occurrence of the igneous rocks, which will be found treated of in its place in the following pages, would seem to point to their classification according to their chemical and mineralogical characters, so that any resemblance or difference that may exist between them, may be traced through the lapse of geo- logical time, the relative dates of their appearance being obtained by means of the accumulations with which they may be associated, and to which relative geological dates can be assigned. Having entered upon these characters in the sequel, the following sketch of the more prominent of the igneous rocks may here suffice : Granitic Rocks. Those composed of a granular mixture of quartz, felspar (whether orthoclase, albite, or labradorite), and mica, with, occa- sionally, the addition of schorl and some other minerals. As the aspect of these rocks varies considerably according to original chemical composition or the mode of cooling, a great variety of appearances are assumed, to which names have been assigned. It thus becomes desirable that these characters should be given whenever it can be accomplished, and that the mere term granitic be accompanied by mineralogical detail, and by a state- ment of the chemical composition, so that correct data may be collected for a proper appreciation of the real differences and resemblances of the rocks commonly thus named. Felspathic Rocks. The separation of these from the foregoing may often be regarded as somewhat imaginary, as indeed is the case with definite classifications of the great bulk of the igneous rocks, passing, as they some- times do, into each other in masses of no very extraordinary volume. The variety known as compact felspar is most frequently a compound of the 6 xviii INTRODUCTION. elements of some felspar, with a surplusage of silicic acid beyond that required for the silicates of that mineral, so that when opportunities have occurred for crystallization of the parts, the result has been a compound of felspar and quartz, or a pegmatite, as it has been sometimes termed, in that case a modification of the granitic rocks when the same minerals may alone constitute a portion of a general mass. The trachytes of active vol- canos and those termed extinct, and of comparatively recent geological date, may represent the more pure felspathic rocks, when wholly formed of felspars, though it would appear that similar rocks are also found amid the igneous products of very ancient geological periods. Felspathic matter, that is, the various component substances in proportions which would form minerals of the felspar family (allowing for that substitution of one substance for another, termed isomorphism), if crystallized, should at least constitute the great bulk of these rocks, whatever others may be entangled among them. ffornblendic Rocks. These, including among them the rocks in which augite is substituted for hornblende, form a somewhat natural division, so far as the prevalence of these minerals may be sufficient to give a character to the mass of an igneous rock, inasmuch as silicate of lime is a marked ingredient, in addition to the silicate of magnesia, another essential sub- stance, and protoxide of iron, generally present, sometimes replacing much of the lime and magnesia. In this division, therefore, are included the dolerites and basalts of active and extinct volcanic products, and the green- stones, generally of more ancient date. In dolerites, silicate of lime is also present in the labradorite, when that member of the felspar family is mingled with the augite of that rock. Taken as a whole, the hornblendic or augitic rocks are compounds of those minerals and some member of the felspar family, there being sometimes an excess of silica beyond the amount required for the various silicates in the hornblende or augite, and felspar ; this excess, then, as it were, thrust aside as quartz. Serpentinous Rocks. To a certain extent these also appear a somewhat natural group of igneous products, especially when viewed with reference to a peculiar aspect, and to the presence of silicate of magnesia (constituting the bulk of the rock) and combined water. In the sequel we have endea- voured to show the correspondence between the varieties of serpentine, considered the most pure, and olivine, a common mineral in certain molten products of active and extinct volcanos. The rocks of this division vary, however, somewhat materially in their constituent substances, and in the proportions of them. Taking bronzite to be the mineral usually named diallage, it would appear little else than the silicate of magnesia of the matter of the purer serpentine mingled with a minor proportion of protoxide of iron, and a little alumina, crystallized, a small quantity of water also forming a part of it. The mineral now chiefly named diallage contains INTRODUCTION. xlx sufficient lime in addition to make it essentially a silicate of lime and magnesia, with also a marked quantity of oxide of iron. In the compound, sometimes largely crystallized, termed diallage rock (gabbro), and not unfre- quently associated with serpentine, the so-termed diallage has to be care- fully examined. In all these rocks, whatever their variations, magnesia is a marked ingredient. Porphyritic Hocks. Though, no doubt, various kinds of mineral matter which have been in a molten state may be porphyritic, that is, have some mineral or minerals crystallized out and apart from the mass of the remainder of the rock, it seems neyertheless convenient, for the present, to notice these rocks as a group. Even amid vitreous matter, from comparatively quick cooling after fusion, definite chemical combinations may be crystal- lized, and dispersed through such matter. This can be artificially accom- plished in our laboratories, and silicate of lime in crystals can be obtained dispersed through ordinary glass. In the arrangement of particles, beyond the vitreous condition, forming the compact and stony state, the porphyritic character is not rare among rocks ; crystals, such as those of felspar, being dispersed amid a base of compact mineral matter. When the latter is chiefly felspathic, the rock is usually known as felspar porphyry. In like manner crystals of other minerals are also thus dispersed amid a similar base, such as those of quartz and mica. The base or general mass of the rock is occasionally granular, such as a compound of felspar and hornblende, constituting greenstone, with dispersed crystals of felspar or hornblende, such base having thus advanced to a state of confused crystallization. These are usually termed greenstone porphyries. In like manner certain granites become porphyritic, from separate crystals of felspar being scattered among the general compound, confusedly crystallized, and the rock is then called -a. porphyritic granite. Even serpentines become in a manner por- phyritic when crystals of bronzite or diallage are dispersed through a base of that rock. The apparent conditions are, that the chemical composition and the mode of cooling of the general mass are such that certain consti- tuent substances can combine and form separate and definite crystallized bodies, the remainder of the rock either not attaining the state when definite mineral compounds can be formed, or only doing so after the pro- duction of the first-formed minerals, and then in a confused manner, not interfering with the forms of the crystals first produced. With regard to the mineral accumulations derived either directly or indirectly from the igneous rocks, and spread over areas of varied extent and form, by means of water, there is a large mass, more or less characterized by the presence among it of the remains of" animals and plants which have existed at different periods, and so perishing, that portions of them, commonly only the harder xx INTRODUCTION. parts, have been entombed in the mineral accumulations of such different times. Observation has shown that these accumulations have succeeded one another, as the various detrital deposits in lakes and seas now succeed those which have preceded them, so that, when the ancient sea or lake bottoms, which, elevated into the atmosphere, now constitute so large a portion of dry land, can be studied in cliffs or other natural sections, or by artificial cuttings or perfora- tions, their manner of succession can be -ascertained. The more investigations have advanced, the more does it appear that these organic-remain bearing, or fossiliferous rocks, as they have been termed, have been deposited and arranged as similar accumulations now are in rivers, estuaries, lakes, and seas. Hence, the geologist, in endeavouring to ascertain the range of such fossiliferous deposits at any given time upon the earth's surface, has to consider the relative amount and position of the land and waters of that time, with all their modifying influences, as also the various conditions under which the life of the period may have been distributed, and its remains entombed amid the detrital and chemical deposits of the day. In fact, he has, from all the evidence he can collect, to suppose himself studying the state of the earth's surface, at such given time, as well with respect to its physical condition as to the existence and distribution of life upon it. Viewing the fossiliferous rocks in this manner, it may be that some of those divisions among them, which it has been found convenient to make for their more ready description, and the tracing of certain states of a sea-bottom over minor areas, have been too minute, regarded as divisions applicable to the surface of the earth generally, since it is not to be supposed that particular mud or sand banks, however considerable locally, were more likely to have been formerly continued, even at intervals, over the earth's surface than they now are. At the same time such minor divisions, showing the constancy or modification of conditions, as the case may be, over the minor areas, are important, inasmuch as it is by a correct appreciation of this detail and the careful consideration of how much may be regarded in that light and how much as more general, that we learn the true value of the latter, INTRODUCTION. xx i and the restrictions which should be placed upon our views derived from the former. Assuming the general condition of the earth's surface during the accumulation of the varied deposits in which the remains of animal and vegetable life have been entombed, to have been for- merly much as at present, regarding the subject on the large scale, and without reference, for the moment, to the variable distribution of land and water, or to whether the heat in the earth itself may or may not, in remote times, have had a greater influence on the life of those times than at present, the sea would appear to have been the chief receptacle of the various mineral accumulations of all periods, so that classifications of the fossiliferous rocks, founded on a succession of deposits in it, would probably be alike the most useful and natural. The manner in which marine inver- tebrate animals now live, and the mode in which the remains of similar animals occur amid the fossiliferous rocks, are such, that this division of life seems now very generally admitted as the most appropriate on which to base classifications founded on the distri- bution of animals, the remains of which are discovered entombed in rocks. We must refer to succeeding pages for notices of the manner in which the remains of life are now preserved in mineral deposits, and for certain points connected with the occurrence of such remains in the accumulations of various geological dates which it appears desirable to bear in mind while studying the fossiliferous rocks. It will be sufficient here to mention that, after first duly ascertaining the actual relative superposition of the various mineral accumulations themselves for evidence of their real succession, and examining the remains of anftnal and vegetable life which have been found in them, it has been inferred that certain minor and major divisions may be effected in the general mass which shall represent the kinds of sea-bottoms marking given and succeeding geological times. Without, in the least, doubting that very great modification may be found needed in classifications based upon the examinations of even considerable areas, when an effective classification, representing the main facts connected with the accumulation and spread of fossiliferous rocks over large portions of the earth's surface, may be necessary, it still becomes xxii INTRODUCTION. desirable to have that which may satisfy the requirements for the time being. The following sketch, therefore, of the general divi- sions at present considered desirable for the area of Western Europe, and supposed, in part at least, to be found also convenient for the mode of viewing the fossiliferous deposits in many other parts of the world, may be useful, especially as respects the major divisions. STRATIFIED AND FOSSILIFEROUS ROCKS. I. Tertiary, or Cainozoic. II. Secondary, or Mesozoic. III. Primary, or Palaeozoic. /. Tertiary, or Cainozoic. !a Mineral accumulations of the present time. 6 Pleistocene, c Pleiocene. B. Middle a Miocene. C. Lower . . .- Eocene. //. Secondary, or Mesozoic. a Chalk of Maestricht and Denmark. b Ordinary chalk, with and without flints, c Upper Green Sand. d Gault. e Shanklin Sands, Vecten, Neocomian, or Lower Green Sand. (a Wealden clay . . \ Organic remains in these are of a 6 Hastings sands . > fluviatile, lacustrine, or estuary c Purbeck series. . J character.* 'a Portland oolite or limestone/t* 6 Portland sands, c Kimmeridge clay. d Coral rag, and its accompanying grits. e Oxford clay, with Kelloways rock. / Cornbrash. g Forest marble, and Bath oolite. h Fuller's earth, clay, and limestone. i Inferior oolite, and its sands. *k Lias, upper and lower, with its intermediate marlstone. * The recent researches of Professor Edward Forbes among the Purbeck series have fully illustrated the prudence of not trusting to fresh-water molluscs as characterizing particular divisions in deposits, at least those ranging downwards to that part of the fob- siliferous series, he having ascertained that it required most careful critical examination to distinguish the fresh-water shells of that series, as it occurs at Purbeck, from those of certain existing fresh-water molluscs in England and part of Europe. t The minor divisions of this group have been given with reference to those usually A. Cretaceous Group C. Jurassic or Oolitic Group* INTRODUCTION. (a Variegated marls, Marnes Irise'es, Keuper. 6 Muschelk a lk.f c Red sandstone, Gres Bigarre, Bunter sandstein. 777. Primary, or Palaeozoic. !a Zechstein, Dolomitic or magnesian limestone. b Rothe todte liegende, lower new red conglomerate and sandstones, Gres Rouge. B. Marine equivalents of % . a Coal measures, Terrain Houiller, Stein Kohlen Geberge. a Carboniferous and mountain limestone, with its coal, C. Carboniferous limestone Group. sandstone, and shale beds in some districts. Calcaire carbonifere, Bergkalk. 6 Carboniferous slates and yellow sandstone. D. Devonian group . . a Various modifications of the old red sandstone series. a Upper ; Ludlow Rocks, Wenlock shale and limestone, E. Silurian Group Woolhope Limestone. b Middle ; Caradoc sandstone and conglomerate. c Lower; Llandeilo, Bala and Snowdon beds. (a Barmouth sandstones, Penrhyn slates, Longmynd rocks, &c. Various rocks subjacent to the Silurian series in Wales and Ireland. employed in England for the sake of English observers. Many modifications have been shown to be effected in other European countries. Of these divisions those of the Oxford clay and lias would appear much extended. * The Trias and Permian groups afford an example, as regards the British islands, of a classification taken from organic remains in preference to the mode of occurrence of the rocks themselves, these groups here constituting parts of a general series of deposits with a somewhat marked general character, known as the new red sandstone. Certain general physical conditions were prevalent during the accumulation of these deposits in Great Britain, and certain portions of Western Europe, at the time that a modification in the life of the period was apparently effected in the same area and those adjacent to it on the north and east. t In the collections lately brought to England by Captain Strachey, Bengal Engineers, after an examination of the Himalaya range, the forms of certain organic remains from the Thibet side of those mountains remind the geologist of those found marking the Mus- chelkalk of Germany ; an interesting circumstance, considering the range of that rock in Europe. J When the great thickness of these deposits in Europe and America is considered, it becomes very desirable to find their marine equivalents, inasmuch as the conditions under which the great mass of these coal measures has been accumulated, as has been noticed'in the sequel, could scarcely constitute other than minor parts of those generally prevailing at the time. It is easy to conceive, as has indeed been done, that their marine equivalents might contain either the organic remains usually found in the deposit beneath them in parts of Western Europe, or those found in the group above them, or a mixture of both. In Northern England the alternations of conditions by which coal beds were included in the carboniferous limestone series, did not interrupt those for the existence of a marked kind of marine animal life in the same localities. xx iv INTRODUCTION. ALTERED OR METAMORPHIC ROCKS. , With the classification of detrital and chemical accumulations, effected by the aid of water, and of earlier geological date than those last mentioned (the Cambrian), there are many difficulties. Indeed the limits which may be assigned to the latter, in descending order, are, in the present state of our knowledge, most uncertain. In the district of the Eongmynd (Shropshire) the Cambrian group attains a thickness of 26,000 feet, almost entirely composed of detrital deposits. The same group, as exhibited in North Wales, not only presents a considerable depth of similar accumulations, but also shows, by pebbles in its conglomerates (vicinity of Bangor, Llan- beris, &c.), that sands were firmly cemented into sandstones, and these ground into shingles, by water action, prior to the production of such conglomerates. These conglomerates, which also contain rounded fragments of hornblendic and felspathic igneous rocks, of a general character similar to those subsequently vomited forth in the same region amid the Silurian deposits, may only constitute portions of a series in the same manner that many other con- glomerates are included in groups of rocks bearing given names ; in fact, be the beaches of different portions of the time required for the whole deposit ; yet they, with the muds, silts, and sands of the period, become important as pointing to causes in action at that time similar to those from which the like accumulations have been effected in after geological periods. Thus, as far as researches have yet gone, we do not arrive at physical conditions differing, as regards the production of detrital mineral accumulations, in any essential manner, that can be determined, from those which have afterwards influenced the accumulation of similar kinds of mud, silt, sand, and gravel, whether now constituting hard consoli- dated rocks, or found in a state more resembling that in which they were originally formed. The aid to the classification of rocks, once supposed to be derived from certain of them having a crystalline or semi-crystalline aspect, yet still preserving a general stratified arrangement of parts, various modifications of them bearing distinct names, such as gneiss, mica slate, and others, is now well known to be unsatisfac- INTRODUCTION. xxv tory, inasmuch as such rocks have been ascertained to be of different geological dates. Whenever any heated mass of igneous rocks has been thrown into juxtaposition with detrital accumula- tions of various kinds (mud, silt, sand, or gravel), the conditions under which the latter are then placed become favourable for the modification of their component parts. Various circumstances, heat being regarded only as one of them, then so act that, besides a tendency of similar matter to gather itself together in irregular forms, particles can often so freely move and adjust themselves, that even minerals of distinct characters are formed, rarely, and sometimes not hitherto, discovered amid any other than these modified or altered rocks. As such modifications or alterations would be expected to depend upon the general chemical character and physical structure of the deposits acted upon, these minerals are found to be combinations of the substances which could readily move and unite in a definite manner. Thus, the altered rocks afford such minerals as andalusite (especially a silicate of alumina, the base of clays), chiastolite (another mineral, in which silicate of alumina is the chief ingredient), cyanite (another form in which the same substance is essential), staurolite (where silica, alumina, and peroxide of iron are required), and the garnet, with all its differences arising from isomorphism, and in which silica may be prominently combined with alumina or iron, magnesia or lime, as the case may be. While minerals of such kinds could be developed in these altered rocks, we should also anticipate that micaceous and siliceous sands or sandstones, as also those in which frag- mentary portions of felspar were mingled, and especially when the latter were not decomposed, would be much modified by a free movement of certain substances. We might expect much oblitera- tion of the grains of the sand, a disposal of the silica in planes, either of original deposit or of cleavage, should that have been effected in the rocks acted upon, as also that the micaceous and felspathic portions might be more gathered together in places, and even readjusted in crystals, since we know, as regards the solution of the component matter of such minerals, that in some veins evidently filling fissures, quartz, mica, and felspar are found either alone, or mingled with other minerals in a manner pointing to o xrri INTRODUCTION. their production from solutions, their component parts derived from the adjacent rocks. The like circumstances acting upon more simple substances, formed into beds of rock, such as limestones, and the combinations of ordinary calcareous matter with carbonate of magnesia in various proportions, would necessarily also produce modification in the arrangement of the component particles, a confused crystalline adjustment of them being effected, such as that seen in statuary marble, when conditions were most favourable. When these bodies were less pure, mingled with detrital matter of the ordinary kinds, the circumstances would be favourable to the development of different mineral substances, such as garnets and others,* amid the general mass. Looking at the varied modes in which detrital and chemical accumulations have been formed, and the different manner in which they can be acted upon by the influences noticed, either on the minor or large scale, the general result could scarcely be otherwise than of the most varied kind. To attempt, therefore, a classification of these modified or altered rocks relatively to geolo- gical dates, would be obviously useless. In considering rocks of this kind it is needful also to bear in mind the general conditions under which beds of detrital or chemical deposits may be modified, or altered from their original state of accumulation, by other conditions than those of the contact or juxtaposition of mineral matter in a state of igneous fusion. Independently of chemical changes effected by the arrangement of the substances in different states of combination, adjusting them- selves according to their affinities and the conditions under which they are then placed,t the circumstances which would arise when * In this manner crystals of quartz have been sometimes produced in beds of statuary marble, as, for example, that of Carrai'a. f We find quartz rocks (that is, grains of quartz, accumulated as sands, and firmly cemented together by silica, the separation of the old surfaces of the sand-grains from the siliceous cement sometimes obscure) as the continuation of ordinary beds of quartzose sandstone, the latter sometimes slightly consolidated, and have simply to infer, to account for the facts observed, silica infiltrated so as to consolidate the beds more in certain situa- tions than in others. Such quartz rocks have often been supposed " altered or meta- morphic" in the sense used for some of the same general aspect acted upon, with others, by juxtaposed igneous matter which had been in a fused state from heat; whereas they INTRODUCTION. xxv ii such beds of rock were deeply buried beneath great accumulations of mineral matter, have to be carefully considered, if the tempera- ture increases in the manner usually inferred as we descend beneath the surface of the earth, even to moderate distances. Huge masses, representing former wide-spread portions of the earth's surface, might thus be placed under conditions similar to those which produce modification and alteration when igneous matter rises from beneath and is forced amid or against detrital and chemical deposits. When again upheaved, as we know great and wide-spread masses of rock have often been during the lapse of geological time, it would be anticipated that similar matter, acted upon in a similar manner, would present like results, and there is much reason to consider that such influences have been the causes of the modification and alteration we sometimes find. By carefully regarding altered or metamorphic rocks on the large scale, and with reference to all the conditions under which they may be produced, they are found to constitute a mass of mineral matter of much importance, showing us, in their most crystalline readjusted state, the extremes to which such matter may be modified without the mingling of parts inferred when in a state of igneous fusion. Igneous rocks themselves are often modified, their component particles having, as it were, striven to adjust themselves in a perfect manner as in the detrital and chemical deposits. Thus, ordinary greenstone can be sometimes observed to have its component minerals, hornblende and felspar, presenting the aspect of the rock known as hornblende rock, and beds of similar matter, either abraded from solid greenstones or vomited forth as ashes, and arranged in beds by the agency of water, to become the rock known as hornblende slate. Thus, then, without attempting to classify these modifications and alterations in the arrangement of the component parts of detrital, chemical, and are merely more firmly cemented and purer quartzose or siliceous modifications of com- mon hard grits, dispersed amid soft marls and shales in so many deposits. Again, the original crystalline accumulations of more chemically-formed heds have to be duly regarded, and separated from the " altered or metamorphic rocks" under notice, as we know that even confused crystalline deposits have been thus produced. xxviii INTRODUCTION. even igneous accumulations, geologically, as regards relative dates, they still, to a certain extent, constitute a class very convenient for investigation, it being always borne in mind that it is de- sirable only so to regard them, in the present state of our knowledge. I THE GEOLOGICAL OBSERVER. CHAPTER 1. DECOMPOSITION OF ROCKS. FORMATION OF SOILS. DECOMPOSITION OF GRANITIC ROCKS. DECOMPOSITION OF SANDSTONES AND LIMESTONES. INFLUENCE OF STRUCTURE AND ORGANIC REMAINS ON DECOMPOSITION. DECOMPOSITION OF ROCKS CONTAINING IRON. As geological knowledge advances, the more evident does it become that we should first ascertain the various modifications and changes which now take place on the surface of the earth, carefully con- sidering their causes, and then proceed to employ this knowledge, so far as it can be made applicable, in explanation of the geological accumulations of prior date. This done, we should proceed to view the facts not thus explained, with reference to the conditions and arrangements of matter which the form of our planet, the known distribution of its heat, the temperature of the surrounding space, and other obvious circumstances, may lead us to infer would be probable during the lapse of geological time. The geological observer cannot be long engaged in his researches before he will be struck with the tendency of rocks to decompose by the action of atmospheric influences upon them. He will soon perceive that this decomposition is both chemical and mechanical ; that certain mineral bodies more readily give way before these in- fluences than others ; and that from altered conditions, as regards such influences, the same kinds of rock will more easily decompose in one situation than in another. It is in consequence of this, decomposition that we have soils supporting that growth of vegetation upon which animal life 2 DECOMPOSITION OF ROCKS. [Cn. I. depends ; for soils are but the decomposed parts of more or less consolidated sea or lake bottoms and of igneous accumulations, with the remains of the vegetation which has grown on them, and of the animals which have lived upon the plants. From the varied configuration of surface the decomposed portions of rocks, forming soils, may not always cover those from whence they were derived, for they may and sometimes have been carried, mechanically sus- pended in water, to various distances, and there deposited, in such a manner as to be mingled with the decomposed portions of other rocks, or wholly cover over the latter. Be this, however, as it may, the decomposed parts of rocks form the base of the soils, affording soluble mineral matter to the plants requiring it, and presenting a physical structure capable of supporting their growth. The decomposition of rocks, in its various stages, will require much attention, so that the observer may properly classify the facts coming within the range of his researches. Among rocks of igneous origin, such as granites, greenstones, and the like, he will find that the decomposition of felspar is among the chief causes of the disintegration of the igneous masses of which this mineral may form a part. It would be out of place here to enter upon the composition of the various minerals of the felspar family ;* it will be sufficient to refer to those portions of them which are soluble, such as the silicates of potash or soda, as the case may be. These silicates, from the action of carbonic acid in the atmosphere, derived from the decay of vegetation, or brought into contact with them by waters containing it in sufficient abundance, are often readily de- composed. The particles once loosened by decomposition, and some of them carried off in solution, rains and changes of tem- perature, particularly in regions visited by frosts, act mechanically, and the surface of the rock, under favourable conditions, is removed. From a repetition of these causes the rock becomes decomposed to various depths, according to circumstances. In cases where the remaining portions are either too large or so situated as not to be readily carried away, a coating of the disintegrated insoluble part * The four minerals of this family which chiefly enter into the composition of rocks, are orthoclase, albite, labradorite, and oligoclase, the general chemical composition of which may be regarded as follows : Silica. Alumina. Potash. Soda. < Lime. Orthoclase 65'4 18 16'6a Albite . 69*3 19-1 11-66 Labradorite 33*7 29-7 4-5 12-1 Oligoclase 63 24 '9 12 -lc a. Including a little soda and lime. b. In part often replaced by lime or potash. c. Commonly, also, containing potash and lime. CH. I.] DECOMPOSITION OF ROCKS. remains, and to a certain extent protects the solid rock beneath irom that decomposition which it would otherwise have suffered. In many granitic regions ample opportunities are afforded of observing the amount of decomposition thus produced ; high tors or bosses of rock rising above a surface in a decomposed state Fig. 1; (fig. 1), while hard masses, having the fallacious appearance of boulders, rounded by attrition, are sometimes included in the loose decomposed granite, as represented beneath (fig. 2). Fig. 2. This illustration is taken from part of the road between Oke- hampton and Moreton Hampstead, Devon, a represents the vege- table soil ; b decomposed granite ; c c solid rounded masses of un- decomposed granite, included in the decomposed part ; and d d solid granite. In such a section as this, great care should, however, be taken to ascertain that c c are not transported boulders of granite, in- cluded in smaller granitic gravel, as sometimes happens with granitic drift, near the sources whence it has been derived. Fortunately in this case the observer would be assisted by the presence of large crystals of felspar disseminated through all parts of the rock, both decomposed and undecomposed, and which are beautifully preserved, remaining uninjured in their forms and in their relative positions throughout the decomposed granite. In granitic regions, sections such as that beneath (fig. 3), in Fig. 3. which a represents the vegetable soil, as it is commonly termed, B2 4 DECOMPOSITION OF ROCKS. [Cn. I. b the decomposed, and c the solid granite, are not unfrequent. Sections of this appearance should, also, be carefully examined, and it be clearly ascertained that the granitic particles at b are of the same kind, and in the same general relative positions, as those at 5 for the temperature of the greatest density of sea water, we shall have to consider that the salts in solution produce no influence upon such density, the water alone having to be regarded. It would be very desirable that experiments respecting the density of sea water at different temperatures should be repeated in the laboratory, and that observations should be made at different seasons upon the temperature of deep fresh-water lakes, in order to see if we are in any way to regard the temperature obtained in the sea of 39'5, observed by Sir James Ross, as a result to which some modifying influence may be attributed. * Some very interesting observations respecting the surface density of the sea off the coast of British Guiana were made by Dr. Davy (Jameson's "Edinburgh CH. VI.] SEDIMENT IN TIDAL SEAS. 99 covered by deposits from ocean currents, conveying detritus from the great continents, Australia, and the larger islands of the world, the oceanic islands may collectively furnish matter of importance. The observer will find that many of these islands rise from com- paratively considerable depths, so that detrital matter derived from them by the action of breakers (and they are very commonly exposed to a nearly-constant abrasion by the surf), moved by the tidal waves sweeping by the islands, and thence delivered into any ocean currents passing near, may be carried by the latter to con- siderable distances. These oceanic islands are found to be chiefly of two kinds, the one of igneous, the other of animal origin. With respect to the former, we have not only to consider the detritus they may now furnish by the action of breakers upon them, but also the transportable matter which may have been ejected from the igneous vents while they rose, by the accumulation of molten rock, cinders, and ashes. Instead of simply accumulating around the igneous vent, as would happen, with certain modifications from the distribution of wind- borne ashes and small local movements of water in tideless seas, not only might there be a to-and-fro distribution of the volcanic matter carried various distances in mechanical suspension from the tidal wave acting against the new obstacle to its movement, but the finer substances could also be borne away by any ocean current passing near, and thus such substances be carried far onward in the direction of its course. As soon as any igneous matter is raised above the sea level, so soon is it attacked by the breakers, and only Journal," vol. xliv, p. 43, 1848). He found that where the Demerara river meets the sea, near George Town, the density of the water was 1-0036, and subsequently as follows : 1. 11 miles off shore = 1-0210 2. 19 .. = 1-0236 3. 27 4. 35 5. 43 6. 51 = 1-0250 = 1-0236 = 1-0250 = 1-0258 7. 80 = 1-0266 The specific gravities of Nos. 4 and 5 were considered to have been influenced by heavy showers of rain which fell while the steamer on which Dr. Davy was on board passed. This modification in the density of the surface waters, by tropical rains, is well shown by the observations of the same author, off Antigua and Barbadoes. Towards the end of a very dry season, the specific gravity of the surface water, off the former, was found to be 1-0273, while, after three months of heavy rains, off Bar- badoes, the specific gravity was reduced to 1-0260. The positions of these two islands give such observations considerable value. With respect to the matter mechanically held in suspension in the waters off British Guiana, Dr. Davy states that, for many miles near the land, it was sufficient to give a light-brown tint to the sea, like the Thames at London-bridge. It was only at about the distance of 80 miles from shore that the waters presented the blue colour of the ocean. H 2 100 DISTRIBUTION AND DEPOSIT OF [Cn. VI. in proportion to its solidity and mass can the portion above water, and removed from the destructive action of the surf, remain to be more slowly wasted by atmospheric influences, and to be clothed with vegetation, if within climates fitted for its growth. Many an island in the ocean can be regarded as little else than the higher part or parts of a volcano, or some more extended system of volcanic vents, rising above its level, the mass and kind of matter ejected being sufficient to keep it there. As might be expected in a great volcanic region like that of Iceland, igneous vents have opened in the sea near its shores, as well as upon the dry land. A volcanic eruption is recorded as having taken place in 1783, about 30 miles from Cape Reikianes, and another off the same island about 1830.* In 1811 a volcanic eruption was effected through the sea off St. Michaels, Azores, and eventually, after the ejection of much matter, columns of black cinders being thrown to the height of 700 and 800 feet, an island was formed, about 300 feet high, and about one mile in circumference. Fortunately the formation of this island was observed and recorded. It was first discovered rising above the sea on the 13th June, 1811, and on the 17th was observed by Captain Tillard, commanding the " Sabrina" frigate, from the nearest cliff of St. Michaels. The volcanic bursts were described as resembling a mixed discharge of cannon and musketry, and were accompanied by a great abundance of lightning. The following (fig. 67) was a sketch made at the time, and will well illustrate the manner in which ashes and lapilli may be thrown into any ocean current or tidal stream passing along, and be borne away by it. This island, to which the name of Captain Tillard's frigate was assigned, subsequently disappeared, but whether simply by the action of the breakers alone, or from the subsidence of the main mass beneath, or from both causes, accounts do not enable us to judge.f * In 1783, the eruptions of several islands were observed as if raised from beneath, and, during some months, vast quantities of pumice and light slags were washed on shore. " In the beginning of June, earthquakes shook the whole of Iceland ; the flames in the sea disappeared, and a dreadful eruption commenced from the Shaptar Yokul, which is nearly 200 miles distant from the spot where the marine eruption took place." (Sir George Mackenzie's Travels in Iceland.) f This is not the only instance of a volcanic eruption forming a temporary island above the sea-level among the Western Islands. It is recorded in the MS. Journals of the Royal Society (a collection containing a mass of curious information respecting the progress of science after the foundation of the Royal Society), that Sir H. Shercs informed a meeting, of January 7th, 1690-91, "That his father, passing by the Western Islands, went on shore on an island that had been newly thrown up by a vol- cano, but that in a month or less it dissolved, and sunk into the sea, and is now no more to be found. CH. VI. 1 SEDIMENT IN TIDAL SEAS. Fig. 67. 101 No doubt very many of the supposed banks in the ocean upon which the surf is stated to have been seen breaking, and never afterwards found, may be very imaginary, but it is still possible, that here and there statements of this kind may be founded upon more positive evidence ; and that, making all allowance for in- correct views as to the latitude and longitude of the supposed banks, some due to the upraising of volcanic cinders and ashes have been observed, these finally so cut away that the sea no longer broke over them. However this may be, we can scarcely suppose that over the floor of the ocean all the eruptions from every volcanic vent upon it have reached above the surface of the water and remained there as islands, or that some, which have accumulated matter to depths not far beneath the surface waters, may not occasionally so vomit forth cinders and ashes, that these substances remain for a time above water until removed by the influence of breakers. 1/1 CHAPTER VII. CHEMICAL DEPOSITS IN SEAS. DEPOSITS IN THE CASPIAN AND INLAND SEAS. CALCAREOUS DEPOSITS. FORMATION OF OOLITIC ROCKS. SALTS IN SEA JWATER. CHEMICAL DEPOSITS NOT NECESSARILY HORIZONTAL. WE have previously adverted to the mixed deposits of calcareous and sedimentary matter in tideless or nearly tideless seas, from which alternate layers of argillaceous limestones and clays, or lines of argil- laceous limestone nodules in the latter might result. According to the specific gravities of the waters of such seas, arising from the different amount of matter in solution in them, will, as we have seen, depend the distances over which river waters can flow out- wards, supposing such rivers,, for illustration, to be equal in volume and velocity, and as respects the amount of matter in solution or mechanically suspended. In this respect, the Caspian, the Black, and the Baltic Seas would all differ, the latter most approaching in the character of its waters to a fresh- water lake. Comparatively, these bodies of water would appear to afford greater tranquillity than tidal seas for the production of chemical deposits, always allowing for the depths to which their waters may be dis- turbed by surface causes, such as winds and changes in atmospheric temperature. In tideless seas, such as the Caspian, where the substances brought down in solution by the rivers accumulate in compara- tively still water, we should expect deposits which could not be effected with equal facility in the ocean, even in those parts which adjoin coasts. In the one case, evaporation keeps down the body of the water, probably even diminishing it? volume during a long lapse of time ; while, in the other, these solutions enter the great mass of ocean waters, and become so lost in it, that certain of them may only, under very favourable conditions, be able to accumulate as a coating or bed upon any previously-formed portion of the ocean floor. The way in which the tidal wave thrusts back river waters twice in each day (taking the subject in its generality), CH. VII.] CHEMICAL DEPOSITS IN SEAS. 103 mingling the common sea waters with those of rivers, up the estuaries, is alone a marked difference from the outpouring of the rivers, with their contained solutions unmixed until the river waters flow over the sea. Instead of comparative quiet along- shore, except where disturbed by the action of surface waves, the whole body of water along tidal coasts is kept in motion, moving alternately one way or the reverse, and not unfrequently in various directions, in consequence of the modification of the bottom, and the mode in which the tidal wave may strike variously- formed or combined masses of dry land. We have above called attention to the differences in tideless or nearly tideless seas, arising from differences between the evapo- ration of their surfaces, and their average supply of water from rivers or rains. Not only should we thence expect the modification of sedimentary deposits previously mentioned, but modifications also in the chemical coatings. An isolated area, like the Caspian, if the evaporation of its waters be greater than its supply, may, during such decrease, present us with conditions favourable to a de- posit of some of its salts, while the main mass of the waters may yet be well able to hold much saline matter in solution. Any shallow parts adjoining the shores becoming isolated, and therefore cut off from the river supplies afforded to the main body, may readily be deprived of all their water by evaporation, and a sheet of saline matter be the result. Indeed, in this manner, any substances in solution would become deposited, and how far they might remain exposed without being removed by atmospheric influences, would depend upon the climate of the locality. That any such beds, the result of the evaporation supposed, may be covered by ordinary sedimentary deposits, due to geological changes of the locality, will be obvious. Around such bodies of water as the Caspian, the observer pos- sesses good opportunities for studying subjects of this kind, which are of considerable interest geologically, when we consider the mode of occurrence of gypsum and rock-salt in many situations, the not unfrequent connexion of these substances, and the kinds of sedimentary matter with which they are often associated. It may be also deserving of attention to consider in such parts of the world the probable annual evaporation of the surface of seas like the Caspian, and the annual supply of waters from rivers and rain.* * It is interesting to consider, in any given land where such bodies of water may be found, even though of much less size, and where it seems certain, from geological evidence, that the present area occupied by such waters is less than formerly, how far 104 CHEMICAL DEPOSITS IN SEAS. [Cn. VII. It may have happened from geological changes, such as might readily convert the Persian Gulf into an isolated sea, by raising the bottom between Cape Mussendom and the opposite coasts at Grou and Sereek, or the Eed Sea, into another, by raising the bottom at Bab-el Mandeb, that these masses of water no longer communicated with the main ocean. Looking at the climatal conditions, and the absence of any great drainage from adjoining land flowing into it, the Eed Sea would lose its waters from evaporation, while with respect to those of the Persian Gulf, it would depend upon the difference between the evaporation and supply of water chiefly obtained from the Euphrates, Tigris, and their tributaries. From existing information, we should anticipate that this supply would not equal the evaporation, so that both bodies of water might become Caspians. It would be well if observers, when among such parts of the world, would gather information sufficient to show us the probable results of such alteration of conditions, especially as respects the deposits of substances now in solution in these seas, and their inter- mixture with common detrital matter. Observations directed to such points can scarcely fail to be valuable with respect to geological theory. Under the supposition of the conversion of the Eed Sea into a Caspian, not only might there be a mixture, under favourable conditions, of chemical deposits and detrital accumu- lations, but coral banks and reefs would be also included in them. By a glance at a map of Asia, it will be seen that a very large area, extending along 70 degrees of longitude from the Black Sea into China, with a varied breadth of 15 to 20 degrees of latitude, does not drain directly or indirectly into the ocean. There is reason to believe that it is a mass of land which, from geological changes* has been cut off from such drainage, the Caspian, the Sea of Aral, with numerous smaller bodies of water, now receiving such drainage waters as evaporation from the surface of this great area will permit, when gathered together in different positions. the climatal conditions may so influence the evaporation and supply of water that a kind of balance is established. We may, for illustration, suppose that, in the first place, the climatal conditions are such, after the separation of a mass of sea waters from con- nexion with the ocean, that a considerable diminution of the volume of the separated water, and consequently, in all probability, of the area occupied by it, takes place. Then will arise the local conditions, under which this diminution may either continue or a balance of evaporation and supply become established. Evaporation, all other things being equal, will depend upon the area of water exposed. If large rivers, such as the Volga, for example, entering the Caspian, bring much sediment into the sea or lake, they tend to make it shallow, and also, by their deltas, to diminish the area, so that the conditions, as to general area, depth, and consequent volume of the water, alter. This alone might destroy any balanced conditions. CH. VII.] CHEMICAL DEPOSITS IN SEAS. 105 The evaporation may completely overpower the supply of water in certain parts of such an area, the salts in solution in the pre- existing waters forming sheets of matter corresponding with the minor areas or lakes when such solutions became in a condition to permit deposits, the least soluble substances being the v first thrown down. A deposit of a particular substance once effected, similar matter would be more readily withdrawn from the solution by the attraction of the first deposits of such substance. In a dry climate, such portion of the common detritus, as did not become consoli- dated, would be swept about by the winds, forming deserts, such as we find in the region noticed, the great Chinese desert of Kobi, or Shamo, being the largest of them. In all such lands the explorer will not lose his time by carefully examining the shores of these various inland seas and lakes, observing the physical con- ditions which may produce the isolation of shallow parts. It would be well also to study deposits of saline matter with reference to their origin from conditions, which may have readily obtained, in consequence of geological changes, by the separation of shallow-water indentations fringing the ocean, particularly in warm and dry climates,* as well as by the partial or total evapo- ration of salt lakes. Amid the great flats which here and there occur on the shores of tidal seas, and which may become dry at certain times, so that patches of sea -water irregularly scattered over them are evaporated, leaving the salt, we have no doubt conditions, particularly in dry and warm climates, for the accumulation of thin sheets of salt, or other substances in solution, which, under favourable circumstances, might be covered up, and, to a certain extent, be preserved by detrital mud ; but these deposits would scarcely have the importance of those previously noticed. At the same time, such Situations should be examined with reference to the chemical accumulations which may be thus intermingled with detrital matter. * In all cases, where practicable, it is desirable to obtain information as to the matters in solution in the various inland seas and lakes. They are known to differ in this respect, as might be anticipated. Thus, according to M. Eichwald, the waters of the Caspian contain much sulphate of magnesia, in addition to the other salts held in solution. Those who are possessed of sufficient chemical knowledge, if they have with them any of the little portable chests of the needful substances and apparatus, will have a local means of a qualitative analysis. It would be well if they could perform a quantitative one on the spot, seeing the difficulty of conveying bottles of water, to be kept, perhaps, a long time, and amid high temperatures. When the ob- server may not be a chemist, he may still assist, under favourable conditions as to transport, by obtaining the waters and putting a sufficient quantity into a clean bottle, immediately sealing it up carefully and tight, and forwarding it, as soon as circum- stances may permit, to some experienced chemist for examination. 106 CHEMICAL DEPOSITS IN SEAS. [Cn. VII. With respect to deposits from chemical solution, the calcareous may be considered as the most important geologically. We have previously adverted to their production in the air, and in fresh- water lakes. The cases of consolidated beaches on some coasts, like those noticed in Asia Minor, may be regarded as in a great measure due to the evaporation of the water containing the bicarbonate of lime in solution, as it percolates through these beaches. In the same manner, we seem to obtain their consolidation in some places by the oxides of iron and manganese, and by other substances. Eespecting the actual formation of beds of limestone in the deeper sea by chemical deposit alone, though we feel assured that it is effected, the exact manner is scarcely yet well determined. The rivers flowing into both tideless and tidal seas alike transport calcareous matter in solution into them, though very variably ; in scarcely appreciable proportions in some, abundantly in others. So long as the carbonic acid needful for the solution of the carbonate of lime remains, the latter will continue in the waters, but should it be withdrawn, either by evaporation of the sea waters in shallow places, or by separation in any other way, the carbonate of lime, if the lime be not taken up in any other combination, will be deposited. With regard to shallow situations in tidal seas, particularly in warm climates, and where pools of water are left for sufficient time at neap tides, we should expect an evaporation of the water, at least in part, and a loss of the carbonic acid, enabling any carbonate of lime present to be held in solution, so that there was a consequent deposit of calcareous matter. This may be well seen where waters highly charged with bicarbonate of lime flow slowly into some nook or bay, on tropical coasts, and even in localities where the rise and fall of tide is small, as, for instance, around Jamaica. It is in such situations, under favourable conditions, that the little grains termed oolites, formed of concentric coatings of calcareous matter, may be sometimes observed to form. A slight to-and-fro motion, produced by gentle ripples of water, may occasionally be seen to keep the carbonate of lime depositing in movement and divided into minute portions, so that instead of a continuous coating of calcareous matter upon any solid substances beneath, a multitude of these little grains is produced. As might readily be anticipated, a small fragment of shell and even a minute crystal of carbonate of lime is sufficient to form a nucleus for the concentric coatings of these oolitic grains. An observer would do well, when an opportunity of this kind may present itself, to watch the mode in which the grains may be CH. VII.] CHEMICAL DEPOSITS IN SEAS. 107 mechanically accumulated, like any other grains of matter, by the wash of the sea, or the drift caused by tidal streams, as he will thereby be the better enabled to judge of the differences or resem- blances he may find between these accumulations and the beds formed of oolitic grains in the calcareous deposits of various geolo- gical ages. While the mode in which calcareous matter may be deposited on the shores of seas may thus be advantageously studied, that in which it is effected in deep water must necessarily be matter of inference. By the means previously noticed, a large collective amount of carbonate of lime, held in solution by the needful addition of car- bonic acid, is discharged by rivers into the sea ; more, no doubt, in some localities than in others, but still as a whole, somewhat widely- Although we might expect solutions of a great variety of substances in the sea, the drainage of the land supplying them constantly, our knowledge on this subject would be more advanced than it is at present, if waters were more collected in different parts of the world, and off a variety of coasts, than they have been. According to Professor Forchhammer, the greatest amount of saline matter in the Atlantic Ocean is found in the tropics far from land, in such places the sea-water containing 3 * 66 parts of saline matter in 100. He states, that the quantity diminishes in approach- ing the coasts, on account of the rivers pouring their waters into the sea, and that it also diminishes on the most western part of the Gulf Stream, where the proportion is 3*59 per cent. Professor Forchhammer proceeds to observe, that by the evaporation of the Gulf Stream waters, the quantity of saline matter increases towards the east, and reaches 3 65 per cent., in N. lat. 39 39' and W.long. 55 16'. Thence it decreases slowly towards the N.E. ; and at a distance of 60 to 80 miles from the western shores of England, the Atlantic contains 3*57 per cent, of solid substances in solution. The same proportion of salts is found all over the north-eastern part of the Atlantic, as far north as Iceland, at distances from the land not effected by the outflow of rivers.* * It is desirable that in all researches as to the amount of the saline contents of the ocean, the depth from which waters for examination may be taken, be regarded. With respect to the specific gravity of sea water at different depths, Sir James Ross mentions (Voyage of Discovery and Research in the Southern and Antarctic Regions), that in lat. 39 16' S. and long. 177 2' W. (there being no bottom at 3,600 feet), the specific gravity at the surface was 1-0274 ; at 900 feet, 1-0272 ; and at 2,700 feet, 1 '0268, all ascertained at 60 Fahrenheit. He further states that his daily experience gave this diminished kind of specific gravity in the depths. As evaporation would tend to render the surface waters more saline, it may be deserving of attention how far this cause may operate downwards in the sea. 108 CHEMICAL DEPOSITS IN SEAS. [Cn. VII. With respect to the chemical character of the saline substances in the waters of the Atlantic, it would appear that they do not differ so much as might be supposed. At the same time, Professor Forchhammer's researches lead him to consider that lime is rather rare around the West India Islands, where myriads of polyps employ it for their solid coral structures ; the proportion of lime to chlorine being there as 247 to 10,000, while the same substance is more common in the Kattegat, where part of the lime brought by nume- rous rivers into the Baltic is carried to the ocean. In the Kattegat the proportion of lime to chlorine is as 371 to 10,000. In the Atlantic Ocean 17 analyses gave 297 to 10,000; and between Faroe and Greenland 18 analyses afforded 300 to 10,000.* Eesearches of this kind, limited as they are at present, are still sufficient to point out the modifying influences of proximity to land, of the heat of the tropics, of the melting of ice in the polar regions, and of oceanic currents flowing from one region, where certain conditions prevail, to another where these may be modified. As geologists, we have to inquire if the salts in solution, and derived by means of rivers from the land, are thrown down on the sea-floor, either within a moderate distance from the land, or further removed in deeper oceanic waters. If we take the calcareous matter, we find that it can be transported, by means of rivers flowing outwards, for various distances over the heavier sea waters, to be still further carried outwards and into greater depths of water, probably, if an ocean current seizes on the river waters thus situated. No small aid would be afforded if, when fitting oppor- tunities presented themselves, waters from the streams which might thus be traversed were carefully examined with reference to their chemical character. In warm climates there might be much evaporation from the upper part of river waters thus slowly passing along the surface of the seas, productive of results, as regards matter in solution, of appreciable value. When we consult analyses of sea waters, to ascertain the condition in which lime may be present in them, we find enough to show that much is to be learnt by experiments made with the aid which the present methods of analysis can afford. We can readily under- stand that while lime may be pouring into some parts of the ocean, as a bicarbonate kept in solution by the proper amount of carbonic acid, it might be converted into solid matter by animal life in another, in regions where a balance of supply is not kept up, so * Forchammer. Memoirs of the British Association for the Advancement of Science, vol. xv. p. 90. CH. VII.] CHEMICAL DEPOSITS IN SEAS. 109 that eventually very unequal quantities are distributed in solution. But it would be well to ascertain such facts carefully, and especially with reference to the combination in which the lime may be found in the different regions of the ocean.* With respect to the deposit of carbonate of lime from sea waters, Dr. Lyon Playfair suggests that, as river waters generally contain in solution a small quantity of silicate of potash, the carbonic acid, dissolved in sea water, enabling the carbonate of lime to be therein held in solution, would act on this silicate, decomposing it, and forming a carbonate of potash. The solvent being thus removed from the carbonate of lime, the latter would be precipitated, and a new portion would be formed from the double decomposition of the newly-formed carbonate of potash on the sulphate of lime and chlo- ride of calcium when present. He suggests that this process of de- composition may account for the silica so frequently found in lime- stones. It is, however, to the action of vegetation, where this can flourish, on sea waters, that Dr. Lyon Playfair attributes a more general deposit of any carbonate of lime from them. He remarks, that marine, like terrestrial plants, constantly require and take away carbonic acid from the waters around them, so that the quan- tity necessary to keep any carbonate of lime in solution, and which may find its way into the sea waters, being removed, the carbonate of lime is thrown down. Independently of the soluble matter thrown into the sea by rivers returning to it frequently that which in anterior geological times was accumulated in it, we have to reflect that the volcanic action which we know has been set up upon the ocean-floor, sometimes throwing up matter above the surface of the sea, forming islands, must as a whole have caused no small amount of soluble matter to be vomited forth. Looking at the gases evolved and substances sublimed from sub-aerial volcanos, we should expect many combi- * We are indebted to Schweitzer for a very careful analysis of the waters of the English Channel. No doubt it is only good for the locality, one not favourable for a knowledge of the composition of oceanic waters, being too much shut in by land, from which river waters, differently charged with saline matter, are discharged. His analysis is as follows: Water ----- 964 '74372 Chloride of Sodium - - 27-05948 ,, Potassium - - 0-76552 ,, Magnesium 3 '66658 Bromide of Magnesium 0*2929 Sulphate of Magnesia - - 2-29578 Lime - - - - 1-40662 Carbonate of Lime - - - 0-03301 With, in addition to these constituents, distinct traces of iodine and ammonia. 110 CHEMICAL DEPOSITS IN SEAS. [CH. VII. nations to be formed and decompositions to arise. Seeing also the soundings around certain oceanic and volcanic islands, no slight pressure would have been exerted upon the earlier volcanic action beneath the seas, a modifying influence alone of no slight importance. Surrounded by seas of inferior temperature, closing in upon the volcanic vent as the heated waters rose upwards, there would be a tendency to have certain substances, only soluble at a high tempe- rature, thrown down where the cooling influences could be felt ; as also, when these substances may be borne upwards by the heated waters, to have them distributed by any oceanic currents acting over the locality, supposing that the heated waters either rose to, or were produced at distances beneath the surface of the sea where these currents could be felt. Without entering further upon this subject, we would merely desire to point out that, in volcanic regions, the sea may not only receive saline solutions marked by the presence of certain substances not so commonly thrown into it by rivers else- where, but that also submarine volcanic action may be effective in producing chemical deposits, either directly, or indirectly, which, under ordinary conditions, would either not be formed, or not so abundantly.* With regard to the mode in which chemical deposits may be accumulated, it is very needful to consider that horizontality is not essential to them. They may be formed at considerable angles, against any previously-existing surface offering the needful condi- tions. Numerous deposits from solutions are effected as well on the sides as on the bottoms of vessels containing them.f Hence we may have deposits on the large scale, giving rise to deceptive appearances. Let , for example, in the annexed section (fig. 68) Fig. 68. e d be the surface of a fluid, such as the sea, from which the beds, b, have been deposited from chemical solution (limestones for instance) * It would be very desirable to ascertain points of this kind, so far as examining the sea waters around volcanic regions may enable the observer to do so ; and more especially when, by any fortunate chance, opportunities are afforded after any sub- marine volcanic action may be evident or supposed. t Pipes conveying waters containing much bicarbonate of lime, or many other bubstanccs in solution, arc well known to be often coated all round. CH. VII.] CHEMICAL DEPOSITS IN SEAS. Ill upon the pre-existing surface, c d, of a stratified rock, c c, and it might, if only a portion of such a section was subsequently exposed, be concluded that there had been movements of the land tilting up these beds at e, when in reality there has been perfect repose as regards their relative position, since the time of their deposit. Even when, as a whole, somewhat horizontal accumulations of this kind might be expected, they are often found to have moulded themselves upon the irregularities of ground upon which they were thrown down. : CHAPTER VIII. PRESERVATION OF REMAINS OF EXISTING LIFE AMID MINERAL MATTER. OF PLANTS AND VEGETABLE MATTER. BOGS. DISMAL SWAMP.' RAFTS IN THE MISSISSIPPI. ANIMAL REMAINS ON THE LAND. VER- TEBRATA. OSSIFEROUS CAVERNS AND LAKE DEPOSITS. INSECTS. LAND MOLLUSCS. EFFECTS OF SHOWERS OF VOLCANIC ASHES. ESTUARY DEPOSITS. FOOTPRINTS ON MUD. THIS is a subject of much importance to the geologist desirous of reasoning correctly upon the mode in which the fossiliferous rocks may have been accumulated. The habits of plants and animals engage the attention of the naturalist, and by his aid most im- portant benefits are conferred upon the geologist. He is thus enabled to infer how plants or animals, found existing under certain conditions, may contribute by their remains to the mass of mineral accumulations now taking place, these occasionally even forming thick beds, spread over considerable areas, without the admixture of mud, and sometimes of any sediment derived from the decompo- sition or mechanical destruction of previously-existing rocks. The observer should, in the first place, direct his attention to the manner in which the remains of terrestrial life may be entombed. Though when terrestrial plants die, the substances of which they are composed are, as a mass, returned to the atmosphere and soil whence they have been derived, the movements of animals which may feed upon them being regarded as so far local, that keeping to the grounds where their food is presented to them, their droppings restore to the soil what the plants had removed from it, the car- nivorous animals which consume the graminivorous, returning that which the latter did not prior to death, there are still conditions under which parts of existing vegetation may become permanently preserved. Exposed to atmospheric influences after death, vegetation decays according to the structure of the different plants and the climate of CH. VIIL] PRESERVATION OF ORGANIC REMAINS. 113 the locality. The rapidity with which decomposition is effected in certain tropical regions is well worthy of attention. We not unfrequently find the outside of a large and prostrate tree retain- ing its form, and while the whole of the inside is hollow, filled with leaves that have fallen into it, and teems with animal life. This kind of decay is still more instructive when upright stems of plants, in tropical low grounds, liable to floods, retain their outside portions sufficiently long to have their inside hollows partially or wholly filled with leaves and mud or sand, the whole low ground silting up, so that sands, silt, and mud accumulate around these stems, entombing them in upright positions, without tops, though their roots retain their original extension. The study of the sedimentary accumulations of river deltas, amid the rank vegetation of some tropical countries, is very valuable as respects certain deposits in which the remains of vegetation form a conspicuous and important portion. Behind mangrove swamps much that has a geological bearing may be frequently seen ; and indeed amid them, the observer not forgetting to direct his atten- tion to the mode in which animal as well as vegetable remains become mingled with, and finally covered over by, sedimentary matter. Not only in the tropics, but in other regions, large tracts of marsh land, interspersed with shallow lakes, are highly favourable to the accumulation of vegetable substances. The leaves of trees, growing in such situations, falling upon the patches of water, take a horizontal position, spreading in a layer in certain climates and seasons over their surfaces. These leaves gradually soak up water, and sink to the bottom. If, from time to time, flood waters bring fine mineral matter in mechanical suspension into such situations, it settles, and thus the leaves become preserved in thin layers alternating with the clayey sediment. Should it so happen that waters, charged with calcareous matter in solution, find their way either gradually and constantly, or by sudden rushes in floods, we may have the leaves or other remains of plants preserved in a deposit of carbonate of lime, more or less pure, according to the presence of any other matter brought into the lakes in mechanical suspension or chemical solution. The manner in which bogs are formed should also be studied. Many no longer exhibit their progress over shallow lakes, while others will show it. In the latter case we find aquatic plants, like the large rushes and water lilies, accumulating mud about their roots, as also decaying vegetation, upon which finally the bog I 114 PRESERVATION OF ORGANIC REMAINS [Cn. VIII. plants advance, the chief of which, in our climate, is the Sphagnum palustre. As these decay beneath, a new growth continues above, up to levels where the requisite moisture can be obtained.* Trees are very frequently seen in these bogs (some of which are very extensive), in a manner showing that the conditions favourable for the growth of various trees have from time to time obtained, so that distinct levels of them have been found occasionally in the same bog. The extent of bogs is very variable, as also the bottoms on which they repose. Sometimes the latter are formed of shell marls, accu- mulated at the bottoms of the shallow lakes, anterior to the advance of the aquatic vegetation over them. The thickness of bogs neces- sarily varies : in some 10 to 30 or 40 feet is not uncommon. Of the pauses in the accumulation of bogs, sufficient to permit a growth of trees upon them, as also a surface upon which habitations may be constructed, perhaps as good an example as any is that of the ancient wooden house discovered in June, 1833, in Drumkelin Bog, on the north-east of Donegal. It was 16 feet below the sur- face of the bog before the upper part was taken off, and 4 feet beneath the cuttings of the time, standing itself upon 15 feet more of bog, so that the total thickness at that place had been 31 feet. The house itself was a square of 12 feet sides, and 9 feet high, and was formed of two floors, the roof constructed with thick planks of oak, the wood employed for the whole dwelling, upon which no iron had been used. Upon clearing away the bog from the level of the house, a paved pathway was discovered extending several yards from it to a hearthstone, covered with ashes, some bushels of half-burned charcoal, some nut-shells, and blocks of wood partly burned. Near the house there were stumps of oak trees, which grew at the time it was inhabited. A layer of sand had been spread over the ground before the erection of the house. All seems to have marked a state of repose in the growth of this part of the bog ; so that a change of conditions affecting the drainage would seem needful to account for the accumulation of 16 feet more above the surface, after the time when this wooden house was constructed. It may have been that one of those burstings of parts of a bog, * Those travelling in North Wales will find, opposite Cwm-y-glo, below the bridge crossing the outlet of Llyn-Padarn (the lower Llanberis lake), a good example of a lake filling up, with the advance of water lilies and other aquatic plants upon a still remaining portion, while bog plants and bog creep on behind them. At the proper season, the locality is brilliant with thousands of water lilies thus advancing. It is easy to see that this was once a third Llanberis lake, but, being shallow, was the first to be nearly filled up. CH. VIII.] AMID MINERAL ACCUMULATIONS. 115 some of which are recorded, had overwhelmed this locality, soft boggy matter having gradually accumulated to a higher level under favourable circumstances in some place adjacent. Bogs are very irregularly dispersed, forming unequal patches as to area and thickness. The surface occupied by the bogs of Ireland alone, has been estimated at 2,800,000 acres. From the humic acid in them, animal and vegetable substances are often found well preserved, and, in consequence, numerous relics of ancient times have been handed down to us, which, unless en- tombed in bogs, would have remained unknown. Other things have evidently been lost in them, and have been brought to light by the progress of the turf-cutter. Many of the beautiful bronze swords, spear-heads, and other ornaments and weapons of its ancient inhabitants, have been thus preserved in Ireland. As might be expected, also, the remains of animals are found which have perished in the bogs. Of bog-like accumulations in a warm climate, the " Dismal Swamp," as it is called 40 miles long, from north to south, and 25 miles in its greatest breadth, from east to west partly in the State of Virginia and partly in North Carolina, seems an excellent example. Sir Charles Lyell describes this swamp as " one vast quagmire, soft and muddy, except where the surface is rendered partially firm by a covering of vegetables and their matted roots." * From the nature of the mass, which appears to be chiefly formed of vegetable matter, spongy for the most part, logs and branches of trees intermingled in it, water is so disseminated that the central portions of the swamp are the highest, rising on all sides above the surrounding firm and dry land, except for about 12 or 15 miles on the western side, where rivers flow into it from more elevated ground. The greatest height of the central part above the sides is estimated at about 12 feet, and in such central portion there is a lake, 7 miles long and 5 miles wide. The greatest depth of this lake is 1 5 feet ; the sides are composed of steep banks of the vege- table mass, and the bottom is chiefly formed of the same matter in a highly-comminuted state, with sometimes a white sand, about a foot thick. Eivers flow out of the swamp from all other parts of its margin except that mentioned. It is a highly-interesting fact as connected with this swamp, one having many geological bearings, pointed out by Sir Charles Lyell, that the surface supports a growth even of trees. He mentions * Lyell's Travels in North America, vol. i., p. 143. i 2 116 PRESERVATION OF ORGANIC REMAINS [Cn. VIII. the juniper trees (Cupressus thyoides) as standing firmly in the softest places, supported by their long tap-roots. With other ever- greens these trees form a shade, under which grows a multitude of ferns, reeds, and shrubs. The great cedar ( Cupressus disticha) also flourishes under favourable conditions. Trunks of large and tall trees lie buried in the swamp. They are easily upset by ex- traordinary winds and covered in the mire, where, with the excep- tion of the sap-wood, they are preserved. Much of this timber is found a foot or two from the surface, and is sawn into planks half under water. Bears inhabit the swamp, climbing the trees in search of acorns from the oaks, and gum berries. There are wild cats also, and occasionally a wolf is seen ; so that there must often be conditions for the" loss of these animals in the mire, and for the preservation of their bones. Indeed, in such a region as this, occupying an area of several hundred square miles, the amount and mixture of animal and vegetable matter, which may be collected in one great extended sheet, is not a little remarkable. Rivers, in some regions, carry forward not only the small plants with the leaves and branches of the larger, but multitudes also of trees are thus sometimes transported, part of them retained within the sedimentary deposits of the rivers themselves, part swept out seawards. It is not among the long-cultivated lands that the amount of plants, great and small, carried downwards by rivers, is best observed, though during floods in them large trees are occa- sionally borne down their courses. It is in regions where man has not by his labours modified the growth of vegetation, or the course of rivers, that the transport of plants by running waters can be well studied. We then have conditions resembling those under which vegetable remains may in this way have been mingled with the sedimentary deposits of previous geological periods. On this account, the courses of rivers, such as those of the Mississippi and its tributaries, are still highly instructive, though in various ways other rivers, pursuing their courses through lands not yet culti- vated in any part by man, may be still more so. The snags of the Mississippi, or great trees carried away from its banks, or those of its tributaries, and which are anchored, so to speak, by their roots upon the bottom of the stream, their heads bending with its strength, are well-known examples of the partial stoppage of trees on their course downwards. The same river, or rather one of its delta streams, named the Achafalaya, furnishes us with a good instance of a krge accumulation of some of these drift trees within the last 80 years. About that time since numbers of these drift trees got CH. VIII.] AMID MINERAL ACCUMULATIONS. 117 entangled in the channel, so that they no longer passed freely down it. Eventually they formed a mass, termed the Eaft, distributed irregularly, and rising and falling with the waters, for a distance of twenty miles, closely matted together in some localities. In 1808 the cubic contents of this collection of drifted trees was estimated at 286,784,000 cubic feet.* If by any change of conditions the channel of the Achafalaya became little supplied with water, and the raft consequently fell in the channel and was covered over with fine sediment derived from muddy waters quietly working their way into the old river course, a long line of lignite, corresponding with twenty miles of the old channel of this river, might be the consequence. When we regard the great rivers of the world, we can scarcely avoid considering that a large amount of plants and trees, differing in kinds and structure according to climates, must be annually entombed, in a manner to prevent that decay they would have suf- fered if left, after death, solely to atmospheric influences. No doubt much of this vegetation is still decomposed after transport by the rivers to their deltas, yet much also must be entombed in deposits excluding ordinary atmospheric influences, and leaving the plants under conditions favourable for their gradual alteration into lignite, or to the more advanced state of coal, should geological changes so permit. In deltas, also, we have, in the pools and lakes formed by the advance of the sediments thrust forward by the rivers, circum- stances in many regions favourable to the growth of aquatic and swamp vegetation. In such situations, as they fill up by the occasional inflow of the muddy waters of the rivers in flood, and by the growth and partial decay of the vegetation, we have also conditions suited to the preservation of some of the plants, or their parts, often in the positions in which they grew, mingled with carbonaceous matter and beds of sediment. It may so happen, in rivers where sands as well as mud are forced forward, that by the occasional shifting of a stream, or the breaking away of a bank, previously barring the entrance of any portion of a main stream, sands may be thrust forward over accumulations of * The 20 miles of length were estimated at 10 miles, this distance being considered as representing a close packing of the trees. The average breadth was taken at 220 yards, and the depth at 8 feet. (Darby, Geographical Description of the State of Louisiana.) Rafts of this description, but of less size, are, as might be expected, found in other divisions of the Mississippi and its tributaries. Captain Hall (Travels in North America, vol. iii., p. 370) mentions being a witness of one of those falls of the banks of the Missouri, covered with trees, which throw so much drift wood into the Mississippi, the banks of the latter also contributing largely to the general mass. 118 PRESERVATION OF ORGANIC REMAINS [Cn. VIII. this kind, their deposit marked by successive lateral and sloping additions, such as have been previously mentioned. With regard to the preservation of animal remains on dry land, or in fresh water, we have to recollect that the rapacious animals very frequently devour the bones of the vertebrata which they destroy, and that the scavenger animals eat up those which the former may have left unconsumed, so that few bones generally remain exposed on dry land to be decomposed by atmospheric influences. It is very probable that in deserts, the bones of animals which have perished in them may be often buried beneath great sand-drifts, there to remain, perhaps, if decomposing causes be slight in such situations, until geological changes may again bring such deserts beneath waters, and consolidation or removal of the sands be effected, as the case might be. We have seen the bones of rabbits and birds exposed by a shift of some of our coast sand-hills, by which portions of old accumulations, marked by successive growths of vegetation, have been carried off by the winds. Vertebrate animals are, in some countries, overwhelmed by the fall of parts of mountain sides or cliffs, so as to become buried deeply in situations where their bones are under conditions favour- able for preservation. Occasionally, they are destroyed by the partial fall of sea cliffs on tidal coasts, while wandering beneath them when the tide may be out, their harder parts, perhaps, washed out to sea when the breakers may have subsequently removed the fallen mass. Such harder parts may thus become mingled with any sedimentary accumulations then forming, should they not be ground to pieces on the coast by the breakers. While studying the mode in which the remains of vertebrate animals may be preserved without the aid of streams, pools, or lakes of fresh water, it will be observed that the clefts of rocks, in countries where such occur, are places into which more animals fall than might at first sight be thought probable. In some of our limestone districts, where caverns are found open to the surface, many an animal is lost, notwithstanding the precautions usually taken, so that we are prepared to expect that, in uncultivated regions, animals chased by others, coming suddenly upon the brink of a fissure and unable to clear it at a bound, often get precipitated into it. How far their remains may be preserved will necessarily de- pend upon circumstances. While even inaccessible to scavenger quadrupeds, many of these fissures are open to scavenger birds who descend and devour the flesh, leaving the bones. Scavenger insects can readily also consume the softer parts. The ultimate CH. VIII.] AMID MINERAL ACCUMULATIONS. 119 preservation of the bones from the decomposing effects of atmo- spheric influences would depend upon their exclusion from them. The accumulation of clayey matter in the fissures, washed in from the tops or sides during rains, mingled often with fallen portions of rocks, forming the sides of the fissure, will tend to this end. Still better, however, would be their entombment by calcareous stalagmites and stalagtites, where these were formed in the fissures of limestones. In the latter case, we might have an ossiferous limestone breccia rising to the surface irregularly, the width vary- ing with the form of the walls of the original fissure. Caves, inhabited for a length of time by the same kinds of animals, during which they brought in their prey, so that such parts of themselves or of this prey which may have remained un- consumed accumulated, also afford opportunities for the preserva- tion of vertebrate animal remains, according to circumstances. If these remains, even teeth, continued long under the decomposing conditions likely to obtain in such situations, without some pro- tection afforded by clay in some caves, by stalagmites in limestone caverns, or by numerous fallen fragments, few traces would be ex- pected, while, if these protecting influences existed, such remains might often be preserved. It is, however, to the aid of water we have to look for the en- tombment of vertebrate remains in the largest quantities, though, no doubt, the labours of Buckland and others have taught us how much may be preserved in fissures and caverns. We have already noticed the loss of animals in bogs and swamps. In some regions, the collective amount of those which perish in this manner must be considerable. We have reason to believe that many mammals perish in lakes, sometimes sinking into soft ground on their borders, at others while endeavouring to cross them. In the former case they may be preserved, as in bogs and common swamps, in a nearly vertical position, their bones occurring relatively to each other as in life. In the latter, their bones may often be scattered. After decomposition had sufficiently advanced, so that the dead body floated, it may be either drifted to a shallow or deep side of the lake, supposing, for illustration, that both existed. If to the latter, and decomposition had still further advanced, and probably also the scavenger animals, both of the air and water, had consumed no small portion of it, the body might descend into deep water, with the bones still, as a whole, in their relative positions, so that if detrital or chemical deposits were there taking place, they would be in the condition to be so preserved. If drifted and stranded on 120 PRESERVATION OF ORGANIC REMAINS [Cn. VIII. a shallow part of a lake, the body would be liable to be attacked with facility by scavenger land quadrupeds, which might not have ventured into the water of the deep parts of the lake for this pur- pose. In many instances, as those who may have seen the dead bodies of animals under such circumstances are aware, the bones would be eventually much scattered, part of them pulled upon the dry land and decomposed, if not eaten, while another part may, under favourable circumstances, again enter the lake, and be there enveloped by deposits in the progress of formation. Whether land animals floated or not after being drowned in lakes must often depend upon the consumption of their flesh while submerged. The various regions of the world furnish us with different creatures inhabiting such pieces of water. In many warm climates, the bodies would soon be attacked by reptiles, capable of easily destroying their softer parts. In some countries, the croco- dilian family would speedily proceed to devour them, and not the less greedily that some decomposition had taken place. By their aid some animals might get dismembered in such a way that the bones became finally much scattered, and the parts of the same animal be somewhat spread among lacustrine deposits. The croco- dilians themselves add not a little to the remains of terrestrial ver- tebrata entombed in lake accumulations, by seizing animals on the shores and dragging them into the water.* With respect to the remains of aquatic reptiles and fish in lakes, the voracity of many of these creatures is commonly so great, and the system of mutual prey so incessantly kept up among them, that entire skeletons would have to be preserved under very favourable conditions. The deltas of the great rivers, especially those in tropical regions, will afford opportunities for the study of the manner in which the remains of aquatic reptiles may become em- bedded in detrital matter. We have seen the caiman of Jamaica, when pursued, so bury himself in the mud of the lagoons, in which he delights to live, that occasionally there must be some difficulty of withdrawal from it. Floods in rivers, particularly those of large size, flowing amid great plains, where the sudden rise of water covers a large area in a short time, concealing the more shallow portions, would appear * The caiman of the great West India Islands in this way frequently obtains dogs, and sometimes goats, incautiously approaching a place where he may be lurking, perhaps half depressed in mud, with the tip of his snout at the surface of the water. The caiman is considered by the negroes so fond of dogs' flesh, that when a bent man- grove tree, with a running noose, may be placed to catch one, a dog in a stout stockade, in the line traced out for the caiman, is thought one of the best baits. CH. VIH.] AMID MINERAL ACCUMULATIONS. 121 the means by which many mammals are swept off their feeding- grounds, drowned, and their dead bodies buried amid the detritus borne down at the same time. At such times, also, bones of mammals which remain strewed about in the more exposed situations, not consumed or decomposed, get mingled with the mud, silt, or sands, carried forwards, and finally deposited. To delta accumulations, whether in lakes or seas, such floods must, in certain climates, often bring down terrestrial mammals, mingling their remains with those of many reptiles. Though, from their powers of flight and consequent escape, we should not expect to find birds caught by floods so as to be carried away, drowned, and, under favourable circumstances, their harder parts entombed, yet, as we do occasionally, though rarely, find the body of a land bird borne down a stream in countries and at times of the year when we have no reason to suppose that it has been shot or otherwise destroyed by man, perhaps we may look to this cause as one, however occasional and rare, by which remains of birds may be preserved. It is in districts where great floods suddenly rise over very extensive flat lands, particularly at times when the young of many birds inhabiting and breeding upon them are unable to fly far or at all, that we anticipate the more frequent surprises of this kind. Land birds occasionally fall into lakes and perish. We have seen instances in which land birds chased by hawks have fallen into lakes. Accidents causing death also now and then happen to the waders frequenting the margins of lakes, as also to birds which live habitually on their waters, either sup- porting themselves by fishing in the shallow parts, like the swans, or by the aid also of diving, like the duck tribe. The preservation of their bones, once at the bottom, in lacustrine accumulations, would be the same as with other animal remains. Under all circumstances, perhaps, to floods passing over extensive flats, raising to the surface of the water the bodies of birds which have perished by natural deaths, and which may be capable of floating, or sweeping forwards the bones of others, not yet con- sumed by scavenger animals, we may look for the chief causes of the transport by water and entombment of the remains of birds in the resulting deposits.* * Neither should we forget, when considering the manner in which birds' bones may be preserved within the boundaries of land, that they may get entangled among travertines, and thus may be entombed in lines and patches corresponding with such calcareous deposits as they form in streams or pools, as under favourable circum- stances in Italy. In the great deserts of the world, birds, such as ostriches, perishing, their remains 122 PRESERVATION OF ORGANIC REMAINS [Cn. VIII. During floods also conditions are very favourable to the sweeping off of numerous insects, even those having the power of flight being caught up in the waters before they could escape. Multi- tudes of these insects are no doubt consumed by fish, yet the remains of others may readily be so mingled up with the sediment of the flood waters where it can be deposited, as to remain per- manently encased by mud, silt, or sand. Seeing the avidity with which, in general, insects cast by myriads, as they sometimes are, on the surface of lakes or pools of water, are devoured by fish, when we discover their remains embedded in calcareous matter, as they have been, we should expect circumstances ill-suited to the habits of insectivorous fish and aquatic reptiles. It may be that in waters in certain pools or lakes charged with large quantities of carbonate of lime in solution by means of the needful carbonic acid, the latter may be so abundant as to drive off the insectivorous fish, and insect-eating aquatic reptiles. We find the remains of land molluscs mingled with soils in many localities in sufficient abundance to show how capable the shells of these animals are of preservation when circumstances will permit. Though light as regards the absolute weight of each shell, the specific gravity of land shells is considerable, more approaching that of arragonite than of common calcareous spar.* In soils, the shells are ill placed for resisting decomposition beyond a certain amount of time, the waters containing carbonic acid readily percolating to them, so that in such situations they are, if not lately embedded, usually brittle, and not unfrequently broken. Among blown sands land shells are often abundant, some land molluscs especially delighting in such habitats. In volcanic countries, or those over which, from their proximity to such countries, volcanic ashes may be scattered, and sometimes abundantly, land shells, and, indeed, various other land animals, may be completely covered over with coatings sufficient not only may be often covered over by great sand drifts, and remain so long beneath, even supposing some change of drift to expose them, as to be no longer available as food to the animals which would otherwise consume them. Some may remain permanently covered, until, as previously mentioned, by a change of geological conditions, these deserts may be again submerged, and their sands be either removed or consolidated into rocks. * When experimenting some years since upon the specific gravity of shells, we found those of the following land molluscs to be : Helix Pomatia 2 -82 Bulimus decollatus 2-85 undatus 2-85 Auricula bovina 2-84 Helix citrina . . .2-87 CH. VIII.] AMID MINERAL ACCUMULATIONS. 123 to kill them, but to aid in the preservation of their hard parts. The fall of large quantities of ashes and cinders, discharged in some volcanic eruption, would appear to cause a greater sudden entomb- ment of terrestrial animals, with the probability of preserving their more solid parts entire, than can be obtained without the aid of water, even including the moving sands of deserts. Volcanic districts are, in temperate and tropical regions, often fertile, abound ing in vegetable and animal life, so that in regions, such as Sum- bawa and Java, for example, land animals, including an abundance of molluscs, may be readily buried beneath discharges of lapilli and ash, such as were vomited forth from the volcano of Tomboro, in Sumbawa, in April, 1815.* * The eruptions commenced on the 5th April, and continued more or less until the 10th, when they became more violent. A Malay prahu was on the llth, though distant from Sumbawa, enveloped in utter darkness from the ashes in the air. Upon landing afterwards on the island, the commander found the country covered to the depth of three feet by ashes and cinders ; and difficulty was experienced in sailing through the cinders floating on the sea. At Macasar, 217 nautical miles from Tomboro, the volcanic discharges were heard to such an extent that, supposing there was an engagement with pirates near at hand, the East India Company's cruiser " Benares," was despatched with troops on board to look after them. The following account, by the commander of the " Benares," obtained by Sir Stamford Raffles, will show the amount of ashes and cinders vomited forth : Proceeding south to ascertain the cause of the explosions heard, at 8 o'clock on the morning of the 12th, " the face of the heavens to the southward and westward had assumed a dark aspect, arid it was much darker than when the sun rose ; as it came nearer it assumed a dusky-red appearance, and spread over every part of the heavens ; by ten it was so dark that a ship could hardly be seen a mile distant ; by eleven the whole of the heavens was obscured, except a small space towards the horizon to the eastward, the quarter from which the wind came. The ashes now began to fall in showers, and the appearance was altogether truly awful and alarming. By noon the light that remained in the eastern part of the horizon disappeared, and complete dark- ness covered the face of day. This continued so profound during the remainder of the day that I," continues the commander of the " Benares," " never saw anything to equal it in the darkest night ; it was impossible to see the hand when held close to the eyes. The ashes fell without intermission throughout the night, and were so light and subtile that, notwithstanding the precaution of spreading awnings fore and aft as much as possible, they pervaded every part of the ship." " At six o'clock the next morning it continued as dark as ever, but began to clear about half-past seven, and about eight o'clock objects could be faintly observed on deck. From this time it began to clear very fast The appearance of the ship when daylight returned was most singular ; every part being covered with falling matter. It had the appearance of calcined pumice-stone, nearly the colour of wood ashes ; it lay in heaps of a foot in depth on many parts of the deck, and several tons of it must have been thrown overboard ; for though an impalpable powder or dust when it fell, it was, when compressed, of considerable weight. A pint measure of it weighed twelve ounces and three-quarters ; it was perfectly tasteless, and did not affect the eyes with a painful sensation ; had a faint smell, but nothing like sulphur ; when mixed with water it formed a tenacious mud difficult to be washed off." Approaching Sumbawa on the lath, the "Benares" encountered an immense quantity of pumice, mixed with numerous trees and logs with a burnt and shivered appearance. The fall of ashes at Bima, 40 miles from the volcano, was so great as to break in the Resident's house in many places. The Rajah of Saugar described some of the stones which fell there to have been as large as two fists, though not generally 124 PRESERVATION OF ORGANIC REMAINS [Cn. VIII. The great eruption of Vesuvius in 79 furnishes us with an ex- cellent example of the manner in which the surface of a country may be covered up by the discharge of volcanic ashes and lapilli, so that various works of art and use are preserved for our instruc- tion. Pompeii not only shows us paintings still remaining on the walls of the houses, but also a great variety of delicate articles, ex- tending to those of the women's dressing-cases. At Herculaneum we have even the writings of the time on papyri, in part still legible. We see an abundance of men's works as they were overwhelmed by the discharge of the ashes and cinders upon them, and often in a condition, after being thus buried beneath mineral matter, permeable to water, for 1800 years, which might not at first be expected. So little general injury seems to have been sustained by the town, even by the shocks of explosions so near, or earthquake movements, that the crushing in of house-tops by means of the weight of ashes and cinders, and the filling up of all corners by the finer dust, appear to have been the chief effects produced. Walking in the street of tombs at Pompeii it seems to require little else than the presence of persons clothed in the cos- tume of the place when overwhelmed by cinders and ashes, to have that street presented to us as it appeared 1800 years since. As showing that not only bones may be preserved under such conditions, but the form of the flesh itself which clothed them, two remarkable instances have occurred at Pompeii, where parts of the human form retained their external shape, the enveloping ash having been sufficiently consolidated, before the decomposition of the fleshy parts. The thickness of the ashes and lapilli which covered up Stabia?, Pompeii, and Herculaneum in 79, has been estimated as varying from 60 to 112 feet in depth.* There are few things we can consider more suddenly destructive of terrestrial animal and vegetable life than these great volcanic eruptions, particularly within areas where several feet of lapilli and ashes can be accumulated over a considerable area within a few days. The whole surface previously clothed with vegetation, above the size of walnuts. A great whirlwind is mentioned by the Rajah, " which blew down nearly every house in the village of Saugar, carrying the tops and light parts along with it. In the part of Saugar adjoining Tomboro, its effects were much more violent, tearing up by the roots the largest trees, and carrying them into the air, together with men, houses, cattle, and whatever else came within its influence." Many thousands of lives were lost, and the vegetation of the north and west sides of the peninsula was completely destroyed, with the exception of a high point of land where the village of Tomboro previously stood, and where a few trees still remained. Life of Sir Stamford Raffles. * Daubeny, "Description of Active and Extinct Volcanoes," 2nd edit., 1848, p. 221. CH. VIIL] AMID MINERAL ACCUMULATIONS. 125 with a multitude of land molluscs and insects, with many birds and mammals, may be all covered with a tnick coating of these volcanic products; many of the molluscs and insects close to the plants on which they may have been feeding. In regions where bogs pre- vail, large tracts of these vegetable accumulations may be buried, with any birds, insects, or molluscs frequenting them, by a thick layer of ashes and lapilli, the subsequent consolidation of which, by geological causes, might produce the deceptive appearance of a molten rock having flowed over them without producing those effects which would, under the latter supposition, have been anticipated. Indeed, when we have to study the fossil vegetation of some regions, a reference to the conditions under which trees and even bogs may be covered by volcanic ashes is one by no means to be neglected.* In tideless seas, terrestrial animal and vegetable substances, borne down floating on the rivers, necessarily pass out over the dense waters of the sea to various distances, according to circumstances, and may be transported still further than the force of the river waters have carried them by favouring currents, should there be such, or by winds, the latter capable of driving them about in various directions, should they change. The body of a drowned animal, the decomposition of which is sufficiently advanced to give it the specific gravity capable of floating (and it should be recollected that it would float easier in sea than in fresh water, as regards its own specific gravity), may be thus drifted a considerable distance until eaten, or too much decomposed to float. Small animals may be readily consumed, bones as well as flesh, by the larger voracious fish; but the bones of the larger mammals might, under favouring circumstances, find their way to the bottom, even in deep tideless seas, like parts of the Mediterranean, to be there mingled with the remains of molluscs or other creatures inhabiting the same depths. The observer has, in like manner, to consider the various land plants and trees which can be carried long distances, sometimes with live creatures still upon them, parts of the latter subsequently, at least those which may escape the voracity of marine animals, * It is stated that in consequence of the great eruption of Skaptar-jokul in 1783, the atmosphere over Iceland was impregnated with dust for a long time. Traces of this dust were observed in Holland. It is evident that bogs in Iceland may readily become buried beneath volcanic ashes and cinders under such conditions. We may take the great explosion of the Souffriere, in Guadaloupe, in 1812, as an example of the destruc- tion of vegetable and animal life, and of a considerable covering of both in many places in a tropical region. It was during this eruption that ashes were conveyed to Barbadoes by an upper current of wind, opposite to the trade wind. 126 PRESERVATION OF ORGANIC REMAINS [Cn. VIII. scattered over various depths of the sea bottom. It will require little attention to see how often the dead shells of land molluscs get thrust out seawards, their modes of floatation at first being such as to keep them above water. The positions necessary for this pur- pose will depend upon the state of the sea surface at the time. If, notwithstanding the state of weather which may have caused floods in the interior of adjoining lands, lifting off the dead shells from the low grounds in multitudes, the sea be moderately calm, the land shells will be carried on with the river waters, but if there be a breaking sea they soon get upset and sink. In such situations we have also to regard the mingling of detrital with organic matter, which may be effected by the push- ing forward of the sands and gravel on the bottom of the rivers. Many a drowned animal may thus become mixed up with a delta advance, and many a river and land mollusc be included amid a general subaqueous drift. Trees often get entangled and buried on the coast, as well as floated off seaward. Thus in tideless seas we have the ready means of transporting terrestrial and fluviatile vegetable and animal remains to various distances seaward, some under favourable circumstances, capable of being embedded in marine deposits at various depths, while others are included amid the detrital accumulations formed by the action of the rivers, thrusting out silt, sand, and gravel from the shores, not forgetting any calcareous deposits which may sometimes be added. In estuaries we obtain a state of things somewhat different. In them a check is afforded at each flood tide, to all borne floating out by rivers, so that when great freshets prevail in the rivers, all caught up by the floods in the interior and floated off low grounds, or borne to the main streams by tributaries, are arrested in their progress. The floating bodies of animals, trees, and smaller plants, are thus not permitted to escape directly seaward, but are lifted by the height of the tide over any low grounds bordering the estuary, these flats, at such times, being more than commonly covered with water. "When the ebb tide lowers the waters, the various substances floated over the estuary lowlands not unfrequently remain upon them, more particularly if any wind prevailing at the time forces them on the edges of the flooded lands. There is often a curious mixture of terrestrial, fluviatile, estuary, and more marine animal and vegetable remains, scattered over the estuary flats after such floods, more particularly should it happen, as it sometimes does on the western parts of the British Islands, that a heavy gale, accom- panied by much rain, occurs at a time of spring tides, so that the CH. VHI.] AMID MINERAL ACCUMULATIONS. 127 high tides combined with an on-shore wind, rising the sea waters still higher, are met by strong freshets from the land. Under ordi- nary conditions, fringes of estuary fuci, mingled with land plants, estuary crustaceans and molluscs and land shells, with here and there the remains of some creature, more strictly marine, are fami- liar to all visiting estuaries. Although amid the deltas of rivers delivering their waters into tideless seas, among the lagoons formed and the coasts adjoining, there may be variable mixtures of fresh and sea waters, affording proper places for the growth and increase of vegetables and animals fitted for living in brackish water, the conditions are different from those of an estuary. In the one case the waters are stationary, except so far as floods from the interior may force forward an extra amount of fresh water, or a prevailing on-shore wind may drive in a greater volume of sea water ; while in the other, large tracts are sometimes bare at one time and covered by water at another, the amount of the saline mixture being variable also, depending on the state of the tide and the volume of fresh water falling for the time into the estuary. And here it is necessary to remark that the observer should not consider as an estuary one of those great indentations of a coast, commonly termed an " arm of the sea," and which is but the consequence of the sea level cutting a previously-formed inequality of the land surface, not unfrequently the prolongation of some valley. No doubt the one kind of coast may sometimes shade into the other, but as regards the kind of life inhabiting estuaries, we should consider brackish water as essential to the latter ; at all events to such an extent that at low tide a river, the waters of which become fresh or brackish, should occupy the channel left. Under the conditions of an estuary silting up in the manner previously noticed, it must necessarily happen that the molluscs and other creatures inhabiting different surfaces, or small depths beneath them, died, such harder parts of them as might be pre- served remaining at levels corresponding with such surfaces, here and there mingled according to circumstances, with vegetable and animal remains, drifted as above mentioned. It will be well to examine the manner in which the different parts of an estuary surface may vary at the same time as to the animal life existing upon it, from the creatures inhabiting the little rills of water which only get checked at spring tides, otherwise meandering amid the higher estuary mud or clay- flats, to those in or upon the sands in the more exposed situations, covered by every tide. 128 PRESERVATION OF ORGANIC REMAINS [Cn. VIII. The manner in which terrestrial animals may become caught in the softer places should also receive attention, especially where springs, readily finding their way beneath silt and sand, form quak- ing or quick sands which engulf them, their bones remaining after the flesh has been consumed by the scavenger animals. An observer should by no means neglect the foot-prints of terrestrial animals, nor indeed of any leaving marks or trails, such having lately, and very deservedly, become of geological importance. These foot- prints are often excellently well preserved upon the mud or clay flats, or gently-sloping grounds of estuaries. Very many estuaries around the British Islands afford abundant opportunities for the study of the mixed foot-prints of birds and mammals upon the mud or clay, more especially during the heats of summer, and at neap tides, when extensive surfaces, covered at spring tides, may be bare and exposed to the drying influence of the sun. We have often seen the foot-prints of common gulls, where these birds have been busy around some mollusc, crustacean, or fish drifted on shore, and sufficiently in a fresh state for their food, most beautifully impressed upon clay or mud, hard dried by the sun, the courses of the birds, sometimes single, at others in pairs or more numerous, well preserved. In the same way the tracts of other birds are common, crossed here and there by those of rabbits, hares, stoats, and weazles, and occasionally of dogs. In some localities, after an area of mud or clay, thus trod upon during the difference of time between the spring and the neap tides, has been well dried by the heats of the summer sun, with deep cracks formed from loss of moisture, pieces of the most instructive kind may with care be taken away, further dried and preserved, and even baked into a brick substance, if the composition of the clay be well suited to the purpose. Mingled with these marks we have often also the trails of molluscs, as also those of estuary crustaceans, striving to regain the water, after finding themselves left by the tide. It might at first be supposed that the rise of the tides over this, for the time, somewhat hard surface, marked by the foot-prints and trails of different animals, would entirely obliterate all traces of them. How far this may be effected will, however, depend upon circumstances. If the rise of the tides from neaps to springs were accompanied by much ripple or waves from winds, it would be anticipated that the fine detritus constituting the mud or clay would, when remoistened, be readily caught up in mechanical suspension, so that all traces of foot-prints and trails would be removed. In all situations where such ripple or waves could be CH. VIII.] AMID MINERAL ACCUMULATIONS. 129 felt this would be expected. All parts of estuaries are, however, rarely exposed to such influences at the same time : many a nook remains tranquil ; and in those where the accumulation of detritus is in progress, and films or fine layers of mud succeed each other, if one becomes hardened before another is deposited, a line of separation more or less permanent is usually established between them. We are sometimes able to separate these layers from each other, after careful drying, so that foot-prints are seen upon many surfaces, beneath each other. We have been fortunate in this respect with some portion of sun-dried mud of the Severn estuary, and Sir Charles Lyell has pointed out the manner in which the foot- prints of the sandpiper (Tringa minuta) are not only preserved in the red mud of the Bay of Fundy (a locality so favourable from its tides, for the exposure of much ground at the neaps), but also repeated upon the different layers of accumulation. In some estuaries, long necks of sands and sandhills so, in part, cross their mouths, that bays of still or comparatively still water, occasionally of considerable area, occur behind them, the main streams of tide flowing elsewhere. Let us assume, for illustration, that fig. 50 (p. 55) represents some estuary of this kind, and that, instead of a shingle beach, d is a tract of sandhills, perhaps extend- ing several miles in length, then e would be the kind of bay noticed, left in comparative quiet, as regards the stream of tide, flowing chiefly on the opposite coast. Much would of course depend upon conditions as to the kind of deposits effected at e, but under the supposition that the set of the tides was such as not to cause a sweep of the stream round this bay, it would be favourable for the occasional deposit of the finer sediment or mud borne down the river, /, by floods. At the same time it would be exposed to the drift of sand from the sandhills, d. In such localities, we have seen the foot-prints of mammals and birds, hardened in the sun, well strewed over by the drift sand from the sandhills ; and it should be observed, that the same winds which were powerful enough to disturb the sandhills and cause the drift, would be prevented by the shelter afforded behind the same hills from disturbing the bay waters near the shore, these waters being under the lee of the sandhills, so that even in the shore and shallow waters the sand may be drifted over the mud or clay, filling up the hollows of the foot-prints. Should the general surface of the land be subsiding gradually, as regards the sea level, it will be obvious that great estuaries may present conditions highly favourable to the preservation of the foot- 130 PRESERVATION OF ORGANIC REMAINS. ^ [Cn. VIII. prints ol animals, the actual remains of which, amid the detrital accumulations, may be most rare. Many aquatic birds frequenting estuaries at particular times, often when driven to seek their food in such situations, from tempestuous weather in their more common sea haunts, may thus leave their foot-prints, the conditions for the preservation of whose bones in the estuary deposits themselves would be of the most rare kind, indeed not to be expected, except under the accident of some individual being killed when up the estuary. With the most truly estuary birds, those which build and commonly live on estuary shores, the case might be different. Upon the supposition of a gradual change in the level of the sea, the land descending, we might have sands abundantly thrust forward over clay with foot-prints and trails. A lowering of a mass of sandhills, partly barring the mouth of an estuary, would at once place much arenaceous matter within the transporting influence of the tidal waters, to be drifted over mud flats, formed previously behind them. In some regions the mass of sand, either accumulated as partial and sub-aerial bars, or more gathered together by the sides of estuary mouths, to be again thrown into tides, however eventually other sandhills and tracts might arise (conditions con- tinuing favourable), would be considerable. That the remains of cetaceans should be found amid estuary accumulations, as also those of numerous fish, some of them more known as purely marine than estuary, will not surprise those who may have seen the porpoises dashing up the estuaries of our coasts in chase of fish which they have driven before them, and their oc- casional entanglement in shoal waters, when left by a quick-falling tide. Other cetaceans also get sometimes stranded. It is more common to find the chased fish, especially the smaller fry, driven on shore. The birds, no doubt, then pick up the fish abundantly, so that only a minor portion may leave their hard remains for entombment, and doubtless, also, the cetaceans often escape in the pools where they may be caught upon the rise of the tide, but there are still many chances for the preservation of the harder parts of these animals amid estuary accumulations which should not be neglected. CHAPTER IX. ORGANIC REMAINS IN MARINE DEPOSITS. MODIFICATION OF CONDITIONS ON COASTS OF AMERICA. OF PACIFIC OCEAN. OF THE INDIAN OCEAN. OF COASTS OF AFRICA AND EUROPE. OF ARCTIC SEA. DISTRIBUTION OF MARINE LIFE. MODIFICATIONS FROM TEMPERATURE AND PRESSURE. FROM LIGHT AND SUPPLY OF AIR. RESEARCHES OF PROF. E. FORBES IN THE AEGEAN SEA. ZONES OF DEPTH. PROF. LOVEN ON THE MOLLUSCS OF NORWAY. ZONES OF DEPTH IN THE BRITISH SEAS. ORGANIC REMAINS DEPOSITED IN THE DEEP OCEAN. ON COASTS. IT is in connexion with the sea, looking at the evidence afforded us by the various fossiliferous rocks of different geological ages, that we should look for the preservation of the great mass of animal remains amid the detrital and chemical deposits of the time. We have seen that, by means of rivers and winds, various plants and animals, or tlieir parts, may be borne into the sea, and that in es- tuaries we may have a mixture of terrestrial and marine remains, and of others suited especially to such situations. In respect to estuaries, some so gradually change into arms of the sea, to be seen on the large scale in the Gulf and River of St. Lawrence, and other situations, and equally well in numerous localities of far less area, in various parts of the world, as for instance, in the Bristol Channel and the Severn estuary, that no marked distinctions can be drawn between the one and the other. Viewing the coasts of the world generally, we not only have to regard all the modifications for the existence of marine animal life, arising from the more or less exposed or sheltered situations of headlands, bays, and other forms of shore, but also the mingling of fresh waters with the sea under the various circumstances con- nected with the drainage of the land into the sea. Let us consider the modifications of condition for the existence and entombment of marine animal life from Cape Horn to Baffin's Bay. First, there is the difference of climate, producing modifications of no slight order, more especially in moderate depths. From Cape Horn to the K 2 132 PRESERVATION OF ORGANIC REMAINS [Cn. IX. West India Islands, with the exception of the Straits of Magellan, there is an unbroken oceanic coast, subject to the action of the tides, upon which bodies of fresh water are thrown by drainage channels in different places, the chief of which are the Eio de la Plata, the Rio de San Francisco, the Tocantins, the Amazons, and the Orinoco rivers, delivering the portion of rains and melted snows not taken up by the animal and vegetable life, or required for the adjustment of springs or other interior conditions of a large part of South America. After a line of coast little broken by rivers, we find ex- tensive estuary conditions at the mouth of the Plata, and not far beyond Lake Mirim, about 100 miles long, a body of water apparently cut off from the ocean by coast action, and draining into another lake or lagoon, Lago de los Patos, having a channel still open to the main sea, and about 150 miles long, with an extreme breadth of about 50 miles. In these two bodies of water, receiving the drainage of the adjoining land, there are necessarily modifications of the ocean conditions for life, and for the entomb- ment of its remains outside in the main sea. A range of coast succeeds, to which comparatively small rivers discharge themselves, until the San Francisco presents itself, and so on afterwards until the mouths of the Para and Amazons join in forming (including between them the Island of Marajo) great estuary conditions, the tides being felt up the latter river, it is stated, 600 miles, so that there are several in the river at the same time. The mouths of the Orinoco present us with delta-form accu- mulations, and then comes the Carribbean Sea influenced by the ponded-back waters of the Gulf of Mexico, so that a kind of tideless sea shades into one where the tides are more felt. More northerly the Gulf stream is seen, transporting warmer waters to colder regions, and skirted by a shore, marked by a line of lagoons for above 200 miles on the coast of Florida, one of them named the Indian river, about 110 miles in length, with an extreme breadth of 6 miles ; another, the Mosquito lagoon, being about 60 miles long, with the like extreme breadth. Thence a much-indented shore, on the minor scale, continues until we come to Cape Fear (Carolina), where the lagoon conditions obtain, a kind of barrier, broken by passages termed inlets, permitting the ingress and egress of sea waters. In Core, Pamlico, Albemarle, and Currituck Sounds, we find a great body of water of an irregular shape, measuring along the line of barrier separating them, except where broken by inlets from the ocean, about 160 miles in length. Rivers drain into this body of water in various directions, so that estuary conditions obtain CH. IX.] AMID MINERAL ACCUMULATIONS. 133 in different places, while the great barrier banks, a point of one of which forms Cape Hatteras, place it under a modification of the conditions outside in the main sea. More northward, we obtain the great indentation of Chesapeak Bay, with its minor breaks into the land, the chief of which is the Potomac ; and then the Delaware Bay, with its river extending inland, the lagoon coast and its inlets continuing from Cape Charles (north entrance of Chesapeak Bay) towards the Delaware, and from near Cape Mary (Delaware Bay), about 85 miles to the northward. Next follows the mouth of the Hudson, and the modifications arising from the shelter of Long Island up the sound at its back, the lagoon character still apparent on part of its ocean coast. After shores variously indented, we reach the Bay of Fundy with all the modifications due to the great rise of tide (p. 78) at its northern extremities. This is succeeded by the great estuary conditions of the St. Lawrence, and finally the large indentions of Baffin's Bay and Strait and Hudson's Bay and Strait, and all the other channels of the cold regions of North America communicating with the Atlantic Ocean. It is impossible, when directing our attention to this long line of coast, so variously modified in character, and necessarily so different in climate, not to see how very modified must also be the conditions for the existence of life and the preservation of any of its harder parts. One contemporaneous coating of sedimentary or chemically deposited matter must include the remains of very different creatures, either living upon or in the surface accumulations, as well as the vegetable and animal remains drifted into it from the land. The molluscs inhabiting the coasts of the cold regions would be expected to differ materially from those in the tropics, and the plants and ter- restrial animals and amphibious creatures of the latter would vary from those in the former. The organic remains buried in the deposits of the Gulf of Mexico, though entombed at the same time as those in Baffin's Bay, could scarcely be expected to offer tjie same cha- racters. If, instead of the eastern coast of America, we look to the western, the first marked difference which presents itself is the absence of great rivers up the whole of the southern Continent and the land connecting it with the wide-spread northern part. Numerous shel- tered situations are to be found amid the islands and inlets extending from Cape Horn to, and including the island of, Chiloe ; after which, for about 6000 miles of coast, to the Gulf of California, the shores are little broken by indentations, except at Guayaquil and Panama, and do not present a single estuary of importance as on the eastern 134 PRESERVATION OF ORGANIC REMAINS [Cn. IX. side of the continent. The mixture of fresh water with the oceans on either side is very different, as are also the conditions for estuary life and the transport of terrestrial and fluviatile organic remains for entombment in the coast sedimentary accumulations. Even after we have passed the Gulf of California, and the Colorado delivering its waters at its head, there is, for about 2000 miles, from Cape S. Lucas to Vancouver's Island, a slightly- indented coast and a minor discharge of drainage waters, with the exception of those delivered by the Columbia or Oregon. Subsequently more north- ward, for about 800 miles, islands and inlets are common, offering modifications for the existence of marine life, as regards shelter and exposure to waves produced by winds, to Sitka Island and Cross Sound. After which comes the variously-indented coast extending to the Aleutian Islands, and so on to Behring Straits. Though we have the same range through climates, the character of the two coasts of the American continent varies so materially that we can scarcely but expect very important modifications, as well in the life as in the physical conditions under which it is placed. We have not only to regard the very great difference in the amount of fresh waters discharged on the east and on the west, with its consequences, but also the ponded waters of the Mexican Gulf and their continuation into the Carribbean Sea, with the result, the Gulf Stream, on the one side and not on the other, not neglecting the important difference presented by the great Mediterranean Sea, of Hudson's Bay and Baffin's Bay on the east, and the kind of coast found on the west. To this also should be added the great barrier offered by America to the passage of tropical marine animals from one ocean to the other.* It may be useful to glance at the great modification of conditions on the western side of the Pacific. Though a great portion of the drainage of Asia is disposed of in other directions, the surplus waters of a l^rge area still find their way to the east coast. The * According to M. Alcide d'Orbigny, of 362 species of molluscs in the Atlantic and Great Oceans, there is only one common to both, Siphonaria Lessoni. Of these 362 species, omitting the last, 156 belong to the Atlantic, and 205 to the Great Ocean. He also remarks that, if the two sides of the American continent be compared, the proportion, in the Atlantic, of gasteropod to lamellibranchiate molluscs, is 85 to 71, while in the Pacific it is 129 to 76. Of 95 genera considered to be proper to the shores of South America, 45 only are common to the two seas. This M. D'Orbigny attributes to the steep slopes of the west side, the Cordilleras rising near the coast, and rocks being more numerous than sandy shores, so that gasteropods would be expected to be more common, while the Atlantic coasts present mud, silt, and sand in great abundance, with gently-sloping shores for a large proportion of their length. Recherches sur Ins lots qui President a la Distribution des Mollusques Cotters Mar ins. Comptes Rendues, vol. xix. (Nov. 1844). Ann. des Sciences Naturelles, Third Series, vol. iii., p. 193 (1845). CH. IX.] AMID MINERAL ACCUMULATIONS. 135 Saghalian river throws its waters, derived from a considerable area, behind the island of the same name, to be driven into the Okhotsk Sea on the north, or the Japan Sea on the south, as the case may be. Both these seas are, to a certain extent, separated from the main ocean by the range of islands, composed of the Kourile and Japanese islands, extending from Kamschatka to Corea, the Japan Sea especially, from the great mass of island land interposing between it and the Pacific, offering the character of a Mediterranean Sea. Proceeding southerly we arrive at the Yellow Sea, which receives the abundant drainage effected by the Hoang Ho and its tributaries, and more southerly still we find the body of fresh water discharged into the sea by the Yang-tse-kiang. Thence, to the south, until the Si-kiang with its tributaries presents itself in the Canton estuary, comparatively minor rivers flow into the ocean, the coast being much indented, smaller rivers and streams often discharging in the upper parts of the indentations. The Island of Hainan, with the great promontory stretching to meet it from the main Chinese land, forms the Gulf of Tonquin, into which the San-koi and other rivers discharge their waters. The amount of fresh water poured into the sea on the eastern coast of Cochin China is subsequently of no great importance, and it is not until we arrive at the delta of the Maikiang or Camboja that the sea is much influenced by the influx of fresh waters, an in- fluence again, however, to be repeated at the head of the Gulf of Siam, by the outpouring of the Meinam, a river remarkably paraUel with the Maikiang for about 700 miles, the latter holding a singularly straight course, as a whole, to the N.N.E., for about 1750 miles.* The remaining portion of the Asiatic continent, formed by the Malayan promontory, throws no important body of fresh waters into the sea in the form of a main river. From Kamschatka nearly to the equator we thus have a con- tinental barrier, for the most part not wanting in the outflow of bodies of fresh water, sufficient to produce marked influences on parts of the coasts, and consequently upon the conditions under which animal life may exist along it, and the remains of terrestrial and fluviatile plants and animals be drifted outwards into any sedimentary or chemical deposits now forming adjoining it. Minor parts of the ocean are also, to a certain extent, separated off by islands, the range of the Philippines and Borneo, in addition to those mentioned, tending to portion off the ocean down to the * Considering the inference to be correct, as it appears to be, that the Latchou is the upper part of the Maikiang. 136 PRESERVATION OF ORGANIC REMAINS [Cn. IX. equator, so that, as a whole, a marked modification of physical conditions is observable on the east and west coasts of the Pacific Ocean. From the equator southward we have no longer a mass of un- broken land on the west to compare with the continuous continent of America on the east. A barrier to the free passage of tropical animal life, supposing other conditions equal, is not presented on the west. Although much land rises above the surface of the sea, the mass of Australia not so very materially of less area than that of Europe, and Borneo and New Guinea exposing no inconsiderable surfaces, there are channels of water amid them permitting tropical marine creatures to extend themselves under fitting circumstances. Though, with the exception of Australia, the various islands may not offer areas sufficient for the accumulation and discharge of fresh waters equal in one locality to some of the great rivers of the world, collectively they embody conditions for the outflow of much fresh water around many of them, so that estuary and brackish water conditions obtain, and consequently physical circumstances fitted for the modification of life. So far as the eastern coast of Australia is concerned, it presents about 2000 miles of shore not more broken or affording more fresh water than the opposite coast of South America. The western part of the Pacific differs from the eastern portion in the multitude of points and small areas through which the floor of the ocean reaches the atmosphere, productive of a com- bination of influences affecting animal life and the accumulation of its harder remains. While on this subject, it may be well to call attention to the material changes which would be effected if, by any of those al- terations of the level of sea and land which the study of geology teaches may be reckoned by differences very far exceeding the depths required, channels of communication were established between the Atlantic and Pacific Oceans by a sufficient subsidence of the Isthmus of Panama, or the communication cut off between the Pacific and Indian oceans by an uprise of the land and sea bottom between Australia and the Malayan Peninsula, one stretching through Timor, Floris, Java, and Sumatra. If the multitude of oceanic islands in the Western Pacific did not too much break up currents, we may suppose a certain amount of ponding up of waters inside the Moluccas, Borneo, and the Philippine Islands somewhat resembling that now effected behind the West India Islands, with perhaps also a modification of the Gulf Stream, escaping along the coast of China. Startling as, at first sight, such changes may appear, CH. IX.] AMID MINERAL ACCUMULATIONS. 137 the geological student has to accustom himself to consider modi- fications in the distribution of land and water, and elevations and depressions of a far more extended kind when he comes to reason upon facts connected with the accumulation and distribution of mineral and organic matter constituting rocks, formed at various geological periods. In the Indian Ocean we have shores confined to the tropical and temperate regions. For nearly 2000 miles the coast of Australia, from Cape Leeuwin to Cape Bougainville, presents us with no known great river pouring out a volume of water sufficient to in- fluence an extended area. The same with the island range of Timor, Floris, Java, and Sumatra, and up the Malay Peninsula, to the head of the Gulf of Martaban, where the Irawaddy thrusts out its delta and discharges a volume of fresh water, the drainage of a large area. From thence to the mouths of the Ganges no important amount of fresh water is carried out into the sea. The great volume thrown into the sea by this river has been already men- tioned (p. 85). Hence to Cape Comorin we find rivers of varied magnitude, the most important of which are, proceeding south- wards, the Mahanuddy, Godavery, Kistna, and Coleroon, draining, with minor streams, the great area of Southern India. As a whole, the Bengal Sea and Martaban Gulf receive a considerable quantity of fresh water, the discharge of which conveys a mass of detritus into the sea, and produces conditions in the waters and the sea bottom, which, beyond Cape Comorin, are not found for about 1000 miles, until we reach the Gulf of Cambay , into which the Nerbudda and other rivers discharge themselves. We find another volume of fresh water thrown into the sea by the Indus, still more northerly, after which we obtain the moderate outflow of fresh water of the coast of Beloochistan, the great indentation of the Gulf of Oman, and its continuation the Persian Gulf, the nearly-dry coast of Arabia, to the Arabian Gulf and its long-continued indentation, the Red Sea. From Cape Guardafui to the Cape of Good Hope, for about 4400 miles, the sea seems little influenced by any considerable discharge of fresh water on the coast, excepting in such places as at the mouths of the Zambesi and two or three other localities. Looking at the Indian Ocean as a whole, any influences upon marine animal life from fresh waters poured into the sea, with the greater amount of terrestrial and fluviatile plants and animals drifted into the ocean by rivers, would be chiefly found in the Bengal Sea (including the Martaban Gulf) and upon the north-east 138 PRESERVATION OF ORGANIC REMAINS [Cn. IX. shores of the Arabian Sea, with one or two places on the east coast of Africa. Excepting Madagascar and Ceylon, the area occupied by islands is inconsiderable. The coasts bounding it on the east are those chiefly of considerable islands (the mass of Australia better deserving the name of a continent), so that in the tropical regions there is a free communication by means of sea channels with the Pacific. On the west, Africa bars all direct communi- cation with the Atlantic, though at the same time the region terminated by the Cape of Good Hope and Cape Agulhas, trends southward, so comparatively little southward of the tropics, and currents (p. 93) so set from the Indian Ocean, round Cape Agul- has and up the south-western coast of Africa, that there is no great land boundary between tropical marine life in the one ocean and the other.* The Indian Ocean is now cut off from marine com- munication with northern regions (however this may have been effected in former geological times, even as late as the tertiary period, by means of waters uniting the Eed and Mediterranean Seas"), while it is well open to all marine life which may enter it, under fitting conditions, from the south. Herein it differs from the Atlantic and Pacific Oceans, which range from the Northern to the Southern Polar regions. In the run of the African coast which bounds the Atlantic for so long a distance on the east, fresh waters flowing outwards through great drainage channels seem chiefly to occur at the Orange Eiver, the Nourse, the Coanza, and the Congo, or Zaire, on the south of the equator, and at the Quorra, Gambia, and Senegal, on the north. The coast northward of the Senegal bounds for about 1000 miles the Atlantic on the one side, and the great African Desert on the other. From the Desert to Cape Spartel minor streams only fall into the sea. The great indentation of the Mediterranean then succeeds. The European rivers discharged into the Atlantic, or the tidal seas and channels communicating with it, are inconsiderable streams as compared with the great rivers of the world ; indeed a large portion of the European drainage finds its way into the Mediter- ranean, Black, Caspian, Baltic, and Arctic Seas. Such drainage as falls into the Caspian is evaporated in that sea, and that not so treated in the Black Sea is evaporated in the Mediterranean ; with all which directly finds its way into the latter. So that from the * Due regard has, however, to be paid to the temperature of the current, considered to be that of the mean of the ocean, which flows for some distance up the west coast of Africa, from the Cape of Good Hope, as also to that stated to run from the south end of Africa some way up the eastern coast. CH. IX.] AMID MINERAL ACCUMULATIONS. 139 Baltic alone the drainage waters of Europe find tlicir way into the Atlantic, in addition to those which flow directly into it, or the tidal channels and seas communicating with it. Enough, however, escapes in this way to give a varied character to the coast con- ditions, as regards the mingling of fresh with sea waters, under which aquatic life may be found and, in part, entombed, and the remains of terrestrial and fluviatile plants and animals be also accumulated. In the Arctic Ocean, the coasts present us with much mingling of fresh water and sea, the drainage of a large portion of Asia and of a minor portion of Europe falling into it ; part of the fresh water discharged into great indentations or arms of the sea, such as the White Sea and the Gulfs of Obi, leaiseisk, Khatangskii, and Kolima ; part through deltas, as the Petchora and Lena ; and part in a more ordinary form. The fresh water so supplied to the coasts of these regions is interrupted or lessened during many months of the year by the climate ; much of it being arrested in the form of ice, to be let loose in the warmer months. The ice, also, in the seas of these high latitudes, necessarily modifies the coast conditions for life as it exists in the temperate and tropical shores of the world. The drainage delivered into the same ocean from North America is less important than from Europe and Asia. Of the North American rivers flowing into this ocean, the Mackenzie would appear the most important, succeeded by the Back and Slave Kivers. The land and sea are so mingled on the north coast of America, and the ice and snows so abundant, that the shore waters become much influenced thereby. Looking to the Southern Ocean, we find the ice and snow of the Antarctic land most important, as regards the shore conditions. A great barrier of ice, indeed, there occupies the position of the coast for a great extent, so that both in the Arctic and Antarctic regions we have to regard ice accumulated round the land, or formed in the sea, as most materially influencing the existence of marine life and the preservation of its remains amid sedimentary and chemical deposits. In such regions, also, we see the extension of marine life (vege- table and animal), and of air-breathing creatures (birds and mam- mals) feeding upon it beyond the range of terrestrial vegetation, and of animals directly consuming it or the creatures which first feed upon it. Though such is the general fact, the conditions for the entomb- ment of the remains of terrestrial animal and vegetable life in the 140 PRESERVATION OF ORGANIC REMAINS [Cn. IX. Arctic and Antarctic regions are, as respects the present distribu- tion of land and sea, different. In the former, we have the de- livery of important rivers into the sea, an abundance of water being discharged during the warm season when the ice is broken up at their mouths, and the interior ice and snows are melting. The Obi and its tributaries alone drain a large Asiatic area, extending from lat. 47 to 67. The Jenisei, rising from theTangnou and Little Altai Mountains, likewise flows through 20 of latitude to 70 N., while the Lena and its tributaries, considered to drain 785,565 square (English) miles, rises (in lat. 57) from the Jablonnoi or Stannovoi Mountains (the eastern portion of which looks upon the sea of Okhotsk), delivering itself into the Arctic Ocean, in about lat. 73 38' N. Other rivers, also, flow northerly for considerable distances from the south, such as the Dvina, Petchora, Khatanga, Anabara, Olia, Olenek, lana, and Kolima. In Northern America, also, the rivers, though not numerous, flowing northerly, still show a drainage extending to the south for several degrees of latitude, though much interrupted by lakes.* Thus the Mackenzie, deliver- ing itself into the Arctic Ocean in about 69 N., flows from the Slave Lake by an outlet in about 61 N., giving 8 degrees of lati- tude for this course, during which the river receives the drainage from the Great Bear Lake. Eegarding the Slave Lake as a mere interruption, by which the waters are spread over a wider space in a depression, the waters discharging themselves by the Mackenzie are derived from a drainage extending over a considerable area (estimated at about 510,000 square miles), and reaching down to lat. 52 30' N., by means of the Slave Eiver (running out of the western end of Athabasca Lake), and the Athabasca (flowing into the same lake also at its western extremity). In the northern parts of Europe and Asia, 3,000,000 square miles of which have been estimated as draining into the Arctic Ocean, and in some portion of North America, there are, therefore, conditions, particularly during the floods caused by the melting of the ice and snows, for thrusting forward the remains of terrestrial and fresh- water life into the northern seas, there to be mingled with detritus, upon the transport and accumulation of which ice has an important influence. We should expect that amid the intermixed land and sea, terrestrial animals often perish while crossing among the ice, at times when the latter is breaking up in the channels * Collectively the lakes of North America constitute a marked feature in the physical geography of that part of the world. The volume of water in the chief lakes has been estimated at 11,300 cubic miles. CH. IX.] AMID MINERAL ACCUMULATIONS. 141 and gulfs, so that their bones are, under favourable conditions, pre- served in any sedimentary matter accumulating beneath. No such conditions prevail in the southern continent, which navigators have lately made known to us. No great rivers there flow outwards, and neither terrestrial plants nor animals, directly or indirectly living upon them, furnish their remains for mixture with any sedimentary deposits which may be forming. All aid which great river drainages afford to the latter is cut off, and the little detritus that can be obtained from the land seems only capable of being so derived directly in the few localities exposed to the breakers during the short period of the year when the shores are not bounded entirely by ice. For the finer matter, not ice-borne, entombing the remains of life, we may probably look, as affording the chief supply, to ashes and lapilli vomited from volcanos, and scattered over adjacent seas. Enough has been stated to show the unequal conditions as to climate and the mingling of fresh with sea waters along coasts. The observer has next to consider the varied character of the shores themselves as regards the shallowness or depth of the adjacent seas, and the modifications of temperature, pressure, access of light, and conditions of intermixed air thence arising. It has been above seen (p. 96), that the volume of ocean is so arranged as to the specific gravities of its waters, that an equal temperature, con- sidered to be 39 5', reigns in the sufficiently deep parts from pole to pole, water of higher temperature rising above these more dense waters in tropical regions, and of a lower temperature towards the poles. Though this even temperature may prevail at the proper depths, it is necessarily modified as the seas become shallow, or currents may transport warmer or colder waters, as the case may be, from one oceanic area to another. As the coasts are approached in the parts of the world where warmer waters float above those of 39 5', we have conditions under which the temperature decreases downwards below the level of the sea to 7200 feet, and upwards in the air to the greatest heights of land. Viewing the subject generally, therefore, and as far as temperature is concerned, marine animals which could support a decrease of temperature equal to about 39 5' (the surface temperature being taken at 78), could live from the level of the sea to the greatest depths in the equatorial ocean.* A higher temperature may be found under favourable conditions of shallow water and small tides in some * It will be at once obvious that this difference of temperature is easily sustained by many land animals in different parts of the world. 142 PRESERVATION OF ORGANIC REMAINS [Cn. IX. tropical regions. In the polar areas, included within the belts of equal temperatures from the surface to the bottom of the sea, and within which colder waters, as a whole, float above those of 39 5', there is a different state of things. Still regarding the subject merely with respect to temperature, the animals capable of living in the tropical regions, and unable to support lower temperatures than 39 5', could not occupy the higher waters. While in the equatorial parts of the world the temperature of the ocean may not be very materially altered on its surface, it is dif- ferent with those portions of its higher waters exposed to the changes of winter and summer, so that the temperature of the surface waters is there more considerably modified, especially upon coasts. The animals living in the shallow waters of such regions are, therefore, liable to an amount of difference in temperature not experienced by those inhabiting the seas of the tropics. This is more particularly the case on the shores of tidal seas, with their estuaries, where, even at high water, large tracts of coasts may only be covered by shallow waters, becoming dry at low tide. As regards mere temperature, it will be apparent that a vast volume of the southern ocean might be tenanted by similar life, extended over its floor at any depths from about 7200 feet at the equator, 3600 in lat. 45 S., the surface in 54 to 58 S., and 4500 feet in lat. 70 S.; and, probably, under the needful modifi- cations, considering the different distribution of land and water on the south and on the north, in a similar manner towards the northern regions. Modifications, also, arising in the various seas communicating with the main ocean, and more or less separated from it, such as the Mediterranean, in the western part of which the waters beneath 200 fathoms have been supposed to remain at about a temperature of 55,* must also be borne in mind. With differences of depth, the observer has to consider the dif- ferences of pressure to which any animal or vegetable living in the sea would have to be subjected, so that such life would be very differently circumstanced, though under equal temperatures, at the depth of 7200 feet at the equator, and in the shallow waters of the * M. Berard found, at a depth of 1200 fathoms (without reaching bottom), between the Balearic Islands, a temperature of 53 '4, the surface water being at 69 -8, and the air at 75 '2. From other observations in the western part of the Mediterranean, at the respective depths of 600 and 750 fathoms, and another not stated, it was found that the water was still at 55 -4, though the temperature of the surface water varied materially. M. D'Urville remarks that these experiments accord with some made by himself, also in the Mediterranean, at 300, 200, 250, 600, and 300 fathoms, when he obtained the respective temperatures of 54 -5, 54 -1, 57 3, 54 -6, and 54 -8 Geological Manual, 3rd edit,, p. 25. CH. IX.] AMID MINERAL ACCUMULATIONS. 143 oceanic regions where that of 39 5' rises to the surface. We cannot suppose an animal so constructed as to sustain a pressure of more than 200 atmospheres at one time, and of 2 or 3 atmospheres at another. A creature inhabiting a depth of 100 feet would sustain a pressure, including that of the atmosphere, of about 60 pounds to the square inch, while one at 4000 feet, no very im- portant depth, would have to support a pressure of about 1830 pounds to the square inch. Animals, among other conditions for their existence, are adapted to a given pressure, or certain ranges of pressure, so adjusted that they can move freely in the medium, either gaseous or aqueous, in which they live. All their delicate vessels, and the powers of their muscles are adjusted to it. When the pressure becomes either too little or too great, the creature perishes ; and, therefore, when act- ing freely in such a medium as the sea, an animal will not readily quit the depths in which it experiences ease. All are aware of the adjustment of an abundance of fish to the depths, to or from which they may frequently descend, by means of the apparatus of swim- ming bladders. This arrangement, however, only changes their specific gravities as a whole, the relative volume occupied by the air or gases in the swimming bladders, being the chief cause of difference, though, no doubt, also the squeezing process at great depths would diminish the volume of such other parts of their bodies, as were in any manner compressible, the reverse happening with a rise from deep waters to near the surface. So adjusted to given depths do these swimming bladders appear for each kind of fish, that it has been observed that the gas or air in the swimming bladders of fish brought up from a depth of about 3300 feet (under a pressure of about 100 atmospheres), increased so considerably in volume, as to force the swimming bladder, stomach, and other adjoining parts outside the throat in a balloon-formed mass.* * Pouillet, Eleraens de Physique Experimental, torn, i., p. 188. Seconde Edition. As regards the pressure and different depths in the sea, Dr. Buckland mentions (Bridgewater Treatise, vol. i., p. 345) that " Captain Smyth, R.N., found, on two trials, that the cylindrical copper air-tube, under the vane attached to Massey's patent log, collapsed, and was crushed quite flat, under the pressure of about 300 fathoms (1800 feet). A claret bottle, filled with air, and well corked, was burst before it descended 400 fathoms (2400 feet). He also found that a bottle filled with fresh water, and corked, had the cork forced in at about 180 fathoms (1080 feet). In such cases the fluid sent down was replaced by salt water, and the cork which had been forced in was sometimes reversed." Dr. Buckland adds that Sir Francis Beaufort had informed him that he had frequently sunk corked bottles in the sea more than 600 feet deep, some of them empty, others containing some fluid. " The empty bottles were some- times crushed, at others the cork was forced in, and the fluid exchanged for sea water. The cork was always returned to the neck of the bottle, sometimes, but not always, in an inverted position." Dr. Scoresby (Arctic Regions, vol. ii., p. 193) gives an account 144 PRESERVATION OF ORGANIC REMAINS [Cn. IX. While thus some kinds of marine animals have the power to adjust their specific gravities to the medium in which they may be placed, some molluscs, such as the nautilus, possessing it, others appear unable, under ordinary conditions, to raise themselves much above the sea bottom. It will be evident that the more their com- ponent parts are incompressible, and the fluids in them agree with the specific gravity of the sea in which they live (and the specific gravity of the sea does not appear to vary from any increase of saline matters in it to great depths, though water being slightly compressible, it will become more dense according to depth), the less they would experience the difficulties of a change of depth. On the contrary, the more any parts may be compressible, and air or gaseous matter be included in their bodies, the less would they suffer changes of depth with impunity. That light should have its effect upon marine as upon terrestrial vegetation we should expect, the light of day being important as well to one as the other, viewing the subject as a whole. It would evidently, also, be important to all marine creatures possessing the. organs of vision, so that we should anticipate that the great mass of fish, crustaceans and molluscs which possessed eyes, would occupy situations and levels in the sea where they could obtain the light needful to them. The Pomatomus Telescopium, caught at considerable depths in the Mediterranean (near Nice), is considered to afford an example of adjustment to the minor amount of light reaching its ordinary abode, its eyes being remarkable for their magnitude, and apparently constructed to take advantage of all the rays which can penetrate the depths at which it lives. While, however, light may be absolutely needed for the existence of some marine life, it is not obviously necessary to others, those not possess- ing eyes. Many marine creatures seem to flourish under conditions in which it can be of little value, at the same time the influence of light may often be of importance where it is not suspected. It is not improbable that to the power of obtaining a proper amount of disseminated atmospheric air in waters, we may look for a very important element in the existence of animal and, indeed, of vegetable life in the sea. To the one and the other oxygen seems essential. At the junction of the sea and atmosphere, we have the best conditions for the absorption of the air by water, the agitation of the surface from the friction of the atmosphere on the sea, particularly during heavy gales of wind, being especially of a boat pulled down to a considerable depth by a whale, after which the wood be- came too heavy to float, the sea water having forced itself into its pores. Cn. IX.] AMID MINERAL ACCUMULATIONS. 145 favourable for a mechanical mixture of the two, assisting, the ab- sorption of the air.* Of the amount of air, or rather of the ap- parently needful element, oxygen, at various depths in the sea, we seem to possess no very definite information, so that researches on this head are very desirable. From observations by M. Biot, on the gaseous contents of the swimming-bladders of fish, it has been inferred that such contents probably vary according to the depths at which such fish live. He found these swimming bladders nearly filled with pure nitrogen when they were those of fish inhabiting shallow waters, and with oxygen and nitrogen, in the proportion of T V of the former to T V of the latter, when those of fish living at depths of from 3000 to 3500 feet. According to M. Aime, the amount of air disseminated in the waters of the Mediterranean, opposite Algiers, is nearly constant from the surface to the depth of 5250 feet.f We might assume that, from its immediate contact with the air, surface waters would more readily obtain any needful dissemination of it than those situated at greater depths, so that the mode* of consuming oxygen would be adjusted to such conditions, animal life inhabiting great depths being so formed as to require it at con- siderable intervals. In tidal seas we find certain molluscs adjusted to live in situations exposed to the atmosphere during the fall of every tide, while others inhabit places always covered by sea, except, perhaps, at equinoxial spring tides. From inhabiting shores some molluscs are commonly considered as littoral species, while others are well known as rarely obtained except in deep waters. Although general views may have been some time entertained with respect to the modification of marine life, depending upon the temperature, pressure, light, and ability to procure oxygen under which it may be placed, J it could scarcely be said that we had sufficient data for the philosophical consideration of this subject until the labours of Professor Edward Forbes in the British and vEgean Seas supplied the necessary information. Professor E. Forbes pointed out that with regard to primary * The friction of air upon fresh-water lakes produces the same result, inter- mingling the air and water. Cascades and waterfalls intermix them in many rivers, those especially in which fish swimming high, or inhabiting minor depths, most flourish. t Comptes Rendue de 1'Acade'mie des Sciences, 1843, vol. xvi., p. 749. J The author entered somewhat at length on this subject, in 1834, in his Researches in Theoretical Geology, chapters xi., xii., and xiii. To this work a table was appended by Mr. Broderip, containing all the information then known respecting the depths and kind of bottom at which recent genera of marine and estuary shells had been observed. L 146 PRESERVATION OF ORGANIC REMAINS [Cn. IX. influences, the climate of the Eastern Mediterranean was uniform, and that the absence of certain species in the JEgean Sea, charac- teristic of the Western Mediterranean, was rather due to a modi- fication in the composition of the sea water, from the impouring of the less saline waters of the Black Sea, than to climate.* The influx of river water produces its consequences ; and it is remarked that, among 46 species of testacea collected on the shore at Alexandria, there were 4 land and fresh-water molluscs, 3 of which are of truly subtropical forms, f so that while in one part of the Mediterranean forms of this character are mingled with the ordinary marine testacea, in another, as at Smyrna or Toulon, the Melanopsis is mixed with them near the former, and characteristic European pulmonifera near the latter. It is also shown by Professor E. Forbes, that while vegetables of a subtropical character may be borne down by the Nile, into the Mediterranean on the one side, accompanying the remains of crocodiles and ichneumons, the Danube may transport parts of the vegetation of the Austrian Alps, with the relics of marmots and mountain salamanders, the marine remains mingled with these contemporaneous deposits retaining a common character. With respect to modifications in conditions arising from depth, Professor E. Forbes divides the Eastern Mediterranean into eight regions, each considered to be characterised by its fauna, and also by its plants, where they exist. Certain species were found con- fined to one region, and several were ascertained not to range into the next above, whilst they extended into that beneath. " Certain species," he adds, "have their maximum of development in each zone, being most prolific of individuals in that zone in which is their maximum, and of which they may be regarded as especially characteristic. Mingled with the true natives of every zone are stragglers, owing their presence to the action of the secondary in- fluences which modify distribution. Every zone has also a more or less general mineral character, the sea bottom not being equally variable in each, and becoming more and more uniform as we descend. The deeper zones are greatest in extent ; so that whilst the * He attributes to this cause the dwarfish character of the molluscs, with few ex- ceptions, when compared with their analogues in the Western Mediterranean. " This is seen most remarkably in some of the more abundant species, such as Pecten oper- cularis, Venerupis irus, Venus fasciata, Cardita trapezia, Modiola barbata, and the various kinds of JBulla, Rissoa, Fusus, and Pleurotoma, all of which seemed as if they were but miniature representations of their more western brethren. To the same cause may probably be attributed the paucity of Medusa;, and of corals and corallines. Sponges only seem to gain by it." Report of the British Association, vol. xii., p. 152, (Meeting of 1843). f Ampullarin ovata, Paludina vnicolvr, and Cyrena oriental!*. CH. IX.] AMID MINERAL ACCUMULATIONS. 147 first, or most superficial, is but 12, the eighth, or lowest, is above 700 feet in perpendicular range."* While tracing the first region or littoral zone, which is thus limited to 12 feet, all the modifications arising from kind of bottom, rock, sand, or mud, are shown to have their influences, and the effects of wave action, bringing up the exuviae of animals inhabiting the next region beneath, are pointed out. The second region is estimated at 48 feet, ranging from 2 to 10 fathoms ; the third at 60 feet, between the levels of 10 to 20 fathoms; the fourth at 90 feet (20 to 35 fathoms); the fifth at 120 feet (35 to 55 fathoms); the sixth at 144 feet (55 to 79 fathoms); the seventh at 150 feet (80 to 105 fathoms) ; and the eighth, all explored below 105 fathoms, amounting to 750 feet, more than twice, the depth of all the other regions taken together, the total depth amounting to 1380 feet. So complete are the modifications in invertebrate life, produced by the conditions in these various zones or regions, that only two species of molluscs were found common to the whole eight viz., Area lactea and Cerithium lima, " the former a true native from first to last, the latter probably only a straggler in the lowest." Three species were found common to seven regions ;t nine to six' regions :J and seventeen to five regions. With regard to geo- graphic distribution and vertical range in depth, Professor E. Forbes remarks that those species which possess the one exhibit the other, more than one-half of those having an extensive range in depth, extending to distant localities, in nearly every case to the British seas, some still further north, and some in the Atlantic, far south of the Straits of Gibraltar. He concludes "that the extent * British Association Reports, vol. xii., p. 154. f Nucula margaritacea, Marginella clandestine and Dentalium 9-costatu* ; the second considered to have possibly dropped into the lower zones from floating sea-weeds. J Corbula nucleus. Necera cuspidata. Pandora obtusa. Venus apicalis. Neoera costellata. Tellina pulchella. Venus ovata. Cardita squamosa. Area tetragona. Pecten polymorpha. hyalinus. Turritella 3-plicata. Triforis adversum. Columbella linncei. Cardita trapezia. Modiola burbot a. Crania ringens. Natica pulchella. Pissoa ventricosa. . cimicoides. reticulata. Trochus exiguus. Columbella rustica. Conns mcd'dfrrnnpvs. Terebratiila detrvncata. L 2 148 PRESERVATION OF ORGANIC REMAINS [On. IX. of the range of a species in depth is correspondent with its geo- graphical distribution."* As regards the influence of light, Professor E. Forbes presents us wit]?, facts connected with the molluscs and other animals, deserving much attention and extended research, due allowances being made for the modifications produced, as. he points out, and to be attributed in many cases to an abundance of nullipores, and to a beautiful pea-green sea-weed, Caulerpa prolifera. The majority of shells in the lower zone were found to be white, or, when tinted, of a rose colour., few exhibiting any other hues. In the higher zones, the shells, in a great many instances, exhibited bright combinations of colour. The animals also of the testacea and radiata, in the higher zones, were much more brilliantly coloured than iu the lower, where they are usually white, whatever the colour of the shell may be.f The researches of Professor E. Forbes have led him not only to attach great value to the bottom in or on which marine animals may live (and it will be obvious that creatures whose habits may be suited to mud would find themselves ill at ease upon rocky ground alone), but also to point out the effects produced by the accumula- tion of the harder parts of successive generations of marine animals * Reports of British Association, vol. xii., p. 171. " If," observes the Professor, " we inquire into the species of Mollusca which are common to four out of the eight ^Egean regions in depth, we find that there are 38 such, 21 of which are either British or Biscayan, and 2 are doubtfully British ; whilst of the remaining 15, 6 are distinctly represented by corresponding species in the north. Thus among the Testacea having the widest range in depth, one-third are Celtic or northern forms : whilst out of the remainder of JEgean Testacea, those ranging through less than four regions, only a little above a fifth are common to the British seas. One half of the Celtic forms in the JEgean, which are not common to four or more zones in depth, are among the cosmopolitan Testacea, inhabiting the uppermost part of the littoral zone." t Professor E. Forbes adds, "In the seventh region, white species (of Testacea) are also very abundant, though by no means forming a proportion so great as in the eighth. Brownish-red, the prevalent hue of the Brachiopoda, also gives a character of colour to the fauna of this zone ; the Crustacea found in it are red. In the sixth zone, the colours become brighter, reds and yellows prevailing, generally, however, uniformly colouring the shell. In the fifth region, many species are banded or clouded with various combinations of colours, and the number of white species has greatly diminished. In the fourth, purple hues are frequent, and contrasts of colour common. In the second and third, green and blue tints are met with, sometimes very vivid, but the gayest combinations of colour are seen in the littoral zone, as well as the most brilliant whites." Respecting the colour of the animals of Testacea, the genus Trochus is selected as " an example of a group of forms mostly presenting the most brilliant hues both of shell and animal; but whilst the animals of such species as inhabit the littoral zone are gaily chequered with many vivid hues, those of the greater depth, though their shells are alm6st as brightly coloured as the covering of their allies nearer the surface, have their animals for the most part of an uniform yellow or reddish hue or else entirely white." Reports Brit. Assoc., vol. xii., p. 173. CH. IX.] AMID MINERAL ACCUMULATIONS. " U> upon the same bottom, thus, in fact, altering its condition, so that they may die out from this increase.* He considers that until the old conditions be restored by a new accumulation of detrital matter different from that presented by the animal exuviae, the same animals would not find the kind of bottom suited to them ; and the geological bearing of this view is shown to be illustrated by the bands or layers of fossils so frequently found interstratined with common sedimentary matter. | In conclusion, Professor E. Forbes adverts to the evidences of the existing fauna of the ^Egean which would be presented if its bottom were elevated into dry land, or the sea filled up by sedimentary deposits. While the remains of some animals would afford the needful evidence of their existence, and occur under circumstances whence the probable depths at which they lived might be inferred : of other anmals, very abundant in the present seas, no trace would be found. J While Professor E. Forbes was thus investigating the conditions under which marine life existed in the Eastern Mediterranean, it fortunately so happened that Professor Loven was engaged in re- searches leading to general and similar conclusions respecting the modifications in marine life on the coast of Norway. Though both localities are so far similar that the shores are for the most part rocky, and deep water to be often obtained near the coast, they differ as to climate, and as to the sea being tideless in the Eastern * He illustrates this point by beds of scallops (Pecten opercularis), or of oysters, which, when considerably increased, give rise to a change of ground, by the accumu- lation of their shells, so that the race dies out, and the shelly bottom becomes covered over by sedimentary matter. Edinburgh New Phil. Journal, vol. xxxvi., p. 324. t In his paper on the light thrown on Geology by Submarine Researches, being the substance of a communication made to the Royal Institution of Great Britain, on the 23rd February, 1844 (Edinburgh New Phil. Journal, vol. xxxvi., p. 318, 1844), Pro- fessor E. Forbes, while remarking on all varieties of sea bottom not being equally capable of sustaining animal and vegetable life, observes, " In all the zones of depth, there are occasionally more or less desert tracks, usually of sand or mud. The few animals which frequent such tracks are mostly soft and unpreservable. In some muddy and sandy districts, however, worms are very numerous ; and to such places many fishes resort for food. The scarcity of remains of testacea in sandstones, the tracks of worms on ripple-marked sandstones, which have evidently been deposited in a shallow sea, and the fish remains often found in such rocks, are explained in a great measure by these facts." % The following are the inferences on this head, inferences extremely valuable re- specting the animal life existing at different geological periods : "1. Of the higher animals, the marine Vertebrata, the remains would be scanty and widely scattered." 2. Of the highest tribe of Mollusca, the Cepholopoda, which though poor in species is rich in individuals, there would be but few traces, saving of the Sepia, the shell of which would be found in the sandy strata forming parts of the coast lines of the elevated sea-bed." " 3. Of the Nudibranchous Mollusca there would not, in all probability, be a trace to assure us of their having been ; and thus, though we have every reason to suppose 150 PRESERVATION OF ORGANIC REMAINS [Cn. IX. Mediterranean and oceanic off Norway. While adverting to the modifications of life at different depths,, Professor Loven attributes much of the character of the submarine life off the coasts of Norway from analogy that those beautiful and highly-characteristic animals lived in the tertiary periods of the earth's history, if not in older ages, as well as now, there is not the slightest remain to tell of their former existence." " 4. Of the Pteropoda and Nucleobranchiata, the shell-less tribes would be equally lost with the Nudibranchiata, whilst of the shelled species we should find their remains in immense quantity, characteristic of the soft chalky deposits derived from the lowest of our regions of depth." " 5. The Brachiopoda we should find in deeply-buried beds of nullipore and gravel, and from their abundance we could at once predict the depth in which those beds were formed." " 6. The Lamellibranchiate mollusca we should find most abundant in the soft clays and muds, in such deposits generally presenting both valves in their natural positions, whilst such species as live on gravelly and open bottoms would be found mostly in the state Of single valves." " 7. The testaceous Gasteropoda would be found in all formations, but more abundant in gravelly than in muddy deposits. In any inferences we might wish to draw regard- ing the northern or southern character of the fauna, or on the climate under which it existed, whether from univalves or bivalves, our conclusions would vary according to the depth in which the particular stratum examined was found, and on the class of mollusca which prevailed in the locality explored." " 8. The Chitons would be found only in the state of single valves, and probably but rarely, for such species as are abundant, living among disjointed masses of rock and rolled pebbles, which would afterwards go to form conglomerate, would in all proba- bility be destroyed, as would also be the case with the greater number of sublittoral Mollusca." " 9. The Mollusca tunicata would disappear altogether, though now forming an im- portant link between the Mediterranean and more northern seas." " 10. Of the Arachnodermatous Radiata, there would not be found a trace, unless the membranous skeleton of the Velella should, under some peculiarly favourable circum- stances, be preserved in sand." " 11. Of the Echinodermata, certain species of Echinus would be found entire ; species of Cidaris, on account of the depth at which that animal lives, would not be unfrequent in certain strata, as the region in which it is found bounds the great lower- most region of chalky mud ; the spines would be found occasionally in that deposit, far removed from the bodies to which they belonged. Starfishes, saving such as live on mud or sand, would be only evidenced by the occasional preservation of their os- sicula. Of the extent of their distribution and number of species no correct idea could be formed. Of the numerous Holothuriadce and Sipunculidce, it is to be feared there would be no traces. The single Crinoidal animal would be easily preserved entire, but its ossicula and cup-like base would be found in the more shelly deposits." " 12. Of the Zoophyta, the corneous species might leave impressions resembling those of Graptolites in the shales formed from the dark muds on which they live. The Corals would be few, but perhaps plentiful in the shelly beds, mostly, however 5 fragmentary. The Cladocora ccespitosa, where present, would infallibly mark the bounds of the sea, and, from the size of its masses, might be preserved in con- glomerates where the testacea would have perished. The Actiniae would have dis- appeared altogether." " 13. Of the sponges, traces might be found of the more silicious species when buried under favourable circumstances." " 14. The Articulata, except the shelled annelides, would be for the most part in a fragmentary state." " lf. Foraminifera would be found in all deposits, their minuteness being their pro- tection ; but they would occur most abundantly in the highest and lowest beds, distinct species being characteristic of each." CH. IX.] AMID MINERAL ACCUMULATIONS. 151 to variations in the sea-bottom, always, however, making allowances for the depth,* thus agreeing with the general views of Professor Forbes. While marine life is thus found adjusted to different zones of depth on the ocean shores of Norway and the east part of the Mediterranean, always carefully considering the local and physical conditions, it becomes the more interesting to have direct evidence of the adjustments which may be effected on the great and gentle slopes bounding some coasts, such as those so important on the eastern coasts of America. Eespecting these great detrital fringes oif coasts, among which may be classed, though very small, com- paratively, the shallow seas around the British Islands, the area of " 16, Tracts would be found almost entirely deficient in fossils, some, such as the mud of the Gulf of Smyrna, containing but few and scattered ; whilst similar muds in other localities would abound in organic contents. On sandy deposits, formed at any considerable depth, they would be very scarce and often altogether absent. Fossili- ferous strata would generally alternate with such as contain few or no organic re- mains. "Whilst at present the littoral zone presents the greatest number and variety of animal and vegetable inhabitants, including those most characteristic of the Medi- terranean Sea, when upheaved and consolidated, their remains would probably be im- perfect as compared with those of the natives of deeper regions, in consequence of the vicissitudes to which they are exposed, and the rocky and conglomeratic strata in which the greater number would be embedded. A great part {' Cape Forward, Captain King found no bottom in the Straits of Magellan with a line of 1,536 feet, CH. XIIL] EFFECTS OF GROUNDED ICEBERGS. 247 or which may have fallen from cliffs upon coast ice, with the addition even of the remains of littoral or shallow-water molluscs, or of other marine animals, such as the bones of fish, whales, and seals carried off by the coast ice. A good example of the removal of a block of rock by coast ice, so far from the polar regions as Denmark, is mentioned by Dr. Forchhammer, who states that one, about 4 to 5 tons in weight, and resting on the shore, was encased in coast ice during the winter of 1844, and carried out to sea with the ice in the following spring, leaving, as it moved seaward, a deep furrow in the sandy clay of the shore, not quite obliterated six months afterwards.* As modifying the accumulations which may be formed on the bottoms of seas liable, from time to time, and, sometimes, as a whole, periodically, to sustain icebergs grounded upon them, the observer has to bear in mind that not only may the icebergs, by being forced against banks, jumble together, and singularly mingle beds of clay and sand, even occasionally adding transported fragments to the disturbed mass, but also act as rocks round and amid which streams of tide, or sea-currents, may become for the time modified. We should expect this to be most experienced in the regions where, from the general intensity of the cold, the icebergs could the longest remain. Sir James Boss mentions that the streams of tide were so strong amid grounded icebergs at the South Shetlands, that eddies were produced behind them,| so that, as far as such streams were concerned, they acted as rocks. Navigators have observed icebergs sufficiently long aground in some situations, that even mineral matter might be accumulated at their bases in favourable situations, while streams of tide may run so strongly between others, that channels might be cut by them in bottoms sufficiently yielding, and at depths where the friction of these streams could be ex- perienced. Much modification of sea-bottoms might be thus pro- duced by grounded icebergs, not forgetting those seasons of the year when many become joined together by ordinary sea-ice, con- * Forchhammer, ' Bulletin de la Socie'te Ge'ologique de France," 1848. He observes, respecting the transport of blocks and pebbles on the coast of Denmark by coast ice, that although the latter envelopes the blocks and pebbles on the shore, to enable these to be borne away, it is necessary that the thaw or rupture of the ice should coincide with a rise of the waters. Respecting blocks and fragments of rock borne out by the ice from the Baltic, by means of the current setting through the Kattegat in the spring, Dr. Forchhammer mentions that, in 1844, a diver found the remains of an English cutter, blown up during the bombardment of Copenhagen in 1807, covered by blocks, some of which measured from six to eight cubic feet. The same diver affirmed that all the wrecks he had visited in the roadstead of Copenhagen were more or less covered by rock fragments. t Ross, u Antarctic Voyage." 248 GENERAL GEOLOGICAL EFFECTS OF ICE. [Cn. XIII. stituting part of a mass to be dealt with on the large scale, when such ice is broken up. However firm the grounded icebergs may, like so many anchors, often tend to hold the main mass, it is not difficult to conceive that conditions arise by which many are dragged, cutting and ploughing up the sea-bottoms in their courses. Ice thus transports portions of rocks, either in the shape of glaciers, descending under the needful conditions in various extra- tropical regions, as floating ice down rivers, as coast ice, as frag- ments of glaciers descending into the sea, or as masses which, having been aground, capsize, and bring up a portion of the bottom on which they previously rested. Huge fragments of rock are by these means moved to distances from their parent masses, of which no other known power, now in force on the surface of our globe, appears capable. It has been seen that glaciers increase and de- crease according to the variations of the climates under which they are formed. What the amount of that increase and decrease may be under the conditions now existing, and where glaciers have been noticed, seems not well ascertained, though the differences in their volume and extent would appear to have been greater than was once supposed. Be that as it may, they distribute rock fragments outwards from mountain regions, these generally angular, unless ground between the glacier sides and bottom, the larger blocks and fragments remaining where the glaciers left them, while minor portions and finely -comminuted mineral matter are thrown into the torrents and rivers, to be disposed of by them according to their powers. River ice may carry detritus entangled in it, distributing the mineral matter over areas corresponding with their courses^ and which may be sufficiently flooded by them, transporting many a block and fragment which the power of the stream could not otherwise have moved. With the exception of rock fragments, which may have fallen from cliffs overhanging the rivers and not afterwards have been rounded, which may have been broken up from the sides in the manner previously noticed (p. 244), or which may have been left by some prior geological condition of the area, we should expect much of the detritus borne down by river-ice to be composed of the ordinary pebbles, sand, and mud of river courses. The sea deals with any ice-borne detritus received from rivers, or from the coasts, according as it is tideless or tidal, and as the portions into which these are carried may be in movement as sea and ocean currents, or the ice be acted on by the wind. Looking at the northern regions, where rivers of sufficient importance discharge themselves, carrying ice outwards, and coast ice is CH. XIII.] GENERAL GEOLOGICAL EFFECTS OF ICE. 249 common, it may be anticipated that much coast shingle, with rounded river pebbles, lumps of the frozen mud, and sands of estuaries, the occasional remains of marine animals, and now and then those of terrestrial animals, suddenly swept outwards by the river floods, would be strewed about upon the sea-bottom. Many a bone of elephants, rhinoceroses, and other animals, imbedded in the mud, sand, and gravel, of these regions, may also, after having been washed out of the beds which contained them, be ice-borne into the sea, and be mingled with remains of existing animals. To these may be added angular fragments carried out by the ice of rivers, or borne by coast ice from beneath cliffs whence such frag- ments have fallen upon it, independently of those carried into parts of the same seas by icebergs detached from the terminal part of glaciers. Although the arctic seas are so shut in by the lands of America and Asia, a comparatively small opening (Behring's Strait) only occurring between them, a space sufficiently wide exists between America and Europe, notwithstanding the interruption presented by Iceland, to permit the escape outwards of a certain portion of ice. We have seen that over the bottom of part of the North Atlantic blocks and fragments of rocks, with minor detritus, are now being strewed, without reference to its inequalities. In the antarctic seas very different conditions present themselves. Great rivers bearing ice-borne blocks and fragments of rocks, with minor detritus, are not found. The land, now commonly supposed to occupy so large an area in the South Polar regions, supports little else than water in its solid form, and the coast, for the most part, seems so encased by huge icy barriers, that common coast ice would there appear considerably limited, as compared with the arctic regions, in its power to carry off rounded boulders and shingles. Such glaciers as reach the sea, transporting fragments from the inland cliffs amid which they may moVe, would appear the principal agents in carrying mineral matter directly from the land, allowing for a portion transported by coast ice. The ice aground off Victoria Land would nevertheless appear to have the power of transporting much detritus when broken up into icebergs and upset, strewing blocks and minor fragments, sand and mud, over a part of the Southern Pacific. The South Shetlands, South Orkneys, South Georgia, Sandwich Land, and the lands more or less encased with ice between the South Shetlands and Victoria Land doubtless also contribute, by means of glaciers, coast ice, and probably also, as capsized grounded ice, blocks and frag- 250 GENERAL GEOLOGICAL EFFECTS OF ICE. [Cn. XIII. ments of rock (some rounded), sand and mud, to the bottom of the Southern Atlantic, and the ocean southward of Africa and Austra- lia. The southern portion oF America adds its glacier-borne frag- ments, and thus, both on the north and on the south, portions of rocks, formed in the colder, are ice-borne, and left beneath the seas of the more temperate, regions of the earth. I J l*/k CHAPTER XIV. INFLUENCE OF A GENERAL INCREASE OF COLD. MODIFICATIONS OF TEMPE- RATURE FROM CHANGES IN THE DISTRIBUTION OF L4ND AND SEA. ERRATIC BLOCKS. EFFECTS OF GRADUAL RISE OF THE SEA BOTTOM STREWED WITH ICE-TRANSPORTED DETRITUS. EFFECTS OF A SUPPOSED DEPRESSION OF THE BRITISH ISLANDS. INCREASE OF ALPINE GLACIERS. TRANSPORT OF ERRATIC BLOCKS BY GLACIERS. FORMER EXISTENCE OF GLACIERS IN BRITAIN. ELEVATION OF BOULDERS BY COAST ICE DURING SUBMERGENCE OF LAND. ERRATIC BLOCKS OF THE ALPS. ERRATIC BLOCKS OF NORTHERN EUROPE. ERRATIC BLOCKS OF AMERICA. THE geological effects now due to ice being as previously repre- sented, it becomes desirable to consider those which would probably arise either from a general diminution of temperature on the surface of the globe, or from partial changes of that temperature. With respect to the first we have to look to some general cause common to the whole globe. Whatever the conditions for the dis- tribution of temperature may have formerly been, we see that the influence of the sun now causes the heat of the tropics, and the different exposure of the polar parts of the earth's surface to it, the great variations of seasons there experienced. Any changes of sufficient importance, therefore, in the influence of the sun, which should produce a corresponding change on the face of the earth, so that the line of perpetual snow, as it is termed, should descend lower towards the sea in the equatorial, and cut its level at less high latitudes in the polar regions, would materially alter the climates of many parts of the world. Geological effects due to ice would be more widely spread than they now are, and the equatorial space within which ice-transported masses of rock and other detritus cannot be borne, would be more limited. Glaciers, where they could be formed, would not only become more extended than they now are in certain mountainous regions, but ranges of mountains, amid which they do not at present occur, the line of 252 EFFECTS OF PARTIAL INCREASE OF COLD FROM [Cn. XIV. perpetual snow not descending sufficiently low, would contain them ; so that, in the one case, mineral matter would be distributed by them over a wider area ; and in the other, over districts where no transport of the kind exists at the present time. Fragments angular, subangular, and rounded, would be distributed by river- ice and coast-ice, where none such are now formed, and sea-bottoms would then be strewed over by them, where, at present, nothing of the kind is carried. Animal and vegetable Hie would be adjusted to the new conditions (that adapted to the colder climates of the earth moving more towards the equator), its remains, at least such as were preserved, spreading over those of the animals and plants which flourished in the same regions under higher temperatures. The like general effects would be expected if \ without supposing a diminished influence of the sun, our whole solar system, moving through space, should pass from the temperature now inferred to be that of the portion amid which that system takes its course (p. 206) to one less high. And it may well deserve the attention of the geologist to consider the effects which would follow such a change, even to the amount of a few degrees, as commonly measured by thermometers. In his observations on the distribution of masses of rock, apparently ice-borne to their present positions, and about to be noticed, it is very desirable that he should regard the subject generally as well as locally, so that, whatever may eventually appear the right inference to be drawn from the facts recorded, such as may bear upon the former should not be omitted in the search for the latter. As regards the evidence of many climates having remained much the same, with certain modifications, during those comparatively few revolutions of our planet round the sun, of which we have any records, and from which we may infer that the climates generally of the surface of the globe have not suffered material alteration since the historical period, as it has been termed, the geological observer will soon perceive that he is forced to con- sider it as affording him very limited aid in his inquiries respecting the former climatal conditions of the earth. The present different conditions as to the production of ice capable of transporting mineral mutter, in the manner above noticed, in the northern and southern cold regions of the globe, are sufficient to prove that partial changes of great importance may arise from differences on the surface of the earth itself. Every -day experience in geological research will show the observer that he has to consider the surface of the earth to have been in an unquiet state from remote geological times to the present, and that while CH. XIV.] CHANGES IN LAND AND SEA OF NORTHERN REGIONS. 253 he so often stands, amid stratified deposits, on ancient sea-bottoms now elevated to various altitudes above the ocean level, many a region shows that its area has more than once been beneath that level and above it. Thus, although a mass of land may now rise above the sea-level at the South Pole, separated by a broad band of ocean from other great masses of land to the northward, producing certain effects as regards the climate of that part of the globe, and the northern polar regions are otherwise circumstanced, it by no means follows that such has always been the case, even in more recent geological times. If we change the conditions of the two polar regions, a difference of results is obtained of an important geological character. Mr. Darwin has skilfully touched upon the effects which would follow such a modification of conditions, and which require to be borne in mind in researches of this kind.* In like manner any elevation or depression of a considerable area of dry land, which should raise parts of it above, or lower others, now above, beneath the line of perpetual snow, would produce mo- difications in the transport of mineral matter which could be effected * He transports, in imagination, parts of the southern region to a corresponding latitude in the north. "On this supposition," he observes, " in the southern provinces of France, magnificent forests, intwined by arborescent grasses, and the trees loaded with parasitical plants, would cover the face of the country. In the lati- tude of Mont Blanc, but on an island as far eastward as Central Siberia, tree-ferns and parasitical orchideae, would thrive amidst the thick woods. Even as far north as Central Denmark, humming birds might be seen fluttering about delicate flowers, and parrots feeding amidst the evergreen woods, with which the mountains would be clothed down to the water's edge. Nevertheless, the southern part of Scotland (only removed twice as far to the eastward) would present an island " almost wholly covered with everlasting snow, and having each bay terminated by ice-cliffs, from which great masses, yearly detached, would sometimes bear with them fragments of rock. This island would only boast of one land-bird, a little grass and moss ; yet, in the same latitude, the sea might swarm with living creatures. A chain of mountains, which we will call the Cordillera, running north and south, through the Alps (but having an altitude much inferior to the latter), would connect them with the central part of Denmark. Along this whole line nearly every deep sound would end in ' bold and astonishing glaciers.' In the Alps themselves (with their altitude reduced by about half), we should find proofs of recent elevations, and occasionally terrible earth- quakes would cause such masses of ice to be precipitated into the sea, that waves, tearing all before them, would heap together enormous fragments, and pile them up in the corner of the valleys. At other times, icebergs, charged with no inconsiderable blocks of granite, would be floated from the flanks of Mont Blanc, and then stranded in the outlying islands of the Jura. Who, then, will deny the possibility of these things having taken place in Europe during a former period, and under circumstances known to be different from the present, when, on merely looking to the other hemi- sphere, we see they are under the daily order of events?" Mr. Darwin then calls attention to the island groups, " situated in the latitude of the south part of Norway, and others in that of Ferroe. These, in the middle of summer, would be buried under snow, and surrounded by walls of ice, so that scarcely a living thing of any kind would be supported on the land;" Narrative of the Surve) ing Voyages of the Adventure and Beagle, vol. iii. p. 291. 254 EFFECTS OF DEPRESSION AND RISE [Cn. XIV. by ice. If the region comprising the Alps were raised 3,000 feet above its present relative level, the area fitted for the formation of glaciers would be greatly extended, many a valley would be filled with ice, and many a mountain would contribute its glacier, not so filled or contributing at the present moment. Blocks and minor fragments of rocks would be ice-borne over, and left at distances from the main range not now attained ; and, under the supposition of a gradual rise of land, many modifications would attend the change in the perpetual snow line, whence the glaciers for the time took their rise. Many a ravine and mountain side would be grooved and scratched not now touched by glaciers, and huge masses of rock be accumulated in heaps or lines, in localities where no ice now transports such masses. Assuming a depression of the same area, if we take the present relative levels only into consideration, the transport of glacier-borne blocks and fragments of rock, with the polishing, grooving, and scratching of valleys and their sides by the moving ice, would be limited to the areas now occupied by glaciers, duly allowing for their extension and contraction within the range of he present climatal conditions. Thus, by the elevation and depression of large areas of dry land, very varied conditions for the existence, extension, or contraction of glaciers, with their geological consequences, may arise, without reference to those due to floating ice, excepting such as could be formed in great lakes, such as that of Geneva, for example, where effects similar to those observed in northern America would be produced. On the shores of such lakes coast ice would be formed, enclosing fragments of the rocks, and the shingles of beaches, to be borne away, should circumstances permit, if raised to an altitude permitting a depression of temperature sufficient for the production of such ice. There is also no difficulty in imagining conditions under which glaciers could protrude into large fresh-water lakes, carrying rock fragments with them, and having their extremities broken off and floated away with their detrital loads, under proper depths of water, as now takes place in the sea in the polar regions. Such masses of ice, though not moved onwards by streams of tide or ocean currents, would still be under the influence of the winds, to be driven to, and stranded in minor depths, where the ice could melt, and leave any blocks or fragments entangled Montigny ...*.. i Mimisse ... .5 Mimisse ......) Geneva . .... 3 Martigny > j Playau 5 Chasseron 3 Playau } Chasseron . . .3 Plan-y-beuf ) Great St. Bernard ... 7 Chasseron 5 Martigny .3 Playau . ..... 3 Grimsel 7 Chasseron 3 Playau 5 Neve of the Ober Aer . . ] Grimsel I Level of the Roches Mouton- j ne'es J Grimsel . \ Brunig . . . J M. Elie de Beaumont remarks that he docs not know in the Alps any glacier which moves through any considerable extent, such as a league, with a slope much less than 3. Cn. XIV.] GENERAL DECREASE OF EUROPEAN GLACIERS. 267 perhaps the extension of a small glacier previously formed at p, or altogether new ; and thus blocks and glaciers may descend against the extension of the main glacier to m. The face of the Alps, as regards snow and ice, would be most materially changed by a descent of the snow-line, so as to be of about the same altitude as that of Chiloe, and a further decrease of temperature would neces- sarily still further extend the glaciers. Assuming a depression of this kind, the observer has to take into consideration the rise of the sea-bottom to the present European levels of sea and land, accompanied by an elevation of general tem- perature to that now found. As the land rose, beaches would be left in various situations, showing the different alterations of the relative levels of sea and land. Should considerable pauses in the elevation of the land have taken place, these would be marked by lines of cliff, where the rocks could be sufficiently worn by the breakers. The production of coast ice would gradually become less, so that its formation would cease in the southern lands, and the glaciers generally would decrease, leaving their lines of moraines, and many angular blocks of rock, perched on the sides of mountains, as in the following sketch (a, b, fig. 101), at altitudes corresponding Fig. 101. with the volumes of their transporting glaciers at the periods of their chief extension down valleys, where only a remnant of such glaciers may be now left at their higher extremities, or even, as in the British Islands, no portion of one may remain. The land continuing to rise, not only would the previous sea- bottom, with its varied accumulations (in some of which the remains of animal life would be entombed, often in regular beds of sand, silt, 268 TRANSPORT OF ERRATIC BLOCKS [Cn. XIV. and mud), be brought within the destructive influence of the breakers, as above noticed (p. 258), but rivers also would begin to flow amid the old sea-bottom. According to circumstances, such rivers would present varied characters, and some would carry forward ice-borne detritus to the sea, or leave it on their courses, as it might happen, until only certain of them, those now possessing the needful conditions, so transported mineral substances. From the interest which has been excited respecting the transport of erratic blocks, many of great volume, by means of ice, a mass of information has been collected, rendering the submersion of a large portion of Northern Europe, Asia, and America, accompanied by a considerable depression of temperature, extremely probable. The effects of floating ice have for a long time engaged attention. Professor Wrede, of Berlin, would appear to have been among the first to account for the erratic blocks on the south of the Baltic, by means of floating ice, there having subsequently been a change of level in that region, by which the sea-bottom became dry land.* Sir James Hall also long since referred to floating ice, combined with earthquake waves, as a means of transporting erratic blocks ; f and its aid, under various conditions, has been sought in explanation of the transport of large and often angular blocks of rock from their parent masses to considerable distances. Though Professor Play fair long since (1802) pointed out glaciers as having been the means of carrying erratic blocks, J even (in 1806) inferring that those on the Jura may have been transported by the extension of ancient Alpine glaciers to that range of mountains, the subject engaged no great attention for some time. M. Venetz appears to have been the first who, having had occasion to study glacier * " Geognostical Researches relative to the Countries on the Baltic, and particularly to the Low Lands at the Mouth of the Oder, with Observations on the gradual change of the Level of the Sea in the Northern Hemisphere, and its physical causes, as quoted by De Luc, Geological Travels, 1810." Professor Wrede supposed a slow change in the centre of gravity of the earth, so that the waters retreated from the northern hemisphere, leaving the sea-bottom dry, with the ice-borne blocks of rock upon it. He calculated the ice needed to float an erratic block, estimated to weigh 490,000 Ibs., occurring at the mouth of the Oder. f " On the Revolutions of the Earth's surface" (1812), Transactions of the Roynl Society of Edinburgh, vol. vii., p. 157. After noticing the removal of a block of rock four or five feet diameter, being a boundary mark between two estates on the shore of the Murray Frith, by the tide, while encased in ice, for 90 yards, and also the magnitude and effects of earthquakes, he asks, respecting the erratic blocks of Northern Europe, if both combined would not produce the effects required, u the natural place of these blocks being covered perfectly with ice, in the state best calcu- lated for fulfilling the office here assigned it," p. 157. He inferred that in the Alps similar waves, assuming the fitting conditions, would wash off portions of glneiers with their load of blocks. * Playfair, <; Illustrations of the lluttonian Theory," 349. CH. XIV.] BY GLACIERS AND COAST-ICE. 269 movements, subsequently (1821) took the same view;* one adopted afterwards (1835) by M. de Charpentier,-|- and further extended (in 1837) by M. Agassiz. f The subject then attracted more general interest, especially from the writings of M. de Char- pentier and M. Agassiz,|| and the consideration of the effects produced by existing glaciers and floating ice, with the probability of a colder state than at present of the northern portions of Europe, Asia, and America, at a comparatively recent time, now form one of the usual objects of geological investigation. Sir Charles Lyell long since called attention to the distribution of blocks and minor fragments of rock over the sea-bottom by means of icebergs, and to the manner in which such detritus would be found scattered over various levels, if this sea-bottom were upraised and formed dry land.lf Subsequently (in 1840) after noticing the action of drift ice, charged with mud, and blocks of rocks, he pointed out the manner in which floating ice may, by grounding upon coasts or banks, so squeeze the upper layers of mud, sand, and gravel, that contorted masses of these layers may repose upon undisturbed and horizontal beds beneath.** It was, however, in consequence of a visit to this country by M. Agassiz, in 1840, and upon the extension of his views respecting glaciers to the British Islands,-)")- that the former existence of glaciers in them has attracted * Venetz, " Bibliotheque Universelle de Geneve," torn, xxi., p. 77, and " Denk- schriften der Schweizerischen Gesellschaft ; " 1 Band, Zurich, 1833. f De Charpentier, *' Notice sur le cause probable du Transport des Blocs Erratiques dc la Suisse, "Annales des Mines," 3me Series, torn, viii., 1835. t Agassiz, " Address before the Helvetic Society of Natural Sciences, at Neufchatel," 1837. "Essai sur les Glaciers et sur le Terrain Erratique du Bassin du Rhone," Lausanne, 1841. || Etudes sur les Glaciers," 1840. f " Principles of Geology," 1832. ** In a communication on the Boulder Formation or Drift, and associated fresh- water deposits, composing the mud cliffs of Eastern Norfolk, ' Proceedings of the Geological Society of London " (January, 1840), vol. iii., wherein the contortions observed on that coast are thus explained. ft In the " Proceedings of the Geological Society of London," vol. iii., p. 328 (1840), M. Agassiz has given a summary respecting his views of the former existence of glaciers in the British Islands. Ben Nevis, in the north of Scotland, and the Grampians in Southern Scotland, are considered by him as the great centres of dispersion of erratic blocks by glacier ice in that part of Great Britain. He pointed out the mountains of Northumberland, Westmoreland, Cumberland, and Wales, as well as those of Ayrshire, Antrim, Wicklow, and the West of Ireland, as also centres of dispersion, " each district having its peculiar debris, traceable in many instances to the parent rock, at the head of the valleys. Hence," observes M. Agassiz, " it is plain the cause of the transport must be sought for in the centre of the mountain ranges, and not from a point without the district." The Swedish blocks on the coast of England do not, he conceives, contradict this position, as he adopts the opinion that they may have been transported by floating ice," p. 329. He considered that the best example of glacier striated rocks in Scotland is to be seen at Ballahulish. 270 EVIDENCE OF GLACIERS FORMERLY [Cu. XIV. attention. Numerous facts have since been adduced in support of this opinion by Dr. Buckland, Sir Charles Lycll, Professor James Forbes, Mr. Darwin, and others.* The amount of submergence. at * Dr. Buckland (" Proceedings of the Geological Society of London," vol., iii. p. 332, 1840), in his paper, " On the Evidences of Glaciers in Scotland and the North of England," points out localities which he infers show the remains of moraines near Dumfries, in Aberdeenshire, in Forfarshire, at Taymouth, Glen Coficld, and near Callender, with evidences of ancient glaciers on Schiehallion, in and near Strath Earn, and nearComrie; and of glacial action at Stirling and Edinburgh, lie also mentions moraines in Northumberland, the evidence of ancient glaciers in Cumberland and Westmoreland, and the dispersion of Shap Fell granite by ice. In his address to the Geological Society of London, as its President, in February, 1841, Dr. Buckland gave a condensed statement of the progress of investigations on this subject during the preceding year, one in which the " Glacial Theory/' was so much considered. Dr. Buckland subsequently, in his memoir on the Glacia-Diluvial Phenomena in Snowdonia, and the adjacent parts of North Wales (December, 1841), "Proceedings of the Geological Society," vol. iii., p. 57D, described the rounded and polished surfaces, often accompanied by grooves and scratches, attributed to glacier action, in the valleys of Conway, of the Llugwy, of the Ogwyn, of the Sciant, and of Llanberis, of Gwyrfain or Forrhyd, of the Nautel or Lyfni, and of the Gwynant. Sir Charles Lycll, in his paper "On the Geological Evidence of the former existence of Glaciers in Forfarshire," stated that though, for several years he had attributed the transport of erratic blocks, and the curvature and contortions of the incoherent strata of gravel and clay, resting upon the unstratified till, to drifting ice, he had found difficulty in thus accounting for certain other facts connected with the subject, until Professor Agassiz extended his glacial theory to Scotland. After a description of various minor districts, Sir Charles Lycll observes, " that it is in South Georgia, Kerguelen's Land, and Sandwich Land, we must look for the nearest approach to the state of things which must have existed in Scotland during the glacial epoch." Professor James Forbes, in his " Notes on the Topography and Geology of the Cuchullen Hills, in Skye, and the traces of ancient glaciers which they present," (Edinburgh New Philosophical Journal, 1846, vol. xl., p. 76), points out groovings and scratchings upon polished rocks of a marked kind. He observes, respecting the valley of Coruisk, that *' the surfaces of hypersthcne, thus planed or evened, present systems of grooves exactly similar to those so much insisted on in the action of glaciers on subjacent rocks, and as evidence of glaciers in parts of the Alps and Jura, where they are now awanting. These grooves or striae are as well marked, as continuous, and as strictly parallel to what I have elsewhere shown to be the necessary course of a tenacious mass of ice urged by gravity down a valley, as anywhere in the Alps. They occur in high vertical clifFs, as near the Pissevache ; they rise against opposing promontories, as in the valley of Hasli ; they make deep channels or {lutings in the trough of the valley, as at Pont Pelissier, near Chamouni ; and as at Fee, in the Valley of Saas. At the same time these appearances have a .W/KT/O/ /////, above which the craggy angular forms are almost exclusively seen, where the phenomena of wearing and grooving entirely disappear. In short," adds Professor Forbes, "it would be quite impossible to find in the Alps, or elsewhere, these phenomena (except only the high polish which the rocks here do not admit of) in greater perfection than in the Valley of Coruisk." Other evidence of the like kind is also adduced. , Mr. Darwin, in his " Notes on the effects produced by the ancient Glaciers of Caernarvonshire, and on the boulders transported by floating Ice " (Philosophical Magazine, 1842, vol. xxi., p. 180), after mentioning the labours of Dr. Buckland, on the same country, and that Mr. Trimmer had first noticed (" Proceedings of the Geological Society," vol. i., p. 332, 1831) the scoring and si-rati-hing ot rocks in North Wales, adduces additional evidence of glacial action in that district. He observes CH. XIV.] IN GREAT BRITAIN AND IRELAND. 271 this period has been variously estimated. Mr. Darwin infers, from a large greenstone boulder on Ashley Heath, Staffordshire, at 803 feet above the sea, and apparently derived from Wales, a consider- able depression of England beneath the sea, and that Scotland, from other data, must have been submerged 1,300 feet.* Looking at the heights to which gravels extend in Wales, often apparently the remains of masses of coast shingles and sand, a like, if not a greater depression beneath the present sea level would be there required. In Ireland, we find large blocks of granite sometimes perched on the heights, amid grooves and furrows on the surface of the rocks beneath, at altitudes of 1,000 feet and more. In some cases we almost seem to have before us a portion of the very blocks which scratched and scored the subjacent rock-surfaces. f Erratic blocks, occasionally of considerable magnitude, arc found, in some localities, at various elevations above rocks of their kind, and from which they are considered to have been detached. Although it is obvious that each fragment, so detached, has deprived the mass of rock whence it has been derived, of so much of its volume, and perhaps also of its height, as regards elevation above that, " within the central valleys of Snowdonia, the boulders appear to belong entirely to the rocks of the country. May we not conjecture," he continues, " that the ice- bergs, grating over the surface, and being lifted up and down with the tides, shattered and pounded the soft slate rocks, in the same manner as they seem to have contorted the sedimentary beds of the cast coast of England (as shown by Mr. Lyell) and of Tierra del Fuego ? " * * * The drifting to and fro and grinding of numerous icebergs during long periods near successive uprising coast lines, the bottom being often stirred up, and fragments of rocks dropped on it, will account for the sloping panes of unstratified till, occasionally associated with beds of sand and gravel, which fringes to the west and north the great Caernarvonshire mountains." Mr. Darwin further remarks (p. 186), as not " probable, from the low level of the chalk formation in Great Britain, that rounded chalk flints could often have fallen on the surface of glaciers, even in the coldest times, I infer, therefore," he continues, '* that such pebbles were probably enclosed by the freezing of the water on the ancient sea-coasts. We have, however, the clearest proofs of the existence of glaciers in this country, and it appears that, when the land stood at a lower level, some of the glaciers, as in Nant Francon, reached the sea, where icebergs charged with fragments would occasionally be found. By this means we may suppose the great angular blocks of Welsh rocks, scattered over the central counties of England, were transported." The deposits of this date in Ireland have occupied the attention of several geologists, among whom may be mentioned, Mr. Weaver, Mr. Griffith, Colonel Portlock, Mr. Trimmer, Professor Oldham, Mr. Bryce, Dr. Lloyd, Mr. Hamilton, and Dr. Scouler. * Philosophical Magazine, 1842, vol. xxi., p. 186. t Although in several parts of Ireland the facts relating to the transport of erratic blocks can be well studied, and the altitudes at which they and the smoothing and scratching of surface rocks are found well observed, there are few places where the latter can be seen in greater perfection than the beautiful neighbourhood of Glcn- gariff, county Cork. The scoring and rounding of the sides and bottom of the valley from the lower part of the demesne of Glengariff to Bantry Bay, and thence to the southward, in the direction of Cape Clear, arc particularly worthy of attentive study. 272 ICE-RAISED ERRATIC BLOCKS AND SHINGLES [Cn. XIV. the sea level, and consequently that if multitudes have been thus detached, previous heights, composed of such rocks, may have been much reduced by the loss thus sustained, there are instances where it would not appear a sufficient explanation to infer that a transport of erratic blocks had been effected by ice in such a manner, that, while higher portions of the parent rock floated away at the required levels, the remaining lower portions were denuded, in the usual manner, as the land emerged. To account for such instances, Mr. Darwin considers that we should regard the probable effects of submerging land, where coast ice could be formed, upon blocks of rock which may have been ice-transported to its shores. He points out that erratic blocks and other portions of the beaches of such shores might gradually be raised as the land became sub- merged, so that finally coast detritus, including the blocks of rocks ice-transported from various distances, would be elevated to heights above that at which it was accumulated or stranded. Blocks, with other coast fragments and shingle, would thus, when the land again emerged from beneath the sea, be found raised above the level at which the remains of their parent rocks are now found.* Respecting the erratic blocks of the Alps, and of the adjoining countries, a large mass of information has been collected.! The main fact of the blocks and associated minor detritus having been transported from the higher Alpine mountains outwards on both sides the main ranges, showing that the cause of their dispersion had been in the Alps themselves, forms the base of the chief modern hypotheses connected with the subject, whether the sudden melting of snows and glaciers by the heat and vapours accompanying the last elevation experienced in these mountains,! * Darwin, " On the Transportal of Erratic Boulders from a Lower to a Higher Level." Journal of the Geological Society, 1849, vol. v. Mr. Darwin remarks that the fragments of rock " from being repeatedly caught in the ice and stranded with violence, and from being every summer exposed to common littoral action, will generally be much worn ; and from being driven over rocky shoals, probably often scored. From the ice not being thick, they will, if not drifted out to sea, be landed in shallow places, and from the packing of the ice, be sometimes driven high up the beach, or even left perched on ledges of rock." t A valuable summary of the labours of geologists on this subject will be found in the " Histoire des Progres de la Ge'ologie, de 1834 a 1845," torn, ii., chap. 5, by the Vicomte d'Archiac. Appended to it is a list of the publications which may advan- tageously be consulted. J As regards the transport of blocks of rock by the sudden melting of snow from the escape of gases rising through fissures during the elevation of mountain chains, the observer will find the subject carefully treated in the' "Note relative a 1'une des causes presumables des phenomenes erratiques,' by Elie de Beaumont (Bulletin de la Societe' Geologique de France, t. iv. p. 1334, 1817). On the supposed heat of the gases required for the melting of the snow, M- Elie de Beaumont remarks, after CH. XIV.] COAST-ICE DURING SUBMERGENCE OF LAND. 273 the former great extension of Alpine glaciers, or the latter com- bined with a considerable submergence of land, so that the sea entered many of the valleys of the Alps, coast ice being possibly also produced. Von Buch, De Luc, Escher, Elie de Beaumont, and other geologists, long since pointed out that, from the mode of occur- rence of the Alpine erratic blocks, the great valleys of the Alps existed prior to their dispersion, and much observation has been directed to the sources whence particular kinds of blocks have been derived.* The magnitude of the blocks on both sides of the Alps, in connection with the distances they must have travelled from their parent rocks, has also long engaged attention. The Pierre d Bot, above Neuchatel, and represented beneath (fig. 102),f affords a good example of an erratic block, perched on the side of Fig. 102. noticing many circumstances bearing on the subject, that " it is unnecessary to attri- bute to the gaseous current, considered to have been disengaged from fissures in the ground, a temperature higher than that needed to overcome the atmospheric pressure. Little would be gained by giving this current a very high temperature." . . . " The hypothesis which admits the erratic thaw to have been produced by vapours of moderate temperature, appears to me," he continues, " also that according to which nature would have worked with the minimum loss of heat." * With reference to the mode of distribution of the erratic blocks in the basin of the Rhone, as also to the kinds of rocks so distributed, M. Guyot has remarked (Bulletin de la Soc. des Sciences de Neuchatel, 1846, Archives de Geneve, Sept., 1847) : 1. That a kind of rock which is abundant in one part of the basin, is rare, or absent, in another. 2. That the blocks of different kinds, commencing with the locality of their origin, form parallel series, preserved in the plain ; blocks of the right side of the valley keeping to the right, of the left side to the left, while those of the centre preserve their central position. 3. That groups composed of a single kind of rock, to the exclusion of others, are here and there found in the midst of various rocks. These views M. Guyot considers as borne out by numerous facts, and he infers that the blocks have been distributed by glaciers in the manner in which similar blocks now are by the moraines of acfual Alpine glaciers. He states that similar facts are observable in the valleys of the Rheuss and Rhine. t Taken from a view in the "Travels in the Alps of Savoy," &c., by Prof. James Forbes, 2nd edition. T 274 ERRATIC BLOCKS OF THE ALPS. [Cn. XIV. the Jura, far distant from its source. This granite mass is estimated as containing about 40,000 cubic feet, and considered to have been transported 22 leagues from the crest of the Follaterres on the north of Martigny.* The blocks on the Jura have always attracted much attention from the circumstance that they must have been transported over the great valley of Switzerland, intervening between that range and the Alps. The blocks on the Chasseron are estimated as rising to the height of about 3,600 feetf On the southern side of the Alps striking masses of erratic blocks are to be seen in the vicinity of the Lakes of Como and Lecco. They will be found high up the northern side of Monte San Primo, a moun- tain well separated from the high Alps by the intervening Lake of Como. The following (fig. 103) is a section of this mountain, showing the manner in which the erratic blocks rest upon it. Fig. 103. P, Monte San Primo ; B, bluff point of Bellaggio, rising out of the Lake of Como, C ; a a a a, blocks of granite, gneiss, &c., scattered over the surface of the limestone rocks, II II, and the dolomite ddd. V, the Commune di Villa, where a previously- existing depression has been nearly filled with transported blocks and minor detritus. - On the north side of the Alpi di Pravolta, E, the block represented beneath, (fig. 104), is seen, one however not Fig. 104. = ^8* * M. d'Archiac remarks (" Histoire des Progres de la Geologic," t. ii., p. 249), that granite and gneiss generally form the blocks of the largest size. " A block of granite, on the calcareous mountain near Orsieres, contains more than 100,000 cubic feet. Above Monthey, many blocks derived from the Val de Ferret, and which have thus travelled a distance not less than 11 leagues, contain from 8,000 to 50,000 and 60,000 cubic feet." ..." The blocks of talcose granite of Steinhof, near Seeberg, one of which measures 61,000 cubic feet, has travelled about 60 leagues." Considering the 40,000 cubic feet supposed to be contained in the Pierre a Bot, as French measure it would weigh about 3,000 tons. t Necker, " Etudes Geologiques dans les Alps," vol. i. Paris, 1841. CH. XIV.] ERRATIC BLOCKS OF NORTHERN EUROPE. 275 so remarkable for size, as for showing the little attrition it could have suffered during its transport from the higher Alps to its pre- sent position. A large amount of information has been obtained respecting the distribution of erratic blocks in Northern Europe, and the sources in Scandinavia whence they have been detached.* The area over which they have been so distributed has been shown in a map by Sir Roderick Murchison, M. de Verneuil, and Count Keyserling,*j- the boundary line exhibiting the southern and eastern limits of the erratic blocks extending from Prussia, to Voroneje, in Russia, and thence northwards to the Gulf of Tcheskaia, on the North Sea. It is remarked that from the German Ocean and Hamburg on the west, to the White Sea on the east, an area of 2000 miles long, varying in width from 400 to 800 miles (which may, perhaps, be roughly estimated at about 1,200,000 square miles) , is more or less covered by loose detritus, amid which there are blocks of great size, the whole derived from the Scandinavian mountains. While regarding the kind and extent of country thus more or less covered with erratic blocks, and the position which the Scandi- navian mountains would occupy relatively to a large submerged area, the opinion that glaciers, icebergs (detached from them), and coast ice, may have been the chief means of dispersing the blocks and other detritus from a large isolated region, as that of Scandinavia would then be, appears far from improbable. Careful examination of the Scandinavian region itself shows that the whole land has been elevated above the present level of the adjoin- ing seas in comparatively recent geological times, and there has been found a scoring of subjacent rocks, and dispersion of blocks outwards from it, according with this view.J * The observer would do well to consult the Rapport sur un Memoire de M. Durocher, entituled " Observations sur le Phenomene Diluvien dans le Nord de 1'Europe," by M. Elie de Beaumont (Comptes Rendus, torn, xiv., p. 78, 1842), where- in an excellent summary and general view of the subject, including the marking of subjacent rocks, up to the date of the observations, will be found. He should like- wise consult the " Geology of Russia in Europe and the Ural Mountains," 1845, by Sir Roderick Murchison, M. de Verneuil, and Count Keyserling ; chapter xx., Scandinavian Drift and Erratic Blocks in Russia ; and chapter xxi., Drift and Erratic Blocks of Scandinavia, and Abrasion and Striation of Rocks ; and also the " Histoire des Progres de la Geologic de 183 i a 1845," torn, ii., premiere partie, Terrain Quater- naire ou Diluvien. Formation erratique du Nord de 1'Europe. Paris, 1848. Not- withstanding the title, this valuable work contains information up to the date of publication. A most excellent and impartial summary of the labours relating to this subject, with original observations, will be found in this ' History.' f " Geology of Russia in Europe and the Ural Mountains," 1845. j M. Daubre'e states (Comptes Rendus, vol. xvi., 1843), that the traces of transport of detritus and of friction diverge from the higH regions precisely as in the Alps. This was observed up to an elevation of 3,800 feet (English). M. de Bohtlingk (Poggendorff's Annalen, 1841,) states that Scandinavian blocks have been transported T2 276 ERRATIC BLOCKS OF NORTH AMERICA. [Cn. XIV. In the region occupied by these erratic blocks, ridges of them and other detrital matter have been observed to run in lines, often for considerable distances. These are commonly known as skars, or osars.* Count Rasoumouski would appear (in 1819) to have been among the first to remarks upon those in Russia and Germany, observing that they usually occurred in lines having a direction from N.E. to S.W. M. Brongniart pointed out (in 1828), that those of Sweden, though sometimes inosculating, took a general direction from north to south.t Much discussion has arisen respecting the origin of these lines of accumulation. Upon the supposition that lines of blocks may have been accumulated by glaciers, and the drift of iceberg and coast ice in particular direc- tions, and that upon the uprise of such lines of deposits, breaker action had been brought to bear upon them for a time, we should expect very complicated evidence. In Northern America erratic blocks are found to occupy a large area, some being strewed as far south as 40 N. latitude. Here, as in Northern Europe, the general drift of detritus appears to be from the northward to the southward, and blocks perched at various altitudes, scored and scratched surfaces of subjacent rocks, and osars or lines of accumulation J occur in the same manner. Such similar effects point to similar causes, and hence the explanations from the coast of Kemi into the Bay of Onega, and from Russian Lapland into the Icy Sea, that is, in northerly, north-westerly, and north-easterly directions, as quoted also in the " Geology of Russia," vol. i., p. 528. * It is worthy of remark that similar accumulations of this date, in Ireland, are known as Escars. f " Annales des Sciences Naturelles," 1828. M. d'Archiac observes (" Histoire des Progres de la Ge'ologie," 1848, torn, ii., p. 36,) that " the form of the osars, their dis- position, and their parallelism with the furrows and scratches of erosion, naturally lead to the idea of a current which has swept the southern part of Sweden from N.N.E. to S.S.W. M. Durocher has found, with M. Sefstrom, that the osars were heaped up on the southern side of the mountains which, in that direction, opposed their course. The osars in Finland, though less marked, have a direction from N. 25 W. to S. 25 E., one which, with the preceding, represents the radii of the semicircle in which the great erratic block deposit of Central Europe occurs " In the " Geology of Russia in Europe and the Ural Mountains " will be found the views of its authors respecting skars or osars. A figure is given of an iceberg aground, and the consequences of its melting stated, lines of angular and rounded blocks being strewed, as the ice dissolved, by a current acting constantly in one direction. J An interesting account of two remarkable trains of angular erratic blocks in Berkshire, Massachusetts, is given by Professors Henry and William Rogers, in the " Boston Journal of Natural History," June, 1846. These two trains, one extending for 20 miles, both previously noticed by Dr. Reid and Professor Hitchcock, were traced to their sources. The blocks are generally large, the smaller being several feet in diameter. One weighs about 2,000 tons. The blocks gradually decrease in size to the S.E., those which have travelled farthest being the most worn. They are stated not to mingle with the general drift beneath them, the boulders and pebbles in which bear " the traces of a long-continued and violent rubbing." " Other long and narrow lines of huge erratic fragments are seen elsewhere in Berkshire, and abound, CH. XIV.] ERRATIC BLOCKS OF SOUTH AMERICA. 277 offered have been of a similar general character.* A large amount of information has also been collected respecting the occurrence of these blocks, and of the polishing and scoring of subjacent rocks, f It is stated that the divergence of any blocks,, such, for example, as those of the Alps, is not observed inkhe United States. Pro- fessor Henry Rogers points out that the scorings do not radiate from the high grounds; but that, amid the mountains of New England and in the great plains of the west, and in Pennsylvania, Vermont, and Massachusetts, they preserve a south-east direction at all their elevations ; the lower parts of the great valleys being alone excepted. In the mountainous portions of the region, the heights and flanks exposed to the north and north-west are the most polished and scored. Blocks of large size have been found in New England, New York, and Pennsylvania, from 1,000 to 1,500 feet above the sea. Erratic blocks are also found in South America. Mr. Darwin discovered them up the Santa Cruz river, Patagonia, in about 50 10' S. latitude, and at about 67 milee from the nearest Cordillera. Nearer the mountains (at 55 miles) they became " extraordinarily numerous." One square block of chloritic schist measured 5 yards on each side, and projected 5 feet above the ground ; another, more rounded, measured 60 feet in circumference. "There were innu- merable other fragments from 2 to 4 feet square. "J The great plain on which they stood was .1,400 feet above the sea, sloping gradually to sea cliffs of about 800 feet in height. Other boulders were found upon a plain, above another, elevated 440 feet, through we think, in nearly all the mountainous districts of New England. One such train, originating apparently in the Lennox ridge, about two miles on the south of Pitts- field, crosses the Housatonic Valley, south-easterly, as far at least as the foot of the broad chain of hills in Washington. Some very extensive ones are to be seen on the western side of the White Mountains. * These will be found in the works and memoirs of Hitchcock, Mather, Emmons, Hall, Rogers, Hubbart, Redfield, Jackson, Christy, Ch. Martins, and other geologists. f We are indebted to Dr. Bigsby for an early notice of the erratic blocks of North America. (Trans. Geol. Soc., London, vol. i., second series.) In 1833, Professor Hitchcock (" Report on the Geology of Massachusetts," art. Dilu- vium,) adduced abundant evidence of the northern origin of these blocks in the districts described by him. The like was also done at an early date for other portions of North America, by Messrs. Lapham, Jackson, Alger, and others. The observer will find an able summary of the facts known in 1846, on this subject, in Professor Hitchcock's Address to a meeting of the Association of American Geologists in that year. Professor Henry Rogers also treated in a general manner of the American erratic blocks in his Address to the same scientific body in 1844, (American Journal of Science, vol. xlvii.) Another general summary, up to 1848, is given by the Vicomte d'Archiac, (" Histoire des Progres de la Geologic," torn, ii., chap. 9, Terrain Quaternaire de 1'Amerique du Nord). t Darwin, " On the Distribution of Erratic Boulders, and on the Contemporaneous TJnstratified Deposits of South America." G col. Trans., second series, vol. vi. p. 415 278 ERRATIC BLOCKS OF SOUTH AMERICA. [Cn. XIV. which the same river flows, and at 800 feet above the sea. In the valley of the Santa Cruz, and at 30 or 40 miles from the Cordillera, (the highest parts in this latitude rise to about 6,400 feet,) blocks of granite, syenite, and conglomerate, not found in the more elevated plains, were detected. Mr. Darwin infers that these are not the wreck of those observed on the higher plain, but that they have been subsequently transported from the Cordillera. He had not opportunities of observing other erratic blocks in Patagonia, but refers to the great fragments of rocks noticed by Captain King on the surface of Cape Gregory, a headland, about 800 feet high, on the northern shore of the Strait of Magellan. Mr. Darwin also describes rock fragments of various dimensions and kinds in Tierra del Fuego and the Strait of Magellan, amid stratified and un- stratified accumulations of a similar general character to those of this geological date in Europe.* Many of the erratic blocks are large, one at St. Sebastian's Bay, east coast of Tierra del Fuego, was 47 feet in circumference, and projected 5 feet from the sand beach. The general drift of these deposits is considered to be from the westward, the manner in which the transported fragments of rock would be carried by a current similar to that which sweeps against the present land. On the north of Cape Virgins, close out- side the Strait of Magellan, the imbedded fragments are considered to have been transported 120 geographical miles or more from the west and south-west. On the northern and eastern coasts of the Island of Chiloe, extending from 43 26', to 41 46' S. latitude, Mr. Darwin detected an abundance of granite and syenite boulders, from the beach to a height of 200 feet on the land. He infers that these boulders have travelled more than 40 miles from the Cordillera, on the east.t * At Elizabeth Island, Strait of Magellan, there occurs, " fine-grained, earthy or argillaceous sandstone, in very thin, horizontal, and sometimes inclined laminae, and often associated with curved layers of gravel. On the borders, however, of the east- ward part of the Strait of Magellan, this fine-grained formation often passes into, and alternates with, great unstratified beds, either of an earthy consistence and whitish colour, or of a dark colour and of a consistence like hardened coarse-grained mud, with the particles not separated according to their size. These beds contain angular and rounded fragments of various kinds of rock, together with great boulders." Geol. Trans., second series, vol. vi., p. 418. Variations of these accumulations are noticed as occurring in other places, and two sections of contorted and confused beds at Gregory Bay are given, and Mr. Darwin infers that this disturbance may have been produced by grounded icebergs. t " The larger boulders were quite angular." ..." One mass of granite at Chacao was a rectangular oblong, measuring 15 feet by 11 feet, and 9 feet high. Another, on the north shore of Lemuy islet, was pentagonal, quite angular, and 11 feet on each side; it projected about 12 feet above the sand, with one point 16 feet high: this fragment of rock almost equals the larger blocks on the Jura." Geol. Trans., second series, vol. vi., p. 425. CHAPTEE XV. MOLLUSC REMAINS IN SUPERFICIAL DETRITUS. ARCTIC SHELLS FOUND IN BRITISH DEPOSITS. EVIDENCE OF A COLDER CLIMATE IN BRITAIN. EXTINCT SIBERIAN ELEPHANT.' CHANGES OF LAND AND SEA IN NORTH- ERN EUROPE. EXTINCTION OF THE GREAT NORTHERN MAMMALS. RANGE OF THE MAMMOTH. FROZEN SOIL OF SIBERIA. UPON the supposition of the submergence of a large portion of the present dry land of Northern Europe, Asia, and America, beneath seas upon which ice was formed, and into which glaciers protruded in lower latitudes than at present, we should expect to discover in the marine deposits of these regions, and of the period now upraised into the atmosphere, evidences of the marine animal life of the time having corresponded with the low temperature to which it was then exposed. This evidence is considered to have been found. As regards the British Islands, Mr. Trimmer pointed out, in 1831, that amid the detrital accumulation referred to this date, and at a considerable height above the sea (since ascertained to be 1,392 feet), upon Moel Trefan (one of the hills on the outskirts of the chief Caernarvonshire mountains), fragments of Buccinum, Venus, Natica, and Turbo of existing species were found. He also stated that on the flanks of the Snowdonian mountains, and between them and the adjoining sea, in the Menai Straits, there were large accumulations of boulders and fragments derived from a distance, (among them chalk flints,) mingled with others of a local kind. Mr. Trimmer subsequently (1838) published a more general state- ment on the same subject, noticing various localities where he and others had found shells, of a similar character, in deposits referred to this date.* * The first communication was made to the Geological Society of London (Pro- ceedings of that Society, vol. i.) ; the second to the Geological Society of Dublin m a memoir, in two parts, entitled, On the Diluvial or Northern Dnft on the Eastern 280 MOLLUSC REMAINS IN SUPERFICIAL DETRITUS. [Cn. XV. Commenting on the facts observed by Mr. Trimmer on Moel Trefan, Sir Eoderick Murchison (in 1832) inferred from the previous discovery of shells of existing species in the Lancashire gravels and sands by Mr. Gilbertson, one which he was enabled to confirm from actual observation, and from finding similar accumu- lations over a large tract of country, that the materials of the ancient shore of Lancashire and of the estuary of the Eibble, were deposited during a long protracted period, and " were elevated and laid dry after the creation of many of the existing species of molluscs."* Numerous facts of the like kind were noticed by different observers ;t but the inference as to a temperature less at that geological time than at present, as shown by the remains of molluscs, does not appear to have taken a distinct form until Mr. Smith, of Jordan Hill, published his views on the subject in 1839.J He discovered shells in places where their animals had lived and died, in the counties of Lanark, Renfrew, and Dumbarton, and hence inferred their entombment by depression, a half-tide deposit being converted into one in a deeper sea, From these and other researches, Mr. Smith obtained a mass of evidence which led him to conclude, from the remains of the molluscs discovered in deposits of this date in different localities, that the climate of the British Islands had then been colder than it now is, more especially as Arctic molluscs, not and Western side of the Cambrian Chain, and its Connexion with a similar Deposit on the Eastern side of Ireland, at Bray, Howth, and Glenismaule." (Journal of the Geological Society of Dublin.) Mr. Trimmer mentions that, prior to his discovery of the shells on Moel Trefan, Mr. Gilbertson had found shells of existing species in gravel and sand near Preston, Lancashire, and that Mr. Underwood had observed furrows and scratches on the surface of rocks laid bare among the Siiowdonian moun- tains, when the great road from Bangor to Shrewsbury was in progress. * Address, as President, to the Geological Society of London, February, 1832. Proceedings of that Society, vol. i, p. 366. t Among the observations of the time, and as important for the locality noticed, should be mentioned those of Sir Philip Egerton, " On a Bed of Gravel containing Marine Shells, of recent Species, at Wellington, Cheshire" (Proceedings of the Geological Society, vol. ii., p. 18;), April 1835). Sir Philip notices the remains of Turritella terebra, Cardium edule, and Murex arenaceus, and infers that there had been an alteration of 70 feet in the level of land and sea, as regards the locality, since the deposit was formed. In 1837, Mr. Strickland ("On the Nature and Origin of the various kinds of transported Gravel occurring in England," read at the British Association in that year) took a general view of the stratified and unstratified cha- racter of these deposits, and divided them into 1. Marine drift, formed when the central portions of England were under the sea; and, 2. Fluviatile drift, when they were above its level, forming dry land, the first composed of () erratic gravel, without chalk flints ; (6) erratic gravel, with chalk flints ; and (c) local, or non- erratic gravel. t "On the late Changes of the relative Levels of the Land and Sea in the British Islands" (Memoirs of the Wernerian Natural History Society, Edinburgh, vol. viii., p. 49, &c.) In this memoir Mr. Smith most carefully cites all those who had previously discovered facts relating to the subject, giving an account of these facts. CH. XV.] ARCTIC SHELLS IN BRITISH DEPOSITS. 281 now found round the British coasts, were obtained from these accumulations.* Professor Edward Forbes, in 1846, availing himself of the information then existing, and of his own researches on the same subject, pointed out that the total number of species of molluscs discovered in the deposits of the British area, and referred to this geological time, was about 124, all, with a few exceptions, now existing in the seas around the British Islands, and yet indicating by their mode of assemblage a colder state of the area than at present, f While carefully noticing the error which might arise * Alluding to the researches of M. Deshayes, to whom the unknown shells dis- covered were transmitted, and who stated that those still found recent, but not in the British seas, occur in northern latitudes, Mr. Smith remarks that this view confirmed that which he had previously entertained from finding many of the shells common with those obtained by Sir Charles Lyell, at Uddevalla, in Sweden, and figured by him (Phil. Trans., 1835) ; from having been informed by the same geologist that the Ftisus Peruvianus still inhabited the Arctic seas ; and from Mr. Gray (of the British Museum) having, from a cursory examination of the shells discovered, remarked that they had all the appearance of Arctic shells. Mr. Smith adds, " In the Clyde-raised deposits, shells common to Britain and the northern parts of Europe occur in much greater abundance than they do at present. The Pecten Islandicus, which has pro- bably entirely disappeared, and the Cyprina Islandica, which, if found recent in the Clyde, is extremely rare, are amongst the most common of the fossil species." Most valuable catalogues are appended to the memoir of Mr. Smith, consisting of lists of recent shells in the basin of the Clyde and north coast of Ireland (including land and fresh-water shells) ; of shells from the newer Pliocene deposits of the British Islands (also including land and fresh-water shells) ; and of recent species (then new) from the Firth of Clyde. f Professor E. Forbes, "On the Connexion between the distribution of the existing Fauna and Flora of the British Isles, and the Geological Changes which have affected their Area during the Period of the Northern Drift" (Memoirs of the Geological Survey of Great Britain, vol. i., p. 367, &c.). The Professor observes that, "as a whole, this fauna is very unprolific, both as to species and individuals, when compared with the preceding molluscan fauna of the red and coralline crags, or that now inhabiting our seas and shores. This comparative deficiency depends not on an imperfect state of our knowledge of the fossils in the glacial formations on that point we now have ample evidence but on some difference in the climatal conditions prevailing when those beds were deposited. Such a deficiency in species and indivi- duals of the testaceous forms of mollusca, indicates to the marine zoologist the pro- bability of a state of climate colder than that prevailing in the same area at present. Thus the existing fauna of the Arctic seas includes a much smaller number of testaceous molluscs than those of Mid-European seas, and the number of testacea in the latter is much less than in South-European and Mediterranean regions. It is not the latitude, but the temperature which determines these differences." " That the climate," he subsequently observes, " under which the glacial animals lived, was colder, is borne out by an examination of the species themselves. We find the entire assemblage made up, 1st, of species (25) now living throughout the Celtic region in common with the northern seas, and scarcely ranging south of the British Isles ; 2nd, of species (24) which range far south into the Lusitanian and Mediterranean regions, but which are most prolific in the Celtic and northern seas ; 3rd, of species (13) still existing in the British seas, but confined to the northern portion of them, and most increasing in abundance of individuals as they approach towards the Arctic circle ; 4th, of species (16) now known living only in European seas, north of Britain, or in the seas of Greenland and Boreal America; oth, of species (6) not now known existing, 282 EVIDENCE OF A COLDER CLIMATE IN BRITAIN, [Cn. XV. from neglecting the occurrence of species at different depths in the sea, he observes, that among those found in these deposits, and in situations where they must have lived and died, there are shells, such as the Littorince, the Purpura, the Patella, and the Lacunce, " genera and species definitely indicating, not merely shallow water, but, in the three first instances, a coast line."* Taking a general view of the flora of the British Islands, and of the probable sources whence its parts have been derived, Professor Edward Forbes has inferred that a portion was obtained from northern regions when the higher parts of these islands were alone above the sea, at a time corresponding with that when the marine molluscs living in the seas around them were of the character above noticed, and when the climate was colder than it now is, the evidence of the land flora thus corroborating that afforded by the remains of the marine molluscs. Under such conditions he infers that " plants of a subarctic character would flourish to the water's edge." The whole area being subsequently upraised, in the manner above noticed, the previous islands would become moun- tain heights, and the plants, uplifted with them, not being deprived of the climatal conditions fitted for them, continued to flourish and be distributed as we now find them.t and unknown fossil in previous deposits. Two other species, from southern deposits in Ireland, were, one the same as one ( Turritella incrassata) still existing in the South- European, though not in the British seas, and the other ( Tornatella pyramidata) extinct, but found fossil in the crag." Professor E. Forbes remarks, that it is "of consequence to note the fact that the species most abundant and generally diffused in the drift are essentially northern forms, such as Astarte elliptica, compressa, and borealis, Cyprina cammunis, Leda rostrata and minuta, TMina calcarea, Modiola vul- garis, Fusus bamfius and scalariformis, Littorince and Lacuna, Natica clausa and Buccinum undatum; and even Saxicava rugosa and Turritella terebra, though widely distributed, are much more characteristic of North-European than of Southern seas." * " Memoirs of the Geological Survey of Great Britain," vol. i., p. 370. The Pro- fessor adds, " a most important fact, too, is that among the species of Littorina, a genus, all the forms of which live only at water-mark, or between tides, is the Littarina expansa, one of the forms now extinct in the British, but still surviving in the Arctic Seas." f " Memoirs of the Geological Survey," vol. i. Professor E. Forbes divides the general flora into five parts, " four of which are restricted to definite provinces, whilst the fifth, besides exclusively claiming a great part of the area, overspreads and com- mingles with all the others." With regard to his general view, the Professor takes ? as his main position, -that " the specific identity, to any extent, of the flora and fauna of one area with those of another, depends on both areas forming, or having formed, part of the same specific centre, or on their having derived their animal and vegetable population by transmission, through migration, over continuous or closely-contiguous land, aided, in the case of Alpine floras, by transportation on floating masses of ice." As respects the vegetation to which reference is made in the text, Professor E. Forbes observes, "The summits of our British Alps have always yielded to the botanist a rich harvest of plants which he could not meet with elsewhere among these islands. The species of these mountain plants are most numerous on the Scotch mountains com- CH. XV.] FROM MOLLUSC REMAINS AND EXISTING FLORA. 283 As confirming his views respecting the effect of great cold at this period upon the marine molluscs in the seas around the British Islands, Professor E. Forbes found, while dredging, that there were depressions off the coasts in which molluscs of Arctic cha- racter still remained, as if imprisoned in cavities during the general rise of the sea-bottom, so that while their germs still found the needful conditions for their development in such depressions, when they passed beyond them, they perished. Quitting the minor area of the British Islands, and extending our views to the great region ranging from Scandinavia eastward along Northern Asia to Behring's Straits, we should, in the higher latitudes, expect no great aid, as regards evidences of a colder climate having^ more prevailed at that geological time than at present, from the remains of marine molluscs entombed amid detritus,* or from the existing flora there found. Under the hypothesis of a depression of land, accompanied by increased cold, it is not difficult to conceive that the marine fauna and terrestrial flora of the region became adjusted to the conditions obtaining at the different times, the one accommodating itself to the new shores, the other creeping to the proper grounds, as the sea-bottom changed and the general temperature became lowered or elevated. The discovery, however, of large animals entire in ice, or frozen mud or sand, with their flesh and hair preserved, in high northern latitudes, and of kinds not now existing there, has been considered as affording somewhat of the evidence required. It is now about half a century since that the body of an elephant, of a species not now living, but the remains of which are widely paratively few on more southern ridges, such as those of Cumberland and Wales. But the species found on the latter are all, with a single exception (Lloydia serotina), inhabitants also of the Highlands of Scotland ; whilst the Alpine plants of the Scotch mountains are all, in like manner, identical with the plants of more northern ranges, as the Scandinavian Alps, where, however, there are species associated with them which have not appeared in our country." * The well-known mass of shells at Uddevalla, in Sweden, raised to the height of 216 feet above the level of the sea, and beneath part of which M. Alexandre Brong- niart long since found Balani still adhering to the supporting gneiss rocks on which they grew (" Tableau des Terrains que compose 1'Ecorce du Globe," p. 89), is described as composed of species still existing in the neighbouring seas. A list of these shells was given by M. Hisinger, " Esquisse d'un Tableau des Petrifactions de la Svede," ed. 2me, Stockholm, 1831. Professor E. Forbes has pointed out that this accumula- tion of shells was noticed by Linnaeus in 1747, and that the species discovered by him are now known as Balanus Scoticus, Saxicava rugosa or sulcata, Mya arenaria, Littorina littorea, Nytilus edulis, Fusus scalariformis, Pecten Islandicus, Fusus antiquus, and Balanus sulcatus. In 1806, the Uddevalla shells, and others of existing species, raised above the present level of the sea in Norway, were observed by Von Buch. They were also described by Sir Charles Lyell, in his account of the rise of land in Sweden, " Philosophical Transactions," 1835. 284 EXTINCT ELEPHANTS AND RHINOCEROSES [CH. XV. dispersed amid the later geological accumulations of the northern portions of the northern hemisphere, its flesh so fresh that bears and wolves devoured it, was found frozen in 70 N. latitude, near the embouchure of the Lena in Siberia.* The body of a rhi- noceros also, of a species now extinct, whose hard remains are also discovered in somewhat similar positions, had been obtained in the state of a mummy by Pallas thirty years previously, in latitude 64 N., from the banks of the Wiljue, which falls into the Lena, the carcase smelling like putrid flesh, the hair still partly on the body. These discoveries long since led to speculations respecting a change of climate in Siberia, one suddenly destroying the animals mentioned by cold, so that their carcases were preserved. Pro- fessor Playfair (in 1802) would appear to have been the first to infer that the elephants and rhinoceroses of Siberia, now extinct, may have been fitted for a cold climate, though the elephants of the present day inhabit regions of a higher temperature, and that "they may have migrated with the seasons, and by that means have avoided the rigorous winters of the high latitudes. "f He also con- sidered that they might have lived farther to the south than the localities where their remains are now found, and " among the valleys between the great ranges of mountains that bound Siberia on that side." Sir Charles Lyell, in 1835, took a similar but more extended view of the subject. Adverting to the mode of occur- rence of the abundant remains of elephants in the deposits of Siberia, an abundance so great that a trade in their tusks for ivory has long been established, to the deposits themselves in which they are discovered having been formed beneath the sea, since they contain the remains of marine shells ; and to a slow up- heaval of the borders of the Icy Sea, as is now taking place, he con- sidered that a considerable change in the physical geography of the * Mr. Adams, who carefully preserved what remained of this animal, relates that it was first observed as a shapeless mass by Schumakof, a Tungusian chief, and owner of the peninsula of Tamset, in 1799 ; that this ice-covered mass fell upon the sand in 1803, and that, in the next year, the chief cut off the tusks, the fossil ivory, if it may from its comparative freshness be so termed, found in these regions, being an article of commerce. Mr. Adams, visiting the spot two years afterwards, obtained the skeleton, still in part covered by the fleshy remains, with portions of its hair, which, together with the tusks, subsequently purchased, is now preserved in the Museum at St. Petersburg ; and a description is given of it in the " Memoirs of the Imperial Academy of Sciences," vol. v., of which a translation was published, with a figure, in London, in 1819. t Playfair's "Illustrations of the Huttonian Theory," Edinburgh, 1802. j "Principles of Geology," 4th edition, 1835. This fossil ivory is still imported from Russia into Liverpool, where it finds " a ready sale to comb-makers and other workers in ivory." Owen, "History of British Fossil Mammals," p. 249. CH. XV.] OF NORTHERN EUROPE AND ASIA. 285 whole region had been effected, a great increase of land northwards being the result of a long-continued and slow uprise of land and sea-bottom. He inferred a general decrease of temperature, so that the elephants and rhinoceroses, though they may have been fitted to live in colder regions than any of the kinds now existing, gradually perished. Sir Eoderick Murchison and his colleagues, in the examination of the geology of Russia and the Ural Mountains, adopted similar general views, inferring that the Ural, Altai, and neighbouring regions of Siberia, were above the sea when these great mammals existed, and that they lived in herds adjacent to lakes and estuaries,* into and down which their remains were swept. It would appear, especially by the researches of M. Middendorf, that the shells found with these remains are of kinds now existing in the seas of the region, so that the molluscs of that time and the neighbouring seas have not been exposed to conditions effecting their destruction. M. Middendorf also mentions, that in 1843, the carcase of an elephant was found in the Tas, between the Oby and Yenesei, in about latitude 66 30' N., "with some parts of the flesh in so perfect a state, that the bulb of the eye is now preserved in the Museum of Moscow."! Sir Roderick Murchison, M. de Verneuil, and Count Keyserling also remark, when describing the range and boundaries of the erratic blocks of Russia, that the area of the districts of Perm, Viatka, and Orenburg, was probably " above the waters and inhabited by mammoths "J at this period. With regard to the probable habits and food of the elephant (Ekphas primigenius) and the rhinoceros (R. tichorhinus), the researches of Professor Owen have shown, that on physiological * " Geology of Russia in Europe and the Ural Mountains," vol. L, p. 500. t The discoveries of M. Middendorf, of 1843, were communicated to Sir Charles Lyell in 1846 (" Principles of Geology," 7th edition, 1847). " Another carcase, together with another individual of the same species, was met with in the same year (1843), in latitude 75 15' N., near the river Taimyr, with the flesh decayed. It was embedded in strata of clay and sand, with erratic blocks, at about 15 feet above the level of the sea. In the same deposit, M. Middendorf discovered the trunk of a larch tree (Pinvs larix), the same wood as that now carried down in abundance by the Taimyr to the Arctic Sea. There were also associated fossil shells of living northern species, and which are moreover characteristic of the drift, or glacial deposits of Europe. Among these Nucula pygnuea, Tellina calcarea, Mya truncata and Saxacava rugosa, were con- spicuous." Lyell's Principles, 7th edition, p. 83. J Alluding to their map, it is further observed that this probably happened, " when the erratic blocks were transported over the adjacent north-western line marked in the map, as the extreme boundary of the granitic erratics, which were, we believe, stranded on or near the shelving shore of this ancient land." Geology of Russia, vol. i., p. 522. " History of British Fossil Mammals and Birds," 1846. To the previous inference that the elephant, from its warm, woolly, and hairy coat, was an animal fitted to live 286 PROBABLE HABITS OF NORTHERN ELEPHANTS. [Cn. XV. grounds the Elephas primigenius " would have found the requisite means of subsistence at the present day, and at all seasons in the sixtieth parallel of latitude," so that by adopting, with Professor Playfair and Sir Charles Lyell, the inference that this animal migrated northwards during the warmer parts of the year, as many northern mammals now do, the mammoth, as that kind of extinct elephant has been termed, would have lived easily on the land considered to have been above water at this period. The Professor adds, "in making such excursions during the heat of that brief season (the northern summer), the mammoths would be arrested in their northern progress by a condition to which the rein-deer and musk-ox are not subject, viz., the limits of arboreal vegetation, which, however, as represented by the diminutive shrubs of Polar lands, would allow them to reach the seventieth degree of latitude." With regard to the habits and food of the two-horned rhinoceros,* found frozen in Siberia, the inferences do not appear so clear as for the mammoth. From the greater amount of hair found on the extinct and frozen rhinoceros, noticed by Pallas, than upon existing rhinoceroses, he seems to have concluded that it might have lived in the temperate regions of Asia. Professor Owen remarks that, " although the molar teeth of the Rhinoceros tichorlimm present a specific modification of structure, it is not such as to support the inference that it could in a cold climate (the skin of the carcase from the Lena, and the ground on which it fell, affording many pounds weight of reddish wool and coarse long black hairs), Professor Owen showed that its teeth especially were adapted for the apparently cold climate in which its remains have been so abundantly detected. " The molar teeth of elephants possess," observes the Professor, '* a highly-complicated, and a very peculiar structure, and there are no other quadrupeds that derive so great a proportion of their food from the woody fibre of the branches of trees. Many mammals browse the leaves ; some small rodents gnaw the bark ; the elephants alone tear down and crunch the branches, the vertical enamel-plates of their huge grinders enabling them to pound the tough vegetable tissue and fit it for deglutition. No doubt the foliage is the more tempting, as it is the most succulent part of the boughs devoured ; but the relation of the complex molars to the comminution of the coarser vegetable substance is unmistakeable. Now, if we find in an extinct elephant the same peculiar principle of construction in the molar teeth, but with augmented complexity, arising from a greater number of triturating plates, and a greater proportion of the dense enamel, the inference is plain that the ligneous fibre must have entered in a larger proportion into the food of such extinct species. Forests of hardy trees and shrubs still grow upon the frozen soil of Siberia, and skirt the banks of the Lena as far north as latitude 60. In Europe arboreal vegetation extends ten degrees nearer the pole ; and the dental organization of the mammoth proves that it might have derived subsistence from the leafless branches of trees, in regions covered during a part of the year with snow." p. 267. * The horns of this rhinoceros have been ascertained to have been of large size. One of the horns of an individual, probably the front or nasal horn, in the Museum at Moscow, measures, according to Professor Owen, nearly three feet in length. CH. XV.] LAND AND SEA CHANGES IN NORTHERN EUROPE. 287 have better dispensed with succulent vegetable food than its existing congeners; and we must suppose, therefore, that the well-clothed individuals who might extend their wanderings northwards during a brief but hot Siberian summer, would be compelled to migrate southward to obtain their subsistence during winter."* Considering the general evidence thus adduced as to the climate of Northern Europe at this geological time, we have to suppose a considerable depression of a large area beneath the level of the Atlantic ; an increase of cold, causing glaciers to descend into the sea in Scandinavia, and even in the British Islands; a great increase, if not extension into the sea, of the glaciers of the Alps, icebergs and coast ice distributing masses and minor fragments of rocks over a considerable European area, as also the shingles of beaches, sand, and mud, accompanied by the transported remains of terrestrial and marine creatures, and a movement of land plants, with terrestrial and marine animals, in accordance with the low temperature then existing. The amount of land rising above the sea, prior to the inferred depression, is uncertain. It may have been more or less than that which we now find, though deposits of varied thickness were accumulated at this time, and now constitute a part of the dry land of Europe, and probably also a portion of the bottom of the adjoining seas. Respecting the great mammals, the carcases of which have been so well preserved in Siberia, and admitting, with Professor Owen, their perfect fitness to have lived in a climate such as that at present found in Northern Europe and Asia, up to a high latitude, we have to consider that at the time of greater cold, their food being adjusted to it, their range, even in the summer season, would be more limited northward, not only by any coasts which might then be thrown back by the depression beneath the sea level, but also by the supposed decreased temperature. The great rivers, flowing northward, would, as Humboldt, Sir Charles Lyell, and Sir Roderick Murchison have pointed out, be then under similar conditions to the present, their embouchures exposed to lower temperatures than their courses in more temperate regions, such courses, though somewhat shorter, being still liable, as now, to be blocked up by ice at their mouths. In such a state of things there is little difficulty in inferring that the elephants and rhinoceroses lived, as they are supposed to have done, in a climate of low * "History of British Fossil Mammals and Birds," 1846, p. 353. 288 EXTINCTION OF GREAT NORTHERN MAMMALS. [Cn. XV. temperature, and that their remains were buried in the detritus accumulated in lakes and at the embouchures of the northern rivers of the time, numerous carcases being washed out to sea and preserved amid ice, or frozen mud and sand, among deposits con- taining the remains of marine molluscs, such as are now living in the adjoining Arctic sea. The cause of the extinction of the great mammals mentioned requires much consideration, and a careful observation of the facts connected with the entombment and preservation of their remains. Humboldt has remarked that the low temperature at present experienced across Poland and Eussia to the Ural mountains, " is to be sought in the form of the continent being gradually less intersected, and becoming more compact and extended, in the increasing distance from the sea, and in the feebler influence of westerly winds. Beyond the Ural, westerly winds blowing over wide expanses of land, covered during several months with ice and snow, become cold land winds. It is to such circumstances of configuration and of atmospheric currents that the cold of Western Siberia is due."* By the immersion of the present dry land to the extent supposed,! unaccompanied by the general decrease of temperature inferred in Northern Europe, there might, no doubt, be reason to expect that such northern portions of European and Asiatic Eussia as were above water would have a higher tempera- ture than at present, but how far this would be met by such a decrease of the present temperature of Scandinavia, the British Isles, and a portion of Central Europe, that glaciers descended to the then sea level, it is more difficult to infer. Because icebergs may have floated from Scandinavia, and have become stranded on the shores of the districts of Perm, Viatka, and Orenburg, and thence along the line pointed out by Sir Eoderick Murchison, M. de Verneuil, and Count Keyserling to the westward, it is not a necessary inference that the temperature of those regions, making every allowance for the influence of multitudes of icebergs at certain seasons, had been very low, more than that the tempera- ture of Newfoundland should be that of Greenland and Baffin's Bay, whence the icebergs stranded near it are derived. Even supposing that as the land rose the temperature of Siberia became such as we now find it, it does not seem to follow, judging from the researches ' * Cosmos, 7th Edit. (Sabine's Translation), vol. i., p. 323. f The observer would do well to refer to the map given by the authors of the Geology of Russia in Europe and the Ural, for the area bounding the occurence of erratic blocks. CH. XV.] RANGE OF EXTINCT NORTHERN ELEPHANTS. 289 and reasoning of Professor Owen, that the mammoths necessarily perished from cold or the want of food.* Assuming that the great cold was unfavourable to their continuance in Siberia, that the country towards the mountains on the south was equally so to their habits, and that thus they may have been there extirpated, the same reasoning does not seem to apply so well to the districts on the west of the Ural. It is now well known that the mammoths must once have ex- isted widely spread over the northern portions of Europe, Asia, and America ; whence the inference, on the hypothesis that they all pro- ceeded from a common stock, or centre, that they spread themselves over continuous portions of land, dry for the time, however now separated they may be by seas. Their remains are not uncommon in Great Britain, though less so apparently in Ireland, and Professor Owen has pointed out the connection of these Islands with Europe when these and other contemporary animals passed into them.f The * On this subject Professor Owen remarks, that " with regard to the geographical range of the Elephas primigenius into temperate latitudes, the distribution of its fossil remains teaches that it reached the fortieth degree north of the equator. History, in like manner, records that the rein-deer had formerly a more extensive distribution in the temperate latitudes of Europe than it now enjoys. The hairy covering of the mammoth concurs, however, with the localities of its most abundant remains, in showing that, like the rein-deer, the northern extreme of the temperate zone was its metropolis. Attempts have been made to account for the extinction of the race of northern elephants by alterations in the climate of their hemisphere, or by violent geological catastrophes, and the like extraneous causes. When we seek to apply the same hypothesis to explain the apparently contemporaneous extinction of the gigantic leaf-eating megatheria of South America, the geological phenomena of that continent appear to negative the occurrence of such destructive changes. Our comparatively brief experience of the progress and duration of species within the historical period, is surely insufficient to justify, in every case of extinction, the verdict of violent death. With regard to many of the larger mammalia, especially those which have passed away from the American and Australian continents, the absence of sufficient signs of extrinsic extirpating change or convulsion, makes it almost as reasonable to speculate with Brocchi, on the possibility that species, like individuals, may have had the cause of their death inherent in their original constitution, independently of changes in the external world, and that the term ot their existence, or the period of exhaustion of the prolific force, may have been ordained from the commencement of each species." History of British Fossil Mammals and Birds, p. 269. f " History of British Fossil Mammals and Birds," 1846, Introduction, p. xxxvi. " If," Professor Owen observes, " we regard Great Britain in connection with the rest of Europe, and if we extend our view of the geographical distribution of extinct mammals beyond the limits of technical geography, and it needs but a glance at the map to detect the artificial character of the line which divides Europe from Asia, we shall there find a close and interesting correspondence between the extinct Europa?o- Asiatic Mammalian Fauna of the pliocene period and that of the present day. The very fact of the pliocene fossil mammalia of England being almost as rich in generic and specific forms as those of Europe, leads, as already stated, to the inference that the intersecting branch of the ocean which now divides this island from the continent did not then exist as a barrier to the migration of the mastodons, mammoths, rhino- ceroses, hippopotamuses, bisons, oxen, horses, tigers, hyaenas, bears, &c., which have left such abundant traces of their former existence in the superficial deposits and caves of Great Britain." U 290 ELEPHANT REMAINS OF ESCIISCHOLTZ BAY. [Cn. XV. depth of Behring's Straits is comparatively trifling, varying from 132 to 192 feet, so that we feel little surprise in finding a.t Esch- scholtz Bay, in about 66 20' N. on the North American shores, inside the Straits, the remains of the Elephas primigenius, asso- ciated with the bones of the urus, deer, horse, and musk ox, in a cliff about 90 feet high, extending about 2 J miles in length. These remains were first noticed by Dr. Eschscholtz (during Kotzebue's voyage), in 1816, and the bones were supposed to be imbedded in ice; but the observations of Captain Beechey's party, in 1826, showed that the ice was merely superficial, arising from the freez- ing of water descending over the face of the cliff, and that the remains of these mammals were really imbedded in a deposit of clay and fine quartzose and micaceous sand. A smell, as of heated bones, was observed where the animal remains abounded.* This facing of ice having been thus deceptive, Dr. Buckland was led to inferf that there also might have been some error respecting the elephant of the Lena having really been encased in ice, and not in mud, the face of which was covered by ice, as at Eschscholtz Bay. Correct observations respecting the mode of occurrence of the animals preserved in a comparatively fresh state, x with their fleshy portions in part or wholly remaining, are some- what important, inasmuch as, if found in ice, we have to infer either that such ice had always remained unthawed in the atmo- sphere (at least so far as the portions enveloping the animals were concerned), from the time when these mammals were encased in it to the present time, or that it became depressed beneath detrital accumulations of the period, and also remained unthawed, until the whole being elevated again into the atmosphere, it became, with the accumulations among which it had been buried, exposed to the climatal and denuding conditions of the present day. Though there would be difficulty in submerging ice, from its specific gravity, beneath water, and especially sea-water, unless sufficiently well loaded with detritus to render this of the proper kind, it may readily happen that, in very cold climates, coast-ice may be anchored, so to speak, in such a manner, by penetrating amid shingles, sand, or mud beneath, that it could be covered over in part, or in thickness, according to variations in seasons, by detrital matter, so as to be in the condition to descend, thus covered over, to those depths where it could remain unthawed, with any animals * " Beechey's Voyage to the Pacific and Behring's Straits." The bones were examined, and the animals to which they belonged were determined, by Dr Buckland. t Ibid. CH. XV.] ICE-BEDS BENEATH DETRITAL DEPOSITS. 291 entombed in it. Indeed, certain facts noticed by travellers and voyagers in the Arctic regions would lead us to infer that this might be the case, and accounts are given of beds of actual ice being found beneath detrital deposits in those regions.* Descended to a proper depth beneath the surface, but not sufficient to bring it within the influence of the heat found to exist beneath certain depths in different parts of the globe, ice might remain there, only to be thawed by a great increase in the temperature of the general climate, or by being again elevated, with a sufficient denudation of ' , protecting detritus, so that the heat of the atmosphere in summer would dissolve it, and disclose any animal remains which may have been therein preserved. At the same time, mud and silt, into which the bodies of such animals as the elephants and rhinoceroses, above noticed, may have been borne during floods, could readily have become frozen, and covered with other detritus, and thus descending, have retained, from what we learn of the depth to which the frozen ground extends in Siberia a depth apparently very different from that found in North America, in the same lati- tudes the remains of the animals in as fresh a state as when first embedded in them, to a level, beneath that of the sea, of 400 feet, if the cold approached that now experienced in northern Siberia.t 9 ^J * M. Middendorf informed Sir Charles Lyell, that in 1843, " he had bored in Siberia to the depth of 70 feet, and, after passing through much frozen soil mixed with ice, had come down upon a solid mass of pure transparent ice, the thickness of which, after penetrating two or three yards, they did not ascertain." Principles of Geology, 7th Edition, p. 86. f The depth to which frozen mud and sand could descend in these regions, without being thawed by the influence of terrestrial heat beneath, would appear from the information of M. Helmersen (" Observations on a Pit sunk at Jakoutsk," Ann. des Mines de Russie, vol. v., 1838), to be between 300 and 400 feet. On the 25th April, 1837, the temperature of the bottom, 378 (English) feet deep, was 31 '1, the strata on the sides of the pit at 75 feet being 21 '2 Fahr. The accumulations passed through were composed of clay, sand, and lignite, mixed with ice. Some experiments made by M. Middendorf, as reported to the Academy of Sciences of St. Petersburg in 1844, showed that, in a shaft and the galleries of some works near the Lena, and at a depth of 384 (English) feet, the frozen crust was still not passed through, though a marked gradual increase of temperature was observed in the descent. While, in one series of experiments, a thermometer, in the ground, 7 feet from the surface, gave on the 25th March, 1 Fahr., the temperature gradually advanced to 26 -6 Fahr. According to M. Erman (" Proceedings of the Academy of Sciences at St. Petersburg," 1838), the depth of ground thawed in September, 1838, in Northern Siberia, was 4 feet 8 inches in woody tracts, and 6 feet 8 inches in the marshy situations. From Sir John Richardson having found the depth of the frozen ground not to exceed 26 feet at Fort Simpson, on the Mackenzie, a station in the same latitude as Jakoutsk (62 N.), M. d'Archiac has inferred (" Histoire desProgres de le Geologie," vol. i , p. 88), that the cold must be far more intense in Northern Asia than in North America, at these high latitudes. Under this view, the bodies of animals could now be preserved in Northern Siberia, by descending and ascending land, which could not be so preserved in North America. U 2 CHAPTER XVI. OSSIFEROUS CAVERNS AND BRECCIA.- FORMER CONNEXION OF BRITAIN WITH THE CONTINENT. MAMMOTH REMAINS FOUND IN BRITISH SEAS. OSSIFEROUS CAVE OF KIRKDALE. MUD IN OSSIFEROUS CAVES. GENERAL STATE OF OSSIFEROUS CAVERNS. HUMAN REMAINS IN PAVILAND CAVE. CAVES FORMERLY DENS OF CERTAIN EXTINCT CARNIVORA. HUMAN RE- MAINS IN OSSIFEROUS CAVERNS. COMPLICATED ACCUMULATIONS IN CERTAIN CAVES. PEBBLES IN OSSIFEROUS CAVES. DEPOSITS IN SUBTER- RANEAN RIVER CHANNELS. OSSEOUS BRECCIA IN FISSURES. CHANGES IN THE ENTRANCES OF CAVES. OCCURRENCE OF MASTODON REMAINS. ASSOCIATION WITH THOSE OF THE MAMMOTH. EXTINCT MAMMALS OF CENTRAL FRANCE. THE bodies of elephants and rhinoceroses being found so well preserved in Siberia, and nowhere, as has often been remarked, are the remains of the Elephas primigenius more abundant than in the lowlands, adjoining the icy sea of Northern Asia,* it is desirable to consider the remains of the same kinds of elephant and rhinoceros, with those of contemporary mammals, found embedded amid accumulations in caves and clefts of rock. The connection of the British Islands with the continent of Europe and * Dr. Mantell states (" Wonders of Geology," vol. i., p. 148, 6th Edition, 1848) that, a company of merchants having been formed in 1844, to collect fossil ivory in Siberia, sixteen thousand pounds of jaws and tusks of mammoths were obtained during the year, and these were sold at St. Petersburg, under the denomination of Siberian ivory, at prices from 30 to 100 per cent, above those of recent elephantine ivory. From the researches of M. Hedenstrom, multitudes of the remains of elephants, rhinoceroses, oxen, and other mammalia, occur in the frozen ground between the Lena and the Kolima, and he mentions that one of the islands of New Siberia, or the Liakhor Islands, in the Arctic Ocean, off the coast of Siberia, between the embou- chures of the Lena and Indigirka, is composed of little else than a mass of mammoth bones, which has been worked for many years by the traders for the fossil ivory it yields. This statement is confirmed by those of other travellers. The high preservation of fossil ivory is not confined to Siberia. Mr. Bald mentions (Wernerian Transactions, vol. iv.) that tusks found between Edinburgh and Falkirk were made into chessmen. CH. XVI.] CONNECTION OF ENGLAND WITH THE CONTINENT. 293 Asia has been above noticed (p. 289), as needed for the migration of the Ullephas primigenius and Rhinoceros tichorhinus into the former, the remains of these mammals so occurring as to leave no room for doubting, that the animals themselves here found the conditions fitted for their existence and increase.* The observer has carefully to weigh the evidence afforded as to the precise geo- logical period when these great mammals thus prospered upon lands now divided from the continent by sea, which it would * Respecting the mammals existing at this time in the area of the British Islands, Professor Owen remarks, (" History of British Fossil Mammals" Introduction^) after noticing the probable disappearance of the mastodon from it, that " gigantic elephants of nearly twice the bulk of the largest individuals that now exist in Ceylon and Africa, roamed here in herds, if we may judge from the abundance of their remains. Two-horned rhinoceroses, of at least two species, forced their way through the ancient forests, or wallowed in the swamps. The lakes and rivers were tenanted by hippo- potamuses as bulky and with as formidable tusks as those of Africa. Three kinds of wild oxen, two of which were of colossal size and strength, and one of these maned and villous like the bonassus, found subsistence in the plains. Deer, as gigantic in proportion to existing species, were the contemporaries of the old Uri and Bisontes, and may have disputed with them the pasturage of that ancient land; one of these extinct deer is well-known under the name of the ' Irish Elk,' from the enormous expanse of its broad-palmed antlers [the Professor states elsewhere, Hist. Brit. Foss. Mammals, p. 467, that the remains of this animal have been found in the ossiferous cavern of Kent's Hole, Devon] ; another had horns more like that of the wapiti, but surpassed that great Canadian deer in bulk ; a third extinct species more resembled the Indian hippelaphus ; and with these were associated the red-deer, the rein-deer, the roebuck, and the goat. A wild horse, a wild ass or quagga, and the wild boar, entered also into the series of British pliocene hoofed mammalia. " The carnivora, organized to enjoy a life of rapine at the expense of the vegetable- feeders, to restrain their undue increase, and abridge the pangs of the maimed and sickly, were duly adjusted in numbers, size, and ferocity to the fell task assigned to them in the organic economy of the pre-Adamitic world. Besides a British tiger of larger size, and with proportionally larger paws than that of Bengal, there existed a stranger feline animal ( Machairodus) of equal size, which, from the great length and sharpness of its sabre-shaped canines, was probably the most ferocious and destructive of its peculiarly carnivorous family. Of the smaller felines, we recognise the remains of a leopard, or large lynx, and of a wild cat. " Troops of hyaenas, larger than the fierce crocuta of South Africa, which they most resembled, crunched the bones of the carcases relinquished by the nobler beasts of prey ; and, doubtless, often themselves waged the war of destruction on the feebler quadrupeds. A savage bear, surpassing in size the Ursus ferox of the Rocky Mountains, found its hiding-place, like the hyaena, in many of the existing limestone caverns of England. With the Ursus spelceusvi&s associated another bear, more like the common European species, but larger than the present individuals of the Ursus Arctos. Wolves and foxes, the badger, the otter, the foumart, and the stoat, complete the category of the pliocene carnivora of Britain. " Bats, moles, and shrews were then, as now, the forms that preyed upon the insect world in this island. Good evidence of a fossil hedgehog has not yet been obtained ; but the remains of an extinct insectivore of equal size, and with closer affinities to the mole-tribe, have been discovered in a pliocene formation in Norfolk. Two kinds of beaver, hares and rabbits, water-voles, and field-voles, rats and mice, richly represented the Rodent order. The greater beaver ( Trogontlierium') and the tail- less hare (Lagomys} were the only sub-generic forms, perhaps the only species, of the pliocene Glires that have not been recognized as existing in Britain within the historic period. The newer tertiary seas were tenanted by cetacea, either genetically or specifically identical with those that are now taken or cast upon our shores." 294 MAMMOTH REMAINS IN SEAS OFF BRITISH COASTS. [Cn. XVI. appear scarcely probable they safely crossed, either by will or accident. The geological time when the needful connection was formed between the British Islands and the continent of Europe, so that these and other contemporary mammals freely roamed from the one part of a general area to the other, is, therefore, a matter of no slight interest. It has to be borne in mind that, during any modified distribution of land and sea formerly existing, by which deposits were accumu- lated, and the carcases of animals were floated out to sea, or swept into fresh-water lakes, so that their harder parts became embedded in calcareous matter, mud, silt, or gravel, the lighter portions of the accumulation, amid which they were entombed, would, as now in the German Ocean and some other parts of the sea adjacent to the British Islands, be liable to be washed off, either at the proper depths beneath the surface of the sea by the action of the wind- waves, or on the shores by the breakers, when changes of level of the sea and land so took place that this action could be experienced. Tusks, teeth, and the bones of the JElephas primigenius have thus been fished up by the trawlers and dredgers on the south-east of England, and in a state sometimes showing little marks of attrition, bearing more the appearance of having been merely relieved, by the wave action, of the mud, silt, or sand which once enveloped them.* Supposing the elephants and rhinoceroses, with other * Professor Owen, in his ' History of British Fossil Mammals," mentions (p. 246), that " most of the largest and best-preserved tusks of the British mammoth, have been dredged up from submarine drift near the coasts. In 1827, an enormous tusk was landed at Ramsgate ; although the hollow-implanted base was wanting, it still measured nine feet iu length, and its greatest diameter was eight inches ; the outer crust was decomposed into thin layers, and the interior portion had been reduced to a soft substance resembling putty. A tusk, likewise much decayed, which was dredged up off Dungeness, measured 11 feet in length; and yielded some pieces of ivory fit for manufacture. Captain Byam Martin, who has recorded this and other discoveries of remains of the mammoth in the British Channel (Geological Transac- tions, second series, vol. vi., p. 161), procured a section of ivory near the alveolar cavity of the Dungeness tusk, of an oval form, measuring 19 inches in circumference. A tusk dredged up from the Goodwin Sands, which measured 6 feet 6 inches in length, probably belonged to a female mammoth." * * "This tusk was sent to a cutler at Canterbury, by whom it was sawed into five sections, but the interior was found to be fossilized and unfit for use." * * " The tusks of the extinct elephant, which have reposed for thousands of years in the bed of the ocean which washes the shore of Britain, are not always so altered by time and the action of surrounding influences, as to be unfit for the purposes to which recent ivory is applied." Mr. Charlesworth, after mentioning that a large lower jaw of a mammoth, of which he gives a figure (" Magazine of Natural History, new series," vol. iii., p. 348, 1839), had been dredged up off the Dogger Bank, in 1837, and quoting Mr. Woodward (" Geology of Norfolk"), as stating that more than 2,000 elephants' teeth had been dredged up oft' Hasbro', on the Norfolk coast,*in 13 years, relates that a mammoth's tusk, dredged up by some Yarmouth fishermen oft' Scarborough, about 1836, was so slightly altered in character, that it was sawn up into as many pieces as there were men in the boat, each CH. XVI.] MAMMOTH REMAINS IN SEAS OFF BRITISH COASTS. 295 contemporary animals, the remains of which are found with those of these mammals, to have been spread over the land prior to the great depression, accompanied by increased cold, as above noticed, and that they gradually retreated before the advance of the sea diminishing the amount of low ground, the original connection between the British Islands and the land of the continent may have more resembled that shown as the boundary of the 600 feet depth, (figs. 65 and 102,) than that which we now find. In such a state of this part of Europe there would be an ample area of continuous dry land for the range of the elephants, rhinoceroses, and their contemporary, but the now extinct, species of hippo- potamus, oxen, deer, tiger, leopard, hyaena, bear, and other mammals. Accumulations of bones could readily, as the land became depressed, be washed out of any lacustrine or fluviatile accumulations amid which they might have been embedded, and be mingled with marine remains of the gradually-encroaching seas, sometimes being worn and re-embedded in gravel, at others less mutilated, or even uninjured, amid more tranquilly-formed deposits. Occasionally some, or portions, of the original lacustrine or fluviatile deposits, containing remains of these animals, may never have been disturbed to any great extent, so that the deposits and the included bones became covered by the marine accumu- lations of the time.* claiming his share of the ivory. One portion was preserved in the collection of Mr. Fitch, of Norwich. A large humerus was, in 1837, trawled up in mid-channel between Dover and Calais, in 120 feet water. A large femur was also found while trawling, about half-way between Yarmouth and Holland in 150 feet water, and the lower jaw of a young animal was dredged up off the Dogger Bank. Other instances of elephant remains, brought up from the sea-bottom off the English coasts are also known. A tusk of the Hippopotamus major was dredged up from the oyster-bed at Happisburgh. * Professor Owen (" Hist. Brit. Fossil Mammals," p. 347), quotes a notice in a Cambridge paper of 26th February, 1845, in which mention is made of high tides having much uncovered the lignite beds at the base of the cliffs near Cromer, Norfolk, and that among the fossil remains of that bed, the lower jaw of a rhinoceros, with seven molar teeth in good preservation, together with the molars of the elephant, hippopotamus, and beaver were discovered. The jaw was examined by Professor Owen, and ascertained to have belonged to a young Rhinoceros tichorhinus. Mr. Strickland pointed out, in 1834 (" Account of Land and Fresh- water Shells found associated with the Bones of Land Quadrupeds beneath diluvial gravel, at Cropthorn, Worcestershire," Proceedings Geol. Soc., vol. ii., p. Ill), that "a layer of fine sand, containing 23 species of land and fresh-water shells, with fragments, more or less rolled, of bones of the hippopotamus, bos, cervus, ursus, and canis," reposes on the lias clay of that district. Professor Owen adds the mammoth and urus to this catalogue (" Hist. Brit. Fossil Mammals," p. 258). " The sand passes upwards gradually into gravel, which extends to the surface, and differs in no respect from the other gravel of the neighbourhood, being composed principally of pebbles of brown quartz, but occasionally containing dhalk flints, and fragments of lias ammonites and gryphites. The bones, though most abundant in the sand, are interspersed also in the 296 CONDITIONS FOR THE PRESERVATION OF [Cn. XVI. Upon the hypothesis, that these animals could have spread under such conditions, and prior to the submergence previously noticed, a time would come when the depression of the old land would be such, that, as regards the British Islands, no sufficient or fitting dry land would be found for them, supposing that the diminished temperature did not destroy them. While assuming that such may have been the conditions in this particular case, it by no means follows, with submerging dry land over a large portion of Europe, that abundant space was not left, even in Northern Asia, for the existence and increase of the Elephas primigenius and the Rhinoceros tichorhinus. The land may not have experienced a contemporaneous depression, or, if so, not one cutting off all the needful feeding-grounds for the support of these mammals. Thus, in several parts of Europe, when the sea-bottom emerged, the former land, variously modified during its submersion, coated more or less with the detritus drifted over and thrown down upon it, and embedding the remains of such animals as perished during the submergence, there might be many sources whence the elephants, rhinoceroses, and other contemporary animals, could spread over the new land as the fitting conditions obtained. It is not difficult to conceive that these mammals may thus have revisited the area of the British Islands, again connected with the main land, so that their remains may be found in lacustrine and fluviatile deposits above the marine accumulations formed during the interval of depression.* As there is evidence in Western Europe of oscillations, as regards the relative level of sea and land, in the more recent geological time, requiring much attention on the part of the observer, he will have carefully to consider their gravel ; but the shells are confined to the sand." Two of the species of shells were considered to be extinct. From the fluviatile habits of some of these molluscs, Mr. Strickland inferred, that the deposit occupies the site of an ancient river bed. He at the same time pointed out " the greater change which has taken place in the mammifers of this island than in the molluscs, since the era when the gravel was accumulated ; and the little variation which the climate appears to have undergone since the same epoch." He also adverted to similar deposits, previously known at North Cliff, Yorkshire, Market Weighton, and at Copford, near Colchester. The section given by Sir Roderick Murchison, M. de Verneuil, and Count Keyser- ling (" Geology of Russia in Europe and of the Urals," vol., i. p. 502), would appear to show, that as respects a part of Russia, and beneath a covering of "clay drift, con- taining numerous bones and teeth of the mammoth, 50 feet thick," there was a " band of finely-laminated sand, full of shells, specifically identical with those which inhabit the adjacent river Don." The sand reposes upon a tertiary limestone. * Localities are mentioned where, in the British Islands, bones of these and of con- temporary mammals have been found entombed in fluviatile or lacustrine deposits, supposed to be above the accumulations referred to the period when erratic blocks and other ice-transported detritus were strewed ove*r the sea-bottom in this part of Europe. Cn. XVI.] MAMMOTH REMAINS IN EUROPE. 297 influence on the spread of mammals, such as those under consider- ation. Assuming, however, only one submersion sufficient to. disconnect the British Islands, followed by an elevation restoring the connection, it would be inferred that lacustrine and fluviatile accumulations would be the highest amid which we should expect to discover the remains of the Elephas primigenius and his con- temporary mammals, partly extinct, partly now existing. Amid any changes arising from the depression and elevation of land and adjacent sea-bottoms, should animals have lived in caves, carrying in their prey, should they had been carnivorous, or have fallen into fissures in the manner previously mentioned (p. 118), their remains, so preserved, would appear the most safe from re- arrangement by waves, tidal streams, or ocean currents. Though the bones of extinct bears and other animals found in caves had pre- viously attracted much attention, it was from the discovery of the remains of mammals in a cavern at Kirkdale, in Yorkshire, in 1821, and from the descriptions of all the circumstances attending the mode of occurrence of these remains, and of the condition of the cavern itself, subsequently given by Dr. Buckland, who visited the spot a few months only after the discovery, that ossiferous caves attained a new interest. This cave was found by cutting back a quarry, as many others have also been. Its greatest length was found to be 245 feet, and its height generally so inconsider- able, that in two or three situations only could a man stand upright. The following (fig. 105) is the section of it, as given by Dr. Buckland ;* a, a, a, a, being horizontal beds of limestone, in which the cave occurs; b, stalagmite incrust- F,g. 105. ing some of the bones, and formed before the mud was introduced ; c, bed of mud contain- ing the bones ; d, stalagmite formed since the introduction of the mud, and spreading over its surface ; e, insulated stalagmite on the mud ; f, /, stalactites depending from the roof. " The surface of the sediment when the cave was first opened was nearly smooth and level, except in those parts where its regularity had been broken by the accumulation of stalagmite, or ruffled by the dripping of water ; its substance was an argillaceous and slightly-micaceous loam, composed of such minute particles as could easily be suspended in muddy water, and mixed with much calcareous matter, that seems to have been derived in part from the dripping of the roof, and in part from comminuted bones. "| The * " Reliquiae Diluvianae," 1823. t Ibid. 298 OSSIFEROUS CAVE OF KIRKDALE. [Cn. XVI. remains of hysena, tiger, bear, wolf, fox, weasel, elephant, rhino- ceros, hippopotamus, horse, ox, three species of deer, and some other animals, were found to be so strewed over the bottom of the cave when the mud was removed, the proportion of hysena teeth over those of other animals so great, and the bones of other animals so broken and gnawed, that Dr. Buckland considered the Kirkdale cave to have been the den of the extinct hyaenas, the remains of which were found in it, during a succession of years. He further considered that they brought in, as prey, the animals, the bones and teeth of which were mingled with their own, and that these con- ditions were suddenly changed by the irruption of muddy water into the cave, burying all the remains of the animals, in an envelope of mud, including the fasces of the hyaenas, which occurred in the Kirkdale cave, precisely as such now do in the dens of existing hyaenas. Many bones were found to be rubbed smooth and polished on one side ; a fact showing, Dr. Buckland infers, that one side had been exposed to the walking and rubbing of the hyaenas. There would thus appear to have been a hole or cavern at first raised above common detrital accumulations, and freely communi- cating with the atmosphere, when the stalagmite b was formed ; then a change by which water containing fine mineral detritus was introduced, the latter subsiding from the water, which may have completely filled the whole of the cavern ; and, thirdly, a time when the cave was out of the reach of water, again freely communicating with the atmosphere so that stalagmites were thrown down upon the even floor of mud. The stalactites depend- ing from the top may have been partly formed during both periods when the cavern communicated with the atmosphere. Stalactites would not be formed if the cave were full of water, since the solution of the bicarbonate of lime, even supposing such to have passed through into the cavern, would then mingle, in the usual way, with the general volume of the water. As regards the introduction of fine sedimentary matter into caves during a submersion of previously dry land beneath the sea, the resulting mud not containing the hard parts of marine animals, much would necessarily depend upon the circumstances under which the entrances, or fissures communicating with the old surface of dry land, were placed. Should the entrances be blocked up by beaches or shingles drifted over them (independently of any which may have been closed by the accumulation of fallen frag- ments before submersion) as the land descended and the coast con- CH. XVI.] INTRODUCTION OF MUD INTO OSSIFEROUS CAVES. 299 ditions changed, the shores ranging gradually to higher levels, the matter of fine mud could be water-borne through the shingles or fragments. Such muddy water once in the cavern, either from this source, or entering amid other cracks and chinks, the resulting mud would settle over the floor, enveloping all within its reach in a mass of fine sediment. In either case, any germs of marine animals secreting hard parts, and entering with the water, would scarcely be properly developed in such a situation. Ossiferous caverns being merely those amid caves in general which, from fitting circumstances, mammals have made their dens, or into which they have fallen or been drifted, all the sinuosities and irregularities of such cavities, both as regards horizontal and vertical range, have to be expected in them. They are found to be variously filled in different localities, so that it becomes difficult to point out any particular arrangement of parts common to the whole. At the same time, the following longitudinal section (fig. 106) may afford somewhat of a general view of many which have been discovered. In it ?, I, I, represent the section of a limestone hill (these caverns being like caves in general most Fig. 106. I common in limestone rocks), in which there is a cavern, b, b, communicating with a valley, v, by an entrance, a. A floor of stalagmite, d, d, covers bones and fine sediment accumulated in the cavities, c, c. A column of stalactite and stalagmite is represented between the two chief chambers of the cave, and which may or may not have blocked up the passage from one to the other. Any circumstances having removed a covering of the entrance, a, or the latter being even constantly open and well known, an observer, if not informed respecting ossiferous caverns, might easily enter such a cave and remark nothing more than the chambers, the stalactites depending from the roof or covering the walls, and a floor partly rock, partly formed of stalagmite ; and even, if the passage between the chambers be closed by stalactite 300 CONDITIONS FOR STALACTITE AND STALAGMITE, [Cn. XVI. and stalagmite, return from the outer cave without being aware of the chamber beyond it. It will be, at once, apparent, seeing that the bones in ossiferous caves may either have been chiefly collected by predaceous animals, have fallen into them from openings in the ground above have been drifted into them, or be the remains of mammals which have entered and died in the caves that great attention should be paid to the mode in which the bones may be accumulated, and to their whole, fractured, gnawed, or other state. Very careful and com- plete sections require to be made of the ossiferous accumulations, and these should not be confined to one portion of a cavern ; for, during a long lapse of time, an open cave may have been variously tenanted or strewed with bones. If an observer be in search of evidence of ossiferous caves having been the dens of predaceous animals, not only the marks of their teeth upon the remains of such bones as may not have been consumed are valuable, but also the mode of occur- rence of faecal remains, and the rubbing and polishing of portions of the walls, especially in the narrower passages, are important. With respect to stalactitic and stalagmitic incrustations, they may have happened at all times when a cavern was above the sea or water-drainage of the time, so that the atmosphere entered it, and bicarbonate of lime percolated in solution through the containing rock into the cave. Thus bones, as in the Kirkdale cave, may have been embedded in this calcareous substance, as well prior to the introduction of any fine sediment by means of water, as after- wards. It is the repose of stalagmite upon an even flooring of the sedimentary matter enveloping the bones, which shows an alteration of conditions, one from a state of things when stalagmite could not be accumulated on the bottom of the cave, to that which permitted it As the remains of mammals of existing kinds, such as the red deer,* of the roebuck,! badger, J polecat, stoat, || wolf,f fox,** * In Kirkdale Cavern, Yorkshire, and Kent's Hole, Torquay; Buckland, "Reliquiae Diluvianae," and Owen, " Hist. Brit. Foss. Mammals." f Fissure in limestone, with the remains of Rhinoceros tichorhimis, Caldy Island, Pembrokeshire; Owen, "Hist. British Foss. Mammals," p. 488. Dr. Buckland mentions an antler, " approaching that of the roe," in the Paviland Cave. $ Kent's Hole, Torquay; Owen, " Hist. Brit. Foss. Mammals," p. 110. Belgian Cave, Dr. Schmerling. Berry Head, Devon ; Owen,' " Hist. Brit. Foss. Mammals," p. 113. || Kirkdale Cave; Buckland, "Reliquiae Diluvianae." Kent's Hole, Torquay; Owen, " Hist. Brit. Foss. Mammals." ^f Kirkdale Cave ; Pavilaud Cave ; Oreston, Plymouth ; Kent's Hole, Torquay. Buckland, "Reliquiae Diluvianae ;" Owen, "Hist. Brit, Foss. Mammals." ** Kent's Hole, Torquay ; Oreston, Plymouth. Owen, " Hist. Brit. Foss. Mammals." CH. XVI.] HUMAN REMAINS IN PAVILAND CAVE. 301 water-vole, field-vole, bank-vole, hare and rabbit,* have been dis- covered in caves mingled with those which are extinct, and as the remains of man have been detected in similar caverns, it becomes needful most carefully to study the circumstances under which all these remains may occur ; so that while, on the one hand, we do not neglect the kind of evidence which might thus show the con- temporaneous existence of mammals now partly extinct, and partly living, t and also of man with the same kinds of animals, on the other, the accidents which may have brought such apparently contemporaneous mixtures together may be duly regarded. Thus, had not Dr. Buckland employed the needful caution, human remains (those of a woman) in Paviland Cave, Glamorganshire, might have been regarded as proving the contemporaneous existence of man and of the Elephas primigenius, Rhinoceros tichorhinus, and Hycena spelcea. In this case, the cave had evidently been employed as a place of sepulture by some of the early inhabitants of that part of Wales, and the ground containing the remains of the extinct animals moved.J * Buckland, " Reliquiae Diluvianse ;" Owen, " Hist. Brit. Foss. Mammals." f The following list of animals, the remains of which have been found in the caves of the British Islands, is given by Professor Owen, in his " History of British Fossil Mammals :" Vespertilionoctula, Rhinolophus ferrum-equinum, TJrsus prisons andspelceus ; Meles taxus, Putorius vulgaris and ermineus ; Lutra vulgaris (from Durdham Down, Bristol, on the authority of Mr. E. T. Higgins) ; Canis lupus and vulpes ; Hyaena spelcca, Felis spelcea and catus ; Machairodus latideus, Mus muscuJus, Arvicola amphibia, agrestis, and pratensis ; Lepus timidus, and cuniculus ; Lagomys spel.* With respect to the kind of openings through which the gaseous and mineral substances are vomited forth, there has existed much difference of opinion. While some geologists infer that the rocks through which the volcanic forces found vent had been so acted upon that they were upraised in a dome-like manner, the gaseous products bursting through the higher part, driving the lighter substances into the atmosphere, if the dome were elevated into it, and raising the viscous molten rock, so that it flowed out of the orifice ; others consider that there has been a simple fissure or aper- ture in the prior-formed rocks through which the volcanic products were propelled, the solid substances accumulating round the vent, so that a deceptive dome-like appearance is presented. The following sections (figs. 113 and 114) may assist in show- ing the differences between the "craters of elevation," first brought under notice by M. Von Buch, and so ably illustrated by M. Elie de Beaumont and other geologists, and the " craters of eruption," as they have been termed. Fig. 113 represents a portion of Fig. 113. ,-<::<^-. ./^5^l^^^,, * Humboldt (Kosmos) refers to the relative height of volcanos as probably of con- sequence if we should assume their seat of action at an equal depth beneath the general surface of the earth. He refers to eruptions being commonly more rare from lofty than from low volcanos, enumerating the following : Stromboli, 2318 feet (English) ; Guacamayo (Province of Quiros), where there are almost daily detona- tions; Vesuvius, 3876 feet; Etna, 10,870 feet; Peak of Teneriffe, 12,175 feet; and Cotopaxi, 19,070 feet. CH. XVII.] CRATERS OF ELEVATION AND OF ERUPTION. 319 deposits more or less horizontally arranged, fractured and upraised in a conical or dome-shaped mass, a portion of them, g a c b h, being divided and rent at c, so that volcanic forces, pressing through, find vent. For the sake of illustration, the rocks broken are assumed to be accumulated in beds. This is by no means essential, the mass disrupted may have been composed of certain crystalline rocks, such as granite, to be hereafter noticed, bearing no marks of stratiform arrangement. If now ashes and cinders be thrown out of this vent, and accumulate in more or less conical layers., one outside the other, until at g and h, the original and up- heaved beds are concealed, and a crater presents itself at v, through which similar substances continue to be thrown, it may be very difficult to distinguish such an arrangement of parts from those effected by the propulsion of similar substances through a simple longitudinal crack, as represented in fig. 114. In this section, a Fig. 114. series of beds, a b (for more contrast represented as previously upraised in a mass, and as all sloping or dipping in one direction), is traversed by a crack, which, though it divides the beds, has not been accompanied by upheaval or depression of one side or the other. Through this vent, c, cinders and ashes are supposed to have accumulated in conical layers, as before, the apex crowned by the crater, v. It will be obvious that in both cases, if the volcanic accumula- tions had been subaerial, even with the addition of the flow of lava currents, and of cracks amid the ashes and cinders filled with molten rock, (which have been excluded from the sections to render them more simple,) much difficulty would arise from the general similarity in the arrangement of the volcanic products exposed to sight, unless denudation from atmospheric influences, or the sinking or blowing off of a large part of the volcano, afforded a better insight into the general structure of the mass, so that, as 320 FOSSILIFEROUS VOLCANIC ASHES AND ' [Cn. XVII. shown by fig. 113, portions of subjacent and tilted beds of dis- similar rocks could be seen, as at g and h, or of similar volcanic accumulations, as in fig. 114. Evidence of a better kind would be expected, should the ashes, cinders, and molten matter have been accumulated both beneath and above a sea level, the action of the breakers denuding the general mass, so that more illustrative sections would be afforded. Thus, if upraised above the sea level, the original dome or cone-shaped rocks, a b (fig. 113), though covered, for a time, by a mass of matter, g v h, the result of a high state of activity in the volcano, may finally become visible, and afford the information sought. In the same manner, evidence of another kind may be obtained, as .regards the accumulation from simple volcanic eruption, by marine denudation, as shown in fig. 114. In both sections it is supposed, that volcanic action not ceasing, conical accumulations may continue to be formed inside a crateriform cavity, more or less occupied by water, cliffs all round facing an active volcanic vent, as at/ (fig. 113). Under even these favourable circumstances, the observer should employ great caution. The facts presented to him may require no little comparison and classification ; for in such localities, more especially, he has to consider how far the relative levels of the sea and land may have remained the same since the various accumula- tions before him have been effected. Let it be supposed, for illustration, that he detects organic remains in beds surrounding the interior basin of water, in which the volcanic island still vomits forth various gases and products. Should the deposits g and h (fig. 113) be of the- more recent geological times, commonly marked by the presence of the remains of molluscs, not much, if at all, differing from those still existing in the vicinity, and should the mineral composition of the including beds not be decisive on the point, the subject may not be so clear. By reference to the section (fig. 114), it will be seen that if the line, d e, representing the present level of the sea, be raised, and, consequently, the whole mass of rocks, including the supporting deposits, a c b, relatively depressed, the layers now above the sea, being then below it, molluscs may have lived upon and amid these layers while they successively constituted the sea-bottom, as upon any other sea- bottoms, and as many molluscs must now do around volcanic islands. There is no difficulty in considering that, during a long lapse of time, breaker action aided in the re-arrangement of many sub- stances, including animal remains, on the subaqeous slopes of vol- CH.XVII.] CINDERS RAISED ABOVE THE SEA LEVEL. 321 canos, the angle of the beds varying according to obvious conditions. Any change in the relative levels of the sea and land, which the observer, as he pursues his researches, will find to have been so frequent, and often so considerable, that should raise the general mass (fig. 114), so that d e be the line of sea level, would expose the edges of these fossiliferous beds facing the interior. And it should be borne in mind, that in many localities calcareous beds, and even limestone, may become mingled with such deposits during their submarine accumulation. When studying the fractures and contortions of rocks, as well on the small as the large scale, there will be frequent occasion to remark, as will be more particularly shown hereafter, the mixture of flexures and fissures, and the extension of the one into the other. The subjoined example (fig. 115) of the termination of a Fig. 115. fracture and flexure, occurring amid the slightly -inclined beds of lias near Lyme Regis, Dorset, may aid in illustrating a point of much interest connected with the present subject, namely, that in the more marked instances adduced of " craters of elevation," a considerable break or outlet is often found on one side. The plan (fig. 115) shows an alternation of the thin-bedded limestone of the lias of Dorsetshire with shale, the whole broken through by a crack, a, I, the continuation of one where there is dislocation producing movement on the sides, and which terminates in a boss at 5, with somewhat diverging small cracks. The interior is composed of limestone, round which shale, covering it, is exposed by the pear- shaped protrusion, outside which is another limestone bed, c c c, dipping outwards from the central portion, b, the whole taking a more horizontal character towards a, where, for a certain length, the plane surface is merely broken by a fissure. With proper forces and resistances employed, a like disposition of parts could be obtained on the large scale. If, as on the subjoined plan (fig. 116), such a state of things had been brought about on the large scale, and volcanic forces had been enabled to find vent at different points, there may be good 322 VOLCANIC ACCUMULATIONS BENEATH WATER. [Cn. XVII. evidence of a crater of elevation, and of other volcanos presenting no such evidence, all situated on a great line of fracture ending in a dome-like flexure, with, perhaps, a common communication Fig. 116, between them. At d a c, the beds broken through would dip, with radiating cracks, around a gorge opening in the direction of the main fissure towards b, while surrounding the volcanic vents, e and f 9 the strata may be horizontal. Under such circumstances, the volcanic accumulations being continued, so that they may be intermingled, if the whole be regarded with reference to depression beneath the sea, or elevation above its level, the observer will perceive that numerous complications would arise, requiring no slight care properly to appreciate. As there is every reason to infer that volcanic substances have been, and are ejected at various depths beneath the sea level, as well as above it, the modifications of the products arising under the former conditions have to be properly estimated, more particu- larly when we have to associate such modifications with changes in the relative levels of sea and land, so that accumulations formed at various depths beneath water may be mingled with those gathered together in the atmosphere. That subaqueous would gradually approximate to subaerial deposits, as the accumulations round volcanic vents rose from different depths in the sea above its level, will readily be understood. When eruptions pierce through the sea level, ashes, cinders, and stones are gathered round a crater, and vapours and gases are evolved, as happened off St. Michael's, Azores, in 1811 (p. 100), and in the Mediterranean, between Pantellaria and the coast of Sicily in 1831 (p. 70). At such times the volcanic forces so accumulate mineral substances around the vent, and so, for the time, overpower the action of the sea, that it is not until these forces have been expended, or greatly abated, that the breakers can abrade the land, and, supposing no subsidence, or falling in of the mass of volcanic matter thus raised, and the latter sufficiently incoherent, level off the accumu- lations to the depths to which waves can mechanically act. CH. XVIL] VOLCANIC VAPOURS AND GASES. 323 Fully to appreciate the modifications which may arise in volcanic action at various depths in water, productive of effects which can only be inferred, very careful study of the gases and vapours evolved, of the chemical composition and mineralogical character, and of the mode of occurrence of the solid substances thrown out from subaerial volcanos, is needed. With regard to the vapours and gases evolved, the chief appear to be aqueous vapour or steam, sulphurous acid, sulphuretted hydrogen, hydrochloric acid, and carbonic acid. Steam is a very common product, and, as Dr. Daubeny has remarked, is sometimes emitted for ages from volcanic fissures.* Hydrochloric acid is also common in various parts of the world. Sulphurous acid has been inferred to predominate " chiefly in volcanos having a certain degree of activity ; whilst sulphuretted hydrogen has been most frequently perceived amongst those in a dormant condition." f Carbonic acid is observed at the close of eruptions, or in extinct volcanos, and is stated to be emitted more from the bases and neighbourhood of volcanos, than from their craters.J Besides these gaseous products, which can be collected when volcanic vents can be approached sufficiently near for the purpose, it is now considered that there is an inflam- mable gas occasionally evolved from some craters and volcanic fissures, which gives the flame often mentioned, but at one time much doubted. Of what kind this gas may really be, appears as yet uncertain, and may long remain so, inasmuch, as it seems chiefly evolved under conditions, such as violent eruptions, unfavourable to examination. Flame observed by Professor Pilla to be emitted from the crater of Vesuvius, in June, 1833, was of a violet-red colour, and the gas producing it inflamed only when in contact with the air. As Dr. Daubeny observes, hydrogen * " Description of Active and Extinct Volcanos," 2nd edit., p. 607 ; 1848. There would appear to be a constant emission of steam from Tongariro, New Zealand, a volcanic mountain rising to about 6,200 feet above the sea. From time to time hot water and mud are ejected, and pour down the mountain side, " coupled with ejections of steam and black smoke, with a noise like that of a steam-engine, but no lava or scorise." Ib. p. 429. f Daubeny, "Voleanos," 2nd edit., p. 608. As regards the discharge of sul- phurous acid and sulphuretted hydrogen, the one more in some places than in others, Dr. Daubeny remarks, that the presence of the one does not prove the entire absence of the other, since these two gases when they meet decompose each other, forming water and depositing sulphur ; and " that merely the portion of either which exceeds the quantity necessary for their mutual decomposition will escape from the orifice ; so that the gas which actually appears indicates only the predominance of the one, and not the entire absence of the other." I Daubeny, " Volcanos," 2nd edit., p. 612. Edinburgh New Philosophical Journal, 1843. From observati6ns made by him in the crater of Vesuvius in 1833 and 1834, Professor Pilla concluded that flames never appear at Vesuvius but when the volcanic action is energetic, and is accompanied Y 2 324 VOLCANIC SUBLIMATIONS. [Cn. XVII. and its compounds not inflaming when steam or hydrochloric acid are mingled with them in certain proportions, and both these being abundantly evolved in most eruptions, an inflammable gas might escape into the air thus mixed, without being inflamed. Hence, though this gas may be often present during violent erup- tions, it may not always be so under conditions for supporting flame. As regards the sublimations from volcanos, we should anticipate that they would be varied, seeing that the conditions under which volcanic forces and products may find vent could scarcely but be variable also. Among the most common is chloride of sodium, or common salt, one which is important from being found connected with volcanic action in such different parts of the earth's surface. Specular iron ore is often found sublimed in chinks and cavities, as is also muriate of ammonia in certain volcanos. Respecting sulphur, it has been inferred to be derived "either from the mutual decomposition of sulphurous and sulphuretted hydrogen gases, or from the catalytic action exerted upon the latter gas by porous bodies, assisted by a certain temperature."* The sublima- tions of the sulphurets of iron and copper, chloride of iron, oxide of copper, muriate and sulphate of potass, selenium, and others, though apparently accidental, have been shown by M. Elie de Beaumont t to have an important bearing upon the filling of mineral veins, as will be hereafter stated. The ashes, cinders, and molten rock ejected, may often be con- sidered as little else than modifications of the same substance, at one time kept in a state of fusion, vapours and gases piercing through it, at another driven off by these vapours and gases in portions of different volume, more or less impregnated with them, with a development of gaseous substances in a state of great tension ; that they do not appear when the action is feeble ; that their appearance always accompanies explosions from the principal mouth, where, however, they cannot be observed except under favourable circumstances ; that they likewise show themselves in the small cones in action, which are formed in the interior of the crater, or at the foot of the volcano ; and that, finally, they are not visible except in the openings which are directly in communication with the volcanic fire, and never on the moving lavas, which are at a distance from their sources. * Daubeny, "Active and Extinct Volcanos," 2nd edit., p. 615. After quoting M. Dumas (Annales de Chimie, Dec. 1846), as having shown that " where sul- phuretted hydrogen, at a temperature above 100 Fahrenheit, and still better when near 190, comes into contact with certain porous bodies, a catalytic action, as it is called, is set up, by which water, sulphuric acid, and sulphur are produced," Dr. Daubeny points out that the vast deposits of sulphur, associated with the sulphates of lime and strontia of Western Sicily, may have been thus produced. f " Sur les Emanations Volcaniques et Me'talliferes." Bull, de la Soc. Ge'ol. de France, 2nd serie, t. iv., p. 1249. CH. XVIL] MOLTEN VOLCANIC PRODUCTS. 325 so as to be rendered cellular ; these portions finally so triturated and worn into fine grains and powder, that while part may fall with the cinders in a conical form around the volcanic vent, another portion may be so light as to be borne great distances by the winds, as from St. Vincent's, in the West Indies, above the trade-winds, far eastward over the Atlantic. The rock in fusion, while occasionally, but somewhat rarely, up- lifted in a volcanic vent to, or so near the lip of the crater as to flow over the outside in a viscous stream, more frequently breaks through different portions of the side ; a result which would be anticipated from the pressure of a substance of the kind, and from rents formed in the sides of a volcano during violent eruptions. After ejection its solidification will necessarily depend upon the conditions to which it is exposed, the volume of the molten mass thrown out being duly regarded. Like all other mineral bodies of the like kind, if rapidly cooled, lavas form glasses, commonly known as obsidians, when associated with volcanic products; if slowly cooled, and in sufficient volume, crystallizing, as is easily illustrated by experiment.* The heat of lava currents would appear to vary, a circumstance to be expected, as whatever may have been the temperature of the molten mass when in the volcano, that of its exclusion would depend upon the cooling influ- ences to which it may have been exposed before it flowed outside the volcano, and could be examined. It has been inferred that * So far back as 1804, the experiments of Mr. Gregory Watt (" Observations on Basalt, and on the Transition from the Vitreous to the Stony Texture which occurs in the Refrigeration of Melted Basalt," Phil. Trans., 1804), proved, as respects basalt (that of Rowley Hill near Dudley), when a mass of it weighing seven hundred- weight was melted and slowly cooled in an irregular figure, that according to the rate of cooling of the various parts, was the structure, one passing from the vitreous to the stony. The silicates forming common glass may, as is well known, by slow cooling, be made to pass into a stony state. We have made many hundreds of experiments upon the melting and recrystallization of igneous rocks, even succeeding in the reduction of certain granites into a glass, and again rendering this glass stony. The varied chemical composition of the substances which may be reduced to the vitreous state is quite sufficient to show that obsidian is a mere rock-glass which can be formed under the requisite condition of comparatively rapid cooling from very different compounds. We have reduced portions of some stratified rocks to this state. This is by no means difficult to accomplish when a moderate amount of lime is present, so that silicate of lime may be produced and act as a flux, as in the ordinary smelting of the argillaceous iron ores of the coal measures. By a little management, slates and shales, with the requisite dissemination of carbonate of lime, may be converted into excellent pumice, intumescence being produced in the melted viscous substance by the carbonic acid. The experiment requires, however, to be carefully watched ; for if the crucible be not removed in time, the carbonic acid escapes, and the vitreous sub- stance alone remains, which may readily, if thought desirable, be, by slow cooling, rendered stony. To produce crystallization by very slow cooling requires great care, and, for the most part, a somewhat large portion of rock. FLOW OF LAVA STREAMS. [Cn. XVII. the temperature at which lava will continue to flow is sufficient to melt silver, lead being rendered fluid in about four minutes. Whatever the requisite heat may be,* lava is found to retain it for a long series of years. Being a bad conductor of heat, as rocks in general are, lava, when subjected to the comparatively low temperature, to which it is exposed after ejection, soon covers itself with a coating of solidified matter. This is necessarily broken as the flow of the viscous mass continues beneath it, and it will be more or less scoriaceous, according, as in cooling, it retains any cellular texture from the passage or dissemination of vapours and gases through it. Hence the surface of lava currents is often broken and rugged, as is represented in the accompanying view of one at Vesuvius (fig. 117).f Under the conditions usually obtaining during the Fip. 117. flow of lava, the viscous current, at a moderate distance from the place of its actual discharge, may be considered as moving in a kind of pipe, this breaking from time to time, as the molten rock in the interior tends to drag the parts becoming solid with it. In this manner the pipe will even convey the lava current up rising ground, should the resistance of its sides be equal to the * In our experiments, ordinary greenstone, when pounded fine, and placed in a crucible, usually melted at about the heat required for melting copper : experiments, however, on so small a scale may be very deceptive. t Taken from Abich's Views of Vesuvius and Etna. CH. XVII.] VAPOURS AND GASES IN MOLTEN LAVA. 327 pressure exerted upon them. As a high angle of descent would be unfavourable to the proper slow cooling and quiet adjustment of particles needed for crystallization, MM. Dufrenoy and Elie de Beaumont consider that beyond a moderate angle lava does not take a crystalline texture. That the external character of a lava current should conform to the velocity of its flow, this depending, other conditions being equal, upon the amount of slope, would be anticipated. The observer should, however, be aware that when crystalline minerals may be found in lava, it does not always follow that their particles have separated out from the other component parts of the mass, after , the whole has been in a molten state. They seem to have been sometimes formed prior to the outflow of the lava. Of this a good example is stated to have occurred at Vesuvius in April, 1822, when fine crystals of leucite were included in a lava stream which issued from the base of a small cone occupying the crater, the comparative infu- sibility of the leucite crystals preserving them entire amid the melted rock.* In like manner should the lava be in part com- posed of a remelted rock containing disseminated minerals, which resisted the heat to which the whole was exposed, such minerals might upon an outflow accompany the lava stream, and be again dispersed amid the new mass, otherwise, perhaps, not crystalline.! It would scarcely be expected that a molten mass, known to be driven about in a crater by vapours and gases, could either overflow the lip of that crater, or burst out from the sides of a volcano with- out having some portions of these vapours and gases intermingled with it, ready to escape into the air. This it would accomplish the easier as the lava was the more fluid, and its temperature high, the vapours and gases then striving most to increase their volume. In proportion as the molten rock cooled, and the expansive power of the vapours and gases decreased, cavities would remain, cor- responding in size to the equalization of the resistance of the cool- ing rock on the one hand, and the expansive power of the vapours and gases on the other. As these conditions varied so would the results ; and thus according to circumstances the hollows formed * Daubeny, quoting Professor Scacchi, of Naples, " Volcanos," 2nd edit., p. 230. f In regions where volcanos traverse igneous rocks of an older date, remelting portions of them, it is easy to conceive occurrences of this kind. Should a felspathic porphyry, containing crystals of quartz, or mica, be thus remelted, and the heat be only capable of fusing the felspathic matter, these minerals may be left untouched. In experiments made for this purpose, we have often found this view borne out, and the quartz disseminated through many slags, as, for example, in many of the first copper slags in the furnaces at Swansea, affords another example of the like kind. 328 FLATTENING OF VESICLES IN LAVA. [Cn. XVII. would differ from good-sized caves, lined with picturesque stalac- tites of lava, to small vesicles. Vapours and gases sometimes con- tinue to escape for a long time through the chinks and cracks of cooling lava. The cavities thus produced in lavas will necessarily take dif- ferent shapes, according to varying conditions. Lava poured out so as to form a broad and comparatively deep mass, with little movement of importance after its outflow from a volcano, the fluid state long preserved, would have its cavities, large and small, placed under different circumstances, from a stream cooling more rapidly, yet still, from moving on greater slopes, continuing steadily to advance for a long distance. In the latter case the hollows and vesicles would be elongated in the direction of the flow, spherical cavities pulled out into almond-shaped forms, and irregular hollows still exhibiting a stretching in the line of move- ment. This elongation of vesicles may be so continued that, as in the subjoined section (fig. 118), they may become completely Fig. 118. flattened, the tenacity of the lava being of a proper kind. If c d be a surface on which a lava stream moves, and e f a portion where its viscosity is such that by moving in the direction, e f, spherical hollows take almond-shaped forms, the lava becoming more tenacious towards the surface, c d, these almond-shaped vesicles would become flatter at a b, so as finally to present, in section, mere streaks or lines, giving a laminated appearance to that portion of a lava cur- rent when cooled. Upon the solidification of the portion a b, the movement continuing, and the upper part gradually taking the tenacity previously possessed by a b, the like appearance of lamina- tion might happen there. Thus, as the upper part of a sheet of lava may, as regards loss of fluidity, and the friction of the viscous upon the solid outside portion, be also placed in a somewhat similar condition as the lower part, a laminated character may more or less be given to a considerable portion of a stream pf lava. The conditions needed, no doubt, require nice adjustment, but they are such as would appear occasionally to prevail. Mr. Darwin, describing the laminated obsidian beds of the Island of Ascension, and comparing them with the zoned and laminated Cn. XVIL] LAVA EXCRESCENCES FROM ESCAPE OF GASES. 329 character of obsidians and different volcanic rocks of other localities, mentions, with another cause of lamination, the stretching and flattening of vesicles by the now of those rocks in a pasty state.* It has also been noticed by Humboldt, and other geologists, and is often to be seen in cabinet specimens. Sometimes vapours and gases escape through molten lava, either for the time occupying portions of craters, or flowing as streams, producing the most fantastic forms. The annexed sketch (fig. Fig. no. 119) represents a somewhat regular ac- cumulation of lava from this cause. It was seen by Mr. Dana, in the crater of Kilauea, in Hawaii, and rose as a whole, to the height of about 40 feet. " It had been formed over a small vent, through which the liquid rock was tossed out in dribblets and small jets. The ejected lava falling around, gradually raised the base ; the column above was then built up from successive drops, which were tossed out, and fell back on one another; being still soft, they adhered to each other, lengthening a little at the same time while cooling, j The following is also (fig. 120) an example of the like kind observed by Dr. Abich,J at Vesuvius, in 1834, scoriaceous lava being gradually built up into a hollow column by the additions of portions of pasty matter adhering to each other when thrust out of the general molten mass by a current of vapour or gas. In a similar manner, great blisters are sometimes raised, which bursting on one side, parts, sufficiently hard, remain, and any molten lava inside flowing out, singular cavities are left. Indeed, the varieties of hollows left by the consolidation of lava, and arising either from the intermixture of vapours and gases, or from the flow of the fluid rock, partially or wholly, out of inequalities in lava streams, or their tubular cases, would appear to be endless. Much instruction may be derived from studying the eruptions from small vents, either in the craters of volcanos, when such can * " Geological Observations on the Volcanic Islands visited during the Voyage of H.M.S. Beagle," p. 62, &c. t " Geology of the United States' Exploring Expedition," 182842, p. 177. Mr. Dana mentions other similar examples, some on a miniature scale, about Mauna Loa. The figure of a man has been added to the original sketch by Mr. Dana, in order to give a general idea of the height of the volcanic projection. i ' Geologischer Erscheinungen bcobachtet am Vesuv und Aetna," Berlin, 1837. The height of the excrescence represented is only eight feet. 330 ERUPTIONS FROM SMALL VENTS. [Cn. XVII. be approached, or on their sides, where they are also sometimes found, not only as respects vapours and gases, but also the discharge and mode of accumulation of fluid and viscous lava, cinders, and Fig. 120. In some vents the molten rock is not much intermingled with the vapours and gases, at others it becomes frothy by intimate admixture with them ; the mineral matter occupying much the less portion of the compound. Occasionally the uplifting of the mass merely raises the lava, so that it falls over the accumulations around the vent, not uncommonly more or less conical ; at other times portions of the molten mass are suddenly caught and whirled high up into the air, acquiring a spheroidal form by their motion.* * Respecting these volcanic bombs, as they have been termed, Mn Darwin, remark- ing on those found in the Island of Ascension (Volcanic Islands, p. 36), which exhibit a cellular interior, inside a shell of compact lava, observes, that *' if we suppose a mass of viscid, scoriaceous matter, to be projected with a rapid, rotatory motion through the air, whilst the external crust, from cooling, became solidified, the centri- fugal force, by relieving the pressure in the interior parts of the bomb, would allow the heated vessels to expand their cells ; but these being driven by the same force CH. XVIL] VOLCANIC CONES. 331 When only ejected short distances, they fall around, squashing into irregular and rough discs, and by their multiplication forming a coating, which may, or may not, be intermingled with scoriaceous cinders, now and then discharged in showers. Small lava streams sometimes burst from these conical accumulations, and the re- sistances of the sides being overcome, and cracks being formed, the molten matter may be seen to rise in them. Many of the effects of volcanic action on the large scale may thus, in miniature, be conveniently observed. Although conical accumulations round an aperture mark the effects of volcanic action from it, driving out ashes and cinders, large and small, with patches of frothy molten rock, and streams of fluid and viscous lava, more or less radiating from a vent, the whole braced together by more or less vertical bands of lava, which have entered cracks, effected from time to time in the general mass; this is not necessarily the case with all, nor with all parts of a mountain of which one or more of these conical accumulations may form a part. As respects cones, the accompanying* view of Fig. 121. against the already-hardened crust, would become, the nearer they were to this part, smaller and smaller, and less expanded, until they became packed into a solid con- centric shell." * Taken from the Voyage de Humboldt et Bonpland, Atlas Pittoresque, PL X , Paris, 1810. Explosions from Cotopaxi are heard at great distances. In 1744, the bellowings from the mountain were heard at Honda, 200 common leagues distant. Humboldt and Bonpland heard them day and night at Guayaquil, 52 leagues distant in a straight line. They were like repeated discharges of a battery. Ihiring the 332 VOLCANOS OF HAWAII, [Cn. XVII. Fig. 122. Cotopaxi will illustrate the production of one of great | size, its beautifully regular shape* showing how well adjusted the volcanic forces, and the substances acted on, must have been for its formation. As to its volume, that of course affords no measure of the time which the cone may have taken for its production, but it shows the great mass of volcanic matter which seems thus heaped by successive coatings into this shape. f The manner in which such a mountain may be braced together by lava currents and dykes we know not. Certainly the general form would lead us to infer a great amount of d ashes and cinders, including among the latter large ejected masses of viscous, scoriaceous, and frothy (pumicy) lava, p forced through a vent keeping in one place during the g accumulations. Mauna Loa, and Mauna Kea, in Hawaii, also lofty vol- canic mountains, the former considered to be 13,760 feet, and the latter 13,950 feet above the sea,J appear to afford much modification in structure from that found at Coto- paxi, one also marked by their outlines, as shown by the accompanying sketch of Hawaii (fig. 122), taken from the eastward. Maps and descriptions show that Hawaii (which, as Mr. Dana remarks, is one of a group about 400 miles in length, ranging from N.W. to S.E., the islands j composing it being merely the higher points of mountains 3 rising above the sea) is a mass of volcanic matter, with three principal elevations, Loa, Kea, and Hualalai.|| The I . eruption of April, 1768, the quantity of cinders vomited from the crater was so great, that in the towns of Hambato and Tacunda night was pro- longed to three o'clock on the 5th, and the inhabitants were obliged to go about with lanthorns. The eruption of 1803 was preceded by the sudden melting of the snow on the volcano. For 20 years previously neither vapour nor smoke had issued from it. In a single night, the cone became so much heated, that, the snow being melted, it appeared black from the scoriae alone. * Humboldt (Kosmos) points to the form of Cotopaxi as at once the most regular and most picturesque of any volcanic cone which he had ever seen. t According to Humboldt (Kosmos), Cotopaxi rises to the height of 19,070 (English) feet above the sea. J According to Dana, " Geology of the United States' Exploring Expe- dition," 183842. Ibid., p. 159. || Mr. Dana remarks (' Geology U. S Exploring Expedition," p. 158), " Besides the three lofty summits there are great numbers of craters in all conditions scattered over the slopes, some overgrown with forests, while about others streams of lava, now hard and black, may be traced along their route for miles. Areas, hundreds of square CH. XVIL] CRATER OF KILAUEA, HAWAII. 333 remarkable crater in activity is that of Kilauea, on the flank of Loa, and distant about 20 miles * from its summit. Ellisf described the crater as situated on a lofty elevated plain bounded by precipices, apparently sunk from 200 to 400 feet below its original level. " The surface of this plain was uneven, and strewed over with loose stones and volcanic rock, and, in the centre was the great crater." * * * "Immediately before us yawned an immense gulf, in the form of a crescent, about two miles in length, from N.E. to S.W., nearly a mile in width and apparently 800 feet deep. The bottom was covered with lava, and the S.W. and northern parts of it were one vast flood of burning matter in a state of terrific ebullition, rolling to and fro its * fiery surge' and flaming billows. Fifty-one conical islands, of varied form and size, containing so many craters, rose either round the edge, or from the surface of the burning lake ; 22 constantly emitted columns of grey smoke, or pyramids of brilliant flame ; and several of these at the same time vomited from their ignited mouths streams of lava, which rolled in blazing torrents down their black indented sides into the boiling mass below." * * * " The sides of the gulf before us, though composed of different strata of ancient lava, were perpendicular for about 400 feet, and rose from a wide horizontal ledge of solid black lava of irregular breadth ; but extending completely round, beneath this ledge, the sides sloped gradually towards the burning lake, which was, as nearly as we could judge, 300 or 400 feet lower. It was evident that the large crater had been recently filled with liquid lava up to this black ledge." The descriptions of this crater given by other observers,! corre- sponds generally with that of Mr. Ellis, due allowance being made for modifications, such as might be expected in a volcanic vent of any kind. The following is an eye-sketch plan (fig. 123) of Kilauea, made during the visit of the United States' Exploring miles in extent, are covered with the refrigerated lava flood, over which the twistings and contortions of the sluggish stream as it flowed onward are everywhere apparent ; other parts are desolate areas of ragged scoriae. But a few months before our visit (1840) a surface of 15 square miles had been deluged with lava, which came by an underground route from Lua Pele (Kilauea)." * Mr. Dana gives the distance as 19 8 miles, and the height of Kilauea as 3/)70 feet above the sea, quoting Mr. Douglas (" Journal of the Geographical Society," vol. iv.), as estimating it from barometrical measurements at 3,845 '9 3,873 -7 feet, and Strzelecki at 4,101 feet. | " Tour in Hawaii." London, 1826. J Mr. Douglas, " Journal of the Geographical Society," vol. iv. ; Captain Kelly, " American Journal of Science," vol. .xl. ; Count Strzelecki, " New South Wales and Van Diemen's Land," and others. 334 CRATER OF KILATJEA, HAWAII. [Cn. XVII. Expedition, under Captain Wilkes, to Hawaii. It well exhibits the cliffs surrounding the cavity, seven miles and a half in circuit, as also the great ledge above mentioned. Combined with the follow- ing section given by Mr. Dana,* it strongly suggests the idea of Fig. 123. an extensive area of molten rock, rising and falling according to the uplifting force of the time, this somewhat suddenly changing, as is not unfrequent in volcanic action, so that, in some states, vertical walls would be formed from the lowering of the fused mass, while at others this might again fill the cavity, and even overflow. In the following section (fig. 124), which is taken across Fig. 124. the shortest diameter (scale equal for height and distance), m m' is the whole breadth of the crater in that line, o n, o' ri the black ledge, p p' the bottom of the lower pit, n p, n' p' the walls of the lower pit, 342 feet in height, and m o, m o' the walls above the black ledge, 650 feet in height. Mr. Dana describes the beds exposed by the cliffs, as nearly horizontal, and the crater as being, at the time of his visit (November, 1840), somewhat in a tranquil state ;t * " Geology of the United States' Exploring Expedition," p. 174. f The descriptions given of the arrangement of the beds, and of other facts con- nected with this crater, have an important theoretical bearing. " These bluff sides of the pit," he observes, " consist of naked rock in successive layers ; and in the distance they look like cliffs of stratified limestone. The layers vary from a few inches to 30 feet in thickness, and are very nearly horizontal. They are much fissured and broken, and some have a decidedly columnar structure. Open spaces or caverns and rugged cavities often separate the adjacent layers, adding thus to the broken cha- racter of the surface, and at the same time giving greater distinctness to the strati- fication. The black ledge varies in width from 1,000 to 3,000 feet. With such Cn. XVII.] CRATER OF MAUNA LOA, HAWAII. 335 one, however, still variable. During subseqent examinations by the United States' Exploring Expedition, in December, 1840, and January, 1841, the lava was observed both to rise and fall in a marked manner, independently of minor oscillations. On one occasion, Dr. Judd had but just time to escape from a sudden uprise and overflow of one of the molten pools, which discharged a mass of liquid lava, not only over the spot where he was standing immediately prior to this upburst, but also over a mile in width an4 a mile and a half in length. The volume of lava then ejected was afterwards estimated by Captain Wilkes at 200,000,000 cubic feet. The next morning (January 17, 1841) the molten lava of the chief lake was ascertained to have subsided about 100 feet. Proceeding from Kilauea to Mokua-weo-weo, the crater on the summit of Loa, over a slope described as one, on the average, of only 6, a volcanic vent is found of much the same general character as that of the former. The annexed sketch (fig. 125) is a reduction of that given by Captain Wilkes.* The deepest part of the crater is nearly circular, and about 8,000 feet in diameter. The walls are described as nearly vertical, stratified like those of Kilauea, 784 feet high on the west side, 470 on the east.t An eruption of this crater occurred in 1843, so that Mauna Loa may be considered as still active. It has been remarked by Mr. Dana, as an interesting dimensions, it is no unimportant feature in the crater. The lower pit is surrounded by vertical walls, which have the same distinctly-stratified character as those above, and are similar in other features. More numerous fissures intersect them, indicative of the unstable basis on which they rest." * * * The south-west extremity formed a partly-isolated basin, of an oval form, and contained a large boiling lake. " The rest of the bottom of the pit, at the time visited by the author, was a field of hardened lava, excepting two small boiling pools, one on the western side, the other near the eastern," p. 174. Describing the day scene, Mr. Dana states, that the " incessant motion in the blood- red pools was like that of a cauldron in constant ebullition. The lava in each boiled with such activity, as to cause a rapid play of jets over its surface. One pool, the largest of the three then in action, was afterwards ascertained by survey to measure 1,500 feet in one diameter, and 1,000 in the other, and this whole area was boiling, as seemed from above, with nearly the mobility of water." * * * " On descending afterwards to the black ledge, at the verge of the lower pit, a half-smothered gurgling sound was all that could be heard from the pools of lava. Occasionally there was a report like that of musketry, which died away, and left the same murmuring sound, the stifled mutterings of a boiling fluid," p. 171. " The dense white vapours rose gracefully from many parts of the black lava plain, and the pools boiled and boiled on without any unnecessary agitation. The jets playing over the boiling surface darted to a height of 10 or 12 yards, and fell again into the pools, or upon its sides. At times, the ebullition was more active, the cauldrons boiled over, and glowing streams flowed away to distant parts of the crater : and then they settled down again, and boiled as before, with the usual grum murmur. Thus simple and quiet was the action of Loa Pele," p. 176. * " Narrative of the United States ' Exploring Expedition," vol. vi. t Dana, " Geology of Exploring Expedition," p. 206. 336 CRATER OF MAUN A LOA, HAWAII. [Cn. XVII. fact, that this outbreak did not affect Kilauea, though a great lateral crater on the same volcanic dome, 10,000 feet lower down its side. The mass of lava seems to have burst out of cracks around the summit of the mountain, and in one instance a subter- ranean channel for a portion of the molten rock was observed.* Fig. 125. * Mr., Dana quotes from the " Missionary Herald," vol. xxxix., p. 381, and vol. xl., p. 44, an account of this eruption, in order to render it more publicly known to scientific persons. For the like reason we insert a portion of it here, not only as it is in itself geologically important, but also as it is stated that only a somewhat limited number of Mr. Dana's valuable work has been printed. The Rev. T. Coan states that, " On the morning of January 10, 1843, before day, we discovered a small beacon-fire near the summit of Mauna Loa. This was soon found to be a new erup- tion on the north-eastern slope of the mountain, at an elevation of near 13,000 feet. Subsequently, the lava appeared to burst out at different points lower down the moun- tain, from whence it flowed off in the direction of Mauna Kea, filling the valley between the mountains with a sea of fire. Here the stream divided, one part flowing towards Waimea, northward, and the other eastward towards Hilo. Still another great stream flowed along the base of Mauna Loa to Hualalai in Kona. For about four weeks this scene continued without much abatement. At the present time, after six weeks, the action is much diminished, though it is still somewhat vehement at one or two points along the line of eruption." Mr. Coan ascended the mountain, passing fields of scoriaceous and smoother lavas, and regions at times still steaming and hot. " Soon," he continues, " we came to an opening in the superincumbent stratum, of 20 yards long and 10 wide, through which we looked, and at the depth of 50 feet we saw a vast tunnel, or subterranean canal, lined with smooth vitrified matter, and forming the channel of a river of fire, which swept down the steep side of the mountain with amazing velocity. As we passed up the mountain we found several similar openings into this canal, through which we cast stones ; these, instead of sinking into the viscid mass, were borne along instantly out of our si.uht. Mounds, ridges, and cones were also thrown up along the line of the lava stream, from the latter of which, steam, gases, and hot stones were ejected. At three o'clock we reached the verge of the great crater, where the eruption first took place, near the CH. XVII.] RARE ERUPTION OF ASHES FROM KILAUEA. 337 From all accounts respecting the mineral volcanic products in Hawaii, the ejection of cinders and ashes would appear to be com- paratively rare. They are, however, occasionally thrown out, though in quantities relatively too small to produce much influence in the arrangements of the other and common volcanic products which have accumulated in a molten state. It is stated that, during an eruption of Kilauea in 1789, ashes and cinders were abundantly ejected, darkening the air, and destroying some men forming part of an army then on its march; and Mr. Dana mentions that near Kilauea, " a few miles south and south-east, great quantities of a pumice-like scoria, with stones and sand, are believed to have been thrown out at this time/'* An uplifting of liquid lava in the craters, and a rending of the solid rocks around them, or further down on the flanks of the great volcanic mounds, through which the molten rock is discharged, would appear the characteristic action of the Hawaian volcanos. Mr. Dana has well stated this mode of eruption, which he terms the quiet mode. Alluding to Kilauea, he remarks, " The boiling pools of the lower pit have gradually filled this (the lower) part of the crater with their overflowings, each stream cooling, and then, in a few hours or days, followed by another and another overflow in different parts of the vast area, till the rising bottom became as highest point of the mountain. Here we found two immense craters close to each other, of vast depth, and in terrific action." To queries transmitted by Mr. Dana, Mr. Coan replied as follows : " The angle of descent down which the lava flowed from the summit to the northern base of Mauna Loa is 6 ; but there are many places on the side of the mountain where the inclination is 10, 15, or 25, and even down these local declivities of half a mile to two miles in extent, the lava flowed in a continuous stream. This was the fact, not only during the flow of several weeks on the surface, but also in that wonderful flow in the subterranean duct, described in the "Missionary Herald." There was no insurmountable barrier in the way of the flow from the summit of Mauna Loa to the base of Mauna Kea, a distance of 25 or 30 miles. The stream sometimes struck mounds or hillocks, which changed its course for a little space, or around which it flowed in two channels, reuniting on the lower side of the obstacle, and thus sur- rounding and leaving it an island in the fiery stream. Ravines, caves, valleys, and depressions were filled up by the lava as it passed down the slope of the mountain, and between the two mountains. In conclusion, I may remark that the stream was continuous for more than 25 miles, with an average breadth of 1^ mile, and flowed down a declivity, varying from 1 to 25." Dana, " Geology of the United States' Exploring Expedition," p. 210. * Mr. Dana (" Geology of the United States' Exploring Expedition," p. 181) quotes from a " History of the Sandwich Islands," published by the Rev. J. Dibble, at Lahainaluna (Island of Maui), in 1843, an account from the lips of those who were in the body of men thus partly destroyed by the eruption. A large volume of cinders and sand is noticed as thrown to a great height, and as falling in a destructive shower for many miles around. Some of the men appeared to have been killed by this shower of cinders and ashes, and others to have perished from an emanation of heated vapour or gas. Z 338 ESCAPE OF LAVA THROUGH FISSURES AT KILAUEA. [Cn. XVII. . high as the black ledge." * * * " The black ledge is finally flooded, and the accumulation reaches the maximum which the sides of the mountain can bear." The pressure increases, and passages are broken out for the molten rock. " In some cases, on the side of the island where the escape takes place, the first indication of the eruption is the approach of the flowing lava. We should not imply that the land is proof against earthquakes, for slight shocks not un- frequently happen, and they have been of considerable force during an eruption. But earthquakes are no necessary attendants on an outbreak at Kilauea. It is a simple bursting or rupture of the mountain from pressure, and the disruptive force of vapours, in consequence of which the mountain, thus tapped, discharges itself." * The mode of fissuring seems to have been well observed in the eruption from Kilauea in June, 1840. The fissures are noticed as at first small. Those through which the molten lava poured formed series at intervals. Through the last twelve miles there were several rents, two or three in some places running nearly parallel. The mass of lava derived from these several fissures reached the sea, on coming into contact with which it became shivered like melted glass cast into water. Into the sea it continued to flow for three weeks, and the waters were so much heated that the shores were strewed with dead fish for the distance of twenty miles. The depth of the lava is considered to have averaged 10 or 12 feet, though in some places only 6 feet thick. The area covered by it was estimated at about fifteen square miles. The lower pit of Kilauea, calculated to have held 15,400,000,000 cubic feet of molten matter, was emptied by the outflow of the lava through the fissures, f The settling down of the lava in Kilauea would appear always to accompany these eruptions.^ The lavas of Hawaii seem to have been usually very fluid, judging from the mode in which they occur. That they are so now in Kilauea seems generally admitted. The production of the capillary volcanic glass, known at Hawaii as Pele's Hair, is an interesting example of this fluidity. Mr. Dana, who witnessed its formation * " Geology of the United States ' Exploring Expedition," p. 195. t Mr. Dana estimates that this gives the best measure of the amount of lava poured out during this eruption. As measured by the amount of matter observed on the surface, a much less quantity was erupted, estimated in this way at 5,000,000,000 cubic feet. t A very considerable eruption from the mountain, of which Kilauea forms a part, is recorded to have recently taken place, during which a large volume of lava was ejected. Pele is the reputed principal goddess of the volcano. Cn. XVII.] GREAT FLUIDITY OF THE KILAUEA LAVA. 339 at one of the pools of melted lava, states that " it covered thickly the surface to leeward, and lay like mown grass, its threads being parallel, and pointing away from the pool. On watching the operation a moment it was apparent that it proceeded from the jets of liquid lava thrown up by the process of boiling. The currents of air, blowing across these jets, bore off small points, and drew out a glassy fibre, such as is produced in the common mode of working glass. The delicate fibre floated on till the heavier end brought it down, and then the wind carried over the lighter capillary ex- tremity. Each fibre was usually ballasted with the small knob which was borne off from the lava-jet by the winds."* In the flow of the lava outburst from Kitauea in June, 1840, the molten rock, as it passed amid forests, not only enclosed the stems of individual trees, leaving cylindrical holes from the total or partial destruction of. the wood, but sometimes also adhered to the branches, descending from them in the "form of stalactites. In these latter cases the heat is described as having been barely suf- ficient to scorch the bark, though the branches were clasped by the molten rock. Should the branch have contained much fluid matter, we can suppose that, the heat and fluidity of the lava being great, aqueous vapour from the bark may have prevented actual contact with it for a time sufficient for the passage of the lava stream at a height at which the branches could be entangled in it.-f- The lava descending suddenly from this height, by the lowering of the general level of the fluid stream as it passed onwards, the sudden exposure to the atmosphere would preserve, by quickly * " Geology of the United States' Exploring Expedition," p. 179. f The summary given by Mr. Dana of the effects of this flow of lava amid the forest ground, is highly interesting in many respects. " The islets of forest trees," he states, " in the midst of the stream were from one to fifty acres in extent, and the trees still stood, and were sometimes living. Captain Wilkes describes a copse of bamboo which the lava had divided and surrounded ; yet many of the stems were alive, and a part of the foliage remained uninjured. (" Narrative of Exploring Expedition," vol. iv., p. 184). Near the lower part of the flood the forests were destroyed for a breadth of half a mile on either side, and were loaded with the volcanic sand ; but in the upper parts Dr. Pickering found the line of the dead trees only 20 feet wide. The lava sometimes flowed around the stumps of trees, and as the tree was gradually consumed, it left a deep cylindrical hole, sometimes 2 feet in diameter, either empty or filled with charcoal. (Mr. Dana refers here to similar facts observed by M. Bory de St. Vincent at the Isle Bourbon, " Voyage aux Isles d'Afrique," 1804). Towards the margin of the stream these stump holes were innu- merable, and in many instances the fallen top lay near by, dead, but not burnt. Dr. Pickering also states that some epiphytic plants upon these fallen trees had begun again to sprout." The fact is then mentioned of the lava depending in stalactitic forms from the branches of the trees, " and although so fluid when thrown off from the stream as to clasp the branch, the heat had barely scorched the bark."" Geology of the United States' Exploring Expedition," p. 191. z 2 340 EFFECTS OF VERY LIQUID LAVA ON TREES. [Cn. XVII. cooling, the adhering and depending portions, so that little heat acted on the branches.* The case would be different where the heated lava continued to surround the lower parts of the trees. Whatever may have been the moisture preventing immediate contact, as the cooling proceeded, a time would come for the scorch- ing, if not actual burning of the included stems. * The results attendant on plunging a highly-heated body into a liquid have been long known at the manufactories of crown-glass. These have of late attracted much attention, especially from the experiments and reasoning of M. Boutigny, the vapour or steam preventing actual contact in the first instance, so that the plunged body does not acquire the temperature that might at first be expected. In crown-glass works it has been, from time immemorial, the practice to plunge the melted and very highly- heated glass in some of the first processes, after removal from the melting-pot, into cold water, to reduce the temperature. This does not fracture the glass, the steam produced preventing contact between the highly-heated glass and the water. In after processes with the same piece of glass, a mere drop of water is employed to sever a large attached glass stem, the heat being now so reduced that contact, with its conse- quences, is immediate. It is not a little interesting, at great crown-glass works, to see both effects, frequently produced at the same time, and within the distance of a few feet, CHAPTER XVIII. VOLCANOS AND THEIR PRODUCTS CONTINUED. VESUVIUS. ETNA. VOL- CANOS OF ICELAND. STROMBOLI IN CONSTANT ACTIVITY. INTERVALS OF LONG REPOSE. SUDDEN FORMATION OF JORULLO OF MONTE NUOVO. FALLING IN OF PAPANDAYANG, JAVA. SUBTERRANEAN LAKES WITH FISH. DISCHARGE OF ACID WATERS. INUNDATIONS FROM VOLCANOS. CHEMICAL CHARACTER OF VOLCANIC PRODUCTS. TRACHYTE AND DOLE- RITE. COMPOSITION OF FELSPATHIC MINERALS OF LAVAS OF OBSI- DIANS OF OLIVINE AND LEUCITE. DIFFUSION OF MINERALS IN VOL- CANIC ROCKS. FUSIBILITY OF MINERALS IN VOLCANIC ROCKS. SINKING OF MINERALS IN FUSED LAVA. FUSION OF ROCKS BROKEN OFF IN VOL- CANIC VENTS. COTOPAXI and the volcanos of Hawaii, though useful in showing how modified the results of volcanic action may be, and pointing to differences in that action of no slight importance to the observer, seeking for facts to guide him to the knowledge of its probable cause, have yet been so recently known to us, that, when studying the changes which may have taken place in volcanic vents, he must look to volcanic lands of which there may be records extending back a few centuries, at least, for the requisite data. Fortunately for this inquiry the volcanos of Italy have engaged attention for many centuries. Vesuvius offers an excellent instance of a volcanic vent which, after remaining long dormant, somewhat suddenly be- came active, nearly 1800 years ago, and has more or less continued, with intervals of various length, in that state ever since. After a repose, not known to have been interrupted during a long period,* suddenly, on the 24th August, 79, after earthquakes of several days' duration, cinders and ashes were furiously driven out, partly, no doubt, a portion of the old volcanic accumulations. Their * A very excellent condensation of our information respecting the ancient, inter- mediate, and modern states of Vesuvius, will be found in Daubeny's " Description of Active and Extinct Volcanos," 2nd edition, 1848. 342 ERUPTIONS OF VESUVIUS. [Cn. XVIII. abundance was so great that three cities, Stabiae, Pompeii, and Herculaneum, were overwhelmed by them (p. 124). We may assume that lava currents were also vomited forth out of the volcano at this eruption, one, apparently, of the greatest of Vesu- vius on record. There then seems to have been a state of repose, or at least of only minor movements insufficient to create attention, for 134 years, when another eruption occurred, succeeded by a similar interval of quiet for 269 years, when there was an outburst so considerable as to cover a portion of Europe with ashes.* There were then intervals between the eruptions, the accounts of which have reached us, of 40, 173, 308, 43,t 13, 89, 1, 167, 194, 131, 29, 22, 12, 3, and 1 years, bringing them down to 1698. " From that time to the present," observes Dr. Daubeny, respecting Vesu- vius, " its intervals of repose have been less lasting, though its throes perhaps have diminished in violence ; for the longest pause since that time was from 1737 to 1751, and no less than eighteen distinct eruptions are noticed in the course of little more than a century, several of which continued with intermission for the space of four or five years." J Even supposing the earlier recorded eruptions of Vesuvius to be only approximations to the real number, some being omitted which would now not fail to be noticed, the irregularity of the intervals of considerable activity would still be so far marked as to point to inconstancy in the final conditions upon which a marked eruption depends. At the same time, also, the different intensity of the eruptions themselves leads to the same inference. Not only was the crater of Vesuvius so tranquil, prior to the great outburst of 79, as to be clothed with vegetation, that crater occupying the de- pression now known as the Atrio del Cavallo, (the present Monte Somma forming a portion of its ancient walls,) but also between the eruptions of 1500 and 1631, the crater of the period was covered with herbage, as those of earlier times may have been * When reference is made to the depth of cinders and ashes now found covering Stabiae, Pompeii, and Herculaneum, it is needful to recollect that a portion of them may have been accumulated during eruptions such as this, and at other subsequent times. t A great eruption, in 1036, during which much lava was poured out, as is stated, from the crater as well as from the sides. J " Description of Volcanos," p. 226. " In the interval between the eruption of 1500 and 1631, the mountain put on the appearance of an extinct volcano, the interior of the crater, according to Braccini, being, in 1611, covered with shrubs and rich herbage, the plain called the Atrio del Cavallo overgrown with timber and sheltering wild animals, whilst in another part there were three pools, two of hot and one of cold water, and two of these impregnated with bitter salts." Daubeny, " Description of Volcanos," p. 235. Cn. XVIII.] ERUPTIONS OF ETNA. 343 between other long intervals of repose following the great eruption of 79. Etna also becomes valuable for the length of time during which its outbursts have been noticed. According to researches respect- ing the earlier eruptions of this volcano,* the year 480 B.C., or thereabouts, would appear that to which any marked outburst can be traced during historic times. This would give us about 2,332 years, for, if not of all, of at least a considerable number of the chief eruptions of this volcano, the geological records of the activity of which would appear to extend far beyond this comparatively limited time. Taking the early historic notices for the value they may possess, including that of 480 B.C., there have been marked eruptions recorded between that date and the commencement of the Christian era. With the exception of a lapse of time between 396 B.C., and 140 B.C. (256 years), the outbursts noticed occurred at intervals of 53, 31, f 5, 10, 3, 66, 12, and 8 years. They thus correspond in frequency with those recorded between A.D. 1284 and the present time.J From A.D. 40, or thereabouts, to 1169, the eruptions from this volcano did not, apparently, receive much attention. If we assume that this lapse of time had not been one of repose more than those which preceded and followed it, Etna seems to have been a somewhat active volcano for the time above mentioned (2,332 years). The volcanos of Iceland have also been known as more or less in activity during a long lapse of historic time. Of the known marked outbursts of Hecla there have been 23, including that of 1845, since 1004 or 1005. These have varied in intensity and in the length of the intervals of repose between them. The eruption of 1845 appears to have driven out a vast abundance of cinders and ashes, the latter carried, by the movements of the atmosphere to * A table of the dates of the eruptions of Etna and Vesuvius, taken from Von HofFs " Geschichte der Veranderungen der Erdoberflache," with some few additions, is given by Dr. Daubeny, in his " Description of Volcanos," 2nd edition, p. 289. f Respecting this eruption of 396 B.C., Dr. Daubeny mentions (" Description of Volcanos," p. 283), that the stream of lava which then stopped the march of the Carthaginian army against Syracuse, is to " be seen on the eastern slope of the moun- tain, near Giarre, extending over a breadth of more than two miles, and having a length of 24 from the summit of the mountain to its final termination in the sea. The spot in question is called the Bosco di Aci ; it contains many large trees, and has a partial coating of vegetable mould, and it is seen that this torrent covered lavas of an older date which existed on the spot." I From 1284, the intervals of repose have been, in years, 45, 4, 75, 33, 1, 1, and 82. Then a continuance of small eruptions for 58 years (1566 to 1624), after which the intervals were 9, 11, 9, 15, 13, 6, 1, 5, 8, 21, 12, 12, 8, 4, 4, 3, 14, 1, 6, 5, 6, 1, 1, 2, 7, 2, 8, 12, 1, and 10 years (in December r 1842), calculated from the table in Daubeny's " Description of Volcanos," p. 289291. 341 ERUPTIONS FROM THE ICELANDIC VOLCANOS. [Cn. XVIII. great distances.* From Kattlagiau-jokull there have been seven eruptions since the year 900. While thus these volcanos have vomited forth molten rock, cinders, and ashes at intervals for 845 and 950 years, eruptions from other vents of the same great volcanic mass of Iceland, such as Krabla, of which there were four outbursts during the last 125 years, have also taken place. Another great volcano, Skaptar-jokull, previously dormant, as far as the historic records of that land extend, suddenly became active in 1783. During this eruption, which was preceded by earth- quakes over the whole of Iceland, and the ejection of volcanic matter in the adjacent sea, considerable masses of lava were thrown out, according to Sir George Mackenzie, -j- from three different points, about eight or nine miles from each other, on the lower flanks of the mountain, spreading in some places to the breadth of many miles. Of the 20 volcanic vents, as Dr. Daubeny has pointed out, which have ejected lava, cinders, or ashes during the 950 years since Iceland was colonized, " eleven have had but one eruption, and amongst these four only occurred within the last cen- tury ; whilst of the remaining nine, Myrdalls-jokull, Skaptar-jokull, Sandfells-jokull, Skeidarar-jokull, Reykianes, Hecla, and Krabla alone would appear to be active at present ; Trolladyngia having had no eruption since 1510; Orcefa-jokull none since 1362; and others having been for a nearly equal time in a state of qui- escence. "J While Vesuvius, Etna, and volcanos in Iceland thus afford in- formation as to alternate, but irregular, intervals of repose and * " On the 2nd of September, 1845, the day of the eruption of Hecla, a Danish vessel, near the Orkney Islands, at a distance of 115 Danish miles (about 500 English) from the volcano, was covered with ashes." Daubeny, " Volcanos," 2nd edition, p. 307. According to Professor Forchammer (Poggendorff's " Annalen," vol. Ixvi , 1845), the cinders and ashes, so abundantly discharged, were ejected from three vents on the south-west slope of Ilecla, and the lava from a fourth, situated a little distance beneath them. The eruptions continued in force on the 12th of the following month (October), the lava still flowing. The eruption did not finally cease, though there were intervals of repose, until the commencement of March, 1846. t " Travels in Iceland," 2nd edition. Noticing this eruption from Skaptar-jokull, in 1783, Sir George Mackenzie states, that in January of that year, volcanic eruptions, represented as accompanied by flame, rose through the sea, about 30 miles from Cape Reykianes, and that several islands were observed, as if upraised, a reef of rocks now existing where these appearances occurred. " The flames lasted several months, during which vast quantities of pumice and light slags were washed on shore. In the beginning of June earthquakes shook the whole of Iceland ; the flames in the sea disappeared ; and the dreadful eruption commenced from the Skaptar-jokull, which is nearly 200 miles distant from the spot where the marine eruption took place." The eruption of 1783, is stated to have thrown out such an abundance of cinders and ashes that the whole island was covered by them. The ashes were wind-borne as far as Holland. J Daubeny, " Description of Active and Extinct Volcanos," 2nd edition, p. 306. Cn. XVIII.] CONSTANT ACTIVITY OF STROMBOLI. 345 activity,* the eruptions themselves, differing in intensity, the two former during the lapse of more than 2,000 years, and the latter approaching towards a period of l,000,t Stromboli, a volcanic vent rising through the sea between Naples and Sicily, has been equally marked, for more than 2,000 years, as exhibiting the same amount of activity. " No cessation," as Dr. Daubeny remarks, " has ever been noticed in the operations of this volcano, which is described by writers antecedent to the Christian era in terms which would be well adapted to its present appearance.''^ There seems a constant boiling of molten matter in the crater, a louder explosion occurring at regular intervals with an escape of steam, and the throwing out of blocks of lava to a considerable height. From the smaller and lower of three apertures within the crater, " a small stream of lava, like a perennial spring, is constantly flowing." || Not only do ancient and modern records thus afford the needful information respecting both intermittent and continued volcanic action for 2,000 years and more, but also as regards the cessation of the same action for so long a period, that the volcanic vents so cir- cumstanced form a kind of transition from active volcanos to those commonly termed " extinct/' The last stream of lava which issued from Monte Rotaro, in Ischia, is that of 1302, known as Arso. The only traces of volcanic action now existing in this island are its hot springs. Thus no eruptions of molten rock, * Selecting Hecla from the table given by Dr. Daubeny (" Volcanos," p. 314) and taken from Garlieb (" Island rucksichlich seiner Vulcans," &c., Freiberg, 1812), with additions, it would appear that its marked eruptions, commencing with that of 1004, have occurred at intervals of 25, 75, 9, 44, 47, 18, 72, 46, 34, 16, 46, 74, 44, 29, 36, 6, 11, 57, 35, 26, 12, 6, and 73 years, the last terminating with the eruption of 1845. The intervals between the outbursts of Trb'lladyngia, commencing with the eruption of 1150, were 38, 171, 116, and 35 years. For 340 years (since 1510) this vent has been quiet. While Hecla has shown the most constancy in position amid the vol- canic vents of Iceland, active at various intervals for the last 846 years, and while single eruptions have only been known at other points, certain vents have shown themselves active during the lapse of the same time for a few years only. Thus eruptions are recorded at Reykianes as occurring in 1222, 1223, 1226, 1237, and 1240, altogether only for 18 years, since which time they have ceased. At Krabla, also, they commenced in 1724, were repeated in 1725, 1727, 1729, and in 1730, after which none have occurred. At Skeidarar-jokull eruptions began in 1725, were repeated in 1727 and 1728, and terminated with one in 1753. The outburst of Sandfells-jokull in 1748 is recorded as continued, probably, with intervals of repose, to 1752, the eruptions being mentioned as annual for that time. t The volcano of Eldborgarhraun, in Iceland, is inferred to have had an eruption in the year 850. J " Description of Volcanos," 2nd edition, p. 247. Hoffmann, Poggendorff's " Annalen," 1832. j| Daubeny, " Description of Volcanos," p. 247. "It flows down the mountain," Dr. Daubeny states, " in the direction of the sea, which, however, it never appears to reach, becoming solid before it arrives At that point. Some portions, however, of the congealed mass are continually detached, and roll down into the sea." 346 SUDDEN PRODUCTION OF JORULLO, MEXICO. [Cn. XVIII. cinders, or ashes have taken place at that old volcanic vent for about five centuries and a half. It may not be improbable, from ancient writings and modern appearances, that at the promontory of Methana (formerly Methone), on the coast of Greece, volcanic forces were in activity, and had not finally ceased in the time of Strabo, though since then that volcanic vent has remained qui- escent.* Dr. Daubeny infers, from Livy and Julius Obsequens, that the volcanic action observable in the Alban Hills (Central Italy) may have continued down to historic times. t The eruption at the promontory of Methana may, it would appear, have been sudden, a considerable mound having been thrust up, or accumulated in a short time, in the manner of Jorullo, j which rose above the Mexican plain, in about four months, to the height of 1,600 feet, or the still more rapid production of the Monte Nuovo, near Naples, which, in about two days, attained an altitude of 440 feet, with a circumference of about a mile and a half. These sudden outbursts are important as regards the causes of volcanic action, more especially when no appearance of a previous volcanic vent, seems to have presented itself. It would appear that prior to June, 1759, the area upon which Jorullo now stands was covered by plantations of indigo and sugar, bounded by two brooks, the Cuitimba and San Pedro. In June, subterranean noises, accom- panied by earthquakes, commenced, and lasted fifty to sixty days. In September, all appeared again tranquil, but on the 28th and 29th of that month the subterranean noises were repeated, and accord- ing to Humboldt, an area of three or four square miles rose up like a bladder. This uprise is considered to be marked by an elevation of 39 feet around the edges of the ground thus moved ; one continued to the height of 524 feet towards the centre of the present volcanic district. The subsequent eruption was very violent, fragments of rock being ejected to great heights, cinders and ashes * Daubeny, " Volcanos," p. 328. It would appear that at that time the volcano was sometimes so hot as to be inaccessible, and to be visible afar off at night, the sea also being heated near it. The hills of the peninsula, according to Virlet (" Expe- dition Scientifique de Moree," 1839), are 741 metres (2,431 English feet) above the sea, and he infers that among the igneous rocks of different dates there found, the last volcanic action, here noticed, occurred on the western part of the peninsula, where the trachyte presents a black and scoriaceous aspect. t He observes (" Volcanos," p. 170) that, " there are indeed some passages in ancient writers which might lead us to suppose a volcano to have existed among these mountains even at a period within the limits of authentic history, for Livy notices a shower of stones, which continued for two entire days, from Mount Albano, during the Second Punic War ; and Julius Obsequens, in his work ' De Prodigiis,' remarks that in the year 640, A.U.C., the hill appeared to be on fire during the night." I Daubeny, " Description of Volcanos," p. 327. CH. XVIII.] SUDDEN PRODUCTION OF MONTE NUOVO. 347 thrown out in abundance, and the light emitted being visible at considerable distances. The Cuitimba and San Pedro poured themselves into the new volcanic vent. " Thousands of small cones, from 6 to 10 feet in height, called by the natives Hornitos (ovens), issued forth from the Malpays. Each small cone is a fumerole, from which a thick vapour ascends to the height of from 22 to 32 feet. In many of them a subterranean noise is heard, which appears to announce the proximity of a fluid in ebullition." Six volcanic masses, varying from 300 to 1,600 feet in height, were thrown up from amid these cones, out of a chasm having a N.N.E. and S.S.W. direction. From the north side of the highest (Jorullo) a considerable quantity of lava was ejected, containing fragments of other rocks. The great eruptions terminated in February, 1760. Respecting Monte Nuovo, the first indications of its production were noticed on the 28th of September, 1538, when, according to an eyewitness,* the sea-bottom near Puzzuoli became dry for 1300 yards, and the fish left upon it were carried away in waggons. At eight o'clock next morning the ground is reported to have sunk, where the volcanic orifice afterwards appeared, about 13 feet. At noon the earth began to swell up, and became as high as the Monte Rossi, and from the vent formed, fire, stones, and ashes were ejected, so that finally the hill took the form now seen. For 70 miles around the volcano the country was covered with ashes, killing birds, hares, and smaller animals, and breaking down trees. Monte Nuovo is 439 English feet high, and has a crater in its centre 420 feet deep, according to M. Dufrenoy. At the bottom there is a cavern, at the extremity of which Professor James Forbes found a spring issuing with a temperature of 182 * 5. These instances of the sudden production of volcanic vents on dry land (and when we consider the chances for observing and recording them, they were probably far more numerous within the last 1,000 years) are sufficient to show that the uprise of volcanos through the sea would be expected amid and around volcanic islands and regions. In the atmosphere they retain their forms, such as are presented at Jorullo and Monte Nuovo : raised through the level of the sea, the stability of such portions depends, as above mentioned (p. 70), upon the power of the volcanic mass to resist the action, first, of the breakers, and, secondly, of the wind- waves, where the former may have cut it down to the proper depths. * Francesco del Nero. A letter of his to Nicolo del Benino of Naples, and sent to Rome in 1538, was first published in Leonhard's " Jahrhuch fur Geologic," 1846, and Daubeny gives a translation of it, " Description of Volcanos," 2nd edition, p. 208. 348 FALLING IN OF PAPANDAYANG. [Cn. XVIII. The volcanic outbursts of this kind between Pantellaria and Sicily, off the coast of Iceland, and among the Azores, have been already noticed (pp. 70 and 100). To these may be added (as showing how much depends on the opportunity and ability to have such submarine terminating in subaerial eruptions recorded, and the range of time during which they have been known), the mud, smoke, and flame noticed by Strabo as rising through the sea amid the Lipari Islands, and the flame also rising therfe above its level during the Social War, as mentioned by Pliny.* Volcanic accumulations would appear sometimes to rest upon considerable hollows, and also to have large cavities distributed among them, the portions covering or surrounding which being either unable to resist the pressure of the superincumbent weight, even in the tranquil periods of a volcano, or broken through during eruptions, the volcanic matter falls in, or water retained amid the cavities is ejected. Of the falling in of volcanic accumulations, depressions sometimes taking the place of protrusions, many instances are given ; but of those which happen to have become known, the disappearance of Papandayang, a volcano of Java, in 1772, would seem to be most remarkable. Papandayang, formerly one of the largest volcanos in Java, was situated on the south- western part of that island. After a short but violent paroxysm, and about midnight, between the llth and 12th of August, a luminous cloud enveloped the mountain. The inhabitants of the sides and foot of the volcano betook themselves to flight, " but before they could all save themselves, the whole mass began to give way, and the greatest part of it actually fell in and disappeared in the earth." This was accompanied by sounds like the discharge of heavy cannon, and an abundance of volcanic substances were thrown out and spread around the adjoining country. The area thus swallowed up was estimated as measuring fifteen by six miles. Forty villages are stated to have been partly swallowed up, and partly destroyed by the volcanic substances thrown out, and 2,957 inhabitants perished. Persons sent to examine the locality found the heat of the substances surrounding it, and piled up to the * Detailing the evidence on his head, Dr. Daubeny ("Description of Volcanos," p. 253) asks if the comparatively recent origin of the Island of Lipari itself may not be inferred from its present fertility as compared with the sterility ascribed to it by Cicero. He also points to the fresh condition of the craters of this island, as observed by Hoffman, the hot springs and stufes at San Calogero, near the town of Lipari, and the statement of Strabo that this island emitted a fiercer fire than Stromboli, as per- haps showing that an active volcano may have existed in it even within the historical period. CH. XVIIL] FISH LIVING IN CAVITIES OF VOLCANOS. 349 height of three feet, so great, that they were unable to approach the spot six weeks afterwards.* Cavities amid volcanic accumulations may not only be partially or wholly filled by water, the condensation of aqueous vapours, finding their way into them, or rain or melted snows upon the ex- terior of a volcano percolating to them, but the waters also may be sometimes of a temperature and kind, permitting the existence and increase of animal life. Humboldt records, that "when, in the night of the 19th June, 1698, the summit of Carguairazo (18,000 French feet in heightf), fell in, leaving two immense peaks of rock as the sole remains of the wall of the crater, masses of liquid tufa, and of argillaceous mud (lodazales), containing dead fish, spread themselves over, and rendered sterile a space of nearly two square German miles. The putrid fevers, which seven years before prevailed in the mountain town of Ibarra, north of Quito, were attributed to the quantity of dead fish ejected in like manner from the volcano of Imbaburu."J The fish here noticed (Pimelodus cydopum), Humboldt further informs us, "multiply by preference in the obscurity of the caverns ;" possibly, also, there may be something in the temperature of the waters. He observes, that it was in consequence of these discharges of waters, pent up in volcanic cavities, that the inhabitants of the plains of Quito became acquainted with these little fish, called by them Prefiadilla. That the waters of such hollows and cavities are not always thus fitted for the existence and increase of animal life would be ex- pected, when the observer reflects upon the varied conditions under which they are likely to occur. As an example of the effects produced by the admixture of gaseous volcanic emanations with the waters in such reservoirs, we may adduce the great flow of acid water which accompanied an eruption of the Javanese volcano of Guntur, or Gounung Guntur, in 1800, when not only streams of lava were poured out (a rare circumstance, it would appear, among the Javanese volcanos, commonly ejecting little else than cinders and ashes), but also an acid torrent. A river * Dr. Horsfield, as quoted by Dr. Daubeny, " Description of Volcanos," 2nd edition, p. 406. t 19,200 English feet. j Kosmos, 7th edition (Sabine's Translation), p. 222. This fact has long since been mentioned by Humboldt in his earlier works. In a letter to Dr. Daubeny (" Description of Volcanos," p. 409), Mr. Beete Jukes, alluding to the almost entire absence of hard rock on the surface of the ground in the volcanic districts of Java, infers, that the Javanese volcanos " had long ceased to erupt lava, and have for ages been burying the previous streams under piles of ashes and powder." 350 DISCHARGE OF ACID WATERS FROM VOLCANOS. [Cn. XVIII. descending from this volcano is described as suddenly swelling, " charged with a large quantity of white, acid, sulphurous mud." On the 8th of October of that year, " the waters came pouring down into the valley, carrying everything before them, sweeping away the carcases of men and sundry animals, and covering the face of the country with a thick coat of mud." On the 12th, a still greater " deluge of mud" came down the valley. Such sudden increases of the volume of water would seem to point to its dis- charge from extensive cavities where it was, for the time, pent up, and where it became impregnated with sulphuric acid, derived, as Dr. Daubeny points out, from the decomposition of sulphuretted hydrogen gas.* Without uselessly multiplying examples of the discharge of con- siderable volumes of water, apparently pent up in the hollows of volcanos, it may be mentioned that, in 1755, a volume of water was suddenly discharged from a cavern below the great crater of Etna, and that, dashing over the snows and side of the mountain, it destroyed and carried before it a large amount of matter. Torrents of water are stated to have issued from Vesuvius during the great eruption of 1631, but whether from caverns amid the accumulations, or as the result of the somewhat sudden condensation of large volumes of aqueous vapour discharged from the crater, is not clear. Be this as it may, the collection of waters amid volcanic accumulations, would appear the needful consequence of the exist- ence of such cavities, and of the condensation of aqueous vapour in, or the infiltration of rain or melted snow into them. These outbursts require to be carefully distinguished from the torrents descending the sides of volcanos more or less covered by snow, either in the higher northern and southern latitudes, or rising above the line of perpetual snow in the temperate or tropical regions. The suddenly- melted snows of Cotopaxi (fig. 121) pour down the furrows on its sides, as in the eruption of 1803, when, in a single night, the snows disappeared from the cone, and the re- sulting torrents of water transported cinders and ashes into the Eio Napo and the Eio de los Alaques.*f Humboldt refers generally to the high volcanos of the Andes as thus, by the sudden melting of their snows transporting smoking scoriae among the lower lands, and producing great inundations.J Similar effects necessarily * Daubeny (" Description of Volcanos," p. 408), quoting Boon Mesch, " Dissertatio de Incendiis Montium Javge," 1826, who obtained his information from lleinwardt, the Dutch traveller in Java. f " Voyage de Humboldt et Bonpland," Atlas, Art. Cotopaxi, Paris, 1810. j Kosmos, 7th English edition (Sabine), p. 221. CH. XVIII.] MINERALOGICAL STRUCTURE OF VOLCANIC ROCKS. 351 follow similar causes in the temperate regions. Probably, however, the consequences of the sudden melting of snow and ice from volcanic action are nowhere so great as in the higher latitudes, where large glaciers, holding, or supporting mineral matter, are broken up, and partly melted and partly in fragments, are hurried onwards to the lower levels. The accounts given of the effects thus produced in Iceland show, that the torrents, so caused and intermingled with ice, are of no slight geological importance. Although, under ordinary circumstances, so little mineral matter appears capable of being moved on Victoria Land (p. 249), it is easy to conceive that, during considerable eruptions of such volcanos as those of Mount Erebus and Mount Terror, great heats may sud- denly melt the snows clothing these mountains, producing large volumes of water, which may continue liquid for a time sufficient to furrow into, and carry off scoriae and ashes, usually bound to- gether by, and, to a certain extent, not unfrequently interstratified with, the great snow covering of those regions. It is needful well to consider the mineralogical structure and chemical composition of the various volcanic products, whether these may be in the form of lava streams, of molten rock which has risen in, and more or less filled fissures, of scoriaceous sub- stances of considerable bulk, or of those lighter bodies commonly known as pumice, cinders, and ash. Though much has been accomplished, more especially of late years, respecting this know- ledge, the discoveries in chemistry greatly advancing such inquiries, and though some apparently sound general conclusions have, from time to time, been formed, it will be evident, before certain of these can be fully admitted, however they may be applicable to the particular localities noticed, that, looking at the distribution of volcanic vents over the surface of the globe, an observer possesses ample opportunities, by careful research in various parts of the world, of still further advancing our knowledge in this respect. Whether the solid volcanic rocks are crystalline, stony, or vitreous, will, as we have seen (p. 325), often in a great measure depend upon the conditions as to cooling, to which they have been exposed, all other circumstances being the same.* Hence the * It is very essential, in such investigations, to bear the other equality of con- ditions in mind, for there may be circumstances much modifying the external parts of lava currents. Thus M. Dufrenoy (" Memoires pour servir a une Description Ge'o- logique de la France/' t. iv.) mentions having found that two-thirds of the interior of a lava current near Naples were formed of a mineral which could be acted upon by acids, while the surface was principally composed of one not so attackable. In like manner, also, as has been remarked by Mr. Dana (" Geology of the United States' 352 TRACHYTE AND DOLERITE. [Cn. XVIII. chemical composition of volcanic rocks, which have been ejected and flowed in a molten state, may often be the same, notwith- standing the different states of mineral aspect. If the adjustment of the particles composing certain crystalline minerals has been pre- vented by the absence of the needful conditions, such as by a sudden refrigeration of the mass, these particles would remain diffused. The solid volcanic products most studied and known to us have been divided into rocks named trachyte and dolerite. Minerals of the felspar family constitute essential portions of these rocks, en- tering more extensively into the composition of trachyte than into that of dolerite. Both rocks may be also viewed as silicates, chiefly of alumina, lime, magnesia, potash, and soda. Trachytes are indeed considered "chemically trisilicates, with or without an excess of silica."* Trachyte may, however, according to the definitions given, also contain free silica or quartz, and the separate minerals mica, hornblende, or augite. Dolerite is composed of the felspar known as labradorite and of augite, and the term augite rock is sometimes given to this compound. In this latter rock the pro- portion of silica is diminished, and that of lime and magnesia increased, -f- This classification of the more solid volcanic products into two main divisions, however convenient as affording facilities for investigation, is found to need such modification, that an inter- mediate class of rocks, termed trachyte-dolerites, has been proposed by Dr. Abich, in which the composition partakes of the mineral characteristics of both trachyte and dolerite. With respect to changes -in chemical composition, Dr. Daubeny remarks, that " the gradual increase of soda is likewise a remarkable circumstance, modern lavas appearing to contain a much larger quantity of it than the volcanic products of ancient periods, and various minerals being hence produced in which this alkali is predominant (natrolite, nepheline, thomsonite, &c.)| Exploring Expedition," p. 203), a body of molten and very liquid lava kept long boil- ing or simmering in a volcanic vent, like Kilauea, in Hawaii, may have certain of its parts separable, the more especially as the temperature may increase in any column of lava in proportion to the pressure upon its parts. * Daubeny, " Description of Volcanos," 2nd edition, p. 15. f Respecting the diminution of silica, Dr. Daubeny observes (" Description of Volcanos," 2nd edition, p. 17), that it is " indicated by the substitution of labradorite for orthoclase, or, in other words, of one atom of silica instead of three, coupled with the presence of hornblende or augite, in both which minerals the silica bears a still smaller proportion to the base with which it is combined." Rammelsberg (" Dic- tionary of Mineralogy," Berlin, 1841) is quoted as pointing to augite as R,'S, where R is either lime, magnesia, protoxide of iron, or protoxide of manganese, the silica being sometimes also replaced by alumina, as is also the case in hornblende. J ** Description of Volcanos," p. 19. CH. XVIIL] SPECIFIC GRAVITIES OF VOLCANIC ROCKS 353 It is highly needful that the observer should most carefully study the mode of occurrence of these rocks in volcanic districts, as he will readily perceive that if their somewhat general sequence be from the trachytic to the doleritic compounds, as has been supposed, an im- portant change had been effected as to the conditions under which the earlier and later substances have been ejected from volcanos. The subject requires to be regarded on the large scale, and due weight given to those modifications arising, as will be further noticed hereafter, from the admixture of matter derived from various rocks, amid which mineral volcanic products may have had to pass before they were finally ejected. In such examinations the chemical composition of the rock, more especially when the minerals noticed may be either ill developed, or their component parts have been unable to collect together in definite arrangements, is evidently of importance. The rock-glasses, or obsidians, may as well belong to one class as the other, and so also certain stony varieties, wherein any real development of dis- tinct minerals has not been effected. Dr. Abich has proposed the relative specific gravity of volcanic rocks as affording great aid in ascertaining the amount of silica in them, a view in which Dr. Daubeny would appear to concur, remarking that in these rocks " the specific gravity of the mineral is inversely as the amount of silica, and directly as that of the other bases, so that a near approxi- mation may often be obtained to their chemical composition by merely ascertaining their weight."* When assuming chemical composition from mineral structure, and that the substances constituting the base of certain definite forms are constant, it is necessary not only to distinguish the minerals themselves, but also to give due weight to the replacement of some substances by others, without altering the form of the * Description of Volcanos," p. 13. The following table is given in illustration : Specific Gravity. Silica per Cent. Trachytic porphyry 2-5783 69-46 Trachyte '. 2-6821 65-85 Domite 2-6334 65-50 Andesite 2-7032 64-45 Trachyte-dolerite 2-7812 57-66 Dolerite 2-8613 53-09 Clinkstone, with a specific gravity of 2-5770, and containing 57-66 of silica, and glassy andesite, specific gravity 2-5851, with silica 66-55, not harmonizing with this view, it is remarked, that though clinkstone chemically resembles trachyte-dolerite, it "has a different mineral composition, for it appears to be a mixture of a zeolitic mineral with glassy felspar," and that " probably the same may apply to glassy andesite." 2 A 354 CHEMICAL COMPOSITION OF VOLCANIC ROCKS. [Cn. XVIII. mineral.* The amount of matter of different kinds which may, as it were, be entangled with that which gives the form, has likewise to be regarded, the entangled matter being sometimes more con- siderable than might at first be supposed, compelled, as it were, by that considered essential to the mineral, to take the arrangement of parts belonging to it.f Carefully searching for facts illustrative of the conditions under which the mineral matter ejected from volcanos may have been derived in the first place, or modified afterwards, it is essential to apply for aid to chemistry as well as mineralogy, important as the latter may be. The passage of vapours and gases through, or their entanglement in, lavas, whether solid, somewhat vesicular, or highly cellular, as pumice, is often sufficient to produce modifications re- quiring great attention. Again, after cooling, with cavities in them of various sizes, containing matter partly gathered together out of the mass of the containing rock, and partly from extraneous sources, lavas may not only be modified in their composition, but mineral substances may be formed in them of a different character from those which would have separated out from the original fused rock. Again, also, lavas, from exposure to atmospheric influences, may have lost some of the soluble substances originally entering into their composition. Thus no little care is required in the selection of portions of a volcanic rock which shall properly represent its original condition, as regards its chemical character. As the felspathic minerals enter so largely into volcanic rocks, and indeed constitute a considerable part of igneous rocks, viewed gene- * Before engaging in investigations of this kind, the observer should make himself acquainted with the bodies termed isomorphous, or those which replace each other without causing any alterations in the structure of minerals. In inquiries into the chemical composition of rocks a knowledge of these substances is highly important. Thus, for example, magnesia, lime, protoxide of iron, and protoxide of manganese, replace each other in any proportion. As M. Dufrenoy has well remarked (" Traite de Mineralogie," torn, i., p. 19), " it is not necessary ,'in order to present the same com- position, that minerals should exactly contain the same weight of their simple con- stituent substances ; it is sufficient that there is an exact relation between the bases and the acids they contain, or between their isomorphous substances." f This power of one compound to compel others to take its crystalline form is of no little importance, in estimating the chemical composition of rocks. These admixtures are clearly mechanical in some instances, as, for example, in the well- known crystallized sandstone, as it is sometimes termed, of Fontainebleau, where grains of siliceous sand, in large quantity, are entangled in carbonate of lime, so crystallized as to include them without destroying its form. Artificial compounds may be made, in which large proportions of some substances may be mingled with others, the fundamental crystalline form of the former remaining uninjured ; thus, for instance, M. Beudant succeeded in producing crystals of the form of sulphate of iron, which contained 85 per cent, of sulphate of zinc, the remaining 15 per cent, only being the proportion of the substance giving the form to the crystals (" Annales des Mines," 1817, t. ii., p. 10). CH. XVIIL] COMPOSITION OF THE FELSPARS. 355 Formulas. CO '* = c : CO :Ctt SPEj{ i*| j*| + -1- + .* s .* i * : 33 | pi I 00 Ci O> O CO O 00 8 8 00 I 8 8 8 4 S 2 S S 00 O 0> 8 S S O (N 2 CO -* CO 00 Ci t^ m Tf "- ^ 1 4 . OS t^ t^ 00 co ', O CO O g g 1 o i CO S CO 1 g I 1 MO 53-10 16-58 9-96 3-34 50-55 20-30 8-60 5-20 49-10 22-28 7-32 3-88 50-98 22-04 8-39 5-94 48-02 17-50 7-70 0-24 Magnesia . . . Soda Potash . . . . 1-16 9-46 2-23 4-17 1-21 8-42 2-52 3-20 2-92 9-04 3-06 2-40 1-23 8-12 3-54 9-84 2-40 12-74 1-56 Increase .... 0-24 1. Lava of Palo. 2. Lava of 1834, taken immediately below the Piano. 3. Lava of Ganatello. 4. Lava from La Scala. 5. Monte Somma, mean of two analyses. Comparing the composition of the lavas of Vesuvius with those of Monte Somma, M. Dufre'noy points out, that while the latter are almost unattackable by acids, those of the former are in a great measure soluble in them, in about the proportion of 4 : 1 ; and that while the lava of Monte Somma contains a large proportion of potash, in that of Vesuvius soda predominates.-)- * " Ueber die Natur und den Zusammenhang der vulkanischen Bildungen," Brunswick, 1841. t " Parallele entre les differents products volcaniques des environs de Naples, et rapport entre leur composition et les phenomenes que les ont produit ;" Memoires pour servir a une Description Geologique de la France, t. iv., p. 381, (1838). M. Dufrenoy adds, that this difference of composition is also apparent in the minerals -common to the two lavas, the augite of Monte Somma having a base of iron, while that of Vesuvius enters among the calcareous varieties, such as sahlite. CH. XVIII.] COMPOSITION OF CERTAIN PUMICES. 357 Respecting lava replete with vesicles or cells (pumice), the fol- lowing analyses are taken from Dr. Abich :* 1 2 3 4 5 6 7 8 9 Silica 60-79 61-08 62-42 62-29 62-04 68-11 69-79 73'77 73-70 Silica and Titanic acid . Alumina, .... Oxide of iron . . . 1-46 16-43 4-26 O.r>q 1-45 17-34 7-77 O.co 0-74 14-72 6-84 O'lS 16 : 89 4-15 16 : 55 4-43 1*23 8-21 8-23 12-31 4-66 10-83 1-80 12-27 2-31 0-62 1'46 5 25 1-24 1-31 0-14 T68 1-21 65 Magnesia .... Potash . 0-79 11.05 4-02 2 '85 3-28 4. 74 0-50 6-21 0-72 6-39 0-37 8 32 0-68 6 69 1-30 4-29 0-29 4-52 Soda .... . 2'97 1 82 1-55 3-98 3-66 1-60 2-02 3-90 4-73 1. Pumice from Teneriffe. 2. From the Island of Ferdinandea. 3. From the volcano of Arequipa, Bolivia. 4. From the Island of Ischia. 5. From the Phlegrean Fields. 6. From the Island of Pantellaria. 7. From the Island of Santorino. 8. From Llactacunga. According to Professor B. Silliman, jun., the modern lava and volcanic glass of Kilauea, Hawaii, not only contain a considerable amount of oxide of iron, but also soda, to the exclusion of potash, all the constituent substances varying much in their relative pro- portions.-f- * " TJeber die Natur und den Zusammenhang der vulkanischen Bildungen," Brunswick, 1841. t Dana, " Geology of the United States' Exploring Expedition," p. 200, whence the following analyses are extracted : 1 2 3 4 Silica. .... 39-74 51-93 50-67 59-80 Alumina 10-55 14-07 . . . . Protoxide of iron . 22-29 10-91 33-62 31-33 2-74 6-20 3-66 Magnesia . . 2-40 1-73 1-13 1-71 Soda 21-62 6-31 10-52 4-83 1 . Dark-coloured Pele's Hair. 2. Scoria. 3. Compact vitreous lava. 4. Compact stony lava. 3 and 4 are from the same specimen, the former constituting the exterior portion of the latter. Mr. Dana also gives the following' analysis of Pele's Hair by Mr. Peabody, which agrees with the above as to the large proportion of protoxide of iron, but differs from it by giving potash : Silica SO'OO Protoxide of iron . . . .28-72 Lime . 7-40 Alumina 6-16 Potash 6-00 Soda . 2-00 358 COMPOSITION OF OBSIDIANS. [On. XVIII. The following arc analyses of rock-glasses, or obsidians, from different parts of the world, showing their variable composition : 1 2 3 4 5 6 T Silica . . . Alumina Oxide of iron 60-5? 19-05 4-22 0-33 62-70 16-98 4-98 0-39 74-05 12-97 2-73 0-12 74-80 12-40 2-03 84-00 4-64 5-01 70-34 8-63 10-52 0-32 69-46 2-60 2--60 Lime ... Magnesia . . Potash . . Soda 0-59 0-19 10-63 3-50 1-77 0-82 6-09 4-35 0-28 4*15 5-11 1-96 0-90 6-40 2-33 3-55 4-56 1-67 3*34 7-54 2-60 7-12 5-08 . Lipari (Abich). 6. India (Damour). 7. P 4. asco 1. From Teneriffe (Abich). 2, Island of Procida (Abich). Telkebanya (Erdmann). 5. Iceland (Thomson). (Berthier). As olivine and leucite are minerals often entering largely into volcanic rocks, it is useful that the observer, while estimating the chemical composition of those in which they may occur, should bear in mind that the former is a silicate of magnesia and pro- toxide of iron [(Mg, Fe) 3 Si], and the latter a silicate of potash and alumina (K 3 Si 2 + 3 AI Si 2 ).* He should also recollect that * The following analyses may aid in showing the similar composition of olivine from various localities. Several others might be added of the same kind : 1 2 3 4 5 Silica 40-09 40-45 41-54 41-44 40-12 Magnesia 50-49 50-67 50-04 49-19 44-55 Protoxide of iron . 8-17 8-07 8-66 9-72 15-32 manganese Alumina. . . . 0-20* 0-19 0-18* 0-19 0-25 0-06 0-13 0-16 0-29 0-14 * Peroxide of manganese. 1. From the Vogelsberg, Giessen (Stromeyer), contains also 0*37 protoxide of nickel. 2. Kasalthoff, Bohemia (Stromeyer), contains also 0-33 protoxide of nickel. 3. Isceweise (Walmstedt). 4. Le Puy, Vivarais (Walmstedt), contains also 0-21 of lime. 5. Monte Somma (Walmstedt). As respects this mineral, it is highly interesting to find that the olivine found in the meteoric iron of Siberia and Otumba, South America, should possess a similar composition. 1 2 Silica 40-86 38-25 , Magnesia . . . Protoxide of iron . 47-35 11-72 49-68 11-75 0-43 0-11 1. From Siberia (Berzelius). 2. From Otumba (Stromeyer). With respect to leucite, the two following analyses, the first from Vesuvius, by 359 CH. XVIII.] CHIEF SUBSTANCES FORMING VOLCANIC ROCKS. augite is a silicate of lime and magnesia* (Ca 8 Si 2 + Mg^ Si a ), labradorite, a silicate of alumina, lime, and sodaf (R Si + Al Si), orthoclase (potash-felspar), a silicate of alumina and potash J (K Si -f Al Si 3 ), albite (soda-felspar), a silicate of soda and alumina (Na Si + Al Si 3 ). The chief substances entering into the composition of volcanic rocks are the silicates of alumina, oxide of iron, lime, magnesia, potash, and soda, the fusibility of the different compounds of which, constituting distinct minerals, varies, the rocks into which augite, or that into which silicate of lime enters most largely, being the easiest of reduction to the fluid state by heat. As labradorite like- wise contains a considerable proportion of silicate of lime, and is more fusible than orthoclase (the potash-felspar), both the minerals entering into the composition of dolerite render it much more fusible than trachyte, chiefly formed of the potash-felspars. Sili- cate of lime may indeed be considered as a characteristic substance in the dolerites, while it, is comparatively rare in the trachytes, that is, of those in which true orthoclase predominates. || In localities, therefore, where trachytic have clearly preceded dole- Arfvedson, and the second from Monte Somma, by Awdejew, will serve to show the proportion of the constituent parts : 1 2 Silica .... Alumina. . . . Potash .... Soda 56-10 23-10 21-15 56-05 23-03 20-40 1-02 Peroxide of iron . 0-95 * In the very numerous analyses which have been made of augite, the silica varies from 47-05 (Arendal, Gillenfelder Maar Eifel) to 57-40 (Tjotten, Norway), the lime from 17-76 (Tyrol) to 25 '60 (Achmatowsk), and the magnesia from 6-83 (Finland) to 18-22 (Vallee de Fassa). There is usually protoxide of iron varying from 4-31 (Tyrol) to 26-08 (Tunaberg, Sweden), as also alumina from 0-14 (Daleearlia,) to 6-67 (Gillenfelder Maar Eifel). f R being taken as lime and g soda, the chemical composition of labradorite is considered to be = 53-7 silica, 29-7 alumina, 12-1 lime, and 4-5 soda. There are usually also small portions of potash varying from 1-79 to 0-3. j The chemical composition of orthoclase is inferred to be 65-4 silica, 18 aluminn, and 16-6 potash, a little soda and lime being included in the latter. Nicol, " Manual of Mineralogy," p. 119. Albite is considered to be essentially composed of 69 '3 silica, 19-1 alumina, 11-6 soda, part of the last often replaced by lime or potash. Nicol, "Manual of Mineralogy," p. 124. l| In those compounds referred to orthoclase, in which soda is more abundaht than potash, it may be much doubted how far they really deserve the name, unless it be inferred, with Dr. Abich, that soda and potash are both isomorphous and di- morphous. 360 DIFFERENT FUSIBILITY OF VOLCANIC MINERALS. [Cn. XVIIL ritic rocks, the more fusible have succeeded the least fusible pro- ducts a fact of no little theoretical value. With respect to the diffusion of certain minerals, such as olivine and leucite, through the mass of a volcanic rock, having once -been formed, that is, the component particles of the silicates of magnesia and protoxide of iron of the one, and the silicates of potash and alumina of the other, having been placed under the conditions permitting them freely to move and become aggregated in the definite and needful manner, these minerals may become so many comparatively infusible bodies amid a more fusible mass. Hence, by the application of a certain amount of heat, the containing substance, should it, for example, be any of the doleritic mixtures, may be fused, while these bodies may remain unmelted, retaining their forms and general characters, until finally acted upon by the surrounding molten mass, with its large proportion of silicate of lime and alumina, forming a flux, and perhaps by a more ele- vated temperature. It is easy, therefore, to conceive that, as has been above mentioned, a lava stream may be ejected containing leucites, and olivines derived from the remelting of a previously- formed volcanic rock. Judging from the specific gravity of dole- rites, when cold and solid (2-94 2-96), leucite crystals (spec, grav. 2*4 2*5) would easily be upborne, rising towards the top of the rock in its fluid state, ready to be ejected in a lava stream. This would not be the case with olivine, the specific gravity of which (3 -3 3 '5) is greater than that of the dolerites, so that if the latter, containing disseminated olivine, were remelted, this mineral, from its little fusibility and greater weight, would have a tendency to descend, like any substance mechanically suspended in a fluid lighter than itself. As to augite, disseminated crystals of it would, from their ready fusibility be soon melted, though their specific gravity would be 3 2 3 5. It may not be out of place to remark, as it has been thought that trachytic may have been formed from felspar-porphyritic and granitic rocks of much older date, that upon the heat to which the one or the other would be exposed, might depend the melting of these rocks either partially or wholly. However silica, if mingled with some other substances, may be readily fusible, when once separated, as quartz, it is highly refractory, even 1 if surrounded by fusible silicates, as may be readily tried in the laboratory, and seen daily in the slags in many great metallurgical works. The felspathic portions, which, in some granitic and felspar-porphyric rocks, contain soda as well as potash, are not difficult of fusion, as Cn. XVIII.] POSITION OF MINERALS IN MOLTEN ROCK. 361 may easily also be found by experiment. With the mica, much depends upon whether it is a potash, lithia, or magnesia mica. The second of these fuses more easily than the first, and both more readily than the third, which we have found, in experiments, still crystallized, after the fusion of a felspar-porphyry, through the base or paste of which crystals of it were disseminated. There appears no difficulty in conceiving that, upon the melting of a granite, composed of orthoclase, quartz, and magnesia-mica, the first, after fusion, may again crystallize, and envelope the two latter, held mechanically suspended in the molten fluid during the fusion of the felspathic portion. Even supposing some of the quartz to have been fused (being surrounded by a substance acting as a flux), upon the recrystallization of the orthoclase, we should expect that the extra amount of silica, not required for the formation of that mineral, would be excluded as quartz. As to the position of any unmelted quartz and mica of a granite, the felspathic portion of which was alone fused, if the latter were wholly composed of ortho- clase (sp. grav. 2 '53 2*58), the quartz (sp. grav. 2-6) might have no great tendency to descend in the fluid body. The mica would more readily fall down, its specific gravity, for the potash kind, being 2*8 3*1, and for the magnesia species, that which is somewhat common in granites, 2 '85 2-9. In some felspar- porphyrites mica or quartz, and sometimes both, are, with felspar, and occasionally other minerals, well crystallized, so that supposing the descent of the mica through the molten mass, and the quartz more mechanically suspended in it, an ejected upper portion may contain the quartz crystals, and a subsequent lava, the mica, sup- posing that it remained still unfused. The attention of the observer is called to this mode of viewing the subject, so that, even on the minor scale, while some trachytic rocks are before him, he may duly estimate the sinking or rising of certain minerals in a fluid mass of molten rock, the higher or lower parts of which may be poured out of a volcano, as its sides may either hold firm, so that lava overflows the crater, or be fissured, letting off the fluid matter at a lower level. Viewing the whole height of a volcano known to us as a minor fractional part of the depth to which the molten matter, partially from time to time thrown out, may descend, certain minerals which have remained unfused upon the partial melting of felspar-porphyritic or granite rocks, (at first taking their relative positions according to their specific gravities,) may be. subsequently melted, their elements mingling with the general mass, to be afterwards elevated and 3G2 SINKING OF MINERALS IN FLUID LAVA. [Cn. XVIII. ejected. Thus supposing much magnesia-mica to have descended in the molten fluid, upon the partial melting of the granite which contained it, the magnesia, upon the final fusion of the micaj might wholly, or partially aid in the production of olivine in sub- sequently ejected lava.* The sinking of minerals in fluid lava long since engaged the attention of Von Buch, who founfl felspar crystals more abundant in the lower that in the higher part of a current of obsidian at TenerifFe, and Mr. Darwin has more recently directed attention to it.f Volcanos having apparently pierced through rocks of most varied chemical composition, as shown by the fragments of them so often ejected,! ^ ma 7 ^ e assumed that portions of such rocks, when not thrown out as fragments, may be often fused in the interior of the volcano, their elementary substances mingling with the general molten mass. While such ^fragments are sometimes little altered, as if, after being broken off suddenly from their parent rocks, they had not been exposed to a heat sufficient to effect much change in them, others appear to have been acted upon in various degrees, so that modifications in the arrangement of their component particles are produced, and even additions to, or subtractions of some of the latter themselves are effected. * In some of the micas the magnesia amounts to more than 25 per cent. One from Lake Baikal, analyzed by Rose, gave 25-97 of this substance, and another from Sala afforded Svanberg 25-39 per cent. f Von Buch, " Description des Isles Canaries ;" and Darwin, " Geological Observ- ations on the Volcanic Islands visited during the voyage of the 'Beagle.'" After quoting the labours of Von Buch, and the experiments of M. de Dree (mentioned by him), in which crystals of felspar in melted lava were found to have a tendency to descend to the bottom of the crucible, Mr. Darwin discusses at length the subject of the relative specific gravities of minerals in fluid lavas. " In a body of liquified rock," he remarks, " left for some time without any violent disturbance, we might expect, in accordance with the above facts, that if one of the constituent minerals became aggregated into crystals or granules, or had been enveloped in this state from some previously existing mass, such crystals or granules would rise or sink according to their specific gravity. Now we have plain evidence of crystals being embodied in many lavas, while the paste or basis has continued fluid. I need only refer, as instances, to the several great pseudo-porphyritic streams at the Galapagos Islands, and to the trachytic streams in many parts of the world, in which we find crystals of felspar bent and broken by the movement of the surrounding semi-fluid matter." J They have long been known on Vesuvius, where the fragments of limestone, on the Monte Somma portion of that volcano, have attracted much attention. A frag- ment of fossiliferous limestone has there also been found. Without entering gene- rally upon the various instances of the ejected fragments of rocks from volcanos, it may be useful to recall attention to those of limestone, dolomite, and sandstone, thrown out when the volcanic island rose through the sea between 1 Pentallaria and Sicily, in 1881 (p. 70), as showing that they may be sought for in such cases. j Dr. Daubeny notices the probable conversion of the ordinary Alpine limestone of the vicinity of Naples into granular limestone by heat, as seen in the fragments of the latter limestone found at the Monte Somma, and he quotes the researches of Dr. Faraday, as showing that carbonic acid cannot be expelled from limestone unless steam be present. OH. XVIIL] FUSION OF ROCKS BROKEN IN VOLCANIC VENTS. 363 By breaking through, and entangling portions of limestones and dolomitic rocks, much lime and magnesia may be obtained by fusion, useful in affording materials for the production of the silicates of lime and magnesia of augite, and the silicate of magnesia of olivine. So also with other accumulations disrupted and par- tially malted. Indeed, upon estimating whence the mineral matter of a volcano may have been derived, it becomes not only desirable to consider the probable composition of any igneous rocks which may have been remelted, and the circumstances attending this refusion, but also the aqueous deposits of various kinds which may have become more or less exposed to fusion during the time that a volcano has been ejecting mineral matter, either as molten rock, cinders, or ashes.* * As respects the volcanic region of Naples, and the fragments of rocks which have been ejected by Vesuvius, it is interesting to consider the modifications of igneous matter which might arise from the addition of lime and magnesia to any fundamental igneous product derived from great depths. Dr. Abich (Ueber die Natur und den Zusammenhang des vulkanischen Bildungen, Explanation of Plates, p. iv.), gives the following analyses of the dolomites and limestones of that vicinity : 1 2 3 4 5 6 7 8 9 10 Carbonate of lime . . . 52 30 5(5-57 54-10 65-20 98-00 98-04 98-08 98-17 96-72 98-40 Carbonate of magnesia . . Oxide of iron and alumina . 46-97 43 43 39-00 0-94 34-79 2-30 ]-96 1-78 1-48 1 69 0-32 1-51 o Silica and bitumen . . . 5-25 1-00 1. From Capri. 2. Valle di Sambruo, between Minuri and Majuri. 3. Minuri. 4. Between Vico and Sorento. 5. Valle di Sambruo. 6. Monte St. Angelo, Castella- mare. 7. Punta di Lettere, Castellamare. 8. Capri. 9. Vico. 10. Capri. CHAPTER XIX. VOLCANOS AND THEIR PRODUCTS CONTINUED. LAMINATION OF STREAMS OF LAVA. LAMINA OF SPHERULES IN OBSIDIAN. COMPOSITION OF VOLCANIC ASHES. VOLCANIC TUFF. PALAGONITE TUFFS OF ICELAND. MODIFICA- TION OF VOLCANIC TUFF BY GASES AND VAPOUR.' SOLUTION OF PALA- GONITE TUFF IN ACIDS. SOLFATARAS. THE GEYSERS AND THEIR MODE OF ACTION, ICELAND. SULPHUROUS WATER AND GYPSUM DEPOSITS OF ICELAND: FUSIBILITY OF VOLCANIC PRODUCTS. FISSURES IN VOLCANOS FILLED WITH MOLTEN LAVA. LAVA EJECTED THROUGH FISSURES. DIRECTION OF FISSURES IN VOLCANOS. ONE kind of lamination observed in igneous rocks has been above noticed (p. 328) as due to the elongation and compression of vesicles, so that by their extreme flattening this structure is pro- duced. In the cases of minerals ejected in an unfused state, the lava current in which they are included moving onwards, so that they would adjust themselves according to their forms and the different velocities of movement produced by friction against the supporting rocks, or any casing of more consolidated portions of the molten stream, we might expect a certain amount of arrangement in planes, or of lamination to be produced. Mixtures of substances of different kinds may sometimes also be so juxtaposed before ejec- tion, that when flowing as a lava current they formed separate layers, the thinner, other circumstances being the same, when the more elongated.* Looking also to the spherical bodies, commonly * Mr. Darwin (Volcanic Islands, p. 70), when describing the Island of Ascension, enters largely into the causes of lamination in volcanic rocks, seen there and in many other parts of the world. Among other remarks, he concludes " that if, in a mass of cooling volcanic rock, any cause produced in parallel planes a number of minute fissures or zones of less tension (which, from the pent-up vapours, would often be expanded into crenulated air-cavities), the crystallization of the constituent . parts, and probably the formation of the concretions, would be superinduced or much favoured in such places ; -and thus a laminated structure of the kind we are consider- ing wouM.be generated." The lamination of'molten matter is often well exhibited in the slags which have flowed from furnaces, especially in some iron-works. CPI. XIX.] LAMINAE OF SPHERULES IN OBSIDIAN. 365 formed of radiating crystals of part of tlie compound, observed when glasses are passing into the stony form (examples of which are not unfrequently produced artificially), we should anticipate, under the circumstances of lava passing into the stony from its fluid condition, and movement still prevailing in the mass, that the cooling portions, more especially adjacent to the ground over which the whole was passing, might sometimes have their parts so acted upon that planes, composed of little spherules, might be formed ; even alternations of them produced as successive portions of the fluid lava became exposed to similar conditions. Obsidian is but the vitreous state of melted rock, and all the conditions obtaining when artificial glasses are passing into the stony state, such as those producing separate crystals of certain silicates, and the arrangement into spherules, has to be looked for as well in the one as in the other, the modifications depending on the kind and abundance of the different silicates, with due regard to the conditions under which the general mass may have moved or remained quiet. The obsidians, in certain volcanic countries, are especially advantageous for studies of this kind, and will well repay the attention of an observer.* He will also find examples of lamination in volcanic rocks which have passed the vitreous state, or intermixture with that state in cooling, and it will be desirable that such, as well as * Dr. Daubeny points out (Description of Volcanos, p. 256), with respect to the obsidian of Lipari, that " some of its varieties possess a remarkable resemblance to certain products obtained by Mr. Gregory Watt (Philosophical Transactions, 1804) during the cooling of large quantities of basalt, an incipient crystallization Loginning to manifest itself in the midst of the vitreous mass in the appearance of white or lighter-coloured spots, which appear to be made up of points radiating from a common centre. In many of the Lipari obsidians, however, these round spots are composed of concentric laminae, and are disposed in general in lines, so as to give a resem- blance of stratification to the mass. In other cases, the whole mass is made up of globules of this kind, which are hollow internally, and are sometimes cemented by black obsidian." Mr. Darwin gives (Volcanic Islands, p. 5465) an interesting account of laminated volcanic beds alternating with and passing into obsidian at the Island of Ascension. After describing these beds, he remarks, that " as the compact varieties are quite subordinate to the others, the whole may be considered as laminated or striped. The laminae, to sum up their characteristics, are either quite straight, or slightly tortuous, or convoluted ; they are all parallel to each other, arid to the intercalating strata of obsidian ; they are generally of extreme thinness : they consist either of an apparently homogeneous, compact rock, striped with different shades of, gray and brown colours, or of crystalline felspathic layers in a more or less perfect state of purity, and of different thicknesses, with distinct crystals of glassy felspar -placed lengthways, or of very thin layers chiefly composed of minute crystals- of quartz and augite, or composed of black and red specks of an augitic mineral and of ah ox^de of iron, either not crystallized, or imperfectly so." Mr. Darwin also mentions the occurrence of layers of globules or spherulites in the transition of one class of beds into the other, one kind of spherulites white, or translucent, the other dart-brown j>r opaque, the former distinctly radiated from a centre, the "latter more obsctJtely so^, ' - 1 < 366 COMPOSITION OF VOLCANIC ASHES. Cn. XIX. the obsidians, should be well examined for evidence either of movement while consolidation was being effected, or for the simple and very gradual crystallization of parts during any long period which the whole body of rock may have taken to cool.* In such researches the observer will have to recollect, that the top of a lava stream is so far differently circumstanced from the lower portion, that, while the former is exposed to the atmosphere and all its changes, the latter rests upon a bad conductor of heat, so that somewhat modified effects may often be produced, as regards the arrangement of the component substances, in the one part and the other. With regard to the cinder and ash accumulations on the sides of volcanos, the adjacent country, and the far-distant regions to which the latter may be borne, it would be expected that their chemical composition would be similar to the lavas, for the time, of their respective volcanos, should any be thrown out, subject to such modifications as their more complete exposure to the vapours and gases rushing out might occasion. We should anticipate that during the eruptions of trachytic lavas the cinders and ashes would be likewise trachytic, and so with the other kinds of volcanic rocks. Thus, should trachytic have preceded doleritic eruptions, in any localities, the ashes and cinders of the one would have preceded the other.t Ashes and cinders being so exposed, particularly the former, to be intermingled with, and surrounded by, these volcanic vapours and gases, much would depend, as to any modification or change in the original mineral substance, upon the time during which this action might last, as also upon the kinds of the vapours While remarking on the spherulites in obsidians and in artificial glasses, Mr. Darwin calls our attention to the observations of M. Dartigues (Journal de Physique, t. lix, pp. 10, 12, 1804), on the difficulty of remelting spherulitic and devitrified glasses without first pounding them and mixing the whole well together, the separation of certain parts from the general compound in the spherules or crystals rendering this necessary. * In all such researches the slow cooling of a lava stream has to be well considered. Dr. Daubeny mentions, that he found the temperature of the lava stream, ejected from Vesuvius in August, 1834, to be 390 Fahr., four months after its outflow, the thermometer placed upon the lava, after the scorise on the surface had been removed. Dani ell's pyrometer gave similar results when introduced into a cavity of the lava (Description of Volcanos, p. 229). f M. Dufrenoy (Examen chimique et microscopique de quelques cendres vol- caniques ; Memoires pour servir a une Description Geologique de la France, t. iv.) considers that volcanic ashes are most frequently composed of distinct minerals, therein differing from the powder produced by the trituration of rocks, usually formed of the union of several minerals. He therefore infers that volcanic ash " is rather the result of a confused crystallization, produced under the influence of brisk agitation, such as in the saltpetre prepared for the manufacture of gunpowder, than the product of the trituration of lavas in volcanic vents, though the ashes, collectively, do not the less represent the composition of the lava." CH. XIX.] MODIFIED COMPOSITION OF VOLCANIC ASHES. 367 and gases to which the ashes or cinders may be exposed. In all cases it would be expected that where the cinders and ashes were the most abundantly and speedily accumulated, as upon the cone or sides of a volcano, the effects arising from an intermixture of the acids and vapours with the ashes and cinders would be the most considerable. For instance, where hydrochloric acid is much min- gled with the ashes and cinders, the whole piled around a crater in a hot moist state, such portions as were soluble in that acid might be much acted upon. The like also with sulphurous and carbonic acids. In considering the original composition and subsequent modifi- cation which any mass or layers of volcanic cinders may have sustained, it is also needful for the observer to search for evidence as to the probability of these cinders and ashes having been arranged, as now found, either in the air or beneath water, such, for instance, as is afforded by the occurrence of shells or other organic remains among them,* or by layers of detritus or chemically- deposited matter, showing a subaqueous accumulation. Ashes and cinders descending into water, and afterwards arranged by it, would pro- bably be well washed, so that little change would be effected after- wards by any acids adhering to, or mingled with them. The term tuff, or tufa, is not uncommonly given to the ash and cinder accumulations of volcanic regions. Dr. Abich has given the following analyses of the tuff of the Phlegrean Fields, Posilippo, and the Island of Vivara, the two former being termed trachy tic tuff, the last basaltic tuff: 1 2 3 4 5 6 T Silica . . 51-65 52-80 54-41 54-57 56-63 45-50 51-08 Alumina . 15-08 15-83 15-40 17-93 15-33 16-05 13-71 Oxide of iron 6-21 7-57 7-74 5-49 7-11 11-69 13-16 Lime . . 5-43 3-13 3-17 0-77 1-74 5-03 7-09 Magnesia . 1-18 0-84 1-50 0-77 1-36 3-20 4-72 Potash . 6-19 7-86 7-54 5-23 6-54 4-12 2-94 Soda . . 1-01 2-90 2-87 6-40 4-84 2-28 2-94 1. Yellow tuff, from JNola. 2. Yellow tuff, from Posilippo. 3. White tuff, from Posilippo. 4. Tuff, from Epomoeo. 5. From the crater of Monte Nuovo. 6. Yellow tuff, from the Island of Vivara. 7. Grey tuff, from Vivara. f * So long since as the time of Sir William Hamilton, shells were detected in the tuff of the vicinity of Naples. They have also been noticed in other localities in that vicinity, and are described as those of species still living. f Mr. Dufre'noy (Memoires pour servir a une Description Geologique de la France, t. iv., p. 384) observes, that the tuffs of Posilippo, Pompeii, and Ischia (the two former analysed by M. Berthier, the last by himself), present nearly the same general characters, with the exception of that of Pompeii, which contains nine per cent, of 368 PALAGONITE TUFF OF ICELAND. [Cn. XIX. Looking at the varied manner in which ashes and cinders may be accumulated, either wholly in the atmosphere or beneath water, to the substances with which they have been mingled in the crater of a volcano, and which may more or less coat or impregnate them afterwards, and to the infiltrations through beds and masses of them subsequently to their deposit, either adding to, abstracting from, or modifying the arrangement of their component substances, we should expect that at times even very solid rocks may be produced, at first sight presenting little of the aspect of an accumulation of fine powder and cinders. Mr. Darwin describes a tuff, apparently of this kind, at Chatham Island (Gralapagos Archipelago), one evi- dently formed at first of cinders and ashes, but now having a somewhat resinous appearance resembling some pitchstones. He attributes this alteration to " a chemical change on small particles of pale and dark-coloured scoriaceous rocks ; and this change could be distinctly traced in different stages, round the edge of even the same particle/'* In Iceland, a tuff apparently also in a changed or modified con- dition from that of its original accumulation, and named. palagonite- tufF,-)* would seem to be of much importance. According to Professor Bunsen (of Marbourg), the palagonite-tuff of Iceland has a density of 2'43, and contains nearly 17 per cent, of combined carbonate of lime, a substance which he infers was infiltrated, adding weight to the opinion, that the entombment of Herculaneum and Pompeii was produced by an alluvion of the tuff forming the flanks of Monte Somma, water having greatly aided the filling up of the edifices in the two towns. Remarking on the trachytic tuff of the Phlegrean Fields, Dr. Daubeny observes (Description of Volcanos, p. 16), that the analysis of it proves that, " like pumice, it is only a metamorphosed condition of trachyte." He considers tuff, pumice, and obsidian, as all modifications of the same basis, the two former containing " water chemically combined, namely, yellow tuff, three atoms ; white tuff, two atoms ; pumice, one." " Now lava," he continues, " although commonly accompanied at the time of its eruption by abundance of steam, and containing, even for several months afterwards, entangled with it a large quantity of this and other volatile matters, holds no water in chemical combination, so that the fact with respect to tuff and pumice shows, that these formations have been placed under circumstances of another kind than those of molten lavas." * Volcanic Islands, p. 99. Mr. Darwin describes this tuff, where best charac- terized, as "of a yellowish-brown colour, translucent, and with a lustre somewhat resembling resin ; it is brittle, with an angular, rough, and very irregular fracture, sometimes, however, being slightly granular, and even obscurely crystalline ; it can easily be scratched with a knife, yet some points are hard enough just to mark common glass; it fuses with ease into a blackish-green glass. The mass contains numerous broken crystals of olivine and augite, and small particles of black and brown scoriae : it is often traversed by thin seams of calcareous matter. It generally effects a nodular or concretionary structure. In a hard specimen, this substance would certainly be mistaken for a pale and peculiar variety of pitchstone ; but when seen in mass, its stratification, and the numerous layers of fragments of basalt, both angular and rounded, at once render its subaqueous origin evident." f From Palagonia, in Sicily, where a similar tuff is found. CH. XIX.] MODIFICATION AND CHANGE EFFECTED IN TUFFS. 369 water. The following is the composition assigned to this rock by him : Silica 37-947 Sesqui-oxide of iron 14-751 Alumina 11-619 Lime 8-442 Magnesia 5-813 Potash 0-659 goda 0-628 Water 16-621 Residue 4-108* It will be obvious, that in volcanic-tuff accumulations much will depend, as respects subsequent modification and change, upon any foreign matter with which they may be mixed, so that when, as beneath water, calcareous matter (often, perhaps, derived through animal life,) as well as clay or other fine sediment, not directly de- rived from volcanic eruptions, is mingled with them, and the whole is heated or raised above the water, effects would be produced not precisely corresponding with those where the modifying action has been alone exercised upon the direct products of volcanos. Tuffs of this kind can scarcely but be often formed, and their examination in connexion with volcanos now in action, or which, geologically speaking, have recently been in that state, will be found important as explaining the origin of certain mixtures of igneous and sedi- mentary rocks, even amid very ancient deposits. In regions, such as Iceland, where volcanic action is widely spread amid its mineral products rising above the level of the sea, and where modifications due to the action of vapours and gases passing through lava streams, cinders, and ashes, may be so great, there would appear good evidence of the changes to which such mineral products may be exposed. Professor Bunsen has pointed out several which he considers to be now in progress in Iceland. "The Icelandic mineral springs," he remarks, " to which belong all the systems of geysers and suffiones, are distinguished from all others in Europe by the proportionally large quantity of silica which they contain ; and, if we except the acidulous springs which are confined to the western part of the island, the so-called beer-springs (olkilder) of the natives, we may divide the springs of Iceland into * On the intimate connexion existing between the pseudo-volcanic phenomena of Iceland: A Memoir translated by Dr. G. E. Day, Chemical Reports and Memoirs, Works of the Cavendish Society, 1848. From the chemical composition noticed, Professor Bunsen derives the formula Mg3 | Ca3 Ui 2 + 2 J> * 3 U Na3 J 2 B 370 SOLUTION OF PALAGONITE IN ACIDS. [Cn. XIX. two main groups, according to their chemical properties, one of which would comprise the acid and the other the alkaline silica springs." Whether the water of these springs has been derived directly from the atmosphere by means of rain, or melted snow and ice, or from sea- water finding its way to the interior of volcanos, the aqueous vapours thence thrown off being condensed in their rise upwards, to it and to the substances with which it can mingle, we have to refer many modifications which evidently take place in the mineral matter through which it passes. The experiments insti- tuted on this subject, and the conclusions deduced from them, and from a personal examination of the springs of Iceland, by Professor Bunsen, are highly valuable. With respect to the action of pure heated water alone for some hours upon the palagonite-tuff above noticed, he found that at the temperature of 212 Fahr. (100 cen- tigrade) or 226 4 (108 cent.,) silicic acid, potash, and soda, were dissolved.* When the water was saturated with carbonic acid, and allowed to act upon pulverized palagonite, all the constituents, with the exception of alumina and oxide of iron, were dissolved in the form of bi-carbonates.f When the palagonite was heated for ten hours, in water saturated with sulphuretted hydrogen, sulphide of iron was formed, and the solution contained silica and the sul- phides of calcium, magnesium, sodium, and potassium.j Pala- * Cavendish Society's "Works ; Chemical Reports and Memoirs, 1848, p. 364. " 1,000 grammes of water after 12 hours' digestion yield, in this manner, a solution containing the following proportions : Grammes. Silica . . . . 0-03716 Soda 0-00824 Potash . . . 0-00162 Total . . 0-01702" t " 1,000 grammes of this water, after four hours' digestion, yielded the following constituents : Grammes. Silica . . . 0-09544 Bi-carbonate of lime . , , magnesia , , soda . , , potash 0- 16893 0-05333 0-06299 0-00189 Total. . . 0-38368" J The solution contained, for 1,000 grammes: Grammes. Silica. . . . 0-1175 0-2748 0-0727 0-0438 0-0410 Sulphide of calcium . , , magnesium , , sodium , , potassium . Total . 0-5498 CH. XIX.] SOLUTION OF PALAGONITE IN ACIDS. 371 gonite was found to be " entirely dissolved in hydrochloric and sulphurous acids, except a small quantity of silica left as a residue."* Thus many and great modifications and changes may be effected in this variety of volcanic tuff; pointing to those which may take place in other volcanic regions, the results in each depending on local conditions. In many districts, as well those in some portions of which volcanic action is now well exhibited, as in those where it is be- coming extinct, as far as respects the ejection of molten rock, cinders, and ashes, discharges of aqueous vapours are effected ; sometimes alone, at others accompanied by some of the usual volcanic gases. Some mention has already been made (pp. 15 and 18,) of thermal or warm springs found to rise as well in regions not marked by volcanic action on the surface as in those where that action is now apparent, or may be inferred to have existed at no very distant geological period. t In some volcanic countries the various modifi- cations under which aqueous vapour, and the gases connected with volcanic action are emitted, can be well studied. The observer can readily suppose that while in the great eruptions these are so driven off as to have effected little combination while in the crater, minor action would leave sufficient time for the condensation of the aqueous vapour into water, and the combination of the latter with * "We see," observes Professor Bunsen, "from the relations existing among these salts themselves (alluding to those mentioned in the text and previous notes), and with the silica, that the constituents of palagonite take very different parts in the decomposition which is induced by hot water, carbonic acid, and sulphuretted hydrogen respectively ; whilst, as we have already seen, this mineral is entirely dissolved in hydrochloric and sulphurous acids, except a small quantity of silica left as a residue. The alkaline siliceous springs, in which there is a smaller quantity of this volcanic gas, assume, consequently, a very different character from the waters of the sumones ; since it is evident, that the composition of the water and the nature of the argillaceous deposits produced from these actions, must stand in a definite relation to the greater or smaller resistance opposed by the separate constituents of palagonite to the action of the weaker volcanic acids, that is to say, to the water, carbonic acid, and sulphuretted hydrogen gas." .... "When the alkaline silicates, removed by the heated water from the palagonite, are brought into contact with carbonic, hydrochloric, and sulphuric acids (the latter of which is formed by the oxidation of the sulphurous acid through the oxide of iron in the palagonite), these alkalies must be converted into carbonates, sulphates, and chlorides, whilst the silicic acid remains dissolved in the alkaline carbonates and in the water, and is partially separated from them by evaporation, as siliceous tuff, a fact already observed by Black in 1792." f While noticing the dispersion of hot springs, and their issue from all kinds of rock, Humboldt (Kosmos) mentions that the hottest permanent springs yet known are those discovered by himself, " at a distance from any volcano the ' Aquas calientes de las Trincheras,' in South America, between Porto Cabello and New Valencia ; and the ' Aquas de Comanzillas,' in the Mexican territory, near Guanaxuato." The first of these has a temperature of 97 centigrade (206 6' Fahr.), according to M. Bous- singault, who visited this spring in 1823. 2 B 2 372 SOLFATARA: [Cn. XIX. volcanic gases, the whole acting upon the rocks through which it has to pass, abstracting matter from them as above noticed. The Solfatara, near Puzzuoli, has long been known in the volcanic region of Naples, from the emission of aqueous vapour and certain gases, manifesting a kind of subdued volcanic action un- accompanied by the ejection of lava, cinders, or ashes.* Dr. Dau- beny found the gas evolved to be sulphuretted hydrogen, with a minute portion of muriatic acid.t Solfataras, or modifications of them, are noticed as existing in many volcanic regions in different parts of the world. Professor Bunsen has shown the connexion of the solfataras of Iceland (the Ndmar of the Icelanders), with the acid springs of that country. He remarks that they " owe their slight acid reaction more commonly to the presence of a small quantity of ammonia-alum, or soda, and potash-alum, than to their inconsiderable traces of free sulphuric or muriatic acids. "} While such springs in Iceland thus illustrate the condensation of some of the aqueous vapours, mixed with gases discharged in that volcanic region, the Geysers also well illustrate that of the aqueous vapours under other conditions. Allusion has been previously made (p. 15) to those long celebrated discharges of steam and water, the Geysers, and to siliceous deposits from them. According to Professor Bunsen, the thermal group to which the Geysers be- long, occurs southward from the highest point of Hecla, and about 20 geographical miles from it. Their main direction is about N. 17 E., " almost parallel with the chain of Hecla, and with the general direction of the fissures." The rock beneath the incrust- ations of the springs is palagonite-tuff, a vein of clinkstone running lengthwise from the western margin of the springs. The following are analyses by Dr. Sandberger and M. Damour, of the water of the Great Geyser : * An ancient lava current, of a trachytic kind, is supposed to be traceable from the mountain to the sea. t " Description of Volcan^s," p. 21 1. After pointing out the probable effects of the two gases upon the trachyte of the mountain, the sulphuretted hydrogen uniting with the bases of the several earths and alkalies, and its consequent decomposition, Dr. Daubeny accounts for the absence of muriatic compounds with these bases, by noticing that, " if they existed they would be immediately decomposed by the sul- phuric acid generated ; and that muriatic acid itself is incapable per se of decom- posing trachyte, except it be concentrated, and the rock pounded, as shown from the fact of its continuance during so many ages in the domite of Auvergne in a free condition." J Cavendish Society Works; Chemical Memoirs and Reports, 1848, p. 327. The cause assigned by Professor Bunsen for the alternate states of repose and activity of this great natural fountain, is very different from that usually inferred. By very careful experiments by M. Descloizeaux and himself, it was ascertained, 1. " That the temperature of the column of the Geyser decreases from below upwards, CH. XIX.] THE GEYSERS, ICELAND. 373 Sandberger. Damour. Silica. . 0-5097 . 0-5190 Carbonate of soda . . 0-1939 , . 0-2567 Carbonate of ammonia . 0-0083 . . Sulphate of .soda . . Sulphate of potash . . Sulphate of magnesia . Chloride of sodium. . 0-1070 . . 0-0475 . . 0-0042 . . 0-2521 . . 0-1342 . 0-0180 . 0-0091 . 0-2379 Sulphide of sodium . Carbonic acid . . . Water . 0-0088 . . 0-0557 . . 0-0088 . 0-0468 998-7695 Not only do the vapours and gases escaping from volcanic vents decompose, under variable conditions, the rocks through which they as had already been shown by Lottin and Robert. 2. That, setting aside small dis- turbances, the temperature goes on increasing regularly at all points of the column from the time of the last eruption. 3. That the temperature in the unmoved column of water did not, at any period of time up to a few minutes before the great eruption, reach the boiling-point that corresponds to the atmospheric and aqueous pressure at the point of observation ; and 4. That it is at mid-height in the funnel of the Geyser, where the temperature approaches nearest to the boiling-point, corresponding to the pressure of the column of water, and that it approaches nearest to this point in proportion to the approximation of the period of a great eruption." Diagrams are given in illustration, and indeed are almost necessary to the view taken. It may, however, be stated, that there is a constant addition of heated water below in the tube or funnel, and an evaporation of the water above, and that the whole is in such a condition that every cause that tends to raise this column of water only a few metres would bring a large portion of it into a state of ebullition, Vapour is gene- rated, and it is calculated that an excess of 1 (centigrade) over the corresponding boiling-point of the water, " is immediately expended in the formation of vapour, generating in the present case a stratum of vapour nearly equally high with the stratum of water 1 metre in height. By this diminution in the superincumbent water a new and deeper portion of the column of water is raised above the boiling-point ; a new formation of vapour then takes place, which again occasions a shortening in the pressing liquid strata, and so on, until the boiling has descended from the middle to near the bottom of the funnel of the Geyser, provided always that no other circum- stances have more speedily put an end to this process." " It appears from these considerations, that the column of water in the funnel of the Geyser extending to a certain distance below the middle, is suddenly brought into a state of ebullition, and further, as may be shown by an easy method of computation, that the mechanical force developed by this suddenly-established process of vaporiza- tion is more than sufficient to raise the huge mass of the waters of the Geysers to that astounding elevation which imparts so grand and imposing a character to these beautiful phenomena of eruption. The amount of this force may easily be ascertained by calculating from the temperature of the preceding experiments (those above alluded to), the known latent and specific heat of the aqueous vapour, and the height of the column of vapour, which wou^l be developed by the ascent, to the mouth of the Geyser, of a section of the column of water. If we designate the height of such a column of water in the funnel of the Geyser by h ; its mean temperature expressed in centesimal degrees by t ; the latent heat of the aqueous vapour by w ; the density of the latter compared with that of the water by s ; and the co-efficient of the expansion of the vapour by d\ we shall find that the excess of heat of the water above the boiling-point under the pressure of one atmosphere is t 100. But the height h, of the section of the column of water, which at the mouth of the Geyser, that is to say, under the pressure of one atmosphere, would be converted into vapour by the quantity of heat, t - 100, would be to the whole height of the water column A, as (t - 100) : w. A column of water of the height wouhl therefore be eva- porated at the mean temperature t, if the water were under the pressure of one 374 SILICEOUS DEPOSITS FROM THE GEYSERS. [H. XIX. rise or against which they may be driven in the atmosphere, and often the latter extends to some distance from the place of escape (well shown in the case of winds prevailing in particular directions), but deposits of different kinds are effected, thrown down from the waters containing them. Professor Bunsen has carefully investi- gated the well-known siliceous deposits from the Geysers of Iceland. Referring to the analysis of the water of the Great Geyser, above mentioned, and remarking that the silica is dissolved in the water by alkaline carbonates, and in the form of a hydrate, he observes that "no trace of silica is precipitated. on the cooling of the water, and it is only after the evaporation of the latter that silica is de- posited in the form of a thin film on the moistened sides of the vessel where evaporation to dryness takes place, whilst the fluid itself is not rendered turbid by hydrated silica until the process of concentration is far advanced." Professor Bunsen then points out, that, in consequence of these circumstances, the incrustations increase in proportion as the surface of evaporation expands with the spread of the water.* The same land presents us with other deposits from waters and atmosphere. Hence it directly follows, that the height H, of the column of vapour sought at 100 (centigrade) and 0-76 metre (29-921 English inches) will be H = h (* ~ 10 ) C 1 + 1QO <*) w s On applying this formula to the value of the numbers found by observation, we obtain the remarkable result that, in the period of time immediately preceding an eruption, a column of water of only 12 metres (39 feet 4-442 in. English) in length, which projects 5 metres (16 feet 4-851 inches English) to 17 metres (55 feet 9 '294 inches English) above the base of the tube, generates for the diagonal section of the Geyser, a column of vapour 638-8 metres (2093 feet 2-245 inches English) in height (assumed to be at 100 centigrade, and under the pressure of one atmosphere), this column being developed continuously from the upheaved mass of water, as the lower strata reach the mouth of the Geyser. The whole column of the Geyser, reckoned from the point where the temperature amounts to 100 centigrade down to the base, is capable, according to a calculation of this kind, of generating a similar column of vapour, 1041 metres (3,415 English feet) in height." Bunsen, On the intimate connection existing between the Pseudo-Volcanic Phenomena of Iceland ; Works of the Cavendish Society, Chemical Reports and Memoirs, pp. 346349. * Chemical Reports and Memoirs ; Works of the Cavendish Society, 1848, p. 344. Professor Bunsen remarks, that the peculiar fojms of the Geysers result from certain conditions. " As," he observes, " the basin of the spring has no part in this incrusta- tion, it becomes converted into a deep tube as it is gradually inclosed by a hillock of siliceous tuff, combining, when it has reached a certain height, all the requirements necessary to convert it into a Geyser. If such a tube be narrow, and be filled with tolerable rapidity by a column of water strongly heated from below by the volcanic soil, a continuous Geyser must necessarily be produced, as we find them in so many parts of Iceland. For it will easily be understood that a spring, which originally did not possess a higher temperature at its mouth than that which would correspond to the pressure of the atmosphere, may easily, when it has been surmounted by a tube, formed by gradual incrustation, attain at its base a temperature of upwards of 1000 centigrade (212 Fahrenheit), owing to the pressure of the fluid resting in the tube." CH. XIX.] SULPHUROUS WATERS OF ICELAND. 375 gaseous emanations, of importance geologically, showing some of the modes in which mineral matter may be accumulated or disseminated. Here again the researches of Professor Bunsen supply us with valu- able information. He points out that the acid silica springs, besides inconsiderable traces of hydrochloric and sulphurous acids, and small quantities of ammonia-alum, or potash and soda-alum, contain " sulphates and chlorides of calcium, magnesium, sodium, potassium, and iron, also silica and sulphurous acid, or in the place of the latter, sulphuretted hydrogen gas. They are especially characterised by deposits of gypsum and sulphur. Professor Bunsen found the com- position of the water, in 10,000 parts, taken from the Keykjahlider solfatara, in August, 1846, to be : Sulphate of lime 1-2712 Sulphate of magnesia 1*0662 Sulphide of oxide of ammonium . . . 0*7333 Sulphate of alumina 0*3261 Sulphate of soda . ....... 0*2674 Sulphate of potash 0*1363 Silica 0*4171 Alumina* 0*0537 Sulphuretted hydrogen 0*0820 Water 9995*6467 The clay and gypsiferous accumulations resulting from these waters, or rather from the general action of their constituent parts upon the rocks traversed by them, and upon each other, possess much interest. The palagonite-tuff is decomposed, and clay, often variegated in. colour, is deposited, and sulphate of lime is also formed. The gypsum occurs as isolated crystals, and " in connected strata and floor-like depositions, which not unfrequently project as small rocks, where the loose soil has been carried away by the action of the water. These depositions are sometimes sparry, corresponding in their exterior very perfectly with the strata of gypsum so fre- quently met with in the marl and clay formations of the trias."f * It is remarked respecting the alumina, " the small quantity of which brings it within the limits of the errors incidental to the experiment," that it may have been dissolved in excess by the alum of the water. Chemical Reports and Memoirs, 1848, p. 332. f Bunsen, Works of the Cavendish Society, Chemical Reports and Memoirs, 1848, p. 336. " Their deposition," the Professor adds, " is owing to the fact that has not hitherto been sufficiently regarded in the explanation of geological phenomena, viz. : that substances crystallizing from solutions are more readily deposited on a surface identical with their own (although at a considerable distance from the limits of their solubility), than on substances different from themselves. These depositions of gypsum increase, therefore, in these formations, in the same manner as we observe small crystals to enlarge in a solution, without any deposit being formed ort-the sides of the vessel ; much salt being removed from the solution (not by a change of tem- perature, but owing to the cohesive force emanating from the crystal), so that no further deposit can be made on the particles of bodies of a different nature. The 376 NITROGEN OF VOLCANOS. [Cn. XIX. In the clay deposits from the Icelandic fumeroles iron pyrites is found in small crystals, mentioned by Professor Bunsen, as " often very beautifully developed." The sulphur accumulations of Ice- land, the Professor attributes to the same cause, as Dr. Daubeny does generally (p. 324), namely, the reciprocal reaction of sulphurous acid and sulphuretted hydrogen gas. With regard to the presence of nitrogen, ammonia, and their compounds, so frequently observed in connexion with volcanic action, opinions seem somewhat divided : while some consider that the nitrogen is actually evolved from the craters and other volcanic vents, part, perhaps, of the air disseminated in water rinding its way to volcanic foci, others infer that ammoniacal products, found in connexion with volcanos, have had a different origin. Dr. Dau- beny, treating of volcanic action, remarks : se Nor is the access of atmospheric air to volcanos more questionable than that of water ; so that the appearance of hydrogen united with sulphur, and of nitrogen, either alone or combined with hydrogen, at the mouth of the volcano, seems a direct proof that oxygen has been abstracted by some process or other from both."* On the other hand, Professor Bunsen seems disposed to consider the sublimations of muriate of ammonia as due to the overflow of vegetation by lava currents, at the same time referring to nitrogen and its compounds, and to their being scarcely ever absent from volcanic exhalations, adding that ' ' they undoubtedly belong originally to the atmosphere, or to organic nature, their occurrence being due to the water which holds them in solution and conveys them from the air to the subterranean depths."-)- If we assume that water from the atmosphere, or sea, process of crystallization here comes within the domain of mechanical forces, since it causes, by the expansive growth of the layers of gypsum, the upheaval of the moist- ened clay deposit, or compresses it towards the exterior, as the first-named masses increase in quantity." * Daubeny, Transactions of the British Association for the Advancement of Science, vol. v., for 1837, and Description of Volcanos, 2nd edition, 1848, p. 655. f Psuedo- Volcanic Phenomena of Iceland, p. 330. " In July, 1846," observes Professor Bunsen, " only a few months subsequent to the eruption of the volcano (Hecla), when I was sojourning in that district, the lower portion of the lava stream appeared studded over with smoking fumeroles, in which so large a quantity of beautifully-crystallized muriate of ammonia was undergoing a process of sublimation, that, notwithstanding the incessant torrents of rain, hundreds of pounds of this valuable salt might have been collected. On surveying the stream from the summit of Hecla, it was easy to perceive that the formation of muriate of ammonia was limited to the zone in which meadow lands were overflowed by the lava. Higher up, where even the last traces of a stunted cryptogamic vegetation disappear, the formation of this salt likewise ceased. The large fumeroles of the back of the crater, and even of tlie four new craters, yielded only sulphur, muriatic and sulphuric acids, without exhibiting the slightest trace of ammoniacal products. When we consider that, according to Boussingault, an acre of meadow land contains as much as 32 pounds of CH. XIX.) FUSIBILITY OF MINERAL VOLCANIC PRODUCTS. 377 more or less containing air (p. 145), does find a passage into vol- canos or their foci, it may not be improbable that both causes can contribute to the effects recorded. It will have been seen that volcanic products are, as a mass, easily melted, notwithstanding that, during times when the particles of matter could freely adjust themselves in a definite manner, cer- tain mineral bodies were formed of a less fusible character, and that from the decomposition of ejected lava, cinders, and ashes, certain other substances are produced, such as siliceous and purer argil- laceous deposits of a more refractory kind. Looking, however, at the mates of matter, it continues readily fusible, whether partaking of the trachytic or doleritic character, or of a mixture of both. Sheets of trachytic or doleritic tuff could be easily acted on by heat, so that even altered as they may have been previously, a considerable area, if a sufficiently-elevated temperature could be applied to it, would begin to yield, rising upwards if any elevatory force were acting from beneath, a fracture finally being effected, so far as any sufficient coherence of parts remained, where the resist- ance became unequal to the force employed. That volcanic action does work through points, however these may be complicated, as from time to time changed, volcanic craters show, and that it has at least sometimes continued to find vent through the same points, or thereabouts, for long periods, is not only attested by volcanos which have been observed during the historic period, but also by those to the products of which, inter- mingled with other accumulations, geological dates may be assigned. That the temperature of volcanos may even change externally, so that snows reposing upon them at one time are suddenly melted from them at another, we have evidence both in high northern latitudes (Iceland) and in the warm regions (Cotopaxi). Volcanic products being, like rocks generally, bad conductors of heat, these changes are sufficient to show the variable amount of temperature to which portions, at least, of volcanic mountains naay be exposed from changes in the conditions to which it may be due. That the nitrogen, corresponding to about 122 pounds of muriate of ammonia, we shall hardly be disposed to ascribe these nitrogenous products of sublimation in the lava currents to any other circumstance than the vegetation which has been destroyed by the action of the fire. The frequent occurrence in Southern Italy of tuff decomposed by acid vapours containing muriate of ammonia, likewise confirms the hypothesis regarding the atmospheric origin of this salt. For the same body of air which can annually convey to a piece of meadow land a quantity of ammonia corresponding to these large nitrogenous contents, must at least be capable of depositing an equal quantity of alkali on tuff-beds saturated by acid water ; which may be actually observed in some rare instances both in Southern Italy and Sicily." 378 FISSURES FILLED BY MOLTEN LAVA. [H. XIX. layers and variably-shaped masses of substances composing volcanos, no matter how accumulated, have been exposed to tension and sub- sequent fracture, is proved by the rents in them which have been filled by molten matter. In the view beneath (fig. 126), in the Fig. 126. Val del Bove, Etna,* exhibiting the now hard lava protruding from that weathering which the whole has experienced from atmospheric influences, we see that the original volume of the previously- deposited volcanic products has been increased by the amount of the molten matter so introduced. The following section (fig. 127), Fig. 127. a b c d f showing the same thing, is not unfrequent amid volcanic craters and ravines. In it, e f represent a thickness of volcanic tuff or lava layers, traversed by dykes (as they are termed), a b c d, of lava which has entered fissures made by the rupture of the beds ef, the force acting from beneath, so that while, for the length seen, some fissures have their walls equidistant from the top to the bottom * Taken from Dr. Abich's " Views of Vesuvius and Etna." CH. XIX.] FISSURES FILLED BY MOLTEN LAVA. 379 of the section, at a and c they diminish upwards. In all these cases, the layers or beds are not disturbed further than by fracture, those on either side of the dykes preserving their continuous lines of original accumulation. This need not always be the case, as will be readily inferred, since after a fracture is made, should the liquid or viscous lava rise with much force from considerable pressure of a column of molten rock with which it may be connected, with a comparative cooling of the upper part of the lava as it rose, in- creasing the solidification of the particles, the upper layers of tuff or lava broken through may be heaved upwards by the friction of the uprising lava, this even overflowing, as appears to be frequently the case. Of this kind the section beneath (fig. 128), taken from a view by Dr. Abich, of a dyke exposed by the fall of part of the crater of Vesuvius, is probably an instance. Fig. 128. The observer has next to consider the magnitude and direction of these fissures. Perhaps volcanic islands, such as those in the Atlantic and Pacific, are as favourable situations for studying vol- canic fissures as are to be found, not, however, that great facilities do not present themselves elsewhere.* The rent or series of rents which traversed the side of Kilauea, during the ejection of lava in 1840, is not only remarkable for its length, but also for the com- * Respecting the Hawaiian group, Mr. Dana, (Geology of the United States' Exploring Expedition, p. 282), infers : 1. That there were as many separate rents or fissures in the origin of the Hawaiian Islands as there are islands. 2. That each rent was widest in the south-east portion. 3. That the south-easternmost rent was the largest, the fires continuing there longest to burn. 4. That the correct order of extinction of the great volcanos is nearly as follows (leaving out Molokai and Lanai, which were not visited by Mr. Dana) a, Kauai ; 6, Western Oahu ; c, "Western Maui, Mount Eeka; d, Eastern Oahu; e, North-western Hawaii, Mauna Kea ; /, South-east Maui, Mount Hale-a-Kala; and #, South-east Hawaii, Mauna Loa. " This order," he observes, " is shown by the extent of the degradation on the surface. Each successive year, since the finishing of the mountain, has carried on this work of degradation, and the amount of it is, therefore, a mark of time, and affords evidence of the most decisive character." (p. 283.) 330 LAVA EJECTED THROUGH FISSURES. [Cn. XIX. parati vely tranquil manner in which it was effected, the inhabitants not being aware of its formation by any earthquake motion, but from rinding a torrent of liquid lava poured out.* Fissures so pro- duced would seem to point to much softening of the subjacent rocks, so that when fractures were formed, though many miles in length, comparatively little resistance, from cohesion, remained in the rocks. From an examination of Maui (Hawaiian group), Mr. Dana infers that at its last eruption, a huge segment of that volcano must have been broken off, by which two great valleys were formed (one two miles wide), through which great opening the lavas were poured out,-)- Here would appear to have been far greater resistance and a more sudden overpowering of it by the force exerted. According to the accounts given, Jorullo was the result of an uprise of ground, finally traversed by a fissure (p. 346). With respect to fissures traversing active volcanos from which lava has issued, there is abundant evidence. The great out- flow of lava from Skaptar-jokull, in 1783 (p. 344), was from fissures at the base of the mountain. Great fissures have been made in Etna, and the numerous subordinate craters seem little else than points in those formed at various times through which volcanic matter has been ejected.! Eespecting the fissures on Etna, M. Elie de Beaumont remarks, that they occur, for the most part, in nearly vertical planes, often so cutting the crater, as it were, to star it, the lower part of the fissures usually filled with lava, the higher with scoriae, and with pieces of tuff and lava fallen from the upper part. He mentions that the fissure formed in 1832, was so far a shift, or fault, that one of the sides of the dislocation rose about a yard higher than the other. The great fissure, which in 1669 traversed the slope of the great gibbosity of Etna and the Piano del Largo, is described as having ranged from near Nicolosi to beyond the Torre del Filosofo, and to have been about two yards wide at the surface, a livid light being emitted from the incan- descent lava rising in it.|| An observer should carefully ascertain the directions of such rents or fissures whether large or small, and * Dana, " Geology of the United States' Exploring Expedition," p. 217. t Ibid, p. 259. J It is stated that there are 52 of these subordinate volcanic hills on the west and north of the summit of Etna, and 27 on the east side ; " some covered'with vegetation, others bare and arid, their relative antiquity being probably denoted by the progress vegetation has made upon their surface." Daubeny, Volcanos, 2nd edition, p.^272. llecherches sur la Structure et sur 1'Origine du Mont Etna, par M. L. Elie de Beaumont ; " Memoires pour servir a une Description Geologique de la France," 1838,t. iv.,p. 111. || Ibid. p. 108. CH. XIX.] DIRECTION OF FISSURES JN VOLCANOS. 381 always with reference to the complication which may arise from variable resistances, even in the prolongation of the same fissures, to the force employed, seeing especially if the greater fractures have continued to preserve any definite directions at different in- tervals of time. M. Elie de Beaumont has called attention to the fact that these fractures so often starring Etna, and into which the molten laVa is introduced, there hardening and remaining, must produce a tume- faction or elevation of the whole, each eruption of the mountain, so characterized, having a tendency to elevate the mass of the volcano.* The same reasoning is applicable to all volcanos rent and fissured in a similar manner, and the abundance of the resulting dykes of lava is often a prevailing feature in many. They are sometimes in- terlaced so as to show differences in date, hose of one time cutting those of another, exhibiting proofs of repeated fractures through the same general mass of volcanic matter. In examining some volcanic regions, the observer will have to consider, as above noticed (p. 323), the probable differences which would arise in the structure and arrangement of the accumulations from a part or the whole of them having been produced beneath water. At considerable depths in the ocean, beyond those at which an equal temperature of 39-5 (p. 96) would appear to prevail, not only the pressure but also the constancy of that temperature and the mass of water possessing it have to be borne in mind. As- suming a communication made, whether by an elevation of the sea- bottom and the bursting of a tumefaction formed by forces acting from beneath, or by one of those adjustments of the earth's surface by which more or less considerable fractures are produced, he will have to recollect that a great volume of water, with a low tem- perature, would be at once brought to bear upon it, and that not only are the usual volcanic gases absorbed by water, but that the very pressure itself might tend to drive them into the liquid state.-f- It would be out of place here to enter into the probable effects pro- duced under such conditions, further than to notice that, supposing the communication made, and the elevatory force sufficient to lift a body of molten lava, so that it could pass out of the volcanic orifice or crack, the observer has to consider the effects which would follow. However any intense heat might permit the existence of * " Memoires pour servira une Description Geologique de la France," t. iv., p. 118. t Dr. Faraday has shown (Philosophical Transactions, 1823), that sulphurous acid gas becomes liquid under the pressure of two atmospheres, at a temperature of 45 Fahr. ; sulphuretted hydrogen under that of 17 atmospheres at 50 ; carbonic acid under 36 atmospheres, at 32 ; and hydrochloric acid gas under 40 atmospheres at 50. 382 SOFTENING AND RAISING OF TUFFS AND LAVAS. [Cn. XIX. the vapours and gases observed at subaerial volcanos, before the rupture was effected, as soon as the water came into contact with them, a ready supply of that at 39 -5 pouring in, the more heated water ascending, as so heated, they would disappear as they rose. If disseminated amid the lava thrown out, the great pressure upon the latter would produce its effects upon them, while the low tem- perature would soon act on the external liquidity of the lava itself. From such a state of things to the minor" depths and surface of the water, great modifications would be expected, solid lava, (supposing the struggle between the forces brought into action to be such, as, on the whole, to permit a gain on the side of the volcanic products), probably prevailing beneath, while there was an admix- ture of more scoriaceous matter above, as the accumulations rose into the atmosphere. The whole mass would be liable at all times to be cracked and fissured, molten lava rising into the rents accord- ing to the pressure of the time upon it, and the tumefaction mentioned by M. Mie de Beaumont progressing. The extent to which sheets of matter could be spread in various directions and at different times around submarine volcanic vents, would necessarily depend upon circumstances; among them, the absence of piles of cinders and ashes into cones, such as are formed by the discharge of vapours and gases through lava into the atmo- sphere, being important, so that when fissures were produced, molten matter flowed more freely out, in the manner, so far as liquidity and the absence of cinders and ashes are regarded, of the streams which poured out on the flanks of Mauna Loa, in 1843. It has been seen that volcanic vents may remain for a long time dormant or closed, and then lava, cinders, and ashes be driven out of them. The probable differences which would arise in such cases with volcanic accumulations beneath the sea and those above its level, require attention. It may be inferred that, beneath given depths of water, where the rents have been more frequent, and the lavas have more frequently been thrown out, there may be such a mechanical resistance to the new application of an elevatory force, that an increase of heat may soften and even melt some of the prior- formed accumulations, for the most part readily fusible. Thus a dome-shaped mass may be raised, not finally splitting, in given localities, at its surface, until even above water, and the quiet cracking of Mauna Loa for the length of many miles, would appear to show us that conditions may arise, even in subaerial volcanos, permitting the heating and softening of a volcanic crust to within comparatively moderate distances from that crust. CHAPTER XX. VOLCANOS AND THEIR PRODUCTS CONTINUED. THE CALDERA, ISLAND OF PALMA. SECTIONS OF ETNA AND VESUVIUS. FORM AND STRUCTURE OF ETNA. ORIGIN OF THE VAL DEL BOVE, ETNA. FOSSILIFEROUS VOLCANIC TUFF OF MONTE SOMMA. MIXED MOLTEN VOLCANIC ROCKS AND CON- GLOMERATES. MODIFICATION OF SUBMARINE VOLCANIC PRODUCTS. PEAK OF TENERIFFE. SANTORIN GROUP. ISLAND OF ST. PAUL, INDIAN OCEAN. DISTRIBUTION OF VOLCANOS IN THE OCEAN -ON CONTINENTS AND AMID INLAND SEAS. VARIABLE PROXIMITY OF VOLCANOS TO WATER. EXTINCT VOLCANOS. MINERAL AND CHEMICAL COMPOSITION AND STRUCTURE OF BASALT. THAT dome-like elevations of volcanic products have been formed, MM. Von Buch, Elie de Beaumont, Dufrenoy, and others consider they have sufficient proof. Some notice has been above given (p. 318), of the equivocal appearances which may be presented by the "craters of elevation" and the "craters of eruption." The Caldera, in the Island of Palma, Canaries, is adduced by Yon Buch as a good example of the " craters of elevation." A large precipitous cavity or crater is there surrounded by beds of basalt and conglo- merate, composed of basaltic fragments, dipping regularly outwards, and is broken only by a deep gorge on one side, through which access can be obtained to this central cavity. White trachyte, and a compound of hornblende and white felspar, are also noticed among these rocks. There being no mixture of scoriae or ashes, and the beds of molten rock as well as the conglomerate presenting a uniform stratification, it is inferred that the whole was formed under different circumstances, such as beneath water, from the ordinary eruptive accumulations of a volcano, and had been up- raised in a dome-like manner, until finally the rupture was effected, and the least resistance being in one direction, the lateral gorge was produced, the whole presenting the appearance of the pear-shaped termination of the fissures in figs. 115 and 116. M. Elie de Beaumont has "given a valuable description of Etna, 384 ETNA. [Cii. XX. CH. XX.] SECTIONS OF ETNA AND VESUVIUS. 385 2 02 O C3 r* 1 I S S * I "I s a O o 2c 336 FORM AND STRUCTURE OF ETNA. [Cn. XX. illustrating the same views.* The Val del Bove shows an accumulation of many hundred layers of fused rocks, somewhat resembling the modern lavas of this mountain, interstratified with others composed of fragmentary and pulverulent substances, the beds varying in thickness from half a yard to several yards, those of fused rock commonly thinner than the fragmentary deposits. The surfaces of the former are rough, and their outer part is penetrated by cells to the depth of 8 or 12 inches; whence beds only 20 inches or 2 feet thick are cellular throughout. The thicker beds are solid in the middle and resemble certain trachytes, labradorite replacing orthoclase. The fragmentary beds are true tuffs, their structure similar to those found in the Cantal and Mont Dore, the component fragments composed of the same substances as the fused beds, sometimes scoriaceous, at others compact. The beds of the Yal del Bove undulate in different directions, varying from horizontality to inclinations of 20 and 30, without their struc- ture or thickness being altered in a constant manner. They are traversed by a multitude of lava dykes, the rock of which is of the same kind as that of the fused beds. Though these take various directions, it is considered that there is a tendency on the whole to one ranging E.N.E. The general form of Etna will be seen from the accompanying- view (fig. 129) taken from one of those in the atlas of M. von Walterhausen.~f* By it the very slight rise of the general mass to the central crater will be observed, as also the great break in front, known as the Val del Bove, where the more ancient vol- canic accumulations are considered to be exposed. While this view may thus be useful in showing the general outline of the mountain, so far as views embracing considerable areas may do so, the annexed section from west to east (fig. 130),J will more cor- rectly give the real shape of Etna, taken through the Val del Bove, therefore somewhat across the outline represented in the view, and exhibit the steep descent through the cliffs of that great break or depression on the flank of the mountain. * *' Memoires pour servir a une Description Geologique de la France," torn. iv. According to M. Elie de Beaumont the order of the Etna formations is as follows : 1. Granitoid rocks, known only by ejected fragments. 2. Calcareous and arenaceous rocks. 3. Basaltoid rocks. 4. Rolled pebbles, forming aline of hills at the junction of the Plain of Catania and the first slopes of Etna. 5. Ancient lavas, forming the escarpments round the Val del Bove ; and 6. Modern eruptions (p. 53). -j- Maps and Views of Ftna. J Taken from Dr. Abich's * Erlauternde Abbildungen Geologischer Erschcinungen beobachtet am Vesuv und Aetna," pi. 9, Berlin, 1837. In this section, the scale for height and distance is the same. CH. XX.] ORIGIN OF THE VAL DEL BOVE, ETNA. 387 M. Elie de Beaumont quotes M. Mario Gemellaro as pointing out that the central mass of Etna is composed of two cones passing into each other; one, interior, formed of ancient volcanic pro- ducts, the other, exterior, formed of modern accumulations. These two cones are upon the same axis, the ancient nearly east from the modern cone, whence they do not completely embrace each other, the modern not altogether covering the ancient, and the products of the latter being exposed on the eastern side of the mountain, especially in the Val del Bove*. This considerable and sudden gap in Etna, M. Elie de Beaumont agrees with Sir Charles Lyellt in referring to a great subsidence of that part of the mountain, in the same manner as the much larger volcanic mass of the Papan- dayang fell in at Java in 1772, and that of Carguairazo subsided on the 19th of July, 1698 ; a volcano considered previously to have rivalled its neighbour Chimborazo in height. M. Elie de Beaumont refefs to the possibility of the lava which lifted the ancient mass of Etna having been abstracted, so that the needful previous support of the portion now occupied by the Yal del Bove being removed, depression was the result. He considers Etna to have been an irregular crater of elevation, the uplifting force not having there acted in the same simple manner as at the Isle of Palma, Teneriffe, and Monte Somma ( Vesuvius).:]: M. Elie de Beaumont infers that the first-formed deposits were nearly horizontal, successive fissures presenting channels for the out- pouring of very fluid lava which spread round in various directions, in the manner that sheets of basalt are seen to have done in many countries, and especially in Iceland, cinders being also ejected, so as to alternate with the fluid rock. Upon the accumu- lations thus produced, the elevatory force is considered to have acted in the manner described. In like manner, the part of Vesuvius known as Monte Somma has been inferred to be the remains of a crater of elevation, which has been broken through, so that it became, in a great measure, * " Memoires pour servir a une Description Geologique de France," t. iv., p. 124. t " Principles of Geology," 4th edition. J " Memoires pour servir a une Description Geologique de France," t. iv., p. 188. M. Elie de Beaumont further observes, that " the force which raised the gibbosity of Etna appears to have acted not on a single and central point, but in a straight line, represented by the axis of the ellipse, of which the southern, northern, and eastern flanks of the Val del Bove form a part ; and it seems to have acted unequally on the different parts of this straight line, so that its western extremity, answering to the actual volcanic vent, has been more raised than the rest. An elevation of this kind could not be produced without the upraised masses being broken, and the rents ought chiefly to coincide with the line of elevation, or diverge, radiating, from its extremities." 2c2 388 FOSSILIFEROU TUFF OF MONTE SOMMA. [Cn. XX. covered by the eruptions of more modern times, those chiefly which followed the great outbreak of 79. The section* (fig. 131) will serve to show the general outline of this volcano, as well as the portion of the cone (Monte Somma) which existed prior to that eruption. After describing the volcanic tuff of the environs of Naples, showing that it is composed almost entirely of the debris of trachyte, the greater portion of the fragments contained in it pumice, that it was, in part at least, fossiliferous,f and inferring that it was arranged in beds under water, { M. Dufrenoy observes that the tuff of Monte Somma is the continuation of the same accumulations, and quotes M. Pilla as having discovered fossil shells in it. He points out that among the limestone fragments in the tuff of Monte Somma some are covered by small serpulce of the same species as those which adhere to the rocks on the coast of Sicily, and that these fossils not being in the least altered, they prove, even more than the general disposition of the tuff^that it has been formed beneath water, and subsequently raised to its present height on the Monte Somma. This tuff, with its fragments or pebbles of limestone (commonly saccharoid) and of micaceous rocks, M. Du- frenoy considers, with the lava associated with it on Monte Somma, to have been upraised, in a vaulted manner, by volcanic forces acting from beneath, the eruptions finally finding vent through this elevated mass, and forming the present Vesuvius. The rocks composing Monte Somma and Vesuvius are pointed out as dif- ferent. While the lavas of Monte Somma resemble crystalline rock, such as granite and trachyte, the Vesuvian products are scoriaceous. The former are composed of leucite, augite, labradorite, and some * From Abich's " Erlauternde Abbildungen Geologischer Erscheinungen beobachtet am Vesuv und Aetna," pi. 9. Berlin, 1837. The scale for this section is the same for height and distance, but it differs from that of the section of Etna on the same page. f M. Dufrenoy (Memoires pour servir a une Description Geologique de la France, t. iv., p. 240) adds to the evidence of Mr. Poulett Scrope, who pointed out (Geol. Transactions, 2nd series, vol. ii., p. 351) that this tuff contained the remains ofostrea, cardium, buccinum, and patella, of species now living in the Mediterranean ; and of Sir Charles Lyell (Principles of Geology) respecting the fossiliferous tuff of Ischia, that ostrea, cardium, and pecten have been obtained from the quarries in the hill of Posilippo, an ostrea and a pectuncuhis at St. Elmo, above Naples, and fossils in other places. J M. Dufre'noy mentions ("Me'moires pour servir, &c., t. iv., p. 238J, that this tuff often presents cavities from 6 inches to 2 feet in height, almost always taking a vertical direction. They are numerous in the tuff of Naples itself, and in the escarp- ments on the high road to Nesita. Their parallelism leads him to infer that they have been caused by the abundant escape of gas which traversed the beds before their solidification. M. Dufrenoy Cp. 241) also notices concretions in the tuff, chiefly in the argillaceous beds. The fossils found by M. Pilla are Turritella terebra, Cardium ciliare, Corbula gibba, and a portion of an Echinite. CH. XX.] MIXED MOLTEN ROCKS AND CONGLOMERATES. 38.) rare nodules of olivine ; while the latter, when compact and crys- talline, are formed of crystals of the order of felspar, but differing from ordinary felspar, albite, and labradorite. They, moreover, contain crystals of green augite, some nodules of olivine, and some rare plates of mica. As with the lava and tuff beds of the Val del Bove, Etna, the lava and tuff of Monte Somma are traversed by numerous lava dykes. Without multiplying examples of mixed beds of conglomerate, tuff, and lava, so occurring as to render it probable that these volcanic accumulations had been effected beneath water, mention may be made of the evidence on that head obtained by Mr. Dana among the islands of the Pacific, as the descriptions given of such aggregations at Oahu, and Maui (Hawaiian group), and Tahiti, possess much interest in their geological bearings. Some of the rounded masses in the conglomerate of Kauai are stated to contain 30 cubic feet, lying against each other, the interstices between filled in with pebbles and finer matter. At Oahu there are some finely-laminated tuffs, which as much point to their formation beneath water as the conglomerates.* Molten rocks and con- glomerates are mentioned as alternating at Tahiti, some of the stones in the latter being six inches in diameter .| In such islands we have merely the upper portions of volcanos above the level of the sea. As respects the evidence of their uprise from situations where large rounded blocks and pebbles could be formed, the simple tumefaction of the volcanic mound, from the causes above noticed (p. 381), would alone aid the uplifting of beaches and various deposits, in minor depths, to heights proportionate to the introduction of the matter filling dykes traversing the mass, and to the extension of the various deposits of ashes and cinders, and of lava, by heat, as covering after covering was accumulated, independently of any great force applied from beneath, and tending to dome out and perhaps throw off the flanks. The observer will have to consider the probable figures which beds would take round volcanic islands. If we are to suppose some volcanos, now inland in various parts of the world, to have been once islands, the depth of water around them at different times would much influence the arrangement of their mineral products. We should expect the deposits which now take place * Dana, "Geology of the United States' Exploring Expedition," pp. 239, 261, 267, 268. f Ibid., p. 295. Mr. Dana describes the general dip of these beds to be from the central part of the island outwards, the central rocks being more compact than those on the exterior and less vesicular, more trachytic and syenitic Cp. 296 J. 330 PEAK OF TENERIFFE. [Cn. XX. around and amid the Hawaiian Islands to be much modified, as regards general arrangement, from those of any volcanos in shallow seas. We must refer to previous notices of volcanic ash and cinder accumulations in tideless (p. 69) and tidal seas (p. 99), and the working out of soft from hard volcanic matter by the breakers (fig. 80, p. 192, as pointing to these modifications. To these may be added the condition of volcanic islands, such as those in various parts of the ocean exposed to almost ceaseless breaker action, separating the harder from the softer volcanic products, the volcanic mass gradually rising, from time to time ; great subaerial eruptions, and perhaps submarine also, being effected. Very com- plicated arrangement of parts could scarcely but arise, and much admixture of molten matter with conglomerates and finer volcanic sediment, be the result, and this independently of any accumu- lations at considerable depths, which, as they rose, would become acted upon by the breakers in a similar manner ; to be afterwards covered with subaerial accumulations, should volcanic action con- tinue above water in the elevated mass. The accompanying view of the Peak of Teneriffe (fig. 132), by M. Deville,* taken from Fig. 132. near Santa Ursula, may serve to illustrate the slope of that moun- tain, considered fundamentally due to the elevation of beds of tuff and molten rock around the central portion, upon which the sub- Etudes Geologiques sur les lies de Tene'riffe et de Fogo," 1847. CH. XX.] SAW TOWN GROUP. 391 aerial eruptions have formed the present Peak, as also the cutting back of the mass at the level of the sea by the breakers. As to the elevation of volcanic accumulations, the island of Santorin has attracted considerable attention, not only from the discovery of organic remains in the tuffs, now raised above the sea, but also from its general form and its history. Dr. Daubeny infers with respect to this island, or rather islands, the larger known to the ancients as Thera, and the second in size as The- rasia, though there may be some uncertainty as to the whole of the little group having been thrown up in historical times that some considerable convulsions may have occurred, to which early ancient writers refer.* Whatever obscurity may hang over the exact times at which the Santorin group, or parts of it, were up- raised above the level of the sea, there appears none as to changes having been effected in its isles and islets by volcanic action from remote historical times. Nearer our own times, it seems certain that a portion of this volcanic mass was raised above water in 1573, forming a rock known as the little Kaimeni ; that there was an eruption of pumice near Santorin in 1638; and that, in 1707, a new rock rose between the Little and Great Kaimeni, " which increased in size so rapidly, that in less than a month it became half a mile in circumference, and had risen 20 or 30 feet above the level of the water, constituting a third island, which was called New Cammeni, a name which it still bears."-)- Some persons landing on the up- raised rock to collect oysters adhering to it, were compelled to leave it from the violent shaking of the ground. The commence- ment of the island was first observed on the 23rd of May, 1707. In July, black smoke accompanied the upheaval of the rocks, and much sulphuretted hydrogen appears to have been discharged. Stones, cinders, and ashes were shortly afterwards ejected ; showers * Daubeny, *' Description of Volcanos," 2nd edition, p. 320. Dr. Daubenj 1 refers to the statement of Pliny, that 130 years after the separation of Therasia, the island of Thera was thrown up ; "a statement," he observes, " confirmed by Justin and Plutarch, as to the fact, though not as to the date." * * * " It is to this event that Seneca seems to refer, where he speaks of an island thrown up in the JEgean Sea, by an accumulation of stones, of various sizes, piled one upon another." * * * " Pliny also speaks of another phenomenon of the same kind, as happening in his own time, for he tells us that in the reign of Claudius, A.D. 46, a new island called Thia appeared near Thera. Bui as he mentions it as only two stadia distant from Hiera, it is possible that the island may have been joined to the latter by a subsequent revolution, as by that recorded to have taken place in the year 726, by which Hiera is said to have been greatly augmented in point of size." t Daubeny (Description of Volcanos, p. 321^, who quotes from Father Goree, an eyewitness of the fact, seen from Scaro and all that side of Santorin. 332 SANTORIN GROUP. [Cn. XX. of the two latter spreading to considerable distances. The volcanic action continued for nearly a year, more or less ; indeed, during ten subsequent years.* As Dr. Daubeny remarks, this elevatory action has not yet ceased, inasmuch as a reef found by the fisher- men to have been raised, during a short time, to within 30 or 40 feet of the surface, was in 1829 ascertained, by M. Lalande, to have no more than 9 feet water over an area of 2,400 by 1,500 feet, the ground gradually sinking around from the centre, and less water, by two feet, was obtained about two months afterwards by M. Bory de St. Vincent. The accompanying (fig. 133) is a map of these islands, with a Fig. 133. a, a, a, a, Thera or Santorin ; b, Therasia. * " In July, the appearances were more awful, as all at once there arose, at a distance of about 60 paces from the island already thrown up, a chain of black and calcined rocks, soon followed by a torrent of black smoke, which, from the odour that it spread around, from its effect on the natives in producing headache and vomiting, and from its blackening silver and copper vessels, seems to have consisted of sulphuretted hydrogen. Some days afterwards the neighbouring waters grew hot, and many dead fish were thrown upon the shore. A frightful subterranean noise was at the same time heard, long streams of fire rose from the ground, and stones con- tinued to be thrown out, until the rocks became joined to the White Island originally existing. Showers of ashes and pumice extended over the sea, even to the coasts of Asia Minor and the Dardanelles, and destroyed all the products of the earth in San- torino. These, and similar appearances, continued round the island for nearly a year, after which nothing remained of them but a dense smoke. On the 15th July, 1708, the same observer (Father Goree) had the courage to attempt visiting the island, but when his boat approached within 500 paces of it, the boiling of the water deterred him from proceeding. He made another trial, but was driven back by a cloud of smoke and cinders that proceeded from the principal crater. This was followed by ejections of red-hot stones, from which he very narrowly escaped. The mariners remarked, that the heat of the water had carried away all the pitch from their vessel." Daubeny, Description of Volcanos," p. 322. CH. XX.] SUBMARINE CHARACTER OF SANTORIN GROUP. 393 sketch of the banks and form of the ground beneath the sea, as shown by the late survey of Captain Graves, K.N. The crateri- form cavity in the centre, with its depth of 213 fathoms (1,278 feet), will be at once apparent, with the shallow depth on the 'west, dividing it from the Mediterranean in that direction. Equally interesting is the deep channel running to the northward, with as much as 990 feet in it, reminding us of those figures and descrip- tions of the craters particularly brought under notice by Von Buch, where a great rent appears to have been effected on one side, form- ing a ravine entrance to their central, and often otherwise inacces- sible interiors. Such a form also will again remind the observer of the pear-shaped termination of fissures noticed above (figs. 115 and 116), one which would so readily accord with the power of a force acting from beneath upwards, so that if this was exerted in the centre of the group of Santorin, and a fissure extended northerly through the deep channel there presenting itself, there appears no mechanical difficulty in supposing a somewhat yielding covering, rendered so, in a great measure, by heat, opening outwards in such a manner that the chief fissure would suffice for any required sepa- ration of parts, a certain amount of cohesion still remaining. It is needful, in estimating the effects which would be produced in this, or in a somewhat similar manner, to regard the whole mass with reference to proportion and real sections,* so that no undue value should be attached to heights or distances ; and also to the masses of limestone of Mount Elias and the hill on the north of it, a range of that limestone on the eastern side of the island (marked by straight lines on the map, fig. 133) having to be taken into ac- count. It is also easy to conceive indeed, the variable intensities of different eruptions from the same volcanic vent point to the fact that the action which at one time may elevate a considerable mass, may at another, and after time, be unable to cause more than a central movement in a volcanic vent.t Professor Edward Forbes and Captain Spratt, R.N., who visited Santorin in 1841, state that " the aspect of the bay is that of a great crater filled with water, Thera and Therasia forming its * Those constructed with the same scale for heights and distances. t When noticing the uprise of ground and eruptions witnessed at Santorin by the Father Goree, in 1707 and 1708, Dr. Daubeny calls attention, as important to the natural history of volcanos, "that in this case, as in many others, the mountain appears to have been elevated before the crater existed, or gaseous matters were thrown out. According to Bourguignori, smoke was not observed till 26 days after the appearance of the raised rocks." " Description of Volcanos," p. 322. 394 RAISED FOSSILIFEROUS TUFF OF SANTORIN. [On. XX. walls, and the other islands being after-productions in its centre."* In the Little Kaimeni they found the elevated sea-bottom, formed of fine pumiceous ash, to be fossiliferous.-J- They were informed that similar beds of shells* were found on the cliffs of Santorin itself. " In the main island, the volcanic strata abut against the limestone mass of Mount St. Elias, in such a way as to lead to the inference, that they were deposited on a sea-bottom, on which the present mountain rose as a submarine mass of rock."| The following view (fig, 134), kindly communicated by Captain Spratt, K.N., is highly illustrative of the general appearance of the interior of the Santorin group, of the position of the central islets, and of the kind of stratification which occurs around the central opening. In a group of this kind, independently of any eruptions through the central cavity or crater, which it would appear have taken place even in later historic times, breaker-action upon the interior cliffs, upon the softer substances especially, would tend to degrade them, and deposit the detritus, so derived, in the central depres- sion, a deep cavity in a tideless sea, as the Mediterranean may be considered, as far as regards geological effects. In like manner, also, heavy seas rolling over the gap facing the south-west, between Therasia and Cape Akroteri (Santorin), where Aspro Island rises above the shallow bank connecting the chief islands (the brim of the volcanic basin, slightly covered with water), tend to force in detrital matter. Deposits at the bottom of the central cavity, varying in depth from 960 to 1,278 feet in its curved passage round the Kaimeni, would seem well at rest, except as regards upheaval or depression from volcanic action there prevailing. In cases where animal life might become extensively destroyed by the boiling of the waters, a ready supply of the germs, even of * In a letter from Professor E. Forbes to Dr. Daubeny, quoted by the latter in his ' Description of Volcanos," 2nd edition, p. 324, 1848. t Professor E. Forbes informs me that the following shells were there obtained: Pectunctulus pilosuSj Area tetragona, Cardita trapezia, Trochvs ziziphinus, T.fanulum T. exiguus, T. Coutourii, Turbo rugosus, T. sanguineus, Phasianella pulla, Turritella, tricostata, Neara cuspidata, Cerithium Lima, Pleurotoma gracile. J " My own impression is," adds Professor E. Forbes, " that this group of islands constitutes a crater of elevation, of which the outer ones are the remains of the walls, whilst the central group is of later origin, and consists partly of upheaved sea- bottoms and partly of erupted matter, erupted, however, beneath the surface of the water." ( Commencing with the southern point of Therasia, the soundings show this submerged brim of the crater to be successively 42, 36, 54, 66, 54, and 42 feet (depths at which wind-wave action would cause much disturbance on the bottom, especially during heavy gales), a point, marked as a sunken rock, rising higher between the south end of Therasia and Aspro Island. CH. XX.] SANTORIN GROUP FROM THE NORTHWARD. /,< '/! I 395 Mm /i mJl-j H 9n .1 J1-.0 /7 (I 01 I 402 MINERAL COMPOSITION OF BASALT. [Cn. XX. Greece, when including its islands. In Asia Minor, extinct volcanos are found, and more especially in the wide district of the Katakekaumene.* With respect to the Holy Land, the destruction of Sodom and Gomorrah has been attributed to volcanic eruptions, and volcanic accumulations are elsewhere noticed in the same land, and in Persia, and its adjoining countries. Doubtless, also, many other regions, not yet explored by the geologist, will be found to present similar accumulations, and indeed they have been noticed in the great continent of America. In various parts of the world, as well in regions where lava streams intermingled with ash and cinders, either piled up conically, or more evenly distributed, are not apparent, as in those where active or extinct volcanos exist, certain rocks are found to which the name basalt has been given. In the application of this name, care has not always been taken to distinguish the same compound considered chemically and mineralogically, so that in the matter of fusibility alone, substances so termed differ somewhat materially, f Fine varieties of greenstone (diabase), consisting of orthoclase and hornblende have as often been termed basalt, as those of labradorite and augite (dolerite). If, with M. Eose, hornblende and augite be considered only modifications of the same mineral, this would leave the difference of these two varieties of basalt to consist in that of the two felspars. The basalt of the Mont Dor has been stated to contain both the augite and hornblende forms of this mineral. Basalt has again been supposed essentially to consist of augite, magnetic iron, and a mineral of the zeolitic family .J The un- * Messrs. Hamilton and Strickland (Geol. Trans., 2nd series, vol. vi., 1841, and " Travels in Asia Minor," 1842, by the former geologist) consider the volcanic products of the Katakekaumene, as referable to three periods. The volcanic accumulations of the last period are as fresh as amid active vents, the ashes and scoriae still loose and piled up as after immediate ejection, the lava streams rugged, a few straggling plants alone finding fitting conditions for their growth. t During the experiments on the fusibility of rocks, to which allusion has been above made, we found marked differences in that of the so-termed basalts. Allowing for changes by the different conditions under which substances, originally similar, may have been placed, so that while one may have been deprived of certain substances, another may have mineral matter added to it, there were still evidently original differences. It has been stated by De Saussure (Journal de Physique), that basalt melts at 76 of Wedgwood. The experiments of Sir James Hall (Trans. Royal Soc. of Edinburgh, vol. v.) went to show that whinstone, or basalt, as it has been called, from the vicinity of Edinburgh, became soft at a temperature from 28 to 55 Wedgwood, a heat, as Dr. Daubeny remarks (Description of Volcanos, p. 616), inferior to that of a common glass-house. J Referring to the composition of this zeolitic mineral, Dr. Daubeny observes (Description of Volcanos, p. 18), that it " is such as to imply that it may have been formed out of labradorite by the addition of water, the presence of which in all zeolites is the cause of the bubbling up under the blowpipe, which has occasioned them to be distinguished by that general appellation." Following out this view, it seems highly CH. XX.] CHEMICAL COMPOSITION OF BASALT. 403 certainty in the employment of the term basalt, would appear to require attention. Thus the rocks which encircle the Peak of Teneriffe, and usually noticed under that head, are referred by Dr. Abich to his class of trachyte-dolerites. While endeavouring to trace the sources whence certain igneous rocks may have been obtained, even sometimes with reference to the melting of masses which may have been accumulated by means of water, or have been intermingled with such deposits, mineralogical and chemical distinctions, as far as they can fairly be carried out, would appear very desirable.* While at times sheets of basalt cover exten- sive areas, at others they are mingled with ordinary volcanic products, apparently, therefore, ejected under dissimilar conditions. Basalt is sometimes highly vesicular, at others very compact ; these modes of occurrence are observable over areas of different extent, both considerable and limited. As regards the relative antiquity of basalt, we find it noticed as well among the ancient as the more modern volcanic products of central France, and among the more modern of the Vivarais in the south of France, as also in those of the EifeLf It is noticed as intermingled among the ancient volcanic rocks of the Siebenge- desirable to consider how far a change may be brought about in a compound of augite, magnetic iron, and labradorite, so that the latter became modified by water after ejection. The vesicles of basalts, as, for example, those of the north of Ireland, are often filled with zeolitic minerals, the results of infiltrations into them, quite as much as agates, &c., also found amid the same rocks. In fact, incer tain districts, the vesicles are filled with a variety of substances, the zeolites forming only a part of them. * As respects the chemical composition of basalt, including that of Teneriffe (trachyte-dolerite of Dr. Abich), the following table of basalt, from Saxony, (1), by Mr. Phillips, from Banlieu (2), by M. Beaudant, and from Teneriffe (3), by Dr. Abich, may be useful : 1 2 3 Silica 44-50 59-5 57-76 Alumina 16-75 11-5 17-56 Protoxide of iron . Peroxide of iron. . . Oxide of manganese Lime 20-00 6 '12 9-50 19-7 0-5 1-3 4-64 2-09 0-82 5-46 2-25 2-76 Potash 1-6 1-42 Soda 2-60 5-9 6-82 Chlorine Water 2-00 0-30 f On this point, Dr. Dauberiy remarks (Description of Volcanos, p. 42), when mentioning the occurrence of basalt with the fresh-water limestones, near Clermont, and the proof by M. Elie de Beaumont of this basalt forming dykes amid the fresh- water formation of the Limagne (Memoires pour servir, &c., torn, i.), that while it occasionally underlies the trachyte and subjacent tuffs of the districts, "its general relation to both these rocks indicates that it is of more modern eruption." ' 404 GLOBULAR STRUCTURE OF BASALT. [Cn. XX. birge, as also of later date in the same district. Basalt is described as among the ancient igneous rocks of Iceland. It occurs in many parts of the world where its relative date is not so apparent, some- times forming the isolated caps of hills, and resting upon other rocks, in a manner pointing to the considerable or partial destruc- tion of some great sheet of this rock. This has been supposed the case with the basaltic hills in parts of Germany. The largest area occupied by basalt seems to be in India, where rocks of this class appear to occupy one of 200,000 square miles.* With respect to this rock, a fine exhibition of it is found in the north of Ireland, where the Giant's Causeway and the adjacent country have long attracted attention. Though on a much smaller scale, the island of Staffa, Hebrides, has also long been equally celebrated for its basalt. In the north of Ireland, its eruption was posterior to the formation of the chalk of the same district, but the portion of the tertiary period to which this should be referred is not clear. Though by no means confined, among igneous rocks, to basalt, the spherical and columnar structures often developed in that rock have also long attracted much attention. The minor spherical structure seen on the small scale in some volcanic rocks, and also in artificial glass, and which has been previously noticed, would appear to have been produced on the larger scale, under certain conditions, in basalts. Sometimes this globular structure, as shown Fig. 13 during the decomposition of the rock, is irregular, so that the whole has the appear- ance of balls of various dimensions piled up without much order (fig. 137) ; at others, a great order prevails, and the concretions are either roughly arranged above one another in wide spheroidal shapes, or so pressed against each other as to produce prisms, sometimes of very symmetrical forms. In 1804, Mr. Gregory Watt showed by his experiments on basalt, that when, in the cooling of a molten mass of that rock, this structure was developed, and " two spheroids came into contact, no penetration ensued, but the two bodies became mutually com- pressed and separated by a plane, well defined and invested with a rusty colour," and he observed, when several spheroids met, that they formed prisms, f * Lieut.-Colonel Sykes (Geological Transactions, 2nd series, vol. iv., p. 409) observes, that in the Dukhun there are proofs of a continuous trap formation, covering an area of from 200,000 to 250,000 square miles. f Observations on Basalt, and on the transition from the vitreous to the stony texture which occurs in the gradual refrigeration of melted basalt, Phil. Trans. 1804. CH. XX.] COLUMNAR STRUCTURE OF BASALT. 405 From the arrangement observed by Mr. Gregory Watt, he in- ferred that " in a stratum composed of an indefinite number, in superficial extent, but only one in height, of impenetrable spheroids, with nearly equidistant centres, if their peripheries could come in contact on the same plane, it seems obvious that their mutual action would form them into hexagons ; and if these were resisted below, and there was no opposing cause above them, it seems equally clear that they would extend their dimensions upwards, and thus form hexagonal prisms, whose length might be indefinitely greater than their diameters. The further the extremities of the radii were removed from the centre, the nearer would their approach be to parallelism ; and the structure would be finally propagated by nearly parallel fibres, still keeping, within the limits of the hex- agonal prism with which their incipient formation commenced; and the prisms might thus shoot to an indefinite length into the undisturbed central mass of the fluid, till their structure was deranged by the superior influence of a counteracting cause." It will require the careful study of this class of rocks, more particularly in a decomposed state, for the observer to ascertain the extent to which the view of Mr. Gregory Watt may be applicable. Where one plane of a sheet of basalt may have been exposed to cooling influences, so that the spheroidal structure could be first developed in it, and in the manner suggested, and also so that no other spheroidal bodies could be developed in the general body of the rock, and thus interfere with the extension of the original spheroid, there would not appear much difficulty in following this view. In those basaltic dykes that are sufficiently common in some districts, where we may suppose that the walls of the fissure, which had been filled by the *ig. molten rock, presented equal cooling conditions, we some- times see, as in the subjoined section (fig. 138), that the prisms shoot out at right angles to the walls of the containing / rock (5c), as if each set commenced at the sides (d and e), confusion arising at the central portion (#/)* In cases, also, when * It sometimes happens that the central portions of a basaltic dyke are more prismatic than the sides, as if the cooling had been too rapid at the sides for the pro- duction of this arrangement of parts. Again, the prisms are sometimes found ranging from wall to wall of the fissure, as if artificially-cut prismatic blocks of rock had been piled in it on their sides. 406 JOINTED COLUMNS OF BASALT. [Cn. XX. not a trace of joints can be observed, as in the annexed section (fig. 139), where the columns ( 7 lowei down, at 1798 feet.* With respect to the temperature obtained in artesian wells, it is desirable that the mode of occurrence of the rocks of the district in which they may be situated should be carefully considered, so that the rise of thermal springs beneath the mineral accumulations traversed may not complicate the heat found. As, for example, should it happen that in a country affording a section similar to that beneath (fig. 160), a series of nearly horizontal deposits b, c, Fig. 160. rests upon a previously-disturbed assemblage of beds, d, traversed by faults, e and /, formed prior to the accumulation of the upper beds, b, c, and that thermal waters rise through these faults, as they often do, and as previously noticed (p. 22), the ordinary supply of rain water entering at g, and passing to a lower porous bed c, the temperature of the earth at any artesian boring, situate at h, might, to a certain extent, be obtained, while another well sunk at i, being immediately near a supply of thermal water through the fault e, would give a more elevated temperature, the higher in * It is calculated that, taking the constant temperature (53) of the caves of the Paris Observatory, 91| feet beneath the surface, these temperatures would give an increase in heat at the rate of 1 centigrade (l-8 Fahrenheit), for 32-3 metres, nearly 106 English feet (105 feet 11-659 inches). 2H 466 VARIABLE TEMPERATURES ARISING FROM THE [Cn. XXV. proportion to the volume and velocity (p. 22) with which the previously -suppressed ready out flow can nowmore freely find vent. This is by no means an unnecessary circumstance to be regarded, since in districts such as that of the neighbourhood of Bath, there is evidence of faults, some of them of considerable size, having been formed anterior to the accumulation of the new red sandstone and oolitic series of that country, the surface of the fractured and con- torted rocks being covered by the nearly horizontal beds of these deposits, and as the thermal waters of Bath (116 Fahrenheit) appear to rise through one of the old fissures, a ready vent for them occurring through the superincumbent beds, as at I (fig. 160). The observer would do well to search in such suspected districts for the temperature of waters pouring abundantly through any faults as at k. And it is worthy of remark, that in the district above noticed, a thermal spring (temp. 74) appears among the older broken and disturbed rocks at the Hotwells, Bristol. Eegarding the temperature found at different depths in artesian wells, and the variations sometimes observed therein, it will have to be borne in mind that the different seams of rock whence water may be obtained, though not in sufficient abundance for the supply sought, will, from any different porosity in them, only permit waters to permeate or flow through them in such a manner that a given quantity can pass through each in a given time, thus influencing the circulation of any heat which they may carry with them from one part of a series of beds to another. If the following section (fig. 161) represent that of certain beds of rock traversed in sinking an artesian well, a, a being a clay, such as the London clay; b a porous bed of sand and gravel, gathering surface waters at c; d chalk; e sands receiving surface waters from /; g clay or marl, and i other sands or gravel gathering surface waters at h, we have very different porosities of the beds Fig. 161. which can permit water to pass somewhat freely through them. Upon perforating through these beds, as at m, their relative per- meability to water would influence the temperature in such an artesian well, a highly-porous bed, b, carrying its surface waters more readily to the well, to rise through it, than the chalk, d, CH. XXV.] UNEQUAL PERCOLATION OF WATER THROUGH ROCKS. 467 beneath ; as would probably also happen with the sands lower down at e. In all cases of this kind, an observer has to allow for the friction of the water through, and capillary action in the rock, which can only permit the circulation of this water according to needful conditions, so that it can only be delivered into the artesian well at a certain rate in each case.* In the section (fig. 161), a smaller well is represented as sunk at n, to the sands, e f, and on the bend of the beds, where, in consequence, the heated waters are more able freely to ascend, and be replaced by heavier and colder water, always supposing the whole to have a temperature above 40 ' Fahrenheit. In this case it might be inferred that, from the greater facility of percolation from the surface, /, to the bottom of the well, n, on the one side, the water would produce a lower tem- perature in the rock through which it passed, than in the same bed of rock at m. Ato and p, part of the curves of two porous, inter- stratified with less permeable beds, k I, are represented, (forming thus, as it were, flat pipes,) for the purpose of showing that, if the curve were continued downwards on the left, water percolating in them, heated beneath, and not easily escaping upwards, might possess a somewhat higher temperature than at the same depths from the surface in the adjoining beds, (supposed, for illustration, to be equally porous,) not having the same facilities offered for obtaining, by circulation according to temperature and densities, colder waters from above. To whatever extent water, permeating amid rocks, may modify their temperature, the greatest density of water will have its influence. In all regions where the annual temperature is such as to exceed that of the greatest density, however the surface, and corresponding depths beneath, may be acted upon, after 60 or 80 feet, the hotter waters would tend to rise and the colder to descend. In those, however, where the temperature is such that the water takes a contrary course, instead of cool waters descending to modify any heat which the containing rock might otherwise possess, they would ascend. For example, in the Siberian shaft (p. 291), descending beneath the 378 feet at which a temperature of 31*1 was obtained, and allowing the same rate of increase as was found * It is often practically found, in borings for common wells, that the relative porosity of the rock or rocks traversed, and the consequent possible delivery of water into them, have not been sufficiently regarded. Though certain loose sands and gravels may afford a volume of water considered most abundant, so far as the supply sought is regarded, rocks generally are but filters of various degrees of porosity, and only capable of permitting water to pass through them in a given quantity and time. 2n2 468 VARIABLE TEMPERATURES ARISING FROM THE [Cn. XXV. from the depth of 75 feet, namely, about 1 (Fahrenheit) for each 30 feet,* it would only be at a depth of about 630 feet that water at 39-5 (Fahrenheit) would be found. As to the springs which rise from fissures, such as those above mentioned (p. 465), a lower temperature, without due precaution, will often be obtained than should be assigned them, even sup- posing that the waters, as they flow upwards from various depths, lose much of their original temperature, and acquire that of the rocks amid which they rise.f The heat of ordinary springs has also to be carefully considered with reference to the kind and mode of occurrence of the hard rocks or less coherent accumulations of matter whence they issue. If we suppose a in the following section (fig. 162) to be a porous sandstone, resting upon a bed of t clay, b b', the rain-waters, absorbed by the former, are prevented from permeating downwards by the latter, so that the water not retained amid the sandstone, issues as springs, on the side of the hill, at the top of the subjacent clay. Should another sandstone, or any other rock, through which water may readily percolate, c c\ occur beneath the clay, this porous stratum also based upon an impervous bed, d d' 9 the atmospheric waters, with any water derived from the springs above and absorbed by the lower porous rock, could alone find a natural outflow, as springs, on the side of the hill d t; while in the opposite direction, d' t', they would saturate that portion of the bed, laterally aiding, by their super- abundance, if we infer the needful facility of passage, the springs between c d. The dotted line t t', representing any depth be- neath which an uniform temperature is preserved throughout the year, should water percolate slowly to the surface, there would be all other things being equal a tendency between a and #, and c and d, on the one side, and a and b' on the other, to have springs issue with nearly equal temperatures. At w, also, if a well be sunk, the temperature of the water being within the depth of * This rate of increase of temperature very nearly coincides wi(h that obtained at Grenelle, namely, l-8 (Fahrenheit) for 53 feet. t It is commonly needful to clear away the ground, so that a thermometer may be plunged in the water where it rises amid the rocks themselves. And this is especially necessary when the volume of water is far from considerable, and flows away slowly. Those thermometers in which the bulb and a portion of the glass project beyond the graduated scales, when handled carefully, will be found the most useful instruments. CH. XXV.] UNEQUAL PERCOLATION OF WATER THROUGH ROCKS. 469 variable temperature, we should expect it to be much of the same kind, the supply being derived laterally from the same reservoir which supplied the springs between c and d, and the impervous bed d (impervious so far as regards the ready passage of water through it) preventing appreciable communication with, and cir- culation of, waters of a higher temperature beneath. Should waters find their way, as springs, by means of joints or fissures, from the reservoirs in both porous beds, a and c (?, beneath the line of variable temperature, more rapidly in some places than in others or the beds themselves differ materially in the facility with which water can pass through variations may be expected, im- portant or not, according to circumstances, in the temperature of springs issuing from them. All other things being equal, the lower reservoir assuming that the temperature increases from the sur- face downwards would be expected to supply the water with the more elevated temperature. It becomes needful, therefore, that after other conditions have been ascertained, the quantity of water delivered by a spring in a given time, and the rapidity with which it flows, should be duly regarded. With respect to the temperatures of those waters which, in lime- stone districts especially, rush out, often in considerable volume and with much force, from subterranean channels, and which result from the loss of many minor streams and of rain-water amid fissures and cavernous rocks, they may be often' very deceptive. Should the waters have been absorbed partly as streams, previously ex- posed to the temperature of the climate of the region, and partly derived from slow percolation through chinks, joints, and the minor cavernous structure of the rock, a mixed heat would 'follow, afford- ing no correct data as to the temperature of the subterranean chan- nels through which the waters have passed. When, also, the whole is derived from the absorption of atmospheric waters by channels of various kinds, the rapidity of passage of the waters downwards to j the great drainage stream, and the differences in this respect have to be considered, as also the chances, not uncommon in some dis- tricts, that great fissure waters, derived from considerable depths, may not be mingled with the general volume of those discharged. Hence much care is required in investigating the temperatures of waters thus discharged, however desirable it may be that they should be properly ascertained. While on the one hand the observer has to regard the adjustment of water, permeating amid the fissures and joints, or the mass of rocks, to its greatest density, and the variable mechanical manner. 470 TEMPERATURE OF WATERS RISING IN FISSURES. [Cn. XXV. in which this may be effected, he has also to consider the depths at which water itself may cease to exist ; assuming the increase of temperature from the surface downwards, whatever its rate, locally or generally, to be certain, as the general evidence would lead us to believe. Should it be inferred that the rate of increase of heat usually supposed probable, namely 1 Fahrenheit for each 50 to 60 feet, is too great, and that sufficient information as to this rate has not yet been obtained, if we take only 1 for every 100 feet, we still seem to obtain a comparatively minor depth, allowing for increase of pressure from the superincumbent water, with the friction on the sides of any fissures, for that portion of the earth's crust in which water may be considered to circulate under the most favourable con- ditions. Taking the ordinary mode of calculation, allowing for pres- sure at increased depths, and assuming every facility of movement of the waters in a fissure, it may be estimated that at a comparatively moderate depth steam would be found instead of water. Waters in fissures, rushing upwards with a rapid rate of outflow and in considerable volume may (as noticed p. 19) bring with them a greater temperature than those finding their way upwards with less velocity and in smaller quantity, the one heating the waters com- municating with them laterally in their course upwards, beyond the temperature due to the containing rocks themselves, and the ordinary percolation of water through them ; the others being cooled by these lateral waters. In certain districts, such as those where volcanic fires have once found vent, and which may be now concealed by various overspreading aqueous accumulations, there may be influences of this kind much modifying the exact depths at which certain temperatures would otherwise be found. No doubt, supposing a general 'source of heat to exist in the earth governing the outer temperature of its crust on the great scale, these would be merely local variations, yet, when endeavouring to ascertain the distribution of heat in the globe, all such variations require attention, so that the disturbing circumstances may be duly separated from the essential causes of the increase of heat downwards from its surface. CHAPTER XXVI. MODE OF ACCUMULATION OF DETRITAL AND FOSSILIFEROUS ROCKS. DETRITAL ROCKS CHIEFLY OLD SEA-BOTTOMS. MIXTURE OF BEDS WITH AND WITHOUT FOSSILS. VARIABLE MODE OF OCCURRENCE OF ORGANIC REMAINS. SEA- BEACHES OF THE SILURIAN AND DEVONIAN PERIODS OF THE NEW RED SANDSTONE TIME, MENDIP HILLS, ETC. OF THE LIAS PERIOD. VARIED MODES OF OCCURRENCE OF THE LIAS. BORING MOLLUSCS OF THE IN- FERIOR OOLITE TIME. OVERLAP OF INFERIOR OOLITE, MENDIP HILLS. LIAS CONGLOMERATE PIERCED BY BORING MOLLUSCS. LAND OF THE TIME OF THE LIAS. EVIDENCE OF LAND FROM FRESH-WATER DEPOSITS. EF- FECTS PRODUCED ON COASTS, RIVERS, AND LAKES, BY CONTINUED ELEVATION OF LAND ABOVE SEA. ELEVATION OF LAND OVER A WIDE AREA. EFFECTS OF CLOSING THE STRAITS OF GIBRALTAR. UNEQUAL ELEVATION OF ! LAND. LAKES ON THE OUTSKIRTS OF MOUNTAINS. MIXTURE OF ORGANIC REMAINS OF DIFFERENT PERIODS FROM SUBMERGENCE OF LAND. VARI- ABLE EFFECTS OF SURMERGENCE OF PRESENT DRY LAND. THE observer, having well considered the manner in which the accumulations of mineral matter are at present effected, chemically and mechanically, through the agency of water, as also the mode in which the remains of animal and vegetable life may be entombed amid such accumulations, has to study the various layers, beds, or other forms of mineral substances formed by aqueous means, and in which organic remains are more or less distributed in various parts of the world. In one respect he has an advantage over his previous investigations, inasmuch as while he could then often only infer that which takes place beneath seas and lakes, he has in these rocks frequent opportunities of obtaining direct evidence of that which actually occurred beneath them, the large proportion of these beds being the bottoms of various sea3 ?or bodies of fresh water, deposited over each other, and subjected to variation from local causes. Inasmuch as the dry land of the world is thus little else than the bottoms of seas and lakes, intermixed with igneous matter vomited upwards at different times from beneath the surface of the earth, some of the latter spread at once on this surface, at other times only laid bare by the removal of superincumbent deposits, the 472 DETR1TAL ROCKS CHIEFLY OLD SEA-BOTTOMS. [Cn. XXVI. observer will have to dismiss from his mind the existing dry lands and waters of the world and substitute such other distributions of them as may best accord with the evidence which, from time to time, he will obtain. No matter how highly raised into mountains, or slightly elevated in plains, these ancient bottoms of oceans, seas, and bodies of fresh water may now be, they did not constitute dry land when formed, and consequently /waters once occupied the areas where they now occur. We have seen that, to produce detrital ac- cumulations, certain conditions of dry land are needed, whence their component parts have to be derived; and, therefore, to form the ancient sea-bottoms of any given time, dry land appears required out of an area so circumstanced, and yet so near to it as to afford the' materials found. Considerations of this kind demand an en- ,1 larged view of the physical geography of different geological times, and such a disregard of the existing distribution of land and water that while all due, weight is allowed for the employment of a given amount of mineral m after, -over certain large areas, in the produc- tion of detrital accumulation^ of different dates the wearing away of one portion raised above- the ocean presenting materials for an .equal and subsequent deposit beneath it'iri an adjacent situation; and consequently, that oscillations in the relative levels of the ex- isting areas of our present continents may keep such matter much in one large area, the mind of the' observer must not be too much occupied by the presefrt arrangements of land and water "on the surface of the Garth. While evidence is sought amid detrital or fessiliferous accumu- lations, of the mode iivwhich the mineral, matter of rocks has been chejnically or mechanically gathered together, and the observer /^endeavours to trace among them former beaches, estuaries, bays, promontories, shallow and deep seas, fresh- water lakes, and the other modifications of water around and- amid dry land, he has at the same time most carefully to study the mode^ of occurrence of any organic remains found in these- accumulations. ' He will have to see if there be evidence that the animals or plants lived and died in or upon the beds where their remains are now found ; or whether, after death, such remains were drifted into these situa- tions. He will also have most carefully to refer to the distribution of the animals and plants existing at any given geological time, according to conditions, regarding that distribution as well on the large scale as with respect to any minor area. With respect to the class of rocks usually named fossiliferous, this term has to be regarded in an extended sense. It is by no CH. XXVI.] MIXED BEDS WITH AND WITHOUT FOSSSILS. 473 means required that the various beds composing any given series of sea-bottoms, should all contain organic remains in certain localities. Frequently, as in the subjoined sketch (fig. 163), representing a series of beds of rock, abed and e, exposed on a cliff, one of them only, such as d, 'may contain them, the others Fig. 163. ! not affording any animal or vegetable exuviae. Thqse beds are not, however, the less interesting on that account, inasmuch as ome cause for this difference may present itself , by .diligent investigation, of importance as bearing- upon the conditions'; dr .their modifica,- tions, under which .the whole series * may . have Been formed. Should the beds be of different Substances as, for example, should a b and e be formed of sands .df different kinds consolidated, as ' hereafter to be noticed, into "sandstoaes ; , 6, conglomerate of pebbles derived from the subjacent or adjoining rocks, cemented by magnesio-calcareous matter; c, red marls; d, d, line showing how denudation might cause successive accumulations to appear as of one time. sections exhibit the beaches, usually composed of shingles of car- boniferous limestone, now cemented by magnesio-calcareous matter, jutting, as it were, into the red mud of the time, (now red marls,) having extended over one portion of it, during the submergence, and having been covered by another as this proceeded. One section (fig. 165) shows only the accumulation of the red mud (marl), while the other (fig. 166) exhibits a subsequently-formed deposit of dark mud, sometimes calcareous, alternating with an argillaceous limestone, together known as the lias. In the red Fig. 166. a, a, beds of disturbed carboniferous limestone ; 6, b, conglomerate of pebbles derived from the subjacent or adjoining rocks, cemented by magnesio-calcareous matter ; c, red marl ; d, lias ; B, Blaize Castle Hill, near Bristol ; s, Mount Skitham. mud no traces of a marine organic remain have been detected in that district, though more northerly streaks of them are found ; but even supposing some rare organic remains may hereafter be discovered, there is still evidence of a beach resting on a coast, this beach thrown up by seas during a period when the dry land was becoming gradually submerged, and a change was effecting in the existing conditions, so that the adjacent sea was no longer without animal life, or at least only contained a small portion of that affording harder parts for preservation amid the deposits of the time, but swarmed with molluscs, fish, and reptiles. The manner in which the filling up was effected is well shown in the section near Blaize Castle (fig. 166), though evidence of this kind is to be found as well elsewhere ; one patch of lias (d, on the right of the figure) nearly reaching over the old beach, as it actually does at no great distance on the westward, where it covers up the carboniferous limestone on the margin of the coal-field from 478 MENDIP HILLS AND ADJOINING DISTRICT. [Cn. XXVI. Fig. 167. 1. Old Bed Sandstone. 2. Carboniferous Limestone. 3. Coal Measures. 4. Dolomitic Conglomerate.. 5. New Red JVIarl and Sand- stone. 6. Lias 1. Inferior Oolite. 8. Alluvial. CH. XXVI.] MENDIP HILLS AND ADJOINING DISTRICT. 479 Redland, near Bristol, to Alverston, on the north. It will be perceived that if the denuding causes, which have removed so much of the deposits of a later geological date than these old shingle beaches, had carried off all traces of them in the section near Compton Martin, Mendip Hills (fig. 165), so that a surface corresponding with the line d d, had only been exposed, there would have been great difficulty in assigning the different parts of this shingle (now conglomerate) covering to their relative geological dates ; though, with the old mud and sands outside of them, deposited at successive times, the relative date of the parts is sufficiently obvious. While on the subject of this district, it may not be uninstructive, as it is one fertile in information, within so very small an area, to call attention to the successive coatings of fossiliferous accu- mulations as they followed one another, each spreading over a part of a preceding deposit, as the dry land of the Mendip Hills and adjacent country sank, and as it would appear, gradually, beneath the sea. For this purpose the accompanying map (fig. 167) may be useful. In it the different deposits represented consist, in the ascending series, of (1) old red sandstone ; (2) car- boniferous or mountain limestone ; (3) coal measures ; (4) dolomitic or calcareo-magnesian conglomerate and limestone ; (5) the new red sandstone and marl ; (6) lias ; (7) inferior oolite, and others of the lower part of the series, known as the oolitic or Jurassic ; and (8) alluvial accumulations, deposits from branches of the adjacent British Channel, where these found their way amid the sinuosities of the land, often covering a plain whereon forests once grew, at a higher relative level of sea and land than now exists, the outcrops of these sheets of concealed vegetable matter and trees forming the " submarine forests" of Stolford and other places on the present coast (p. 448).* The darkly-dotted patches in the map (conglomerates, 4) will serve to show the mode of occurrence of the beaches surrounding * The names of the various places marked by crosses and letters in the map (fig. 167) are as follow: a, Tickenham; 6, Nailsea ; c, Chelvey; d, Brockley ; c, Kingston Seymour ; /, Wrington ; ^, Nempnet ; A, Congresbury ; /, Banwell ; m, Locking ; n, Bleadon ; o, Lympsham ; />, Burington ; q, Compton Martin ; r, Hinton Blewet ; s, East Harptree ; , Lilton ; v, Chew Stoke ; ar, Chew Magna ; y, Stowey ; a', Shipham ; &', Biddesham ; c', Badgworth ; d', Weare ; e', Axbridge ; /', Chapel Allerton; g', Chedder; h', Priddy ; z", Binegar; ', Chewton Mendip; Z', Wedmore ; m', Radstock ; n', Kilmersdon ; o', Draycot ; />', Stoke Rodney ; q f , Westbury ; r", Wookey; s', Dinder; *', Crosscombe ; t/, North Wooton; w/, Wells; x, Shepton Mallet; y', Downhead; z', Mells; a", Elm; b", Whatley ; c", Nunney ; d", Cloford ; e", East Cranmore ; and/", Chesterblade. 480 ANCIENT BEACHES NEAR CLIFTON, BRISTOL. [Cn. XXVI. the older rocks of the Mendip Hills, and an adjoining portion of country near Wrington, /. Although, from the travelling upwards of continuous portions of these beaches during the gradual sub- mergence of the dry land, and the subsequent wearing of the rocks, including all in the district, up to the time of its alluvial plains inclusive, they may not give the exact representation of the beaches of one time, they will still serve to show the manner in which they were accumulated round this old portion of dry land. Taken in connexion with similar facts observable even so near as Gloucestershire and Glamorganshire,* and looking at the size of the rounded fragments sometimes found in them, the effects of considerable breaker action is observable on the shingles, and they seem to have been well piled up at the bottom of old bays and other localities where favourable conditions for their production existed. The following section (fig. 168) will show one of these a, a, limestone, intermingled with sandstones and marls, of the upper part of the carboniferous limestone series of the district, brought in by a large fault, on the N.W. of the Windmill Hill, Clifton ; 6, boulders and pebbles, in part subangular, of the sub- jacent rocks, cemented by matter in part catcareo-magnesian, variably consolidated ; c, conglomerate or breccia, in which the magnesio -calcareous matter is more abundant, becoming more so at rf, where it further assumes the character of the more pure dolomitic limestone in which pebbles and fragments do not occur. ancient beaches facing the gorge of the Avon, near Clifton, Bristol, in a depression between Durdham Down and Clifton Hill, in which some of the rounded portions of the subjacent rock cannot be much less than two tons in weight, requiring no slight force of breaker action to move them and heap them up as now seen. The submergence of this dry land continuing while geological changes were being effected over a wide area, (in which this district occurred as a mere point,) and so that, without reference to the modifications of deposits produced elsewhere, the red sedi- ment of the seas near the shores of the land, then above water in the area of the British Islands, was succeeded by others in and above which animal life swarmed, the beaches moved upwards on See the Geological Map, " Memoirs of the Geological Survey of Great Britain," vol. i., pi. 2, in which a large area occupied by accumulations of this class and time will be found represented. CH. XXVI.] LIAS NEAR SHEPTON MALLET, SOMERSETSHIRE. 481 the slopes of adjacent rocks. Thus the rolled pebbles of the latter, and of the cliffs of the time, were occasionally intermingled with the remains of the animal life then existing. Near Shepton Mallet (#', in the map, fig. 167), where the lias (6) rests both on the old red sandstone (1), and the carboniferous limestone (2), there is much of this old shingle (now conglomerate).* The following (fig. 169) is a section, exhibited close to Shepton Mallet, on the Bath road, wherein a line of pebbles (b) is strewed over the previously upturned edges of supporting carboniferous limestone (a, a), and constitutes a continuation of some more arenaceous and pebble beds, presenting much the appearance of a shore, not far distant, f Fig. 169. d d y d The lias at c, covering this pebble or shingle bed, has been thrown down (as it is termed) by a dislocation, or fault /, so that beds above that at c, are seen at d, d, d, the latter again broken through by a dislocation at g, and the whole surface of the hill being so smoothed off by denuding causes, that a gently-sloping plane is alone seen. Before we quit this section, it may be mentioned, that an observer in search of the different conditions under which fossi- liferous deposits may have accumulated will here see that much less mud must have been mixed with the calcareous matter of the lias than is usual in the district, and which is to be found not far distant from this locality. The lias limestone beds (d, d, d} are here thick, for the most part, and in purity more resemble the car- boniferous limestone (a, a) on which they rest, showing a cleaner state of the sea where they were formed than in those areas over which the usual mud, and muddy and silty limestones of the lias were accumulated. Coupled with the evidence of beaches, this greater freedom from mud would seem to point to the greater proximity of a shore with minor depths of sea, near and at which the waters were generally more disturbed, so that the lighter sub- * These conglomerates, which are abundant, and wherein the pebbles are chiefly derived from the adjacent carboniferous limestone, have been long since pointed out by Dr. Buckland and the Rev. W. Conybeare (1824), " Observations on the South- western Coal District of England ;" Geological Transactions, 2nd series, vol. i., p. 294. t This was well seen further up the road, in 1845, at which time some new cuttings were in progress. 2i 482 LIAS RESTING ON DISTURBED [Cn. XXVI. stances being readily held in mechanical suspension, they were easily moved away by tides and currents to more fitting situations for deposit. This character of a less muddy condition of the lias is by no means confined to the vicinity of Shepton Mallet ; it is to be seen in several places in that part of England and South Wales. It is well shown in parts of Glamorganshire, where, indeed, as in the vicinity of Merthyr Mawr care is required not to confound some of the lias with the carboniferous limestone to which it there bears no inconsiderable mineralogical resemblance. Here, again, the observer finds this character in connexion with old conglomerates, resem- bling beach accumulations of the time of the lias, pointing to the probable proximity of dry land, such as may be readily inferred to have then existed in the great coal district on the north of it, even now, after so much abrasion, during depressions and elevations beneath and above the sea during a long lapse of geological time, rising high above these deposits. In the same neighbourhood (Dunraven) there is also good evidence of the lias reposing upon a clean surface of carboniferous limestone, as will be seen in the an- nexed sketch (fig. 171) and in the subjoined section (fig. 170), Fig. 170. wherein a represents disturbed strata of the latter, and b beds of the former, resting on their edges. In the section (fig. 170) the lower beds (b) of the lias are light-coloured, and contain fragments from the subjacent carboniferous limestone, these succeeded by argillaceous grey limestones at c. /, / are dislocations or faults, traversing the beds. In this case, though an observer might sus- pect the vicinity of a coast from the fragments in the lower lias, he would desire further evidence, and by search he would find, Fig. 172. round the point d, in the sketch (fig. 171), a conglomerate (b, b, fig. 172), reminding him "of a beach interposed, to a certain extent CARBONIFEROUS LIMESTONE. 483 2i2 484 VARIED MODES OF OCCURRENCE OF THE LIAS. [Cn. XXVI. and level, between the beds of lias (d, d) and a worn slope of 'sup- porting carboniferous limestone beds (a, a), which, here, from a local curvature are brought into a horizontal position. At c, in this section, the whitish variety of the lias of the district is found in a great measure free from muddy admixture. It is even oc- casionally dolomitic, and somewhat crystalline in this vicinity.* Returning to the minor area of the Mendip Hills for evidence respecting the dry land and shores of the locality and period, we find, as the land became more and more depressed beneath the sea, that the lias, as it were, crept up the sides of the steeper shores, accumulating more muddy matter outwards, depressions being filled up, sometimes even on the shores when sufficient tranquillity permitted such a deposit, fine sediment accumulating above fine sediment, so that there was a kind of passage of the lias into the fine red marls beneath (fig. 173).t Fig. 173. a, gray lias limestone and marls ; b, earthy whitish limestone and marls ; c, earthy white lias limestone ; d, arenaceous limestone ; e, gray marls ; #, red marls ; /t, sand- stone, with calcareous cement ; i, blue marl ; A, red marl ; I, blue marl ; and m, red marls. The observer next finds limestone beds (7), known as the in- ferior oolite, resting (map, fig. 167) from Cranmore (e"), on the south, to M ells (z?) on the north, upon various older accumula- tions ; old red sandstone (1), carboniferous limestone (2), coal measures (3), and lias (6), passing over the nearly-horizontal beds of the latter as well as the variously-curved beds of the three former. The remains of animals of marine character show that this accumulation was effected in a sea, and therefore, that the depression of the land above mentioned had continued; but, as no distinct beaches have yet' been discovered in connexion with this * Part of these lower beds of the lias of the district is known at Sutton Stone, and has been employed for architectural purposes during many centuries, being well fitted for them. t The section selected is from the vicinity of Shepton Mallet, reference to which has been previously made in the text, p. 481. CH. XXVI.] BORING MOLLUSCS OF THE INFERIOR OOLITE. 485 calcareous deposit, the probable boundaries between the sea and the land are not so apparent. The whole of the Mendip Hills may have been beneath the waters, though the relative levels of the different parts of the general masses of rock, notwithstanding the changes in these levels produced by various dislocations, effected during a long lapse of geological time, would lead us to infer that portions of dry land may still here and there have risen above the sea in that minor area. Be this as it may, when this overspread of calcareous matter (inferior oolite) took place, passing over the old margin of the lias, there were bare patches of carboniferous limestone (2) in the sea, and into these the boring animals of the time burrowed. Their remains are now found in the holes worked by them ; and when good surfaces are exposed, an observer might imagine himself walking on limestone rocks, dry at low tides, in which the lithodomous molluscs of the present day were in the cells hollowed out by them. Not only are these old surfaces thus bored by the rock-burrowing molluscs of that period (the time of the inferior oolite deposit), but here and there as, for example, near Nunney ( covered by a sandstone bed, b, is surrounded by other sandstones, c, 18 5. still standing in their places of growth, are seen above each other. Sir Charles Lyell describes ten forests of this kind, as occurring above each other, in the cliffs between Minudie and the South Joggins, at the head of the Bay of Fundy. The thickness of the mass of beds containing the upright stems is estimated at about 2,500 feet, and the usual height of the trees is from six to eight feet, but one was seen apparently 25 feet high and four feet in diameter, with a considerable bulge at the base. All these stems appeared to be of the same species.* We are indebted also to~ Mr. Logan for a very detailed account of these coal measures. In his description of the Sydney coal-field, Island of Cape Breton,f Mr. Eichard Brown notices many upright stems of plants in different beds. Among the sections given, the annexed (fig. 185) will be useful, as * Lyell, " Travels in North America," vol. ii., pp. 179-188. t Brown, " Section of the Lower Coal Measures of the Sydney Coal Field, in the Island of Cape Breton," (Quarterly Journal of the Geological Society of London, vol. vi., p. 115). After adverting to the descriptions of the coal measures of Nova Scotia by Sir Charles Lyell (Travels, &c.) and by Mr. Logan (Section of the Nova Scotia Coal Measures at the Joggins), Mr. Brown estimates the productive coal measures of Cape Breton at more than 10,000 feet in thickness. The Sydney portion, described in this communication, was, by measurement, 1,860 feet thick. The dip is mentioned as at an angle of 7. 506 TERRESTRIAL PLANTS IN SUCCESSIVE PLANES. [Cii. XXVII. showing this occurrence of many vertical stems above each other.* In it, a represents sandstones, b shales, c coal, and d the beds, usually argillo-arenaceous, in which the roots (Stigmaria) are in their positions of growth. The total thickness of the deposits amounts to 92 feet, and in it occur four planes of upright steins, (the second showing different levels of growth in it,) and six ancient soils, surmounted by as many seams or beds of coal of very different depths, the most considerable being six feet, and the least seam, one of mere carbonaceous matter, one half-inch thick. It will no doubt at once suggest itself that such accumulations of mud, silt, sand, and sometimes gravel, intermingled with layers of fossil vegetation, these layers based upon a soil, probably moist or wet, in which the roots of certain plants freely grew, while vertical stems occurred, as much sometimes as 15 or 20 feet high, and two to four feet in diameter, even planes of these old forests being found above each other in limited sections, must have been gradually submerged, so that, at intervals, the soil was sufficiently exposed to, or near the atmosphere, that the plants entombed amid them could come under their proper conditions of growth. A trough or other cavity, or slightly-inclined plane of shore, gradually rilled up to the level of the atmosphere, would only give one layer of vegetation, whereas, in some coal districts, where the seams of coal are reckoned with the soils on and in which their constituent plants grew, 50 or more intervals for growth may have to be accounted for. A submersion of the ground on which the plants flourished, so that at times the mud, silt, or sand of the time accumulated at a greater rate than this submersion could keep them beneath the level of water, or during which, though the descent of the land may have been, as a whole, constant, there were minor amounts of movement (by which after a subaqueous area had been filled up to the atmosphere, there were pauses when the plants could grow), would alike appear to explain the facts observed. The section of the 1,860 feet in which the upright stems of the Sydney beds (Cape Breton) occur, shows that there were more than 40 periods in the general descent of the mass when there were soils in which the roots (Stigmaria) of the plants of the time and locality found their needful conditions for growth, those for the accumulation of * In this section the beds are reduced to horizontally, and are on a proportional scale, the relative thickness of the beds being taken from the detailed description of them by Mr. Richard Brown (Journal of the Geological Society, vol. vi., p. 120.) CH. XXVII.] THICKNESS OF SOUTH WALES COAL MEASURES. 507 the vegetable matter above them having varied materially.* When we turn to the sections of the European coal-fields of this kind, similar evidence presents itself.! In the section of the Bristol coal measures between the Avon and Cromhall Heath, there were no less than 50 periods during which the conditions for soils obtained, and roots (Stigmaria) were freely developed in them, these soils topped by a growth and accumulation of plants, apparently requiring contact with the atmosphere for their exist- ence. The general thickness of that series is about 5,000 feet and it is based upon an accumulation, chiefly sandy, about 1,200 feet thick. The Glamorganshire coal-field gives a still greater deposit of mud, silt, sand, and gravel, intermingled with soils in which roots of some, at least, of the plants of the time spread out freely, most frequently, though not always, covered by beds or seams of coal, the thickness of which necessarily depended upon the duration of the conditions needful for the growth and accumu- lation of their component plants. The mass of these various beds in the neighbourhood of Swansea may be estimated at about 11,000 feet ; so that if accumulated by subsidence, horizontal beds piled on each other, it would have to be inferred that in this part of the earth's surface, and at that geological time, there had been a some- what tranquil descent of mineral deposits, sometimes capable of supporting the growth of plants requiring contact with the atmo- sphere, but most commonly beneath water, for a depth by which the first-formed deposits became lowered more than two miles from * The detail of the general mass is thus summed up by Mr. Brown : Arenaceous and argillaceous shales Bituminous shales . . . Carbonaceous shales . . Sandstones . . v -' Conglomerate . , Limestone , Coal . . . -. . Underclays . . . . Total . From this it would appear, that while the calcareous matter (limestone), gravel (conglomerate), and mud-mingled organic matter (bituminous and carbonaceous shale), were of little importance, the mass was composed of silt and mud (arenaceous and argillaceous shales), and of sand (sandstone), the former double the thickness of the latter. The more pure vegetable matter (coal) amounts to about ^jth part, and the soils (underclays) to somewhat less than ^th part. t See the detail of the coal-fields of South Wales, Monmouthshire, and Gloucester- shire (Vertical Sections of the Geological Survey of Great Britain, Sheets 1-11), and descriptions of portions of the same districts (Memoirs of the Geological Survey of Great Britain, vol. i. pp. 161-212). 508 FALSE BEDDING IN COAL-MEASURE SANDSTONES. [Cn. XXVII. their original position. It may be inferred that this thickness is not really that of the general mass, as the component beds might have been accumulated one against each other, as happens in single sandstone and conglomerate beds (figs. 38, 57), and as no doubt has more often to be taken into account than it has been, in the calculations of thickness. It may, however, be remarked, that in these coal deposits, where planes of vegetation of a peculiar kind seem so frequently to have been based on very soft soils, and the whole has been so intermingled with continuous accumulations of mud, that the general sections appear often to point to great thick- ness, more particularly when the component beds are., after dipping downwards, found rising with similar characters at a considerable distance. Nevertheless, the unevenness in many of the deposits should be well considered, and the probable value of the general decrease of the whole thickness from such causes be duly esti- mated. Though the fine mud of the time (now argillaceous shales) gives little information as to deep or shallow water in which it may have been deposited from mechanical suspension, the sand- stones of the coal measures very frequently show that they have been far more the result of sands drifted along the bottom of moving water, than of having been mechanically suspended in it. Indeed, the accumulation of the sands is much that which would be expected from a pushing forward of the bottom detritus into a shallow depression, where the conditions may have been so changed by alteration of levels that the sand of a higher situation, and nearer its source of supply, was readily transported into it. Sections of the subjoined kind (fig. 186) are of the commonest occurrence Fig. 186. in many parts of the British coal measures, and they would ap- pear not less common in the great coal deposits of* North America and parts of Europe, the geological age of which has been consi- dered somewhat equivalent. By careful removal of the upper sur- faces of these beds, the overlaps of the differently-drifted laminaa may be seen, and occasionally still better in coast exposures. CH. XXVIL] SURFACES OF COAL-MEASURE SANDSTONES. 509 The following (fig. 187) is a sketch* of the upper surface of a bed of sandstone exposed on the coast near Nolton Haven, Pembroke- shire, showing the different margins of the sand, as its various drifts proceeded. Fig. 187. An observer having thus obtained evidence of the apparent growth and accumulation of terrestrial plants in place, and the rooting of at least some of them in soils beneath of such a character that fine rootlets could spread freely amid their parts, has to look carefully into the species of this and other plants entombed in the general mass, endeavouring to see if there may not be some drifted amid the mud, silt, and sands, and even included among the coal itself, which may differ from those inferred to have grown on the spot. There would appear much to accomplish on this head, at the same time, however, it seems probable that while some plants have thriven in the planes of vegetable matter now converted into coal, others, even trees, have been borne into the general mass of vegetation, by water transporting them, as many a river now does. Matted masses of plants are often discovered among the sandstones, as if drifted by some stream, transporting such plants on its sur- face, while it pushed onwards the sands beneath it, streaks of such intermingled vegetation sometimes extending many yards in length, and occurring amid sandstones, the component sands of which have been thus accumulated. The following is a sketch (fig. 188) of the upper surface of part of" one of these vegetable drifts at Pem- brey, Caermarthenshire, in which multitudes of the stems of Sigillarice and Lepidodendra, chiefly the former, and now con- verted into coal, are crossed and matted together in all directions. These drifts of plants, now forming streaks of coaly matter in * By Professor John Phillips, when examining that part of South Wales with the author. 510 DRIFTS OF MATTED PLANTS IN COAL MEASURES. [Cn. XXVII. the sandstones or shales including them, are sufficient to show that though numerous coal beds may be the result of the growth Fig. 188: of a peculiar vegetation in place, the roots of which required and penetrated a suitable soil beneath, it might so happen that exten- sive and deep accumulations of drifted plants may wholly form coal beds under favourable circumstances, so that an observer, while investigating coal deposits, should carefully weigh any evi- dence of this kind, as well as that pointing to the growth of plants in the situations where their remains now constitute coal. The two modes of accumulation are by no means incompatible with each other. On the contrary, they may be often intermingled, sometimes conditions prevailing more, or even entirely, in favour of one instead of the other. At the same time it may be remarked that, as careful investigations have proceeded, the evidence of the growth in place of the mass of plants now constituting extensive coal beds, during the time when the chief coal accumulations of Europe and America were effected, has been gaining ground, inas- much as the soils beneath most of the coal beds and containing roots (Stigmaria) have been very commonly found.* * These soils, though far from having been acknowledged as such, have long been known, and employed as guides by the working colliers, whose experience taught them their frequent occurrence beneath beds of coal, the more especially where they constitute, as they frequently do, excellent materials for the 'fire-bricks so often required in our Coal districts, for the different metallurgical and other uses for which that fuel is employed. The name given to these ancient soils varies in different districts underclay, ttottomstone, and undercliff are not uncommon names in South Wales and the west of England. The ganister of Yorkshire and Derbyshire is a bed or beds of this kind. Though so long known to the coal miner, they have been rarely noticed until lately in colliery sections. CH. XXVII.] PARTIAL REMOVAL OF COAL BEDS. 511 An observer will not long have been engaged in the exami- nation of extensive coal districts without usually finding that, while certain beds of coal can be traced outcropping for long dis- tances, and found also beneath the surface at various depths, accord- ing to circumstances, others are more local, mere patches, as it were, amid sheets of vegetable matter far more persistent over wider areas. In like manner some of the former mud, silt, or sands, accumulated at the same time, present a more common character, scattered over extensive districts, than others, the 'muds usually, as might be expected from their component parts having been dif- fused in a fine state of mechanical suspension in water, being the most persistent. Taking the chief sheets of coal as guides, duly weighing the kind and amount of distribution of the accompanying ancient mud, silts, sands, and gravels, and reducing the section and plan, so that all embarrassments of contorted or simply tilted beds, with any fractures or dislocations which the whole accumu- lation may have sustained, be removed, it will be seen how far these sheets of interstratified matter may extend in a manner requiring an even, or nearly even surface, over a wide space. To accomplish such an object, it will be obvious that an observer should free himself from mere local variations, and attend to the evidence presented on the large scale. Thus it may be required that all the coal districts of Great Britain and Ireland, whether remaining as patches, reposing on older rocks, or simply exposed by the action of denuding causes which have removed some cover- ing of subsequent deposits, should be regarded as a whole, and with reference to any portion of dry land of which they may have constituted an addition, and from which the needful supply of mud, silt, sands, or gravel, now forming its accompanying beds of shale, argillaceous and arenaceous, sandstone and conglomerate, was derived. With regard to the sheets of vegetable matter, now constituting coal beds, they sometimes present traces of water action on their surfaces, much reminding us of the erosion to be seen upon ex- tensive areas of bog, channels being cut out by drainage and run- ning waters. Sands have been sometimes drifted above such sheets of vegetable matter, before they Fig. 189. became consolidated, mud, or even sands, first covering them, being removed, as in the following section (fig. 189) where d is a coal bed reposing on an ancient soil, e, full of roots 512 CHANNELS ERODED IN COAL BEDS, FOREST OF DEAN. [Cn. XXVII. (Stigmaria), and c is mud (shale) first covering the vegetable matter (coal), but which was subsequently cut into by the water drifting the sand (sandstone) b, a deposit covered subsequently by mud (shale) a. In this manner many a portion of the bed once resting on coal may be found swept away in parts, even to the removal of portions of the coal beds themselves. The Forest of Dean presents an excellent example of channels cut in the vegetable matter (now forming coal) of a particular portion of the coal measures there seen. The chief channel represented in the annexed plan (a, b, fig. 190), has long been known to the colliers of the district as the " Horse." Mr. Buddie very carefully examined the circumstances connected with the " Horse" and its tributaries (c, c, c), known as the " Lows," whence it would appear that when the vegetation was in an easily-removable state, like that of some bogs, drainage water had cut out a main and subsidiary channels, into which a subsequent deposit of sand was thrown down, covering over the whole surface, as any sand deposit might now do a great area of bog if submerged.* As proving that the unequal action of water was not confined to that on the surfaces of the sheets of vegetable matter, it is needful to remark that careful observation will frequently show this to have happened with other portions of the coal measures. The following section (fig. 191), observed on a cliff, composed of these * The "Horse" has been followed in the working of the coal-bed in which it occurs (that named the Coleford High Delf) for about two miles, and it has been found to vary in breadth from 170 to 340 yards. Quartz pebbles are observed in some portions of the sandstone covering up the " Horse " and the " Lows," as also fragments of coal and ironstone. Buddie, *' Geological Transactions," vol. vi. CH. XXVII.] LAPSE OF TIME DURING DEPOSIT OF COAL MEASURES. 513 rocks, between Little Haven and Gouldtrop Road, Pembrokeshire, may serve to illustrate this circumstance. Herein a deposit of Fig. 191. / mud (shale), a a, seems to have been cut into by a furrow at b, extending to f f, e e' 9 so that, other things being equal, these exuviae are similar in sections of the detrital accumulations which do not correspond with the general planes of those deposits, but with others representing their littoral, shallow, or deep-water conditions, as the case may be, of succeeding times. These modifications, from the causes noticed, have to be well considered when certain organic remains are viewed as character- istic, as it has been termed, of the accumulations of a particular geological time, those to which some name may have been assigned. When any such are found more in abundance in, or seem confined to, the deposits of some particular area, and appear to be the exuviae of animals which have lived at or near the localities where they are obtained, the kind of bottom, probable depth of water, and proximity to or distance from the dry land of the time have to be sought, so that the conditions under which the creatures themselves flourished may be duly appreciated. In such re- searches it will be often found that the kind of bottom appears to have materially influenced the abundance and distribution of these particular animals, so that, when a change was effected in the sedimentary matter deposited, they moved elsewhere, even re- turning in the same abundance as before to the same area, should 2N 546 MODIFICATION FROM VARIED SEA-BOTTOMS. [Cn. XXVIII. the conditions fitted for them have been re-established. If, in the following plan (fig. 207), the shaded portions represent minor areas Fig. 207. of mud, distributed amid sands, it would be expected that the creatures whose habits induced them to prefer the one to the other would keep within the respective variations of sea-bottom, so that if, in the course of accumulation, this bottom became modified, sands drifting or being thrown over the mud, or the latter over the former, the animals would follow the modifications according to their habits. Thus in any given sections of these sea-bottoms streaks of different kinds of them may be found accompanied with peculiar organic remains, the animals from which they were derived merely shifting their ground as circumstances arose, thus introducing interlacings, as it were, of different kinds of sea- bottoms. Looking at the conditions which at the present time appear to govern the existence of marine life both as regards the relative position of different portions of it and the distribution of similar animals, very great care seems to be required in assuming particular species as characteristic of particular geological periods without reference to their mode of occurrence at the time. It would seem very needful that the probable habits of these species should be well considered, so that proper importance should be assigned to other and contemporaneous species whose remains may be equally of value in continuous or contemporary accumulations formed under modified conditions elsewhere. Unless this be done, it may often happen that littoral species, very characteristic of the shores of a particular region, will be uselessly sought for amid con- temporaneous accumulations in the deep seas of other regions, while not a trace can be found of deep-sea species, abundant else- where at the same geological time, amid shallow water and littoral deposits. The calcareous and fossiliferous accumulations of different dates are frequently of so mixed a character as to require much care. They are often mere beds of organic remains ; these cemented together by the carbonate of lime,, which, after the deposit, has CH. XXVIII.] INFUSORIAL REMAINS. 547 been formed at the expense of the organic remains themselves. At other times, however, they have been clearly produced by de- posits from solutions of the bicarbonate of lime, in the manner previously mentioned (p. 106). Some limestones require very careful examination in order to ascertain their mode of formation' Thus it has been observed that beds presenting no appearance of organic remains to the naked eye, may yet be found to be almost wholly composed of them when the microscope is employed and due precautions taken. In this manner many beds of the moun- tain limestone series of the British Islands have been found replete with the remains of life where none were at first suspected. Even when upon exposure to atmospheric influences fossils of far larger dimensions, readily visible to the naked eye, and extending to half an inch or more in length or breadth, are found in fair abundance, it sometimes occurs that the ordinary fracture of the limestone bed may not readily show them. We do not here include the remains of encrinites, echinites, and some other fossils, which, from their rhomboidal fracture, a little practice will enable an observer readily to distinguish ; but others, where they are far from being easily detected. The most beautiful shells will occasionally thus present themselves upon searching a weathered surface, not a trace of which can be obtained by ordinary observation. It is now known that certain beds, as well siliceous, calcareo- siliceous, as calcareous, are made up almost wholly of minute organic remains, far too small to be seen by the unassisted eye. For our great progess in this order of investigation geologists are indebted to M. Ehrenberg, who has shown how much infusorial remains are diffused, even producing deposits of considerable im- portance, and most materially adding to the volume of others. Whether or not some of these microscopic minute bodies may be vegetable instead of animal, their geological importance remains the same, if indeed it be not increased from such deposits, alto- gether or in a great measure composed of myriads of microscopic organisms, being referable to both animal and vegetable life.* While on the subject of deposits chiefly formed of organic re- mains, the probable chemical composition of these remains, when first introduced amid the accumulations in which they are found, should not be neglected. In this manner it may be seen that the magnesia, so much more commonly distributed amid limestones than has often been inferred, may sometimes be due to such remains, * As to some of these supposed infusoria being vegetable, see note, p. 233. 2N2 548 CHEMICAL COMPOSITION OF ORGANIC REMAINS. [Cn. XXVIII. particularly where many corals are present.* The like also with the phosphates of lime, silica, and other substances. Whole layers may be formed of the harder parts of infusoria, so that when these are siliceous, they, and the spiculse of many sponges, may serve to diffuse no small amount of silica amid deposits of a dif- ferent character. By careful investigation of the conditions under which the re- mains of various fresh- water or marine animals may be found in rocks, the deposit of which by means of water is evident, and also by well-directed attention to the mode in which the remains of terrestrial life, not forgetting those of insects,f may have become intermingled with them, the observer will frequently find himself most materially aided in a knowledge of the probable physical geography of different areas, often considerable, at given geological times. With this knowledge and a due regard to the varied dis- tribution of the life of the time, and the abundance and kind of mineral matter deposited at the same period, he may be enabled to trace the changes and modifications which have taken place contemporaneously in the rivers or lakes, amid the lands, or in the seas, at different times in such portions of the earth's surface. Kegarding that surface as a whole, it is difficult to conceive that the distribution of life, allowing for great changes in that distri- bution during the lapse of time, could not have been adjusted to conditions as they successively arose, and which modified it more in one locality than another, so that great care seems required properly to separate the local from the general effects produced at assumed equal periods, or during a long succession of them. Modern investigations, while they, on the one hand, lead us to infer many great changes in animal and vegetable life during the accumulation of the various deposits in which its remains have been preserved,^ teach us, on the other, that forms once supposed * In some investigations undertaken by Mr. Maule at the Museum of Practical Geology, for the purpose of tracing the various changes which organic remains may have undergone under different conditions of entombment, he found magnesia, even to the extent of 6 and 7 per cent, in some recent corals. f In countries, and especially in tropical islands, such as the West Indies, where the off-shore or land-winds are at times somewhat strong, multitudes of insects are often borne out to sea, where, though the greater proportion may become the food of marine creatures, some fall in situations to be entombed amid mud, silt, or sand. Those accustomed to pass along such coasts are familiar with this fact. Our own coasts in summer weather present many instances of insects surprised and drawn off coasts seaward by the sudden setting in of the land-wind in the evening. J Referring to various general works containing lists of the remains of animal and vegetable life considered characteristic of the different deposits which it has been thought convenient to separate and class under particular names, it is only required CH. XXVIII.] CAUTION AS TO CHARACTERISTIC FOSSILS. 549 only confined to the more modern accumulations have existed in far more remote times.* While it is probable that the evi- dence of great changes having taken place during the lapse of geo- logical time in the vegetation and animals which have existed on the earth's surface will be only confirmed by extended research, it seems equally probable that investigations carried out with proper regard to the varying physical geography of different geological periods will show the necessity of tracing the probable causes pro- ductive of new adjustments of lands and waters at those different times, and of studying the distribution of the lifef of such times to point to such animals as the trilobites, among the more ancient accumulations, and to the ammonites of the middle portion of the fossiliferous series, to show that certain marine creatures which have now ceased to exist once swarmed in particular areas at given times, and have not lived after those times. No doubt the preserva- tion of the parts of many terrestrial animals requires a combination of favourable circumstances, so that no great surprise is to be experienced when we obtain few traces of such animals amid the contents of the old sea-bottoms usually presented to our examination. The remains of the marsupial mammal {Phascolotherium Bucklandi, Owen) and of the insectivorous mammals (Amphitherium Prevostii, and Am. Bro- deripii, Owen) in the Stonesfield slate (oolitic series), near Oxford, are sufficient to introduce caution into general reasoning as to the existence or non-existence of terrestrial mammals at different geological times. Speaking of the conditions under which these remains occur, Dr. Buckland remarks (Bridgewater Treatise, vol.i., p. 121) that " at this place (Stonesfield) a single bed of calcareous and sandy slate, not six feet thick, contains an admixture of terrestrial animals and plants with shells which are decidedly marine; the bones of Didelphis (Amphitherium and Phascolotherium) , Megalosaurus (a great saurian 40 or 50 feet long, partaking, according to Cuvier, of the structure of the monitor and crocodile), and Pterodactyle (a flying saurian), are so mixed with ammonites, nautili, belemnites, and many other species of marine shells, that there can be little doubt of this formation having been deposited at the bottom of a sea not far distant from some ancient shore." With respect to the wing-covers of insects found in the same deposit, Dr. Buckland remarks (Bridgewater Treatise, vol. L, p. 411) that they are all coleopterous, '-and in the opinion of Mr. Curtis, many of them approach nearly to the Buprestis, a genus now most abundant in warm latitudes." * As regards the forms of molluscs, the genera Avicula, Modiola, Tei-ebratufa, Lingula, and Orbicula are found from the Silurian rocks upwards to the present day. The like with Turbo, as a restricted genus, and also with Nautilus, with slight varia- tions in form. With respect to those remarkable and beautiful animals the star- fishes, Professor Edward Forbes states, that species of the genus Uraster are found in the Silurian rocks closely resembling the existing northern forms (Decade I., " Memoirs of the Geological Survey"), and that in the lias, Uraster Gamy I (Decade III.) is only critically to be distinguished from the common Uraster rubens, now inhabiting the British seas. According also to the Professor, the Terebratula striatula, of the cretaceous series, cannot be distinguished specifically from the Terebratula caput-ser- pentis of the same seas. t It is very desirable, in the enumeration of organic remains discovered in different beds and localities, properly to represent the abundance of the individuals of each species. This mode of investigation has received careful attention during the pro- gress of the Geological Survey of the United Kingdom. Without due precaution of this kind, the remains of a single individual figures as prominently as those of many hundreds, and a correct view of the correspondence or difference between the various portions of contemporaneous accumulations as to the life entombed in them becomes 550 EFFECT OF GENERAL HEAT ON DISTRIBUTION. [On. XXVIII. in accordance with those laws which appear to govern that distri- bution at the present time. At the same time we should not neglect those conditions which would follow a gradual decrease in the heat of the earth, should it eventually be found that a tem- perature more equal over the earth's surface than that afforded by the sun would appear required for the distribution of animal and vegetable life in the earlier periods of its existence on our planet. much impeded. A careful study of the comparative numbers of individuals often shows how much some species of marine molluscs have preferred one kind of sea- bottom to another, while others seem to have flourished equally well through varied changes in the sea-bottoms. It is well to bear in mind that the researches of naturalists teach us that many an area is now little, if at all, tenanted by marine molluscs, such areas being unsuited to their habits, while others adjoining them may be covered by multitudes of various molluscs. CHAPTER XXIX. IGNEOUS ROCKS OF EARLIER DATE THAN THOSE OP MODERN VOLCANOS. SIMPLE SUBSTANCES FORMING IGNEOUS ROCKS. VOLCANIC PRODUCTS AMID THE OLDER FOSSILIFEROUS ROCKS. FOSSILS AMONG OLD IGNEOUS PRODUCTS. VOLCANIC TUFF AND CONGLOMERATES AMONG THE SILURIAN DEPOSITS OF WALES AND IRELAND. OLD VOLCANIC PRODUCTS INTER- MINGLED IN THE DEVONIAN ROCKS OF SOUTH-WESTERN ENGLAND. - IGNEOUS ROCKS ASSOCIATED WITH THE CARBONIFEROUS LIME^fON^] ,0F DERBYSHIRE. RELATIVE DATE OF THE WICKLOW AND WEXEOED GRA- NITES OF THE DEVON AND CORNWALL GRANITES. UNCERTA%" DATES OF SOME IGNEOUS DYKES. ELVAN DYKES IN CORNWALL AND VF$$N.- A NSHIRE. IGNEOUS ROCKS OF THE NEW RED SANDSTONE SERIES OF DEVONSHI] DATES OF CORNISH AND DEVONIAN EL VANS. ELVANS OF WICKLoV WEXFORD. CHEMICAL COMPOSITION OF IGNEOUS ROCKS. EFFECTS OF SILICATE OF LIME IN IGNEOUS ROCKS. As has been previously remarked, the distinction between the products of active and extinct volcanos is rather one of conve- nience than of fact ; and the same may, to a certain extent, be also observed as to the differences between those above noticed (pp. 317 407), and the products about to be mentioned. By arrang- ing igneous products according to the different geological dates to which they may be assigned, the observer has the means of studying not only their modes of occurrence, but also the con- stancy or change of the elementary substances entering into their composition during the lapse of geological time. The igneous rocks known to us by their appearance on the sur- face of the globe, have been found sufficiently well distributed to be available for an approximative estimate of their component elementary bodies. Viewed as a whole, they are chiefly oxides of substances commonly considered simple, one of the oxides, that of silicon, acting as an acid, and combining with a large portion of the other oxides. Silicic acid (silica), free or combined, may be seen more to prevail in certain rocks than in others ; but there are 552 SIMPLE SUBSTANCES FORMING IGNEOUS ROCKS. [Cn. XXIX. few igneous products found in any abundance, which do not mainly consist of silica, or the silicates. The simple substances, with silicen, constituting this mass of matter, whence the sedimentary deposits have been, with minor exceptions, more or less directly or indirectly derived during the lapse of time, have not been found numerous. They are chiefly aluminium, potassium, sodium, cal- cium, magnesium, iron, and manganese, making with silicon eight substances, considered elementary, all combined with another, oxygen, and forming the great volume of the igneous rocks, such as they are known to us. Of other elementary substances enter- ing into their composition on a minor scale, probably sulphur, boron, lithia, and fluorine, may be regarded as the principal bodies, with the addition of hydrogen, so far as it may enter into the composition of any waters that can be regarded as a real com- ponent part of these rocks. Numerous other simple substances, no doubt, may be detected amid these products in different locali- ties, even sufficiently abundant in some to be remarkable; but viewed in the mass, the nine elementary substances above mentioned, with the four others in a minor manner, appear to constitute the great mass of the igneous rocks of all ages. That so much of the great volume of these rocks should consist of the combination of oxygen with a few simple substances, and that the union of oxygen with one of them should constitute such an important compound for further union with the other oxides, are in themselves circumstances of no slight interest to a geologist anxious to trace some connexion between the igneous products of all geological periods and the substances beneath the exterior and consolidated portion of the earth during the same lapse of time. We have elsewhere* estimated silica as constituting 45 per cent, of the mineral crust of the globe, hence the oxygen contained in silica alone would form at least 26 per cent, of that crust. f If the amount of oxygen in the other oxides be included, the percentage becomes largely increased ; so that when this substance is regarded as free from its union with the matter forming rocks, and in a gaseous form, its volume becomes enormous.^ In studying these rocks it may be assumed that the observer would be desirous of ascertaining how far there may be evidence * " Researches in Theoretical Geology," 1831, p. 8. t According to Berzelius, silica is a compound of 48-4 parts silicon and 51'6 parts of oxygen. J The volume of oxygen would be obviously still farther augmented by the addition of that contained in the various waters on the surface of the globe, water being a compound of oxygen and hydrogen. CH. XXIX.] VOLCANIC PRODUCTS AMID THE OLDER ROCKS. 553 of igneous products having been thrown out in the manner of those ejected from active volcanos at different geological times. As it so happens that certain portions of the earth's surface appear to have remained in a state undisturbed by igneous action from very early periods to the present day, while other portions seem frequently to have been subjected to this action during the same lapse of time, all regions, however interesting they may otherwise be geologically, do not present the needful conditions for this kind of investigation. In the British islands, presenting so many coast and other natural sections, as also so cut and pierced by the operations of the miner and engineer, it fortunately happens that amid the older fossiliferous deposits there is evidence of igneous products having been contemporaneously ejected. Igneous rocks are so entangled with detrital accumulations of the Silurian series, especially well exhibited in Wales, and in the counties of Wicklow, Wexford, and Waterford, on the opposite shores of Ireland, that a geologist has excellent opportunities afforded him for observation. He finds that the igneous products, thus associated with these old fossiliferous deposits, may be divided into those which have occu- pied their relative positions in a molten form, and those which have been mingled with them through the agency of water, with also certain accumulations which may even have been piled up in a mechanical manner in air. Having in view the manner in which the products of existing volcanos are thrown out into the air or water, and are commingled, it is desirable that the geologist should endeavour to trace any differences or resemblances he may find when opportunities of the kind noticed present themselves. In the first place he does not possess the advantages of the surfaces usually presented in active volcanic districts, or of those, such as in France (p. 401), which have not been disturbed by the action of seas upon them, but finds masses of mineral matter, of which the igneous products only con- stitute a part, usually thrown out of the positions in which they were originally accumulated, the igneous often bent and contorted with the aqueous deposits with which they are associated. These districts, moreover, are often the mere wrecks of the mud, silt, and sand of former sea-bottoms, combined with the igneous pro- ducts, large portions having been removed by denuding causes, so that not only has the general mass been squeezed, bent, contorted, and sometimes broken, but portions of it (occasionally to be measured by cubic miles) entirely removed, Hence, no slight care and exact research are required to collect the needful evidence, so that 554 FOSSILS AMID OLD IGNEOUS PRODUCTS. [Cn. XXIX. all the parts may, mentally, be again restored satisfactorily to their places. This may often nevertheless be sufficiently accomplished. In examining the igneous products associated with the Silurian rocks in Wales and Ireland, two kinds become somewhat pro- minent, one in which the matter constituting felspar prevails, another in which that forming hornblende is mingled with the first, to an equal and even greater amount. Those accustomed to active volcanic regions might be disposed to see in this circum- stance a general resemblance to trachytes and dolerites (p. 352) therein distinguished, as also to those mixed products which have been named trachyte-dolerites. Proceeding still further in the inquiry, it will be found that certain of these old products are mingled mechanically with substances that have once formed ordi- nary mud, silt, sand, and even conglomerates, reminding us of the mixture of the ashes and lapilli thrown out of existing volcanos, and intermixed with the detrital accumulations forming under the fitting conditions around or near many active igneous vents of the present day. Even lapilli may be detected amid beds which are composed of something more than the igneous substances them- selves. These appearances would alone lead an observer to con- sider the old igneous products before him with reference to certain of the results of modern volcanic action, and he would probably be not the less induced to take this course when he found, as in such districts he often may, organic remains amid them, either alone or mingled with common detritus, preserved precisely as in volcanic tuff associated with the common mud, silt, and sand deposits of the present time. He will sometimes find the organic remains arranged in seams, as amid ordinary detrital accumulations, representing in like manner the bottoms of ancient seas strewed with the harder part of molluscs, and other marine animals of the time. The geologist may occasionally discover organic remains, thus ar- ranged in seams, the matter of the shells of molluscs still preserved in beds of hard and solid rocks, ringing under the hammer, and at first sight appearing as if they had flowed in a molten state. Such beds of consolidated igneous matter, arranged in water, are fre- quently very deceptive, requiring no slight care not to confound them with the rocks which have really flowed in a molten state amid those with which they are often associated. Not only in cases where such beds contain organic remains, but also where no trace of them can be detected, much caution is needed. For the most part microscopic observation will show that they are composed of fragments of igneous molten products, in which the component Cii. XXIX.] VOLCANIC TUFFS OF SILURIAN PERIOD. 555 parts of felspatliic or hornblendic minerals have variously pre- vailed, and that these fragments are angular. When lapilli, especially those having the aspect of pumice, are mingled in these accumulations, there is usually little difficulty in determining their true character, and they then assume the appearance of those resulting from the deposits of the ashes and cinders thrown out of volcanic vents at the present day, more especially of such as have been formed in water by the fall of volcanic ashes and cin- ders in it, and are now elevated above it. They thus resemble the tuffs in the vicinity of Vesuvius, Etna, and some other active volcanos, which have showered ashes and cinders into seas adjoin- ing them, a change of relative level of the land and sea having been effected, and parts of former sea-bottoms having been upraised. Those who have devoted much close attention to the structure of the valcanic tuffs of the present time can scarcely fail to be struck, particularly when they are regarded on the large scale, with the resemblance of many of the accumulations of igneous pro- ducts associated with the Silurian rocks of Wales and Ireland to certain of them, especially to those such as palagonite (p. 368) and some others which have become consolidated and modified in appearance, so that the original small grains of ashes and fragments thrown out of volcanos have become one general and, at first sight, almost homogeneous substance. While in many localities the laminated character of the beds, and the presence of marine organic remains occurring in the same manner as in any other detrital and associated deposit, point to their accumulation beneath the sea, ashes, and sometimes lapilli, vomited forth from the volcanos of the time and locality, and arranged in extensive and compara- tively thin beds; at others the conglomerates and breccias of igneous rocks, mingled confusedly with ancient volcanic tuff, the whole interlaced with dykes and veins of felspatliic and horn- blendic molten rocks of different kinds, remind the observer of a confused mixture of substances in the body of a volcano itself, partly subaerial and partly subaqueous, the general mass buried up by other accumulations as the volcanic rocks gradually descended beneath them. Of the natural sections exposed in Wales and Ireland, though there are many excellent opportunities in various places, the most instructive is probably on the coast of the county of Waterford, between Tramore and Ballyvoil Head, a distance of fifteen miles. Huge masses of these igneous products are there found 111 great variety, and molten rocks of different kinds and shapes will be seen sending out veins, and cutting as well the 556 SILURIAN VOLCANIC TUFF AND CONGLOMERATES. [Cn. XXIX. ordinary detrital deposits, formed prior to these outbursts, as the igneous substances of the time. Conglomerates and breccias are found piled in various forms, cemented by igneous matter, ap- parently thrown out as ashes, and ancient tuffs formed of smaller fragments are observed in different places, while examples are to be seen of deposits of various kinds changed in aspect and character where molten matter has burst in among them. There is also a variety of minor, but collectively important, objects of interest bearing upon the igneous products and their mode of ac- cumulation at this early geological period. As regards the in- trusion of molten matter amid the conglomerates and breccias, the following section (fig. 208) on the west of Kilfarrasy Point may be found illustrative, others of equal interest being, however, suf- ficiently common. a, a, compact igneous rock, in which the substances composing felspar prevail ; b, b, b, conglomerate and breccia formed of various portions of igneous products, chiefly fel- spathic, cemented by matter resembling that of volcanic tuff. Such districts require obviously to be studied on the large scale. For example, the section exhibited on the Waterford coast, excellent as it may be, would scarcely afford the needful evidence, taken by itself. It would be necessary to consider it with re- ference to the mode of occurrence of similar accumulations along their whole range, thence northward through the counties of Waterford, Wexford, and Wicklow. When this is accomplished, the sections afforded on the coast of Waterford are seen to form part of the general evidence pointing to the relative age of the igneous accumulations at this time ; one shown, moreover, to have been anterior to the formation of the old red sandstone of the south of Ireland, inasmuch as these igneous rocks, as also the beds of the Silurian series with which they are associated, were disturbed, bent, and contorted before its accumulation, and this old red sandstone not only reposes quietly upon the disturbed rocks, but also contains worn fragments of the latter, the igneous substances included. CH. XXIX.] VOLCANIC PRODUCTS AMID DEVONIAN ROCKS. 557 The geologist, seeing this considerable resemblance to the products of modern volcanos, and also the general similarity of the elementary substances found in them, as far as researches have yet extended,* (considering both igneous products in their masses,) would be prepared to find evidence of igneous action also bearing a resemblance to that of volcanos at the present time amid any accumulations of intermediate geological date, should the fitting conditions prevail. For evidence of this action he would look necessarily in very different regions ; for not only is it required that there should have been igneous products of this kind at all geological times in various parts of the earth's surface, sometimes in one locality, sometimes in another, but also that in the present arrangement of land and seas they should be attainable for observation. In this research, however, the geologist may again find opportunities in the British islands. In Devon and Cornwall he may obtain evidence of a continuance of the like igneous action at a subsequent period, amid deposits, some of which may be referred to the date of the old red sandstone series of other parts of the British islands, while some are more modern, and others perhaps more ancient. Amid the Devonian and Cornish accumulations of this date he will detect beds apparently also formed of the volcanic ashes of the time, and other arrangements of igneous matter, some rocks evi- dently poured forth in molten masses, and breaking through ordinary and previously-formed deposits. Among numerous localities good sections are afforded at low tide on the Tamar, near Saltash, showing an association of the Devonian rocks, molten products, and other accumulations of igneous origin.f In many situations, the igneous ash of the time graduates into the ordinary * Investigations are now in progress in the laboratory of the Museum of Practical Geology for the purpose of ascertaining the chemical composition of the igneous rocks of different dates, obtained during the progress of the Geological Survey of the United Kingdom. | As we have elsewhere mentioned (" Report on the Geology of Cornwall, Devon, and West Somerset," 1839, p. 63), there is an abundant mixture of igneous and ordinary sedimentary rocks in the vicinity of Saltash and St. Stephen, and thence across the Tamar to St. Budeaux on the east, and towards St. Urney on the west, and in the creeks which run up from the Lyhner to Manaton Castle and St. Urney. The schistose varieties are certainly contemporaneous with the associated sedimentary deposits, while dykes of greenstone and other compounds of hornblendic and fel- spathic matter are seen to cut through the various accumulations, " analogous to those which are produced in the beds of ash, and filled by lava on the flanks of volcanos, in cases where the latter are partly submarine ; traversing shales, clays, and other aqueous deposits, as well as the ash, which in such cases may readily have become interstratified among them." In the continuation of the beds near Saltash, many of the schistose accumulations of ash so graduate into the common kinds of deposit of mud and silt that no correct distinctions can be drawn between them. 558 IGNEOUS ROCKS ASSOCIATED WITH THE [Cn. XXIX. detrital matter associated with it, as well in the continuation of a contemporaneous deposit as in successive deposits, in the one case pointing to a gradual removal from the source of supply, such as a volcanic vent ; in the other, to an unequal supply over the same area, occasionally intermittent, so that common deposits were effected on a sea-bottom at intervals. Good examples of these kinds of igneous accumulations, with an intermixture of solid molten matter, some of the latter showing large-grained compounds of felspar and hornblende, are to be found in the direction of Davidstow and St. Cletha. In certain of the ash-beds much calcareous matter is sometimes found, assisting as a cementing substance. They are so calcareous near Grylls, on the south of Lesnewth, that attempts have been made to burn the compound rock for lime. Upon attentively examining the composition of those beds of igneous substances which are arranged amid the ordinary deposits of the time, it is found to vary much as to the molten products associated with the general mass of deposits. While some, like the trachytic tuffs of modern times, are chiefly formed of the compo- nent parts of felspar, others are more like the dolerite tuffs, and contain substances usually found in augites and hornblendes, while others again partake of the character of both. Seeing that, like the ordinary muds, silts, and sands with which they are associated, they have become consolidated, and like them also have been ex- posed to the passage of water through them, as well when buried deep (by depressions of the general area) beneath their present levels, as when exposed, as now, to atmospheric influences, many of these rocks may not now contain all the substances originally dis- tributed in them, while they, like the other deposits associated with them, may have received additional mineral matter. It will readily be inferred that soluble substances, such as the silicates of soda and potash, may have been removed during the long lapse of geological time in which they may have been exposed to modifica- tion and change. Though there is this difficulty, much may yet be accomplished by accurate analysis of portions carefully selected. In Derbyshire the observer will again see igneous rocks associated with ordinary deposits ; in this case with limes tone, known as the carboniferous or mountain limestone, in such a manner that their relative geological antiquity can be ascertained. Careful investiga- tion shows that in that area, at least, and probably much beyond it (beneath a covering of the sands, shales, and coals, known as the millstone grit and coal measures), and after a certain amount of CH, XXIX.] MOUNTAIN LIMESTONE OF DERBYSHIRE. 559 these limestones had been accumulated, there had been an outburst and overflow of molten rock, irregularly covering over .portions of them. And further, that after this partial overflow, the limestone deposit still proceeded, probably spreading from other localities where the conditions for its accumulation had continued uninter- ruptedly. Occasionally water action upon the igneous products may be inferred prior to the deposit of the calcareous beds upon them, if not also a certain amount of decomposition of the former, the limestones immediately covering them containing, fragments (some apparently water- worn),, and a mingling of the subjacent rock, such as might be expected if calcareous matter had been thrown down upon the exposed and decomposed surfaces of the igneous rock. In some parts of the district another outflow of the same kind of igneous rock again took place, and was again covered by limestone beds, so that in such portions of the area, two irregularly- disposed sheets of once molten rock are included among the mass of the limestone beds. The following section (fig. 209) of part of this district,* by Professor John Phillips, may serve to illustrate the mode of occurrence of these beds of igneous rock, the areas of which do not coincide, so that one outflow did not exactly cover that overspread by the other. In this section, a a are the igneous rocks, locally known as toadstones,j and b b the limestones, c being the covering beds of millstone grit, and// faults. l-'in Cop. Bow Cross. Natural sections (many of which are excellent) and mining operations show that as regards thickness these overflows vary considerably, so much so as to aid the observer in forming some estimate of the localities whence the molten matter, when ejected, may have been distributed around. * A reduced portion of one (No. 18) of the horizontal sections of the Geological Survey of Great Britain. f Professor John Phillips suggests that this name is a corruption of the German word todtestein ; rathe todte liegende (red dead, or unproductive bed), being a term applied by German miners to the unproductive rocks subjacent to the copper-bearing slate of Mansfield and other localities. In like manner, the name JSarmaster, given to those who superintend the distribution of the mines and collect the dues or royalties, has long been considered a corruption of Bergmeister. See Pilkington's " Derbyshire," 1789, p. 110. 560 IGNEOUS ROCKS ASSOCIATED WITH THE [Cn. XXIX. Although there are clays amid the limestones in the relative positions of the igneous rocks, and some of these seem clearly little else than such rocks in a highly-decomposed state, retaining the arrangement of their component mineral substances, as, for example, at the isolated boss of limestone at Crich, protruding (at a distance of 3i miles from the main mass) from the squeezing action to which these rocks and the coal measures above them have been subjected, through the lower part of the latter, known as millstone grit, it would scarcely be safe to conclude that all lying nearly in the same general geological levels were so, inasmuch as some of them may be clays of another character. Care on this head is rendered necessary by finding a clay a true underclay of the coal-measure kind, supporting a thin bed of impure coal in the higher part of the limestone series near Matlock Bath.* In the case of Derbyshire, though there may have been a removal of a portion of the igneous beds by the action of water upon their exposed surfaces (and an attentive examination of the upper over- flow likewise shows a quiet adjustment of the limestone beds formed upon it), no deposits resembling the ash and lapilli beds above mentioned as found in Devon and Cornwall, Wales and Ireland, have yet been detected. There is no evidence showing an accumulation of ash and cinders in the manner of subaerial volcanos. If there had been such, and this had been attacked by breaker action and currents, the geologist would expect to discover some portions included amid the limestone beds, and such have not been found. It may readily have happened, therefore, that the igneous matter was thrown out in a molten state, without any accompaniment of ash and cinders ; and this might have taken place as well beneath the level of the sea as above it. Upon examining the structure of the igneous rock, it is found to be partly solid, and confusedly well crystallized, a compound of felspar and hornblende, with, sometimes, sulphuret of iron. It is partly vesicular, in some localities highly so ; the vesicles, as usual, filled with mineral matter of various kinds, t where the rock has remained unaffected by atmospheric influences, but exhibiting the original and vesicular state of the molten rock where these have * This impure bed of coal was cut while driving the tunnel through the High Tor, for the railway running by Matlock Bath, and is to be well seen, dipping rapidly, with the other beds, in the drift cut into the cavernous mine, part of which is shown by the name of the Rutland Cavern, at the Heights of Abraham. t Carbonate of lime, as might be expected, is a very common substance in these vesicles. CH. XXIX.] CARBONIFEROUS LIMESTONE OF DERBYSHIRE. 561 removed the foreign substances in them. In some localities the scoriaceous character of the rock is as striking as amid many vol- canic regions of the present day. Like more modern igneous pro- ducts, also, it will often be found decomposed in a spheroidal form. The following (fig. 210) is an example of this decomposition at Diamond Hill, on the south side of Millersdale, where the con- cretionary structure has been developed somewhat on the minor scale, and the size of the spheroidal bodies is about that of bombshells and cannon-balls. Fig. 210. It will be thus seen that amid the older fossiliferous deposits igneous rocks may be so associated as to give the relative dates of their ejection, even in such a manner as to lead to the inference that in some cases there have been subaerial volcanic vents at hand, whence molten matter, cinders, and ashes may have been thrown out, as in the present day, the elementary substances of which this ejected matter is composed, reminding the geologist very strongly of those thrown out in a similar manner in modern vol- canos. As has been stated (p. 553), it will require the observer to readjust in his mind the various parts of countries, like those noticed in Cornwall and Devon, Wales and Ireland, replacing the portions now removed by denudation, properly to consider this subject with reference to the relative times when the various igneous products were ejected and accumulated amid the ordinary sedi- mentary deposits of that early geological time. Let the following section (fig. 211) be one of a volcano, so situated that while lava currents and dykes of molten matter (a a a) were thrown out and became mingled with subaerial tuff and volcanic breccias (b b b), subaqueous deposits were formed near and over these products, mingling volcanic and ordinary detritus in the same or associated beds. 2o 562 RELATIVE GEOLOGICAL DATES OF THE [Cn. XKIX. If the volcanic action ceased, and the general area were depressed so that new and ordinary chemical or detrital deposits, d d d, were effected, and the whole was merely tilted, not complicating the subject with squeezing and contortion, and some new surface, n, 8, be given to the general mass, as shown beneath (fig. 212), the ob- Fig. 212. server will at once perceive that the mode of occurrence of the igneous rocks amid the ordinary deposits will require careful con- sideration and study. He will see that a hasty investigation is not likely to afford the requisite data, and that prolonged research is needed for very exact determinations, though he may often find sufficient in a short time, if the natural or artificial sections be favourable, for a just general view of the subject. When igneous products are not associated with ordinary fossi- liferous deposits in the manner mentioned, and often, unfortunately, they cannot be so favourably studied, a geologist may still obtain certain relative dates by their mode of occurrence on the great scale. Fortunately, we may again take the British islands for illustration, as showing how much may be found connected with the subject even in that minor area. When the granite range of Wicklow and Wexford, and which also includes portions of adjacent counties, is examined with reference to the rocks in contact with it, it is seen that certain Cambrian and Silurian rocks, the range of which it traverses in a slanting manner, are upturned, much modified in their mineral structure, when in contact with the granite, and often much broken at the junction ; even huge masses of them included in the latter, granite filling the cavities and fissures thus produced ; so that little doubt is left that these rocks were formed prior to the intrusion of such granite. Thus far the observer merely obtains evidence of no very definite kind as to the actual period of this intrusion, though in the district noticed he would see that this kind of igneous action took place after that which in the same area produced an out-throw of felspathic and hornblendic products as above noticed (p. 553). He only discovers that the one set of igneous products has been uplifted by the other. Continuing his researches, he sees certain conglomerates of the old red sandstone reposing quietly upon the granite, and, when this happens, containing rounded portions of that rock, as well as CH. XXIX.] WICKLOW, WEXFORD, AND DEVON GRANITES. 563 much finer detritus from it. He also finds where the same con- glomerate stretches over the disturbed older rocks, with their included igneous products, that rounded and angular fragments of these products are imbedded in it. He has now the approximate relative date of the granite of the district, so far that it rose up after that portion of the Silurian series was formed which is there disturbed, and prior to such portion of the old red sandstone series as is represented by this conglomerate. We will suppose that he has obtained evidence of a portion of the Silurian series disturbed being the lower, and of the conglomerate representing some higher or middle portion of the old red sandstone series, as found developed elsewhere in the British islands. There would then, no doubt, be something of an interval in the geological series, during which the uprise of the granite may have taken place, never- theless the observer has, by the means employed, arrived at a certain approximation of no slight value as to the real relative date of its protrusion. To show this value, it is only needful to turn to Devon and Cornwall, where at such a comparatively trifling distance, the geo- logist finds a granite of much the same general character pro- truding through the equivalents of those accumulations which have quietly covered the Irish granite mentioned, after its consolidation, the disturbance caused by the uprise of the Devonian and Cornish granite extending to the lower portion of the coal measures, as may be seen around the northern part of Dartmoor, where veins extend from the granite in that direction into these sedimentary rocks, in the same manner as into the Silurian deposits of Wicklow and Wexford. In the case also of the granites of Cornwall and Devon, it becomes necessary to seek for evidence as to any deposits so occurring as to show the geological dates between which their uprise was effected. Throughout the greater part of the district, evidence of the kind required is not to be found, but on the east- ward of Dartmoor, and of the continuation of the deposits which have been disturbed at the time these granites were intruded, beds are found, known as the new red sandstone series, reposing quietly on the disturbed rocks, the lower portion of them containing rounded and angular fragments of the latter. It would thus appear that the approximative date for the elevation of the Cornish and Devonian granites amid the accumulations effected up to that time, was some- where between the lower part of the coal-measure series (including the millstone grit of central England in that series), and the lower portion of the new red sandstone deposits. 2o2 564 UNCERTAIN DATE OF SOME IGNEOUS DYKES. [Cn. XXIX. Thus in south-eastern Ireland and south-western England there is evidence of two protrusions of granite at different geological periods, different rocks of known relative ages being disturbed on the one hand and unmoved on the other, so that approximative dates are obtained for both protrusions. If in the annexed section (fig. 213) a, a, be a mass of granite thrust upwards through sedi- Fig. 213. mentary beds b b, sending veins into fractures effected in them, as well as modifying their mineral structure at the junction, and c be an accumulation containing rounded or angular fragments of a and b, it follows that the relative geological dates of b and c being known that of the protrusion a, a, would be known, also, within greater or less limits as the formation of b and c may be separated or ap- proximate to each other in the geological series. This would be the case of south-eastern Ireland. In that of Devon the disturbed beds b b, altered as before, and with granitic veins a, a, in them, would be covered by beds / reposing quietly on them, and also con- taining fragments of them, with here and there igneous rocks, e, interposed. Usually the relative dates of the rise of molten mineral substances into fissures of prior-formed rocks, such portions of igneous matter, known as dykes, cannot be obtained when these are un- covered by accumulations of which the position in the geological series is known; as, for example, if, in the subjoined section (fig. 214), a and b be dykes of any igneous rocks cutting through Fig. 214. some sedimentary deposit c d, and these be uncovered by any ac- cumulation of ascertained geological date, the exact relative time when the cracks were effected and the molten matter rose in them would remain uncertain. It sometimes happens, 'however, that some evidence as to relative date may be obtained, of a fair ap- proximative kind, even with respect to dykes of this character. It would not be sufficient that they cut one set of rocks, and not an- other, in some given district, without further general evidence, so CH. XXIX.] ELY AN DYKES IN CORNWALL AND DEVON. 565 as to refer them with certainty to a particular time, anterior to the formation of the beds not cut by them, since it may have happened that contemporaneous causes did not act beyond a given area, though in certain of these cases there may appear much to support an in- ference to that effect. For example, numerous greenstone dykes are found to traverse the Cambrian rocks in Merionethshire and Caernarvonshire, while these are not observable amid certain upper Silurian deposits in Denbighshire and Flintshire, and contempo- raneous igneous rocks are associated with intermediate accumulations in Caernarvonshire, and other adjacent counties. It might hence be inferred that, when the igneous eruptions producing the latter were effected, fissures were formed in the still more ancient deposits (Cambrian) and molten matter injected into them, and that igneous action ceasing, the adjoining higher parts of the Silurian deposits were undisturbed by the intrusion of any igneous matter. It is far from im- probable that this inference would, in a great measure, be correct ; but that it is not wholly so, the inspection of dykes of the same kind tra- versing various parts of Anglesea, and seen to cut into the coal measures of the Menai Straits, between Bangor and the great sus- pension bridge, at once shows. It may readily have happened that igneous matter had been thrown into fissures formed at these different times in even the moderate area of Caernarvonshire and Anglesea, and hence it would be hazardous, without other evidence, to decide upon one dyke being separable in geological time from another, even when not far distant from each other, at the same time that many pro- babilities might seem to exist as to the relative date of some of them. The granitic and porphyry dykes in Cornwall and Devon, known locally as elvans, may be taken in illustration of the approximation to relative geological dates occasionally attainable. It has been seen that the granites of that district were upraised posterior to the deposit of the lower part of the coal measures, and anterior to that of the new red sandstone series. Subsequently to the protrusion of the granite, and to, at least, its partial consolidation, fissures were formed traversing both the granites and the various disturbed sedi- mentary rocks adjoining them, and into these fissures molten matter was introduced, as shown previously (fig. 7, p. 9), and as may be further illustrated by the following section (fig. 215), seen on the cliffs at Trevellas Cove, near St. Agnes, where an elvan a, a, cuts through the slates 5, and is traversed by dislocations/, /, one of which materially shifts the rocks, and thereby displaces the elvan dyke, near the sea. With respect to the same fissures having tra- 566 EL VANS IN CORNWALL AND DEVON. [Cn. XXIX. versed both the previously-consolidated rocks and the granite, the Fig. 215. following map (fig. 216) of part of the mining district of Gwennap, Cornwall, may be useful, a, a, being the granite, c, c, the schistose Fig. 216. rocks broken through by it, b, b, the elvan dykes, and s, greenstone, The fissures v, v, v, and d, d, d, were produced at different sub- sequent periods, some of them variously filled by the ores of tin and copper, or other substances, and known (locally) as lodes and cross courses. Upon examining the composition of these elvans, they are found to be formed of matter similar to that of the granites of the districts, usually corresponding with any modifications observable in patches of that rock exposed nearest on the surface. Indeed, they seem merely portions of the same general matter which rose CH. XXIX.] IGNEOUS ROCKS AT CALVERLEIGH, DEVON. 567 * in fissures formed by the cracking of the adjacent granite, only consolidated in its higher parts, such cracks also extending through the various rocks above the granite. The relative date would be only so far thus obtained as to show that the filling of the fissures was posterior to the intrusion of the main masses of granite, some of the latter rock, in its molten state, readily rising into such fissures, formed both in its own higher parts, and in any covering rocks.* Proceeding eastward from the Dartmoor granite to the boundary of the new red sandstone series, where this reposes on the uneven surfaces and indentations of the older and previously-disturbed fossiliferous deposits in that direction, igneous rocks are found associated with its lowest part in some localities, pointing to local igneous action, while these lowest beds were accumulating. Not only are some of these lower accumulations so entangled with the igneous rocks, that there appears difficulty in not considering them of contemporaneous production as a whole ;f but there would also appear to be traces of subaerial action. The latter seems to occur near Calverleigh, where, as in the annexed section (fig. 217), a, a Fig. 217. Culverleif-h. d represent the disturbed beds of the lower coal measures, at part of an ancient gulf amid those rocks ; b, a conglomerate wholly com- posed of portions of these subjacent deposits, cemented by red sandstone and argillo-arenaceous matter, without any fragments of igneous rocks; c 9 felspathic porphyries, and more compact fel spathic rocks, some scoriaceous ; and d, conglomerates and sand- stones, fragments of the igneous rocks, and others of a similar character, being contained in the conglomerates. Along the range * Occasionally fragments have been detached from the adjacent rocks, and en- veloped in the molten matter of the elvans. That at Pentuan is among the best examples of this circumstance. This elvan is a fine-grained compound of felspar and quartz, with crystals of mica. Fragments of the slate rocks traversed are found in it. Occasionally, though rarely, there are portions of quartz which appear to have been broken off some quartz vein in the slates, and thus became, like the other fragments, included in the molten rock. In a branch of the Pentuan elvan, taking a course alongshore to the Black Head, the fragments derived from the adjoining rocks are very numerous, decreasing in abundance from the sides of the dyke towards its central part, in which they are rarely detected. t The intimate connexion of igneous rocks and the red sandstone series at Thor- verton and Silverton was pointed out, in 1821, by the Rev. J. Conybeare, " Annals of Philosophy," new series, vol. ii., p. 161. 568 IGNEOUS ROCKS IN THE LOWER PORTION OF [H. XXIX. of the igneous rocks, particularly on the north of them, there is an arenaceous deposit, here and there mingled with the ordinary sandstone, which bears a great resemblance to a volcanic product, so much so as to lead to the inference that it had been ejected in the manner of volcanic ash, and that, falling into water, it had been mingled with the mud, sand, and gravel, adjoining some volcanic vent of the time.* The igneous rocks of this date can be well studied at the base of the new red sandstone series from Exeter to Haldon Hill. They are seen at Pocombe Hill, resting directly on the edges of the dis- turbed and subjacent coal measures, and are chiefly formed of a siliceo-felspathic compound, with occasional though not numerous vesicles. These igneous rocks are also well exhibited between Ide and Dunchidiock, resting on similar accumulations. Near Western Town, the intimate connexion between them and the red sandstone and conglomerates can be seen. By reference to geological maps, it will be observed that the igneous rocks thus associated with the lower portions of the new red sandstone series near Exeter, Crediton, Thorverton, Kellerton, Silverton, and even near Tiver- ton, have been thrown out in a prolongation of the general direction of the granite bosses and el vans extending from the Scilly Islands to Dartmoor. By examining their component parts, they are observed to be formed of substances corresponding with those found in these granites and el vans. While many of them present a porphyritic character, others are more homogeneous in structure, and some- times vesicular. Much of the lower new red conglomerates and breccias in the neighbourhood of these igneous rocks is composed of fragments derived from them, so that these fragments, if again gathered together, would constitute no inconsiderable mass. Among them many porphyries are found, as well containing quartz as felspar. Masses of the igneous rocks from which they are derived are not often observable, though in such a district the portions visible on the surface afford no measure of the igneous masses which may be buried beneath a thick covering * The facts in this locality would appear to show, that along a range of ancient coast, of a date corresponding to the first production of the new red sandstone series of Devon and Somerset (see Maps of the Geological Survey, Sheets 20, 21 , 22), there was (1) a subaqueous valley, or depression, among the disturbed coal measures, there occurring, the partial abrasion of which, by breakers on the shores adjoining, produced (2) the shingles, and other detritus, now forming a conglomerate. Sub- sequently (3), igneous products were accumulated, probably ejected from a neigh- bouring vent, which, with others in South Devon, were then in action; and finally a partial destruction of these rocks affording (4) some of the materials for a conglome- rate, afterwards formed. CH. XXIX.] THE NEW RED SANDSTONE SERIES IN DEVON. 569 of detrital matter. It is, therefore, important to observe por- phyries in place, sometimes only containing quartz crystals; at others, these mingled with crystals of felspar, associated with the lower part of the new red sandstone series, at Ideston and Knole. Weighing all the facts thus observable, the geologist might be led to infer that the date of at least some of the elvans of Cornwall and Devon, though they are uncovered by deposits affording direct means for approximating to the time when they rose in the fissures where they are found, might not very materially differ from the commencement of those accumulations which constitute the lower portion of the new red sandstone series of that part of England, granitic matter constituting the base of the various rocks ejected, and being merely modified in its aspect according to the varied conditions to which it had been subjected. So much denudation has taken place in this region since these ancient igneous rocks were ejected, that no doubt many a mass showing any connexion which once existed between such igneous rocks as those near Exeter and other adjacent parts of Devonshire has been swept away. As illus- trating a denudation of deposits of the new red sandstone series in Devonshire, so that a portion of them only now remains, we have already noticed the Thurlestone rack in Bigbury Bay (fig. 47, p. 52), a detached piece of the small patch there occurring. Pro- ceeding still further westward to Plymouth Sound, a porphyritic rock, of the same general kind as those which are found near Exeter, is seen cutting through the Devonian rocks at Cawsand, and on the coast thence towards Bedding Point,* forming, as it were, a sort of connecting link between the elvans more westward and the igneous rocks above noticed, and appearing to constitute the denuded remains of the lower part of the new red sandstone series, extending, with an admixture of igneous products, in this direction, a small patch still remaining of the old continuous deposit at Bigbury Bay, and at Slapton, in Start Bay. With respect to the elvan dykes in the counties of Wicklow and Wexford, which in their mode of occurrence and aspect resemble those of Devon and Cornwall, though an observer does not appear to possess the same opportunities of inferring their relative dates, inasmuch as igneous rocks, composed of similar substances with * This porphyritic rock is a compound of felspar and quartz, containing crystals of mica, and, more rarely, of felspar, ft is of a somewhat earthy character, probably, from the effects of decomposition. The colour is reddish, as a mass, mixed occa- sionally with spots of bluish green. 570 CHEMICAL COMPOSITION OF IGNEOUS ROCKS. [Cn. XXIX. these elvans, have not hitherto been detected in the lower part of the old red sandstone covering up the disturbed rocks in which the fissures, filled by them, have been effected ; still, as the old red sandstone contains portions of the granite of the district, and is uncut by the elvans, it might . be inferred that the date of these elvans was not only posterior to the granite, but also anterior to the old red sandstone. They are to the granites of this part of Ireland what the elvans of Devon and Cornwall are to the granite of that part of England. They seem the result of cracks from the cooling and solidification of a crust, so to speak, of the molten mass beneath, such cracks passing through superincumbent rocks adhering to this cooled and solidified crust. The elvans of Wick- low and Wexford can be well studied, not only inland but on the coasts. Good examples of their mode of occurrence at the latter are to be found at Seapark Point, Wicklow. Of the two classes of igneous rocks above noticed, the one chiefly differs from the other chemically, in the presence, in part of one class only, of a larger proportion of lime and magnesia, these some- times replaced by oxide of iron. This difference is principally confined, as a whole, to that portion of one class which contains the mineral named hornblende in which the silicates of lime and mag- nesia, though somewhat variable in quantities, form marked ingre- dients, the lime alone mounting to from 10 to 15 per cent, of that mineral, and the magnesia varying from 15 to 25 per cent. Usually in these rocks the protoxide of iron more or less replaces some of the lime or magnesia of the hornblende. The presence of hornblende, when in proportions extending even to Jth or -J-rd of the mass, renders the rock in which it thus occurs far more fusible than the compounds of felspar, and silica, or of felspar, quartz, and mica, a difference due probably, in great measure, to the silicate of lime acting as a flux. In the other igneous rocks, those which have been ejected in a molten state (not referring to those which have been noticed under the head of modern volcanic products), and in the first place con- fining our attention to the great mass of them composed of two or more of the minerals named quartz, felspar (whether orthoclase or albite), mica and hornblende, as chief and prevailing substances, neither in the compounds of quartz and felspar, nor in that of quartz, mica, and felspar (orthoclase or albite), is there the same amount of lime as when hornblende enters into the mass. The prevailing mica in such rocks seems to be that commonly termed potash-mica, from that substance being a marked ingredient in it. CH. XXIX.] EFFECT OF SILICATE OF LIME IN IGNEOUS ROCKS. 571 In this mineral the lime is usually in very small quantity, com- monly under 1 per cent. In the lithia and magnesia micas it is rare, and, when found, has been so only in very small proportion. In the felspar, also, when either orthoclase or albite, members of this family apparently much distributed amid the older igneous rocks, lime has only been detected hitherto in small quantities, rarely in proportions equal to 1 per cent.* In compounds wherein labradorite is found, the case would be different, since this is a felspar in which lime usually occurs in comparatively large propor- tions, from 10 to 15 per cent. Silicate of lime, therefore, would appear to constitute a marked source of difference between the igneous rocks with and without hornblende and labradorite. With respect to the magnesia in many hornblendes, this also would be a substance of importance when compared with compounds of quartz, felspar (either orthoclase, albite, or labradorite), and mica, unless the latter were magnesia mica, into which this substance is found to enter in proportions varying from 10 to 15 per cent., or those varieties of felspar which have been referred to orthoclase, and yet contain from 10 to 20 per cent, of magnesia. As the trachytes of more modern geological times have been inferred to be some modifications of granites (p. 360), the observer might be induced to inquire how far the old igneous products noticed as occurring like certain of those of the present time may, in like manner, have been modifications of granitic matter beneath them ; how far, in fact, certain of the molten felspathic rocks of the British islands associated with the older fbssiliferous deposits may have been the trachytes of those times, and have been derived from granitic matter below them, such granitic matter afterwards upheaving these earlier modifications of portions of it, when geolo- gical time advanced and with it conditions for such a movement. With the hornblendic compounds there would be the same diffi- culty as with the modern dolerites and lavas of that class, so far as the silicate of lime was concerned, though in both cases, supposing silicate of lime to form a marked part of a fused mass, that it should be ready, as a substance aiding in fluxing others, to be thrown upwards, might be anticipated from the conduct in our furnaces of the slags into which silicate of lime largely enters. * Dr. Abich found 1-26 of lime in the orthoclase of the trachyte of Pantellaria, and 2-06 in the basis of the Drachenfels trachyte. The orthoclase of the older igneous rocks has not hitherto afforded any proportion of this kind, though at the same time, it must be confessed, that the igneous rocks of that date have not, as yet, received sufficient extended examination to arrive at any accurate results as to the chemical composition of the greater masses. 572 COMPACT FELSPAR AND PEGMATITE. CH. XXIX. Eespecting the compound of the matter of felspar and an addi- tional quantity of silica, beyond that required for the silicates in the minerals of that family, good opportunities are often afforded for studying the variable aspect it assumes as the conditions for cooling may have been favourable, or otherwise, to the crystallization of the felspar. When cooled so that the crystallization is not apparent, the compound has a homogeneous aspect, and is commonly known as compact felspar ; when confusedly crystallized and silica is well separated, as quartz, from the other ingredients, it forms one of those binary granitic mixtures sometimes termed granitello and pegmatite. Occasionally crystals of felspar being developed while the remainder of the rock retains its homogeneous character, a variety of hornstone porphyry is produced.* The variable aspect of the less crystalline varieties may be seen in numerous situations, the complete crystallization not so frequently .t In countries in which granitic matter has upheaved the prior superficial accu- mulations of this class, the resemblance of some kinds of products is sometimes so considerable as occasionally to lead to much ambiguity respecting their relative dates. * Of a columnar mass of the latter, the columns in part somewhat bent, a good example may be seen among the igneous products associated with the Silurian series west of Knock Mahon, on the coast of Waterford. t A good example of a binary compound of quartz and felspar may be found among the igneous rocks amid the Cambrian series, close to the town of Caernarvon, on the northward ; part of a portion of molten matter, in which the more common homogeneous mode of occurrence of the silicates of the felspar combined with the quartz, prevails. cit jU( v'' CHAPTER XXX. MODE OF OCCURRENCE OF GRANITES IN SOUTH-WESTERN ENGLAND AND SOUTH-EASTERN IRELAND. GRANITE VEINS. CHEMICAL COMPOSITION OF GRANITIC ROCKS. SCHORLACEOUS GRANITES OF CORNWALL AND DEVON. SLIGHT COVERING OF OTHER ROCKS OVER THE GRANITES OF WICKLOW, WEXFORD, AND CORNWALL. SERPENTINE AND DIALLAGE ROCK OF CORN- WALL. SERPENTINE OF CAERNARVONSHIRE OF ANGLESEA. CHEMICAL COMPOSITION OF SERPENTINE OF SERPENTINE AND OLIVINE COMPARED. COMPOSITION OF GREENSTONE AND SYENITE. RESEMBLANCES AND DIF- FERENCES OF ORDINARY GRANITIC AND HORNBLENDIC ROCKS. GRANITIC, FELSPATHIC, HORNBLENDIC, AND SERPENTINOUS ROCKS EJECTED AT VARIOUS GEOLOGICAL TIMES. RELATIVE FUSIBILITY OF IGNEOUS ROCKS. MODI- FICATION OF THE MATTER OF IGNEOUS ROCKS. ADDITIONAL MINERALS ENTERING INTO THE COMPOSITION OF ORDINARY IGNEOUS ROCKS. GENERAL CHARACTER OF IGNEOUS ROCKS. WHETHER the observer studies the granite of south-western Eng- land, or that of prior elevation in south-eastern Ireland, he finds the same general mode of occurrence, one very different from that of the igneous products associated with the Devonian rocks in one district, and the Silurian rocks in the other. There is"' no inter- stratification and contemporaneous intermingling of parts, but, on the contrary, evident protrusion in mass, and a subsequent filling of fissures traversing the beds of pre-existing deposits. In both districts, the granitic protrusions appear the accompaniments of great contortions, foldings, and even dislocations of prior accumu- lations of all kinds, as if, amid this squeezing and new adjustment of such accumulations, molten matter beneath rose upwards (there being sufficient pressure upon it), and occupied areas where the resistance of any prior superficial covering was insufficient to resist this intrusion. Upon examining the boundaries of the granitic masses observable on the surface, the amount of fractures affected around them, and in the various rocks adjoining, is found to be considerable. Indeed, 574 .MINOR FRACTURES AROUND GRANITE PROTRUSIONS. [Cn. XXX. where opportunities are afforded either by natural exposures or artificial sections, they are seen to be common. Thus, inde- pendently of any great movements or dislocations of prior-formed rocks of all kinds, the margins of the granitic intrusions are them- selves marked by abundant fractures on a minor scale, as if those intrusions had themselves in some measure been connected with their production. As to the extent of the fractures into the adjoining and prior-formed rocks, it may be considered as somewhat insig- nificant when regarded with reference to their mass and that of the granites. In the range of the Wicklow and Wexford granite, not only are these cracks found abundantly, but evidence is also afforded of huge detached masses of detrital rocks being apparently embedded in the external parts of the same granite. This can be well studied in Glenmalure, where such great masses seem as if partly contained in the granite, having floated on that rock when in a molten state, like great icebergs in the sea, and like them also in part submerged. No doubt this may be only appearance, as the parts connecting these masses may have been removed by denuda- tion. At the same time when sections are made of the whole on a scale equal for height and distance, and all the foldings of the older rocks are considered, a great breaking up of the latter seems needed to account for the mode of occurrence of all the rocks. No doubt that much of both the older rocks and the granite of south- eastern Ireland has been removed by denudation effected during a long lapse of geological time, often by abrasion from heavy breaker action, while rising above or descending beneath the ocean level ;* yet there still appears to have been disruption of the prior-formed Cambrian and Silurian rocks. The curve, which agrees with the * It is not a little interesting, in this part of Ireland, to study the denudation with reference to the exposure of both the granite and altered sedimentary rocks (for they and certain associated igneous products of that date are much modified and altered, as will be hereafter noticed) to the same degrading forces. The granite is often of a decomposing kind, while the altered rocks are, for the most part, tough ; hence the exposure of both to the same abrading force has caused the softer substance to be worn away more than the harder. In consequence the tough altered rocks have been the means of preserving much of the granite beneath them from removal. Lugna- quilla, the highest of the range, is capped by these altered rocks, now chiefly mica slates ; and many other examples of heights and flanks of mountains thus preserved may be seen. When this denudation is also studied with reference to an Atlantic exposure, the interest is not lessened, inasmuch as the western flanks of the moun- tains point to more abrasion on that side than on the east, just as would happen from the destructive influence of Atlantic breakers, rolling in, as now, from the westward. It requires very little imagination, when standing upon some parts of this range, to fill up the lower ground with sea, so that the Atlantic may break upon the cliffs beneath, facing the west. In the range of mountains near that named Blackstairs, the cliff character of the western flanks is very marked. Cn. XXX.] GRANITE VEINS. 575 upraised masses of the prior-formed rocks, and fortunately many of these are still preserved, showing the probable extent to which they have been so raised to a height above those crumpled and folded on either side, is of the kind represented beneath (fig. 218). This may not be considerable, yet it seems difficult to obtain the effects produced without much separation as well as disruption of parts of the older rocks. In this section, upon the same scale for Fig. 218. heights and distances, a a, is the intruded granite, c c, the contorted and older rocks on either side, altered near the granite, and b b b, portions of them uplifted, a large mass forming the summit of (L) Lugnaquilla. Upon examining the contents of the cracks in the prior-formed rocks surrounding the granitic masses, they are found filled with the granite in such a manner as to show the comparative liquidity of that substance when the cracks were made and filled, for even fine threads may be occasionally seen, branching out of the main cracks, with granitic matter in them. Though, as might be anticipated, the crystallization of this matter is modified in the finer fissures, from differences of the rate of cooling alone, the contents of the granitic veins generally would point to long-sustained heat among the intruded rocks, the whole having probably required a considerable lapse of time for solidification. The following sketch (fig. 219) of some granitic veins at Wicca Cove, or Pool, near Fig. 219. Zennor, Cornwall, may serve to illustrate the mode of occurrence of many of them, and the annexed section (fig. 220) will show their connexion with an adjacent mass of granite behind the rocks ex- posed in the sketch (fig. 219), a a, being the granitic veins, b b, altered slate, and c the main mass of granite. 576 GRANITE VEINS. [Cn. XXX. The west coast of Cornwall, exposing the junction of the granite of that district with the sedimentary deposits and the igneous rocks associated with them, offers many other illustrative instances, as at Pendeen Cove, Cape Cornwall, Tetterdu Point, Mousehole, and other places. They are also as well and easily seen at St. Michael's Mount. The following sketch (fig. 221), exhibits the section of a somewhat complicated fracture, the shaded parts (a a a) being altered slates, and the dotted portion granite, the mass of the latter occurring on the side b, b. Looking at these veins as a whole, it would often appear as if the prior-formed rocks had not yielded very slowly to the force applied, but in a comparatively sudden manner, the granitic matter being driven into the cracks, formed by heavy pressure, so as to fill up the fine fissures.* There often also seems evidence of cracks having been formed after only a mere comparative * Portions of these rocks are sometimes found completely isolated in the matter of the granitic vein. CH. XXX.] CHEMICAL COMPOSITION OF GRANITIC ROCKS. 577 film of the main mass of granite had been consolidated, granitic veins similar to those amid the prior-formed rocks, and clearly merging into the main mass of granite, being found alike to traverse a certain amount of the external parts of the granite and these other rocks.* In general such veins are easily to be distinguished from the elvan dykes (the result apparently of subsequent action) by their tortuous courses, and by their general resemblance to those first formed amid the older rocks at their junction with the granite.-)* The chemical composition of granitic masses will necessarily engage the attention of the observer, more especially when he con- siders that so much of the detrital deposits of all ages have been derived from granitic matter ; indeed, the volume thus distributed as detrital accumulations must be enormous. As has been seen, the elementary substances forming the chief part of the volume of this rock do not appear to be numerous. For certain of the modifi- cations of mineral structure it may be again desirable to refer to the portions of the British islands already noticed, since the relative ages of the igneous rocks in them are so well shown. Funda- mentally, the constituents of the granites in south-western England and south-eastern Ireland seem little different. The chief variations may probably consist in the greater admixture of schorl with the other constituent minerals in the former than in the latter ; indeed, generally speaking, schorl is rare in the granites of southern Ireland. Such differences can readily be considered as merely local, the same molten matter beneath having supplied the portions upraised at different geological times. Be this as it may, the presence of a mineral in any abundance which contains boracic acid as an essen- tial ingredient,^ is one of importance, more particularly when we refer to the researches of M. Ebelmen, he having shown that by * Instances of this kind are not uncommon, both in south-eastern Ireland and south-western England. They are well exhibited at Killiney Hill, near Dublin, and the large masses of granite brought from thence for the harbour at Kingstown often show them. They are also to be well seen in the granite of the Scilly Islands, and the exposed granites of the Land's End coast, as at Tol-Pedn-Penwith and Lamorna Cove. f In examining granitic countries it is very needful not to confound the filling of joints in granite with quartz, felspar, and mica, in the manner of fissures, including mineral veins, with the granitic veins noticed in the text; such modes of filling being very deceptive, unless due care be employed. They can, however, be usually well distinguished by the manner in which the minerals occur in them, showing a deposit from solutions against the walls of the granitic fissures, the crystals pointing inwards, and arranged in the manner of many common and mineral veins. The analyses of M. Hermann give about 10 per cent, of boracic acid in schorl, 39 of silica, 31 of alumina, a variable quantity of protoxide of iron (4 to 1'2 per cent.), 2 to 9 of magnesia, with a few other subordinate, and, probably, accidental substances, such as lithia, soda, and potash. 2p 578 SCHORLACEOUS GRANITE [Cn. XXX. employing that acid as a solvent, at an elevated temperature, minerals may be produced by the evaporation of this solvent, some of them gems, such as rubies, which are usually termed insoluble, and infusible in our furnaces, a result having a considerable bearing upon the production of many igneous compounds.* Cornwall and Devon present frequent and good opportunities for the study of schorlaceous granites and rocks composed of schorl and quartz (usually termed schorl rocJc) in connection with them. As might be expected from the comparatively easy removal of boracic acid by considerable heat, the chiefly schorlaceous compounds are found at the extreme parts of the granitic masses. They vary from a simple binary compound of schorl and quartz to mixtures of schorl, felspar, quartz, and mica; the latter is, however, not an usual ingredient in the granitic rock when schorl is present in any abundance. Complete passages may frequently be traced between the ordinary compound of quartz, felspar, and mica, by the gradual loss of the felspar and mica, into the simple mixture of quartz and schorl, the mica being commonly the first to disappear. The schorl sometimes presents itself in radiating bunches of crystals, especially amid the quartz, t Here and there different arrangements of schorl * The researches of M. Ebelmen on this subject are marked by the true spirit of philosophic investigation. He sought for a substance which at a high temperature acts like water, as regards others dissolved in it. As by the evaporation of water certain crystalline bodies might be formed, so, he inferred, that by employing those which could be volatilised at high temperatures, yet at a given heat, while in fusion, be capable of dissolving the greater part of metallic oxides, certain calculated propor- tions of some oxides would crystallize, when the dissolving body was evaporated in open vessels at a great heat. Acting upon this view, and selecting boracic acid as the solvent, he was completely successful, producing rubies, sapphires, spinels, chryso- beryl, chrysolite, chromate of iron, and others. Crystals of emerald were formed from pounded emeralds, when fused with boracic acid and a little oxide of chromium. The crystals of chrysoberyl were sufficiently large to have their optical properties tried, and these were found to be identical with those of the natural mineral. t Good examples of nests of schorl in quartz, the crystals radiating, may be seen in the Dartmoor granite, as above Bowdley, near Ashburton. Schorlaceous granite and schorl rock can be also well seen in the same granitic district at Holne Lee, and on the south of the moor, as also near Tavistock. The granite of the Brown Willy mass is not so schorlaceous, though schorl is found, especially towards the south. Near St. Cleer, there are compounds of schorl, felspar, quartz, and mica, similar to some found on Dartmoor. The St. Austell granite is much more schorlaceous, veins of that mineral being common in it. The decomposed granite of that district, furnishing so much clay to the porcelain works of England, is extremely schorlaceous. Singular stripes of schorl rock are found at its outskirts, as between Watch Hill and Long- lane ; on the north and south of Burthy Row, near St. Enoder, and at the long- celebrated Roche Rock. Near Meladore there is an interesting mixture of schorl and quartz, containing large crystals of felspar, some of these decomposed, and crystallized schorl introduced into the cavities left by them. At Calliquoiter Rock there are variable mixtures of schorl, quartz, felspar, and mica, the outside portions formed of the two former. The granite of St. Dennis Hill is in like manner a com- pound of these four minerals. The Cam Menelez granite is not so schorlaceous, OF CORNWALL AND DEVON. 579 CH. XXX.] and of the other minerals are observed. The following (fig. 222) is a somewhat marked instance of the adjustment of varied corn- Fig. 222. 2 feet. pounds round a kind of central nucleus. It occurs in the Dartmoor granite, towards Camwood, a is a cavity not quite filled by long crystals of schorl, crossing in many oblique directions, but with a general tendency towards the centre ; b is an envelope of quartz and schorl, the former predominating ; c, another covering of the same minerals, the schorl being more abundant; and d, a light flesh-coloured granite, the felspar predominating. Large crystals of felspar are not uncommon in the granites of Cornwall and Devon, rendering the rock a porphyritic granite. That of Dartmoor is not unfrequently of this character, as is also the granite of the Brown Willy district, and the same variety may be seen in many other localities.* The granites of south-eastern Ireland are also occasionally porphyritic, from the distribution of felspar crystals amid the ordinary triple compound of quartz, felspar, and mica. Throughout these districts, though the granite may enter the fractures of the adjacent and prior-formed rocks, there is no trace of an overflow of the igneous matter in a molten state, so that the observer is led to infer that, when the intrusion was effected, the though schorl is found, and more especially at the confines of the mass. The Land's End granite is schorlaceous to a considerable extent. A variety of schorl rock, composed of a base of schorl and quartz, with large crystals of felspar, is found close to Trevalga, near St. Ives. Here also, in some parts, the crystals of felspar have been decomposed and removed, and the cavities more or less filled with crystals of schorl. * As chiefly differing from the ordinary granite, that of St. Austell is probably the most marked, a steatitic mineral therein replacing mica to a great extent, particularly in the portions which are found in a decomposed state. Much pinite (a silicate of alumina and magnesia, the latter partly replaced by protoxide of iron) is mingled with a part of the granite near the Land's End. 2i P 2t 580 SERPENTINE AND DIALLAGE ROCK OF CORNWALL. [Cn. XXX. igneous rock was not as now exposed to the atmosphere, or beneath waters in such a manner that it could pass beyond the broken por- tions of the deposits now forming its superficial boundaries, and flow over them in the manner of lava discharged from a volcanic vent. If any portion of these granites did so pass over prior- formed, consolidated, and disrupted rocks, all traces of such over- flows have been removed by denudation. Molten matter in a sufficiently fluid state to enter the smaller ramifications of the cracks around the masses of granite, would readily, if elevated sufficiently high, overflow the disrupted and contorted deposits amid which it was protruded. The covering of the granite of south-eastern Ireland is comparatively slight ; the whole district adjoining the main masses of that rock is so pierced and cut by it, as to show upon the surfaces exposed, that the whole of the prior- formed accumulations has been upborne, so that upon the denuda- tion of the various inequalities, the granite was unequally exposed.* The same may be said with reference to south-western England between Dartmoor and the Scilly Islands.. There is yet another igneous product in a part of this limited area to which a relative geological date may be assigned. This product is serpentine, which is chiefly found in considerable abun- dance in the Lizard district, in Cornwall. It is seen among the Devonian rocks in a manner reminding us of the mode of occur- rence of some of the contemporaneous compounds of felspar and hornblende, which have been associated, in a molten state, with the sedimentary deposits of that date. That it was vomited forth anterior to the granite of the district, would appear from its being traversed by veins of that rock, in the same manner that other rocks of the district are traversed by them. Even allowing that these veins may be of no greater antiquity than the elvans of the same county, this would limit the fissures for their introduction to about the age of the lower new red sandstone deposits of that land. At Clicker Tor, south of Liskeard, serpentine is found amid Devonian slates, and near Veryan, diallage rock (diallage and felspar) is seen associated with similar serpentine, and in a manner pointing to an ejection of these rocks in the same way as certain greenstones amid accumulations of the igneous products of the * The granite of the island of Anglesea, probably of about the same date, is also interesting, as showing how readily it might be concealed from superficial exposure, by a somewhat more thick envelope of the Silurian rocks through which it has risen. Indeed some of the portions exposed are merely minor inequalities cut into by de- nudation. CFI. XXX.] SERPENTINE AND DIALLAGE ROCK OF CORNWALL. 581 district. The position of the Lizard serpentine, and the diallage rock found with it, seems much the same with these minor portions of serpentine more eastward. The Lizard serpentine occupies a somewhat large area, reposing upon hornblende slates and rock, which appear little else than the ordinary volcanic ash-beds above mentioned as intermingled contemporaneously with the ordinary detrital deposits of the time and locality (p. 558).* There is often an apparent passage from the diallage rocks into the serpentine,! while also there seems an intrusion of serpentine amid the former, as between Dranna Point and Porthalla. Though there may be some intermixtures of the serpentine and the diallage rock rendering their relative antiquity a little doubtful in places, as a whole, the latter would appear to have been thrown up after the former. At the junction of the diallage rock of Crousa Downs and St. Keverne, with the serpentine at Coverack Cove, veins of the former cut through the latter.J On observing, also, the connexion of these two rocks, in a range ex- tending from Careglooz through Gwinter towards Groonhilly Downs, the diallage rock seems to have cut through and disturbed the serpentine. Near Landewednack, also, the diallage rock appears to rise through the hornblende slates and cut into the serpentine. This diallage rock, as between Coverack Cove and St. Keverne, * It is not altogether clear whether this alteration may not be due to the influence of some granitic mass beneath, with which the granite veins, traversing the serpentine, may be connected, such granitic mass closer to the latter than might be inferred from the natural sections, inasmuch as beneath the hornblende rocks and slates, there are talco-micaceous slates to a certain extent interstratified with the latter, much remind- ing the observer of the various alterations effected in the proximity of the granites of the district. A glance at the Geological Survey Maps (Sheets 23, 24, 25, 30, 31, 32), or at the Index Map in the Report on the Geology of Cornwall, will show that there may readily be a line of granite concealed beneath the sea, and ranging in a somewhat general manner with the granite from Dartmoor to the Land's End, which has caused the alteration of the rocks into the mica slate and gneiss of the Start Point, and Bull Head, Devon, and produced the gneiss on which the Eddystone Lighthouse, in front of Plymouth Sound, is erected, and the talco-micaceous slates of the Lizard Point. The connexion of the hornblende slates with the latter may be conveniently seen near Poltreath, on the west of the Lizard Town. t As we have elsewhere remarked (Report on the Geology of Cornwall, &c., p. 30), " whatever the cause of this apparent passage may have been, it is very readily seen at Mullion Cove, at Pradanack Cove, at the coast west of the Lizard Town, and at several places on the east coast between Landewednack and Kennick Cove, more par- ticularly under the Balk, near Landewednack, and at the remarkable cavern and open cavity named the Frying Pan, near Cadgwith. It will generally be found that, at this apparent passage of one rock into the other, there is calcareous matter, and a tendency to a more red colour in the serpentine near its base than elsewhere. J The veins of diallage rock in the serpentine between the rivulet in Coverack Cove and the pier at the village will repay examination. Some of them are large grained, the crystals of diallage of considerable size, reminding the geologist of the larger-grained gabbro of Italy. 582 SERPENTINE OF CAERNARVONSHIRE. [Cn. XXX. passes occasionally into a compound, in wjiich hornblende also enters ; so that while in some places it appears a mixture of diallage and felspar, in others it more resembles one of hornblende and felspar. Eegarding a mixed mass of matter in which the propor- tions of the chief substances, silica, magnesia, lime, alumina, and oxide of iron, may be unequally disseminated, such changes may be readily appreciated, the conditions for the adjustment of the substances in crystalline forms being variable.* With respect to the serpentinous rocks in Anglesea and Caer- narvonshire, the relative and approximative dates are not so certain. At Porthdinlleyn, a rock, which has been commonly termed serpentine from its appearance, though not altogether agreeing with the usual varieties of that rock, has apparently traversed the chloritic and micaceous slates of that part of Caernarvonshire ; but being only covered by a raised sea-bottom of comparatively recent geological date (p. 458), the time when this may have been effected remains doubtful, though an impression of its intrusion being even referable to the date of some of the older rocks of the district might exist. In its greenish and red colours, it much resembles the ordinary serpentines. The component parts are much gathered together in some situations in irregular nodules, Fig. 223. between which much red jasper is frequently found, -f- as in the * Looking at the principal ingredients in hornblende and diallage, as given by a mean of three analyses of the former by Goschen, Bonsdorff. and Struve, and by a mean also of three analyses of the latter by Kohler, Regnault, and Von Kobell, the differences between these minerals would be as beneath : Hornblende. Diallage. Silica .... 40-86 52-00 Magnesia . . . 13-54 15-91 Lime .... 12-35 19-59 , Oxide of Iron . . 14-54 7-47 Alumina ... 15-96 3-18 t Judging from the frequency of jasper fragments of precisely the same kind in the superficial drift of the district, fragments of even several hundredweights being found (Aberdaron), there would appear to have been much destruction of rocks similar to that of Porthdinlleyn, perhaps of a softer kind, the jaspet^from its hardness, being preserved and included amid the other hard detritus. CH. XXX.] SERPENTINE OF ANGLESEA. 583 annexed sketch (fig. 223) taken towards the north-western point of the roadstead, where the dark portions represent the jasper, or other siliceous matter between the nodules, sometimes of large size. Of the serpentine in Anglesea, the aspect of which presents much the usual characters of that rock, though some of it may have been in a molten state when included among the beds where it is now found, other portions much remind the geologist of some mingling of calcareous and serpentinous matter, altered from the state of the original accumulation of their component parts. This may be the case with part of the serpentine at Cerig-moelion, as also at Rhoscolyn. There are also some appearances near Amlwch, amid the bedded rocks there found, as if certain of the contemporaneous beds had taken a serpentine character from the conditions fox the adjustment of their constituent ingredients, to which the whole of the associated beds had been exposed, having been favourable to such a modification of parts. As to an accu- mulation of serpentinous matter in the manner of the felspathic and hornblendic rocks so common in North Wales, contempora- neously with the Silurian deposits, there would not appear any par- ticular difficulty, since, even without supposing an outburst of serpentinous matter in the manner of volcanic ashes and cinders, (though why this may not also have happened does seem clear), the wearing away of serpentine rocks, formed at an earlier date, may readily have supplied the detrital materials for deposits, which when consolidated presented the character above mentioned. At all events this appears a mode of occurrence which it would be desirable that the observer should bear in mind, and the more so that in some other localities for serpentine in the British Islands, as, for example, in the county of Galway, there are some inter- laminations and other modes of occurrence of serpentinous and cal- careous matter, suggesting to the geologist that such mixtures may have been arranged in water, the accumulations subsequently acted upon so that the present structure of the rocks was produced.* This brings us to consider the chemical composition of the ser- pentines mentioned, viewed geologically. They are of very varied mixtures of a kind of base of silicate of magnesia with silicate of alumina, and occasionally of soda and potash, as also of oxide of * An examination into the chemical composition of some large pilasters of this serpentinous rock, in the Museum of Practical Geology, London, showed that it was a mixture of silicate of magnesia and carbonate of lime, with minor quantities of oxide of iron and alumina. The interlamination of the chief portions of the mixture is often most marked in parts of this rock. 584 SERPENTINE AND OLTVINE COMPARED. [Cn. XXX. iron. Water is likewise a marked ingredient. Amid all this variety, among which those serpentines may be included through which diallage may be disseminated (a compound common in parts of the Lizard district), more pure serpentine (as it is inferred) is to be found ; that is, the serpentine which has been often considered as a distinct mineral species (how far correctly remains to be de- termined), and which is a silicate of magnesia combined with water, and a minor portion of oxide of iron,* Looking at the chemical composition of the common igneous product olivine, the observer finds that it also is essentially a silicate of magnesia with oxide of iron, the presence of water as an essential ingredient in serpentine being the marked difference between it and olivine. t This is an interesting circumstance, pointing to the very moderate modifica- tions of constituent parts which may produce mineral aspects of such a varied kind. Taken in the mass, the serpentine of the Lizard seems often a compound into which alumina enters as a marked ingredient, thus more resembling, in that respect, the substance named soapstone, occurring in veins in it, and which is a compound of silicate of magnesia and alumina. J As a substance also worthy of notice, since so frequently occurring in small veins in portions of the rock, asbestus should not be neglected, its com- ponent parts being apparently derived from the mass of serpentine amid which it is found. Though the minerals so named appear of varied chemical composition, and have been regarded as members of the hornblende family, the asbestus of the Lizard seems chiefly a silicate of magnesia, more like the selected serpentine inferred to be a mineral species, without its water. Quitting this minor area, mentioned merely because the igneous products noticed may be there referred approximately to certain * The chemical composition of these selected portions of serpentine is inferred to be Mg 3 . fci 2 . + 2 H. t Taking the composition of the serpentine and of olivine from the 13 analyses of each by several chemists, such as are given by Professor Nicol, in his Manual of Mineralogy, the similarity or difference would be as follows : Serpentine. Olivine. t' Silica .... 41-99 41-92 Magnesia ... 40'24 46-67 Oxide of Iron . . 3-38 10-75 Water .... 12-68 The small quantities of alumina, lime, soda, and carbonic acid, in a few of the selected serpentines, and of alumina, lime, and the oxides of manganese, tin, nickel, and chrome in some of the olivines are not here noticed. J According to Klaproth, a soapstone from the Lizard district, contained, silica, 45 ; alumina, 9-25; magnesia, 24-7; peroxide of iron, 1 ; potash, 0-75; and water, 18. Svanberg found in a soapstone from the same locality, silica, 46*8 ; alumina, 9 ; mag- nesia, 33-3 ; peroxide of iron, 0-4 ; lime, 0'7; and water, 11. CH. XXX.] COMPOSITION OF GREENSTONE AND SYENITE. 585 geological dates, and the localities can be easily visited, and passing to more extended and distant regions, the geologist will scarcely fail to be struck with the similarity of various igneous products in each, these being to a certain extent classified. Those which have been termed volcanic and extinct volcanic, with reference to the present time, have already been noticed as presenting certain marked resemblances in different parts of the earth's surface. The same general resemblance will be found in those products in which the minerals of the felspar and the hornblende families prevail, with or without an excess of silica (occurring as quartz), in various regions. Though their real modes of occurrence may not always have been properly ascertained in the numerous and different loca- lities, whence specimens and notices of them have been obtained, and though certain accounts of their manner of association with other rocks may require more attention to the methods of investiga- tion which the progress of knowledge now requires, there is, nevertheless, frequently sufficient to show the great mineral resem- blance of many of these igneous products in widely-distributed parts of the earth's surface. Viewed chemically, there is yet much to be accomplished respecting them, particularly with regard to any modifications as to the prevalence of some simple substances more at one time than at another, as also more in certain regions than in others. Of the class of igneous products to which the name greenstone has been given from that crystalline state wherein the constituent minerals, felspar and hornblende, are distinctly seen associated in variable proportions, to the rock wherein the matter of these minerals has not been exposed to the conditions fitted for its separate adjustment in that crystalline form there are endless varieties. With an excess of silica, beyond that required for the silicates of the component minerals, syenite is produced, quartz being then distinctly added to the other two minerals. Again, it sometimes happens that while there is a granular arrangement of the felspar and hornblende, even occasionally with the addition of quartz, crystals of felspar are disseminated through the mass, forming a greenstone or syenitic porphyry, as the case may be. Some of the compact varieties, termed compact felspar, have already been noticed (p. 572). Altogether the shadows and shades of modification have been found so numerous, depending on variations of chemical composition on the one hand, and on dif- ferent conditions for cooling on the other, that there has been a disposition to seek some term for the whole, which shall leave the exact composition of the rock open to description, while a kind of 586 RESEMBLANCES AND DIFFERENCES BETWEEN THE [Cn. XXX. generic name is preserved. The name of trappean rocks* has been somewhat adopted of late, particularly by British geologists, for this class of igneous products. It is one, no doubt, open to objection if regarded as a name to be preserved; but in the present state of knowledge, this or some other general term has its convenience as massing together certain products of a family character. This class of igneous rocks appears to be found amid accu- mulations of all geological ages, from the older deposits to the accumulations which approximate to the date of those amid which the basalts and associated products, previously mentioned (p. 402), are seen, having been thrown out from some points on the earth's surface, however these may have varied in position. Seeing that their mode of occurrence is such, even amid the old Silurian deposits, as to remind us of the products of modern volcanos, it may be inferred also as probable that from that geological date to the present time, rocks of a similar kind have formed portions of the products discharged from igneous vents, similar to those now scattered over the surface of the earth. Looking at the granitic rocks as a class, they also are found to present a great family resemblance in different parts of the world, though sharp distinctions between them and those previously mentioned cannot always be found, the one class passing into the other, especially when the hornblendic minerals are absent, in a manner resembling the modifications only of some general amount of given substances. When these minerals are present as is some- times the case, the chief chemical differences between such mix- tures and more ordinary granites, appear to consist in the abundance or scarcity of the silicates of lime and magnesia, these substances forming comparatively a small portion of the granitic rocks, viewed on a large scale, while they enter conspicuously into the composition pf the hornblendic rocks.f Where the two classes are found passing into each other, it often becomes desirable to see how far the hornblendic rocks may have been previously thrown out and * This term has been derived from the Swedish word trapp, a stair, it having been once supposed that an arrangement in stair-like forms, on the large scale, was characteristic of these rocks. t With reference to the difference or resemblance between granites and green- stones, as we have elsewhere remarked (Researches in Theoretical Geology, p. 397, 1834), " granites, no doubt, vary in their chemical composition, and so do greenstones, yet they always so differ from each other as masses of matter, that the one can never become the other from mere differences in cooling." If we suppose the felspar to be of the ordinary potash kind, and a granite to be formed of two-fifths of such felspar, of two-fifths of quartz, and one-fifth of mica (containing fluoric acid), and a greenstone CH. XXX.] ORDINARY GRANITIC AND HORNBLENDIC ROCKS. 587 consolidated, and have been reraelted by the granitic rocks, so as to have thus formed an addition to their original molten mass, the whole, upon cooling, having its constituent parts so adjusted as to present the appearances observed. As a common character, the granitic rocks seem to be chiefly formed of silica and alumina, after which come, as principal ingre- dients, potash and soda, the latter sometimes more prevalent pro- bably than has been usually inferred. The silica and alumina often constitute 80 per cent, of the whole mass, thus leaving only 20 per cent, for the other substances. In cases where labradorite is the member of the felspar family present in granitic rocks, either alto- gether replacing other felspars, or associated with them, lime would form an ingredient of importance,* though silica and alumina would still constitute the most marked substances in such rocks. Suffi- cient examination has not yet been given to granitic rocks to show us the relative prevalence of soda, potash, or lime (in cases of labradorite), during the progress of geological time. Taking the granite of Wicklow and Wexford, above noticed, it would appear that soda occurred in some fair abundance in the granitic rocks, protruded in that part of the world, anterior to the accumulation of the old red sandstone. As to the geological times when granitic rocks have risen through prior-formed, and usually disturbed, deposits accumulated to be composed of the same kind of felspar and an equal proportion of hornblende, the calculated differences may be taken somewhat as follows (Geological Manual, 3rd Edition, p. 448-50) : Granite. Greenstone. Difference. Silica . 74-84 54-86 19-98 Alumina 12-80 15-56 2-76 Potash . 7-48 6-83 0-65 Magnesia Lime 0-99 0-37 9-39 7-29 8-40 6-92 Oxide of Iron 1-93 4-03 2-10 Oxide of Manganese 12 Fluoric Acid . . 0-21 O'll 0-75 0-01 0-54 * The presence of lime amid igneous products, though it may there occur as a silicate, is interesting as affording the base of a supply for some, at least, of the calcareous matter required by animal life, or distributed as ordinary limestones. However powerful silica may be, acting as an acid where heat, and especially great heat, is employed, at the lower temperatures it is comparatively weak. As for example, at great heats the silicates of potash and soda are readily formed, whether carbonic acid be present or not, but at low temperatures, solutions of the silicates of potash or soda are easily decomposed by the carbonic acid. So also with silicate of lime, if that substance were in contact, in the presence of water at a moderate tem- perature, with carbonic acid, it would be decomposed, forming carbonate of limo, and if the carbonic acid were in sufficient abundance, bicarbonate of lime, ready to be removed in solution. 588 SERPENTINOUS ROCKS EJECTED AT VARIOUS TIMES. [Cn. XXX. by the agency of water, they would appear to include all from the earliest, even to the production of comparatively recent beds of the tertiary series. Of the latter kind, Mr. Pratt has found instances in Catalonia.* Thus there is no conclusion to be drawn as to the relative antiquity of these rocks from the mere fact of their occur- rence in any particular locality. This has to be sought in the manner in which they may be found associated with other accumu- lations, the relative geological dates of which are determinable. The serpentines, also, and their not unfrequent associate diallage rock, seem to have appeared with somewhat common characters through a long range of geological time. They have been above mentioned as probably of early date in Wales. In Cornwall, though not of equal antiquity, they are apparently still referable to the earlier geological times. In Ireland, also, they seem to have been formed at a remote geological period. Various lands show that they were not confined to those times, but became associated with accu- mulations of less antiquity; and in Italy, where there are many good opportunities of studying these rocks, they have been found amid deposits up to those of the tertiary times included, it being inferred that the rocks in that land which contain the fossils named nummulites were, as pointed out by Sir Roderick Murchison,t ac- cumulated at a time when the lower deposits of the tertiary series were effected in several other parts of Europe. The occurrence of serpentine and diallage rocks amid the Alps, and among the various accumulations of the Jurassic and cretaceous series, usually cutting through them in Italy, and in the continuation of the same accu- mulations, eastward, in different localities into Asia, is a marked circumstance. These rocks were probably ejected from beneath, at various geological times, over the area of Europe, from the early fossiliferous deposits up to some part of the tertiary series included. So much of various parts of the world remaining to be examined geologically, it would be premature to conclude that these rocks have not been ejected at more recent geological times in some localities. It has been seen that into the serpentines, magnesia enters largely, the relative amount of that substance being somewhat characteristic, as lime and magnesia combined, are among the hornblendic rocks. It would not, however, be right to infer that silicate of magnesia is alone to be regarded, since the mixtures in which diallage is dis- * Pratt, MSS. t Journal of the Geological Society of London, vol. v., p. 157. CH. XXX.] RELATIVE FUSIBILITY OF IGNEOUS ROCKS. 589 geminated and even prevails, show that other marked substances have entered into the composition of the mass when in a molten state. In such arrangements of parts of the compound, the ingre- dients needful for diallage have merely separated out from it under the fitting conditions, the lime, oxide of iron, and alumina having probably been in a more disseminated state previously.* Sometimes the base of the rock, still termed serpentine, from its general aspect, and the diallage crystallized out from the general mass, appear of nearly the same composition.! With respect to the fusibility of the igneous rocks generally, they no doubt present considerable differences. At the same time, it is needful to bear in mind, that experiments upon them, in the con- dition in which we find them, do not exactly give us the measure of their fusibility when they were in a molten state. Prior to the adjustment of the parts of many into minerals of a definite kind, they must often have been far more fusible, as can be shown by again placing them under their old condition of a molten mass, producing the vitreous adjustment of parts, so that these definite compounds be not again formed.J It hence becomes desirable to view the fusibility of these rocks, with reference to a complete mixture of all their constituent parts, anterior to the separation of any, or the whole of them into crystalline compounds. * M. Berthier found the diallage from La Spezia, a locality very favourable for the study of serpentine and diallage rock, to be composed of Silica 47-2 Magnesia . 24' 4 Lime .13-1 Protoxide of iron 7*4 Alumina 3- 7 Water 3-2 f According to Dr. Kohler (Thomson's Mineralogy, &c., vol. i., p. 174), the composition of the diallage, and of the rock containing it at Harlzburg, is as follows : Diallage. Rock. Silica 43-900 42-364 Magnesia 25-856 28-903 Protoxide of Iron and Chromium 13-021 13- 268 Protoxide of Manganese . . . 0-535 0-853 Lime 2-642 0-627 Alumina 1-280 2-176 Water 12-426 12-074 J In experimenting upon the fusibility of igneous products, we have often found very considerable difference in that fusibility, after some crystallized and compound rock had been formed into a glass, from that which it had exhibited when first acted upon by the same amount of heat employed. In the same manner, artificial glasses which have been melted and cooled slowly, so as to form a stony mass, or merely exposed to a temperature at which a certain crystallized arrangement of their constituent parts is produced, become more difficult of fusion than when in their first state. 590 MODIFICATION OF THE MATTER OF IGNEOUS ROCKS. [Cn. XXX. If we are to regard certain of these rocks to have been ejected from volcanic vents in the manner of modern volcanos, it seems also needful to consider that they have been accompanied by outbursts of vapours and gases, sublimations of different kinds having taken place at those different times as now. As a substance very common among the compounds of felspar and hornblende, even of those con- temporaneously thrown out amid the older fossiliferous rocks, sul- phur combined with iron is very common, indeed, sulphuret of iron is often a marked ingredient among those which are commonly termed greenstones. Not, however, that it is confined to them, for the more felspathic products often also contain it.* Amid the various modifications and changes of structure to which the deposits associated with certain igneous products have been often subjected, it is to be expected that the latter having been exposed to similar conditions, would, in like manner, have their parts also much modified. Indeed, those igneous products which have been versicular, show, by the various mineral substances found in them, that mineral matter has often been in movement in proper solvents, and passing through its pores, had adjusted itself in the cavities of the versicular rock as definite mineral compounds. Numerous soluble substances, once disseminated amid the general mass of such rocks, may readily have been -transported elsewhere, and aid in forming, by new combinations, less soluble substances. Thus many are found disseminated amid modern volcanic products, which, assuming that they were once disseminated amid those of ancient times, would scarcely be now detected in the latter. As regards the conditions to which igneous rocks of ancient geological date may have been exposed during the lapse of time, it would scarcely be expected, when they may have been subjected to the influence of long continued heat, from any depression to considerable depths, especially beneath a thick covering of other deposits, that any obsidians would preserve their vitreous character, such disap- * It sometimes happens that iron pyrites is found in prior-formed deposits of ordinary detrital matter, adjacent to protrusions and dykes of these igneous rocks, in such a manner, as if either the sulphur, or sulphur and iron had been derived from them. A good example of this mode of occurrence may be seen at Bettws Disserth, on the north of Builth, South Wales, where spheroidal pieces of iron pyrites occur in a Silurian slate adjacent to some hornblendic rocks; these spheroids somewhat abundant in places, and the slate having all the appearance of having been altered by the intrusion of the igneous rock. At the falls of the Wye, near Builth, much iron pyrites is also seen at the contact of some igneous rocks intruded among slates, in like manner altered, certain fossils in them being likewise coated with the same mineral near the contact of the two rocks, though this is not observed at a short distance from it. CH. XXX.] ADDITIONS TO ORDINARY COMPONENT MINERALS. 591 pearing from the usual causes productive of devitrification, the component substances taking a stony form. As to the minerals which appear, as it were, additions in different localities to the general masses of granite, and even to those rocks where hornblende and felspar chiefly constitute^ the component minerals, they are often very various, and, as M. Elie de Beaumont has remarked with respect to granite, much distributed outside their masses.* While they are often merely some other arrange- ments in different proportions of the simple substances contained in the general mass, | at others, they appear as if in some manner the result of an addition derived from the rocks, against which the molten mass has been thrown, and thus formed during the long conti- nuance of those conditions (among which great heat is prominent) that have prevailed after the uprise of such igneous rocks in different localities. Among these minerals, garnets of different kinds may be remarked, as occurring as well in the igneous as in the prior- formed, and subsequently modified rock, against which the former has been thrust. When we consider the various substances which analyses seem to show are, as it were, entangled amid those consti- tuting the chief mass of the igneous matter ejected,J it would be anticipated that when these were relatively abundant, and could make their own adjustments more freely, less controlled by the in- fluences of those forming the chief minerals, compounds would be effected of a definite kind and be separated from the main mass. Thus, occasionally, mixtures would be formed of more than the usual substances, even constituting masses of importance in parts of the earth's surface, where, though the usual free silica and silicates of ordinary granite and other compounds were still the most pre- valent substances, others are present, giving a somewhat modified character to the general rock. With respect to the occasional component parts of granitic * Sur les Emanations Volcaniques et Metalliferes. Bull, de la Soc. Geol. de France, 2nd se'rie, t. iv. (1847). f In talc, a mineral sometimes associated with others in granites, we seem to have magnesia in a certain relative abundance, separating itself from a main mass in which it may usually have been a subordinate substance, talc being essentially a silicate of magnesia. Its formula is considered to be 3 M Si + Mg 3 Si*. J M. Elie de Beaumont, in his table of the distribution of simple substances in nature (Bulletin de la Soc. Geol. de France, 2nd serie, t. iv.), considers the 4 following to be found in granite, viz. : Potassium, sodium, lithium, calcium, magnesium, yttrium, glucinium, aluminium, zirconium, thorium, cerium, lanthanium, didymium, uranium, manganese, iron, cobalt, zinc, tin, lead, bismuth, copper, silver, palladium ?, osmium, hydrogen, silicon, carbon, boron, titanium, tantalum, nobium, pelopium, tungsten, molybdenum, chromium, arsenic, phosphorus, sulphur, oxygen, chlorine, and fluorine. 592 ADDITIONS TO ORDINARY COMPONENT MINERALS. [Cn. XXX. rocks, chlorite should be mentioned as one of some importance, inasmuch as while it shows a modification of the mixture and relative proportions of some of the ordinary constituent ingredients of granitic minerals, silica, alumina, magnesia, oxide of iron, and oxide of manganese, it also points to water as an essential ingre- dient. When disseminated, therefore, among granitic rocks, as it is in the Alps, Scandinavia, and some parts of the British Islands, chlorite becomes a combined mineral of no slight interest, from the addition of water to the other substances present.* As respects the various minerals, which are, as it were, ad- ditional to those usually constituting the mass of the chief divisions of the igneous rocks, not only has the dispersion, in variable pro- portions, of other substances than the usual ingredients to be re- garded, considering these likewise in their greater or less local proportions, but also the additions which may be derived from the melting of parts of prior-consolidated accumulations, even of those thrown down from solutions in water, and fused by the intrusion of the igneous rocks. Though the chief portions of the ordinary detrital deposits are but abraded parts of previously-consolidated igneous rocks which have been worn away, and then dispersed as above noticed (pp. 63 101), this has been most frequently so accomplished that a remelting of the deposits thence formed, would not reproduce the original rock, the various parts having been separated mechanically into different beds, and decomposition having deprived certain of even the separated substances of por- tions of their original ingredients. With respect to the latter, for example, should the silicates of soda or potash have been removed in solution, as has often happened, from a felspar of which they once constituted a part, the matter again fused might not contain any of those silicates, so far as the felspar is regarded, silicate of alumina being then the prevailing substance.f Igneous matter, the usual granite compounds, for instance, melting limestone rocks, the lime might be introduced into the molten mass, and the carbonic acid being thrown off, the silicates of lime be formed, ready for combination in other minerals than those resulting from the mass of the granite, as it rose from beneath. So also with dolomite, which * Taking various analyses, from 10 to 11 per cent, of water enters into the com- position of chlorite. The formula for chlorite is considered to be (Mga Si + 3 R Si) + 9MgH. t This substance constitutes the base of the clays employed in the manufacture of porcelain, and which are formed from decomposed felspars in districts where that mineral has been distributed in sufficient abundance. CH. XXX.] GENERAL CHARACTER OF IGNEOUS ROCKS. 593 could thus furnish not only the lime, but also the magnesia for the production of hornblende, should the other ingredients of that mineral be near and not drawn elsewhere. In this manner it will be obvious very material additions may be made to an original and general mass of rocks in a molten state. There appearing so much of a general character in the various igneous products of different geological times, to call the attention of an observer towards some general cause, which, though much modified under certain circumstances, has yet always exerted an important geological influence, he has carefully to consider the subject, so that, while a proper and close attention may be given to local sources of modification, the great cause of these igneous pro- ducts, taken as a whole, be not neglected. Whatever may have been the conditions under which substances were probably ejected in the manner of modern volcanos in past geological ages, from time to time molten matter of a very common general character seems as if always ready to have been upheaved, in larger masses, whenever there were great disruptions of prior-formed accumula- tions on the earth's surface. Thus, while the minor and perhaps modified manifestations of the conditions for throwing out igneous substances generally, were constant in different points of the earth's surface for the time being, these substances mingled with the ordi- nary accumulations of the day, from time to time a greater amount of molten matter was upheaved, lifting such igneous products as well as their associated sedimentary deposits, as if the former action, however intense, was but superficial as compared with that from which the more wide-spread and important movements were derived. Be this as it may, the igneous products form objects of the greatest interest, whether regarded as the source whence so large a proportion of the detrital accumulations are derived, for the modifications they have so frequently effected in the deposits against or amid which they have risen, or been protruded, for the differences and resemblances they exhibit among themselves, or for the proof they afford that during the long lapse of geological time of which we can obtain traces, and up to the present day, there have been conditions for uplifting mineral matter in a molten state, that matter chiefly composed of the oxides of a few simple substances two of them especially (sodium and potassium) being not only remarkable for their comparative lightness, but also for an avidity for oxygen so great that they will decompose water in order to obtain it. _ / & 2Q CHAPTER XXXI. CONSOLIDATION AND ADJUSTMENT OF THE COMPONENT PARTS OF ROCKS. ADJUSTMENT OF COMPONENT PARTS OF CALCAREO-ARGILLACEOUS DE- POSITS. ARRANGEMENT OF SIMILAR MATTER IN NODULES. CENTRAL FRACTURES IN SEPTARIA. NODULES OF PHOSPHATE OF LIME. SPHEROIDAL CONCRETIONS IN SILURIAN ROCKS. CRYSTALS OF IRON PYRITES IN CLAYS AND SHALES. MODE OF OCCURRENCE OF SULPHATE OF LIME. MODIFI- CATION IN THE STRUCTURE OF ROCKS FROM CHANGES OF TEMPERATURE.- CHLORIDE OF SODIUM DISSEMINATED AMONG ROCKS. IMPORTANCE OF SILICA AND SILICATES IN THE CONSOLIDATION OF DETRITAL ROCKS. ALTERATION OF ROCKS, ON MINOR SCALE, BY HEAT. FORMATION OF CRYSTALS IN ALTERED ROCKS. CRYSTALLINE MODIFICATION OF ROCKS. ALTERATION OF ROCKS NEAR GRANITIC MASSES. READJUSTMENT OF PARTS OF IGNEOUS ROCKS. PRODUCTION OF CERTAIN MINERALS IN ALTERED ROCKS. MINERAL MATTER INTRODUCED INTO ALTERED ROCKS. MICA SLATE AND GNEISS. WHEN the gravels, sands, silts, clays, or mud of various geological times are presented to the attention of the geologist in the form of conglomerates, sandstones, arenaceous and argillaceous slates and shales, their component parts, originally drifted, or otherwise borne into the relative situations where they are now found, have either been joined together by mineral matter, subsequently intro- duced among them, or by a change in the condition of some part or parts of the original deposit which should permit such portions, in an altered form, to cement the remainder. With carbonate of lime, the oxides of iron and manganese and occasionally with silica, as substances cementing fragments of rocks, either angular or rounded, on hill sides or other subaerial localities, where springs containing and depositing those substances occur y we may consider the observer as familiar. That various breccias, conglomerates, and even sandstones so formed, occasionally constitute parts of a series of geological products, may be considered probable. It is easy also to infer that during geological changes, gravels, sands, and mud constituting the margins and bottoms of lakes and seas, may be so CH. XXXI.] ADJUSTMENT OF CALCAREO-ARGILLACEOUS DEPOSITS. 595 placed beneath isolated portions of water, to which the access of rivers or streams may be insufficient to meet the loss by evapora- tion, that certain substances held in solution may be slowly de- posited amid such subjacent gravels, sands, or mud, so as to pro- duce modification, change, or even consolidation of various kinds in them. Independently, however, of these effects, the observer will have to direct his attention to modification, change, and consolidation of a far more general kind, and for which some more general cause appears to be required. He will, in the first place, have to dismiss the view that the relative age of rocks is alone a sufficient cause for the effects noticed; though, taken as a whole, the relative geological age of deposits is so far important, that, other things being the same, there may be a greater chance of the older rocks being consolidated or modified in their structure, inasmuch as they may have been more exposed, during the lapse of time, to the causes productive of such consolidation and change. It may, in the first place, be desirable to consider the modifi- cation of parts which might arise in a bed or mass of mud, or clay after its deposit, the component parts of such mud or clay being variable. We may take, by way of illustration, those alternations of argillaceous limestones and shales, often, calcareous, which are observable in the lias of some parts of Western Europe, and which appear the result of an unequal supply of mud and calcareous matter, sometimes the one and sometimes the other predominating. Examples of irregular deposits of this kind must not, however, be considered as confined to any particular age, since among the older Fig. 224. as well as newer geological accumulations, this kind of deposit may often be found. The above (fig. 224) may be taken as illus- 2 Q 2 596 ARRANGEMENT OF SIMILAR MATTER IN NODULES. [Cn. XXXI. trating alternations of this kind, the surfaces of the beds being irregular. In itself such a section may merely present us with the evidence of alternating conditions, by which carbonate of lime was 'more thrown down at one time than at another, though, with care, forms of the surfaces are often traced which would seem to point to an abstraction of calcareous matter from the adjacent original clays or mud ; a circumstance which becomes more evident where the cal- careous matter in the general deposit has decreased, and many irregular patches of the argillaceous limestone, and nodules of it, are arranged in lines or are more dispersed through the deposit, as shown in the subjoined section (fig. 225). In such cases the cal- Fig. 225. careous matter of given times of deposit, irregular like those where whole sheets of argillaceous limestone were produced, seems gathered to different points in or about the same plane, that upon which the general deposit was accumulated, the matter arranged round these points, thus variously dispersed on the plane, so that two or more nodules may be joined together while others remain isolated. This gathering together of similar matter distributed through a soft muddy or clay mass, would be anticipated, and the more so, when we remember the manner in which similar matter may be gathered together from solutions, dragged away, as it were, forcibly to points where some of it may have been first deposited, as noticed by Pro- fessor Bunsen (p. 375). Facts of this kind are as well seen among the carbonates of iron, of so much value in the coal measures of the British Islands, as amid the accumulations above noticed ; and they, in like manner, point to a separation of the carbonates from the muddy mass, and, for the most part, in planes corresponding with the relative times of their original deposit in the general accumulation, one chiefly detrital, and thrown down from mechanical suspension. It occa- sionally happens that this gathering together of similar matter from amid a mass through which it was originally dispersed, usually in certain planes and thicknesses, can be seen to have taken place so that a certain original lamination of parts is not destroyed. Instances of this kind are to be found in one or two of the ranges of nodules Cri. XXXI.] CENTRAL FRACTURES OF SEPTARIA. 597 in the lias of Lyme Regis, Dorset, where, as beneath (fig. 226), these are seen still preserving the lamination of the general deposit; Fig. 226. an arrangement of parts easily ascertained by breaking the nodules in this plane. In these nodules some organic remain, such as a fish, nautilus, ammonite, or a piece of wood, not unfrequently seems to have formed a point around which the carbonate of lime was aggre- gated, though this has by no means been always the case, since some are occasionally found without organic remains, or only con- tain them in a dispersed state. Such aggregations and separation of parts are at the same time a modification of the original deposit, and a partial consolidation of it. As a proof that the mass was soft when the nodules were formed, it will be often found that while the same kinds of organic remains, and especially thin shells, are flattened, in the same planes, in the associated and adjoining clays, marls, or shales, they are com- paratively well preserved, uncompressed, in the nodules, the con- solidation of the latter having protected them from the pressure to which those had been subjected in the remainder of the deposit, then in a yielding condition. With regard to the relative time and mode of consolidation of the nodules, the observer may be frequently enabled to study it in those commonly known as septaria, where, after the aggregation of the similar matter, such, for example, as the carbonate of lime in many clay or shale deposits, and the carbonate of iron in the coal measures and some other rocks, a splitting of the interior has taken place, and subsequently to a certain amount of consolidation, since the fractures are usually sharp, pointing to a sufficient amount of cohesion of parts. The subjoined section (fig. 227) will show the ordinary Fig. 227. manner in which such nodules are broken in the interior, the cracks not extending to their exterior surfaces, as if there had been a shrinking of parts from the centre outwards, so that the resulting 598 NODULES OF PHOSPHATE OF LIME IN ROCKS. [Cn. XXXI. largest openings were central. In the nodules of this kind, not uncommon in many clays, marls, and shales, the cracks are usually filled according to the character of the general deposit of which the nodules constitute a part; thus carbonate of lime is frequent in those where that substance is much disseminated, and carbonate of iron where the latter is not uncommon. Occasionally other substances are introduced, such as, in the ironstone nodules of many parts of the British Islands, the sulphurets of lead, zinc, and iron, copper pyrites, and certain other minerals. Nodules and other formed bodies of phosphate of lime, also sometimes occur in a manner pointing to the aggregation of their component parts from previous dissemination amid surrounding detrital deposits. Many of the nodules and other forms of phos- phates of lime in the lower parts of the cretaceous series of south- eastern England and in parts of France, seem thus produced. Mr. Austen has informed us,* that the nodules he examined had a con- centric arrangement of parts, like agates, and he points to the pro- bability that the phosphoric acid may have constituted part of the fnecal or coprolitic matter accumulated with other organic bodies, sit the period of the original deposit, and had been disseminated among the sand and ooze of the locality and time. Modern re- searches have shown that phosphate of lime is far more diffused among rocks than was at one time supposed. When free carbonic acid is present in water, the phosphate of lime is, like the carbonate, soluble, though not to the same extent as the latter ; so that con- ditions may readily arise not only for its dissemination, but also for its aggregation into various forms amid rocks through which its particles could move. Not only waters impregnated with free carbonic acid, in the usual manner, would afford the common means of transport for such particles, but also, in the cases referred to by Mr. Austen, for the mixture of coprolitic with vegetable matter, the decomposition of the latter, and often, indeed, of the fecal matter itself, might produce the carbonic acid needful in the required solution. The association of similar matter in nodules, is also sometimes well seen amid deposits of siliceous sands, these aggregated so that the nodules protrude as marked objects on weathered banks or cliffs. Sometimes the nodules are dispersed among the arenaceous accumu- lations, while at others they range in certain general planes, corre- sponding with those of deposit, and thus, in their mode of * Journal of the Geological Society of London, vol. iv., p. 257, 1848. CH. XXXI.] SPHEROIDAL CONCRETIONS IN THE SILURIAN ROCKS. 599 occurrence, resemble the nodules of the carbonates of lime and iron, above mentioned. In certain of the arenaceous deposits the cementing substance of the nodules is occasionally calcareous, apparently aggregated from that matter once more dispersed amid the sands, and deposited amid the grains from solution, as a bicarbonate. The oxides and hydrated oxides of iron are also observed gathered in nodules, either dispersed or in planes, aggre- gating portions of sands. Even amid the older detrital accumulations with which geologists have become acquainted, this structure is observable. The separa- tion of calcareous matter into nodules from among the component parts of an original mud deposit, can be as well seen in the old series of rocks, known as Silurian, such as in portions of the Wenlock shales and limestones of that series, as it occurs in parts of Wales and the adjoining English counties, as in far more modem geological accumulations. So also with the aggregations of siliceous matter in the nodular or spheroidal forms, showing that similar conditions for these arrangements and adjustment of parts have continued to prevail through a long range of geological time. The following section (fig. 228) of part of the upper portion of the Fig. 228. Silurian series (Ludlow Eocks) of Brecknockshire, to be seen at a considerable development of that portion, in Cwm-ddu, near Llan- gammarch, will exhibit the arrangement of parts of this arenaceous rock, in certain beds, in a spheroidal form; layer after layer, as the decomposition of the rock shows, having been arranged round somewhat central points of aggregation dispersed in certain lines of beds. Aggregations of this kind occasionally measure many feet in diameter. Such aggregations are sometimes only to be detected on the face of rocks by lines arising from the stains of peroxide of iron, which, when followed out, are found to correspond with spheroidal surfaces. When, geologically, these adjustments of the parts of deposits may have been effected, it is not easy to infer, since in the instances 600 CRYSTALS OF IRON PYRITES IN CLAYS AND SHALES. [Cn. XXXI. of those in the older accumulations, they may have been produced, as many of those in certain more modern accumulations are seen to have been, before the solidification of the sandy portions around the spheroidal aggregations and nodules, the whole of the bed, or beds, having been submitted to further conditions for consolidation, after the separation of certain portions of them into such aggrega- tions of similar matter. There are certain other separations of the original portions of a deposit, where the particles have possessed such free movement and powers of adjustment, that they have been enabled to gather themselves into crystals. Of this the crystals of the sulphuret of iron amid the mud deposits of all geological ages is an example, as also the crystals of sulphate of lime in numerous clays. Cubes and other forms of iron-pyrites are as common amid the oldest fine sedi- mentary accumulations, occurring in a manner to leave little doubt of the aggregation of their component particles from the mud in which they were diffused, as among the clays of tertiary deposits. That iron-pyrites should be gathered round organic remains in rocks of different ages, particularly in those, such as have been mud and clays, where the movement of its component particles may be inferred to have been, as in the case of the crystals above noticed, somewhat easy, would be anticipated, inasmuch as the production of iron-pyrites in connexion with decomposing animal matter is well known.* Thus we frequently find the sulphuret of iron incrusting organic remains, as crystals, and in more irregular lumps and patches, particularly amid clay and shale accumu- lations. Eegarding sulphate of lime, irrespectively of its distribution in crystals, as selenite, amid clays and shales, it often constitutes considerable nodules, and dispersed irregular masses, as if, inde- pendently of original deposit, or change from the carbonate by the introduction of sulphuric acid amid particles of limestone, it had separated out from the body of the rock, and became aggregated amid a soft muddy deposit, thrusting aside the latter. Certain * Mr. Pepys, in 1811 (Transactions of the Geological Society of London, 1st series, vol. i.), was among the first to publish a very illustrative case of the production of iron-pyrites from the decomposition of the bodies of some mipe in a solution of sulphate of iron. Another illustrative instance of the formation of iron-pyrites upon animal matter in a decomposing state, occurred at the bottom of a mine-shaft, near Mousehole, Cornwall, where a dog had fallen into a solution of iron, and its body was found surrounded by iron-pyrites. In these, and other well-known cases, the hydrogen evolved from the decomposition of the animal matter, is considered to take the oxygen both from the sulphuric acid and oxide of iron, so that iron-pyrites, or bi-sulphuret of iron, is formed. CH. XXXI.] MODE OF OCCURRENCE OF SULPHATE OF LIME. 601 nodular portions so occur in particular lines, that we may suppose them to have been produced much in the same way by segregation as the nodules of the carbonates of lime and iron, above noticed. At the same time beds of gypsum, both on the large and small scale, also so occur amid clays, marls, and shales, especially well seen amid portions of the red and grey marls of the upper new red sandstone series, or trias, that there is much difficulty in deciding as to the probability of their original production from solutions, amid the clays or mud, in a manner similar, as regards general principles, to that noticed by Professor Bunsen, or partly in that manner, and partly by segregation into veins formed subsequently to the general accumulation and its partial induration. The section beneath (fig. 229), seen at Watchet, Somersetshire, amid the marls Fig. 229. of the trias, will illustrate a mode of occurrence of not an uncommon kind, wherein beds of gypsum a, a, a, are united by strings of the same substance traversing the intermediate marls b, b, b, in various directions, and having somewhat the appearance of cracks filled, inasmuch as the fibrous gypsum in them has the fibres usually at right angles to the walls of the containing marls, as if crystalliza- tion had taken place against those walls. No doubt this appearance may be deceptive, but at all events, it becomes an interesting object of inquiry, to ascertain how far, under such modes of occurrence, the evidence may be in favour of an original separation and deposit of the sulphate of lime, contemporaneously with the matter of the marls, or of a segregation of, at least, part of the same substance into veins, from a dispersion of the sulphate of lime amid the body of the accumulation, When the observer reflects upon the different conditions, to which the various deposits in seas and bodies of fresh water may have been subjected, posterior to their original accumulation, he will not fail to appreciate the modifications which the whole mass of many may have sustained. The mere change from being super- ficial, on the bottoms of seas and other bodies of water, to being buried beneath many, and sometimes varied additional accumu- lations, is alone a condition under which new adjustments of parts 602 MODIFICATION IN THE STRUCTURE OF ROCKS [Cn. XXXI. may arise, and this without a change in the relative distance between the surface of the sea, or other waters, and the deposit itself. Should the accumulation above it be thick, changes (p. 444) arise in its temperature, with their consequences as regards the motion of aqueous solutions distributed through beds of different degrees of porosity. The geologist should direct his attention to the still greater causes of modification and change which would follow the sinking of such deposits, as regards the crust of the earth, when they de- scended into comparatively elevated temperatures, so that their component parts, and the various solutions with which they may be moistened, become affected by that temperature. The springs which issue from various rocks, and for which the supply is derived by the simple percolation of atmospheric waters through porous beds of different kinds, until thrown out by less pervious beds (p. 16), suffice to show the amount and kinds of substances soluble under such conditions, and which remain in the various deposits effected beneath the sea or other waters, after many of these accu- mulations have been more or less solidified, and raised into the atmosphere, where they now constitute portions of land above the level of the sea. In the various borings or sinkings for mine- shafts, the driving of extensive tunnels and levels, and in wells of various kinds, especially of those termed artesian, he has also the oppor- tunity of ascertaining the soluble contents of the waters which may be disseminated among the rocks traversed ; and where such waters may be considered as in a somewhat stagnant state, except so far as movement through any fissures, joints, and the pores of the rocks themselves, may be induced by differences of temperature from the surface of the earth downwards towards the interior. There does not exist so much exact information as to the substances in solution among the waters disseminated amid rocks in this manner as is desirable ; neither are the soluble contents of the various waters rising through faults on the surface of the ground, or flowing up at the bottoms of mines, with a temperature sufficiently elevated to render it probable that they rose from greater depths, so well known as is required for properly estimating the amount and kinds of substances, which may be thus circumstanced ; but; there still exists sufficient knowledge on the subject to show the observer the value of investigations in this direction. The waters rising from the chalk at the artesian well in Trafal- gar-square, London, and which are obtained from their dissemina- tion in that rock, show, that in 68-24 grains of solid matter in an CH. XXXI.] FROM EXPOSURE TO CHANGES OF TEMPERATURE. 603 imperial gallon, 18 grains are composed of carbonate of soda ; while the carbonate of lime contained among the solid matter above men- tioned, only amounts to 3'255 grains ; and thus the waters resting, to a certain extent, stagnant in the chalk beneath London, with its thick covering of (London) clay, exhibit a very different character, as to the substances in solution, from that of the spring waters which flow out of the chalk on the surface, where that rock arrives at or adjoins it.* Among the various substances found in solution, either dissemi- nated among the pores of rocks, or which become, as it were, washed out of them in solution, by waters percolating through them and issuing as springs, the observer will do well to recollect the amount of chloride of sodium so often obtained. That it should be a somewhat abundant substance would be expected in deposits of mud, silt, sand, and gravel effected beneath the sea ; as also that, when such accumulations were elevated into the atmosphere, and rain-waters found their way to the chloride of sodium, it should be removed by any springs thence resulting. It will be seen that in the waters disseminated amid the chalk beneath London, this substance was found to constitute somewhat more than two-sevenths of the whole solid contents obtained from it.f Looking at chloride of sodium alone, and its dissemination among beds of quartz or other siliceous sands, and the descent of the whole to some very elevated temperature by depression of the earth's surface in any given region, * The following are the substances contained in an imperial gallon of the waters of the Trafalgar-square well, according to Messrs. Abel and Rowney : Grains. Carbonate of lime 3-255 Phosphate of lime Carbonate of magnes a Sulphate of potash Sulphate of soda . Chloride of sodium Phosphate of soda. Carbonate of soda 0-034 2-254 13-671 8-749 20-058 0-291 18-049 Silica 0-971 Organic matter .... 0-908 In the cases of soluble mineral matter disseminated in rocks, such as the chalk beneath London, it should be borne in mind, that when there is a movement of the contained water among their pores or fissures to supply that raised to the surface by pumping, or rising from boring and overflowing, the original condition of somewhat stagnant dissemination becomes changed by the amount of the water thus required so that when many wells reach into the chalk, as beneath London, a movement of water amid the body of that rock is occasioned towards the various wells, which would not have taken place under ordinary natural circumstances. t As sea, or rather estuary waters, are inferred partly to percolate into the chalk beneath London, some caution is needed as to the source of all the chloride of sodium in the chalk so situated. 604 VARIABLE CONSOLIDATION OF DETRITAL DEPOSITS. [Cn. XXXI. some effect might be anticipated from the production of a silicate of soda, aiding a consolidation of the sands, in the same manner as a salt glaze is produced by the potters. While studying the variable amount of consolidation of rocks, the geologist cannot fail to have his attention arrested by the dif- ferent states, in this respect, in which he sometimes finds the beds amid a series of deposits, grouped together, and which have evidently been subjected to the same general conditions. It would strike him, probably, that the original condition of the deposits could not fail to produce marked differences in this respect. He would anticipate that a bed of pure quartz sand, unmingled with other and muddy matter, might, if cemented by somewhat pure silica, form a substance of a harder and more solid kind than when ordinary sand was deposited, mingled with a certain portion of mud, or when the grains were composed of different substances, so that they could be variably acted upon by the matter forming the cement. In the one case, there may be a rock, commonly known, from its composition, as quartz-rock., wherein it is sometimes even difficult to trace the original grains of sand, their surfaces having been more or less acted upon by the mode in which the infiltration of the cementing silica has been effected ;* while in the other, a sandstone of the ordinary amount of consolidation has been alone produced. The occurrence of certain quartz rocks among the accumulations of all geological ages, and amid other and contem- poraneous beds, can be often well studied ; and sometimes the passage of an ordinary sandstone bed into a quartz rock can be easily traced. Of this, a quartz rock, amid the new red sandstone series near Bridgend, Glamorganshire, may serve for an example, as the same bed can be readily followed from its ordinary sandstone cha- racter on the north of the town, to that of quartz rock on the road to Pyle Inn. Changes of a similar kind are sufficiently common in the course of numerous rocks, as well in single and marked beds, as in numbers of them collectively ; and the observer will, no doubt, have to seek for the causes of these differences as well in the unequal or variable supplies of the cementing matter, according to * The arrangement of parts in certain of these quartz rocks is sometimes such that it requires very careful examination, and even occasionally a thin slicing of a part, so that it can be studied through transmitted light, in order to distinguish the original grains of quartz sand, the cementing and external parts of these grains having become so much blended. For the most part, however, the detrital origin of the quartz grains is sufficiently evident. In examining these rocks, as they are often traversed by veins of quartz, it is needful carefully to distinguish between the latter, which are merely the ordinary infiltrations of silica into cracks and fissures, from the body of the rock itself, a circumstance that has not always received attention. CH. XXXI.] CONSOLIDATION BY SILICA AND THE SILICATES. 605 subordinate local influences, as among the different original com- positions of continuous deposits; the latter often, nevertheless, appearing a sufficient cause, in the same way that, in a series of beds, wherein varieties of this kind are very striking, much original differences are apparent. Certain hard quartzose beds beneath others of coal, between Swansea and the Mumbles, maybe taken in illustration of a probable change effected by the introduction of silica, or some silicates, after their original deposit. In these beds, the roots of a plant (Stigmaria), existing when the coal measures, of which they constitute a portion, were accumulated, once as freely grew, spreading out their finest parts in the evidently yielding ground of the time (p. 501), as in any other of the similarly-circumstanced beds of the same district supporting seams of coal, and known as under-day s (p. 510), though now they are bound up in a hard siliceous rock, upon which atmospheric influences have as little action as on ordinary quartz rocks, the original silty and loosely-aggregated substance of the beds being converted into a hard quartzose substance. Looking at the mass of detrital matter, more or less consolidated by silica or the silicates, the study of the manner in which this may have been effected by them, becomes a matter of no slight interest to the geological observer. He finds silica in a pure or nearly pure state in cavities of various rocks, especially of those of igneous origin, wherein hollows and vesicles have been left, it being seen more or less filling such cavities with agates, onyxes, chalce- dony, and rock crystals, and he can have little doubt that this silica was introduced into the hollows and vesicles by infiltration and in solution. Indeed, the stalactitic forms of the silica often sufficiently show this, certain agates, as is well seen upon their decomposition, being merely forms of this kind eventually filling hollows. At other times, the layers of the siliceous deposits occur in planes, apparently horizontal at the time they were effected. These modes of occurrence show him that silica has been, and can be, disseminated amid the pores of rock, often hard and (so called) compact, its particles finding their way for deposit in a pure or nearly pure state into the vesicles and cavities of such rocks. In investigations of this kind it will be desirable that the observer should bear in mind that certain silicates are not difficult of decom- position, as, for example, those of potash and soda, when free car- bonic acid may be present. Upon looking at this subject generally, such conditions may be inferred not to be so rare as might at first be supposed. In certain regions, the decomposition of the felspar 606 CONSOLIDATION BY SILICA AND THE SILICATES. [Cn. XXXI. alone in granitic and some other igneous rocks, gives rise to solu- tions of the silicates of potash and soda, and the introduction of waters having free carbonic acid, derived from the atmosphere, in them, would separate the potash or the soda, as the case might be, from the silica, the latter being deposited under favourable condi- tions for dissemination amid the pores of rocks.* When we regard the manner in which carbonic acid may arise from the decompo- sition of organic bodies, be mingled with water, and act upon certain silicates, it is also to be inferred that favourable conditions may arise under which silica could be thus thrown down, even when vegetable matter afforded the carbonic acid, amid the pores and cavities of a certain part of the plants themselves, preserving their finest structures.-f- Though silicic acid may thus, under favourable conditions, to which it is here sufficient to direct careful attention, be easily sepa- rated from certain silicates under the common temperatures which are known on the surface of the earth, or found at moderate depths, circumstances with regard to this substance become changed when the heat to which it is exposed in connexion with others is con- siderable. For instance, instead of decomposing the silicates of potash or soda, in the manner above mentioned, the carbonic acid would be driven off, and the silicic acid would remain combined with the alkalies. Again, it is now known that while pure silica, so very important geologically, may be very difficult to dissolve in water at the temperature commonly termed ordinary, when the heat of water is much increased beneath the requisite pressure, it may be considered simply, like many other substances, as more soluble in highly-heated waters than in those of more moderate temperatures. Hence, when the observer regards the facility with which pressure, and elevated temperature may be obtained, by descent beneath the surface of the earth, he will see that no slight modification and change may be effected by the mere lowering of beds, moistened with water to situations where such water could act upon the silicates of the rocks among which it may be dissemi- nated, and even upon silicic acid itself, existing as grains of pure quartz, this solution ready to be effected by, and to produce vari- ous modifications and changes among the substances forming the * Mr. Henry informs me that when experimenting upon silica he found that a silicate of soda was decomposed even by the carbonic acid of the atmosphere and the silica deposited, its state and appearance being much affected by the degree of con- centration of the solution. t The fine structure of fossil siliceous wood is often beautifully preserved. CH. XXXI.] ALTERATION OF ROCKS, ON MINOR SCALE, BY HEAT. 607 original deposit, or other matter subsequently introduced amid its parts.* The action of considerable heat upon rocks, producing change and modification of their component parts, can often be so studied among a minor mixture or juxtaposition of igneous rocks, and those evidently produced by chemical or mechanical deposit in water, as much to assist inquiries into the manner in which more general changes and modifications may be aided, and even sometimes effected on a great scale. In volcanic regions, substances, such as clays, become hard, in fact baked, as any tile or brick may be, by the overflow of a lava current among them, the result being the same as might be expected from our knowledge of the action of heat upon different varieties of clays in our potteries and porce- lain manufactures, some clays burning or baking well, others ill. In such cases the usual result is the production of certain changes by the action of the heat communicated from the liquid lava. A still further modification of parts is effected when, without loss of the original form of the deposit acted upon, some of the con- stituent particles have separated from the main mass in which they were disseminated, and, joining together, have produced crystals, there having existed a power of movement in these particles, similar, so far as regards conditions for separation from the main mass, and the movement obtained, to that above mentioned as hav- ing taken place in yielding deposits, such as clays. This modification in the arrangement of the component parts of rocks is common to the igneous action of all geological times. It can be as well seen amid the accumulations of igneous matter deposited with the old Silurian series of the British Islands, as in various regions among the volcanic products of the present time, and is one requiring some attention, since it might otherwise much interfere with the conclusions of an observer as to the conditions under which the component parts of a rock may have been origi- nally gathered together. A porphyritic character, from the disse- mination of certain crystals, as, for example, those of some of the * It is to be hoped that investigations in this direction mdy more occupy the attention of chemists than has hitherto occurred. The subject is full of interest, and appears one likely to reward the labours of those who, taking a certain class of geological facts for their guide, unite with them the conditions of high temperature beneath great pressure, as also exclusion from the atmosphere, such as may be inferred to exist beneath given depths in the earth, upon the hypothesis that heat increases down- wards towards the central portions of the earth, for at least the distance at which water, should it continue to exist as such, can be heated up to a very elevated tem- perature. 608 CRYSTALLINE MODIFICATION OF ROCKS. [Cn. XXXI. felspars, may be too hastily assumed as indicating the rock, thus characterised, to have been in a complete molten state.* In those instances where the rocks have evidently been fissured prior to the introduction of the igneous matter, which thus forms a simple dyke, as it is usually termed, or some tortuous form of vein, the observer would necessarily infer consolidation sufficient for the production of the fracture, so that any change or modi- fication found in the rock fractured would have taken place after such consolidation. Cases of this kind of alteration are far from uncommon. In studying them it becomes needful to recollect that not only the mere action of heat may be brought to bear under such circumstances, as it might be with regard to the clay of a brick or porcelain vase, but also that moisture and solutions would probably be disseminated in the usual manner amid the pores and cracks of the rock so acted upon, and this often beneath much pressure ; exposure of the changes thus produced being often due to some of, or all, the causes of denudation removing former, and considerable, pre-existing and covering portions of rocks. Changes and modifications in such cases must necessarily depend much upon the substances acted upon, and the manner in which their component parts may have been arranged. The most simple forms of modification are those where some substance, such as com- mon limestone, may have its parts so modified that a crystalline adjustment of them is effected; the portions of rock in contact with the igneous matter being thus altered, the greatest modifi- cation effected nearest the igneous rock and becoming less as the distance from it is increased. Of this kind of modification the often-quoted instance of the chalk in the Isle of Raghlin may be taken as an example. In this case, as shown beneath (fig. 230), Fig. 230. c a c a c a dykes, a a a, of basaltic rock traverse the chalk of that part of Ireland (so much broken up by eruptions of igneous matter at a period subsequent to the chalk), converting that rock, between and adjoining them, into a more crystalline substance, c c, this character gradually disappearing on each side, b b. The alteration at the contact of dykes of igneous rocks is not confined to the more * Modifications of this kind, by which crystals of felspar have been developed in rocks which still preserve their original planes of deposit, are not uncommon. CH. XXXI.] ALTERATION OF ROT5KS NEAR GRANITIC MASSES. 609 crystalline arrangement of the traversed and adjacent beds, certain minerals being very often formed by the movement of their com- ponent particles, under conditions when they could adjust them- selves into crystals, the surrounding matter giving way to their forms. These minerals vary much according to the chemical com- position and physical structure of the deposits acted upon, and also according to the volume as well as kind of the igneous rocks introduced. A far larger amount of modification and change is necessarily effected when the mass of igneous rock, introduced amid prior accumulations, is considerable, and when it may be inferred, as it often can be, that this intrusion has been effected at depths beneath the surface where there was no contact with the atmosphere ; but where, on the contrary, any water distributed amid the pores or crevices of the previously-formed rock, consolidated or otherwise, as may have happened, could not escape, with any solutions it contained, having been confined to a certain range, beyond .which, a continuation of the same, or other rocks, with their dissemi- nated moisture, remained much in its condition prior to the in- trusion. As has been previously remarked, certain of the gran- itic intrusions appear to have effected much change in adjacent accumulations. In various parts of the world, such modifications of previously-formed rocks of all kinds in contact with the in- trusions and upheavals of granitic matter, are most marked, the altered rocks being traceable to their more usual forms of ordinary limestones, argillaceous and arenaceous slates, sandstones, or the like ; some even of these rocks being fossiliferous, and so occurring, that their relative geological age can be readily assigned them. Changes and modifications of this kind can be well seen to have been produced upon the Cambrian and Silurian rocks, prior to the accumulation of the old red sandstone, in parts of Ireland (in the counties of Wicklow, Wexford, &c.) ; and in south-western England, upon deposits of a later date (effected anterior to the new red sandstone) ; the rocks acted upon being of varied composition, including different igneous accumulations, as well thrown out in a molten state, as deposited as ashes and lapilli beneath water (p. 554). In such situations, the observer Avill find, as he might anticipate, the consolidation by silica and the silicates often very considerable, beds of ordinary sandstone sometimes exhibiting their component grains as if passing into the matter cementing them. Judging from the solvent effects of water and steam, at high temperatures, upon the usual silicates employed in glass, when moisture is dis- 2 R 610 ALTERED IGNEOUS ROCKS. [Cn. XXXI. seminated in rocks, and raised to a very high temperature, tinder the conditions above noticed, the silicates, so common among various argillaceous and arenaceous slates and sandstones, would be acted upon, so that considerable consolidation by them became frequent among these accumulations. Such conditions could be scarcely otherwise than favourable to the aggregation of certain substances into a crystalline state upon a more extended scale than in the case of the smaller bodies of molten rock intruded among, or rising through, fractures in prior and consolidated accumulations. Certain igneous rocks seem sometimes to have had the volume of their component minerals increased, as, for example, the crystals of hornblende and felspar to have become enlarged near the contact with the granite. Sometimes either with, or without increase of volume, the particles of hornblende and felspar of an ordinary greenstone become so adjusted as to present far greater brilliancy of aspect, so that the rock takes the appearance of that commonly known as hornblende rock. Good instances of this kind are to be seen in the county of Wicklow. In like manner, the old Silurian volcanic ash-beds of the same district will be seen, under similar conditions, with the brilliant aspect of hornblende slates. The hornblende rock and slate of the Lizard district, Cornwall, seem, in like manner, little else than ordinary greenstone, and the volcanic ash of the Devonian series, modified either by the action of a great mass of serpentine which has flowed over, and remained upon them, or by that of granitic matter beneath. Seeing the slight chemical differences usually noticed between hornblende, augite, and hypersthene, it would be expected that changes would be effected in the aspect of the rocks containing them, under the circumstances mentioned. The hy- persthene rock of some localities, as for example, that of Cocks Tor, near Tavistock, Devon, appears to come under this head. While on this subject, it may be remarked that other modifications are sometimes observable, as if the substances composing the rocks being originally more varied, or certain others having entered among them from without, after exposure to change from the consequences of juxtaposition to great masses of molten matter, minerals appeared not found beyond the limits which may be assigned to these alterations. In some regions, as, for example, in the counties of Wicklow and Wexford, in Ireland, the manner in which andalusite been developed, forcing off other portions of the rock, such as mica ; the old stratification of the deposit being still retained, is uc OW ia ! CH. XXXI.] CERTAIN MINERALS FORMED IN ALTERED ROCKS. 611 highly instructive. Near the intruded granite, the crystals of this mineral are occasionally found of large size ; and while they have, as it were, shouldered off' the other substances in the way of their formation, they sometimes exhibit portions of entangled matter, such as mica, as might be expected in such a mode of production. This mineral, not uncommon under similar conditions, is precisely one of those which would be expected to be thus formed, being essentially a silicate of alumina (the base of the clays), a compound forming a prominent part of the original deposits in which these andalusites become developed. Chiastolite is also a form in which the silicate of alumina appears amid altered rocks, and is one not uncommon among the old sedimentary deposits modified in contact with granite in Devon and Cornwall. Staurolite, another common mineral developed amid mechanically- formed deposits, when acted upon by masses of granitic and of other igneous rocks in a molten state, is again one which might be expected, being essentially composed of silica, alumina, and peroxide of iron, with the addition of a small portion of magnesia. Oyanite is also another form in which silicate of alumina occurs developed amid altered rocks. Garnets are often very common in those which have suffered modification in the adjustment of their component parts. When the observer refers to the chemical composition of these minerals, as shown by various analyses, he will see, by duly considering the isomorphism of certain substances that their component parts may be readily gathered together, under conditions for their movement, from amid rocks apparently of different kinds. While peroxide of iron constitutes a prominent portion of some garnets, it is replaced by alumina in others ; and while lime forms an important substance in most garnets, it may be considered as replaced by protoxide of iron in others.* Amid the various altered rocks in which garnets have been developed, the pushing aside, as it were, of other parts * The following, among the numerous analyses of garnets, may show their varied composition, chiefly due to isomorphism : No. Silica. Alumina. Iron * i Manganese Magnesia. Lime. Potash. Protoxide. Peroxide. 1 39-85 20-60 24-85 0-46 9-93 3-51 2 35-64 30-00 3-02 29-31 2-35 3 42-45 22-47 9-29 6-27 13-43 6-53 4 36-45 2-06 24-48 0-28 0-06 30-76 1. Greenland (Karsten) ; 2 Altenau, Hartz (Trolle-Wachtmeister) ; 3. Arendal (Trolle- Wachtmeister) ; 4. Beaujeux (Ebelmen). 2K2 612 MATTER TRANSMITTED INTO ALTERED ROCKS. [Cn. XXXI. of associated mineral matter, when their crystallization was effected, may be well studied. Among their modes of occurrence, those where they have been developed amid sandstones, as, for example, near Killan, in the county of Wexford, are highly interesting, the grains of sand being forced asunder to permit the development of the crystals of garnets. The transmission of mineral matter from the igneous and heating body into the prior-formed rocks, whether these were or were not consolidated, seems well shown when boracic acid is present among the former. This appears to have been the case around much of the granite in Devon and Cornwall, where schorl, as above men- tioned (p. 578), is not only found disseminated around the general mass of granite in numerous localities, but also constitutes the outer portion of that rock in many situations. The matter of the schorl (chiefly silicic acifl, boracic acid, and alumina)* has passed into the pre-existing and mechanically-formed rocks in many places, among which Fatwork Hill and Castle an Dinas, near St. Columb, Cornwall, may be noticed as localities where this circumstance can be well seen. The boracic acid might, indeed, have solely escaped out of the granitic mass, and meeting with the other essential parts of schorl, have produced the latter mineral amid the grains of the mechanically-formed rocks thus acted upon. With respect to mica, also, its component parts often appear as if introduced from the igneous rocks into many of the sedimentary deposits against, or amid which they have been intruded, at the same time that certain mechanically-formed rocks, such as arena- ceous deposits containing detrital mica, seem merely to have had * The analyses of schorl, by M. Hermann, gave for the composition of black schorl from Gornoseiiit, near Katherenenburg (1), of brown, from Mursinsk (2), and of green, from Totschilnaja, Ural (3), and of black, from Monte Rosa, by Le Play (4):- ! a 3 4 Silica . . . Boracic Acid . Alumina . Protoxide of Iron Peroxide of Iron Manganese Protoxide Magnesia .... 39-00 10-73 30-65 1-58 6-10 9-44 37-80 9-90 30-56 0-50 12-07 2-50 1-42 40-54 11-79 31-77 3*65 0-90 6-44 44-10 5-72 26-36 11-96 6 : 96 0-50 Lithia 0-50 2-09 Potash 2-32 Soda .... 2-09 Carbonic Acid . . Chrome . 50 1-66 1-66 1*17 Water .... 0-60 CH. XXXL] MICA SLATE AND GNEISS. 613 the micaceous matter so acted upon, as to form better developed films of that mineral, its component parts having adjusted them- selves in a manner more resembling an original formation of mica. In like manner, also, ordinary felspar seems to have been thus pro- duced, so that an original deposit in which grains of quartz, felspar, and mica have been accumulated, (the detritus of some pre-existing granitic rock,) would become the laminated compound of that character known as gneiss, while one formed only of grains of quartz and mica would become mica slate.* Paying due regard to the ori- ginal composition of the rocks acted upon, with proper reference to the conditions under which mineral matter may be passed out, and move among the parts of the igneous rocks, gradually cooling down during a long lapse of time, and also amid the pores and fissures of the prior-formed rocks, the observer would anticipate very numerous combinations, producing great modifications and changes in the original composition of deposits. He will find the study one of great interest, well repaying the time he may devote to it. * "When considering the various kinds of modification of parts which deposits may sustain from the contact of great masses of igneous rocks in a fused state, or from descent towards the interior of the earth, so that somewhat similar conditions may be produced, it is not a little interesting for the geologist to direct his attention to those which certain of them would, in consequence, present. If, for example, the thick beds of the millstone grit, forming the lower part of the coal-measure series*(as this grit occurs in the midland and northern counties of England, composed of quartz, mica, and felspar, the latter usually decomposed), were exposed to the conditions for alteration above noticed, the compound would have a granitic appearance, the more especially if the silicates of potash or soda, or both, were introduced, and again united with the remains of the previous felspar. This rock, even as it is, has often the ap- pearance of a somewhat decomposed granite, as is the case with many sandstones of the coal measures of the British Islands generally. CHAPTER XXXII. CLEAVAGE OF ROCKS. CLEAVAGE IN SANDSTONES AND SHALES. CLEAVAGE IN SHALES AND LIMESTONES. MINOR INTERRUPTIONS TO CLEAVAGE ACTION. CLEAVAGE ON THE LARGE SCALE. RANGE OF CLEAVAGE THROUGH CONTORTED ROCKS. DOUBLE CLEAVAGE. RELATIVE DATES OF CLEAVAGE. ELONGATION AND DISTORTION OF ORGANIC REMAINS BY CLEAVAGE ACTION. DIFFERENT DIRECTIONS OF CLEAVAGE IN THE SAME OR JUXTAPOSED DISTRICTS. JOINTS IN ROCKS. DIRECTIONS AND RANGE OF JOINTS. JOINTS IN GRANITE. JOINTS IN SEDIMENTARY ROCKS. JOINTS AMONG CONGLOMERATES JOINTS IN LIMESTONE. MOVEMENTS OF ROCKS AFTER JOINTING. COMPLICATION OF BEDDING, JOINTING, AND CLEAVAGE. WHETHER modifications and changes in rocks have been produced by the depression of deposits to depths beneath the earth's surface, where high temperature may cause the effects noticed, be brought about by similar action from the juxtaposition of great mineral masses elevated or intruded in a state of igneous fusion, or consoli- dation be produced chemically in any other way, an observer may not be unprepared to consider that the matter thus acted upon might sometimes exhibit an arrangement of parts corresponding with some adjustment on the great scale. Whether those arrangements of the parts of rocks to which the terms cleavage and joints . have been applied, may have taken place during their first consolidation, may be due to some action on the large scale after their consolidation as a whole or in part, or sometimes have been produced under both conditions by an influence acting independently of consolidation, the subject is one which would appear to require more extended observation, and a better-digested body of facts than has as yet been obtained. When a geologist has before him, as in the slate quarries of North Wales, a mass of mineral matter which he has reason to CH. XXXII.] CLEAVAGE AND JOINTS OF ROCKS. 615 conclude has once been a bed of clay or mud, now not only conso- lidated, but also rendered highly fissile in planes which do not correspond with that of the original deposit ; such planes consti- tuting a part of certain others traversing the rocks of a district generally, that he should be led to infer, with Professor Sedgwick and some other geologists, that the finely-divided, yet mechanically- deposited matter had been gathered together by some force, reminding us of that which unites the particles of crystals of certain given combinations of substances, might appear probable. Upon studying the districts where the cleavage of rocks occurs, the observer usually finds a considerable degree of uniformity of direction in the course of the cleavage planes, where they cut the horizon, through a variety of rocks, and over considerable areas. In certain of the masses or beds in which this has taken place, the effect appears to have been more easily*produced than in others, and it may be sometimes seen that while, in associated coarse and fine-grained beds, the latter are beautifully cleaved, the former ap- pear unaffected, as if the fine particles of the one could be easily acted upon, while the coarser grains better resisted a new adjust- ment. This conclusion requires, however, much caution, since though the coarser beds may not, at first sight, appear affected by the arrangement of parts producing cleavage, they may be found when broken to exhibit divisional planes, though these may not be so numerous, in the direction of those in the finer-grained beds associated with them. Thus, in the following section (fig. 232), Fig. 232. representing sandstone beds a, a, a, associated with argillaceous beds, 1}, 5, b, while the cleavage may be well developed and easily seen in the latter, in the former it may also be found by fracture of the beds, or by their decomposition from atmospheric influences. This, however, by no means constantly occurs, the conditions under which the fine-grained beds were cleaved, not having apparently been favourable to the adjustment of the parts of the other beds, so that they became thus divisible. 616 CLEAVAGE IN SHALES AND LIMESTONES. [Cn. XXXII. In certain of the carboniferous limestone districts in Ireland, where shales are associated with the limestone beds, the cleavage of the former is apparent, while the latter are either unaffected by it, or so obscurely as not to have their component parts adjusted to the same amount by this action. The following section (fig. 233), at Fig. 233. Clonea Castle, county Waterford, will serve to illustrate this cir- cumstance, a, a, a, being beds of limestone which do not exhibit cleavage, while the shales, , b, usually more or less calcareous, and associated with them, are cleaved. The same locality also shows that where the limestone beds become somewhat argillaceous, partaking partly of the character of the shales, and partly of the more pure limestones, they exhibit marks of cleavage action, as if the particles, being then less coherent, had more readily given way before that influence. The following sketch (fig. 234) may be found useful in illus- Fig. 234. tration of the modified passage of cleavage through dissimilar sub- stances, or of those, the coherence of the parts of which, at the time of the cleavage action, may have been different. It repre- sents a portion of the Devonian series, on the east of Hillsborough, CH. XXXII.] MINOR INTERRUPTIONS IN CLEAVAGE ACTION. 617 near Ilfracombe, North Devon, b, b, b, being thin seams of lime- stone, about two or three inches thick, , , a 9 argillaceous slates ; the argillaceous slates being made so by cleavage. The true planes of deposit are shown by those of the interstratified seams of lime- stone. The planes of cleavage lamination traverse the whole with a general southern dip, slightly interrupted at the seams of lime- stone, where their course is modified ; and though the limestone is divided in the same general direction, it is so in a somewhat con- torted manner, as shown by the carbonate of lime which has been sub- sequently infiltrated and deposited in the fissures so formed. The somewhat contorted modification of the cleavage in the limestone is shown at c. In such cases as these the geologist may consider the limestone to have been consolidated so as to have been capable of a certain amount of fracture, while the mud or clay, now consoli- dated as well as cleaved, so as to form hard slates in the direction of the cleavage, had the whole of its component parts more or less rearranged in certain directions by the cleavage action. At times an interruption may be traced even between argillaceous beds themselves, when these are piled upon each other in a manner marking a pause in the deposit, so that a clear surface has been formed between the production of one bed and the accumulation of another. The following section (fig. 235,) seen near Wivelis- combe, West Somerset, shows that while the planes of cleavage a Fig. 235. take a general direction, a a, they are slightly bent, and undulate where the partings of the beds, b b, occur. Whether, at the time of the cleavage the mere break in the continuity of the argillaceous matter was sufficient to produce this effect, or whether any kinds of substances may have been in solution in water, occupying the interstices between the beds, causing the effects observed, becomes matter for investigation. These interruptions and modifications of cleavage, though im- portant for the study of its cause and mode of action, and which are far from being always observable, become lost when cleavage is 618 CLEAVAGE ON THE LARGE SCALE. [Cn. XXXII. traced through considerable masses of rocks, and viewed on the large scale. We find it, as Professor Sedgwick long since (1835) pointed out, in a large portion of North Wales,* traversing all kinds of rocks in given directions, over wide areas, notwithstand- ing the varied position of their beds. Indeed, it may sometimes be readily traced through them when contorted in all directions, as well horizontally as vertically. As the cleavage thus cuts through even contorted rocks, it must clearly often happen that it passes through their planes of direction at various angles. This will be found to occur more frequently than might, without very careful examination, at first sight appear ; however it may arise that, in certain districts, the general direction of the tiue bedding, or planes of deposit, may be found to coincide with that of the cleavage, as regards horizontal range, whatever variation the dip of the cleavage may exhibit as regards itself, or the true bedding of the rocks. With respect to a mass of rocks of varied kinds, detrital and igneous, with some admixture of limestone, traversed by cleavage ranging diagonally to the general bedding, the chain of hills known as the Chair of Kildare, Ireland, may be taken as a good example, easily visited. The range of the cleavage coin- cides generally with that of the neighbouring districts in the counties of Wicklow and Wexford ; while the beds have a direc- tion diagonally across the cleavage. Among these hills highly- crystalline porphyries will be found cut by cleavage, as well as the argillaceous and arenaceous beds of the Silurian series of which they are principally composed. As illustrative of variation in the dip of the cleavage, and of the true beds seen vertically, and especially when the latter are contorted, the following section (fig. 236) of part of the Devonian series, on the coast between Morte and Bull Point, near Ilfracombe, Devon, may be useful, inasmuch as the true beds, chiefly of argillaceous matter, have been so cleaved in a constant direction, that while the cleavage planes, #, #, some- * " The whole region," the Professor observes, with reference to the country near Rhaiadr to the gorges of the Eolan and Towy, " is made up of contorted strata ; and of the true bedding there is not the shadow of a doubt. Many parts are of a coarse mechanical structure, but subordinate to them are fine crystalline chloritic slates. But the coarser beds and the finer, the twisted and the straight, have all been sub- jected to one change. Crystalline forces have' rearranged whole mountain masses of them, producing a beautiful crystalline cleavage, passing alike through all the strata. And again, through all this region, whatever be the contortions of the rocks, the planes of the cleavage pass on generally without deviation, running in parallel lines from one end to the other, and inclining at a great angle, to a point only a few degrees west of magnetic north." "On the structure of Large Mineral Masses, Trans, of the Geological Society of London," 2nd series, vol. iii., p. 477. CH. XXXIL] CLEAVAGE THROUGH CONTORTED ROCKS. 619 times cut the bedding at right angles, at others they coincide with it, as at b, b. Fig. 236. Among the contorted and cleaved rocks, some will be found which may lead an observer to consider how far the cleavage took place after their consolidation into the hard sandstone, and even quartz rocks which he may find, or had occurred while that con- solidation was in progress. The following sketch (fig. 237) of the Fig. 237. cleavage of hard sandstone beds, with some slate, part of the Cam- brian series, at Bwlchhela, nearly opposite the Penrhyn slate quarries, North Wales, may serve to illustrate this subject, , a, being the lines of cleavage traversing the disturbed beds. Cleavage of this kind is exhibited on a much larger scale at the Holyhead Mountain, Anglesea, amid its quartz rocks, thus rendered fissile in a great measure across the bedding, as is shown in the following section (fig. 238) seen on the clifT opposite the South Stack Light- Fig. 238. house ; the nearly vertical lines, a, a, a, representing the direction of those of cleavage, varying somewhat in their amount of dips, the contorted beds being composed of sandstone and slate ; the former, for the most part, converted into quartz rock. Occasionally a double cleavage will be found, one set of planes cutting another, as in the annexed section (fig. 239), where two 620 DOUBLE CLEAVAGE. [Cn. XXXII. sets, a a and b b, cross the beds c, c, and each other, dividing up the mineral matter of the deposit into elongated forms of a pris- Fig. 239. a c/ ci matic character.* To the same kind of action we may, perhaps, assign those somewhat regularly- formed solids into which arena- ceous beds are occasionally divided in countries where cleavage has been effected. In such cases the cleavage planes cut those of deposit or true bedding, either at right or considerable angles, so as to produce forms of the following kind (fig. 240). The result- Fig. 240. 7 ing portions of rock vary considerably in size. We have seen some not much larger than four times that above represented, though they are usually much larger. As to the relative date of the cleavage, the observer may some- times obtain evidence of importance. Thus in Ireland, in the old red sandstone series of the counties Waterford, Kerry ,t and Cork, which affords such excellent examples of cleavage, it will probably have been effected after that of the Silurian rocks of Wicklow, Wexford, and Waterfoid ; inasmuch as portions of Silurian slates are to be found in the conglomerates of the old red sandstone of Waterford, clearly worn off, in a cleaved condition, from the sub- jacent, upturned and contorted rocks, on which this conglomerate has been deposited. Hence, also, it may be inferred that the cleavage of the mountain or carboniferous limestone shales found in some parts of Ireland, occurred after that of the Silurian rocks above mentioned. In the following section (fig. 24 1), representing * "When the joints, to be hereafter noticed, are numerous and somewhat close to each other, the rock becomes broken up into a multitude of short irregular prismatic portions, of a marked character. t In the road, now so much visited for the beauty of its scenery, between Kil- larney and Glengariff, excellent examples of cleavage will be seen. CH. XXXII.] ELONGATION AND DISTORTION OF FOSSILS. 621 veins of a porphyritic rock, a, a, traversing some Devonian slates, b, b, between Cawsand and Bedding Point, Plymouth Sound, the Fig. 241. slates and the igneous rock presenting one common lamination from cleavage, the latter would be effected after the intrusion of the porphyry. Upon studying the mode of occurrence of the rocks in the district, it will be found, as above noticed (p. 569), that porphyries of this kind might belong to the period of those known as elvans in Cornwall and Devon. By examining the conglomerates usually forming the lower part of the new red sandstone series of Devonshire, the observer detects portions of the prior -formed rocks laminated in a manner so agree- ing with cleavage, that he may infer that the cleavage of the older Devonian rocks was effected before the deposit of the new red sandstone in that district, and subsequently to the intrusion of the granite. Upon following out the modification and adjustment of parts in rocks traversed by cleavage, it will in many districts be seen that there has been a movement and rearrangement of them in direc- tions corresponding with the planes of cleavage. There have often been elongations in those directions, so that any organic remains Fig. 242. contained in the beds become distorted, and seem as if pulled out, as in the preceding sketch (fig. 242), where several shells of 622 DISTORTION OF FOSSILS FROM CLEAVAGE. [Cn. XXXII. jStrophomena expansa have suffered this elongation, in the direction of the plane of cleavage, a b, at Cwm Idwal, Caernarvonshire, the real form of this shell being that represented beneath (fig. 243). Sometimes a fossil, such as a trilobite, may be doubled down on both sides, as over a ridge, in the following manner (fig. 244), the Fig. 243. Fig. 244. sides of a Calymene Blumenbachii, a, having been, as it were, pulled down by the cleavage, the real form of this trilobite being that shown at b* This adjustment of a fossil to the planes of cleavage has been regarded by some geologists as effected by a purely mechanical movement, cleavage being referred to a pressure of the component parts of rocks productive of the effects seen. The observer will do well to consider the evidence adduced in sup- port of this view.")" At the same time he will have to direct his attention to the lamination of clays effected by electrical action, as shown by Mr. Robert Were Fox,J and Mr. Robert Hunt, and bear in mind that great masses of rocks are often extended layers of dissimilar or variously aggregated matter, moistened by saline solutions, in which common salt frequently occupies a prominent place. He has also to recollect that these layers, as has been pointed out by Professor Rogers, || may, under certain conditions, * The specimen whence the sketch is taken is from the lower Silurian rocks of Hendre Wen, near Cerrig y Druidion, North Wales. f See writings of Mr. Sharpe, Geological Journal, vol. ii., p. 74; vol. v., p. 111. j Mr. Robert Were Fox having produced lamination of clays by means of long- continued voltaic electricity, pointed out (in 1837) the bearing of his experiments in the Report of the Polytechnic Society of Cornwall for that year (pp. 20, 21, and 68, 69). He found that the planes of the laminae were formed at right angles to the direction of the electric forces. With reference to the cleavage of rocks, Mr. Fox considered " the prevailing directions of the electrical forces, depending often on local causes, to have determined that of cleavage, and the more or less heterogeneous nature of the rock to have modified the extent of their influence." Experiments carried on by Mr. Robert Hunt, at the Museum of Practical Geology, showed similar results; Memoirs of the Geol. Survey, vol. i.. p. 433. || Athenaeum, Proceedings of British Association, Birmingham, 1849. CH. XXXII.] VARIATION OF CLEAVAGE IN SAME DISTRICTS. 623 be differently heated, one portion descending to the depths beneath the surface of the earth, where a high temperature could act on such parts of the rocks, and the solutions in them, so that thermal electricity may be brought to influence the arrangement of their component parts. Viewing the subject in this light, the particles, all other things being equal, which were the most readily moveable, such as those of clays or unconsolidated accumulations of silt, would appear to be the deposits most readily acted upon. There appears little difficulty in inferring that the particles of matter, such as those which have composed slight organic bodies, or represent such particles at the time when the cleavage action was in force, would yield, like those amid which they were placed, to the same influ- ence, so that, at the final adjustment of all the component parts of a cleaved rock, they would be found so arranged, as regards their original position, as to present an elongated form. Cleavage will sometimes be found to occupy a somewhat isolated position in a given area, as also a portion unaffected by any action of the kind to occur in a generally-cleaved district. Facts illustrating the causes of these differences are very desirable, as are, indeed, all careful observations on the subject of cleavage, considered as a whole. Whatever view may be taken of its cause, extended research is required as to the direction of cleavage through many and variously-situated regions, the composition and mode of occurrence of the rocks traversed, and the changes in the dips of the planes of cleavage, as well as in their directions. In certain districts, where different directions of cleavage seem much to correspond with that of the ranges of the rocks themselves, and these are of different geological ages, as, for example, the north-east portion of North Wales, where certain Upper Silurian deposits have been accumulated upon rocks of the Lower Silurian series, upturned, with others of the Cambrian series supporting them, it becomes desirable to ascertain how far one direction of cleavage is limited to the rocks of one general range and not found in the other. In other words, endeavouring to ascertain how far the cause of cleavage, or its mode of action, may have depended upon the general position of the beds in the rocks themselves. Now, in the case mentioned, the cleavage of the older rocks (Lower Silurian and Cambrian) differs considerably from that found in those more recently deposited (Upper Silurian). When the ob- server connects this with the circumstance that in the same country (North Wales) one set of rocks (the former) had been disturbed and contorted, even in parts probably constituting dry land, 624 REARRANGEMENT OF MATTER BY CLEAVAGE. [On. XXXII. (certain of its rocks, at least, so consolidated that they could be broken off and form the materials for beaches, with vein quartz, pointing to the filling of cracks and fissures,) he may be led to inquire not only into the evidence of the cleavage having been effected in the older deposits anterior to that in the more recent, but also into the probability of this action having coincided with the different geological times during which the consolidation of both may have taken place. In some districts, it requires no slight care on the part of a geo- logist to ascertain whether the lamination of a rock before him may be due to that of original deposit or to cleavage, far more, without some experience, than might at first be thought probable. This difficulty is occasionally also increased by such arrangements of parts, apparently produced during the action effecting cleavage, that the matter of the rock is gathered under somewhat different forms in the planes of cleavage, causing a diversified kind of la%i- nation of a very deceptive kind. Instances of this fact may be well seen in Wales and in southern Ireland. Where such diffi- culties present themselves, the observer should very carefully search for lines of organic remains, which usually afford clear evidence of the true planes of bedding ; a slight seam of such remains may often suffice to place him right with respect to the true bedding of a mass of cleaved rocks. Sandstone, limestone, or other rocks marking the bedding, should also be carefully sought, so that errors, easily committed in some regions, leading to the confusion of the range and direction of the true bedding, may be avoided. In a section such as the following (fig. 245), a stratum Fig. 245. of this kind as at a, may show the true bedding, perhaps otherwise very indistinct, the cleavage, b, being so prominent as to give a false appearance of deposit lamination in another direction. Independently of the arrangement of the component parts of rocks into cleavage, under certain conditions, there is another adjustment of them, to which the term joint has been applied. It is one to which the observer, among consolidated deposits and igneous matter, will often have to direct his attention. Joints are of iar more extended occurrence than cleavage, though they are to be found as commonly in districts affected by the latter, as in CH. XXXIL] JOINTS. 625 others. They are seen as frequently among rocks which have been ejected or protruded in igneous fusion, as among those which are detrital, or which may have been deposited from solution. They traverse the coarsest conglomerates as well as accumulations of the finest sediment, such as once may have been common argillaceous clay or mud. The distinction between coarse cleavage and closely- approximated joints may be sometimes difficult to determine, as, for example, in the section beneath (fig. 246), representing a por- Fig. 246. phyry traversed by planes, dividing it 'into slabs of moderate thick- ness in a given direction, on the summit of the mountain on the south of Clynog Yawr, Caernarvonshire. As a whole, however, the joints so traverse all kinds of rocks, cutting through such varied bodies, those, for example, often gathered together in a coarse conglomerate, and in such definite and perfect planes, that power- ful as cleavage has often been, the action productive of joints some- times appears to have been more capable of dividing the mineral matter brought within its influence. It is often by means of the minor solids into which many rocks are divided from the intersection of joints, or by that of the latter and the planes of true bedding, that they can be employed for useful purposes. Though from the last circumstance long known, joints have only attracted attention as among objects of geological interest within the present century. The joints of granitic rocks appear to have been among the earliest of those observed as having definite, or nearly definite directions for considerable distances in given areas.* The planes of joints in these rocks not only present * With respect to the directions of joints in south-western England, Professor Sedgwick remarked in 1821 (Cambridge Philosophical Transactions, vol. i.) that " whenever any natural section of the country (Devon and Cornwall) exposes an extended surface of the granite, we find portions of it divided by fissures, which often, for a considerable extent, preserve an exact parallelism among themselves." He further adds, that " these masses are not unfrequently subdivided by a second system of fissures, nearly perpendicular to the former, in consequence of which structure the whole aggregate becomes separated into blocks of rhomboidal form." In 1833, Mr. Enys pointed out that the vertical joints of the Penryn granite ranged from 2s 626 JOINTS IN GRANITE. [Cn. XXXII. much interest in themselves, as dividing the matter of the rock, so that in some which contain large crystals of felspar, as in the south-west of England, and other districts, the parts of the divided crystals exactly face one another on each side of the joints, but also as coinciding with a kind of cleavage, usually found ranging parallel to the fissures of jointing, so that the quarry-men will work off minor portions of the granite by, as they term it, taking the grain of the stone, this grain being parallel with the planes of the joints. Though the joints are sufficiently obvious, this grain may not be perceptible to the eye of an observer, at the same time that the quarry-men, working from experience in certain directions and planes, produce the effect desired by forming holes and driving numerous wedges in such planes. The columnar appearance of granite, produced by the occurrence of block upon block, as if artificially piled on each other, is often to be seen on exposed mountain peaks, bosses protruding from more rounded and less elevated masses of that rock, and on sea cliffs. The following (fig. 247) may serve to illustrate the appearance of Fig. 247. jointed granite on sea-coasts, such as those of the Land's End dis- trict, Cornwall. The horizontal planes shown, are due to the structural arrangement of the granite of that county, and are to be N.N.W. to S.S.E., varying but a few degrees from those points. ("On the Granite District near Penryn, Cornwall," London and Edinburgh Phil. Mag., May, 1833.) From very careful research in the granitic districts of Cornwall and Devon we found, as elsewhere stated (Report on the Geology of Cornwall and Devon, 1839), that many hundreds of observations gave about 80 per cent, of cases in which the great joints differed only 14 from N. 25 W., and about 15 per cent, of instances in which they varied between 14 to 20 from that point, leaving 5 per cent, of cases in which the northerly and southerly joints more approximate to the cross joints. The prevailing direction of the joints in the serpentine district of Cornwall ranges within a few degrees of N. 25 W. In this body of rock, as in the various granitic portions of the same district, there are numerous variations in direction, but viewed as a whole the general range of joints is as .above stated. CH. XXXII.] JOINTS AMID SEDIMENTARY ROCKS. 627 seen in many other parts of the world, in accordance with the external form of the general mass. It may be, that in the joints of granitic rocks we only have the structural arrangement of parts effected at their consolidation, so that the different origin of the horizontal from the vertical planes may be somewhat imaginary. The more or less vertical joints of some granitic areas are certainly often continued, as for example in Cornwall, into the sedimentary rocks amid which they may occur, but how far it can thence be inferred that these were produced subsequently to the horizontal planes in the granite may be questionable, inasmuch as the jointing of the whole area may have been effected at one geological period. Indeed, as above noticed (p. 580), the granite may often be inferred to be at comparatively slight depths beneath such districts, sup- porting the sedimentary rocks that have been upraised. Though it may be sometimes doubted, regarding the more or less vertical jointing of granitic rocks, how far this may be considered as originally structural, like the divisions in certain felspathic and hornblendic rocks, giving a columnar character to them, in the manner of basalts (p. 405), there can be no doubt as to the joints in the sedimentary rocks. Here the observer certainly sees the matter of consolidated gravels, sand, silt and clay, or mud, dis- tinctly divided by planes cutting through it in marked directions for considerable distances. Such appearances as the following (fig. 248), are often presented to our attention amid sedimentary Fig. 2 18. d accumulations, particularly when these are well consolidated, two sets of joints, shown by the planes a and b, intersecting at c, and a joint parallel to a appearing at d. The most striking illustrations of the action of the power pro- ductive of joints are to be seen in conglomerates, where a great variety of pebbles, and of different sizes, is sometimes found divided 2s2 JOINTS AMONG CONGLOMERATES. [Cn. XXXII. as smoothly, in given planes, as if these pebbles had been formed of soft yielding substances, and had been cut by some thin sharp instrument, dividing them asunder in one plane. Good illustra- tions of this circumstance may be seen in the conglomerates amid the older rocks, and perhaps are nowhere better exhibited than among the old red sandstone conglomerates in the county of Waterford. Huge masses of the conglomerate, composed of quartz pebbles, and of portions of older arenaceous and other deposits, as also of igneous rocks, in certain localities, may be found smoothly cut through, and separated by joint planes. In the Commerachs, as for example, in the cliffs rising several hundred feet above the lakes, they seem to divide the mass of conglomerate into huge columns. Upon careful examination, the division presents no trace of dislocation or movement, the faces of the divided parts of the pebbles fitting each other exactly. Joints of this kind are very accessible, and readily seen in the old red sandstone conglomerate resting upon upturned Silurian rocks, opposite the town of Water- ford. Of the manner in which these divisional planes pass through conglomerates, without the slightest trace of movement of the beds, or of the pebbles in them, the best opportunities are sometimes afforded on sea-coasts, especially where the beds may be nearly horizontal, and well defined, and where the tide may recede con- siderably from the shore. Of this kind, the following sketch (fig. 249) of a joint, a, 5, traversing a remarkable conglomerate amid the mountain limestone series on the coast, near Skerries, county Fig 249. Dublin, the pebbles being of considerable size, may be found illus- trative. The surfaces of the divided pebbles, composed of portions of Cambrian rocks, probably derived from masses of them still in part remaining in the vicinity, are as smooth as if no divisional plane of the kind passed through them, yet it is one not only cutting CH. XXXII.] ' JOINTS IN LIMESTONES. 629 through this conglomerate, but the limestones with which it is associated. Joints in limestones are often of the most marked kind. In many cases there is no difficulty in distinguishing the bedding from the joints. In others, however, the observer will not find it so easy to determine between the two surfaces, without much care. It sometimes happens, that the joints have a much more marked appearance than the divisions of true bedding. As, for example, in the annexed sketch (fig. 250), wherein the joints are prominently shown, one in particular being somewhat opened at a, while the Fig. 250. true bedding, b b, is more obscure. In such cases, the observer has carefully to search for lines of organic remains, dissimilar beds, or partings of shale or other substances, in order to be sure of the true bedding. The courses of joints, though often of a marked kind through various rocks in the same district, and in the same general direc- tions for long distances, as if the power producing them had been brought into action under some great leading influence, affecting a great mass of mineral matter in that district, however modified in character its parts may be, appear not a little adjusted, as in cleavage, to the main position of the component beds, there being frequently a tendency in joint divisions to take courses at right angles, as a whole, to it. As in cleavage, also, divisions re- sembling jointing, so far as their distance from each other is con- cerned, appear to run through certain beds of a general accumula- tion more abundantly than in others. Of this kind, the divisions through parts of the shales of the lias near Lyme Eegis may be taken as an example. Though joints are not there observed in the mass of the argillaceous limestones composing that deposit, in certain beds of shales, on the west of the town, divisions, perpen- 630 MOVEMENT OF ROCKS AFTER JOINTING. [Cn. XXXH. dicular to the beds, may be seen to run like so many planks on a floor, stretching as far as the beds are exposed at low water. As there appears little reason to doubt that joints, like cleavage, have been formed, under suitable conditions, at different geological times, and as these cleaved or jointed rocks may readily have been moved after they were divided in this manner, it would be expected that, sometimes, the position of the one and the other, as regards their direction with the horizon, is not that in which either the cleavage or jointing was effected. Cleaved and jointed rocks are sometimes found in positions to render such subsequent movements probable. For example, the old red sandstone series of southern Ireland reposes upon Silurian rocks probably cleaved, if they were not also jointed, prior to the accumulation of the former, and the same series is also traversed by similar divisions. Upon studying that portion of Ireland, the observer finds that the old red sand- stone, with also the carboniferous or mountain limestone series resting upon it, has been also disturbed since its deposit ; hence, the lower rocks having been again moved to permit the rolling and bending of the great mass of matter resting upon them, their original planes of cleavage, if not of joints also, can scarcely be in their original position. The probability of such movements may, therefore, somewhat interfere with first views as to the original position of cleavage and joints, and the geologist should bear in mind, that the movement of a body of rock, divided in this manner, into flexures, might be accompanied by the friction of some of the surfaces of the divisional planes upon each other, thus embarrassing his researches into the original condition of such surfaces. Move- ments of this kind may give an uncertainty to the slightly-inclined planes of joints which are sometimes found, though there is, as yet, no evidence to show that joints have originated in a manner to render divisions in the mineral matter improbable at these angles with the horizon. Such planes of joints require to be well distin- Fig. 251. a b a guished from those of true beds, which they often much resemble, as, for example, in the preceding section (fig. 251), where a mass CH. XXXII.] BEDDING, JOINTING, AND CLEAVAGE COMBINED. 631 of argillaceous matter, originally a thick accumulation of clay or mud, though now consolidated into hard rock, shows joint lines, a a a, and sections of the planes of cleavage, b b, but does not exhibit a surface sufficiently large to show the planes of true bedding. Occasionally, the division of an original deposit of clay or silt, by cleavage and joints, becomes most complicated, requiring no slight care on the part of an observer to arrive at the surfaces of the true beds, more especially when organic remains are absent, and the mineral matter is of a common character throughout. Of this kind of complication, the following sketch (fig. 252) of a quarry at Fig. 252. . Brewer's Hill, county Wicklow, may be useful as an illustration. The true bedding is a plane, facing the reader, while there are divisional planes ranging in the direction a a, in that of b b, and in that of c c. CHAPTER XXXIII. BENDING, CONTORTION, AND FRACTURE OF ROCKS. EARTH'S RADIUS COM- PARED WITH MOUNTAIN HEIGHTS. VOLUME OF THE EARTH WITH MOUNTAIN RANGES. EFFECTS OF A GRADUALLY COOLING GLOBE. MOUNTAIN RANGES VIEWED ON THE LARGE SCALE. DIRECTIONS OF MOUNTAIN CHAINS. CONDITIONS EFFECTING THE OBLITERATION OR PRESERVATION OF MOUNTAIN CHAINS. LATERAL PRESSURE ON BEDS OF ROCK AMID MOUNTAINS. BENDING AND FOLDING OF DEPOSITS IN THE APPALACHIAN ZONE, NORTH AMERICA. FLEXURES AND PLICATIONS OF ROCKS IN THE ALPS, AND OF THE RHINE DISTRICT. -IGNEOUS ROCKS AMID CONTORTED BEDS. CONTORTED COAL MEASURES OF SOUTH WALES. CONTORTION OF THE COMPONENT PARTS OF BEDS. THOUGH it has been necessary to allude to the disturbance of various accumulations, as well igneous as those formed by means of water, while noticing rocks of different kinds which have been more or less moved after their deposit or intrusion, it may be desirable to call attention to this subject as one which may also be conveniently considered by itself. It will have been seen, when pointing out the intrusion of igneous rocks, that the disturbance of mineral matter accumulated at one geological period, while the deposits of another were comparatively unmoved, assisted in afford- ing evidence of the relative time when the igneous rock may have been elevated in a molten state from beneath (p. 564) ; and also that the arrangement of conglomerates and sandstones against or around beds of prior-formed disturbed rocks was useful in showing the probability of ancient dry lands having occurred in particular situations, edged by beaches and coast cliffs, (p. 475). Though mountains by no means present us with the only means of studying the bending, contortion, and fracture of rocks on the large scale, they become important from the masses of matter raised in them comparatively high into the atmosphere and sometimes continuous for considerable distances, from the frequent adjustments CH. XXXIII.] EARTH'S RADIUS AND MOUNTAIN HEIGHTS. 633 of lower grounds to them, and the opportunities afforded for obtaining illustrative sections in various planes. A glance at any artificial globe of fair dimensions will be sufficient to show the ranges or chains, as they have been termed, of those mountains which constitute marked ridges upon the surface of the earth. With such a globe before him, and bearing in mind the heights of the various ranges or chains of mountains as compared with the diameter of our planet, an observer may, pro- bably, be led to infer that, however elevated and important these may be considered by those wandering amid their depressions, or striving to ascend their heights, viewed as ridges on the surface of the earth, they con- stitute very minor protrusions, interfering little with the general form of the world. It is somewhat important, in searching for facts illustrative of the production of mountains, that their relative proportion to the volume and diameter of the earth should not be ne- glected. If, in the annexed diagram (fig. 253), a b e, represent a section of a portion of our planet, from its surface a, b, to its centre e ; the thick line, a, b, would be the elevation of even the highest mountains as compared with the radius of the earth. Hence it is not diffi- cult to conceive that the rending of any portion of consolidated or partly consolidated mineral matter, distributed in various ways over the surface, a, b, and the squeezing of the sides of these rents or fissures against each other, (with or without the propulsion upwards of any molten substances amid interstices in the squeezed masses of consolidated or partly-con- solidated mineral matter,) would present ridges of varied forms more or less corresponding with the lines of the fissures. It has been seen that igneous rocks have been ejected in various ways, that mineral matter worn from them by the action of the Fig. 253. ! i ~i i f i i j i i i i i i i i i / i i i ! l 1 1 1 1 I 1 i 1 / i 1 i I i 1 i 1 1 i 1 1 1 1 i / i i i i 1 i I f 1 / 634 MOUNTAINS AS REGARDS VOLUME OF THE EARTH. [Cn. XXXIII. sea and atmospheric influences, or obtained in solutions, has been spread over differently-sized areas, that these have sometimes moved up and down as regards the surface of the ocean, and that, considering rocks to have met with more elevated temperatures when depressed, (particularly when covered by superadded mineral matter above,) than when raised into the atmosphere, modifications have been effected in the arrangement of their component parts. Bearing all this well in mind, and giving considerable latitude to views of the thickness of the earth's surface which may thus have been moved, if we assume this thickness to extend in depth even to 100 miles (a c, b d, fig. 253), we merely arrive at the relative proportion of volume and thickness of the exterior of our planet shown in the accompanying diagram (fig. 254), wherein the depth of the 100 miles is represented by the thick line forming the circle. Fig. 254. Having prepared himself by this general view of the relative importance of the volume and diameter of the earth, and of the mountain ridges on its surface, the geologist will probably feel also disposed to regard the contortions and fractures of various rocks which he may discover in such ridges with reference to some cause acting generally over the surface of our planet, since he finds marked mountain ranges in all extensive areas of dry land. If, upon further investigation, he obtains evidence, as he will not fail to do, that all mountain ranges have not been elevated to their present positions contemporaneously, the deposits of particular geological periods resting upon prior-formed and disturbed beds in some, while in others, equivalents, in geological time, to these un- CH. XXXIII.] MOVEMENTS IN MOUNTAIN RANGES. 635 moved deposits are themselves disturbed and broken, even, perhaps, covered tranquilly by subsequently-formed beds, he may be induced to conclude that whatever the cause of mountain ranges, it may have continued in action during a long lapse of geological time, and may still exist.* Upon connecting the form, volume, and diameter of the earth * The following section (fig. 255) may probably be useful in showing the relative age of disturbed beds of rock in mountain ranges. If the rocks a a, are found resting quietly on the upturned strata, b b t it is inferred that b b have been disturbed prior Fig. 255. to the accumulation of a a ; and, consequently, if a a be a known rock in the geolo- gical series, a relative date is obtained for the movement of the beds b 6, so far as relates to a a. If it should so happen that there are no commonly known deposits absent between them, the approximate relative date of the uplifting of b b is obtained. Should it also occur, in any range of mountains or disturbed country, that other accu- mulations, c, are, in like manner, so placed relatively to the deposits b b, that another and anterior movement of rocks can be inferred, then, in such range of mountains or disturbed district, there would have been two distinct movements, one prior to the production of b b, the other anterior to the accumulation of a a. In the case of beds covering contortions, it becomes very needful carefully to observe them sufficiently on the large scale. For example, let beds a a, in the annexed section (fig. 256), Fig. 256. repose quietly on the contorted strata b &, and let the only portion exposed to view be where they are cut by the line c ; then all the beds would appear undisturbed, and it would be only by moving to the right or left, and where the disturbed strata beneath might chance to be fairly exposed, that the real mode of occurrence may be found. This is by no means so needless a caution as might, at fifst sight, be supposed, particularly when the bends and contortions are upon the large scale. While on this subject it may be useful to notice the imperfect knowledge of the dip, or inclination Fig. 257. of beds, from one view of them only, since they may even appear horizontal, as in the annexed sketch (fig. 257), while in reality they have been much disturbed, forming a 636 EFFECTS ON A GRADUALLY COOLING GLOBE. [Cn. XXXIII. with the relative proportion of the volume and height of mountain ranges, such as those of the Alps, Andes, and Himalaya, it may suggest itself to the observer to consider how far some general cause for these comparatively trifling ridges and rugosities, little inter- fering with the even character of the surface of the world, may not have followed some change in the volume of the earth itself. Should he try the hypothesis of a spheroid, such as that of the earth, losing heat by radiation into surrounding space, by which a given volume of matter parted gradually with its temperature, one sufficient at first to keep the whole in a liquid state, perhaps he might be led to infer that an oxidised and comparatively cooled superficial covering of solidified mineral matter, having a prevailing crystalline arrangement of parts, especially in its lower portion, might be brought under conditions by which it would have to crack and ridge up, with various adjustments as to foldings and fractures, in order to adjust itself to a mass below, gradually ceasing to occupy some originally-supporting space beneath it. Upon this hypothesis, the oxidation of the various elementary substances constituting the mass of the mineral matter known to us on the surface of the globe, has to be regarded, inasmuch as such oxida- tion would add to the volume of the elementary substances on that surface, and thus alone aid in altering the exact fitting of a crust of mineral matter upon the remaining portion of the earth beneath, the elementary substances in which had remained unchanged. Be this as it may, and whatever the hypothesis employed to arrive at the cause of mountain chains, it appears desirable so to examine into the facts connected with the arrangement of the masses of mineral matter of which mountain ranges may be composed, that, while all due regaid be paid to individual chains, observation should portion of some bent or contorted rooks, a8 is shown in the following view (fig 258) Fig. 258. supposed that of the same cape (/>, in both figures) on a coast projecting from the mainland, a a. CH. XXXIIL] MOUNTAIN RANGES VIEWED ON THE LARGE SCALE. 637 also be directed to the subject on the larger scale. The earth so little differs from a sphere in form, that, in investigations of this kind, it may be regarded as one composed of matter upon which some general action, tending to ridge its surface, might also produce results on that surface of a definite general kind, supposing forces and resistances, and all other circumstances, equal. It is in this supposition of exactly equal conditions that there may be much difficulty with such relatively minor volumes of matter as mountain chains, so that even inferring some constant action, it may be so modified by circumstances as to be materially concealed from obser- vation. To the direction of lines of disturbances on the earth's surface, productive of mountain chains, or otherwise, as may have occurred, much attention has been given of late years, in conse- quence of the labours of M. Elie de Beaumont on this subject.* He has inferred that there is evidence to show that, during the lapse of geological time, the disturbances of the earth's crust have been effected in given directions, at certain times, and that these disturbances have taken place along considerable fractions of the great circles of our planet.j He has further considered that there * The first account of the views of M. Elie de Beaumont on this subject was com- municated to the Academy of Sciences of Paris, in June, 1829. t M. Elie de Beaumont remarks, in a communication to the author, in 1831 (Geo- logical Manual, 1831), " Pursuing the subject, as far as my means of observation and induction will permit, it has appeared to me that the different systems (of mountains and disturbed rocks), at least those which are at the same time the most striking and recent, are composed of a certain number of small chains, ranged parallel to the semi- circumference of the earth's surface, and occupying a zone of much greater length than breadth ; and of which the length embraces a considerable fraction of one of the great circles of the terrestrial sphere. * * * The secular refrigeration, that is to say, the slow diffusion of the primitive heat to which the planets owe their spheroidal forms, and the generally-regular disposition of their beds from the centre to the circumference, in the order of specific gravity, the secular refrigeration, on the march of which M. Fourier has thrown so much light, does offer an element to which these extraordinary effects (the elevation of mountain chains) may be referred. This element is the relation which a refrigeration so advanced as that of the planetary bodies establishes between the capacity of their solid crusts, and the volume of their internal masses. For a given time, the temperature of the interior of the planets is lowered by a much greater quantity than that on their surfaces, of which the refrigera- tion is now nearly insensible. We are, undoubtedly, ignorant of the physical pro- perties of the matter composing the interior of these bodies ; but analogy leads us to consider, that the inequality of cooling above mentioned would place their crusts under the necessity of continually diminishing their capacities, notwithstanding the nearly rigorous constancy of their temperature, in order that they should not cease exactly to embrace their internal masses, the temperature of which diminishes sensibly. They must therefore depart, in a slight and progressive manner, from the spheroidal figure proper to them, and corresponding to a minimum of capacity ; and the gradually-increasing tendency to revert to that figure, whether it acts alone, or whether it combines with other internal causes of change which the planets may contain, may, with great probability, completely account for the ridges and pro- tuberances which have been formed at intervals on the external crust of the earth, and probably also of all the other planets." 638 DIRECTIONS OF MOUNTAIN CHAINS. [Cn. XXXIII. have been several distinct systems of disturbance, each marked by a given direction. When more recently describing some lines of this kind which he considers referable to certain systems, succeeding each other r in the order of geological time, and all of relatively ancient geological date, M. Elie de Beaumont takes occasion to remark, after alluding to the systems of small arcs of great circles, that, " the fundamental problem presented by a like system of small arcs observed on the surface of the globe, where they are marked by the crests of mountains or by the outcrop of beds, consists in determining the great circle of comparison, to one of the elements of which each of the small arcs observed is parallel."* Thus while * " The small arcs determined by observation," continues M. Elie de Beaumont (Bulletin de la Soc. Geologique de France, t. 1846-7), " may be generally considered as being 'themselves infinitely small secants, or tangents to so many small circles resulting from the intersection of the surface of the sphere with planes parallel to the great circle of comparison, forming the equator of the whole system. Each of these small circles is a parallel with respect to the equator of the system ; it has the same poles as it, and these poles are the two points where all the great circles per- pendicular to the small arcs, constituting the system of parallel traces determined by observation, intersect. " The problem arising from such a system of parallel traces observed on the surface of the globe consists in determining these two poles, or, which amounts to the same thing, its equator, i.e., the great circle of comparison to which each of the small arcs observed may be considered as parallel. This determination," observes M. Elie de Beaumont, " would be easy, and might be made after two, or at least a few observa- tions, if the condition of parallelism were rigorously satisfied : since, however, this in general is but approximately accomplished, the determination of the great circle of comparison can only follow from the means of numerous observations, well com- bined with each other ; and thus, while the observations are not very multiplied or spread over a wide space, we can only advance towards this determination by suc- cessive approximations." As it would be quite impossible to present a correct view of the different systems of disturbance, without the needful tables and calculations en which he has founded them, and which would be here out of place ; and as it would moreover be extremely difficult satisfactorily to abridge the very condensed statements of M. Elie de Beau- mont, we would refer the geological observer to his memoir in the " Dictionnaire Universelle d'Histoire Naturelle," t. xii. p. 167, and to his more extended and recent general work on this subject, entitled " Notice sur les Systemes de Montagnes," Paris, 1852, in which he has treated the subject still more, at large, and up to the present time, and where all his views respecting the great disturbances on the earth's surface, produced at distinct geological times, will be found. In a note " Sur la Correlation des Directions des differents Systemes de Montagnes " (Comptes Rendus, 9 Septembre, 1850), M. Elie de Beaumont calls attention to the present known directions of mountains, and their adjustment to & pentagonal network formed by the intersection of fifteen great circles of the sphere. For the mode of investigation on which this view is founded, our limits compel us to refer to the memoir itself. M. Elie de Beaumont concludes his note by remarking that " the fifteen circles which divide the surface of the sphere into twelve regular pentagons possess the property of the minimum contour of the system of lines of most easy crushing (plus facile ecrasemenf). If the ridging of the earth's crust were simul- taneously produced, these fifteen circles would, perhaps, be alone traced ; but as the production of the different systems of mountains has been successive, the octahedral, dodecahedral, and others, have probably been the forms necessarily intermediate in passing from one to the other of the fundamental circles." CH. XXXI11.] MODIFICATIONS IN DIRECTIONS OF MOUNTAINS. 639 estimating the directions of disturbance at different geological times, with reference to the views of M. Elie de Beaumont, the observer would have to bear in mind the great circles of comparison to which the directions of any ranges of mountains, or masses of disturbed beds are to be referred. In investigations of this kind, the geologist has to consider not only any exertion of force tending to disrupt portions of the earth's surface, acting generally or partially, but also the kind of resistance offered, one which may be materially modified by any variable thickness of the solid matter acted upon, and by variations in the coherence of portions of that matter. As in all movements of this order, differences in the lines of least resistance to some given force, independently of those in that force itself, would produce very marked differences in the ranges of disturbed rocks, especially on the minor scale, in researches of this kind it may not be easy always to estimate very correctly the value of a so-called minor scale. If an observer, aware of the general geological structure of the British Islands, and of a few thousand square miles of the adjoining portion of the continent of Europe, duly weighing the probability of the mode of occurrence of the different rocks to a depth not extending to even more than three or four miles, suppose this mass of variably- accumulated matter to be ridged, squeezed, and contorted by a force acting in some given direction, so as to produce a lofty chain of mountains like the Alps or the Himalaya, he would expect that very material minor modifications are not unlikely to be produced in the direction of the various parts, and even that these might extend and interfere with the direction of the range itself. If the great masses of igneous rocks, such as the granites of various parts of the area mentioned, are to be inferred as, so to speak, anchored somewhat firmly beneath, a crush, acting upon them and the de- trital accumulations by which they may be surrounded superficially, or be covered by to various depths, would be expected to be marked by an arrangement of the mineral matter in accordance with its different coherence, form, and thickness. As during the progress of geological time so much of the earth's surface, formed of either igneous products or strewed over with detrital, or chemically-deposited matter of various kinds, as also with the remains of animal and vegetable life, has been covered by more modern accumulations of the like kind, even now, over wide- spread areas, concealing them, it becomes no easy task for the geologist to picture to himself the surface conditions of our planet at given periods, so that the disturbed and undisturbed portions 640 OBLITERATION OF MOUNTAIN RANGES. [Cn. XXXIII. may be duly estimated. This becomes the more difficult as his investigations extend to the earlier periods, since not only may so much of the then surfaces of the earth be now buried beneath more modern accumulations, but even the ridging of such surfaces, con- stituting mountains, may have been obliterated by that action of the sea and atmospheric influences to which the term denudation has been applied. Looking at these sources of the removal of mineral matter, and for the moment inferring all other conditions to be equal, the older a range of mountains the less should we expect the remains of it ; and conversely, the more modern the range the more should we expect to find it unaltered in its form and general character. Here at once the differences in the other conditions present themselves. Contemporaneously-produced ranges of mountains, and even portions of them, may have been acted upon very variously. One range, or part of it, in some given area, might remain as when thrust into the atmosphere, modified only by the influences to which it has been therein exposed, while in another area, or part of one, the land may have been depressed beneath and raised above the sea level, even several times, with the attendant consequences of either new coverings or the removal of mineral matter thence arising. Fortunately in Europe and America large tracts are found, where the beds of the older fossiliferous rocks still occupy po- sitions not very different from those of their accumulation, and wide-spread areas have changed their relative levels, as regards that of the sea, so in mass, that these old sea-bottoms became large portions of dry land, without the folding and crushing of their component beds. Other considerable areas of like kinds may pro- bably be detected when extended, and as yet little explored regions become better known. Be that as it may, these old undisturbed portions of the world's surface become important, from pointing out those parts of it which have escaped the ridging, squeezing, and contortions to be found in many other localities. If we could obtain such somewhat widely dispersed, they would aid consider- ably in separating the undisturbed from the disturbed portions of the earth's crust, so far as regards the squeezing or contortion of them, though not, as is obvious, those which may have been lifted and let down bodily in a horizontal or nearly horizontal manner. When we find, as in the great north and south range of the Ural mountains, these same accumulations squeezed and disturbed as a whole, and in a marked line, and the relative date of the disturb- ance can be approximately inferred, as has been done by Sir CH. XXXIIL] LATERAL PRESSURE OF BEDS AMID MOUNTAINS. 641 Roderick Murchison and his colleagues, Count Keyserling and M. de Verneuil,* the geologist obtains a knowledge, not only of the time up to which these portions of the earth's surface may have remained without such disturbance, but also of the direction of the line or lines along which it was effected. The mountain ranges of the world, occurring in so many parts of its surface, seem all marked by evidence of the squeezing and con- tortion of the different accumulations disturbed, as far as researches have yet extended. While some show igneous matter to have risen up in somewhat considerable abundance, and apparently when these disturbances were effected, it is not discovered so com- monly in others. This may merely depend, all other things being equal, upon the amount of mineral matter of another character which has been removed, or upon that matter having been so ad- justed as to conceal the igneous rocks. Much caution is therefore needed when an observer may be engaged in this kind of inquiry. Thus, in some granitic ranges, such, for example, as those above noticed in South-western England and South-eastern Ireland, we may only have the remains of former chains of mountains. To obtain very close approximations in ranges of mountains, to the amount of folding, contortion, or fracture of the various rocks acted upon in the manner mentioned, sections should be formed proportionally representing these circumstances. Usually, however, no great exactitude is attempted, so that sections of mountain districts merely afford very general views on the subject. Even these, nevertheless, are sufficient to show the great lateral pressure to which the whole, abstracting any igneous rocks (apparently introduced during the time or times of such disturbing action), has been commonly subjected. The observer often finds, when following some given series of beds, presenting characters sufficiently marked for the purpose, and properly weighing the evidence as to gaps, due to the openings from fractures, that if such contorted beds could be again laid out flat, as when deposited, Fig. 2J3, a * they would have to be spread over a greater superficial area than they now occupy. Thus, if in the annexed section (fig. 259), * " Geology of European Russia and the Ural Mountains." 2 T 642 EVIDENCE OF LATERAL PRESSURE IN MOUNTAINS. [Cn. XXXIII. the curved and contorted line represents the foldings and con- tortions of a given series of beds, c b, on the flank of some moun- tain chain, such as the Alps, and, allowing for fractures and portions removed, if that line be reduced to a straight one, a 5, it will be evident, that a lateral extension to the amount of the dis- tance a d, will be required for the return of these beds to their original position, supposing, for illustration, the point b to have remained firm. In like manner, if instead of one flank only of a range of mountains, thus exhibiting a folding and contortion of its beds, both flanks do so, and a section across the whole range shows these to be of the kind represented beneath (fig. 260), then the line c d, would represent the distance required for the flattening of the folded and contorted beds, instead of that of a b, giving the distance now occupied by them. If the points a and b, be inferred to have remained relatively firm, as respects distances outside them, then there has been a diminution in the distance be- tween them, equal to c a + b d, the beds previously occupying the distance c d, being so folded and bent as not to extend beyond a b. Hence, also, a motion from c to a, and from d to b is inferred, and supposing the substances of these beds sufficiently yielding, this might be accomplished without a break at /. Considering breaks to have been formed at the chief bends, as at 0, o, o, o, the dis- tances for the relative movement c a, and b d, may be somewhat altered, fractures of the kind represented beneath, in fig. 261, Fig. 261. being to be taken into account, c being a line of fracture along which the beds a are considered to have slid to b. A diminution of the area previously occupied_by~ these folded CH. XXXin.] FLEXURE OF ROCKS IN THE APPALACHIANS. 643 and contorted beds having been thus effected, the observer has to see whether on the one side of a mountain chain or the other, or on both, there may be any evidence in favour of lateral pressure acting from without inwards, or if there may appear any in favour of a great fissure or fissures, in the ranges of the mountains them- selves, against the sides of which the rocks moved, and had ad- justed themselves according to the action of gravity, and the lateral thrust upon yielding materials outwards. The usual im- pression left, by even the general sections given of ranges of mountains, such, for example, as those of the Alps, is, that there has been an elevation of their component rocks in the direction of these main ranges, and that they have adjusted themselves late- rally to meet the force of gravity acting vertically upon the upraised mass.* Inferring the needful pressure, it would be expected, that molten matter beneath the masses moved, would be ready to enter amid any openings effected, as far as that pressure permitted, this intruded matter tending to brace much of the fractured beds to- gether, upon cooling. An intrusion of such molten rocks, might, therefore, be among the consequences of the action producing the elevation of the mountain range, and be more or less important, according to circumstances. Geologists are indebted to the Professors Rogers for observations on an extensive district in North America, one of about 195,000 square miles, which have led them to point out an arrangement of the bends and foldings of disturbed rocks in accordance with the * Respecting the folding of beds by vertical and lateral pressure, Sir James Hall, as long since as 1813 (Transactions of the Royal Society of Edinburgh, vol. vii., p. 86), showed that this could easily be imitated artificially by taking various pieces of cloth, placing them horizontally on some table, c (fig. 262), pressing them downwards by a Fig. 262. weight, a, acting parallel to the plane of the table beneath, and by applying force laterally, 6, b. In experiments of this kind, it is not, however, necessary to have the top weight, a, since if the cloth be in proper quantity, its gravity alone will be suffi- cient to produce the contortions, and a more exact resemblance to nature be obtained. By moving only one side, or both, as thought desirable, a very interesting illustra- tion of the contortions of beds may thus be easily seen. 2 T 2 644 FLEXURE AND PLICATION OF BEDS IN THE ALPS. [Cn. XXXIII. distance from the application of force. A careful examination of the Appalachian zone, as they term that region, showed that it is marked by five great belts, which, when crossed from south-east to the north-west, exhibit the greater flexures in the first belt, or that on the south-east of the Blue Ridge or Green Mountain Chain. The component beds of the belt are doubled into enormous, closely com- pressed alternate folds, dipping almost exclusively to the south- east at angles varying from 45 to 70. In the third belt, the beds are less compressed, the northern side of each anticlinal curve* approaching nearly to verticality. In the fourth belt, that of the central Appalachians of Pennsylvania, Virginia, and Tenessee, the convex and concave flexures progressively expand, the steepness of the north-west side of each anticlinal gradually diminishing. In the fifth belt, that of the coal region of the Alleghany and Cumber- land Mountains, the curves dilate, and subside into broad sym- metrical undulations with gentle dips. The folds and undulations of the beds occur in groups, the several axes being very nearly parallel and similar in the character of flexures, many of the larger anticlinals having a length of 80 or 100 miles.| With respect to dislocations of these beds, two systems are noticed, one of short fractures nearly perpendicular to the direction of the anticlinals, the other ranging with them, and often of considerable amount. The longitudinal dislocations (and some in Virginia have a length ex- ceeding 100 miles) are inferred to be broken flexures, the fracture almost invariably occurring on the north-western or inverted sides of the anticlinals, and having a moderately steep south-eastern dip. Some of these great fractures have thrown the portions of once continuous beds not less than 8,000 feet asunder, measured perpen- dicularly to the surfaces of the strata. After an examination of the disturbed rocks of the Alps, Jura, and of the district of the more ancient fossiliferous rocks of the Rhine, Professor H. Rogers con- * In a vertical section of rocks, of which the following line b, c (fig. 263) repre- Fig. 263. a sents the bends from pressure, a, a, a, would be the anticlinal, and s, s, s, the synclinal curves. t As regards the distances of the contiguous great folds, they are stated to be less than one mile in the south-eastern belt, in the central belt between one and two miles, and in the north-western belt the flexures have an amplitude of from five to ten miles. GH. XXXIIL] EFFECTS OF IGNEOUS ROCKS ON FLEXURE OF BEDS. 645 siders that in these localities also the like flexures and plications are observable.* To produce a system of flexures and plications, such as that described by the Professors Eogers as occurring in North America, would not only seem to require great lateral pressure, but also a somewhat uniform and general yielding of the various beds moved during the whole time that the needful action was prolonged. Had there been large volumes of intermingled or deep-seated masses of igneous rocks, offering different resistances to the force employed, much modifications in the resulting flexures and plications would be expected, the softer and less-consolidated beds being even occa- sionally squeezed over the large masses of the hard igneous rocks. Thus it might happen that when such igneous rocks were in abundance, many masses being deep-seated, the results of an appli- cation of force along an extended line of action would be so modified as to offer considerable difficulty in tracing the various flexures and plications to such line. In all cases, as well that of the great Appalachian zone, as in the masses piled up to more marked heights, such as in the Alps and Himalaya, a shortening of the space previously occupied by the component beds appears required (fig. 260, p. 642). Whether the observer be engaged upon the examination of flex- ures or plications amid ranges of mountains or less highly-elevated portions of country, it is very desirable not only that he should duly appreciate the amount of the folding and bending of the accumulations disturbed, but also the real outline of the districts. Without a proper reference to this outline, the most exaggerated views may be entertained of the importance of heights and depres- sions, especially of mountainous regions, relatively to their dis- * Upon examining the Devonian rocks of the Rhine, Prof. H. Rogers inferred, that the entire region composed of these and the carboniferous series exhibits the effects of the laws of flexure and plication found in the Appalachians, and he points to a section from south-east to north-west, either through the Taunus to Westphalia, or by the Rhine from Bingen to Remagen, or from the Hundsruck to the coal region of Liege, as showing an almost universal south-eastern dip, resulting from the close oblique folds with steep or inverted dips to the north-west of each large anticlinal. He further remarks, that on approaching the northern side of the district the flexures become progressively more open, and that the inequality in the dip of the sides of the anticlinals diminishes, so that in this case also the force would appear to have been applied on the south-east. In the Jura, the Professor considers the anticlinals to have one side of the arch more incurved than the other, but not inverted, and that while the ridges are higher next the great plain of Switzerland, all the individual flexures are steepest towards the Alps. In the Alps, he infers the axis-planes to dip inwards from both flanks towards the central portion, so that the masses are folded in opposite directions ; the plications of the Bernese Oberland dipping south, those of the chain of the St. Gothard and the Simplon towards the north. 646 PROPORTIONAL SECTION OF THE ALPS. [Cn. XXXIII. Fg- 264 . tances ; an exaggeration very detrimental to a just appre- ciation of the relative mass of such mountains as compared with the less elevated and more moderately marked fea- tures of countries amid which they may occur. The accompanying section (fig. 264) may serve to show the relative importance of the elevation and mass of the Alps, from the Jura to the central ridge, in a line traversing the lake of Geneva and the summit of Mont Blanc, the scale being the same for heights and distances.* In this section j represents the Jura; g, the lake of Geneva; v y the Voirons ; m, the Mole; a, the Aiguille de Varens; b, the Breven ; MB, the Mont Blanc, and fiif/f//7///y ' " part of the rocks shown near that town are tilted over, so as to have the false appearance of having been deposited after those which they really support; the mountain limestone series, a, appearing to repose at Tenby, r, (from the part of the curve there visible,) upon the coal measures ; d d, being the level of the sea, Certain lower beds of this limestone series are brought up, by a bend of the strata, at b. c, c, c, represent various shales and sand- stones of the coal measures. There are dislocations, or faults, at/s, and w v is Waterwinch, on the northward of Tenby. A still more considerable apparent inversion, from the same reason, is to be seen time, these may themselves be reheated in part after consolidation in their higher portions, and after the first uplifting, when fissures formed in the prior deposits were even filled with the then molten rock, so that, pressure continuing, these resoftened portions could be squeezed up like the beds of the prior-formed deposits, still further thrusting the latter on one side. 648 CONTORTION OF THE COMPONENT PARTS OF BEDS. [Cn. XXXIII. on the shores of part of Milford Haven (Langum Ferry), at a few miles westward from Tenby, where old red sandstone rests inclined on mountain or carboniferous limestone, and this again upon the coal measures.* In movements of this kinc}, even disturbances in the arrangement of the component parts of the beds themselves would be expected according to their relative positions, and that of such component parts. Thus, with an interstratification of sand and mud, slightly, if at all, consolidated, if a squeezing lateral motion be applied to these beds collectively, they would yield relatively to their respective resistances. Of this class the minor contortion of the component parts of some sandstones interstratified with shale beds, of the older fossiliferous rocks at Bewly Bay, Waterford Harbour, as shown in the accompanying section (fig. 266), may be taken as Fig. 266. an example. The minor portion of the sandstone beds, a, a, a, a, are there seen contorted, as in disturbed masses of rock on the large scale, while the shale b, b, b, 6 ^^^^''J^^^L^ (formerly mud) has slid and adjusted itself in a less marked manner, though its particles may have been also moved. The sliding of more consolidated over less hard beds the ob- server will often find well shown, as also the marks of friction produced upon the adjustment of such consolidated beds as could move upon or against each other, the striation being Fig. 267. often beautifully exhibited.! Pressure movements of this kind may be well seen in Pembrokeshire among the coal measures, some coal beds having so given way before the general force, that their component parts have been squeezed, in the manner represented (fig. 267), into the outer portion of the flexures, a, a, while the roofs and bases of the coal beds are brought into contact between them. * With respect to such inversions, as they are sufficiently common amid series of beds bearing the same geological names, their occurrence in a sequence of accumula- tions is merely the same thing made to appear somewhat more important from different names being assigned to different parts of the accumulations moved. f In the so-much-visited Alum Bay, in the Isle of Wight, where various tertiary beds are turned up vertically, the squeezing of parts of the clays against each other is well exhibited. This is particularly well seen in the white pipe-clay bed, contain- ing fossil plants. CHAPTER XXXIV. FAULTS. PRODUCTION AND DIRECTION OF FISSURES THROUGH ROCKS. EVIDENCE OF THE RELATIVE DATES OF FISSURES. FALLACIOUS APPEAR- ANCE FROM A SINGLE MOVEMENT SHIFTING VARIOUS FISSURES. FISSURES SPLIT AT THEIR ENDS. LINES OF LEAST RESISTANCE TO FISSURES.' RANGE OF MINERAL VEINS AND COMMON FAULTS IN SOUTH-WESTERN ENGLAND. RANGE OF FAULTS NEAR SWANSEA. INCLINATION OF FAULTS. PARTS OF DEPOSITS PRESERVED BY FAULTS. COMPLICATED FAULTS. NOT only has the geologist to direct his attention to the fractures effected by the snapping of plications, when the rocks acted upon have been incapable of further flexure, as a mass or in part ; but also to numerous lines of fracture, sometimes of considerable length, which traverse beds and masses of rocks, where violent squeez- ing into great plications and flexures has not occurred. For such lines of fracture, the mining teim fault has now been adopted.* Sometimes, when even of considerable length, they are accom- panied by very minor dislocations, the sides of the fractures nearly corresponding ; at others, the fracture has resulted in a separation of the beds, perpendicular to their surfaces, of several thousand feet, and yet the fracture not be on the bend of a plication. Being of importance in mining districts, and mineral veins being com- monly the filling up of spaces consequent on them, the range of these fractures becomes better known in such districts than they would otherwise be ; at the same time, however, in numerous other districts, where beds of marked and dissimilar mineral structure occur, they may be readily traced.-)- The range of these fractures and the relative time of their pro- * A term derived from the miners, chiefly those working coal, who, when these dislocations are met with, often find themselves at fault, the amount of the dislocation produced not being always clear. They are also known as troubles by the miners. t The geologist will find faults traced with great care in many of the maps of the Geological Survey of the United Kingdom, as, for example, in Sheets 36, 37, 41, 42, 55, 56, 61, 74, and 79 of the Great Britain series. 650 PRODUCTION AND DIRECTION OF FISSURES. [Cn. XXXIV. duction have of late occupied much attention. Their mode of occurrence has especially engaged the attention of Mr. Hopkins, who has investigated the conditions under which directions would be taken by fissures, either formed at the same time, or at periods subsequently to each other, seeing if the anticlinal lines and other disturbances and dislocations of rocks may not be referable to some " widely-diffused action of some simple cause, general in its nature with respect to every part of the globe, and general in its action, at least with respect to the whole of each district, throughout which the phenomena are observed to approximate, without interruption, to the same geometric laws."* Mr. Hopkins commences, as to the action of an elevating force, with as simple an hypothesis as he conceives the subject will admit. " I assume this force," he observes, "to act under portions of the earth's crust of considerable extent at any assignable depth, either with uniform intensity at every point, or in some cases with a somewhat greater intensity at particular points ; as, for instance, at points along the line of maximum elevation of an elevated range, or at other points where the actual phenomena seem to indicate a more than ordinary energy of this subterranean action. I suppose this elevatory force, whatever may be its origin, to act upon the lower surface of the uplifted mass, through the medium of some fluid which may be conceived to be an elastic vapour, or in other cases a mass of matter in a state of fusion from heat."-f- * Hopkins, Researches in Physical Geology ; Transactions of the Cambridge Philo- sophical Society, vol. vi., part i. t "The first effect of our elevatory force," continues Mr. Hopkins, " will, of course, be to raise the mass under which it acts, and to place it in a state of extension, and, consequently, of tension. The increase of intensity in the elevatory force might be so rapid as to give it the character of an impulsive force, in which case it would be impossible to calculate the dislocating effects of it." He, therefore, always assumes " this intensity, and that of the consequent tensions to increase continuously, till the tension becomes sufficient to rupture the mass, thus producing fissures and disloca- tions," the nature and position of which are his first objects of investigation. " These will," he proceeds, " depend partly on the elevatory force, and partly on the resistance opposed to its action by the cohesive power of the mass. Our hypotheses respecting the constitution of the elevated mass are by no means restricted to that of perfect homogeneity ; on the contrary, it will be seen that its cohesive power may vary in general, according to any continuous law, and, moreover, that this power, iu descend- ing along any vertical line, may vary according to any discontinuous law, so that the truth of our general results will be independent, for example, of any want of cohesion between contiguous horizontal beds of a stratified portion of the mass. Vertical, or nearly vertical, planes, however, along which the cohesion is much less than in the mass immediately on either side of them, may produce considerable modifications in the phenomena resulting from the action of an elevatory force. The existence of joints, for instance, or planes of cleavage in the elevated mass, supposing the regularly- jointed or slaty structure to prevail in it previous to its elevation, might affect in a most important degree the character of these phenomena." CH. XXXIV.] PRODUCTION AND DIRECTION OF FISSURES. 651 After investigating the action of the elevatory force supposed upon a thin lamina, and the direction of the fissures according to various conditions, parallel upon the single application of that force, Mr. Hopkins in applying his researches* to a mass of three dimensions, deduces, among other important conclusions, that, " if the mass be subjected to two systems of parallel tensions of which the directions are perpendicular to each other, two systems of parallel fissures may be produced, of which the directions will be perpendicular to each other." *' No two systems of parallel fis- sures," he infers, " could be thus formed, of which the directions should not be perpendicular to each other'." "If the fissures in either of these systems be near to each other, they could not have been formed by such tensions as we have been considering, in succession. They must have been formed simultaneously in each system. One system, however, might be formed at any time sub- sequently to the other/' The modifications produced by different conditions are pointed out, and Mr. Hopkins remarks upon the sense in which the term parallelism, in these investigations, should be regarded, He observes that, ( ' if the size of the mass be com- paratively small, and its boundary irregular, this property would altogether cease to characterise the phenomena."! Eeflecting upon the modes of accumulation, as well of igneous as of aqueous deposits, and upon their variable admixture in different localities and at different times, the observer will be led to infer that homogeneity of structure in considerable masses of the mineral matter distributed over the earth's surface would not very frequently be found. Bearing this in mind, as also that in the active volcanic districts of the world there is evidence of the varied intensity of igneous action somewhat irregularly distributed beneath a certain * As it is out of place in a work of this kind to enter sufficiently into the investi- gations of Mr. Hopkins, further than to show their general bearing, we would refer for the mode of investigation, and the manner in which the varied results are pro- gressively developed, to the Memoirs themselves, as given in the Cambridge Philo sophical Transactions, where the observer will find the subject fully treated. 1 Mr. Hopkins remarks, that " if we suppose the superficies of our elevated mass to be of finite length, and to be bounded, for instance, by a line approximating to the form of an elongated ellipse, the direction of the fissures in the transverse system, as we approach towards either extremity of the elevated range, will gradually change from perpendicularity with the major axis (the axis of elevation) till they become parallel to it at the extremities of the ellipse, always preserving their approximate coincidence with the directions of the lines of greatest inclination of the general surface of the mass. The fissures of the other system will be approximately perpen- dicular to these lines. In this case, then, the two systems will be no longer charac- terised by any constant relations which their directions bear to that of the axis of elevation, and, therefore, the terms longitudinal and transverse will cease to designate them so correctly as in other cases." 652 EVIDENCE OF THE DATES OF FISSURES. [Cn. XXXIV. amount of the earth's crust, interferences with fractures of the regular kind above mentioned will probably suggest themselves. Nevertheless, it is highly desirable that he should endeavour to classify the fractures found so commonly in various parts of the world with reference to views on the large scale, so that he may look beyond the details of some given locality, and endeavour to arrive at general conclusions as to the cause of any faults and dis- turbances of deposits in it by following out their directions, differ- ences of date, and such other circumstances as the conditions under which they are presented to his attention may permit. The directions of fractures, if even merely those without that movement of either of their sides which should cause them to be faults, having been carefully noted, the relative geological dates of their production may not always be so easy to ascertain. It is found that, in certain districts, we may have several of different geological dates, and yet the whole be uncovered by any deposits of which the relative time of acccumulation may be ascertained, so that the probable date of the whole or some of these faults and fissures may remain uncertain. Unfortunately, this uncertainty too often prevails. At the same time, careful observation will some- times enable the geologist to obtain somewhat fair evidence of the relative dates of these fractures, and from such evidence probable inferences as to those of others may be occasionally drawn. For example, there is evidence of north and south fractures having traversed the old red sandstone, mountain limestone, and coal measures of Somersetshire, anterior to the accumulation of the new red sandstone series of that district, and posterior to the bending and contortion of the former rocks, the faults traversing these contortions even at right angles, and the older rocks having been worn down after the fractures, the lowest beds of the new red sand- stone series of that country reposing tranquilly upon the faulted and abraded older rocks. We may refer, in further illustration of this circumstance, to the geological map of the Mendip Hills, pre- viously given (fig. 167, p. 478), where faults, r, r, r, r, somewhat parallel to each other, and having a north and south direction, cut through old red sandstone (1), carboniferous limestone (2), and coal measures (3), so that, from an irregular curve of these beds having been traversed, scarcely any horizontal movement in the present denuded exposure of this part of the Mendip Hills is seen on the north, while there appears a considerable shift on the south. These faults are observed, as far as the surface is concerned, to stop at the lias (6) and new red sandstone (5) on the north, and the only one CH. XXXIV.] RELATIVE DATES OF DIEFERENT FISSURES. 653 traced completely across to terminate at the inferior oolite (7) on the south. This apparent and superficial termination of the faults, arises from their having been formed anterior to the deposits of the inferior oolite, lias, and new red sandstone. The chief fault is well known to traverse the coal mines beneath a continuation of these rocks, on its range northward, and is ascertained to be covered over horizontally by them all N. W. from Eadstock. Thus, in this case, the date of these faults would be after the disturbance, and the flexure of the coal measures in that district, and anterior to the accumulation of the new red sandstone series (including its dolo- mitic conglomerate) in the same district. Hence other faults in the vicinity having the same range might be inferred to have been contemporaneously produced with them, the more especially as at Wick Eocks, five miles from Bath, there is also evidence of faults traversing the coal measures, these having been subsequently and quietly covered by beds of the new red sandstone series. That all the faults traversing any denuded or uncovered portion of the older rocks of the same district, were of the same relative date, is shown not to be probable by finding some traversing the higher deposits themselves, both on the north and south of the Mendip Hills, the chief of these taking an east and west direction, so that, fortunately, in this limited district, an observer may learn the value of caution, as to the relative dates of faults.* As to the exposure of faults, and inferences as to the dislocation of one series by others, much caution is also often needed. For example, it does not follow, as in the subjoined plan (fig. 268), Fig. 268. that the fissure a b, is posterior to another, c d, and has shifted it at e, because the one line is continuous and the other not, since * As regards these subsequent faults, which have commonly an east and west direction, they are seen to have traversed deposits up to the chalk inclusive. A very considerable fault of the latter kind (see Sheets 18 and 19 of the Geological Survey of Great Britain) brings chalk into contact with the bed known as the Kimmeridge Clay, one of the oolitic series, at Mere, Wilts. Thus, in this district, there is evidence of an east and west disturbance between the deposit of the coal measures and that of the new red sandstone series, and of another posterior to the deposit and consolidation of the chalk. 654 RELATIVE DATES OF DIFFERENT FISSURES. [Cn. XXXIV. such, fractures, under fitting conditions, may have been contem- poraneous portions of some far larger dislocation, of which these are only minor parts, with adjustments due to minor conditions. Such apparent shifting of one fissure by another is of the same kind as those small complicated fractures close to, or forming parts of, the fissures or faults themselves, and of which the following (fig. 269) is an example, from St. Agnes, Cornwall ; small contemporaneous fractures in slate having been filled by peroxide of tin, and so that an apparent heave or shift took place at h h. When such appearances present themselves, it is needful to ascertain that any mineral matter, filling a fissure c d (fig. 268), has been dislocated and traversed by the fissure a b. Fig. 269. Evidence of the kind of dislocation mentioned is often to be found, so that no doubt remains of one fissure or set of fissures having been first formed, and also altogether or partially filled, prior to the production of another or others. Mining districts often present abundant opportunities for investigations of this kind. AS an example, we may notice a well-known district near Eedruth, Cornwall, where, as represented beneath (fig. 270), granite, g, slates, s, elvan dykes, e, e, e, and lodes or mineral Fig. 270. veins, ?, ?, I, are all cut through and dislocated by a fault a b, one of the great cross courses, as they are termed, of that country, CH. XXXIV.] COMPLICATED APPEARANCE FROM A FAULT. 655 having northerly and southerly ranges. This plan is also useful in showing the range of the fissures, e, e, filled with the granitic matter (elvan) introduced after the production of the granitic masses, g (p. 565), and the coincidence in range of parts of the fissures, I, I, of the country, containing copper and tin ores, and subsequently formed, since they traverse these elvans in the ver- tical section downwards. With respect to sections in any planes, the horizontal, for ex- ample, in countries complicated by the occurrence of different rocks, variably situated as respects each other, or by fissures rang- ing differently and filled more or less with mineral substances of various kinds, even by mineral matter which has been raised in them in a molten state, some care is needed, so that an observer may properly appreciate the relative position of the parts of the general solid rock broken, shifted, and, as it were, rubbed down to some given plane. Let, for illustration, the following section (fig. 271) represent one of such a district as that of Cornwall, a b, Fig. 271. I being the surface of the country, e 0, elvan dykes, and I, I, I, lodes or mineral veins. Let this country be now dislocated in a plane perpendicular to the section, so that a b f on the one side be lifted vertically above a b on the other. It will be seen that, on the level a 5, though the amount of vertical elevation has been com- mon to all the lodes and elvans, these now occupy, on the surface a b, very different distances from each other, according to the portions of their various dips or underlies intersected on that sur- face after the movement mentioned. This will be still further illustrated by the subjoined plan (fig. 272), supposed to be taken on the level a b, all above it, after the fault was effected, being considered as removed by denudation, as is commonly the case. As the letters and figures correspond on both the section and plan, it will be found that, while the lodes, I 1, I 2, and the elvan e 1, 656 EVIDENCE OF A SUCCESSION OF FISSURES . [Cn. XXXIV. are shifted to the right, on the side of the dislocation marked B, the lodes l^ and I 4 are shifted to the left ; and that, in the latter I'l e'\ Fig. 272. l'31'll'Q e2Z'5 II 12 el 15 part of the section and plan, a lode or branch from a lode I' 0, appears on the side B, which was not at the surface on the side A, so that three lodes appear on the side B as continuations of the two lodes visible at the surface, on the side A. The elvan e 2, which was close to the lode I 4, on the side A, is apparently re- moved far from it on the side B, and moreover contains the lode I 5 in the latter case, one which was far removed from it, on the surface, on the side A.* , The evidence of a succession of fissures is often extremely in- teresting. While some clearly dislocate and shift the whole of a mass of rocks, with any prior-formed fissures included in them, others appear as mere fissures, with their walls slightly if at all moved from their former relative positions as continuous portions of the same mass of rocks. In the annexed plan (fig. 273), one of Fig. 273. * Figures of this kind serve to illustrate the apparently contradictory facts some- times observable on the sides of dislocations, denuded down to a common level, where elvans, or other dykes, and faults, or mineral veins, dip at various angles in opposite directions. In the illustration given in the text, the motion has been supposed vertical. As such movements are frequently otherwise, when it is desired to see how, by the CH. XXXIV.] FISSURES SPLIT AT THEIR ENDS. 657 the mineral veins of the Charlestown, Pembroke, and Crinnis mines, St. Austell district, Cornwall, it will be seen that the granite boundary, g g, as well as the lodes 1 1, are shifted by the fault or cross course a b ; (the same circumstance attending the fault, c d, though not shown on plan ;) while another, and subsequent fissure, e /, traverses the whole without shifting it. Fissures are often found to split at their ends after no very con- siderable course, when regarded in their horizontal range. Of mineral veins so divided at their extremities, when viewed hori- zontally, the following plan (fig. 274) of the Wheal Fortune range Fig. 274. of mines, Breague district, Cornwall, may be taken as a good ex- ample. The main lode is there seen to be split on both the east and west after a range, as a marked fissure, for about a mile and a quarter (the plan is on a scale of one inch to the mile). The lodes, m, are those of Wheal Friendship mine, and, if prolonged, would also fall into the main vein of the Wheal Fortune mines. These various lodes traverse elvan dykes, e, e, or courses, as they are termed in Cornwall, and are cut by faults or cross courses, d, d, subsequently produced. It should be remarked, with reference to beds or other arrangements of rocks of variable toughness, tra- versed by fissures, that occasionally some care is needed not to be misled by minor appearances, for the fissures taking lines of least resistance may so run against or along harder beds, or dykes of mineral matter, as to lead to false impressions. Thus, in the an- nexed section or plan (it is immaterial which it may be considered), a fissure being opened from d towards e, and encountering an elvan use of such sections, explanations of apparently complicated phenomena may be afforded, it becomes necessary not only to have the sections strictly accurate and proportional in all their details, but also to make the movement correspond with that found among the rocks themselves. If an observer will paint on two pieces of flat glass, a variety of sections of this kind, the same on both pieces, so that when held together they appear as one, and slide the glasses on their flat surfaces, a variety of interesting circumstances will be made apparent as to the consequences of fault movements in different directions ; the surfaces of ground being supposed, as in nature, to be denuded down to some common levels. 2u 658 LINES OF LEAST RESISTANCE TO FISSURES. [Cn. XXXIV. dyke a b, might have resistances to the force employed so ad- justed that it only traversed the latter at ! ? ' /' -* / || ; ' ! ' j k * S > ' 3> a / | | / I 1 2 '/! /: / I s ' / So| iif /i , ^ .1 w to 1 ^D fl '. ! ! ; I \\\ t: \ -M ^ .rt ; *"*"*>. 2 *~ ^S w PH \ ~'--. o ^ -^ ' I : ' \ i * M \ N. ff| /// ; / "^J ' ' ; ' ' ' ;' ?^ \ V -"" -..._ \ ^ j 1 1 I ' ; ; ( 02 ^i ^ . e \ \ "fe "o " ' * " !S <; * % : ; g Q "S |1 "1 t-3 'a) \ 4 3 -2 * /' 1 i t* S '' / "2 Q) ft^ ( < ^ ; i 1 \ .!-,& 111 li P ^ v? V;3 | MJ rfS i o / ' M V J/ / 5 1 , \V\ rX / / -s i > -^ \ ^'>^ ^ e '' / ^ O ^ v * s ^V o ,53 JT3 01 x i " 1 / ^ r a / o o i ) / f , ! * in CH. XXXVII*] SOMERSETSHIRE, AND WALES. 715 and another of 5,000 feet, for the denudation of the same rocks on the north of Bristol.* The annexed section (fig. 306) is one of those given in illustration of this subject, and will serve to exhibit the disturbed beds of carboniferous limestone, old red sandstone, and Silurian rocks, shorn down by denudation in the slightly elevated district of Southern Pembrokeshire, as also, with every allowance for numerous fractures and gaps in the higher part carried off, the great mass of matter thus removed. Viewing such considerable denudations, quite as readily seen in many other, and distant regions, as in the minor districts noticed, and combining them with the elevations and depressions of land that have taken place at all geological times, sometimes slowly moving large surfaces above and beneath the level of the sea, at others squeezing and folding various mineral accumulations into great ranges of mountains, the geologist will be at no loss for evidence, not only that the surface of the earth has long continued in an unquiet state, but also that the same amount of mineral matter may have been repeatedly employed in part, or as a whole, in the production of deposits spread over various areas for the time being, these deposits either fossiliferous or without organic exuviae, as the conditions for the preservation of the remains of the animal and vegetable life of different times, may or may not have pre- vailed. As considerations of this kind constitute a part of those which lead to the most extended views, by the aid of which we endeavour to trace back the past conditions of our planet, they, and the class to which they belong, tending, as they do, to keep atten- tion alive to the greater problems, while the detail necessary for their solution is collected, cannot be too frequently present to the mind of the geological observer. * Referring to the reduction of the Horizontal Section of the Geological Survey, Sheet 17, extending from Glastonbury Tor, across the Mendip Hills, by Clifton, Bristol, to the flat land at the Severn, and to the sketch for filling up denudation, pi. 4, fig. 4, in the " Memoirs of the Geological Survey," vol. i. APPENDIX. GEOLOGICAL MAPS AND SECTIONS Though an observer may be supposed usually to have access to the best maps of any country he may examine geologically, and, in general, to find such maps containing the information which is desirable, as well respecting the natural physical features of the country, as the artificial modifications of, or arrangements on, its surface, so that he can always ascertain his exact position, and possess the power of re- cording any circumstances considered sufficiently important in their true re- lative places on such maps ; it, nevertheless, sometimes happens that the maps of a district are inaccurate, or do not contain those things which are needed. A geologist will not long have endeavoured to record his observations upon maps, before he will ascertain that many a beautiful engraving may be worthless, while some coarse, unpromising plan may be most valuable. In case of need, therefore, it becomes important for an observer to be so far skilled in the construction of maps, as not only to be able to correct one which may be imperfect in an efficient manner, but also to make such a sketch of ground as may suffice for his purpose. - A knowledge of the kind of surveying commonly termed military drawing, will be found most advantageous for his progress. He will scarcely accomplish much on this head by the aid of books alone, and therefore should study it in the field. If possessing a good eye for form, he will by no means find the acquisition of this knowledge difficult, while he will soon perceive that it affords him great additional power in satisfactorily recording his observations. Even in many a map where the lines representing rivers, coasts, and other natural features, are exceedingly accurate, as also those showing the roads, canals, villages, and other artificial arrangements, are equally so, it too often happens that the relief of the ground, the true forms of the inequalities of surface of the hills and mountains, is either not given, or so inaccurately that it would have been better if no attempts had been made to represent it.* Now, the true relief of the surface of a district is often of the greatest * The method, too often adopted, of representing the lines of water-shed as those of the highest grounds in many regions, cannot be too much deprecated, leading, as it often does, to the most imperfect views as to the real inequalities of surface in them, and as to the action of those geological causes which have produced such inequalities. 718 GEOLOGICAL MAPS AND SECTIONS. value to the observer. It is only necessary for him to attempt a record of his observations upon maps with and without a correct representation of this relief, fully to appreciate the difference. A power, therefore, accurately to sketch in the forms required becomes of no slight advantage.* Much may be accomplished by the improved prismatic compass (care being taken in districts where rocks containing much protoxide of iron are present, such as many of those of igneous origin in which hornblende and augite prevail, which will divert the magnetic needle), and by some effective arrangement of a spirit level, by which close approximations to slopes may be obtained. For approximations to heights within a certain range, the aneroid barometer will be found useful, especially in regions where the mercurial barometer and sympesometer may not be easily carried, and where atmospheric changes, affecting such instruments, are not considerable. Possessing these simple instruments, and a fair knowledge of military drawing, the observer may make many a sketch of a country, the geological structure of which would otherwise have been imperfectly represented. Supposing the possession of a proper map, either previously made, or con- structed during the progress of his work, it is needful that a geologist should follow up the rocks he may be investigating, quitting them as little as ' opportunities permit, so that all connected with their modes of occurrence may be carefully ascertained, and all the points of importance be as care- fully entered upon his map. Without caution of this kind, very grave errors may readily be committed, inaccurate inferences being drawn respect- ing the mode of occurrence of accumulations, seen only at different and frequently distant points, which more detailed examination would have pre- vented, and this more especially in districts of highly-disturbed and broken rocks. As to the boundaries of the different mineral masses which it may be thought desirable to insert on geological maps, these necessarily depend upon the scale of the map on the one hand, and the view taken of their relative value on the other. Whatever the scale, it is desirable that the great distinctions considered important should be clearly apparent, and that the detail should be so represented as not to impede a correct view of them. It is better to select such portions of a map for enlargement as may be required for the illustration of any particular detail, than to sacrifice broad comprehensive views for the sake of its introduction when only really needed in particular localities. As with the objects to be represented on geological maps, so with the colouring employed upon them. Comprehensive, clear views should not be * With reference to the sketching of ground, the method of representing a hilly or mountainous country by lines, approximating to those of equal altitudes, as is shown by fig. 180, p.* 493, will be found very serviceable for geological investigations, especially those where relative altitudes are important, or where the small inclinations of beds can only be measured by the heights they occupy, and at distances too con- siderable to be ascertained by instruments constructed to measure the dip of rocks (clinometers). The contour lines, or those of equal levels, upon some of the Ordnance Maps in England and Ireland, and so valuable for many purposes, form an accurate representation of this method. GEOLOGICAL MAPS AND SECTIONS. 719 sacrificed to attempts to introduce detail only important locally, and which can be best shown by the enlargement of such portions of a map. The employment of given colours to represent certain divisions of the geological series has been considered very desirable, so that the eye becoming accus- tomed to them may, as it were, currently read off maps thus coloured. This is certainly important, and might be accomplished, to a considerable extent, in the general maps, national maps for. example, of different countries. Much may be, and has been effected as to the information to be afforded in geological maps, by a mixture of signs and colours ; the latter repre- senting some accumulation, or series of accumulations; and the former certain modifications of it considered important. In this manner, for example, igneous rocks may be represented by some given tint or colour ; and the variations in their mineral structure, so far as regards the surface of the land, by various signs. The like with those divisions in the sedi- mentary deposits of different geological dates to which names have been given, various signs also readily show their mineral structure in different parts of their surface exposure. Among the signs employed amid the stratified rocks, it is very needful to have a sufficient number representing their modes of occurrence as to the position of their beds, showing when these are horizontal, inclined at any particular angle, or contorted ; when the latter, the kind of contortion, and the like. The following signs (fig. 307) have been found useful for this purpose. The point of the arrow, Fig. 307. b o a, shows the dip or inclination of the beds as respects the horizon ; and it is desirable to place on one side of this sign the amount of the dip, such as 5, 15, 23, as it may happen to be. The sign b is intended to point out that, while the general inclination, or dip of the beds maybe in the direction corresponding with that of the arrow, they undulate on the minor scale ; c, shows that the strata are vertical, their range, or strike, as it is often termed, being in the direction of the longest line. Beds much plicated on the minor scale, while they have a general range, are shown by d, the straight line pointing out the general range. An anticlinal ridge is repre- sented by the sign /, the two arrow-heads showing the direction of the dip on either side, and the cross line that of the range of this form of beds ; -e is intended to indicate the occurrence of beds so contorted and folded in various planes, that no definite dip or range of them can be inferred in the locality where this sign may be entered upon the map. The cross g, 720 GEOLOGICAL MAPS AND SECTIONS. represents a horizontal arrangement of the beds. By attention to such, or any other system of signs considered effective, the arrangement of the com- ponent beds of stratified rocks is so exhibited as to present the geologist with evidence enabling him to take a comprehensive view of this part of his subject. By combining any system of this kind with another for the dis- tribution of organic remains, in the fossiliferous rocks, he still further advances his general views ; so that with colours, and with signs for mineral structure, distribution of organic remains, and any movements which the beds may have sustained since their deposit, his map not only becomes a record of his observations, but also presents him with a classified collection of facts from which he may deduce important general conclusions that otherwise might not so readily be attained. Geological maps conveying information only as to the surface arrangement of rocks, vertical sections of the country, either directly obtained from natural or artificial exposures of the various accumulations, or inferred from abundant and satisfactory information, collected at various points on the line of section or within safe distances from it, become essential for a right view of the manner in which the various rocks of a district may occur. Too much stress cannot be laid on the importance of rendering all such sections strictly proportional, so that they should, as much as possible, be miniature representations of nature. The distortions and erroneous conclusions which arise from a want of attention to this point are endless. In the first place, the outlines of countries are usually so exaggerated, as to elevations and depressions, that the real forms of the surface are falsified, and that true appreciation of them, which would lead to just views of their relative im- portance, and of the conditions which have produced them, is superseded by most imperfect ideas as to the real relief of a district, even of such as may be mountainous. True surface sections of the latter are often especially needed to afford the geologist a correct view of the different heights and depressions, so that he may insert his observations on that which will not deceive him in his endeavours to trace the amount and direction of any general disturbance, to ascertain the value of any curves or plication of beds, and to restore the various component parts to their inferred original Tposi- tions in such regions. Though with known altitudes at sufficiently numerous points on any given line of proposed section, the various distances from these points being known, much may be accomplished by a practised and steady eye by sketching in the intermediate ground ; and this may be the only means at command in somewhat rapid excursions :* the line given by instrumental work, when time suffices, is the only real method of obtaining the object sought. All lines of section are thus run on the Geological Survey of Great Britain, and the results thence obtained have been so satisfactory * For example, the section from the Jura to, and across the Mont Blanc, above given (fig. 264, p. 646), was obtained, from known heights of different points on the line, by barometric measurements of intermediate altitudes, and a sketching of the ground on the spot. GEOLOGICAL MAPS AND SECTIONS. 721 that few, once experiencing the advantages so derived, would probably be disposed to abandon this method of observation. In certain districts, such as those where that important product coal is obtained, exact sections of surface are as indispensable as the exact relative positions of the beds them- selves with reference to them, so that the true positions of the coal-beds may appear. With his level or his theodolite, an observer feels that con- fidence in his labours which he might not otherwise possess. Having the surface right, he can enter the dips, and other modes of occurrence of the rocks found, in their real relative situations on his section, and thus possess a collective miniature representation of the needful circumstances, such as no other less correct method will insure, be his powers of generalization what they may. As in many lines of section, all the various accumulations cannot be so traversed, as to have all deposits cut at right angles by them, care is required to represent only the relative thickness of such deposits where the section passes ; that is, the lines of separation of beds, or collections of them, as given in the section, should correspond exactly with those which would appear if the rocks supposed to be vertically cut through, were really so ; and the beds on one side of the cut being removed, the face of the other was exposed, as if on a cliff. By turning to fig. 257 (p. 635), it will be found that a line of section parallel to the cliff represented would even give the beds there shown as horizontal, while they really dipped considerably at right angles to it, as seen in the sketch, fig. 258. It is easy in such cases to notice the true amount and direction of the beds on the section, and thus make the real value of the lines on such section clear. By giving more dip than such lines represent, a greater thickness is shown than really exists, and the total amount of mineral matter which the surface of the ground and the line of section should exhibit, is misrepresented. In addition to these vertical and proportional sections, it sometimes becomes necessary to enlarge a part, so far as regards a column rising vertically to the plane of accumulations. In like manner, also, this should be proportional, and on a scale sufficient to render the object sought by the enlargement clear. The scale of such sections adopted by the Geological Survey is that of 40 feet to the inch ; and it has been found one amply sufficient for very considerable detail, as may be seen by reference to the vertical sections of the coal measures, those which can be used for mining purposes (sections, Nos* 1 to 11 and 16 to 18). Vertical sections, deposits represented as piled one above the other horizontally, whatever may be their real inclinations in different localities, may also be usefully employed for comparing distant accumulations with each other, especially as regards their thickness. As, for example, the following section (fig. 308), serves to show the different thicknesses and modifications of the cretaceous and oolitic groups as developed in southern and northern England. In these sections (the same letters being employed to represent equivalent deposits in both), the cretaceous series in Wilts and 3 A 722 GEOLOGICAL MAPS AND SECTIONS. Somerset is divided into chalk, a ; upper green sand, b ; gault, a clay bed so Fig. 308. WILTS AND SOMERSET. YORKSHIRE t Cretaceous Group. Oolitic Group. Cretaceous Group. Oolitic Group. named, c ; and df, lower green sand ; while in the same series in Yorkshire, 6 and d are supposed to be absent. As regards the oolitic group, e represents the Kimmeridge clay ; jT, coral rag and its calcareous grits ; g, Oxford clay and the Kelloway rock in its lower part; h, Cornbrash and Forest Marble ; t, Bradford clay ; k, great oolite ; Z, Fuller's earth : m, inferior oolite ; ft, marlstone ; o, lias. The superficial gravels, &c., above the chalk, are represented by t. As respects the divisions e, f, and g, the two sections do not much vary, while a considerable difference is seen in the beds A, i, k, and /, as found developed in southern and northern England. There has apparently been a modification of the conditions under which these equivalent portions of the oolitic series were deposited in the two localities, so that while, in the south, marine remains point merely to deposits beneath the waters of a sea, shales and sandstones on the north contain the remains of terrestrial fossil plants (p. 516) so occurring that not only the close proximity of land has to be inferred, but also the existence of marshy land itself, supporting a growth of certain plants (Equisetuni) , entombed as they stood. The different character of the lias on the north and on the south will also appear, this deposit being not only thicker on the north, but also there exhibiting a certain depth of upper lias marls, not continued to Wiltshire, though it can be seen gradually fining off southerly into Gloucestershire. In this manner, it will be obvious that much useful evidence may be embodied ; so that, by the combined aid of the maps, and the sections of various kinds, a sound and comprehensive view of the different rock accumulations of a country may be obtained. ADDITIONAL NOTICES. 723 Page 103. Great Salt Lake of North America. The recent researches of Captain Stansbury, of the United States' Topographical Engineers (Expe- dition to the Great Salt Lake of Utah, 1852), have made us acquainted, from actual survey, with many important circumstances connected with the region in which the Great Salt Lake of North America is found. A con- siderable area, on the western watershed of that continent, is separated from the general drainage. In it a chief depression finally receives the waters of the country, forming a lake from which the evaporation is such that no surplus waters escape out of that area, as a whole, into the general water- shed to the westward. A minor depression first receives the drainage of part of the general area. The waters of this lake, named Utah, are fresh, while those of the other, the Great Salt Lake, are highly impregnated with saline matter. The evaporation over the area draining into the Lake. Utah (30 miles long and 10 miles in breadth,) is such, com- pared with the supply of water, that a river, named the Jordan, by the Mormon settlers on its banks, flows from it and falls, after a course of about 50 miles, into the Great Salt Lake. Notwithstanding this supply of fresh water, as also from other streams, the chief of which enters it on the N.E., rounding the Wahsatch Mountains, on the east, the waters of the Great Salt Lake are so entirely saline as to form a strong brine, one that in 100 parts, by weight, of the water, contains more than 22 parts of soluble salts, common salt (chloride of sodium) by itself constituting 20 parts. The exact amount of soluble substances, as determined by Dr. L. D. Gale, is as follows : Solid contents in 100 parts of the water = 22 '422 parts. Specific gravity of the water = 1 17 Chloride of sodium . . 20 '196 Sulphate of soda . . 1'834 Chloride of magnesium . 0'252 Chloride of calcium . . a trace. The Great Salt Lake is situated, according to Captain Stansbury's maps, between about 40 40' and 41 42' north latitude, and between about 112 D to 113 10' west longitude. The western side of the lake is bounded by a level plain country, little elevated above its waters, so little indeed that the pressure of the wind is sufficient considerably to change the coast- line in that direction, according to its duration, the difference " amounting, in many cases, to miles in width" (p. 186). In dry weather the plain, abounding in soft ground, is covered with a coating of salt. Captain Stansbury mentions that the minute crystals of salt " glisten brilliantly in the sunlight, and present the appearance of a large piece of water so per- fectly, that it is difficult, at times, for one to persuade himself that he is not standing on the shore of the lake itself" (p. 119). The Salt Lake, exclusive of offsets, is 291 miles in circumference, the Wahsatch Mountains rising above it on the eastward ; several islands rise above its waters, which 3 A 2 724 ADDITIONAL NOTICES. would seem generally shallow, 36 feet between Antelope and Carrington Islands, being apparently the most considerable depth. The valley of the Salt Lake is estimated at about 4,300 feet above the level of the ocean. Upon the slope of one of the ridges connected with the plain of the Salt Lake, Captain Stansbury states that " thirteen distinct benches or watermarks were counted, which had evidently at one time been washed by the lake, and must have been the result of its action continued for some time at each level." The highest of these benches was about 200 feet above the present level of the lake. He infers that at some former period a vast inland sea extended over this region (p. 105). We would here seem to have an isolated, and raised portion of the sea exposed, like the region of the Caspian, to conditions under which it had to adjust itself to the supply of the fresh water it could receive from rains or rivers. This supply not being equal to the evaporation, the saline contents gradually formed an increasing relative portion of the waters in the chief depression, until they became that at present found in the Great Salt Lake. Whether this adjustment be now permanent, is probably uncertain ; but even assuming that it might be so, if all other things remained the same, there is a disturbing cause of modification in the amount of detritus now thrown by the streams into this shallow lake. All would appear to add something, but the Bear River is described as bringing down an immense quantity of sedimentary matter at every freshet. The exact difference ' of elevation between the Utah and Great Salt Lakes does not appear to have been ascertained ; but assuming that this level is not considerable, and that formerly a sea extended over the whole, there is no difficulty in seeing that Lake Utah, receiving more water than covered its evaporation (the lake is situated amid mountainous ground), would gradually become fresh from the gradual weakening of its saline solutions, while the lower lake became as gradually more saline from the reverse action. Thus a fresh-water and a very saline lake would have resulted from the imprisonment of parts of the same original sea, acted upon, under modified conditions, in a given area, this as a whole not now obtaining a supply of water beyond its general evaporation. Page 368. Volcanic Hocks of Iceland. Professor Bunsen (of Marbourg), after personally studying the volcanic rocks of Iceland, and carefully investi- gating their chemical relation in the laboratory, has presented us with views of great value and importance, not only respecting the palagonite rocks, but also as regards the volcanic products ejected in a molten state. (On the Processes which have taken place during the Formation of the Volcanic Rocks of Iceland ; Poggendorffs ' Annalen,' 1851, No. 6, and * Scientific Memoirs,' new series, vol. i, p. 33, 1852.) After alluding to the homogeneous character of the mixed silicates ejected in a state of igneous fusion, and the separation of the component matter into different mineral substances, according to the physical conditions to w r hich it has been subjected, he ADDITIONAL NOTICES. 725 divides the Iceland volcanic rocks into two groups, which he names normal-trachytic and normal-pyroxenic. He understands by the former the trachytic rocks richest in silica, and by the latter the basaltic and doloritic rocks poorest in silica. These constitute extreme members of a general mass, and graduate into each other. " The first distinguishing characteristic of the normal-trachytic rocks is, that they represent almost exactly a mixture of bisilicates of alumina and alkali, in which lime, magnesia, and protoxide of iron are either wholly wanting or are present only in insignificant quan- tities." ***** "The normal-pyroxenic rocks, which as basic silicates of alumina and protoxide of iron, in combination with lime, magnesia, potash and soda, form the most extreme members towards the opposite end of the series, present a similar correspondence in their average com- position." * * * * " As the proportion cf the oxygen of the silicic acid to that of the bases is here, with slight variations, as 3 : 1 '998, all these rocks may be regarded as a constant mixture of bibasic silicates, when we consider only their entire mass, without reference to the fact of their con- stituents being grouped into minerals of definite composition." " The quantity of silica almost always bears a constant proportion to the lime and magnesia, while the relation between the quantities of alumina and protoxide of iron is subject to considerable variations." Thus there is a mass of matter having a tendency to arrange itself into two distinct sub-masses, each distinguished by a definite composition, the whole governed in its lithological character by the variable manner in which a mixture was effected, and by the physical conditions to which the molten mass may have been exposed. The Professor gives the following as the composition of the normal- trachytic (1) and the normal-pyroxenic rocks (2,) observing that from it the mean relation between the oxygen of the acid and that of the bases can be calculated, being for the trachytic rock as 3 : 0*596, and for the pyroxenic as 3 : 1 998 : Silica Alumi Lime Magnesia . Potash Soda 100-00 100-00 Selecting silica, as the most convenient constituent of these rocks for calculation, because it can be well determined, and is the least variable in them, in order to estimate the mixture of these normal rocks which may exist in any compound of them, Professor Bunsen remarks that, "If we characterise by S the per centage of silica in a mixed rock, by s the per- (1) (2) * . 76 67 . 48-47 ind protoxide of iron . 14-23 . 30-16 . 1*44 . 11-87 . . 0-28 6-89 . 3*20 . 0*65 . . . 4-18 1-96 726 ADDITIONAL NOTICES. centage of silica in the normal-trachytic, and by a the per-centage of silica in the normal-pyroxenic rocks, then s -S S^ = a in which equation a represents the quantity of normal-pyroxenic rock- mass which must be mixed with one part of normal-trachytic mass, in order to give the composition of the mixed rock in question." * * * "All the other constituents of the mixed rocks are then determined by means of a. For if the weight of the separate constituents of one part of the normal-pyroxenic rock-mass is taken as p p^ ... p n , and in the same manner the weight of the separate constituents in the unity of weight of the normal-trachytic rock-mass as t ^ . . . /, the weight of all the other constituents may be ascertained by means of the equation (a+l) (0+1) ' (a + 1) ' The abstract given by the Professor abounds in valuable research, and should be consulted by the observer anxious to enter upon this very interesting class of inquiries. The memoir is divided into 1, Genetic relations existing between non-metamorphic rocks (from which the notice above given has been taken) ; and 2, Genetic relations of the metamor- phic rocks, the latter being subdivided into 1, Palagonitic rocks; 2, Zeolitic formations; and 3, Formation of rocks by pneumatolytic meta- morphism. In the first subdivision of the second division, the palagonitic tuff is pointed out as a mixture of hydrated and anhydrous silicates, the latter belonging exclusively to the pyroxenic rocks, the former, generally cementing the fragmentary rock, being regarded as a mixture or combi- nation of two silicates, one represented by the formula 3 R O 2 Si O 3 -f- Aq, and the other by 3A1 2 O 3 , SiO 8 -f-Aq. These silicates the Professor considers as appearing to combine in definite proportions, for which he proposes the formula 3 R O 9 2 Si O 3 + 2 Al 2 O 3 Si O 3 4. Aq. He adds, that the palagonitic substance occurs generally as characteristic of the pyroxenic volcanic rocks, mentioning besides Iceland, the basaltic accumulations of Germany and France, the Euganean Islands, Etna, the Azores, Canaries, Cape de Verd Islands, and the Galapagos Islands. Under the head of Zeolitic Formations, their intimate connexion with the palagonitic and pyroxenic classes is pointed out, and under that of the .Formation of Rocks by Pneumatolytic Metamorphism, the products resulting from the action of volcanic gases and vapours upon all the rocks which had been treated of. INDEX. AAR GLACIER, view and description of, 216. Aberystwyth, condition of copper coins found near, 691. Abich, Dr., on the composition of vol- canic tuffs near Naples, 367; section of Etna, 386 ; of Vesuvius, 388. Achafalaya river, raft in the, 117. Action, elevatory, recent, at the Santorin foup, 392. istment of ancient marine life to pths of water and kinds of sea bot- tom, 542. ^Egean Sea, zones of depth in, 146 ; con- ditions of its bed, if converted into land, 149. Africa, coast, conditions of, 138. Agassiz, M., on glaciers, 209, 269. Age of rocks, relative, insufficient of itself for mineral character, 595. Air in sea-water, 144. Albite, composition of, 359. Alps, detritus of, 29 ; glaciers of, 209 ; erratic blocks of, 264 ; flexure of beds in, 644; part of, including Mont Blanc, proportional section of, 646. Alteration of rocks, from descent be- neath earth's surface, 602. Altered rocks, sulphuret of iron in cer- tain, 590 ; formation of crystals in, 608 ; production of certain minerals in, 611 ; mineral matter transmitted into, 612. Amazon River, sediment of the, 63. America, form of its coasts, 131, 133; volcanos of, 399. America, South, glaciers of, 241. Ammonites, multitudes of in lias, Marston Magna, Somersetshire, 538. Ancient volcanic products, organic re- mains in, 537. Andalusite, production of, in altered rocks, 610. Anglesea, igneous dykes of, 565. Animal and vegetable^life, marine, effects of sinking of sea-bottom upon, 543. Antarctic ice barrier, 234; geological effects of, 236. Anticlinal and synclinal lines, 643. Appalachian zone, United States, bend- ing and plication of rocks in, 643. Arago, M., temperature of the atmo- sphere, 207 ; on temperature found at the Artesian well, Grenelle, 465. Arctic glaciers reaching the sea, 226; effects of, 227. Arctic shells in British deposits, 281. Arctic ocean, coasts of, 139; drainage into, 140 ; glaciers and icebergs in, 225. Area, extension of required, in reducing beds, in mountain chains, to horizon- tality, 641. Areas, large, of undisturbed rocks, raised in mass, 640. Argillaceous limestone, concretionary nodules and layers of, 595. Argillo-calcareous nodules, lamination of some, 596. Ari Atoll, 177. Arrangement, diagonal, of the minor parts of detrital rocks, 532. Artesian wells, temperature of waters found in, 18, 465, 466 ; substances con- tained in, at London, 603. Artificial minerals, methods of producing certain, 578. Ascension, Island of, laminated volcanic rocks in, 365. Ashes, volcanic, composition of, 366. Ash-beds, ancient volcanic, deceptive character of, after consolidation, 555. Asia, great central depression of, 104; form of its coasts, 134 ; rivers of, 140. Asiatic volcanos, 400. Atlantic and Pacific Oceans, effects of gradually joining, on marine animal life, 541. Atlantic, ancient exposure of coasts to, 711. Atmospheric influences, effects of, on upper part of mineral veins, 690. Augite, composition of. 359. Austen, Mr, R. C., on Kent's Hole Cave, 302 ; on distribution of detritus in English Channel, 459 ; on the occur- rence of phosphate of lime, 598. Auvergne, extinct volcanos of, 401. Axmouth, destruction of cliffs at, 22. BAB B AGE, Mr., on elevation and depres- sion of coasts, of Bay of Naples, 438 ; on movements of land from increase and decrease of heat, 439, 444. Baku, mud volcanos or salses of, 410. Baltic, deposits in the, 73 ; analysis of the water of, 73 ; effects of ice in, 74, 247. Banwell cave, osseous breccia in, 309. Barrier reef of Australia, 181, 193. Bars at river mouths, 77. Basalt, mineral composition of, 402 ; re- lative fusibility of, ib. ; chemical com- position of, 403 ; relative antiquity of, ib. ; globular structure of, 404 ; colum- nar structure of, 405 ; jointed columns of, 406. Bath, springs of, 18 ; origin of thermal waters of, 466. Beaches, changes of relative level of, on tidal coasts depressed or elevated, 452 ; ancient, among fossiliferous rocks, importance of, 474 ; of the time of the Silurian rocks, 475; of old red sand- stone period, Scotland, ib. ; of Chair INDEX. of Kildare, Ireland, 476 ; of new red sandstone period, ib. Beaufort, Admiral Sir Francis, on in- flammable gas of the Yanar, 410. Beaumont, M. Elie de, on declivities of glaciers, 266 ; on the direction of the fissures of Etna, 380 ; on Etna, 383 ; on the origin of the Val del Bove. 387 ; dis- tribution of mineral substances, 591 ; his views respecting directions of mountain chains, 637; on correlation of directions of mountain chains, 638 ; on metalliferous and volcanic emana- tions, 672 ; on initial volatilization of metallic substances in veins, 674. Becquerel, M., on substances produced by slow secondary electrical action, 674; on partial conversion of steel plate into silver at the Mint, Paris, 694, Beds, formed around volcanic islands, character of, 389 ; formed by unequal drift, 534. Beds of rocks, different consolidation of, in the same group, 602. Beech ey. Captain, on coral islands, 172. Belcher, Sir E., on the movement of a current at 40 fathoms, 528. BelemniteS) multitudes of, in lias, Golden Cap, near Lyme Regis, 538. Bending, contortion and fracture of rocks, 620. Bending and plication of rocks, artificial illustration of, 632. Bermudas, coral reefs at, 199. Berthier, on the analysis of caiamine, 693. Binney, Mr., on Stigmaria, 501. Birds, preservation of their remains, 121 ; foot-prints of, on surfaces of rock, Con- necticut, 524. Bischoff, M. Gustav, experiments illus- trative of deposit of mineral matter in fissures, 669. Black Sja, deposits in the, 73. Bogs^ how formed, 113;. extent and thickness of, 114. Bombs, volcanic, 330. Boracic acid, of Tuscany, 413 Boring molluscs, carboniferous limestone pierced by, at time of inferior oolite, 485 ; lias conglomerate drilled by, 486 ; inferior oolite pierced by during accu- mulation, ib. Boutigny, M., experiments on incandes- cent bodies, 340. Bourbon, Isle of, coral reefs near, 179. Bow Island, account of, 173. Breakers, force of, 47, 49 ; action of, on volcanic products, St. Paul's Island, Indian Ocean, 337; force of, in Scot- land, on side of German Ocean, and of Atlantic, 47. Breaker action, 48; great denudation from, 707 ; on volcanic islands, 390. Breccias, calcareous, 14 ; osseous, 309. Brewer's iiill r county Wicklow, compli- cation of bedding, cleavage and joint- ing near, 631. Bridgend, Glamorganshire, quartz, rock, in trias near, 604. Brighton, raised beach near, 459. Bristol Channel, deposits in, 84, 88, 129 ; beaches of time of new red sandstone near, 476, 480 ; mode of accumulation of dolomitic limestone near, 497 ; foot- prints of animals on muddy shores of, 525 ; faults in, 662. Bristol, amount of denudation near, 715, Britain, climate formerly colder, 282 ; upper tertiary mammalia in, 293. British Islands, map of the 100 fathom line round, 91 ; map of, when depress- ed 1000 feet, 261 ; effects of submer- gence on, 260, 263 ; older igneous pro- ducts of, 553 ; effects of squeezing and elevation of, into great range of moun- tains, 639. British Seas, distribution of marine life in, 152 ; mammoth remains found in, 234. Brongniart, M. Alex., on raised coast of TJddevalla, 441. Brown, Mr. Richard, on vertical plants in coal measures, Cape Breton, 505. Buckland, Dr., on glaciers in Scotland, 270; on the fossil elephant, 290; on Kirkdale cave, 297 ; on osseous breccia, 309; on fossil trees and ancient soils of Isle of Portland, 518 ; on mammal remains, oolitic series, Stonesfield, 549. Buckland, Dr., and Mr. W. D. Cony- beare, on submarine forest, Bridge- water levels, 448. Buddie, Mr., on erosion of coal beds, Forest of Dean, 512. Bunsen, Professor, on the composition of palagonite tuff of Iceland, 368 ; on ac- tion of water and acids on palagonite tuff, 370 ; on the mode of action of the Geysers, Iceland, 372 ; on gypsum de- posits of Iceland, 375 ; on volcanic sublimations of muriate of ammonia, 376. Bunsen Prof., on the volcanic rocks of Iceland, 724; their chemical composi- tion, 725. Bwlch-hela, near Penrhyn Quarries, N. Wales, cleavage through contorted sandstones at. 619. CAIMAN of West Indies, 120. Caiamine, probable origin of, 692 ; com- position of, 693. Calcareous deposits, 106; from volcanic action, 1 10 ; not always horizontal, 110. Caldera, the, Island of Palma, Canaries, 383. Cambrian rocks, conglomerates of, Ban- gor, North Wales, 475 ; alterations of, by heat, 609. Carbonate of lime, deposition of, 12, 13, 106 ; of copper with vegetable remains, 695. Carbonic acid, action of, on certain sili- cates, 606. Carburetted hydrogen, exhalations of, 409. Cardigan Bay, map of, 82 ; tides in, 83. Carglaze Tin Mine, Cornwall, stanni- ferous veins amid joints in granite of, 686, INDEX. 729 Carne, Mr., on character of rocks in Cornwall, affecting contents of mineral veins, 680 ; on modification of mineral Veins, 689 ; on the different dates of mineral veins, 687. Carnon tin Streamworks, human skulls found in, 449. Caspian Sea, nature of its waters, 73, 103, 105 ; deposits in, 102. Caves and minor cavities, metalliferous, of Derbyshire, 682. Cavities, circular, produced during earth- quakes, 426 ; amid rocks, action and re- action of substances in, 674. Cawsand, Plymouth Sound, porphyry of, 569. Central Asia, volcanos of, 400 ; waters of, 492. Central France, extinct volcanos of, 401. Cetaceans, remains of, 130. Chair of Kildare, hills of, range of cleavage diagonally through beds of, 618. Chalk, composition of water of r beneath London, 603; altered by basalt, Isle of Raghlin, 608. Channels, eroded in coal measures, Forest of Dean, 512 ; of erosion in coal mea- sure detrital deposits, Pembrokeshire, ib. Character of surfaces of rocks, 527. Charlestown and Crinnis Mines, Corn- wall, range of mineral veins at, 657. Chemical deposits in inland seas, 102. Chesil Bank, Dorsetshire, 56. Chiastolite, formation of, in altered rocks, 611. Chili, elevation of coast of, during earth- quakes, 432; extent of great earth- quake at, 417. Chloride of sodium in spring water, 14; dissemination of, amid rocks, 603. Chlorite in granite, 592. Clarke, Rev. W. B., on Lafu island, 196. Cleavage, influence of, on the decompo- sition of rocks, 9. Cleavage, 614; in mixed beds of sand- stone and argillaceous matter,. 615 ; in limestone and shale, 616 ; modification of, in passing through thin beds of limestone amid shale, 617; minor in- terruption of passing junction of beds, ib.', on the large scale, 618 ; through contorted beds y 619 ; ranging diagon- ally through bedding, ib.; double, 620 ; relative dates of, 620; distortion of organic remains by, 621 ; different di- rections of, in same or juxtaposed districts, 623 ; gathering of mineral matter in planes of, 624. Cleaved and jointed rocks, subsequent movement of, 630. Cliffs, effects of the sea on, 48. Clonea Castle, Waterford, characters of cleavage at, 616. Clyde, newer pliocene deposits of the, 280. Coal beds, extent of, 511 ; partial re- moval of, during coal measure deposit, 512 ; effects of squeezing upon, Pem- brokeshire, 648. Coal measures, evidence afforded by, 500 ; stigmaria beds of, 501 ; vertical stems of plants in, 502 ; mode of filling up hollow vertical stems of, 504; growth of terrestrial plants in succes- sive planes in, 505 ; thickness of, 507 ; false bedding in sandstones of, 508 ; surfaces of sandstones of, 509 ; drifts of matted plants in, 510 ; extent of coal beds in, 511; partial removal of coal beds of, during general deposit, ib. ; lapse of time during accumulation of, 513 ; pebbles of coal in, 514 ; marine remains in parts of, 515 ; mode of deposit of, 516 ; flexures and plications of, in South Wales, 647. Coal, pebbles of, in coal-measure accu- mulations, 514. Coasts, action of sea on, 46, 49, 51; influence on organic life and preserva- tion of remains, 133, 159 ; distribution of animals on, 157, 159 ; effects of ice on, 245. Coasts, rivers and lakes, effects on, during continued elevation of land above sea, 489. Coast sand-hills, formation of, 59. Cold, effects of its general increase, 251, 264. Colenso, Mr., on beds composing Pen- tuan tin stream works, Cornwall, 450. Colours and signs, advantage of mixture of, in geological maps, 719. Compact felspar, character and com- position of, 572. Complication of surface, produced by smoothing down single dislocation, under certain conditions, 655. Component parts of rocks, consolidation and adjustment of, 594. Component parts of beds, flexures and plications of, 648. Composition of the volcanic tuffs near Naples, 367. Conglomerates and volcanic tuffs, mixed beds of, with lava, in Pacific islands, 389. Conglomerates, joints in, 628. Cooling globe, effects of, on rocks on sur- face, 636, Corals, in British seas, 153 ; general dis- tribution of, 165 ; migrations of when young, 167 ; chemical composition of, 168; conditions of growth, 192. Coral reefs, extent of, 165, 169 ; stratifi- cation of, 195; formation of, 173, 186; conditions under which they occur, 186 ; influence of volcanos on, 190, 204 ; elevation of above the sea, 195. Cordier, M., on mode of obtaining tem- perature of the earth, 464. Cornwall, action of the sea on coasts of, 55; sand-hills on coasts of, 61 ; jointn among granite in, 626; fragmentary lodes in, 696. Cornwall and Devon, granites of, 563 ; influence of dissimilar rocks on mineral veins in, 679; metalliferous districts of, 677. 730 INDEX. Correa de Serra, Mr., ou submarine forests, Lincolnshire, 448. Cotopaxi, structure of its cone, 331, 332 : descent of water from, 350. Couthouy, Mr., on coral reefs, 195 ; ice- bergs, 231. Covering, slight, above granites, in Wicklow, Wexford and Cornwall, 580. Cracked surfaces of deposits, 522. Crag deposits, 314. Crantock Church, Cornwall, built of con- solidated shell sand, 62. Craters of elevation, 318 ; eruption, 319. Crater lagoons, volcanic islands, 394. Cretaceous rocks, overlap of, in England, 521. Crich hill, Derbyshire, lead ore of, 682. Crust of the earth, proportion of 100 miles deep of, to volume of world, 634. Crystalline modification of rocks, 608. Currents in the Mediterranean, 71 ; the ocean, 93 ; influence of, in distributing sediment, 98. Cwm Lrlech, Glamorganshire, vertical stems of plants in coal measures of, 503. Cwm-ddu, Llangammarch, concretionary arrangement of beds of Silurian series near, 599. Cwm Idwal, Caenarvonshire, distortion of organic remains, by cleavage at, 622. Cyanite, formation of, in altered rocks, 611. DANA, Mr. J., on corals, 168 ; on vol- canos of Hawaii, 332, 337 ; on volcanic fissures of the Hawaiian Islands, 380 ; on volcanic islands in the Pacific, 389. Dardanelles, effect of closing the Straits of Gibraltar upon, 491. Darwin, Mr. C., on coral islands, 169; on elevation of coral reefs, 201 ; on glaciers in Wales, 270; on elevation of erratic blocks, 272 ; on the lamina- tion of volcanic rocks, 364 : on vol- canic tuff of Chatham Island, 368. Daubeny, Dr., on globules and lamina- tion of Lipari obsidian, 365 ; on the gas evolved from the Solfatara, Puz- zuoli, 372 ; on nitrogen of volcanos, 376 ; on Santorin group, 391, 392 ; on mud volcanos of Maculaba, 412 ; on boracic acid, 414. Dean, Forest of, removal of coal beds at time of coal-measure deposit in, 512. Decomposition of rocks, 1 ; importance of studying, 11. Decomposition of vegetable matter, 113. Deer, foot-prints of, around trees of sunk forests, South Wales, 450. Deltas in pools of w ater, 24 ; in lakes, 42-45; in tidelees seas, 64; in tidal seas, 79, 89 ; preservation of organic remains in, 117, 126, 128. Delta lands, effects of gradual subsidence of, on vegetation, 516. Denudation, effects of, on surface, after dislocation of various rocks and mine- ral veins, 655 ; or partial removal of rocks, 705 ; island masses of rock left by, 710; contorted rocks worn down by, 711 ; exposure of old rock-surfaces by, 712; amount of matter removed by, 713 ; in South Wales and adjacent English counties, amount of, 713. Densities, relative mean, of surface and mass of earth, 673. Deposits, siliceous, from the Geysers, Iceland, 374. Deposits in river courses, 31 ; in lakes, 42 ; chemical in seas, 102 ; in the Caspian, 103 ; cracked surfaces of, 522. Derbyshire, igneous rocks associated with carboniferous limestone of, 558 ; mineral veins amid limestones and igneous rocks of; 682 ; various modes of occurrence of mineral veins in 683 ; debris on hill sides of, 215. Detrital deposits, accumulation of, 472 ; variable consolidation of, 604. Detrital and fossiliferous rocks, mode of accumulation of, 471 ; chiefly old sea- bottoms, 472; diagonal arrangement of minor parts of, 532 ; consolidation of, 604. Detritus, of the Alps, 29 ; deposition of, 30, 31 ; transport of, by rivers, 24, 29; by tides, 77, 89 ; by currents, 98 ; by icebergs, 230, 236 ; by river-ice, 242 ; with remains of molluscs, 279 ; drift of, from shallow to deep sea-bottoms, 532. De Verneuil, M., on mud volcanos of Taman and Eastern Crimea, 412. Devon and Cornwall, ancient igneous products in, 557. Devonian rocks, contemporaneous ig- neous products in, 557. Devonshire, action of the sea on its coasts, 55. Diagonal arrangement of the minor parts of beds among detrital rocks, 532. Diallage, composition of, 582, 589 ; of La Spezia, 589 ; of Harlzburg, 589. Diallage rock of Cornwall, 580. Diatomaceee, distribution of, 238. Different rocks, influence of, on mineral veins, 680. Dip of beds, fallacious appearances re- specting, 635. Dismal Swamp, 115. Distributon of animal and vegetable life at different geological times, 539. Distribution of land and sea, 253. Dolerite, composition of, 352. Dolomitic limestone, mode of deposit of, near Bristol, 497. Dome-shaped igneous matter, raising and splitting of, 383. D'Orbigny, on distribution of mollusca, 134. Dorsetshire, action of the sea on coasts of, 56. Dranse, temporary lake and floods of, 41. Drawing, military, advantage of, 718. Drifted organic remains, 536. Drifts of matted plants in coal measures, 510. Dry land, in great part bottoms of ancient seas and lakes, 472 ; present, variable effects of submergence of, 498. Dubois de Montpereux, M., on mud vol- canos of Taman, 411. INDEX. 731 Dufrenoy, M.,on composition of volcanic ashes, 366 ; on the composition of volcanic tuffs, 367 ; on fossiliferous volcanic tuff of Monte Somma, 388 ; on structure of volcanic tuff, near Naples ; ib ; on origin of Monte Somma, 388. Dukhun, great area of basalt in,- 404. Duncan, Dr., on foot-prints of animals on surfaces of rocks, Corn Cockle Muir, Dumfriesshire, 523. Dunraven Castle, South Wales, mode of occurrence of lias at, 483. Dykes of lava, Val del Bove, Etna, 378. Dykes amid conglomerates of ancient igneous rocks, 556. Dykes, igneous, unce 564. uncertain date of many, EARTHQUAKES, 415; connexion of, with volcanos, 416 ; areas disturbed by, 417 ; transmission of vibrations of, 418; earth-waves of, 419 ; sea-waves of, 420 ; unequal transmission of, 422 ; local interruptions of, ib ; locally extended range of, 423 ; effects of, on lakes and rivers, 428 ; sounds accompanying, 430 ; fissures produced during, 425; settle- ment of unconsolidated beds during, 426 ; circular cavities produced during, ib; traversing mountain ranges, 424; great sea-wave produced by, 427 ; flame and vapours during, 429 ; elevation and depression of land during, 431 ; action of, on sea-bottoms, 531. Earth, motion of, as affecting currents, Earth's surface unstable state of, 443. Earth, temperature of, 463; radius of, 633 ; mean density of, 673. Earth-wave, of earthquakes, 420. Ebelmen, M., method of producing arti- ficial minerals by, 578. Ecton Mine, Staffordshire, notice of, 689. Effects of earthquakes on sea-bottoms, 531. Effects of gradual subsidence of delta lands on vegetation, 516. - sea-bottom being raised round British islands, 520. Egerton, Sir P., on the ossiferous caves of the Hartz, 303. Ehrenberg, Prof., on coral reefs, 185 ; on infusorial remains in rocks, 547. Elephant, fossil, notice of, 284, 290, 294, 312. Elevation and depression of bottom in the ocean, 526. of land, present, gradual in Norway and Sweden, 440. Elevations of mountain chains, 637. Elvans, of Cornwall and Devon, mode of decomposition of, 4 ; range of, 565 ; composition of, 566 ; dates of, 569 ; of Wicklow and Wexford, 569 ; character of mineral veins traversing, in Corn- wall, 678. Elvan dyke, fallacious appearance of, traversing mineral veins, 658. Emanations, metalliferous and volcanic, 672. England, former connexion of with the Continent, 294, 297. English Channel, tides in the, 80 ; analy- sis of water of, 109 ; distribution of detritus in, 459. Erie, Lake, draining of, 39. Erratic blocks, origin of, 256; trans- portal of by glaciers, 269 ; of the Alps, 272; of northern Europe, 275; of America, 276, 277. formation, d' Archiac cited, 275, 276. Erroob Island, coral reefs with lava, 205. Eschscholtz Bay, elephant remains at, 290. Estuary deposits, 126, 129 ; foot-prints of birds in, 129, 525 ; cetacean remains found in, 130. Etna, eruptions from, 343,350; direction of fissures at, 381; section of, 385; form and structure of, 386. Europe, form of its coasts, 138 ; effects of submergence on, 264, 267 ; changes of land and sea in, 287 ; mammoth re- mains in, 296. Exeter, igneous rocks near, 568. Extent of coal beds, 511. Extinct volcanos, 401. FALSE bedding in coal-measure sand- stones, 508. Faraday, Dr., on the liquidity of gases under pressure, 381. Faults, temperature of waters rising through, 465 ; well seen usually in mining districts, 649 ; of different dates, 653 ; of Somersetshire, in coal measures, and inferior rocks, smoothed off before deposit of new red sandstone, 652 ; caution respecting the shifting of one by another, 653 ; considerable, shifting rocks and mineral veins, near Redruth, Cornwall, 654 ; fallacious appearances arising from, 655 ; different traversing, 657 ; range of, in Cornwall and Devon, 659 ; inlaying mass of coal measures, Nolton and Newgale, Pembrokeshire, ib ; in Somerset and Dorsetshire, 660 ; near Swansea, 661 ; inclination of, 662; parts of deposits preserved by, 663; complicated, 663 ; friction surfaces in, 664. Fawnog, Flintshire, remarkable ' flat ' of lead ore at, 683. Felspars, chemical compositions of vari- ous table of, 355. Felspar crystals, amid altered stratified rocks, 608. , decomposition of, 24. Fingal's Cave, basalt of, 407. Fish, ejected from volcanos, 349 ; fossil, occurring as if suddenly destroyed, 537. Fissures, in volcanos, filled by molten lava, 378 ; earthquake, flame and va- pours from, 429 ; through rocks, pro- duction and directions of, 650 ; relative dates of, 653 ; evidence of succession of, 656 ; in rocks, split at their ends, 657 ; effects, amid mixed rocks, of lines, of least resistance to, 658; filling of with mineral matter, 665 ; filling of 732 INDEX. minor, 667 ; deposits from solutions in, 668 ; opened beneath seas, 671 ; cha- racter of substances filling, 673 ; action and reaction of substances in, 674 ; coating of walls of, by mineral matter, 696 ; coated by dissimilar substances, 697 ; several successive movements in the same, 698 ; sliding of sides of, on mineral matter accumulated at inter- vals, 700; fractures through contents of, 702. Fitton, Dr., on earthy (ancient soil) bed, Vale of Wardour and Boulonnais, 518 ; on fossil shells in the position in which their animals lived, 535. Fitzroy, Capt. R.N., on effects of earth- quake on coasts of Chili, 433. Flames from volcanos, 323. Floods, geological effects of, 26. Fluviatile deposits, mammalian remains in, 295. Foot-prints of air-breathing animals on mud and sand, 128 ; on the surfaces of rocks, 523. Forbes, Prof. E., on the distribution of marine animals in the .ZEgean Sea, 146 ; on zones of depth in, ib. ; in British seas, 152; on the origin of the British flora, 282 ; on Santorin group, 393 ; on movements of coast, Bay of Maori, 442 ; on shells in raised beaches of the Clyde, 459 ; on fossils of Longmynd district, 475 ; on conditions of Portland and Purbeck deposits, 518. "Forbes, Prof. James, on glaciers, 210,213 ;' measurements of the motion of glaciers, 218 ; on glaciers in Skye, 270. Forchhammer, Prof., on the salts in sea- water, 109 ; on the effects of ice in the Baltic, 247 ; on solubility of part of . matter of feJsparSj 670. Forest marbra^diagonal arrangement of organic ri^hs of, 536. Fossil trees and ancient soils, Island of Portland, 517. Fossils, distortion of by cleavage,. 622. Fournet, M., on character of rocks af- fecting mineral veins, 680 ; on gneiss pebbles in mineral veins, 696 ; x>n the deposit of different mineral substances in veins, 698. Fox, Mr. Robert "Were, on electro-mag- netic properties of mineral veins, 675 ; experiments illustrative of cleavage, 622. Fractures, considerable, of beds, amid plicated rocks of mountain chains, 642. Fresh-water deposits, evidence of land from, 488. Friction marks on rock surfaces, 527. Frome, Somersetshire, mode of occur- rence of inferior oolite, near, 486 ; forest-marble of, 536. Fundy, Bay of, foot-prints of animals on muddy shores of, 129, 525. Fusibility of rocks, to be viewed with reference to complete mixture of their component parts, 589. GALE, Dr. L. D., on the solid contents of Great Salt Lake, 723. Gambier's islands, 176. Ganges, bore wave in the, 79 ; Delta of the, 85 ; body of water discharged by, 86 ; detrital matter of, 86. Garnets, in altered sandstone, 612 ; pro- duction of, in altered rocks, 611 ; com- position of, ib. Gases, certain, liquid under different pressures, 381. Geneva, Lake of, soundings in, 42 ; de* posits in, 44; temperature of, 96. Geological maps and sections, 717. survey of Great Britain, maps and sections of, alluded to. 507, 649, 653, 658, 661, 662. Geysers of Iceland, 15 ; situation of, 372 ; mode of action, ib ; mineral contents, 373; siliceous deposits of, 15, 373, 374. Giant's Causeway, jointed columnar structure of basalt at, 407. Glaciers, in the Alps, origin of, 209 ; structure of, 210 ; motion of, 213, 219 ; transport of boulders by, 219 ; rocks grooved by, 220 ; advance and retreat of, 213, 223; supposed extension of, 265; table of their declivities, 266; in Himalaya, 224 ; in the Arctic re- gions, 225 ; in the Antarctic regions, 231; in South Georgia, 239; in the British Islands, 270; straits of Ma- gellan, 240. Glamorganshire, thickness of coal mea- sures of, 507 ; lias of, 482 ; carbonate lime of, ib. Glydyr Vawr, false and irregular beds in, 533. Gneiss, production of certain kinds of, 613. Godolphin Bridge, Cornwall, lode of, 699* Graham Island, formation of, 70. Granite, mode of decomposition, 2, 6 ; relative date of; in Wicklow and Wex- ford, anterior to old red sandstone, 562 ; in Cornwall and Devon, posterior to the coal measures, 563 ; mode of occurrence of, in south-west England and south-east Ireland, 573 ; veins of, 575; schorlaceous, of Cornwall and Devon, 578 ; porphyritic, 579 ; mine- rals, additional to those in ordinary, 680 ; general resemblance of, in dif- ferent regions, 586 ; chemical differ- ence of, from hornblendic rocks, 587 ; prevalence of silica and alumina in, 587 ; of comparatively recent date in Catalonia, 588 ; columnar appearance of, from joints, 626 ; masses of, exposed by denudation amid disturbed rocks, 646. Granitic rocks, chemical composition of, 6, 577 ; alterations of rocks near, 609. Graves, Captain, R.N., survey of San- torin group, 393. Great circles of comparison, for direc- tions of mountain chains, 633. Great Crinnis Lode, Cornwall, change of character of, in range of, 688. Great Salt Lake of North America, 723 ; extent of, ib. ; solid contents of, ib. Greenland, glaciers in, 226, 228 ; gradual depression of land at, 441. INDEX. Greenstone, composition and character of, 585, 587. Crenelle, near Paris, temperature found at Artesian well of, 465. Ground, gradual submergence of, during deposit of coal measures, 506. Ground-ice, formation of, 242. Gulf stream, 94, 132 ; saline matter in, 107. Gypsum, deposits of, Iceland, 375 ; occur- rence of, in the trias, 601. Gwennap, Cornwall, range of elvans, lodes and cross courses in, 566. HABITS, probable, of animals, regarded with reference to distribution of or- ganic remains, 546. Hausman, M., on change of sulphuret into carbonate of lead, in mineral veins, 691. Hawaii, volcanos of, 332. Heat, alteration of rocks on minor scale by, 607. Hecla, eruptions of, 343. Henry, Mr., on deposits of silica, from silicate of soda, 606. Hen wood, Mr., on mines of Cornwall and Devon, 687. Hermann, M., analysis of black schorl, 612. Hillsborough, Ilfracombe, North Devon, cleavage near, 616. Himalaya, snow-line of, 207 ; glaciers of, 224. Hitchcock, Prof., on foot-prints of birds in red sandstone series, Connecticut, 523. H^hnbaum, Dr., on foot-prints of animals on surfaces of rocks, 523. Holy head Mountain, Anglesea, cleavage through quartz rock at, 609. Homogeneity, effects of want of, among rock accumulations, upon production of fissures, 651. Hooker, Dr., on height of snow line, north and south sides of Himalaya, 207 ; on Diatomaceae, 238. Hopkins, Mr. William, on production and" direction of fissures, 650. Horizontal deposits, upon contorted rocks, caution respecting, 635. Hornblende, chemical composition of, 4, 582. Hornblendig rocks, chief chemical dif- ferences of, from granite, 586 ; slate, . produced by alteration of hornblendic ash beds, 610. Horner, Mr., on submarine forest of Bridgewater levels, 448. Horse, in mining, description of the term, 704. Hot springs, 15. Humboldt, Alex, von, on the snow line, 208 ; on mud volcanos, 410 ; on local interruptions of earthquakes, 423; on earthquakes traversing mountain chains, 424 ; on sounds accompanying earthquakes, 430. Hunt, Mr. Robert, experiments illustra- tive of cleavage, 622. Hyena, bones of, in caves, 298. Hypersthene rock, Cocks Tor, Dartmoor, 610. ICEBERGS, range towards the equator, 231, 235, 237-; formation of, 229; geo- logical effects of, 230, 236, 247. Ice, influence of, in transporting mineral matter, 206, 241, 243, 248, 260; effects on sea-coasts, 245 ; of glaciers, struc- ture of, 210. Iceland, submarine eruptions near, 100 ; eruptions of its volcanos, 343 ; com- position of palagonite tuff of, 368 ; geysers of, 15, 373. Iceland, volcanic rocks of, 724 ; chemical composition of the trachytic rocks of, 725. Igneous matter, flow of from submarine vents, 381. Igneous products, more ancient than modern volcanic, 551 ; simple sub- stances composing, 552 ; fossils amid older, in British islands, 554. Igneous rocks, decomposition of, 4 ; ancient range of, in counties Water- ford, Wexford, and Wicklow, decom- position of, 553 ; of Derbyshire, mode of occurrence of, 558; structure of, 560 ; range of, from Scilly Islands towards Tiverton and Exeter, 568 ; chemical composition of ancient, 570; general resemblance of, in various parts of the world, 585 ; altered struc- ture of, 610 ; matter added to, by melt- ing of parts of other iocks, 591 ; gene- ral remarks respecting, 592 ; readjust- ment of parts of altered, 610 ; modifi- cations of, from percolations of solu- tions, 590. Icthyosaurus, preservation of skeletons . of, 539. Ilfracombe, North of Devon,' cleavage through contorted beds near, 617. Imbaburu, fish ejected from, 349. Indian Ocean, currents in, 95 ; coral reefs in, 168 ; form of its coasts, 137. Inferior oolite, boring molluscs of time of, 485; overlap of, Mendip hills, ibid. Infusorial animals, remains of, in rocks, 547. Insects, recent, drifted from land by winds, 548. Insects, remains of in rocks, 122. Inversion of coal measures, mountain limestone and old red sandstone, Langum Ferry, Pembrokeshire, 647. Ireland, distribution of detritus on south of, 460 ; granite of south-eastern, alter- ations of rocks near, 60J ; extent of bogs in, 115. Iron oxides, influence on colour of rocks, 10. Iron pyrites in slate, 590 ; crystals of, in clays and shales, 600. Island masses, left by denudation, 710. Island of Cape Breton, successive growths of terrestrial plants at, in coal-measures, 505. Island of Jura, Hebrides, raised beaches of, 462. Islands, volcanic, of Pacific, 389. 734 INDEX. Isle of Wight, present effects of breaker action on coast of, 712; matter re- moved by denudation in, 713. Isomorphous substances, 354. Isthmus of Panama, effects produced by depression of, 495, 541. JAMAICA, great earthquake at, 426. James, Capt., R.E., on mode of occur- rence of old red sandstone, Ross, Here- fordshire, 533. Java, volcanos in, 348, 349. Joints, 624; approximation of to cleav- age, 625 ; among granitic rocks, 626 ; amid sedimentary rocks, 627 ; among coarse conglomerates, 628 ; in com- pact limestones, 629 ; in lias shales, ib. ; metalliferous deposits in, 685. Jorullo, sudden production of, 346. Jukes, Mr. Beete, on Great Barrier reef, 181, 185, Jupiter Serapis, temple of, Puzzuoli, rise and depression of 436. Junctions of granite and schistose rocks, Cornwall, character of mineral veins traversing, 677. KAIMENI, New, Santorin group, elevation of, 391. Kaup, Prof., on footprints of animals on surfaces of rocks, 523. Keeling atoll, account of, 168. Kent's Hole, Devon, 302, 307. Kettle and Pans, Scilly Island, 6. Kilauea, description of its crater, 333, 338 ; lava flow of, 339 ; analysis of vol- canic glass of, 357. Killarney, Lake of, decomposition of limestone at margin of, 8. Killingworth colliery, Newcastle, ver- tical stems at, 502. Kirkdale bone cave, 297. LABRADORITE, composition of, 359. Lacustrine deposits, 42, 43, 44. Lafu Island, 196. Lagoon Islands, 177 Lakes, formation and removal of by rivers, 39, 40; deposits in, 4245; temperature of, 96; organic remains in, 120 ; of North America, extent of, 140. and rivers, effects of earthquakes on, 428 ; on the outskirts of mountains, 493. great, of North America, effects of submergence of, 499. Land, effects of depression and rise of, .254; ancient, of Silurian period, 475; effects of rise of, over a wide area, 492 ; effects of unequal elevation, ib. ; elevation and depression of, during earthquakes, 431 ; elevation and de- pression of masses of, from variations in their heat, 438 ; quiet rise and sub- sidence of, 435; effects produced by elevation of, 489491 ; varied effects of submergence of, beneath sea, 495 ; depression of beneath sea, effects on distribution of marine life, 543. Landes, sand-hills in the, 61. Land-slips, causes of, 21, 22. Lapilli, volcanic, among igneous pro- ducts, amid Silurian rocks, 555. Lapse of time during deposit of coal- measures, 513. Lateral pressure, evidence of, in chains of mountains, 641. Lava, molten, action of juxtaposed, on subjacent rocks, 607. Lava streams, 326, 339 ; forms of, 328 ; effects of on trees, 340 ; composition of 356 ; lamination of, 364 ; currents, slow cooling of, 365 ; dyke of, crater of Ve- suvius, 379 ; ejected through fissures, 380. Lavas, comparison of those of Monte Somma and Vesuvius, 388. and tuffs, softening and raising of, 382. Lavernock Point, Glamorganshire, com- plicated fault near, 663. Lead, sulphuret of, converted into car- bonate, in mineral veins, 692. Leaders in mining, description of the term, 704. Leucite, chemical composition of, 359. Lias, beaches at the time of, 480 ; resting on disturbed carboniferous limestone, 481 ; of South Wales, 482 ; varied mode of occurrence of, 484 ; land of time of, 487 ; laminated nodules of, 597. Life, effects on distribution of, from elevation and depression of land, 542. animal and vegetable, conditions for distribution of, at all times, 539 ; modifications of, from altered positions of land and sea, 540. Light, influence of, on marine life,144, 148. Lime, bicarbonate of, in solution, 12 ; in seas, 107 ; how deposited, 109. Limestone districts, temperature of waters in, 469. fossiliferous, fragments of ejected, from Vesuvius, 363. Limestone and shale, irregular alternat- ing deposits of, 595. Limestones, how decomposed, 7 ; joints in, 629. Lime and magnesia added to lava by melting of limestone and dolomite, 363. Lipari Islands, eruptions in, 348. obsidian, globules in, and lamina- tion of, 365. Lisbon, great earthquake of, 417. Little Sole Bank, off southern British shores, rugged character of bottom near, 460. Littoral sea-bottom, raised near New Quay, Cornwall, 456 ; at Porth-dinlleyn Caernarvonshire, 458. Logan, Mr., on vertical stems of coal- measures, 501. Lb'ven, Prof., on the molluscs of Nor- way, 149. Lycll, Sir Charles, on glaciers in Forfar- shire, 270 ; on the habits of fossil ele- phant, 284 ; on origin of the Val del Bove, Etna, 387 ; on great Lisbon earth- quake, 417 ; on earthquakes of the Mississippi valley, 429 ; on earthquake in the Runn of Cutch, 434 ; on rise and INDEX. 735 depression of coasts of Puzzuoli, 437 ; on gradual rise of land in Norway and Sweden, 440 ; on vertical fossil forests in coal-measures, Bay of Fundy, 505 ; in foot-prints of birds, shores of Bay of Fundy, 525. Lyme Kegis, landslips at, 22 ; fracture in rocks near, 321 ; joints in shales of lias at, 629 ; faults near, 660. MACKENZIE, Sir G., on the Geysers of Iceland, 15. M~.culaba, mud volcanos of, 412. r\: ~ellan Straits, glaciers of, 240; climate o' F , 240. Maldiva Islands^ 177, 202. Mallet, Mr., on earthquakes, 418, 420. Malvern Hills, 262. Mammals, British, found in caves, 30; entombment of, 119 ; remains of, in British seas, 294 ; in fluviatile deposits, 295 ; extinct, of central France, 306 ; in sunk forests, western England, 449 ; remains of, in oolitic rocks, 549. Mammoth remains, 284, 289, 293, 294. Mantell, Dr., on raised beach near Brighton, 459 ; on Wealden deposits, 519. Maps and sketches, construction of, 718. Marcet, Dr., on density of sea-water, 98. Marine life, distribution of, 142. Marine remains in parts of the coal- measures, 515. Marmora, M. de la, on elevation of coast in Sardinia, 442. Maui, Hawaiian Islands, great volcanic fissure at, 380. Mauna Kea, volcano, 332. Mauna Loa, volcano, 332, 335. Mauritius, coral reefs of, 177. Mastodon, remains of, 293, 315. Mediterranean Sea, volcanic accumula- tions in, 69 ; deposits in, 71 ; currents in, 71 ; distribution of animals in, 146 ; movements of coasts in, 442 ; effects of closing the Straits of Gibraltar on, 491. Mendip hills, beaches of time of new red sandstone at, 476 ; geological map of, 478; lias of, 484; inferior oolite of 484; overlap of inferior oolite at, 485; faults in, 652, 661 ; character of ancient coasts of, 707 ; denudation of rocks in vicinity of, 709 ; amount of denudation at, 713. Merope rocks, Cornwall, 52. Metals, certain, in mineral veins, occur- rence of sulphur and arsenic with, 773; analogous properties of certain ores of, 672. Methone, ancient volcano at, 346. Mexico, Gulf of, deposits in, 75 ; coast of, 132 ; currents in, 94. volcanos of, 400. Miallet, ossiferous cave of, 303. Mica, introduction of matter of, into altered rocks, 613. Mica slate, production of certain kinds of, 613. Millstone grit, granitic character of, if metamorphosed, 613. Mine waters, character of, 675. Mineral matter gathered together in planes of -cleavage, 624; filling of fissures and other cavities of rocks by, 665 ; solubility and deposit of, in fissures, 668 ; replacement of one kind by another, in veins, 694. springs and veins, similar sub- stances in, 672. substances certain, more abundant at crossing of veins, 704 ; infiltration of, into cracks of ironstone nodules, 666. veins, or lodes, character of amid dissimilar rocks, 676, 680; through junctions of granite and schistose rocks, Cornwall, 677; character of, traversing elvans, Cornwall, 678; of Derbyshire, 681 ; directions of, in Cornwall, 687 ; different dates of, ib. ; modifications of, in depth and range, 688 ; character of, on " backs," or upper parts of, 691 ; effects of atmo- spheric influences 011 upper parts of, 691 ; modification of contents of, 692; pseudomorphous crystals in, 694 ; arrangement of mineral matter in, 695 ; fragmentary condition of contents of many, 696. veins and common faults, range of, in Cornwall and Devon, 659. Minerals, different fusibility of, in vol- canic rocks, 360; sinking of unfused,in molten rock, according to specific gra- vity, 361. Mines, temperature of rocks in, 464. Mimisan, destroyed by sand-hills, 61. Mississippi, floods in, 27, 75; delta of, 76; rafts of wood in, 117; extension of earthquakes up the valley of, 424. Mode of occurrence of organic remains, 534. Mode of illustrating movements' from faults, 656. Modifications in the distribution of life from changes in the relative positions of land and sea, 540. Molluscs, distribntion of, 134; entomb- ment of, in detritus while living, 539 ; sudden destruction of multitudes of, 538 ; remains of, range of certain genera through different deposits, 548 ; marine, littoral species of, covering up by depression of coasts, 544. Mollusc shells, in rocks, replacement of, by various mineral substances, 666. Mont Blanc, view of the glaciers of, 212; proportional section from the Jura over, 646. Monte Nuovo, sudden formation of, 347. Somma, Vesuvius, origin of, 387. Moraines, glacier, formation of, 215, 218. Morris, Mr. J., on mammalian remains at Brentford, 312. Mountain ranges, modification in direc- tion of, 639 ; obliteration of, 640. Mountains, production of lakes on out- skirts of, 493 ; ranges of, relative pro- portion of, to volume and radius of earth, 634 ; production of at different geological times, 635 ; direction of, 638 ; evidence of lateral pressure in, 642. 736 INDEX. Movements, several successive in the same fissures, 698. Mud volcanos, 408; of Baku, 410; of Taman, 411 ; of Maculaba, 412. Muriate of ammonia of volcanos, 376. Murchison, Sir R., on the effects of ice in northern rivers, 244, 260 ; on the low- ering of lakes, 254 ; on the elevation of Britain, 280; on the fossil elephant of Siberia, 285, 288, 296 ; on mud volcanos of Taman and Kertch, 413 ; on gradual rise of land in Sweden and Norway, 440 ; on Silurian rocks, 473 ; on Caspian region, 492 ; on vertical stems of plants, oolitic series, Yorkshire, 517 ; on date of rocks containing nummulites, 588; on great area of undisturbed rocks in Russia, 640. NAPHTHA, springs of, 413. Naples, effect on the coast near, 436. Nelson, Capt., on Bermudas, 199. Newfoundland, Bank of, 197 ; map, 198. New red sandstone, beaches of time of, in England and Wales, 476 ; distribu- tion of land and sea at time of, in western Europe, W7 j, of Devon, igne- ous rocks amid lower, 567 ; occurrence of gypsum in, 601. Niagara, Falls of, 39. Nice, osseous breccia at, 311. Nicol, Mr. J., on the composition of fel- spars, 359. Nile, sediment and delta of the, 64 ; body of water from, ib. ; map of its delta, 65. Nilsson Prof, on the coast of Scania, 440. Nitrogen, in connexion with volcanic products, 376 ; evolved from mud vol- canos, Taman, 412. Nodules of impure carbonate of iron or lime, cracking of centres of, 597 ; filling of cracks with mineral matter, 666 ; of phosphate of lime, 598. North America, lakes of, effects of sub- mergence of, 498. Great Salt Lake of, 723. North Devon, denudation of contorted rocks in, 710. North Wales, cleavage of rocks in, 618. Nolton and Newgale, Pembrokeshire, inlaying of mass of coal measures by faults at, 658. Norway, distribution of molluscs on coasts of, 150. Nullipora, nature of, 170. Nunney, Somersetshire, boring molluscs in carboniferous limestone, near, at time of inferior oolite, 485. OBSIDIAN, chemical composition of, 358 ; laminae of spherules in, 365; merely vitreous state of rock, 365. Ocean, influence of its temperature on life, 141 ; influence of depth of, on life, 142 ; floor of, effects of elevation and depression of, 526. Old red sandstone, beaches of time of, in Scotland and Ireland, 475; mode of occurrence of, Ross, Herefordshire. 533 ; cleavage of, 620. Olivine, composition of, 358, 584. Oolitic rocks, probable formation of, 106. series, ancient cliffs of, south-west England, 707. Ordinary springs, temperature of, 468. Organic remains, mode of preservation of, 112; on dry land, 118 ; in the ocean, 156; on coasts; 157, 159; in volcanic tuff, Santorin group, 394 ; mixture of beds with and without, 473 ; em- bedding of in tideless seas, 126 ; in marine deposits, 131 ; variable mode of occurrence of, amid fossiliferous < . . . rocks, 474 ; mixture of, of different periods, 496 ; mode of occurrence of, 534 ; in the positions ~where their animals lived and died, 535 ; drift of, ' by currents, 536 ; diagonal arrangement of, ib. ; among ancient volcanic tuffs, 537 ; viewed with reference to land and sea at all times, 539 ; effects of rise and fall of land, on distribution of, 544 ; particular kinds of, reference to conditions respecting, 546 ; forming beds of rocks, 547 ; sometimes seen only by weathering of rocks, ib. ; che- mical composition of, 548 ; caution re- specting supposed characteristic of deposits, 549 ; distortion of, by cleav- age action, 622 ; alteration of, by mineral matter, 697. Orinoco, delta of, 132. Orleigh Court, Bideford, widely sepa- rated patch of green sand at, 712. Osseous breccias, how formed, 309 ; in fissures, 310 ; at Nice, 311. Ossiferous caves, 119, 297, 309 ; general state of, 298, 304 ; remains found in, 300 ; human remains in, 301, 303, 304 ; dens of extinct carnivora, 302 ; pebbles found in, 306. Overlap of eretaceous beds in England, 521. Owen, Professor, on the fossil elephant, 285, 289 ; on the upper tertiary . mammals of Great Britain, 293. Oxen, foot-prints of, among trees of sunk forests, South Wales, 450. Oxidation of crust of earth, effects of, 636. PACIFIC OCEAN, currents in, 95 ; coast of, 134 ; coral islands in, 172. Papandayang volcano, falling in of, 348. Partial removal of coal-beds during the deposit of the coal measures, 511. Paviland cave, human remains in, 303,314. Pebbles of coal in coal-measure accumu- lations, 514. Palagonite tuff, composition of, 369 ; ac- tion of pure water on, 370 ; of sul- phuretted hydrogen, hydrochloric, and sulphuric acid on, 371. Pebbles, occurrence of, in mineral veins, \ 696. Pele's Hair, 338, 357. Pembre, Carmarthenshire, foot-prints of deer and oxen in sunk foreskof, 451. Pcntuan, character of elvan of, 567. Pentuan tin-stream work, Cornwall, beds composing, 449. Pentland Frith, tides in, 81. re 39816 i*; -,%- v ; >- A %*; ift :. .