TEXT-BOOK OF PETROLOGY TEXT-BOOK OF PETROLOGY CONTAINING SUMMARY OF THE MODERN THEORIES OF PETRO- GENESIS, A DESCRIPTION OF THE ROCK-FORMING MINERALS, AND A SYNOPSIS OF THE CHIEF TYPES OF THE IGNEOUS ROCKS AND THEIR DISTRIBUTION AS ILLUS- TRATED BY THE BRITISH ISLES BY <%~s\JLsr^ F. H. HATCH, PH.D. * 1 y// ED in OK REVISED This book is now published by Messrs. George Allen & Company, Ltd. RUSKIN HOUSE, 44 & 45, RATH BONE PLACE, OXFORD STREET, LONDON, W. APRIL, 1'Jll. Q E4- FIRST EDITION, January 1891 ; SECOND EDITION, October 1892 ; THIRD EDITION, January 1903; FOURTH EDITION, September 1905; FIFTH EDITION, January 1909 j SIXTH EDITION, /w 1910. PREFACE TO THE SIXTH EDITION. IN this edition, besides the correction of a few typographical errors and of some misspellings of place-names, there are several small im- provements suggested by friends. Especially to be noted is a simplification, but without alteration of principle, of the tables of classi- fication of the Plutonic and of the Volcanic Rocks (on pages 153 and 248). A new map, showing the volcanic area of Glencoe and the Etive granite complex, has also been added (on page 371) by kind permission of Messrs. Clough, Maufe and Bailey, and of the Council of the Geological Society. Mr. Pringle has revised the Index. 241722 PREFACE TO THE FIFTH EDITION. ALTHOUGH reprinted in 1903 and again in 1905, this book has not been revised since 1892. During the sixteen years that have elapsed since that date, Petrology has advanced so rapidly that to bring the book up-to-date has meant practically re-writing it. In doing this, my en- deavour has been to retain as far as possible the general arrangement of the Edition of 1892, which I believe has been found useful both by students and by those engaged in teaching. Among new features of the present Edition are fuller references to literature, more complete lists of chemical analyses, and the separation of the part treating of the classification of rock types from that dealing with their dis- tribution as illustrated by the British Isles. Many new cuts have also been added. VI PREFACE TO THE FIFTH EDITION vii The task of bringing the book up to the requirements of modern Petrology has been considerably lightened by the very great assist- ance I have received from many friends. My thanks are particularly due to Mr. R. H. Rastall, Fellow of Christ's College, Cambridge, for much valuable help. It was at his instigation I under- took the revision ; and he has been kind enough to read the proofs and to supply many new micro-photographs (mainly of sections in the Students' Series of the Sedgwick Museum in Cambridge). I have also to thank Mr. George Barrow for valuable information regarding the distribution of the Granites of the Central Highlands, Dr. Flett for reading through some of the parts dealing with the work of the Geological Survey, Messrs. J. Romanes and E. S. Gandy for a careful verification of the physical constants in Chapter III of Part II, and Mr. A. Pringle for reading the proofs and supplying a very full index, which has added very con- siderably to the value of the book. viii PREFACE TO THE FIFTH EDITION For the use of blocks I am indebted to Pro- fessor W. C. Brogger,Dr. J. E. Marr, Messrs. A. Harker and G. J. Williams, and the Director of the Geological Survey; also to the Council of the Geological Society and the Controller of H.M. Stationery Office for permission to reproduce cuts that have appeared in the Quarterly Journal and in the publications of the Geological Survey, respectively. At the end of the book is appended a "Table for the determination of the chief Minerals of Igneous Rocks in thin section," by Messrs. R. H. Rastall and J. Romanes, which will be very useful to beginners in Petrology. F. H. HATCH. Jan. 1st, 1909. PREFACE TO THE SECOND EDITION, I HAVE taken advantage of the opportunity offered by the exhaustion of the first edition of my " Introduction to Petrology " to thoroughly revise the book, while largely increasing its scope. I have been encouraged to do this both by the favourable notices the book has received from its reviewers and the kind hints proffered me by numerous friends. For valuable suggestions and assistance I have specially to thank the following gentlemen : Professors T. G. Bonney and J. W. Judd and Messrs. G-. Barrow, A. Harker, J. Hort Player, J. J. H. Teall, and W. W. Watts. Mr. Pringle has again been kind enough to revise the proofs. I have also to acknowledge my indebtedness to Mr. Hinman's Eclectic Physical G-eography (American Book Co., New York) for the three illustrations forming figures 2, 5 and 6 of my book, 1 F. H. HATCH. July 1 5th, 1892. PREFACE TO THE FIRST EDITION, A LITTLE book that should briefly describe the mineral constituents and internal structures of the Igneous Kocks, their mode of occurrence at the surface and their origin beneath the crust of the earth, has long 1 Being figures 4, 5 and 9 of the fifth edition. ix x PREFACE TO THE FIRST EDITION been a desideratum among English text-books of Science. With the view of filling this gap this treatise has been prepared ; and it is hoped that it will be found useful, not only as an introduction to the subject, but also as a handy work of reference. Von Lasaulx's JEinleitung in die Petrographie served me as a model in the first instance ; but in working out my scheme I have thought it desirable to deviate considerably from the arrangement adopted in that excellent little book. In the verification of data Rosenbusch's Mikroskopische Physiographie (2 vols.), Fouque and Levy's Mindralogie Micrographique, Levy and Lacroix's Les Mine'raux des Roches and Teall's British Petrography have been of great assistance to me ; and the illustrations are, in many cases, taken from these works. I have also to thank the Council of the Geological Society and the Editor of the Geo- logical Magazine for permission to reproduce illustra- tions that have appeared in their journals. The work has profited much by a careful revision of the proof sheets kindly undertaken by Mr. A. Pringle, M.A., B.Sc., of the Museum of Practical Geology, London. F. H. HATCH. Nov., 1890. 28 JERMYN STREET, LONDON, S.W. CONTENTS PART I THE PHYSICAL CHARACTERS OF THE IGNEOUS ROCKS Chapter INTRODUCTION - I. MODE OF OCCURRENCE Stocks, Bosses, Domes, Bathyliths - - - 9 Dykes ... 11 Veins, Apophyses, Tongues - - - - 13 Necks, Laccoliths * 4 Bysmaliths, Sills , 17 Lava-flows - 18 Tufts - 21 II. EXTERNAL STRUCTURE - - - - -22 III. TEXTURE OR MICRO-STRUCTURE - - - - 30 Juxtaposition (Allotriomorphic and Jdiomorphic Granular Textures) - - - - 46 Interposition (Graphic and Pcecilitic Textures) - 46 Porphyritic, Orbicular, and Banded Structures - 48 IV. COMPOSITION - 01 Chemical Composition ----- 51 Mineralogical Composition 55 Changes in Composition Subsequent to Consolida- tion (Pneumatolysis, Contact-Metamorphism, Dynamic Metamorphism, Weathering, etc. ) - 67 xi xii CONTENTS PART II THE CONSTITUENT MINERALS OF THE IGNEOUS ROCKS Chapter Page I. CLASSIFICATION OF THE ROCK-FORMING MINERALS - 67 II. APPEARANCE UNDER THE MICROSCOPE 73 External Contour or Form - . 73 Internal Micro-Structure ... - 75 Double Refraction - . " , . . 79 III. DESCRIPTION OF THE ROCK-FORMING MINERALS - 81 The Silica Group - 81 The Felspar Group ' - - - - - 85 The Felspathoid Group ' . - . - - 98 The Amphibole Group - . - - 104 The Pyroxene Group --.-.. HO The Olivine Group - - - - - 118 The Mica Group 120 The Scapolite Group 124 The Garnet Group --... 125 The Tourmaline Group - 125 The Epidote Group - . . 128 The Phosphate and Fluoride Group - - - 129 The Iron-Ore Group - ]31 The Spinel Group 133 The Zircon Group 134 The Titanic Acid Group - - - 135 The Carbonate Group 138 The Zeolite Group ] 40 The Chlorite Group - - - - . - 141 Other Hydrated Silicates 142 Contact Minerals 145 CONTENTS xiii PAKT III THE CLASSIFICATION OF THE IGNEOUS ROCKS Chapter a 8 e I. THE PLUTONIC ROCKS ...... l&l The Granite Family ...... 160 The Syenite Family ..---- 172 The Monzonite Family 183 The Diorite Family ..... 186 The Alkali-Gabbro Family ..... 188 The Gabbro Family ...... 201 II. THE HYPABYSSAL ROCKS --- 208 The Quartz-Porphyry Family - - - - 211 The Porphyry Family - - - 218 The Porphyrite Family C^ ^- - 219 The Dolerite Family - * . . ? . 221 The Mugearite-Teschenite Family 225 The Aplite-Pegmatite Family - - 227 The Bostonite Family- - - 229 The Tinguaite-Solvsbergite Family - - - 230 The Lamprophyre Family ..... 232 The Monchiquite-Alnoite Family - - 236 The Perknite Family - - - - - - 238 The Picrite Family - - - -240 The Peridotite Family ..... 242 The Serpentines ....... 245 xiv CONTENTS Chapter Page III. THE VOLCANIC ROOKS - - - 248 The Rhyolite Family - 249 The Trachyte Family - 253 The Trachy-Andesite Family 258 The Andesite Family - - 261 The Basalt Family 266 The Alkali-Basalt Family 272 The Limburgite-Augitite Family - - 275 PART IV THE DISTRIBUTION OF THE IGNEOUS ROCKS AS ILLUSTRATED BY THE BRITISH ISLES I. THE DISTRIBUTION OF THE PLUTONIC ROCKS - 279 II. THE DISTRIBUTION OF THE HYPABYSSAL ROCKS 312 III. THE DISTRIBUTION OF THE VOLCANIC ROCKS - - 356 APPENDIX : Table for the Determination of the Chief Minerals of Igneous Rocks in thin Section, by R. H. Rastall and J. Romanes ... - 384 INDEX - - - - - - - - - 390 GENERAL WORKS OF REFERENCE. W. C. Brogger. DieEruptivgesteinedesKristianiagebietes. I. Die Gesteine der Grorudit- Tinguait - Serie, Videnskdbs selskabets Skrifter. I Mathe- matisk - naturv. Klasse. 1894, No. 4, Christiania, 1894. II. Die Eruptionsfolge der tria- dischen Eruptivgesteine bei Predazzo in Slid Tyrol. Ibi- dem, 1895, No. 7, Christiania } 1895. III. Das Ganggefolge des Laurdalits. Ibidem, 1897, No. 6, Chris- tiania, 1898. F. W. Clarke. The Data of Geochemistry, U.S. GeoL Survey, Bull. No. 330, Series E., 1908. E. Cohen. Sammlung von Mikrophotographien von Mineralien und Gesteinen, 1899. W. Cross, J. P. Iddings, L. V. Pirsson, and H. S. Washington. Quantitative Classification of Igneous Rocks, based on Chemical and Min- eral Characters, with an introductory review of the development of system- atic Petrography in the Nineteenth Century, 1903. C. Doelter. Petrogenesis, 1906. . ( Mineralogie Micrographique des roches Fouuue and Michel- 1 , f. , ., , j eruptives fran9aises (with coloured plates), 1879. A. Marker. The Natural History of Igneous Rocks, 1909. J. P. Iddings. Rock Minerals, their chemical and physical characters and their determination in thin section, 1906. J. P. Iddings. Igneous Rocks, Vol. I : Composition, texture and classification, 1909. XV XVI GENERAL WORKS OF REFERENCE A. Osann. H. Rosenbusch. H Rosenbusch. J. J. H. Teall. H. S. Washington. H. S. Washington. F. Zirkel. Beitrage zur chemischen Petrographie. Part I, Molekularquotienten zur Berechnung von Gesteinsanalysen. Part II, Analysen der Eruptivges teine aus den Jahren 1884-1900. Mikroskopische Physiographic der Min- eralien und Gesteine. Vol. I. Die petrographisch wichtigen Mineralieii. Part I, General (by E. A. Wiilfing), 1904. Part II, Special (by H. Rosen- busch), 1907. Vol. II. Massige Gesteine. Part I, Tiefengesteine, Gang- gesteine, 1907. Part II, Ergussgesteine, 1908. Petrographical Tables, an aid to the Micro- scopical Determination of Rock-form- ing Minerals, translated from the German by F. H. Hatch. British Petrography (with coloured plates), 1888. Chemical Analyses of Igneous Rocks from 1884-1900, U.S. Geol. Survey, Prof. Paper No. 14, Series D. & E., 1903. The Superior Analyses of Igneous Rocks from Roth's Tabellen, 1869-1884, U.S. Geol. Survey, Prof. Paper No. 28, Series D. &. E., 1904. Lehrbuch der Petrographie. Vol. I, 1893. Vols. II and III, 1894. PETROLOGY. INTRODUCTION. Petrology is that department of geological science which has for its object the investigation of the characters and relations of rocks, that is to say, of the various materials of which the earth's crust is built up. A rock may be defined as a mineral aggregate, possessing a more or less persistent geological character. In studying rocks, therefore, we have three points of departure : (1) geological character, or mode of occurrence; (2) the nature of the constituent minerals; and (3) the manner of aggregation of the constituent minerals, or rock- texture. In classifying rocks we must allow each of these factors its proper value. The exclusive use f one of them would lead to incongruous results. Two or more rocks, for example, may be composed of the same minerals and yet have originated in totally dissimilar ways, the result being a difference both INTRODUCTION in texture and in mode of occurrence. Thus, a granite, a felspathic grit and a gneiss may aH three be composed of quartz, felspar and mica ; but in texture and in mode of origin they differ widely. The granite has been produced by the consolidation of a molten magma at some considerable depth beneath the surface : hence it possesses a coarse- grained, highly crystalline texture. The grit, on the other hand, is composed of fragments of minerals that have been derived by disintegration from other rocks, and have been transported and deposited by the agency of moving water. Lastly, in gneiss there is a banded structure, which in most cases has been produced by a re-arrangernent of the minerals since the first formation of the rock. 1 The three rocks made use of in our example may be taken as types of the three classes into which rocks are often divided ; viz., Igneous, Sedimentary and Metamorphic. Since the metamorphic division comprises rocks both of igneous (ortho-gneisses) and of aqueous origin (para-gneisses) a two-fold division of rocks would be more logical. Such a division is not practicable, because the original character of a rock is too often obscured by the secondary structures imposed i The banding of some rocks commonly classed as gneisses, however, took place before final consolidation, and is com- parable to ' ; fluxion-structure" in lavas. (See page 49.) INTRODUCTION 3 by metamorphism. But it may be pointed out that, according to an estimate of F. W. Clarke, 1 the crust of the earth is composed of rocks originally igneous, to the extent of 95 per cent., the remaining 5 per cent, being made up of 4 per cent, of shales, '75 per cent, of sandstones, and '25 per cent, of limestones. In the following pages it is proposed to discuss briefly the origin, characters and distribution of the chief igneous rocks. 1 Proc. of the Amer. PhU. Soc., vol. xlv., 1906, p. 21. PART I. THE PHYSICAL CHARACTERS OF THE IGNEOUS ROCKS. CHAPTER I. MODE OF OCCURRENCE. THE Igneous Rocks are those that have been formed by the consolidation of molten material. We have good reason for believing that the temperature of the interior of the earth is very high high enough, indeed, to melt all rocks with which we are acquainted at the surface. Physical and mathematical considera- tions, however, militate against the view that any large proportion of the earth's interior is occupied by liquid matter. It has been customary, therefore, to assume that at great depths the constituents of the globe, although existing at a high temperature, are maintained in the solid form by the enormous pressure to which they are subject at su.ch depths. A different hypothesis has been advanced by Arrhenius. 1 Since it has been shown experi- mentally by Tammann 2 that^at very high pressures the melting point of solids can be no longer raised by 1 Arrhenius : Zur Physik des Vulcanismus, Geologiska Fo'ren- ingens i Stockholm Forhandlingar, Band xxii. (1900), p. 395. For a useful summary of Arrhenius' Theory by R. H. Rastall, see Geol Mag. for April, 1907, p. 173. 2 Tammann : Kristallisieren und Schmelzen, 1903, p. 184. 5 6 PHYSICAL CHARACTERS [PART I increasing the pressure, it appears that at great depths the matter composing the earth's interior must be in the gaseous state; for pressure can no longer keep it solid, or even liquid, by raising the melting point. These considerations lead to the hypothesis that below the solid crust of the earth lies a liquid zone, and that beneath this there is a gaseous material which from its high density probably consists largely of iron. The physical properties of gaseous matter under such conditions must be very unlike those associated with gases at the surface of the earth ; inter alia it would appear to possess high rigidity and low compressibility. It is further calculated that about 80 per cent, of the earth's interior is occupied by gaseous metal (probably iron), 15 per cent, by gaseous rock-magma, and 4 per cent, by liquid magma, leaving 1 per cent, for the solid crust. The discussion of this question need not detain us. For, whether the portion of the earth imme- diately beneath the crust is occupied by rock in the molten condition, or by solid material at a sufficiently high temperature to become liquid under a diminished pressure, the ultimate result is the same : if, by one means or another, the external pressure be overcome, liquid magma will move in the direction of least resistance. According to the nature of the resistance presented by the environment, the molten rock will pierce the solid crust of the earth to a greater or less distance ; perhaps ultimately reaching the surface, where it will flow out in the form of lava, or be reduced by the violence of the explosion to cinders or to fine ash. CHAP, i] MODE OF OCCURRENCE 7 This consideration leads to a two-fold division of igneous rocks; namely, into those that consolidate in large masses at considerable depths beneath the surface, and those that penetrate to upper portions of the crust and are poured out upon the surface. The former are termed plutonic, deep - seated or abyssal, the latter, volcanic. The different con- ditions under which plutonic and volcanic rocks consolidate, are the cause of considerable differences in structure. In the one case, the solidification of the magma under the conditions imposed by a slow rate of cooling, considerable pressure and the presence of occluded water, produces a highly crystalline and coarse-grained texture ; while the rapid hardening of a lava, chilled by being spread over a large area of cold rock, and subject, moreover, to diminished pressure and consequent loss of water, imparts to it a more or less vitreous character. Granite and rhyolite rocks of similar chemical and rnineralogical composition illustrate this difference. The former is wholly crystalline, its large crystals completely filling space to the exclusion of all interstitial matter ; while in the latter the crystals are embedded in a glassy or semi-vitreous paste, the streaky character of which evidences the differential movements of the once viscous lava. Volcanic rocks are, however, connected with their deep-seated or plutonic roots by necks and pipes, or by dykes. The plutonic masses also send off tongues or apophyses into the surrounding rocks, and in some cases even the superficial strata are pierced by dykes and sheets of igneous material that have found no 8 PHYSICAL CHARACTERS [PART I actual outlet. A third division of igneous rocks is therefore necessary to embrace these connecting links between plutonic and volcanic rocks. For this division the term hypabyssal l is used. Plutonic and hypabyssal rocks are intrusive, while the volcanic rocks are extrusive. The field relations of the plutonic masses suggest an intimate connection with the original magma- reservoirs, of which, indeed, in some cases, they may form a part. On the other hand, the shape assumed by a hypabyssal rock is of necessity regulated by the nature of the passage along which it is injected, or, in other words, its shape is that of the channel of escape ; while volcanic rocks, having been erupted at the surface, appear in beds or streams. The plutonic rocks, being essentially deep-seated, can only appear at the surface in places where there has been much denudation, the overlying strata having then been removed by erosive agencies (frost, rain, fluviatile and marine action, etc.) ; unless, as sometimes happens, they have been upraised by powerful earth movements. They are, therefore, exposed more frequently in the older than in the younger strata. For this reason, granites, syenites, diorites, gabbros and similar plutonic rocks were formerly considered by some geologists to have been intruded only in pre-Tertiary times. That this view is erroneous has been shown in places where the denuding agencies have been sufficiently powerful to expose plutonic rocks of comparatively recent age, as 1 Brogger: Die Gesteine der Grorudit-Tinguait-Serie. VidensJc. Skrifter I., Math.-natiir. Klasse, 1894, No. 4, p. 123. CHAP, i] MODE OF OCCURRENCE 9 in the Western Isles of Scotland, where granites and gabbros, etc., have been intruded into bedded basalts of Tertiary age. The following terms are used in describing the chief forms in which the phi tonic rocks occur : Stock or boss. Any irregularly-shaped intrusion which cuts across the structure of the invaded forma- tion. The contacts are often vertical or highly inclined, and there is no visible floor on which the erupted material rests. The word boss is sometimes used jrather loosely, being used also for minor intrusions to which the above term should not strictly be applied. Dome. This term may be usefully employed to describe granite masses whose sides shelve away beneath the surrounding sediments, at first at a comparatively low angle, the exposed area being FIG. I. Section through a granite dome intrusive in the Killas and penetrated by dykes of El van ( E ). ( 'After Mac A lister. ) usually circular or oval. This term implies no adhesion to a particular theory as to mode of origin, as is the case with bathylith and laccolith. Bathylith. This term was introduced to connote the characters of igneous masses, similar to stocks or 10 PHYSICAL CHARACTERS [PART i bosses, but of much larger size, and occurring typically in great mountain ranges (mountain-core, central granite-core, massif, etc.). Suess * defines a bathylith as a stock-shaped or shield-shaped mass which has originated by the melting, and assimilation by the intruding magma, of a portion of the invaded formation. When exposed by denudation, it is found either to maintain its diameter or to grow broader with increasing depth. The assimilation theory thus applied by Suess to explain the origin of granite-bathyliths dates from a paper by Kjerulf 2 in 1879. Michel-Levy 3 also has stated it very clearly in his studies of the French granites. It has not, however, found universal acceptance ; Brogger, 4 for instance, maintains that the granite-massifs of the Christiania district are laccoliths rather than bathyliths in Suess' sense. It is beyond the scope of this book to discuss more fully so controversial a subject, interesting and important as it is. The references given to the literature will enable the student to plunge as deeply 1 Sitzungsberichte der Wiener Akademie, vol. civ. (1895), p. 52. "Eine stock oder schild-formigeDurchschmelzungsmasse, welche mit fortschreitender Abtragung entweder deri Querschnitt behauptet oder breiter wird, bis in die ' e\vige Teufe.' " 2 Kjerulf : Udsigt over det sydlige Norges geologi. Christiania, 1879. 3 Michel-Levy : Contributions a 1'etude du granite de Plamanviile et des granites frangais en ge'ne'ral. Bull, des services de la carte geol. de la France. No. 36, 1893. 4 Brogger : Die Eruptionsfolge des triadischen Eruptiv- gesteine bei Predazzo in Siidtyrol; Videhsk. Sknfter, 1895, No. 7, p. 116. CHAP, i] MODE OF OCCURRENCE 11 into the subject as he may desire. Brogger's and Michel-Levy's works should especially be consulted. While on this subject, it may be pointed out that it has been held that certain rocks of plutonic character may be produced by the fusion of deeply buried sediments even without the aid of magmatic intrusion. Thus Lawson l has described in the Rainy Lake district of Canada vast areas of rocks of plutonic habit which, according to him, have originated by the fusion of sedimentary rocks. This is supposedly brought about by the rise of the geo- isotherms, consequent on the deposition of great thicknesses of sediments in a sinking area. We have now to consider the forms assumed by rocks that have been produced by the injection of the magma, fed frdm deeper-lying reservoirs, into channels or chambers produced by the parting of the invaded formation. It is evident that the injection must have taken place either, across or along the planes of stratification. Dykes and volcanic necks are instances of the former mode of injection, sills and laccoliths of the latter. Dykes. A dyke is a wall-like mass of igneous rock, produced by the injection of molten material, into a fissure. It has nearly or quite parallel walls, and its length is considerably greater than its width. It may vary in inclination from vertical to horizontal, but it is a sine qua non that it cuts across the bedding when the invaded formation is stratified. Should there have been successive injections of similar 1 Report of the International Geological Congress, London, 1888. 12 PHYSICAL CHARACTERS [PART I material in the same fissure, the finally resulting mass is termed a multiple dyke. Should the suc- cessive injections be of different magmas, the resulting mass is termed a composite dyke. The Tormore dykes of Arran are a good example of composite dykes. 1 Examples of both types have been described by Barker in Skye. 2 Dykes sometimes form marked features in the scenery of a district ; for, being more capable of resisting the disintegrating action of the weather than the rocks they traverse, they remain standing, FIG. 2. Section showing granite sending apophyses into a bedded rock. (Afl&r De la Beche.} litce huge walls, when the softer rocks have been worn away. Occasionally, however, they weather more rapidly than the surrounding material, and Mien produce hollow troughs. Quartz - porphyries, larnprophyres, dolerites, etc., occur as dykes ; e.g., the quartz-porphyry dykes of Cornwall, the lam- prophyre dykes of Yorkshire and Westmorland, the 1 Judd : Quart. Journ. Geol. Soc., 1893, p. 564. 2 Barker : The Tertiary Igneous Rocks of Skye. Mem. Geol. Surv., 1904, pp. 201 and 206. CHAP. Ij MODE OF OCCURRENCE 13 dolerite dykes of the North of England and the West of Scotland. Veins. Veins are produced by the injection 01 the magma into smaller and less regular fissures than those that give rise to dykes. When such veins represent the infilling of cracks in the partially consolidated rocks with a residual portion of the same magma, they are termed contemporaneous veins. FIG. 3. Plan showing granite veining slates. (After DC fa Beche.) Apophyses or tongues. These are dykes or veins that are in visible connection with the larger intru- sive bodies which supplied their magma. Owing to the different physical conditions under which the rock composing an apophysis has consolidated, it usually possesses a micro-structure unlike that of the large intrusive body with which it is connected, although H PHYSICAL CHARACTERS [PART I the chemical composition of the two may be identical. Necks. Necks are the filled-up pipes or vents by which the erupted material made its escape. Usually they are plugs of consolidated igneous rock, surrounded by beds of volcanic ejectamenta. Some- times, however, they consist entirely of fragmentary material produced by the violence of the explosions FIG. 4. Volcanic necks in New Mexico; U.S.A. that precede or accompany an eruption. As examples, the necks developed in western -New Mexico, and the Bass Kock and North Berwick Law, in Haddington- shire, may be cited. Laccoliths. A laccolith is a dome-shaped intru- sion ; it may be conveniently regarded as a special case of sill, in which the intrusion of the magma has been accompanied by an up-arching of the overlying beds. As a rule the base remains flat: a laccolith CHAP. l] MODE OF OCCURRENCE grows " by up-lifting its cover." l The ground-plan approximates to a circle, but may be an elongated oval with the long axis in the direction of the strike of the invaded strata. While in sills or sheets the lateral dimensions are very great as compared with the thickness, in laccoliths the thickness approximates -., '', \ FIG. 5. Laccolith forming Mount Hillers, Henry Mountains. to the width. Laccoliths may be multiple (as in the case of the so-called cedar-tree laccoliths), or com- posite (as, for example, the gabbro-laccolith of the Cuillin Hills described by Harker 2 ). The laccolith theory of the origin of igneous rocks has met with very general acceptance, and many large plutonic intrusions are now considered to be laccoliths. Brogger even regards the largest granite 1 Gilbert : The Geology of the Henry Mountains; U.S. Geol. Survey, Washington, 1877. 2 The Tertiary Igneous Rocks of Skye. Mem. Geol. 1904, p. 87. 16 PHYSICAL CHARACTERS [PART i intrusions (e.g., those of the Christiania district) as laccoliths rather than bathyliths. (See p. 10.) Laccoliths were first described by G. K. Gilbert in the Henry Mountains of Utah, where they are very typically developed and well exposed. Many of the W.I5-S. O 509 PS6T IQ00 FIG. 6. Section of a composite laccolith in bedded basalts of Allt an 't-Sithean and Cnoc an 't-Sithean, Sligachan, Skye. The basalt of the laccolith is shown black and the granophyre white. The bedded basalts are cross-hatched. (After Harker : The Tertiary Igneous Rocks of Skye, Mem. Geol. Surv., 1904, p. 209.) intrusive rocks of North Wales have an outcrop which indicates a laccolithic origin ; and the same is probably the case with some of the acid intrusives of the Central Highlands of Scotland. According to Harker, 1 some of the acid and basic intrusions of the Red Hills and the Cuillin Hills in Skye have the general habit of laccoliths ; and in this connection attention must be drawn to the fact that, in the case of very large laccoliths, we have conditions of magma-consolidation closely similar to those of the deep-seated magmas ; consequently, it is to be expected that rocks so formed should have plutonie characters. This, indeed, is found to be the case in 1 The Tertiary Igneous Rocks of Skye. Mem. Geol. Surv., 3904, p. 83. CHAP. l] MODE OF OCCURRENCE 17 the example quoted from Skye, where rocks of plutonic habit (granite and gabbro) are found to grade into rocks of true hypabyssal type (granophyres and dolerites). Bysmalith or plug. As defined by Iddings l a bysmalith is an injected body, having the shape of a cone or cylinder, which has either penetrated to the surface or terminates in a dome of strata like that over a laccolith. The " plutonic plug " of Eussel 2 is a similar conception. Vertical displacement with faulting is the characteristic of this method of intru- sion. Mount Holmes, in Yellowstone Park, is cited by Iddings as a type. Intrusive Sheet or sill 3 A flat bed of rock produced by the uniform intrusion of molten material N Loch an Leo id Carn Dearg G B FIG. 7. Section of a composite sill, Carn Dearg, near Suishnbh, Skye. Scale 6 inches to a mile ; G. Granophyre portion of sill ; B. Basalt portion ; D. Supposed dyke-feeder of sill. (After Harker : Tertiary Igneous Rocks of Skye, Mem. Geol. Surv.,- 1904, p. 211.) between the planes of stratification. When the in- trusion is repeated between many planes of strati- 1 Journ. ofGeol., vol. vi,, 1898, p. 707. 2 Journ. ofGeol., vol. iv., 1896, pp. 23-43. 3 The word sill for a flat bed is derived from the Great Whin Sill of the North of England. 18 PHYSICAL CHARACTERS [PART i fication, it is known as lit-par-lit intrusion. Sills are liable to be mistaken for contemporaneous lava- flows ; 1 from which, however, they may be dis- tinguished by the following points of difference : (1) they break across the bedding, appearing here on one horizon, there on another ; (2) they sometimes send off veins into the strata above them, as well as into those below them ; (3) they alter the beds both above and below them ; (4) they are rarely vesicular 3 FT GRANITE GMEISS QUARTZO-FELSPATHIC FIG. 8. Lit-par-lit intrusion of granite in a foliated rock (gneiss). Summary of Progress, Geol. &urv., 1907, p. 78. or amygdaloidal ; (5) they are not accompanied by tuffs. As in the case of dykes, sills may be multiple or composite. Examples of both have been described 1 Mr. Barker points out that the sill-formed intrusions in the basic lavas of Skye have to a large extent not been discrimin- ated from the flows. Loc. cit.^ p. 29. CHAP, ij MODE OF OCCURRENCE 19 by Harker in Skye. 1 Dolerites frequently occur in intrusive sheets, and constitute one of the most striking features of many igneous areas, for example, Cornwall, the English Lake District, North Wales, Western Isles of Scotland, and Fife. The Whin Sill in the North of England is an intrusive sheet of basic rock, which, though never more than 150 feet thick, extends for a distance of 80 or 90 miles. Lava-flows. Lava is the molten rock poured out from a volcanic vent. The flow is subaerial or submarine, according as the eruption took place on land or on a sea-bottom. In subaerial flows the lava forms beds or streams of greater or less extent, according to the fluidity of the erupted material, the more basic magmas being the more fluid. Thus the Icelandic basalts were so liquid that they flowed in streams for a distance of 40 to 50 miles ; while, on the other hand, the acid trachytic lavas of Auvergne in Central France were erupted in such a viscous condition as to form hummocky or dome-shaped masses of small lateral extent. When the flows took place in an area of deposi- tion, e.g., on a sea-bottom, the lava-beds lay upon, and were covered by sedimentary rocks of practically the same age as themselves. In such cases they are said to be contemporaneous with the strata with which they are interbedded. Submarine eruptions began in pre- Cambrian times, so that contemporaneous igneous rocks occur in formations of various ages. 1 Barker, loc. cit. A resume* of the literature of the subject is given on p. 199. 20 PHYSICAL CHARACTERS [PART i Bedded lavas constitute a notable feature in the physiography of certain parts of the British Isles. Much of the beautiful scenery of North Wales and of the Lake District owes its origin to the great develop- ment of volcanic lavas (rhyolites and andesites), erupted in the Ordovician period. The Cheviot andesites are Old Red Sandstone lavas. Plateaux of Carboniferous basalt cover large areas in the Midland Valley of Scotland ; while bedded basalts of TOFF A FRAGMENTS LAVA ROCK BENEATH OON FIG. 9. Ideal section of a volcano. Tertiary age are extensively developed in the North of Ireland and in the Western Isles of Scotland. Basalt-plateaux of a similar character, but on a much grander scale, occur in some of the Western States of North America (Idaho, Oregon, and Washington) ; and in India an area of 200,000 square miles is covered by the basaltic flows known as the " Deccan traps." On account of the absence of pyroclastic accumulations in these basalt-plateaux, the lava is considered to have been fed through dyke-fissures; and CHAP, i] MODE OF OCCURRENCE 21 this form of intrusion, which is known as "fissure- eruption" has taken place even within historical times in Iceland. 1 Volcanic Ash-beds, or tuffs. When the eruption is accompanied by explosion, the molten rock is ejected in the form of bombs, cinders (lapilli), and fine ash, which are deposited in layers. The consolidation of this pyroclastic material produces volcanic breccias FIG. 10. A thin section of a volcanic tuff", showing its fragmental nature. (After Cole and. Jennings. ) or tuffs, according to the degree of coarseness of the fragments. The fragmentary material ejected from volcanoes often becomes mixed with the ordinary sediment of the sea-bottom, producing ashy limestones and ashy shales. 1 Thoroddsen : Geog. Journ., vol. xai. (1899), p. 500. CHAPTER II. EXTERNAL STRUCTURE. WHILE in the aqueous rocks the most characteristic feature is the stratification of the deposits, the igneous rocks are essentially un stratified. In general they may be said to possess a massive, in contra- distinction to the bedded character of the sedimentary rocks through which they are usually intruded. There are, however, some cases in which rocks of igneous origin are traversed by horizontal divisional planes, which resemble those of stratification. Thus the intermittent outpouring of lava from a volcanic vent produces a rock which has a true bedded character, each bed representing a distinct flow. Again the differential movement of an igneous mass in a plastic or semi-solid condition causes it to assume a laminated or " fluidal " structure on consolidation. Flow-structures of this kind in rocks of igneous origin have occasionally been mistaken for stratifica- tion. 1 Horizontal jointing in massive rocks may also simulate a thick - bedded character. Thus granite, when exposure to the corrosive action of the weather 1 The parallel structures produced in igneous rocks by dynamic or pressure metamorphism might also be instanced as examples of " pseudo-stratification." CHAP, n] EXTERNAL STRUCTURE 23 has brought out its divisional planes, often presents a rudely bedded appearance, which at a distance may b,- mi -folk n f^r Gratification. (S F' "T lad- 11) For the !iiuiiiMi, we iia-e to c>>u id \v m\- the external structures produced as a result of the con- solidation of an igneous rock from a molten condition. c ^iB V._j 4S (j2i -a .^j^-^|W tfftwJlK ^^a^, FIG. II. Mural jointing in granite. The micro-structure or texture of igneous rocks will be treated subsequently. Reference must first be made to the planes that are developed by shrinkage or contraction in passing from the liquid to the solid state. The divisional planes known as joints are found in all varieties of igneous rock ; and the appearance of a rock laid bare in a quarry or other section, is to a large extent dependent on its mode of jointing. The joints may be disposed in an irregular manner, or the parallelism of one or more systems pf divisional planes may impart a more or less definite structure to the rock. The " mural jointing" of granite, which was referred to above, is mainly caused by a system of roughly parallel planes. In the so-called sheet- structure the parallelism is still more pronounced. (See Fig. 12.) A platy structure is produced by the parallelism of plane joints not far removed from one another. This structure is found in dykes in 24 PHYSICAL CHARACTERS [PART i which the divisional planes are parallel to the bounding walls. In some rocks the surfaces of separation appear to form the concentric shells of a large spherical system of jointing. This character is well exemplified in the rock known as phonolite, FIG. 12. Pseudo-stratification in granite due to horizontal jointing or "sheet- structure." Crabtree and Havey Quarry in Sulivan, Maine. (After T. N. Dale.) which has a tendency to split into slightly curved plates. 1 A micro-structure of a similar nature frequently developed in vitreous rocks is known 1 The spheroidal weathering of some greenstones is due to a joint-system of this nature, the existence oi which is only demonstrated by the facility it offers to the progress of decom- position. CHAP. 11] EXTERNAL STRUCTURE 25 as the perlitic structure. (See Fig. 13.) In all such cases the concentric-laminated structure is produced by concentric fissuring due to contraction on cooling. When a rock is traversed by two or more systems of shrinkage -joints, a columnar structure results. Many igneous rocks present this mode of jointing, but the structure reaches its most perfect develop- FIG. 13. PeHite of Tokay, Hungary, under the microscope, illustrating Perlitic and Flow-structures. (After Fouque and Levy. ) rnent in basic rocks : the columns, which are often very long and regular, are bounded by three, four, five, or six planes, producing triangular, quad- rangular, pentagonal, and hexagonal prisms. Where the rook-texture is homogeneous the six-sided prisms are most prevalent, for in this case the centres of 26 PHYSICAL CHARACTERS [PART contraction are equidistant, and the angles of the prisms fit together without any intervening space. The long axes of the columns are perpendicular to the surface at which the greatest cooling took place. In accordance with this law the columns are FIG. 14. Columnar basalt, Giant's Causeway, Ireland. vertical in horizontal sills and flows ; while in dykes they are horizontal if the walls are vertical. Fine examples of columnar jointing are found in the basic rocks of the Western Isles of Scotland as at CHAP, n] EXTERNAL STRUCTURE 27 Staffa and in Skye, also at Elie and St. Andrews in Fif'eshire. Occasionally the columns are sub- divided into segments by a subsidiary cross-jointing, and usually such segmentation is accompanied by a spheroidal tendency, producing ball-and-socket joints. Good examples are seen in the Giant's Causeway in the North of Ireland, and in the Cheese Grotto at Bad Bertrich in tly Eifel. Lavas are distinguished by marked /low-structures (ropy and stringy surfaces, internal streakiness, etc.), due to the rolling-over and pulling-out of the semi- solid mass. They are also characterised by a cellular or scoriaceous character, produced by the elimination of water-vapour from the magma on its eruption. Innumerable steam bubbles are developed in the viscous liquid, and their retention in the solidifying lava produces a vesicular structure, which reaches its maximum development in the substance known as pumice. The vesicles are usually drawn out in the direction of flow, and in extreme cases become extended to long-branching, coral-like tubes as in the " pipe-amygdaloid " of the Volcanic Beds of the Drakensberg and Malutiberg in South Africa, and, more rarely, in the Tertiary basalts of the Western Isles of Scotland. When filled with decomposition - products (calcite, zeolites, chalcedony, etc.), the almond-shaped vesicular cavities are known as amygdales, and the structure as am ygdaloidal. Vesicular structure is not found in the deep- seated rocks, but hypabyssal rocks are occasion-ally i See A. Geikie : Ancient Volcanoes of Great Britain, 1897, p. 17. 28 PHYSICAL CHARACTERS [PART i amygdaloidal. 1 A structure genetically allied to the vesicular is found in some granite rocks that have consolidated under a comparatively shallow cover. In such rocks are small cavities of irregular shape, into which project well-shaped crystals. This means that the minerals, separating from the magma in the neighbourhood of a vapour cavity, attained a more perfect development than those constituting the bulk of the rock. This structure is known as the drusy or mi&rolitic, and is characteristic of the granites of the British Tertiary area (Mourne Mountains, Arran, Skye, etc.). Reference may be conveniently made here to the dark-coloured patches (the heathen of the quarry- man) which are of frequent occurrence in some granites. Some of these are generally, and probably rightly, regarded as having been produced, during an early period of the solidification of the magma, by the segregation of its more basic constituents 2 ; but in many cases, especially when they occur in the marginal portion of the intrusion, they are un- doubtedly included fragments of foreign rocks that have been caught up by the magma during its intrusion. For such inclusions the name xenolith has been proposed. 8 In some cases xenoliths occur in such abundance as to impart to the granite, 1 A. Geikie : Trans. Eoy. Soc. Edin., vol. xxxv., p. 38. * J. A. Philips : Quart. Journ. Geol tioc., xxxvi. (1880), pp. 1-2, and xxxviii, (1882), pp. 216-217. 3 Sollas : Trans. Eoy. Irish Acad., vol. xxx. (1894), p. 502. For inclusions of single crystals of a foreign mineral the same author uses the term xenocryst. CHAP, n] EXTERNAL STRUCTURE 29 by their parallel arrangement, partial fusion and " streaking out," a local banded or gnei.ssose struc- ture. 1 The researches of Harker 2 in Skye show that the character of an igneous rock may be materially modified by the incorporation of xenoliths of another. In such cases the intrusion of the later rock probably took^)lace before the consolidation of the earlier was completed. Certain structures which might perhaps be regarded as external, and as such receive treatment here (e.g., the banded, and the spheroidal or orbicular structures), are for convenience referred to in the next section, which deals with the micro-structures of the igneous rocks. (See page 48.) 1 Lacroix : Le granite des Pyrenees, 2nd Mem. , Bull, des services de la carte geol. de la France, No. 71, 1900, pp. 21 and 25. 2 The Tertiary Igneous Rocks of Skye. Mem. Geol. Surv., 1904. CHAPTER III. TEXTURE OR MICRO-STRUCTURE. THE examination of a thin section of an igneous rock under the microscope shows that it is composed either entirely of crystals, or of crystals embedded in a paste or groundmass of a more or less imperfectly individualised glassy substance, or lastly of glass alone. Rocks of the first type are termed holo- crystalline ; those of the second, hypo-crystalline ; and those of the third, vitreous. The internal structure or texture of a rock depends mainly on the relation existing between the crystals themselves, or between the crystals and the groundmass, and in the latter case, to some extent, on the character of the groundmass itself ; it will vary according to the conditions under which solidification took place, since the crystals were formed by molecular aggregation in a rock-magma during the process of cooling. Rocks that from the nature of their environment were caused to cool rapidly, are com- posed entirely or in great measure of glass ; but when the conditions were such as to allow of a gradual loss of heat, the molecules had time and opportunity to group themselves into definite mineral 30 CHAP, in] TEXTURE OR MICRO-STRUCTURE 31 species, and a crystalline or hypo-crystalline texture is the result. Sometimes it is the whole body of the rock that is vitreous; sometimes there is only a narrow "selvage" of glass, representing the rapidly chilled portion of the magma that carile into immediate contact with the cooling surface. There is, naturally, every grada- .//; FIG; 15. FIG. 16. Glcbulites. Margarites. tion between rocks that consist wholly of glass and those that are completely crystalline. The glassy base, even of vitreous rocks, is seldom completely devoid of crystalline material. Examined with the microscope, it is often found to contain numerous minute, spherical, rod-shaped, and hair-like bodies, FIG. 17. FIG. 18. Trichites. Belonites. which represent the first products of crystallization. These bodies are in general termed crystallites. 32 PHYSICAL CHARACTERS [PART i They are not crystals, for they possess no distinct crystallographic form, nor do they react on polarized light. The different forms have received specific names: thus, the drop-like bodies are termed globulites ; the rod-shaped ones, belonites ; and the coiled and twisted hairs, trichites. (See Figs. 15-18.) Besides crystallites there are small needle- and rod-shaped bodies which are known as microlites. They are distinguished from crystallites by the fact that they react on polarised light (becoming alternately light and dark when rotated between crossed nicols), and can generally be referred to FIG. 19. Skeleton-crystals of olivine. (After Rosenbusch.} some mineral species felspar, augite, olivine, mag- netite, etc. By combination of the simple microlitic forms, curiously forked and gridiron-like aggregates (skeleton-crystals) are produced. (See Figs. 19 and 20.) The linear arrangement of these various bodies, and the way in which they sweep round the larger crystals, serve to indicate the once fluid nature of the vitreous rocks in which they are found (fluxion- or flow-structure). CHAP, in] TEXTURE OR MICRO-STRUCTURE 33 An incipient form of crystallization, often met with hi vitreous rocks, consists in the production of small FIG. 2O. Pitchstone of Arran, under the microscope, showing microliles and keleton-crystals of hornblende in a glassy base. (After Teall.} Spherulitic structure in a felsite of Arran. (After Bonney.} 34 PHYSICAL CHARACTERS . [PART i spherical bodies, known as spherulites. These bodies, usually of the size of a pea or smaller, present, when viewed under the microscope, a divergent fibrous structure, and are characterized between crossed nicols by a black cross, the arms of which are parallel to the short diagonals of the two nicols. These fibro-radial crystalline aggregates were FIG. 22. Spherulitic structure, between crossed nicols. (After Fcuque and Ltvy. ) produced during the final period of solidification, or subsequently thereto ; for streams of microlites some- times pass through them, showing that the formation of crystals and the arrangement of them in lines of flow preceded the growth of the spherulites. CHAP, m] TEXTURE OR MICRO-STRUCTURE 35 The separation of minerals from a silicate-magma which is slowly cooling is determined by the readi- ness with which the different minerals will form supersaturated solutions, by mass - action, by the chemical affinity of the different bases for one another and for silica, and by the pressure to which the magma is subject. As a preliminary to the discussion of the micro-structure or texture of a crystalline or hypo-crystalline rock, we must con- sider the conditions under which minerals are produced in a molten magma that is undergoing solidification. Lagorio 1 has shown that silicate-magmas behave as solutions, and that therefore the laws established with reference to the separation of salts from satu- rated and supersaturated solutions, can be applied to explain the formation of minerals in an igneous magma, which may be regarded as a mutual solution of the silicates that build up the complex minerals of igneous rocks. Crystals tend to form in a magma when it becomes sufficiently supersaturated with any definite compound. The gradual diminution of temperature causes successive supersaturation with different compounds, which would therefore crystallize in the order of their solubility, but for the modifica- tion produced by the relative proportion and chemical affinity of the different bases present as silicates. The order in which the silicates of the metallic oxides tend to saturate molten silicate solutions, is given by Lagorio as follows : first, the oxides of the 1 Min. u. Petr. Mitth., vol. viii. (1887), pp. 421-529. 36 PHYSICAL CHARACTERS [PART I heavier metals (the sesquioxide of iron more easily than the protoxide) ; then follow in order magnesia, lime, soda, alumina, and potash. The influence of mass in determining the formation of minerals is illustrated by the poorness of the acid rocks in iron-ores, the excess of silica in such rocks having united with the iron to form iron -silicate. Again, when iron-ore is developed in acid rocks, it is as magnetite; whereas in basic rocks, ilmenite pre- dominates: the excess of silica in the acid rocks draws the titanic acid to combine with it to form sphene, while in the basic rocks the latter is left free to combine with the iron-oxides. The influence of relative proportion and chemical affinity of the bases on the separation of minerals in a slowly cooling magma, is illustrated by a case quoted by Harker 1 from the Isle of Rum. In a series of rocks composed essentially of olivine and anorthite, the olivine is found to have crystallized out first when the rock is rich in that mineral, while in varieties rich in the felspar, the reverse holds good. Only when the two minerals occur in a certain fixed proportion, one to the other, have they crystallized simultaneously. Vogt's researches on slags 2 show that, when a pair of minerals are in mutual solution, there is a certain definite ratio of the one to the other in which they will crystallize out in intimate admixture. Whichever mineral is in excess of this 1 Science Progress, 1907, p. 245, and Geology of the Small Isles, Mem. Geol. Surv., 1908, p. 85. 2 Die Silikatschmelzlosungen : Vidensk. Skrifter, I. Math.- natur. Klasse, 1903 and 1904, Christiania. CHAP, in] TEXTURE OR MICRO-STRUCTURE 37 (the eutectic l ) proportion, will separate out first. Only when the residual magma contains the two minerals in the eutectic proportion, will they crystallize simul- taneously in intimate intergrowth. The eutectic ratios for certain pairs of minerals crys- tallizing from slags, have been established. For augite and olivine it is as 68 : 32^ for plagioclase and olivine, 70 : 30. Teall 2 suggested in 1888 that the interstitial micro-pegmatite of acid rocks is an eutectic mixture of quartz and felspar, and represents the final mother- liquor from which the other minerals have crystallized out. Subsequently Vogt showed that the eutectic ratio of quartz and felspar corresponded approximately to 26 parts of the former to 74 of the latter. The order of crystallization of a pair of minerals which do not form mixed crystals can be exemplified by means of a temperature-concentration diagram, in which the concentration of the relative proportions of the two components is shown in percentages by abscissae, and the temperature by ordinates. Take for instance the case of the anorthite-olivine magma already quoted. In the diagram (fig. 23) the point A on the concentra- tion line represents 100 per cent, of anorthite, and the point O, 100 per cent, of olivine. Points lying between A and O represent all possible mixtures of these two minerals. Further, A' on the temperature line represents the freezing point of pure anorthite, and 0' that of pure olivine. Starting with pure 1 F. Guthrie : Phil Mag., 1875, p. 20. 2 British Petrography, London, 1888, p. 395-419. 38 PHYSICAL CHARACTERS [PART i anorthite, the addition of an increasing proportion of olivine lowers the freezing point, as shown by the curve A'E', and similarly the addition of anorthite to olivine produces the freezing point curve O'E'. These two curves intersect at K'. The point E at which the vertical from E' cuts the con- centration line, gives the composition of the eutectic of anorthite and olivine. Given a magrna represented in composition and temperature by the point P, and subject to a falling temperature, no consolidation will take place while 0' Concentrati Off FIG. 23. the temperature falls from P to P. But at P' the constituent (here anorthite) which is in excess of the eutectic proportion begins to separate in the pure form, and the system moves (by fall of temperature and change of composition of the residual liquid) along the curve P'E'. On reaching the point E', the whole liquid consolidates as the eutectic, without further change in composition. If, on the other hand, we start at a point Q, representing a liquid in which olivine is in excess, a similar result is obtained, but pure olivine first separates. CHAP, in] TEXTURE OR MICRO-STRUCTURE 39 If in addition to the two minerals a third be present, say pyroxene in the case quoted, Nernst's law of the reduction of solubility between substances having a common ion, 1 appears to govern the order of crystallization. Thus olivine and pyroxene have the ion (Mg Fe) in common, consequently the solubility of the olivine is much reduced, and it invariably crystallizes before the felspar, even when not present in very large amount. 2 The effect of pressure on the separation of salts from solutions has been investigated by Sorby. 3 He has shown that the solubility of salts which increase in volume as they pass into solution, is lowered by pressure. The rock-forming silicates behave in a like manner. Consequently, a magma which at a given temperature and pressure is on the point of crys- tallizing will, if the pressure be relieved, remain liquid, until, by the fall of temperature, the proper point of saturation has been reached. This affords an explanation of the corrosion and resorption of crystals that have been formed in a magma prior to eruption. The relief of pressure consequent upon eruption lowers the fusing point of the mineral, and a portion of it passes into solution, separating out again in many cases during a later phase of con- solidation. Since a decrease of temperature induces crystallization, while a decrease of pressure retards it, it is conceivable that, in a magma subject to changing * Nernst : Theoret. Chem., 3rd edit. (1904), p. 492. 2 Barker : Geology of the Small Isles, Mem. Geol. Surv., 1908, p. 86. 3 Proc. Roy. tfoc., vol. xii. (1863), p. 538. 40 PHYSICAL CHARACTERS [PART I temperature and pressure, the development and retardation of crystallization might be several times repeated, thus affording an explanation of the zones of resorption which are often observable in the larger crystals of hypabyssal and volcanic rocks. In the hypabyssal and in the volcanic rocks there are often two distinct phases of consolidation. The first phase consists in the formation of porphyritic l crystals, or phenocrysts, 2 whose growth is promoted by molecular currents in a magma of extreme liquidity. This has been termed by Rosenbusch the intra- telluric phase of crystallization, signifying thereby that the crystals were produced before the eruption of the rock, while it was still in its original magma- basin. The second phase consists in the comparatively rapid solidification of the magma, which becomes the groundmass in which the phenocrysts are embedded. The groundmass usually contains a second crop of the minerals belonging to the first phase, together with those minerals that only appear in 'the ground- mass. If the crystallization was complete, the groundmass is holocrystalline. More usually, how- ever, a residual mother-liquor remains after the separation of the crystals, and consolidates as glass. This is known as the glassy base. According to Crosby, 3 the coarse granular texture of some deep - seated rocks implies a uniformly 1 So named because such crystals constitute the typical character of the rocks known to the ancients as porphyry. 2 Iddings (Bull. Phil Soc. Wash., vol. xi., 1888-1891, p. 73) from alvw } I show, and /c^o-raAXos, crystal. 3 Amer. Geologist, vol. xxv. (1900), p. 299. CHAP, in] TEXTURE OR MICRO-STRUCTURE 41 slow rate of cooling which enabled the wide-spaced immobile crystals of the first phase to continue to grow until the magma was exhausted without fresh centres of crystallization being set up. Such a granitoid rock may be regarded as consisting wholly of overgrown phenocrysts. In magmas which by their environment are subject to a more rapid rate of cooling, a condition of viscosity may be quickly attained which is too great for free molecular diffusion, and crystallization is started simul- taneously at many narrow-spaced centres. The result is a finely crystalline granular texture, in which the rock may be regarded as consisting en- tirely of groundmass. In porphyritic rocks two phases of consolidation are presented. Such rocks are formed from magmas which at first, under the influence of slow cooling, started the formation of phenocrysts at widely-spaced centres of crystalliza- tion, with free molecular diffusion. A sudden change to a condition of increased viscosity checked the supply of fresh material to the growing phenocrysts, and induced the setting up of many new centres, with consequent rapid consolidation to a groundmass. This abrupt change from centralized to diffused cr} 7 stallization is consequent upon an increased rate of cooling, which, in the case of the volcanic rocks, is brought about by their effusion, and in the case of the hypabyssal rocks, by their trans- ference from deep-seated to superficial portions of the earth's crust. The relief of pressure at the moment of extrusion or injection may, as already pointed out, cause at first a partial resorption of the 42 PHYSICAL CHARACTERS [PART I phenocrysts ; but the increased rate of cooling, aided by the escape of occluded water, speedily induces a magmatic condition analogous to that of an over-cooled or super-saturated solution, and solidification of the magma-residuum speedily takes place. The occasional occurrence of the porphyritic structure in the heart of a bathylith, where the abrupt change in the rate of the cooling postulated by Crosby's theory cannot conceivably take place, may best be explained in the manner suggested by Professor Miers. 1 He has shown that experimental evidence confirms the suggestion of Ostwald that there are two states of super-saturated solution, namely, a labile and a metastable. A super-saturated solution is labile, when crystallization can take place in it spontaneously ; aud at a somewhat higher temperature it is metastable, when crystallization can only be induced in it by the introduction of a solid fragment of the dissolved substance. Applying this to a silicate magma which is at a sufficiently high temperature to be in the raetastable state, is cooling slowly enough to remain in that state for a consider- able time, and is sufficiently viscous to prevent the growing crystals from sinking, we have the conditions suitable for the growth of porphyritic crystals. These must have originated as spontaneous nuclei if the magma had temporarily entered the i British Association Rep., 1905. Presidential Address to Section C. CHAP, m] TEXTURE OR MICRO-STRUCTURE 43 labile state; or they may have been started by inocula- tion or by marginal cooling, which may reduce it locally to the labile state, though the magma as a whole may have remained metastable. If, when the porphyritic crystals have attained a certain size, the magma, by a somewhat sudden increase in the rate of cooling, enters into the labile condition, spontaneous cystallization will be started at many centres, and a groundmass will be produced in which there will be a second generation of one of the constituents of the rock. The special case of the production of the por- phyritic structure in a quartz-felspar magma in which the magma-residuum solidifies as micro- pegmatite, has been explained thus. 1 The magma having reached, by cooling, the labile state of saturation, the crystallization of quartz starts spon- taneously at a number of centres. But by its separation, super-saturation is relieved, the magma becomes metastable, and the crystals already started continue to grow as the temperature falls. Since there are no felspar crystals to start the separation of that mineral, the formation of quartz will continue beyond the limit required by the eutectic ratio. The continued subtraction of quartz then leaves the felspar in excess, so that when the cooling magma reaches the labile stage for that mineral, felspar alone will crystallize. Finally, when the separation of the excess of felspar brings the magma again to the eutectic point, the presence of crystals of both 1 Barker, loc. tit., p. 251. 44 PHYSICAL CHARACTERS [PART i minerals causes their simultaneous crystallization in the form of micro-pegmatite. Having thus briefly discussed the conditions under which rock magmas may be supposed to solidify, we must turn our attention to rock textures, which are genetically dependent on the conditions of solidifica- tion. As already pointed out, the micro-structure of rocks is determined both by the shapes of the constitu- ent crystals and by their arrangement with respect to one another and to the glassy base, if such be present. First with regard to the shape of the crystals : those forms in which the crystal contours are more or less perfectly developed are termed idiomorphic ; while those in which the formation of the crystal contours has been prevented are termed allotriomorphic l (see page 73). In respect to the size of the grain, the structure of a holocrystalline rock may be coarsely crystalline, finely crystalline, or micro-crystalline. These terms explain themselves. A rock which is apparently homogeneous may be found, when examined in thin sections under the microscope, to consist of an aggregate of minute granules, some of which act upon polarized light but are too small for even microscopic determination. This is what is meant by the term cryptocrystalline. The micro-structures or textures dependent on the arrangement or relative position of crystals, may be 1 These terms were introduced by Rosenbusch. Some authors (e.g., Zirkel and Iddings) use Rohrbach's terms automorphic and xenomorphic, which express the same idea. CHAP, m] TEXTURE OR MICRO-STRUCTURE 45 considered under the heads of juxtaposition, inter- position, porphyritic structure, orbicular structure and banded structure. 1 Juxtaposition. a. Allotriomorphic granular is the texture in which the crystals are of uniform size(equidimensional) and without crystal-contours. It is usually termed FIG. 24. Granophyric Structure. the granitic structure, and is presented mainly by the plu tonic rocks. 1 This classification is to some extent based on one given in the Quantitative Classification of Igneous Rocks by W. Cross, J. P. Iddings, L. V. Pirsson, and H. S. Washington ; Chicago, 1903. It is proposed by these authors to connote " the arrangement of the crystalline and non-crystalline parts of a rock" by the term fabric as opposed to texture. 46 PHYSICAL CHARACTERS [PART i b. Hypidiomorphic granular similar to the pre- ceding, but with a part of the crystals showing crystal contours. G. Pan-idiomorphic granular a texture in which all the constituents are bounded by crystal contours. There is often a dominant tabular or prismatic habit. Interposition. a. The graphic or micrographic structure is that in which two minerals mutually enclose one another by interpenetration. The most commonly occurring type is the granophyric structure, in which the two minerals in intimate intergrowth, but retaining their parallel orientation, are quartz and felspar. The intermixed minerals are, in this case, known as micropegmatite. 1 There is a tendency iu granophyric structure towards a divergent or radiate arrangement around a common centre or nucleus, which is usually occupied by a crystal of felspar (see Fig. 24) showing optical continuity with the felspar fibres of the framework. The radial character becomes more pronounced as the crystal needles or fibres became more slender. Such radially fibrous aggregates were named pseudo- spherulites by Rosenbusch ; but every transition appears to exist between pseudo-spherulites of micro- pegmatite and the so-called microfelsite spherulites, in which the component mineral fibres are no longer distinguishable. Teall 2 first called attention to this, 1 Another name used for such intergrowths of threads of felspar and quartz is myrmekite. 2 British Petrography, London, 1888, p. 402. CHAP, in] TEXTURE OR MICRO-STRUCTURE 47 and expressed the opinion that the latter represented a special modification of the former, and that both owed their origin to a simultaneous crystallization of the residual magma as quartz and felspar in the eutectic proportions. (See page 37.) b. The pcecilitic or poikilitic structure 1 is that in which optically-continuous crystals of one mineral play mW^M FIG. 25. Ophitic Structure. the role of matrix for crystals or grains of another which do not show parallel orientation, for example, olivine grains in pyroxene or hornblende. " Lustre mottling" is a consequence of the poecilitic structure. A special type is the ophitic structure, presented by most dolerites. In this the felspar appears in iTerm introduced by G. H. Williams, Journ. of GeoL, vol. i. (1873), p. 176. 48 PHYSICAL CHARACTERS [PAKT i well-defined lath-shaped or tabular crystals, penetrat- ing and enveloped by a groundmass of augite or hornblende. Porphyritic, Orbicular, and Sanded Structures. The porphyritic structure is characterized by the presence of phenocrysts of one or more minerals in a groundmass of different material, which may be either crystalline, hypocrystalline or vitreous. Its origin has been discussed on page 40. The special type known as glomero-porphyritic 1 structure, is produced by the packing together of the porphyritic constituents, as in the Tynemouth dyke and the Peppercraig trachyte. The orbicular structure is a form of crystal aggregation in which incipient forms are arranged radially as spherulites (see page 34), or in which definite minerals are grouped radially or concentric- ally round a common centre to form spheroids. The spherulitic structure occurs in glassy rocks ; the spheroidal structure, in holocrystalline rocks (e.g., spheroidal granite, orbicular diorite, etc.). The banded structure is produced (1) by the linear arrangement of crystallites and microlites, or of crystals ; (2) by the parallel arrangement of layers or streaks (Schlieren) 21 of rock showing different decrees of crystallization, or different proportions of *A term introduced by Judd, Quart. Journ. Geol. $oc., vol. xlii. (1886), p. 71. 2 A German term introduced by E. Reyer. See Doelter's Petrogenesis, p. 86. CHAP, in] TEXTU&E OR MICRO-STRUCTURE 40 the component minerals. 1 A typical example of the first type is quoted by ?&. Geikie and Teall: 2 in a sill of troctolite at Allival, in the Isle of Rum, the parallelism of the felspars is so pronounced as to impart to the rock a lamination which might easily be mistaken for the fissility of a schist. In the same paper the authors give an example of the second type: the gabbro of Druim an Eidhne, in the Cuillin Hills of Skye, which possesses a marked banding, due partly to differences .of texture, and partly to variations in the proportion of the constituent minerals. This structure appears to have resulted from a differentiation of the parent magma into distinct but allied magmas, before eruption, so that the streaky character was impressed on the rock before crystallization commenced. This is shown by the fact that there is no parallelism of the constituent crystals. Harker, 3 however, points out that the heterogeneity which is necessary for the production of banded structure by fluxion, may also result from an admixture of magmas not derived from a common source. The characteristic banded structure of rhyolites, 1 It is worthy of note that these structures are the result of flow before or during consolidation (pseudo-gneissic or primary gneissic structure of some authors), and must be distinguished from the banded structure produced by dynamic meta- morphism, which is the result of molecular re-arrangement and flow after consolidation (true gneissic structure). 2 On the banded structure of some Tertiary gabbros in the Isle of Skye. Quart. Journ. Geol. Soc., vol. 1. (1894), p. 649. 3 Quart. Journ. Geol. Soc., vol. lix., 1903, p. 215. D 50 PHYSICAL CHARACTERS [PART I in which there is an alternation of glassy and crystalline layers or zones, in which spherulites are developed, is explained by Iddings * as depending on the variable amount of water vapour contained in different layers of the flowing magma. 1 Journ. Amer. Geol. Soc., vol. xxxiii. (1887), p. 45. See also Bull. Phil Soc. Wash., vol. xi., p. 446. CHAPTER IV. COMPOSITION. IN discussing this part of the subject we have to distinguish between the chemical and the mineralogi- cal composition of rocks. Although the latter is to some extent dependent on the former, rock- magmas possessing similar chemical compositions may develop different mineral products, when crys- tallized under other physical conditions. Differences in chemical composition, however, produce important mineralogical variations ; and the chemical factor plays an important role in the classification of the igneous rocks. CHEMICAL COMPOSITION. Rock -magmas are, without exception, silicate-mag- mas ; that is, they consist of silica, together with the bases iron, alumina, lime, magnesia, potash and soda. If the silica is in excess of the bases, the solidified rock contains free silica or quartz, and such rocks are said to be acid. If, on the other hand, the silica percentage is low, the rock is said to be basic. In the acid rocks the percentage of silica may rise to 80 ; in the basic rocks it sinks to below 50, and in some ultra- basic PHYSICAL CHARACTERS [PART I .-d even lower, reaching a minimum somewhere uetween 30 and 40. The proportion of silica greatly influences the crystallization of a magma. Acid magmas crystallize with more difficulty than the basic ones : consequently rhyolitic glasses (obsidian, pitchstone, etc.), are common, but basaltic glasses (tachylyte) are rare ; again, at depths at which acid magmas solidify as .fine-grained or porphyritic rocks, basic magmas, other conditions being the same, crystallize as medium-grained or even coarse-grained rocks. Among the bases, alumina occupies an important place, being present in a great number of rock- forming silicates. While the greater proportion of this base is combined with alkalies to form the felspars (or felspathoids), a portion enters into the composition of the ferro-magnesian silicates (mica, hornblende, augite, etc.). The uncombined oxide occurs, somewhat rarely, as corundum. The per- centage of alumina in rocks ranges from nothing, in the non-felspathic peridotites, up to about 20, in certain syenites. The oxides of iron and magnesium also exert con- siderable influence in determining the character of a rock. The percentage of these bases is low in the acid, but rises considerably in the basic rocks. The lower oxide of iron (FeO) and magnesia are com- bined in an important series of silicates the so-called ferro-magnesian minerals : olivine and the pyroxenes and amphiboles, or biotite, with the addition of potas- sium in the last-named. The same bases with the addition of lime form the lime-magnesian silicates CHAP, iv] COMPOSITION 63 (the monoclinic pyroxenes and amphiboles). The oxides of iron are also abundantly present in the more basic rocks as free iron-ore -(magnetite and ihnenite). Lime is an important factor : it takes an important part in the composition of the basic plagioclase felspars, but decreases as the felspars become richer in alkalies. It is also abundant in monoclinic pyroxene, less so in hornblende, and almost entirely absent from the rhombic pyroxenes and biotite. But of all the bases it is perhaps the alkalies, potash and soda, that are of greatest im- portance in determining the constitution of rocks. Thus together with alumina, and in some cases with lime, they form in combination with silica the felspars and felspathoids. According to the predominance of the particular base, we have: potassium silicates (orthoclase and leucite); sodium silicates (albite and nepheline); silicates of sodium and calcium (the basic plagioclase series) ; calcium silicate (anorthite). In rocks having an abnormally high percentage of alkalies, soda-amphiboles and soda-pyroxenes occur (e.g., riebeckite, arfvedsouite, cossyrite, segirine). Potash also enters into the composition of biotite. Among compounds that occur in smaller propor- tions, although widely distributed, are phosphoric and titanic acids. The phosphate of lime (apatite) is present in most basic and in many intermediate and acid rocks ; some apatite-bearing rocks even contain as much as two to three per cent, of phosphoric acid. Titanic acid is rarely completely absent, and may rise to five or six per cent., as in 54 PHYSICAL CHARACTERS [PAKT i certain basalts of Pantelleria. 1 Titaniferous iron- ore (ilmenite) is a frequent constituent of basic rocks ; sphene (titano-silicate of lime) occurs in acid rocks, and perovskite (titanate of lime) in basic rocks. Small quantities of titanic acid appear also to be present in certain varieties of augite and mica. 2 Boron, fluorine and chlorine occur in small quantities in some igneous rocks. The last- named is chiefly present as a constituent of members of the nepheline-leucite group, while boron and fluor- ine are found in the minerals tourmaline, mica, topaz, fluorspar, and axinite, especially in rocks that have been subject to pneumatolytic modification. 3 Sulphur occurs, mainly as a constituent of pyrites, granules of which are dispersed through some igneous rocks. Carbon also occurs, chiefly in the form of graphite. Among the metals, barium, strontium, lithium, chromium, vanadium, manganese, molybdenum, tung- sten, nickel, and members of the platinum group are frequently present in minute proportions, especially in basic rocks. A series of Italian gabbros was found by Vogt to contain traces of chromic acid, ranging from 018 to O30 per cent.; and Dr. Pollard has found both chromium and vanadium in some of the 1 Washington : Quart. Journ. Geol. Soc. , vol. Ixiii. (1907), p. 74. 2 The brown mica of Miask has been shown by Schapfer to contain 4 '73 per cent, of titanic acid ; and Washington (loc. cit.) states that a colourless augite of Central Italy carries 2 '85 per cent. 3 Boron, fluorine and chlorine play an important role as so-called mineralisers in the crystallization of certain rock- CHAP, iv] COMPOSITION 55 Skye rocks. 1 The researches of Sandberger have shown that the metals gold, silver, copper, mercury, and tin are also present in minute, traces in igneous rocks; while native iron, in alloy with nickel and cobalt, has been found distributed in small particles through certain basalts. The mean of a great number of analyses 2 of igneous rocks shows that the elements oxygen, silicon, alumin- ium, iron, calcium, magnesium, sodium, and potassium make up 99 per cent, of their chemical composition, and of these, oxygen alone makes up 47 per cent. MlNERALOGICAL COMPOSITION. With regard to the mineralogical composition of igneous rocks, the acid group is characterized by the presence of free silica or quartz in greater or less abundance, together with a dominant alkali- felspar. The intermediate group comprises rocks witb little or no quartz, in which both alkali-felspars and members of the lime-soda series, as well as the felspathoid minerals (nepheline, leucite, etc.), play an important part. In the basic rocks quartz is absent, and the alkali-felspars generally so. Olivine, on the other hand, is frequently present. The pyroxenites and hornblendites constitute a type of basic rock in which both felspar and olivine are absent. Lastly, the most basic rocks consist for the greater part of olivine, together with 1 The Tertiary Igneous Rocks of Skye. Mem. Geol. Surv., 1904, p. 249. 2 Clarke : Analyses of Rocks, U.S. Geol. Surv., Bull. 168 (1900), p. 15 56 PHYSICAL CHARACTERS [PAIIT i smaller quantities of other ferro-magnesian minerals and iron-ores ; when felspar is present it is the basic lime-felspar anorthite. With regard to hornblende and augite, the former has a predilection for the more acid rocks, while the latter favours the basic rocks. The rocks belonging to the acid group are in general of a light colour, owing to the predominance of siliceous and alumino-alkaline minerals. 1 As the basic character increases the colour becomes darker, owing to the greater abundance of ferro-magnesian minerals and iron-ores. 2 The specific gravity also increases with the basicity, being about 2'6 in acid rocks, and reach- ing as much as 3*6 in ultrabasic varieties. The following estimate of the mean mineralogical composition of igneous rocks made by F. W. Clarke, 3 as the result of the statistical examination of some 700 rocks, is of interest : Quartz - - P* 12'0 Felspars - - 59 -5 Hornblende and > , a Q Pyroxene } Mica '". - - 3-8 Accessory Minerals - - 7*9 100-0 1 The word salic has been coined to connote the siliceous and aluminous character : Quantitative Classification of Igneous Rocks ; Chicago, 1903, p. 116. The same idea is expressed by Brogger's term leucocratic, which has reference to the light colour of rocks rich in salic minerals. 2 The word fertile indicates the ferro-magnesian character, and alferric the presence of alumina and ferric oxide in the minerals : ibid. Brogger's term melanocratic has reference to the dark colour of rocks rich iuftmic and a/ferric minerals. 3 F. W. Clarke: The Data of Geochemistry; Washington, 1908. CHAP, iv] COMPOSITION 67 CHANGES IN COMPOSITION SUBSEQUENT TO CONSOLIDATION. Various changes may take place in the igneous rocks subsequent to their consolidation. They may be classified under the following heads : (1) pneumato- lytic changes, produced by the action of gaseous exhalations which accompanied or immediately followed the intrusion of the magma ; (2) those produced by percolating thermal waters (hydro- thermal metamorphism) ; (3) those due to the intrusion of later igneous masses (contact or thermal metamorphism) ; (4) the crushing down and reconstruction of minerals under the influence of earth-movements (dynamic metamorphism) ; (5) and, finalty, atmospheric or meteoric weathering. The changes brought about by the first two and the last, consist in the decomposition of the original minerals, the formation of new minerals from the same materials, the removal or introduction of new material, etc. As far as they involve a change in the material (chemical) composition of the rock, they are known as metasomatic. Pneumatolytic changes are especially evident in volcanic districts, where the gases, discharged even long after the effusion of lava has ceased, cause wide- reaching changes in the rocks they attack. The decomposition of the volcanic rocks by hydrochloric acid and sulphuretted hydrogen, together with steam, in the solfatara near Naples, is a well-known instance. 1 1 Hence the term solfataric activity. 58 PHYSICAL CHARACTERS [PART I The changes wrought by the gaseous emanations of the vapour vents, or fumaroles, of volcanoes whose activity has not long ceased, or which are in a condition of intermittent quiescence, afford another example. The gases emitted through these channels are hydrochloric and sulphurous acid, carbon di- oxide, etc. The} 7 differ according to the stage of volcanic action which has been reached at the time of emission. The effects of pneumatolysis can also be well studied in connection with certain tourmaline-bearing granite intrusions, where highly-heated water vapour, derived doubtless from the same deep-seated source as the granite itself, and containing compounds of boron and fluorine, and probably also metallic constituents, such as lithium, tin, tungsten, and uranium, has apparently modified certain portions of the granite and the sediments in its vicinity. Thus certain belts in the Cornish granites are converted into greisen (an aggregate of quartz and white mica), or into schorl-rock (quartz and tourmaline). The greisen occurs in veins, usually near the contact of the granite with the country rock. It is a completely recrystallized rock, consisting of quartz and white mica. It is assumed that the solvent vapours, which in this rock have effected the replacement of the felspar by quartz and white mica, came up through cracks which had formed in the already- solidified granite. The rock in the immediate neighbourhood of the cracks was modified in the manner described, and thus " veins " of greisen were produced. The rock which gave rise to the schorl- CHAP, iv] COMPOSITION 69 rock was in some cases a porphyritic granite. The felspar crystals, however, whilst retaining their original shape, have had their substance replaced by a granular aggregate of quartz, tourmaline, apatite, and white mica. The felspar and black mica of the ground-mass have also been replaced by tourmaline and quartz. Occasionally pseudomorphs of tinstone and quartz, after felspar, are found. 1 The so-called kaolinisation of granite, by which the felspathic constituent of the rock is converted into an aggregate of secondary minerals, consisting largely of hydrated silicates of alumina, but including also muscovite and quartz, is also ascribed by some to the action of heated vapours (steam, carbon dioxide and hydrofluoric acid) ascending from below through fissures in the granite. 2 Others, however, hold that this decomposition of granite is, in the main, the result of percolation of atmospheric water carrying dissolved carbon dioxide. When completely decomposed, the rock becomes so friable that it can be broken down by a stream of water, and this fact is made use of in the process by which china clay is produced. Interesting pneumatolytic changes- have been observed in the greenstones (dolerites) that are sufficiently near the granite to have been affected by its fluoric and boric emanations. These rocks being rich in lime, this substance is prominent in the composition of the newly-formed minerals. Thus axinite (boro-silicate of lime and alumina) is 1 Reid and Flett : The Geology of the Land's End District, Mem. Geol. Surv.. 1907, p. 49. Rosier: Neues Jahrb. B.B., 1901, p. 231. 2 Flett, loc. cit., p. 58. 60 PHYSICAL CHARACTERS [PART i characteristic of the pneumatolytic veins of the greenstones. Other secondary minerals are fluorspar, epidote, pale-green augite, green hornblende, brown garnet, sphene and occasionally zinc-blende. 1 The same minerals are produced when the rocks affected are calcareous sediments. Hydrothermal metamorphism is closely allied to pneumatolysis both in its origin and effects ; and it is possible, as suggested by Harker, 2 that the circulating waters that bring it about may, in some cases, be of the same volcanic origin as the lavas whose metamor- phism they subsequently effect. On this assumption a portion of the water, occluded in the magma on its eruption, did not escape in the form of vapour, but remained shut up in vesicles and other interstices of the lava. The changes brought about by the restricted circulation of such heated waters must, on this theory, be regarded as a final phase of vulcanicity. The minerals that form the amygdales of the basaltic lavas probably owe their origin to agencies of this nature. Among them may be mentioned the hydrated silicates known as zeolites ; also chlorite, calcite, chalcedony, opal, and quartz. The materials which went to form these minerals were derived from the constituents of the rock, largely from the felspar, but also from the ferro- magnesian and calcareous minerals. In this connection the changes which, as Daubre'e 3 1 Flett, loc. cit., pp. 38 and 39. 2 The Tertiary Igneous Rocks of Skye, Mem. GeoL Surv., 1904, p. 41, 3 Daubre'e : Sur le Metamorphisme, Paris, 1860. CHAP, iv] COMPOSITION 61 showed, have been produced by the alkaline thermal waters of Plombieres in the Vosges, are of interest. The masonry which supported the aqueducts that conveyed the water to the Roman baths, and through which the hot waters had been percolating for centuries, was found to contain various zeolites, calcite, aragonite, fluorspar and opal. These minerals had been formed by the action of the alkaline waters on the calcareous, aluminous and siliceous materials of the masonry. But circulating thermal waters may also be the means of introducing entirely new material from without, by which the composition of a rock may be considerably changed. Thus the percentage. of silica in some of the Peru andesites has been considerably raised, subsequent to their consolidation, by the introduction of chalcedonic and opaline silica. 1 A similar " siHcification " of some of the Rhenish trachytes (Rosenau in the Siebengebirge, and at Berkum near Cologne) was described by Von Lasaulx ; and like phenomena have been observed among the acid volcanic rocks of Caernarvonshire and of Snap. 2 Contact-metamorphism. With regard to changes produced in igneous rocks by the heat due to later intrusions, there is not much information available. Allport 3 has shown that the augitic greenstones (dolerites) within the contact zone of the Cornish 1 Hatch : Tschermak's Min. u. Pet. Mitth., vol. vii., p. 328. 2 Harker and Marr : Quart. Journ. Geol. Soc., vol. xlvii. (1891), p. 303. 3 Quart. Journ. Geol Soc., vol. xxxii. (1876), p. 407. 62 PHYSICAL CHARACTERS [PART I granites, are converted into rocks in which secondary hornblende and biotite are abundant. Newly-formed augite and epidote also occur. 1 Similar phenomena have been observed in the Harz by Lessen ; and the syenites of Meissen are reported to have effected the conversion of the diabasic rocks into hornblende-schists. Marr and Barker 2 have shown that the Borrowdale andesites undergo marked changes in contact with the Shap granite : the metarnorphism involves the formation of brown mica, sphene, etc., in the body of the rock, while in the vesicles green hornblende is produced by the alteration of the chlorite that originally filled them. Interesting changes as a result of contact meta- morphism have been observed by Harker 3 in the Skye basic lavas, where they have been invaded by large gabbro and granite intrusions. The secondary minerals are chiefly a green fibrous hornblende, a brown biotite, and some epidote. The hornblende is derived from the augite of the original rock, while the biotite comes from chloritic and other alteration products. The felspars are little changed in the least metamorphosed rocks, but in the most altered types they are completely recrystallized. Of especial interest are the changes observed in the amygdales, for here the lime and lime-soda zeolites which filled the vesicular cavities of the lavas, are found to be 1 The Geology of the Land's End District, Mem. Geol. Surv., 1907, p. 37. 2 Quart. Journ. Geol. Soc., vol. xlvii. (1891), p. 266. 3 The Tertiary Igneous Rocks of Skye, loc. cit. , p. 51 CHAP, iv] COMPOSITION 63 converted into lime-soda felspars. In these cases the chemical change brought about by contact meta- morphism involved little more than the expulsion of the water ; this process being the reversal of that by which the zeoites were derived from the felspars of the original lava under the influence of hydrothermal metamorphism. Closely allied to the phenomena of contact meta- morphism are the changes brought about in an invaded igneous rock by the injection of the invading magma. An example of this has been described by Professor Sollas at Barnavave, Carlingford district in Ireland, 1 where a gabbro, in which contraction- joints and fractures, due to earth movements, had been opened, has been intimately penetrated by the fluid magma of a granophyre, so as to produce a new rock of intermediate character. The consideration of variations in the composition of a magma before consolidation, does not properly belong here; but it is convenient, while dealing with the subject of contact alteration, to refer to the changes that may be brought about in an invading magma by the absorption of fragments of the in- vaded rock. This type of contact alteration is well illustrated by the Carrock Fell rocks described by Barker. 2 In this case a heterogenetic or hybrid rock has been produced by the incorporation in a granophyre magma of highly basic portions of a gabbro. The same author has described similar 1 Trans. Roy. Irish Acad., xxx. (1894), p. 479. 52 Quart. Journ. Geol. Soc., vol. li. (1895), p. 136. 64 PHYSICAL CHARACTERS [PART i phenomena in Skye. 1 Zirkel 2 also considers the zircons and sapphires that occur in the basalt of Finkenberg, near Bonn on the Rhine, as xenocrysts (see page 28) derived from included rock frag- ments which, with the exception of these minerals, have been completely absorbed by the basalt. Possibly the kimberlite of the diamond pipes of South Africa, the origin of which has been so much discussed, may have been produced in a similar manner. 3 In this hybrid rock there are, besides xenoliths of a great variety of rocks, xeno- crysts of numerous minerals, among the most important of which are garnet, chrome-diopside, kyanite, graphite, and diamond. The subject of the dynamic metamorphism of the igneous rocks i.e., of the changes produced as a result of mechanical stress is too wide-reaching to be adequately discussed here. It must suffice to allude to some of the commoner modifications. Ir. many cases these are mainly structural ; such as granulation 4 of the constituents (e.g., quartz), their iThe Tertiary Igneous Rocks of Skye, loc. cit. Harker describes on p. 192 a hybrid rock (marscoite) produced by the absorption of granitic material by a basic magma. 2 Abh. math, phys., kgl. Gesettsch. der Wissensch., vol. xxxviii. (1903). See also Lehrbuch der Petrogr., 2nd edit., 1893, p. 593. 3 See A. du Toit : Eleventh Ann. Rep. of the Cape Geol. Comm. (1906), p. 151. 4 Granulation of this type is termed cataclastic in contra- distinction to the protodastic granulation produced by the breaking down of the earlier formed minerals, in a partially consolidated magma, under the influence of differential move- ment or "flow." :J* 1-63 Anatase . 2-52 Melilite ; *: 1-63 Perovskite . . 2-38 Actinolite . j 1-62 Chromite . . 2-10 Calcite v . . ' *'. 1-60 Zirkon . 1-95 Muscovite . . 1-59 Sphene . 1-93 Talc . '. ". 1-59 Garnet . 1-81 Anorthite . *V 1-58 Mgirine . T79 Chlorite . , 1-58 Epidote . 1-72 Serpentine . . 1'57 Kyanite . 1-72 Biotite . . 1-56 Spinel . . 1-72 Kaolinite . 1-54 Zoisite . . 1-70 Quartz V . 1-55 Diopside . . 1-68 Oligoclase . . 1-54 Olivine . 1-68 Nepheline . . 1-54 Axinite . 1-68 Cordierite . . 1-54 Enstatite . 1-67 Albite . . 1-53 Hornblende . . 1-64 Orthoclase . . 1-52 Andalusite . . 1-64 Sodalite . . 1-48 Apatite . 1-63 Fluorspar . . 1-43 THE INDEX OF REFRACTION OF VARIOUS MEDIUMS USED IN THE DETERMINATION OF THE REFRACTION OF MINERALS. Water * \ . 1-333 Monobrombenzol . -561 Olive oil V: . 1*469 Bromoform . . -588 Beech-nut oil . 1'477 Cinnamon oil . . -605 Cedar oil . . 1-516 Monochlornaphthalene -635 Canada balsam . 1-540 Monobromnaphthalene -655 Clove oil . . 1-542 Methylene iodide . -740 CHAP, n] UNDER THE MICROSCOPE 75 INTERNAL OR MICRO -STRUCTURE. The principal variations in the micro-structure of the rock- forming minerals in thin section and mounted in Canada balsam, are those produced by surface-relief, by cleavage and by foreign inclusions. Surface-relief. The unevenness of the surface of a mineral ground to a thin section, is only notice- able when there is a considerable difference between the index of refraction of the mineral and that of the medium in which it is mounted. Minerals with a high index of refraction, like olivine and sphene, for instance, have a dappled or mottled appearance, that is absent from minerals like quartz and felspar of which the index of refraction approximates to that of Canada balsam. It is to be noted that the apparent relief of calcite, which has indices of refraction lower and higher than Canada balsam, changes when the section is rotated in polarised light. Cleavage. The lines of division of a mineral show greater or less distinctness, precision and regularity, according to the degree of perfection of the cleavage. A perfect cleavage gives numerous fine straight lines, while an imperfect one gives lines which are branching and irregular in proportion to the difficulty of separa- tion. The distinctness and thickness of the cracks depend also on the difference between the index of refraction of the mineral and that of the Canada balsam in which it is mounted. Thus the cracks in quartz and the cleavages in felspar are much less noticeable than those in olivine, augite or hornblende. The relative position of cleavage lines depends on the 70 CONSTITUENT MINERALS [PART n direction of the section through the mineral. A prismatic cleavage produces two intersecting sets of parallel Hues, unless the section be in the direction of the vertical axis, in which case the two sets coincide. A basal or pinacoidal cleavage furnishes only one set of lines. Inclusions. During their formation, crystals take up and inclose portions of the matrix from which they have separated, and other foreign bodies. These FIG. 27. Highly magnified Liquid Inclusions in Quartz. (After Fouqut and Lfoy.) inclusions may bear a definite crystallographic rela- tion, in their arrangement, to the shape of the crystal (i.e., they may be central, peripheral, or zonal\ or they may be distributed without definite arrange- ment. They are of four kinds : (1) gaseous, (2) liquid, (3) glassy and (4>} mineral. Gas-inclusions CHAP. Il] UNDER THE MICROSCOPE 77 are small, round, or elliptical cavities filled with air or carbon dioxide. Occasionally they have the inverse shape of their host, being, in fact, negative crystals. In consequence of the difference in re- fractive index between the gas and the mineral sub- stance inclosing it, gas-inclusions are always bounded by a weM-marked dark border. Liquid-inclusions are also round, elliptical, or tubular bodies, having a contour somewhat less strongly marked than that of FIG. 28. Crystal of Augite, containing a vitreous core. gas-inclusions. The liquid, which may consist of water or carbon dioxide under pressure, usually does not quite fill the cavity, and a small movable bubble remains. When sufficiently small, the bubble possesses a constant vibratile motion. The water is sometimes saturated with common salt, and may then contain cubic crystals of that mineral. Liquid- inclusions occur abundantly in many minerals, not- 78 CONSTITUENT MINERALS [PART n ably in quartz. (See Fig. 27.) Glass-inclusions are distinguished from liquid -inclusions by the sharpness of their contours and the fixity of their bubble ; also by the fact that more than one bubble is fre- quently present. The inclosed glassy substance is often coloured or turbid, being related in this re- spect to the glassy base of the rock. (See Fig. 28.) FIG. 29. Glass Inclusions with bubbles and crystallites, highly magnified. (After Fouque and Ltvy. ) Devitrification, or the development of crystallites and microlites, may also have taken place. Mineral- inclusions are distinguished from the foregoing by the possession of idiomorphic contours. They may consist of needles, rods, granules, plates or scales, and are often arranged along planes bearing a definite relation to the symmetry of the crystal in which CHAP, n] UNDER THE MICROSCOPE 79 they are included, being then often the cause of the optical phenomena known as schiller, chatoyancy and asterism. DOUBLE REFRACTION. All doubly refractive minerals are coloured between crossed nicols, the colouration being a result of the interference of the two rays that are produced by the passage of light through such minerals. The degree of colouration (i.e., in Newton's scale of colours) depends on the amount of separation of the two rays, and this varies with the thickness of the section for an}- given mineral, or, the thickness being constant, with the nature of the mineral experimented upon. Thus, in sections of such thickness that the felspar gives a grey between crossed nicols, quartz will give a yellow of the first order, and augite and hornblende, bright blues, greens and reds of the second and third orders. The following figures, representing the maximum birefringence of some well-known minerals, form a useful scale for the comparison of the double refraction colours in thin sections of a uniform thickness. 1 1 It is beyond the scope of this book to deal with the optical determination of minerals under the microscope. For informa- tion on this subject reference may be made to Kock Minerals, by J. P. Iddings, New York, 1906; to Minerals in Rock Sections, by Luquer; to Anleitung zur Gebrauch des Polarisationsmikroskop, by Dr. E. Weinschenk, Freiburg im Breisgau, 1906 ; or to Kurze Anleitung zur mikrosk. Krystalbestimmung, by Schroeder van der Kolk, Wiesbaden, 1898. Messrs. Swift & Sons of London supply a microscope, suitable for petrological work, at a moderate price. 80 CONSTITUENT MINERALS [PART u THE MAXIMUM BIREFRINGENCE OF SOME ROCK- FORMING MINERALS. Rutile 287] Magnesite 202 Dolomite 179 Oalcite 177 Aragonite 155 Very strong. Sphene 091 Zircon 062 Talc 050 Muscovite 043 \ Epidote 038 < Olivine 035 Orthite 032 Actinolite 027 Augite 025 Strong. Sillimanite 020 Diopside 018 Hornblende 016 Wollastonite 015 , Hypersthene. 0121 Andalusite Oil Gypsum 010 > Moderate. Quartz 009 Orthoclase 008 , Zoisite 006 ^ Nepheline 005 Apatite 003 ^Weak. Leucite 001 Pennine (Chlorite) 001 J CHAPTER III. DESCRIPTION OF THE ROCK-FORMING MINERALS. SILICA GROUP. Quartz. Pure silica, SiO 2 . Crystallizes in the rhombohedral system, the crystals being usually bounded by the hexagonal prism (recognisable by its horizontal striations) with pyramidal (rhombohedral) terminations. Sometimes, however, the prism faces are absent, the crystals then consisting of the double pyramid of twelve faces. Unattacked by acids, excepting hydrofluoric acid. Insoluble in potash (distinction from opaline silica). Sp. G. = 2'6. Hard- ness = 7. 1 Index of refraction, T551. Double refraction, moderate. Between crossed nicols the polarization- colours vary, in moderately thin sections, between a light grey and pale yellow of the first order of 1 Mobs' scale, viz., l = talc ; 2 = gypsum ; 3 = calcite ; 4 = fluorspar, 5 = apatite, 6 = orthoclase, 7 = quartz, 8 = topaz, 9 = corundum, 10 = diamond. If the hardness of corundum be represented by 1,000, then, according to Jaggar, the hardness of the other minerals composing Mohs' scale may be expressed by the following figures : topaz, 152 ; quartz, 40 ; felspar, 25 ; apatite, 1'23 ; fluorite, '75 ; calcite, '26 ; gypsum, '04. 81 F 82 CONSTITUENT MINERALS [PART n Newton's scale of colours. Thicker sections, of course, show brighter colours (yellow, red, blue, etc.). Sections perpendicular to the vertical (here coincident with the optic) axis give a uniaxial interference-figure. Quartz may be recognised by its hardness, pellu- cidity, vitreous lustre and irregular to sub-conchoidal fracture. Usually it is colourless, sometimes, how- ever, brownish or yellowish (smoky quartz, cairngorm, FIG. 30. Secondary quartz-aggregate ; between crossed nicols. {After Lindgren.} etc.), pink (rose quartz), purple (amethyst) or bluish (in certain gneisses and in some Palaeozoic and pre- Cambrian grits). Under the microscope it appears in clear, colourless grains, characterised by the presence of innumerable minute liquid-inclusions generally arranged along lines and planes. High inagnifica- CHAP, in] DESCRIPTION OF MINERALS 83 tion discloses in many of these a minute vibratile bubble, occasionally also cubes of salt, showing that the liquid, in some cases at least, consists of a saturated solution of that substance. In other cases, the enclosed substance has been proved to be carbon dioxide. Quartz is the mineral eminently characteristic of the acid rocks. In granites and tonalites it occurs in irregular grains without definite crystalline con- tours ; and in microgranites, felsites and rhyolites, in rudely-contoured, often bi pyramidal crystals, which are, however, generally corroded by the surrounding magma. As a secondary constituent, quartz fills cracks, joints and cavities (vein-quartz) ; similarly it occurs in association with felspar, mica, tourmaline, etc., in the so-called pegmatite-veins. Tridymite. Pure silica, SiO 2 . Triclinic, but affect- ing hexagonal symmetry. In thin, six-sided, colourless tablets, which split up between crossed nicols into a number of depolarizing areas, showing light and dark banding. Insoluble in acids ; soluble in alkalies. Sp. G. = 2'3. Hardness = 7. Index of refraction, 1/477. Tridymite occurs in volcanic rocks (rhyolites, trachytes and andesites), lining their cavities with pretty groups of interpenetrating twins and trins : a well-known occurrence is in the rhyolite of Tardree, Antrim. Under the microscope it is described as occurring in imbricated aggregates. Tridymite is only found in lavas, and appears to have been produced under the influence of some mineralizing agent. Opal. Hydrated silica : SiO 2 + Aq. This mineral occurs in amorphous masses resulting from the solid i- 84 CONSTITUENT MINERALS [PART n fication of colloid or bydrated silica. Colour variable : yellow, brown, etc. Translucent to opaque. Soluble in alkalies (distinction from quartz). Sp. G. = 2 '2. Hardness = 5'5-6. Index of refraction = T45. Opal is a secondary constituent of many acid volcanic rocks (rhy elites, trachytes, and even the less acid andesites). It is also deposited from the FIG. 31. Spherulitic Opal in a chalcedonic matrix, between crossed nicols. (After Fouqut and Llvy.) water of hot springs and geysers (yeyserite). Hyalite, a pure colourless variety, frequently occurs in andesites. Between crossed nicols opal is usually isotropic, but occasionally shows weak double re- fraction. A black cross between crossed nicols is observable in spherulitic opal. (See. Fig. 31.) Chalcedony. Silica, SiO 2 . Mixtures of crystal- CHAP, m] DESCRIPTION OF MINERALS 85 line and amorphous silica. Cream-coloured to colour- less ; translucent to semi-opaque. Insoluble in acids ; partially soluble in alkalies. Sp. G. = 2'59-2'64. A common secondary constituent of igneous rocks. Under the microscope chalcedony has a fibrous structure, giving an imperfect black cross between crossed nicols. Agate consists of alternating layers of chalcedony, jasper, amethyst and other varieties of silica. It is found filling wholly, or partially, the amygdaloidal cavities of old vesicular lavas (melaphyre, basalt, etc.). FELSPAR GROUP. The felspars crystallize in two systems, the mono- clinic and the triclinic ; but the forms of the two systems are very similar, and the angles between analogous faces differ by a few degrees only. The chief monodinic felspars are orthoclase and sanidine. Triclinic are microcline, albite, oligoclase, andesine, labradorite, bytownite -and anorthite. The felspars of the albite-anorthite series are comprised under the general term plagioclase. The commonest combination of faces presented by the felspars consists of the following forms : the basal plane (generally denoted by P), the clino- or the brachy-pinacoid (M), the prisms (T and 1), the two ortho- or the two macro- domes (x and y), the clino- or the brachy-dome (n), and the pyramid (o) (Fig. 32). The principal cleavage is basal (P) ; but this is nearly equalled by the cleavage parallel to M. These CONSTITUENT MINERALS [PART n two cleavage planes are at right angles in orthoclase, but are inclined to one another at less than a right angle in the triclinic felspars. Under the microscope orthoclase presents sections that are uniformly affected by polarized light, or are divided into two areas that present uniform polarization respectively (dual M FIG. 32. Crystal of Orthoclase. P. Basal plane. M. Ciinopinacoid. /. Prism. x and y. Orthodomes. 0- Pyramid. n. A clinodome. (After Rosenbusch.) twinning on the Carlsbad type) ; whereas plagioclase usually has a striated appearance between crossed nicols, due to the different angles of extinction of alternating twin lamellae (polysynthetic twinning on the albite type). 1. ALKALI-FELSPARS. Orthoclase, or potash-felspar. Chem. comp.= K 2 0, A1 2 3 , 6SiO a (65% SiO a , 17% K 2 O and 18% CHAP, in] DESCRIPTION OF MINERALS 87 A1 2 3 ). Monoclinic. Sp. G. = 2'54-2'57. Hardness - 6. Index of refraction, 1'524; double refraction, moderate; it is distinctly less than quartz, sections of ordinary thickness polarizing in grey colours between crossed nicols. Unattacked by acids except hydrofluoric acid. Two types of crystal are common : flat tables (with M largely developed) and stout prisms. A roof -like M FIG, 33. Orthoclase Crystal twinned on the Carlsbad-type, p. Basal plane. M. Clinopinacoid. /. Prism. x. An orthodome. (After Rosenbusch.} termination is produced by the basal plane (P) and the orthodome (x\ which are about equally inclined to the vertical axis, and form with one another an angle of 129 43'. (See Fig. 32.) The crystals pre- sent dual twinning on various types (Carlsbad, Manebach, Baveno, etc.), the commonest being the Carlsbad. When twinned on this type, the crystals are usually united on the clinopinacoid (M\ the 88 CONSTITUENT MINERALS [PART n basal planes (P) of the crystals being inclined in opposite directions. (See Fig. 33.) Twinning of this nature is often revealed in thin sections by the difference in extinction shown by the two halves of a crystal when rotated between crossed nicols. In the Manebach type the twinning-plane is the basal plane (P) ; in the Baveno type it is the clinodome (n). FIG. 34. Crystal of Orfrhoclase twinned on the Baveno-type. p. Basal plane. M. Clinopinacoid. /. Prism. y. An orthodome. (After Rosetibusch.} Orthoclase occurs pure and colourless in the variety termed adularia. Usually, however, it is turbid and cloudy, due to the development of various de- composition products. This decomposition of felspar, which is loosely spoken of as kaolinization, consists in the formation of hydrated silicates of alumina, with liberation of free silica, and removal of the alkalies. Among the secondary minerals thus pro- duced the sericite variety of mica is easily recognised ; CHAP, in] DESCRIPTION OF MINERALS 89 the presence of kaolinite is not so easily demonstrable, although there can be little doubt that it is also formed. Orthoclase occurs as a constituent of the more acid plutonic and older volcanic rocks (granite, syenite, felsite), often in large well-developed crystals (granite- FIG. 35. Orthoclase Felspar, with decomposition products. o. Orthoclase. q. Quartz. m. Secondary muscovite. k. Kaolin. porphyry, felspar-porphyry) ; also in foliated rocks (gneiss and various schists); and, together with quartz and other minerals, in segregation veins (pegmatites). Among the sedimentary rocks it occurs in the so-called felspathic grits or arkoses, which are derived from the disintegration of granitic rocks. 90 CONSTITUENT MINERALS [PAKT n Sanidine is the name given to a clear, glassy, fissured variety of orthoclase occurring in the more recent acid lavas (rhyolite, trachyte, obsidian). It usually occurs in flat tables, twinned on the Carlsbad type ; also in prismatic crystals and irregular grains. Under the microscope it is distinguished from ortho- clase by its pellucMity. Inclusions (glass, etc.) are, however, common. The crystals have often under- FIG. 36. Section of microcline, under the microscope. gone fracturing and corrosion during the eruption of the lava in which they were suspended. Microcline is a triclinic potash-felspar, with pseudo - monoclinic symmetry, being very nearly allied in external form to orthoclase. The angle formed by the basal and the clinopinacoidal cleavage planes is very nearly, but not quite 90 (actually CHAP, in] DESCRIPTION OF MINERALS 89 50'). It is distinguishable from orthoclase, however, by a characteristic cross-hatching that becomes visible when the mineral is viewed between crossed nicols. This structure is generally ascribed to intersecting systems of twin-lamellre, parallel and perpendicular to the edge PM\ it is charac- terised, however, by the fact that the lamellae are spindle-shaped, instead of being bounded by FIG. 37. Perthite : section parallel to the clinopinacoicU The extinction measured to the parallel edges. (See Fig. 36.) angle of microcline is 15 30' edge PM. Perthite. Both orthoclase and microcline are often intergrown with a soda-felspar (albite or oligoclase). The latter occurs in narrow lamellae intercalated along planes parallel to the orthopinacoid. In sections 92 CONSTITUENT MINERALS [PART n parallel to the basal plane or clinopinacoid the in- cluded felspar appears in patches and strips, which are distinguishable from the main mass of the orthoclase or microcline by their twin-striation or by their different extinction-angle. Such admix- tures are known as perthite, micro-perthite, or microcline micro-perthite. FIG. 38. Perthite : section parallel to the basal plane. Considered chemically, the felspars are silicates of alumina and one or more of the bases potash, soda and lime. There are three primary felspar molecules : potash-felspar J(K 2 0, A1 2 3 , 6Si0 2 ) = Or; soda-felspar KNa 2 0, A1 2 3 , 6Si0 2 ) = Ab; and lime-felspar (CaO, A1 2 3 , 2Si0 2 ) = An. They occur in nature as ortho- clase, albite and anorthite respectively. By their combination in definite proportions two important CHAP, in] DESCRIPTION OF MINERALS 93 series of felspars are formed : a lime-soda series and a potash-soda series. The lime-soda series comprises the well-known members : oligoclase, andesine, bytownite and labradorite. The potash-soda series is represented by felspars that have been variously described as perthite or micro-perthite, microcttne micro-perthite, soda-orthoclase or crypto-perthite, and soda-microcline or anorthoclase. Soda-orthoclase (the natron-orthoklas of Forstner) is apparently a homo- geneous mineral consisting of two orthoclase mole- cules in combination with three albite molecules, whereas micro-perthite, which has the same chemical composition, is a mechanical mixture of the two different felspars in intimate intergrowth (See p. 38) Brb'gger, however, regards soda-orthoclase as crypto- perthite, that is to say, as ultra-microscopic perthite, and ascribes its peculiar blue schiller to internal reflection from the minute planes of fracture between alternating fine lamellae of albite and orthoclase parallel to the orthodome (y). The crystals have an unusual form, due to the predominance of the prisms (T and I) and the orthodome (y). Anorthoclase Brogger regards as a soda-microcline, and like that mineral it is referred to the triclinic system. In the same manner as he regards soda-orthoclase as crypto- perthite, so he regards soda-microcline as a crypto- perthitic modification of microcline, that is, an ultra- microscopic microcline-perthite, in which the albite lamellae are too minute for recognition under the microscope. 94 CONSTITUENT MINERALS [PART u 2. LIME-SODA FELSPARS. The lime-soda felspars are collectively known as plagioclase. They crystallize in the triclinic system. The crystal form is similar to that of orthoclase, with the difference that the basal plane (P) instead of being normal to the brachypinacoid (M) is inclined to it (from left to right) at an angle of 86-87. The distinguishing character of the triclinic felspars is the lamellar or polysyn- thetic twinning. The fine lineations produced by FIG. 39. Crystal of Plagioclase twinned on the Albite-type. P. Basal plane. M. Brachypinacoid. T. Prism. x. A macrodome. {After Rosenbusch.} M FIG. 40. Repeated twinning on the Albite- type. p. Basal plane. M. Brachypinacoid. (After Rosenbusch. ) this type of twinning can be easily seen with the pocket lens on the faces of the basal cleavage. In thin section, between crossed nicols, the twinning appears as a parallel striation, consisting of light and dark bands, due to the simultaneous extinction of alternate lamellae. Usually the striation is parallel CHAP, m] DESCRIPTION OF MINERALS 95 to the bracbypinacoid (M) : this constitutes the albite type of twinning. (See Figs. 39 and 40.) A second series of twin-lamellae sometimes .crosses these at right angles, constituting the pericline type of twinning. The researches of Tschermak, and of Day and Allen, 1 have shown that the members of the lime- soda series may be regarded as isomorphous mixtures of albite and anorthite in varying proportions. They FIG. 41. Section of twhwied Plagioclase between crossed nicols. form a graduated series both with regard to their chemical constitution and their optical and other physical properties (specific gravitjr, etc.). It is con- venient to make use of the names oligoclase, ande- sine, bytownite and labradorite, to fix certain points in the series. If we represent the soda-felspar mole- cule K^a-A A1 2 3 , 6Si0 2 ) by Ab, and the lime-felspar molecule (CaO, A1 2 O 3 , 2SiO 2 ) by An, then the inter- 1 Amer. Journ. Set., 1905, p. 93. 96 CONSTITUENT MINERALS [PART n mediate felspars have the composition represented by the following average formulas and percentages : * Name. Formula. Percentage Composition. Si0 2 A1 2 3 Nn a O CaO Albite Ab 68*7 19*5 11*8 Oligoclase Andesine AUA"; 62-0 60*2 24-0 25*2 8-7 7*9 5-3 6-7 Labradorite A^Anj 55*6 28-3 5*7 10*4 do. Ab 1 An 9 51*7 309 4-0 13*4 Bytownite Anorthite An' 47-4 43*2 33*8 36-7 2-0 16*8 201 With regard to the optical properties of the plagio- clases, the extinction angles are smaller for the soda end of the series (albite, oligoclase and andesine) than for the lime end (labradorite and anorthite). In general, a high extinction-angle may be taken as indicating a felspar of the basic (or lime) end of the series ; while a low extinction-angle indicates a felspar of the albite, oligoclase, or andesine type. Microlites consisting of a felspar intermediate in composition between oligoclase and andesine, extinguish straight. The double refraction is moderate. The specific gravity and optical properties of the different members of the series are summarised in the following table: 1 Since the rate of the molecular weight of Ab to An is as 1:1*061, it is approximately accurate to calculate the relative weight of the two felspar substances in the proportion of the number of molecules: thus Ab 3 An 2 = f by weight of Albite to f Anorthite. CHAP, in] DESCRIPTION OF MINERALS 97 Name Formula Sp. G.i Mean Index of Refraction Extinction- angles in sections normal to (010)(001) : Albite .Ab 2-605 1-535 -15 Oligoclase AbgAiii 2649 1-545 + 5 Andesine Ab 2 \n a 2-660 1-549 140 Labrador! te AbjAi^ 2-679 1-558 28 do. AbjAn 2 2-710 1-567 37 Bytownite A^AOg 2-733 1-577 41 Anorthite An 2-765 1-585 43 The plagioclase felspars usually contain numerous inclusions : liquid, glass and mineral matter (haematite, rutile, mica, etc.). These may be irregularly dis- tributed, or arranged along certain crystallographic planes, or in zones parallel to the periphery of the crystal. A different kind of zonary structure is that produced by the presence of felspar layers, marking a gradual change of composition in the crystal during its growth. The chemical analysis of a felspar is therefore not always made on homogeneous material, as the outermost layers of a crystal may have quite a different composition to the central core. The following minerals result from, the decom- position of basic plagioclases : epidote, zoisite, albite (in the so-called saussurite) and calcite. Albite, oligoclase and andesine are not attacked by hydrochloric acid ; labradorite is to a small extent ; while anorthite is decomposed with separation of gelatinous silica. The fusibility is greatest in albite, 1 From determinations made by Day on artificially prepared felspars. G 98 CONSTITUENT MINERALS [PART n and decreases with the diminution of soda and the increase of lime. The specific gravity, on the other hand, is least at the soda end and greatest at the lime end (see table on page 97). Just as orthoclase is characteristic of the acid rocks, so the different plagioclase felspars are found in rocks of intermediate and basic composition: oligoclase and andesine occur in the diorites, por- phyrites and andesites ; labradorite in gabbros, dolerites and basalts ; and anorthite in allivalite, eucrite, etc. Oligoclase also accompanies orthoclase in many granites and trachytes. Albite is frequently present as a secondary constituent (" secondary felspar ") in the so-called felspar- quartz mosaic of mechanically metamorphosed rocks. Its clear un- striated granules are scarcely distinguishable from quartz, but for the fact that they give a biaxial interference-figure in convergent polarized light. THE FELSPATHOID GROUP. Nepheline. Silicate of soda, potash and alumina: (NaK) 2 0, A1 2 O 3 , 2SiO 2 . Pure sodium-nepheline which has been prepared artificially has the composition: SiO 2 = 42-3 AI 2 O 3 = 35-9, Na 2 O - 21'8, but in nepheline as it occurs naturally the proportion of Na 2 O to K 2 O is usually about 5:1. Crystallizes in the hexagonal system. Occurs in small colourless and glassy six- sided prisms (combination of hexagonal prism and basal plane). Soluble in hydrochloric acid with sep- aration of gelatinous silica ; the solution giving cubes of common salt when evaporated. Sp. G. = 2'6 ; hard- CHAP, m] DESCRIPTION OF MINERALS 99 ness, 5'5 6. Index of refraction, T54. Imperfect basal and prismatic cleavage. Nepheline is found in the cavities of ejected blocks (Monte Somma, Lake Laach) ; as an essential constituent of certain lavas (phonolite, nepheline-basalt, tephrite, etc.) ; and as a constituent of the soda series of the plutonic rocks (syenites and alkali-gabbros). Under the microscope it presents hexagonal and four- sided sections ; but occurs more frequently in small colourless granules, the presence of which can often only be demonstrated by gelatinization with hydro- chloric acid and subsequent staining with fuchsine. The four-sided sections extinguish straight between crossed nicols ; while the six-sided ones give a uni- axial interference figure in convergent light. The double refraction is weak, the polarization colour never exceeding a grey tint in sections of normal thickness. It lies between apatite and zoisite in this respect. This mineral is peculiarly subject to alteration, usually to zeolitic products, such as natrolite and hydro-nephelinite ; also to white mica (gieseckite) or to cancrinite. Nepheline is liable to be confused with apatite. The latter mineral, however, occurs usually in longer needles than nepheline, and has a higher index of refraction. Chemically, apatite may be distinguished by the phosphate-test from nepheline, and nepheline by gelatinization and staining with fuchsine from apatite, Elseolite is the name sometimes given to a dull grey or greenish variety of nepheline. It forms irregular masses of considerable size, somewhat re- 100 CONSTITUENT MINERALS [PART 11 sembling quartz, from which it may be distin- guished by its characteristic greasy lustre and brownish colour. Leucite. Silicate of alumina and potash : K 2 O, A1 2 O 3 , 4SiO 2 = Si0 2 , 55'0 ; A1 2 O 8 23'5 ; K 2 O, 215. Regular, the usual form being that of the icosite- trahedron. Colour, dirty white or grey. Attacked slowly by hydrochloric acid with separation of silica. Sp. G. = 2'45 - 2-5. Hardness = 5'5 - 6. Index of re- fraction, 1-508. Fusible with difficulty. Leucite occurs as a constituent of the more recent volcanic rocks : leucitophyre, leucito-tephrite and leucitite ; also in members of the alkali series of the plutonic rocks. FIG. 42. Sections of Leucite, showing the double refraction and inclusions. Under the microscope it presents colourless polyhe- dral (usually eight-sided) sections. Weak double refraction, due to the mineral being in a state of strain : the larger individuals present intersecting systems of alternately light and dark twin-lamellae. When heated to 500 C. leucite becomes normally isotropic. Inclusions (glass, magnetite, belonites, etc.) are common, showing both a zonal and a radial arrangement. CHAP, in] DESCRIPTION OF MINERALS Wl Remarkable pseudomorphs of orthoclase and neph- eline after leucite have been described by Hussak, Derby, J. F. Williams, Teall, Shand, and Knight. 1 These pseudo-leucites, as they are called, whilst main- taining more or less the icositetrahedral form of leucite, have had their leucite substance replaced by an aggregate of orthoclase and nepheline (see Fig. 43). FIG. 43. Crystals of Pseudo-leucite. From Spotted Fawn Creek, Yukon Territory, Canada. (After C. W. Knight}. The crystals are pseudomorphs after leucite, and are composed of the following minerals : orthoclase, scapolite, nepheline, biotite, and a little plagioclase. The latter mineral is sometimes only represented by a decomposition product. Other minerals, such as biot- ite, augite, hornblende, sphene, etc., are probably inclu- sions from the original magma in which the leucite 1 Hussak : Neues. Jahrb., 1887, vol. ii., p. 255, and 1890, vol. i., p. 166 ; Derby : Quart. Journ. Geol. Soc., vol. xlvii (1891), p 251 ; J. F. Williams : Amer. Journ. Sci., vol. i. (1895), p. 194 ; Teall : Geological Structure North- West High- lands, Mem. Geol. Surv., 1907, p. 443 ; Shand : Neues. Jahrb., xxii., 1906, p. 413 ; Knight : Amer. Journ. Sci., vol. xxi., 1906, p. 286. CONSTITUENT MINERALS [PART n crystals were formed. Derby suggests that the crystals of leucite rich in inclusions which were first formed, were subsequently altered to an aggregate of orthoclase and nepheline by some magmatic change before final consolidation took place. Melilite (Humboldtilite). Silicate of alumina, iron, lime, magnesia, and soda : 12(CaMg)O, 2(AlFe) 2 O 3 , 9SiO 2 . Tetragonal ; occurring in small square tablets FIG. 44. Melilite in the lava of Capo di Bove, near Rome. I FIG. 45. Peg-structure in Melilite, highly magnified. and prisms, also in irregular grains. Colour, white to yellow. Gelatinizes easily with hydrochloric acid. Sp. G. = 2-90-2-95. Hardness = 5-5*5. Index of re- fraction, 1*629 ; double refraction, weak the same as nepheline the interference colour in thin section being commonly an indigo-blue. CHAP, m] DESCRIPTION OF MINERALS 103 Under the microscope melilite appears in lath- shaped sections, which are characterized by a striation normal to the long axis (peg structure, see Fig. 45). Occurs as a constituent of certain basalts (melilite- basalt) and nepheline- and leucite-rocks. Sodalite. Silicate of alumina and soda, + sodium chloride: 3(Na O, Al 2 O a , 2SiO 2 ) + 2 NaCl with SiO 2 = 37'2, Al 2 O,=3lV Na 2 O = 25'6, 01 = 7-3 per cent, less 170~ for 01. Regular. Sp. G. = 2'l-2'3. Hardness = 5'5. Index of refraction, 1*484; isotropic. Gelatinizes easily with hydrochloric acid. Occurs in blue, greenish, or colourless grains in syenites and in volcanic ejectamenta. Under the microscope it presents colourless or faintly blue sections, which remain dark when rotated between crossed nicols. Numerous inclusions. Hauyne and Nosean. Isomorphous silicates of alumina, lime, and soda, together with sulphate of soda. The soda end of the isoraorphous series with little or no lime is nosean = 3(Na 2 O, Al O 3 , 2Si0 2 ) + 2 Na 2 SO 4 , with SiO 2 = 31-7, SO 8 = 141, A1 2 O 3 = 26-9, and Na 2 = 27'3. When Na 2 : Ca = 3: 2, the composition of haiiyne is SiO 2 = 32'0, SO 3 = 14'2, A1 2 O 3 = 27'2, CaO = 100, Na 2 O = 16'6. Regular. Sp. G. = 227- 2'50. Hardness = 5-5'5. Index of refraction, T496 ; isotropic. Gelatinize easily with hydrochloric acid ; on evaporation, needles of gypsum are formed in the case of haiiyne, none in the case of nosean. The minerals are very similar in composition and appear- ance. Hauyne, however, is generally blue, and nosean colourless or yellow. Under the microscope these minerals present square or six-sided sections, which 104 CONSTITUENT MINERALS [PART n remain dark when rotated between crossed nicols. The sections are characterized by the presence of numerous minute inclusions aggregated to a marginal or zonal dark band, or accumulated in the centre as a grey shaded patch. In other cases they are arranged FIG. 46. Crystals of Haiiyne, showing the mode of arrangement of the inclusions. (After Roscnbusch. ) along axes intersecting in the centre at 60. (See Fig. 46.) They are essentially volcanic minerals, occurring as sporadic grains in phonolites, andesites and basalt; also in volcanic ejectamenta (Vesuvius, Lake Laach in the Eifel). AMPHIBOLE GROUP. The amphiboles are silicates mainly of magnesia, lime and iron ; other bases occurring in some varieties are alumina, manganese and soda. They are nearly all monoclinic, only one variety being rhombic (anthophyllite) and one triclinic (cossyrite). Sp. G. = 2-90-3-55. Hardness = 5-6. Unattacked by acids. Cleavage, parallel to the prism (as in Fig. 48), well-marked. The angle made by the prism CHAP, m] DESCRIPTION OF MINERALS 105 faces is about 124 (see Fig. 47), which constitutes a useful distinction from augite, in which the prism faces intersect at an angle of about 87 3 . FIG. 47- Back view of a Crystal of Hornblende. p. Orthodome. I m. Prism. r. Clinodome. I b. Clinopinacoid. Under the microscope, sections perpendicular to the vertical axis are generally six-sided (being bounded by two clinopinacoidal and four prismatic FIG. 48. Cross-section of Hornblende, showing the prismatic cleavage. planes), with cleavage-lines intersecting at an angle of 124. (See Figs. 47 and 48.) They frequently show dual, more rarely multiple, twinning. Index of refraction, 1'622-1'72. The double refraction is 106 CONSTITUENT MINERALS [PART n high, even thin sections giving chromatic polarization. The coloured varieties are strongly pleochroic (dis- tinction from augite). By alteration processes (weathering, metamorphism, etc.) the amphiboles give rise to chlorite, serpentine and calcite. 1. MONOCLTNIC AMPHIBOLES. a. Brown and Green Amphiboles. Common Hornblende. Monoclinic. Meta- silicate of iron, alumina, lime and magnesia, with smaller amounts of soda and potash, lime forming about one quarter of all the bases. The so-called " basaltic hornblende " contains a larger proportion of ferric iron and titanium than common hornblende. Colour, black by reflected light ; brown or green by transmitted light. Pleochroism, strong in brown and yellow, or green and yellow tints. Index of refraction, 1*642. Double refraction, high. Extinc- tion angle for the brown variety, 10 to 15 ; for the green, 12 to 20. Occurs in hornblende-granites, syenites and mon- zonites (brown and green varieties poor in alumina) ; diorites and gabbros (mainly the green aluminous variety) ; phonolites, trachytes (usually a green soda- bearing variety), and andesites (brown variety); and as an accessory constituent of basalts. The crystals are among the first-formed constituents, and have often suffered subsequent corrosion in the still molten magma, being then surrounded by a dark-coloured border. CHAP, in] DESCRIPTION OF MINERALS 107 b. Green Amphiboles. Actinolite. Monoclinic. Silicate of iron, magnesia and lime : CaO, 3(MgFe)O, 4SiO 2 . Colour, grass-green. Pleochroism, in green tints, not very strong. Maximum extinction-angle = 15 to 18. Index of refraction, 1-626. Double refraction, higher than that of common hornblende, excepting basaltic hornblende, which has FIG. 49. Crystals of Hornblende, under the microscope, showing the pre- dominance of the prism faces over those of the pinacpids, and the prismatic cleavage. (After Teall.} a much higher birefringence than either. Occurs in long prisms and needles, or in ribbon-like blades and fibres, as a constituent of amphibolite and hornblende- schist. Smaragdite, nephrite and uralite are fibrous green hornblendes allied to actinolite, generally secondary after pyroxene. Such paramorphic horn- blende occurs in gabbros, diabases and epidiorites. 108 CONSTITUENT MINERALS [PART IT Pilite is a similar substance produced by the alteration of olivine. c. White Amphiboles. Tremolite. Monoclinic. Silicate of magnesia and lime : CaO, 3MgO, 4SiO 2 . Colourless. Maximum extinction = 15 CI to 17. Index of refraction, 1-621. Double refraction, the same as actinolite. Occurs in lamellar and bladed aggregates as a constituent of metamorphic limestones (cipollino). Asbestos is a fibrous variety of tremolite; although commercial asbestos chiefly consists of chrysotile (serpentine). d. Soda Amphiboles. Arfvedsonite. Monoclinic. Silicate of soda and iron : (Na 2 Fe)O, Si0 2 , Resembles common green hornblende, but is of a more bluish hue. Maximum extinction-angle varies from 10 to 14 = (80 to 76). Occurs in association with the soda-minerals elseolite, nepheline and leucite in elseolite-syenite, phonolite, etc. Barkevikite is a similar hornblende, but brown in colour and containing titanic acid, and with a positive extinction-angle of from 10 to 14. Index of refraction, 1704. Double refraction, high, but less than actinolite. Katophorite occupies an intermedi- ate position between arfvedsonite and barkevikite. Glaucophane. Monoclinic. Silicate of alumina and soda Na 2 0, A1 2 O 3 , 4SiO 2 . Colour, blue. Pleo- chroism striking, in blue and reddish violet tints. (y = blue, /3 = reddish violet, a - colourless to yellow), (See Fig. 57.) Maximum extinction-angle 3 to 11. Occurs in prisms and blades in schistose rocks CHAP, in] DESCRIPTION OF MINERALS 109 (Anglesey, Isle of Syra, Japan). Gastaldite is a variety. Biebeckite. 1 Monoclinic. Silicate of soda and iron : Na 2 O, Fe 2 O 3 , 4Si0 2 . Colour, deep blue. Pleo- chroism in blue and green tints, (y = green, /3 = blue, a deep blue and nearly coincident with the vertical axis). (See Fig. 57.) Maximum extinction 2 to 7 ( = 88 to 83). Biebeckite occupies a position among the amphiboles equivalent to that of aegirine and acmite among the pyroxenes. Occurs in certain microgranites and trachytes (Socotra, Mynydd Mawr in Wales, Ailsa Craig and Eildon Hills in Scotland). Crocidolite is a similar variety. 2. RHOMBIC AMPHIBOLE. Anthophyllite. Rhombic. Silicate of magnesia and iron : (MgFe)O, SiO 2 . Colourless. Extinction, straight. Index of refraction, 1/633. Double refrac- tion, high, but less than actinolite. Occurs in bladed forms in amphibolites ; also in fibrous zones around the olivines of some gabbros. 3. TRICLINIC AMPHTBOLE. Cossyrite. Triclinic. Silicate of alumina, iron and soda. Under the microscope this mineral appears in prisms of a deep reddish-brown colour, only trans- mitting light in very thin sections. It is only known to occur in the rhyolites of Pantelleria (pantellerites). 1 Discovered by Sauer (1888) in granite collected by Dr. Riebeck in Socotra. 1.10 CONSTITUENT MINERALS PYROXENE GROUP. II The pyroxenes are silicates of iron, magnesia and lime : the bases alumina, manganese, soda and lithia, also occur in some varieties. They are unattacked by acids. Sp. G. = 3-3*5. Hardness = 5-6. Cleavage parallel to the prism (angle = 87), rather imperfect. FIG. 50. Crystal of Atigite. a. Orthopinacoid. b, Clinopinacoid. m. Prism. s. Pyramid. (After Rosenbusch.) The smaller angle between the prism-faces and the inferior degree of cleavage constitute the main points of distinction from hornblende. Under the micro- scope sections perpendicular to the vertical axis are octagonal, being bounded by the faces of the prism and of both pinacoids. With regard to maximum extinction-angles in the monocliuic pyroxenes, these are high (38 to 54) in the diopside-augite series ; 6 to 38 in the 8Bgirine-augite series, and to 6 in the acmite-segirine series. In the rhombic pyroxenes CHAP, in] DESCRIPTION OF MINERALS 111 the extinction is of course straight. Index of refrac- tion, 1-668 (enstatite) to 1-799 (acmite). Double re- fraction, high in the monoclinic augites, but moderate in the rhombic pyroxenes. 1. RHOMBIC PYROXENES. Meta-silicates of iron and magnesia (MgFe)O, SiO 2 . According to the percentage of iron they contain, the following varieties may be distinguished : Enstatite, with less than 5% of FeO. Bronzite, 5 - 14% of FeO. Hypersthene, above 14% of FeO. Amblystegite, up to 31/ of FeO. FIG. 51. Longitudinal section of Enstatite. For practical purposes enstatite represents the meta- silicate of magnesia, or MgO, SiO 2 , with SiO 2 = 60 and MgO = 40 per cent. In the same way hypersthene may be regarded as 3MgO, FeO, 4SiO 2 , with 8iO 2 = 55-5, MgO = 27'7 and FeO = 16'8 per cent.; while amblystegite would have the formula MgO, FeO, 2SiO 3 , in which SiO 2 = 518, MgO = l7'2 and FeO = 112 CONSTITUENT MINERALS [PART n 31 '0 per cent. Intensity of colour and pleochroism increase with the percentage of iron, enstatite being colourless and non-pleochroic in thin section, bronzite slightly coloured and pleochroic, and hypersthene and amblystegite strongly so, The rhombic pyroxenes are characterized by the possession of a fairly perfect pinacoidal cleavage, in addition to the prismatic cleavage common to all pyroxenes. In some varieties the pinacoidal cleavage surfaces have a sub-metallic (bronzy or coppery) lustre. This phenomenon is caused by reflection from the surfaces of innumerable parallel rods and flakes of FIG. 52. Cross-section of Enstatite. foreign matter intercalated along these planes, and said to be of secondary origin. Between crossed nicols the rhomuic pyroxenes extinguish, of course, straight. Index of refraction, T66 to TTO. The double refraction is moderate; in enstatite it is the same as that of quartz, but hypers- thene has a higher birefringence. The . pleochroism of hypersthene is as follows: a = sea-green, /3 = salmon-red, y = reddish-yellow. They occur in various igneous rocks : diorites, monzonites, norites, dolerites, porphyrites, andesites CHAP, m] DESCRIPTION OF MINERALS 113 and some peridotites, and pass by alteration into a fibrous serpentinous substance known as bastite. 2. MoNOCLiNic PYROXENES. a. Brown Pyroxene. Common Augite. Silicate of alumina, lime, magnesia and iron. It may be regarded as made up of m molecules of diopside, CaO, (MgFe)O, 2SiO 2 FIG. 53. Longitudinal section of Augite. FIG. 54. Cross-section of Augite. + n molecules of Tschermak's silicate (MgFe)O, (AlFe) 4 O a , SiO 2 . There is usually a small quantity of soda present, and when the percentage of Na 2 O exceeds 2'5 the mineral is termed segirine-augite, the colour approximating to that of segirine. Sp. G. = 3;30-3'50. Hardness - 5-6. 114 CONSTITUENT MINERALS [PART n This mineral is black by reflected light. In thin section it is colourless or of a pale-green to pale- brown colour, sometimes with a violet tinge, especially FIG. 55. Crystal of Augite showing hour-glass structure. FIG. 56. Crystals of Augite twinned on the Orthopinacoid a. Orthopinacoid. b. Clinopinacoid. m. Prism. s. Pyramid. (After Rosenbnsch.) when the mineral contains titanium. Pleochroism, faint or absent. Index of refraction, 170 to 1'72. High double refraction, but slightly less than actinolite. CHAP, m] DESCRIPTION OF MINERALS 115 Extinction angle, in sections parallel to the vertical axis, reaches a maximum of 54, constituting thus a distinction from hornblende. Slight variations in the extinction angle in different peripheral layers often produce in the crystals a zonal structure, which occasionally assumes the form of an hour-glass. (See Fig. 55.) The cleavage is less perfect than in horn- blende, the cracks being irregular, thick and branching in thin section. Dual and even multiple twinning (parallel to the orthopinacoid), not uncommon. (See Fig. 56.) Augite passes readily into chlorite by the ordinary process of weathering : dynamic metamorphism con- verts it into green hornblende (uralite, actinolite, etc.). Loose crystals of black opaque augite occur among the material ejected from volcanoes (lapilli, cinders, ash, etc.). It is a very common constituent both of plutonic and volcanic rocks, especially of the more basic varieties : gabbro, dolerite, andesite, basalt, etc. Diallage is a laminated variety of augite, usually of a greenish colour, occurring in gabbro. The lami- nation is parallel to the orthopinacoid ; it appears in thin section as a fine lineation, which constitutes the chief diagnostic character of the mineral. The faces of this lamination or pseudo-cleavage, are character- ized by a pearly or sub-metallic lustre, due to the presence of minute included plates and rods. Thin pieces, flaked off' in this direction and examined in convergent polarized light, present the oblique emer- gence of an optic axis (distinction from the rhombic 116 CONSTITUENT MINERALS [PART n pyroxenes). Salite is a similar mineral in which the inclusions are parallel to the basal plane. b. Green and White Pyroxenes. Diopside, Malacolite, Coccolite, Omphacite. Meta-silicates of lime, magnesia and iron : CaO, (MgFe)O, 2Si0 2 . Pare diopside may be regarded as CaO, (MgFe)O, 2Si0 2 in which the ratio between MgO and FeO is variable, MgO being usually in excess. Sp. G. = 3'3. Index of refraction, 1'68 ; double refraction, high, but slightly less than common augite. In colour these non-aluminous monoclinic pyroxenes vary from a very pale-green (malacolite) to a bright grass-green (omphacite, chrome-diopside). Malacolite occurs in grains and prisms in certain diorites, epidiorites and metamorphic schists. Bright green pyroxene (omphacite, chrome-diopside) occurs in some peridotites, also in eclogite, pyroxene-granulite, etc. ; coccolite occurs in metamorphic limestones. Under the microscope malacolite gives colourless sections, which on account of their fairly high index of refraction may easily be mistaken for epidote. It is distinguished from this mineral, however, by an oblique extinction in lath -shaped sections, and by the absence of pleochroism. Wollastonite. Meta-silicate of lime : CaO, SiO.j, with SiO 2 = 5172, and CaO = 48'28 per cent. Mono- clinic. Sp. G. = 2'78 -2-9. Hardness = 4-5 -5. Perfect cleavage parallel to the basal plane and to the orthopinacoid. Colourless, with pearly lustre. Occurs in columnar crystals with fibrous structure, and in CHAP, in] DESCRIPTION OF MINERALS 117 fibres in metamorphic limestones and crystalline schists. Easily decomposed by hot hydrochloric acid ; and the solution, on the addition of dilute sulphuric acid, yields crystals of gypsum. c. Soda-pyroxene. Acmite and .ZEgirine, corresponding to arfved- sonite, glaucophane, etc., among the amphiboles. Ideally pure acmite is a meta-silicate of sodium and ferric iron, and corresponds to the formula Na 2 O, I. Anthophyllite. II. Glaucophane. III. Tremolite, brown horn- blende, etc. IV. Green hornblende. V. Blue amphibole (Silver Cliff). VI. Riebeckite. VI. I. Enstatite, etc. II. Spodumene. III. DFopside, etc. IV. and V. Augite. VI. ^Egirine. FIG. 57. Fe 2 O 3 , 4SiO 2 ; with SiO 2 = 52'0, Fe 2 O 3 = 34'6 and Na 2 O = 13-4; but usually the bases CaO, MgO, and Al 2 O f are present in small quantities and the mineral is termed segirine. When these are increased and 118 CONSTITUENT MINEB^S [PART n the soda diminished, the aegirine passes through segirine-augite into common augite, so that acmite, segirine, aegirine-augite, and augite are members of an isomorphous series. ^Egirine is found in certain members of the alkali series of plutonic and volcanic rocks, as for instance in syenites, nepheline-syenites and in phonolites. The foregoing diagram (after Iddings) shows the relation existing between the morphological axes and the axes of elasticity in the principal amphiboles and pyroxenea The angle given is that of extinction. OLIVINE GROUP. Olivine. Ortho-silicate of iron and magnesia: 2(MgFe)0, SiO 2 . Occasionally the bases CaO, MnO and ZnO are represented in minerals developed in metamorphic limestones, while the iron-magnesium compound or common olivine occurs most commonly in the igneous rocks. The following members of the isomorphous series deserve mention. Forsterite, 2MgO, Si0 2 ; with SiO 2 = 42-9 and MgO = 57-1. Fayalite, 2FeO, SiO 2 ; with Si0 2 = 29'4 and FeO = 70-6. Tephroite, 2MnO, SiO 2 . Monticellite, CaO, MgO, SiO 2 . Titanolivine, with 5-6% of titanic acid. Olivine, 2(MgFe)O, SiO 2 ; with MgO varying from 31-52%, and FeO from 5-30%. Rhombic. Occurs in tabular or prismatic forms, made up of piriacoids and domes, or in irregular grains. Sp. G. = 3'2-3'5. Hardness = 6'5-7. Gela- CHAP, in] DESCRIPTION OF MINERALS 119 tinizes with hydrochloric acid. Colour, btack, olive- green, or yellow. Pinacoidal cleavage, imperfect. Under the microscope olivine presents colourless sections, sometimes bounded by crystallographic contours, oftener by irregular and indented out- lines, due to corrosion. (See Fig. 58.) Its index of refraction is high (1'678), the outlines being strongly marked and the surface of the sections having a roughened appearance. The double refraction is also FIG. 58. Corroded crystal of Olivine. high, the sections polarizing in bright colours. Un- altered olivine is, on account of its optic properties, liable to confusion with diopside, but the'fact that the cleavage is pinacoidal in the former and prismatic in the latter is a useful distinction. Straight ex- tinction in all principal sections. Inter-penetration twins are not infrequent in basalt. (See Fig. 59.) They are best seen between crossed nicols on account of the difference in extinction shown by the two individuals. \ / Olivine is very prone to decomposition; it alters 120 CONSTITUENT MINERALS [PART n readily into serpentinous products (antigorite, idding- site), also into ealcite or dolomite, liraonite, and more rarely into hornblende (pilite, anthophyllite). FIG. 59. Twin-crystal of Olivine. Olivine occurs only in basic and ultrabasic rocks : in gabbros, dolerites, basalts, limburgites, picrites, peridotites, etc. The peridotites are mainly composed of this mineral. Olivine alters readily into fibrous aggregates of serpentine ; the alteration proceeding along cracks, and being often accompanied by a separation of iron-ores (magnetite, limonite, etc.). MICA GROUP. The rock-forming micas may be divided for all practical purposes into white and black micas. In white mica (muscovite), there is usually a con- siderable divergence of the optic axes ; while the dark mica (biotite) is, in most cases, practically uniaxial, i.e., the two optic axes are nearly coincident. There are, however, certain rare varieties of dark mica that have a large optic axial angle. All micas are monoclinic, but with pseudo-hexagonal symmetry (see Fig. 60) ; and a very perfect basal cleavage is CHAP, m] DESCRIPTION OF MINERALS 121 characteristic for the group. The elasticity of the laminae distinguishes mica from other minerals (talc, chlorite, etc.) possessing a similar cleavage. FIG. 60. Crystal of Mica. c. Basal Plane. o. Pyramid. m. Prism. b. Clinopinacoid. (After Rosenbusch.) WHITE MICAS. Muscovite, or Potash-mica. Hydrated ortho- silicate of alumina and potash : K 2 0, 3A1 2 O 3 , 6SiO 2 , 2H 2 O (Tschermak), = SiO 2 , 45'2 ; A1 2 O 3 , 38'5 ; K 2 O, ITS; H 2 O, 4'5.' Not attacked by hydrochloric acid. Sp. G. = 2-76-31 Hardness 2-3. Index of refraction, 1*592. Double refraction, very high, thin sections giving brilliant chromatic polarization between crossed nicols. Sections parallel to the basal plane, however, exhibit a low interference-colour, usually grey. Occurs in granite and gneiss, in plates, leaves, scales and fibres, possessing a silvery white colour and bright lustre. Fragments of muscovite are frequently present in sandstones and shales, and by their parallel arrangement impart to these rocks a fissile character. Under the name of sericite a silvery white or pale- green variety occurs as a frequent secondary con- 122 CONSTITUENT MINERALS [PART n stituent of the metamorphic schists. There is no doubt that in many cases it results from the alteration of the felspar. Its thin laminae wrap round the lenticular masses of quartz and felspar, and play an important part in determining the schistosity of these rocks. FIG. 6l. Section of Mica showing the cleavage. Other white micas are paragonite, a hydrated silicate of alumina, and soda (Na 2 O, 3A1 2 O 3 , 6SiO 2 , 2H 2 O) occurring in schists ; margarite, a hydrated silicate of alumina and lime, in schists ; and lepidolite and zinnwaldite, hydrated silicates of alumina, potash, soda and lithia, in pegmatites and greisens. Under the microscope the white micas give lath-shaped sections, presenting well-marked cleavage-lines, often bent. They also occur in streaky and ribbon-like bands. BLACK OR FERRO-MAGNESIAN MICAS. Biotite, a hydrated silicate of alumina, iron, magnesia and potash (HK) 2 O,2(Mg,Fe)O,(Al,Fe) 2 O 3 , CHAP, in] DESCRIPTION OF MINERALS 123 3SiO 2 . Attacked by hot hydrochloric acid. Sp. Gr. = 2-8-3-2. Hardness = 2 -5-3. Index of refraction, 1*56. Double refraction, very high, the colours being bright except in sections parallel to the basal cleavage, which remain dark when rotated between crossed nicols. Occurs as a constituent of some granites, syenites, diorites, lamprophyres, trachytes and andesites, also, as an accessory in some basalts. Loose crystals of a reddish brown biotite (rubellan) are found embedded in volcanic ash. Under the microscope this mineral appears in plates, scales, and lath -shaped sections, the latter showing intense pleochroism (pale yellow to chestnut brown an black). The principal absorption takes place when the long axis of the lamella, or the trace of the cleavage, is parallel to the short diagonal of the polarizing nicol, thus distinguishing it from tourma- line, which is equally strongly absorptive, but in a direction at right angles to this. Inclusions are fre- quent : tourmaline, zircon, orthite, sphene, and apatite, the last four of which are often surrounded by pleo- chroic borders. Intergrowths with hornblende ana pyroxene are common. Alters easily to chlorite, the alteration being marked by the gradual assumption of a green colour. Lepidomelane is a variety of biotite, rich in ferric iron. Phlogopite is a magnesian mica, rich in potassium, and with variable amounts . Hardness = 6. Index of refraction, 1 '696-1 '7. Unattacked by hydrochloric acid. Under the microscope it presents colourless prisms and granules, resembling epidote, but distinguishable from that mineral by its lower double refraction, which is slightly higher than nepheline but less than orthoclase. Occurs in association with epidote in altered basic igneous rocks and metamorphic schists. PHOSPHATE AND FLUORIDE GROUP. The apatites are phosphates of lime + chloride or fluoride of lime: Fluor - apatite, 3(3CaO, P 2 O 6 ) + CaF 2 ; with P 2 O 5 = 42'3, CaO = 55'5, F = 3\S per cent. less 1-6 O for F. Ohlor-apatite, 3(3CaO P 2 O 5 ) + CaCl 2 ; with P 2 O 5 . = 410, CaO = 53'8, Cl = 6'8 per cent, less 1'6 for 01. There are also intermediate varieties containing both fluorine and chlorine. Hexagonal. Sp. G. = 316-3'22. Hardness = 5. Im- perfect cleavage parallel to basal plane and prism, the latter scarcely noticeable under the microscope. When fresh, colourless and glassy; altered varieties are dull green and red. Soluble in acids without gelatinizatiori : the nitric acid solution gives a yellow 130 CONSTITUENT MINERALS [PART n precipitate with ammonium molybdate (distinction from nepheline). Apatite occurs in long hexagonal prisms and needles; also in short prisms. It is one of the commonest accessory minerals, being found in most igneous rocks. It is especially abundant in the gabbros, dolerites and basalts. It is also one of the earliest-formed constitu- ents ; for its needles are usually found penetrating all other minerals. Under the microscope it is colourless, and presents six-sided cross sections and elongated lath-shaped longitudinal sections. The former remain dark when rotated between crossed nicols, the latter extinguish straight. Double refraction, weak, the polarization colour being grey. Index of refraction, rather high (1'637). There is often a central core of inclusions ; and in such cases there is usually a slight absorption of light when the long axis of the section is parallel to the short diagonal of the nicol. Apatite may be easily mistaken for nepheline. It is dis- tinguished from this mineral by the chemical reactions mentioned above, by the index of refraction, and by the fact that apatite occurs in rocks in slender, needle- like prisms, nepheline on the other hand, in shorter and thicker prisms. Of the two varieties of apatite, chlor- apatite occurs more frequently in basic rocks, and fluor-apatite more frequently in acid rocks. Xenotime ^(phosphate of yttrium, and Monazite (phosphate of cerium and lanthanum) are rare minerals, occurring in sands and probabty derived from igneous rocks. Prof. Derby has described them in the muscovite-granites of Brazil. Xenotime occurs in pyramidal crystals of the Tetragonal System, which CHAP, m] DESCRIPTION OF MINERALS 131 are transparent, yellow, or colourless, with highly polished faces. By alteration it becomes opaque. Refraction and double refraction, very high. Monazite crystallizes in the Monoclinic System. Index of refraction, V811. Double refraction, very high. Fluorspar. Fluoride of calcium: CaF 2? with Ca -511 and F = 48'9. Regular. Sp. G. = 318-3-19. Hardness = 4. Decomposed by sulphuric acid. Octa- hedral cleavage, perfect. Index of refraction, 1'4 >4. Isotropic. In thin section, colourless to violet or blue. Occurs in small quantities in granites, syenites, and especially in nepheline-bearing rocks. IRON-ORE GROUP. The iron-ores that occur as common accessory constituents of rocks are magnetite or magnetic iron- ore, ilmenite or titaniferous iron-ore and pyrites. Magnetite. Magnetic oxide of iron : FeO, Fe 2 O 3 , with FeO = 310, and Fe 2 O 8 = 69'0 per cent. The FIG. 64. Crystals of Magnetite. (After Rosenbusch.} ferrous iron may be partially replaced by magnesium and by manganese. Regular. Sp. G. = 4-9 5'2. Hard- ness = 5 - 5- 6 '5. 132 CONSTITUENT MINERALS [PART n One of the commonest accessory constituents of igneous rocks, occurring in black octahedral grains, skeleton-crystals, and minute particles. Under the microscope it is perfectly opaque, even in the thinnest sections. In reflected light it has a bluish-black me- tallic lustre (distinction from pyrites and ilmenite). (See Fig. 64.) The amount of magnetite present in tiie igneous rocks varies. It sometimes increases locally to a considerable extent, as in the dark patches of granitic rocks. Tornebohm and Vogt have shown that, in certain differentiations of basic igneous rocks, a gradual increase in the percentage of iron-ore may be observed up to 90 per cent. Ilmenite. Titaniferous oxide of iron ; (TiFe) 2 O 3 . Rhombohedral. Sp. G. = 4*5 - 5 '2. Hardness = 5-6. FIG. 65. Section of Ilmenite. Infusible. Attacked with difficulty by aeids. Black and opaque in thin section. Occurs in hexagonal sections, but oftener in irregular grains, and then not easy to distinguish from magnetite. It may be dis- tinguished from that mineral, however, by the fact that it is very liable to alteration into a white opaque substance, known as leucoxene (a variety of sphene), by which it is almost invariably accompanied. The CHAP, in] DESCRIPTION OF MINERALS 133 alteration takes place along three systems of lamellae intersecting at 60, thus indicating the rhombohedral character of the mineral. Ilmenite occurs in basic igneous rocks, especially in gabbro, dolerite, etc. Pyrites. Sulphide of iron: FeS 2 , with Fe = 46'6 and S = 53'4 percent. Regular; in cubes and pentagonal dodecahedra. Sp. G. = 4-9-5'2. Occurs abundantly disseminated in minute yellow specks and grains in both igneous and sedimentary rocks. Under the microscope it is opaque; in reflected light, however, its brassy yellow colour at once characterizes it. SPINEL GROUP. The spinel group consists of a series of minerals crystallizing in the Regular System, and composed of a double molecule formed by the union of a sesqui- oxide (Fe 2 O 3 , A1 2 3 , O 2 O 3 ) with a protoxide (FeO, MgO, MnO, ZnO, CrO). The commonest combinations occurring in rocks are the following : Formula. Colour. Specific Gravity. Hardness. Index of Refraction Spinel Pleonaste Hercynite MgO, A1 2 3 (MgFe)O, (AlFe) 2 3 (FeO, A1 2 O 3 ) red dark green 3.5-4.1 3.6 3.9 8 8 7.5-8 1.715 1.749 Picotite (FeMg)0,(CrFeAl) 2 3 ( yellowish ) ( brown f 4.0 8 Chromite FeO, Cr 2 3 ( dark brown ) ( to opaque ) 4.5 5.5 2.096 Magnetite FeO, Fe20 3 ( black and ) ( opaque 1 4.9-5.2 5.5-6.5 Spinel, MgO, 28.3, and A1 2 O 3 , 71.7 per cent. Hercynite, FeO, 41.4 and A1 2 O 3 , 58.6 per cent. Chromite, FeO, 32, and CrzO 3 , 68 per cent. Magnetite, FeO, 31.0, and Fe20 3 , 69.0 per cent. 134 CONSTITUENT MINERALS [PART n The spinels are abundant in igneous and raeta- morphic rocks, occurring in small grains and octa- hedral crystals. Under the microscope they are isotropic. The dark-coloured spinels (pleonaste, her- cynite, picotite and chromite) occur frequently in peridotites and serpentines. Magnetite has already been described with the iron-ores. The spinels can be distinguished from the garnets by differences in specific gravity and chemical composition. ZIRCON GROUP. Zircon. A double oxide of silicon and zirconium : Zr0 2) Si0 2 , with SiO s = 32>8 and Zr0 2 = 67'2 per cent. Tetragonal ; occurring in small prisms terminated at both ends by pyramids. Colour, red and brown. FIG. 66. Crystals of Zircon . (After Roscnbusch. ) Infusible ; unattacked by acids, except in fine powder with concentrated sulphuric acid. Sp.G. = 4-1-47. Hardness = 7'5. Index of refraction, T952. In thin sections zircon occurs in small, colourless grains, with CHAP, m] DESCRIPTION OF MINERALS 135 well-marked contours, Between crossed nicols the polarization colours are brilliant pinks and greens, the double refraction being very high (between talc and sphene). Zircon has a wide distribution in eruptive (both plutonic and volcanic) and metamorphic rocks. It occurs frequently as an inclusion in black mica, in which case the grains are surrounded by a pleochroic zone. Being undecomposable it is found in the sands derived from granitic and gneissose rocks, and also in the sandstones and quartzites produced by their consolidation. 1 Eudialyte is a hexagonal mineral allied to zircon. Chemical composition, 2Na 2 O, 3(CaFe)O, ZrO 2 , 7SiO 2 . Specific gravity, 2'91-3.0. Index of refraction, T613; double refraction, low. It occurs in nepheline-syenites and related pegmatites in the Christiania district of Norway, Kangerdluarsuk in Greenland, the Kola Peninsula of Finland, and at Magnet Cove, Arkansas. Eucolite is a variety of the same mineral. TITANIC ACID GROUP. Sphene or titanite. Silicate and titanate of lime : CaO, SiO 2 , TiO 2 ; with SiO 2 = 30'6, TiO' 2 =40'8, and CaO = 28'6 per cent. Monoclinic. In flat cuneate crystals of a greenish yellow to blackish-brown colour. Sp. G. = 3-3-37 Hardness = 5-5'5. Index of refrac- tion, 1'93. Unattacked by hydrochloric acid. Under the microscope sphene appears in irregular, strongly 1 A. Dick : On zircons and other minerals contained in sand Nature, vol. xxxvi., p. 91. 136 CONSTITUENT MINERALS [PART n contoured grains, which are occasionally wedge-shaped. Its colour, in thin section, is reddish-yellow. Pleo- chroism feeble. Double refraction, very high (between zircon and calcite), but not producing brilliant chro- matic polarization, the colour effect between crossed nicols being an indefinite brownish grey of a high order. The white opaque substance surrounding ilmenite, and resulting from its alteration, is a variety of sphene, known as leucoxene. Sphene occurs in granites, syenites and crystalline schists; leucoxene in gabbros, diabases, epidiorites and hornblende and chlorite schists. FIG. 67. Crystals of Sphene. Perovskite. Titanate of calcium : CaO, TiO 2 ; with CaO = 411 and Ti0 2 = 58'9 per cent. Regular: in octahedra modified by the dodecahedron and cube. Also in modified cubes. Cleavage, fairly perfect parallel to the cube. Sp. G. = 4-02-4'04. Hardness = 5'5. Index of refraction, high (2'38). Isotropic, but with occasional anomalous double refraction. Colour in transmitted light, grey to violet grey, brown to reddish. Occurs in basic igneous rocks usually in CHAP, in] DESCRIPTION OF MINERALS . 137 association with nepheline, leucite, or melilite, also in crystalline schists. Rutile. Titanic Oxide: TiO a . Tetragonal; cry- stallizing in slender prisms and needles. Frequently twinned, on two types, producing geniculated and heart-shaped forms. Colour, yellow to red. Sp. G. = 4-20-427. Hardness = 6-6'5. Index of refraction, 2759. Double refraction, very high. Unattacked by acids. Distinguished under the microscope by its yellow colour and strongly marked borders, due to the high index of refraction. It frequently presents a peculiar interlaced structure, known as sagenite. (See Fig. 68.) Rutile occurs as prisms and twins FIG. 68. Crystals of Rutile (Sagenite). After Rosenblisch. in the crystalline schists, especially in hornblende schist ; also in sands and sedimentary rocks. Fine hair-like bodies occurring in clay and slates (Thonschiefernddelcheri), are referred to rutile. 1 Anatase. Titanic oxide : Ti0 2 . Tetragonal ; with bipyramidal or tabular habit. Sp. G =3'82 1 Teall : Min. Mac)., vol. vii., 1887, p. 201. 138 CONSTITUENT MINERALS [PART n 3*95. Hardness = 5*5 6. Index of refraction, 2'52. Double refraction, strong. Colour in thin section, yellow, brown or blue. Occurs in small quantities in granites and other igneous rocks, also in sands and clays derived from them. ;.'-*",. CARBONATE GROUP. Calcite. Carbonate of lime : CaCO 3 ; with CaO= 56 and C0 2 =44 per cent. The carbonates of lime, magnesia and iron (calcite, magnesite and chalybite) are isomorphous, and can replace one another in any proportions. Rhombohedral. The crystalline forms assumed by calcite are innumerable : it occurs in rhombohedra, which may be either flat or have any degree of acuteness ; in sharp-pointed scalenohedra (dog-tooth spar) ; in thick-set or slender six-sided prisms crowned by the faces of the rhombohedron ; or in prisms terminated by the basal plane. It also occurs in amorphous masses, and in veins. Sp. G. = 2'72. Hardness = 3 (easily scratched with the knife). Index of refraction, 149 1*66. Cleavage parallel to the rhombohedron, perfect rhombohedra being produced by the fracture. Effervesces with acids. Colourless to white ; but often stained yellow, red and brown by ferruginous impurities. Under the microscope it occurs in irregular grains, traversed by fine cleavage lines, intersecting at an acute angle. Very high double refraction. Between crossed nicols, the polarization colour is an iridescent pinkish grey of a high order ; and broad colour-bands, produced by lamellar twinning, are often visible along the diagonals of the cleavage rhomboids. CHAP, in] DESCRIPTION OF MINERALS 139 As a secondary constituent, calcite occurs in the more basic igneous rocks diorites, gabbros, dolerites, etc. It is the principal constituent of the altered limestones (marbles). It often forms the cementing material of fragmental rocks (e.g., calcareous sandstones). Ohalybite or Siderite. Carbonate of iron : FeCO 3 ; with FeO = 621, and GO 2 = 37*9. Rhombohedral; iso- mor phous with calcite. Sp. G. = 3'83 - 3 '88. Hardness = 3*5-4. Index of refraction, T76. Double refraction, very high. Colour in thin section, grey, yellowish or brown. Occurs frequently in radially fibrous spherulites (spherosiderite). Dolomite is a double carbonate of lime and mag- nesia: CaO, MgO, 2C0 2 . It resembles calcite in its crystallographic and optic properties. Index of refrac- tion, 1*622. Double refraction, slightly higher than that of calcite. Chemically it is less soluble than calcite, being not acted upon by acetic acid or cold dilute hydrochloric acid. Sp. G., 2'8-2'9. Hardness = 3'5-4. Aragonite is the rhombic carbonate of lime : CaCO 3 , with the same percentage composition as calcite. Its crystals are often six-sided, and, especially when twinned, resemble the hexagonal combination of prism and basal plane. Sp. G. = 2 9 3. Hardness = 3'5 -4. Index of refraction, T631. Double re- fraction, slightly less than calcite. It is found in fibrous and spherulitic aggregates in many igneous and metarnorphic rocks. It frequently fills cavities in basalt, or traverses the same rock in strings and veins. Stalactites are often built up of fibrous aggregates of aragonite. The small spherular bodies 140 CONSTITUENT MINERALS [PART u which are formed in warm calcareous springs and give rise to pisolitic and oolitic rocks, consist of radiate and fibrous aragonite. ZEOLITE GROUP. Zeolites. A group of hydrated silicates of various bases : alumina, potash, soda, lime, baryta and strontia. They are secondary products, occurring in igneous rocks as the infillings of amygdaloidal cavities (especially of melaphyres and basalts), or as pseudo- morphs after decomposed minerals (e.g., nepheline). Under the microscope the zeolites are always colourless, and occur in fibrous and radiate aggre- gates. They are easily decomposed by hydrochloric acid, with separation of gelatinous silica. The index of refraction of the zeolites is low (less than that of Canada balsam). All of them also exhibit low double refraction. Some of the more commonly occurring varieties are : heulandite, natrolite, analcime, phillipsite, laumontite, scolecite and apophyllite. The com- position of these is given in the following table; Crystallo- Pencentage Composition. System. Si Oz A1 2 3 CaO Na 2 K 2 Hal Heulandite Monoclinic H 4 Ca A1 2 (Si0 3 )6+3 H 2 O 59.2 16.8 9.2 _ _ 14. Natrolite Rhombic Na 2 A1 2 Si 3 Oio+2 H 2 O 47.4 26.8 16.3 9. Analcime Regular NaAl(Si0 3 ) 2 +H 2 54.5 23.2 14.1 s. Phillipsite Laumontite Monoclinic Monoclinic (K 2 Ca)Al 2 (Si0 3 ) 4 -f4H20 48.8 H 4 Ca Al2 Si 4 Oi 4 +2 H 2 51.1 20.7 21.7 7.6 11.9 - 6.4 16. 15. Scolecite Apophyllite Monoclinic Tetragonal Ca(AlOH) 2 (Si0 3 ) 3 +2H 2 O 45.9 H 7 KCa 4 (Si02)8+4H 2 53.7 26.0 14.3 25.0 5.2 - 13. 16. CHAP. HI] DESCRIPTION OF MINERALS 141 CHLORITE GROUP. Ohlorites. Hyd rated silicates of magnesia, iron and alumina. Monoclinic (with pseudo-hexagonal symmetry). Gelatinize with hydrochloric acid. Sp. G. = 2-60-296. Hardness = 2-2'5. Colour, green : pleo- chroic in green and yellow tints. Occur in six-sided plates, scales, and fibres, sometimes united to spherular and spiral aggregates (ripidolite and helminth'). Basal cleavage, less perfect than in mica ; cleavage-flakes non-elastic. The double refraction is very low, the usual colour between crossed nicols being a grey or steel-blue of the first order. When heated on platinum foil chlorite becomes black and paque. Tschermak has divided the chlorites into ortho- chlorites and leptochlorites, according as they occur well crystallised or in fine scales and fibres. The orthochlorites he regards as mixtures of a serpentine molecule and an amesite molecule. The serpentine molecule (Sp) has the composition H 4 (MgFe) 3 , Si 3 O 9 , and the amesite molecule (At) that of H 4 (MgFe) 2 , Al 2 Si0 9 . The principal chlorites, then, may be represented thus: Pennine 1 Clinochlore / SpAt Prochlorite = Sp 3 At I7 Corundophilite = SpAt 4 Amesite = At Chlorite is essential^ a secondary mineral resulting from the decomposition of dark mica, hornblende and augite. It occurs abundantly in altered igneous 142 CONSTITUENT MINERALS [PART n rocks, especially in the more basic types such as gabbro, diabase, melaphyre, etc., which largely owe their greenish colour to this mineral ; also in epidiorites and chlorite-schists. It is sometimes confounded, under the microscope, with secondary hornblende and with serpentine, with which it is often associated and which it much resembles. It may be distinguished from the former by its low double refraction, and from the latter by its pleochroism. OTHER HYDRATED SILICATES. Serpentine. Hydrated silicate of magnesia and iron; 2H 2 0, 3(MgFe)0, 2Si0 2 . Rhombic or monoclinic. Sp, FIG. 69. FIG. 70. Olivine altering into Serpentine. (After Bonney.) G. = 2-5 - 2-7. Hardness = 3-4. Colour, dull green ; often stained red and yellow with iron oxides. Index of refraction, 1/57. Double refraction, moderate. Straight extinction between crossed nicols. Occurs as an altera- CHAP, in] DESCRIPTION OF MINERALS 143 tion-product of olivine, hornblende and augite. Under the microscope it usually presents a confusedly inter- laced aggregate of colourless blades, scales and fibres, which frequently enclose unaltered fragments of the mineral from the alteration of which it takes its origin. (See Figs. 69 and 70.) Serpentine derived from olivine possesses a " mesh structure " ; that arising from horn- blende is " bladed," while augite often gives rise to a " netted " form of the mineral (antigorite). Pseudophite is an aluminous variety of serpentine produced by the alteration of the felspar in gabbro. Veins of fibrous serpentine (chrysotile) are worked as a source of com- mercial asbestos. Talc. Hydrated silicate of magnesia: H 2 O, 3MgO, 4SiO 2 , with SiO 2 = 63'5, MgO = 317, and H 2 = 4-8 per cent. Rhombic; but with pseudo-hexagonal symmetry. Sp. G = 27-2-8. Hardness = 1. Index of refraction, 1*545 : double refraction, very high (between mus- covite and zircon). Scratched by the finger-nail. Pale-green or colourless. Cleavage, basal. Flakes, flexible, but non-elastic. In thin section, it is found in colourless scales, resembling muscovite. It occurs as an alteration - product of magnesian minerals, mainly in metamorphic schists (talc-schist). Stea- tite or soapstone is a variety of talc. Ottrelite and Chloritoid. Hydrated silicates of alumina, iron, magnesia, or manganese. Ottrelite is probably represented by the formula H 2 O(FeMn)0, A1 2 O 3 , 2SiO 2 , while chloritoid is H 2 O (FeMg)O, A1 2 O 3 , SiO 2 . Monoclinic; but with pseudo-hexagonal symmetry, like mica. Sp. G. = 3*52 -3*57. Hardness = 6'5. Index of refraction, 174. Double refraction, 144 CONSTITUENT MINERALS [PART n high, same as that of hornblende. Colour, bluish- green. Pleochroism in bluish and green tints : a, olive green ; /3, plum colour to indigo blue ; y, yellowish green to colourless. Occur in lustrous 6-sided plates in metamorphic schists. Under the microscope, characterized by lamellar twinning. Kaolinite. Hydrated silicate of alumina: 2H 2 0, A1 2 O 3 , 2Si0 2 ; with Si0 2 =46'5, A1 2 O 3 = .39'5, H 2 O = 14'0. Monoclinic ; but with pseudo-hexagonal sym- metry. Sp. G. = 2-6-2-63. Hardness = 2-27. Cleav- age, basal. Index of refraction, T56. Double refraction, low a means of distinction from FIG. 71. Crystals of Kaolinite, under the microscope. (After Teall.) muscovite. Occurs in colourless six-sided plates and scales, resembling muscovite, in pockets in quartz veins at Amlwch in Anglesey, and at the National Bell Mine, Silverton, Colorado. It is pro- bably one of the decomposition products of felspar. CHAP, in] DESCRIPTION OF MINERALS 145 CONTACT MINERAL GROUP. Andalusite. Silicate of alumina : A1 2 O 3 , SiO 2 , with SiO 2 = 36'8 and A1 2 O 3 = 63'2 per cent. Rhombic. In square thick-set prisms, terminated by the basal plane. Sp. G. = 31-3'2. Hardness = 7-7 '5. Index of refraction, 1*638. Unattacked by acids. Crystals of this mineral usually appear dark- coloured, owing to the presence of included carbonaceous matter. Under the microscope, however, the grains are either colourless or pink ; FIG. 72. Andalusite, under the microscope. (After Cohen.} in the latter case, they often present a marked pleochroism (pale-yellow or pale-green to rose-red). The pleochroism is sometimes manifested in irregu- larly distributed spots and patches. Double refrac- tion, slightly higher than that of quartz, but less 146 CONSTITUENT MINERALS [PART n than sillimanite, which it resembles. Occurs in slates and shales that have undergone raetamorphism in contact with granite ; also in gneisses and crystalline schists, and as an accessory constituent of granite (e.g., at the Cheesewring in Cornwall). Ohiastolite is a variety of andalusite, containing graphitic material arranged along the diagonals of the prism. Occurs in small light-coloured prisms in chiastolite- slate in the neighbourhood of granite (e.g., Skiddaw). Sillimanite. Silicate of alumina: A1 2 3 , SiO 2 , with Si0 2 = 36'8 and A1 2 O 3 = 63'2 per cent., like andalusite. Rhombic. Habit, prismatic without definite terminal faces, often in long and slender crystals ; sometimes fibrous (fibrolite). Cleavage parallel to the brachy- pinacoid, perfect. Sp. G. = 3'23-3'24. Hardness = 6-7. Unattacked by acids. Colour, brown to greyish green ; in thin section, colourless. Index of refrac- tion, fairly high (1/667) ; double refraction, moder- ately high, higher than that of andalusite. Occurs in gneisses and crystalline schists, often in the aureoles of contact metamorphism around granite, in association with cordierite, corundum, andalusite and kyanite. Kyanite. Silicate of alumina: A1 2 O 3 , SiO 2 , with SiO 2 =36'8 and A1 2 O 3 = 63'2 per cent., like sillimanite and andalusite. Triclinic ; usually in long prismatic crystals. Cleavage parallel to the orthopinacoid, perfect, with partings parallel to the basal plane. Sp. G. = 3-5-37. Hardness =4-7. Colour, white to blue. In thin section, colourless to pale blue, with weak pleochroism. Index of refraction, high (172); double refraction, low. Kyanite occurs in gneisses CHAP, m] DESCRIPTION OF MINERALS 147 and crystalline schists in association with garnet, staurolite and sillimanite ; often in the aureoles of contact metamorphism round granite. It is also found in sands and clays in association with rutile, tour- maline, zircon, etc. According to Vernadsky, kyanite, when heated to 1300 C., is converted into sillimanite. Staurolite. Hydrated silicate of alumina, iron and magnesia : 2H 2 O,6(FeMg)O,12Al 2 O 3 ,llSiO 2 . Rhombic. In prismatic forms terminated by the basal plane ; commonly twinned, forming symmetrical Maltese and St Andrew's crosses. Sp. G. = 3-3-38. Hardness = 7 7'5. Index of refraction, high (1*74), double refraction, slightly higher than quartz. Un- attacked by acids. Occurs in the crystalline schists, and in rocks of the granite contact-zone. Oordierite. Hydrated silicate of alumina, iron and magnesia : H 2 O, 4(MgFe)0, 4A1 2 O 3 , 10SiO 2 . If the ratio Mg:Fe = 7:2, then SiO 2 =49'4, A1 2 3 = 33'6, FeO = 5'3, MgO = 10'2, and H 2 O = 1'5 per cent. Rhom- bic ; with pseudo-hexagonal symmetry. Sp. G. = 2'60 -2*66. Hardness = 7 7 '5. Double refraction, slightly lower than that of quartz. Often twinned. Colour, dark blue ; in thin section usually colourless. Feeble pleochroism when coloured. Yellow pleochroic spots or halos surround inclusions of zircon and apatite, but disappear on rotation of the section ; they vanish on heating. Cordierite may be distinguished from andalusite by its lower index of refraction (1*536), a grain placed in Canada balsam being scarcely visible, whereas andalusite stands out with well- marked contours. Cordierite occurs in rounded grains, with complex twinning producing radial extinction, in granites and 148 CONSTITUENT MINERALS [PART u metamorphic rocks (cordierite - gneiss or cordierite- schist); more rarely in volcanic rocks (e.g., basalt, andesite). It alters easily into mica-like decomposition products (pinite, esmarkite, praseolite, gigantolite, etc.). Idocrase or Vesuvianite. Hydrated silicate of lime and alumina: probably H 2 O, 12CaO, 3A1 2 3 , 10 SiO 2 . Also small quantities of manganese, iron, magnesium and alkalies, are often present. Tet- ragonal. In square prisms, with basal plane and pyramid. Sp. G. = 3'35-3*45. Hardness = 6*5. Colour, dark green, brown, red, yellow. Lustre, vitreous. Index of refraction, high (1'72); double refraction, weak. Occurs in limestones that have undergone alteration by contact with igneous rocks. Axinite. A borosilicate of calcium and aluminium : 7CaO, 2A1 2 O 3 -, B 2 3 , 8SiO 2 , in which lime may be partially replaced by manganese, iron, and magnesium. Triclinic. In broad crystals with acute edges. Cleavage parallel to the brachypinacoid, distinct. Sp. G. = 3 271 -3'29. Hardness = 6'5-7'0. Colour, honey-yellow to clove-brown. In thin section, colourless to pale- yellow or violet. Faint pleochroism. Index of refraction, high (1'677) ; double refraction, the same as quartz. Occurs in the contact-aureoles round granite. Topaz. A silicate and fluoride of aluminium : A1 2 O 2 (O,F 2 )SiO 2 . If the ratio of O:F 2 =5:1 then = 33-3, A1 2 O 3 = 56'5, F = 17'6, deducting O for Rhombic; with short prismatic habit and pyramidal and basal terminations. Basal cleavage, perfect. Sp. G. = 3*5- 3*57. Hardness = 8. Colourless, wine-yellow CHAP, in] DESCRIPTION OF MINERALS 149 or tinted blue, red or green. In thin section, colourless. Index of refraction, 1'62. Double refraction, the same as quartz. The lower double refraction is a means of distinction from sillimanite. Occurs in some granites and pegmatites ; also in the aureoles of contact-metamorphism round granite. Datolite. A basic orthosilicate of calcium and boron: H g O,2CaO3 a O 8 ,2SiO a with SiO 2 = 37'6, B 2 O 5 = 21-8, CaO = 35'(fand H 2 O = 5'6 per cent. Monoclinic. In stumpy, prismatic forms ; or in irregular grains. Cleavage, ill-defined. Fracture, conchoidal to uneven. Sp. G. = 2'9-3'0. Hardness = 5-5'5. Colourless in thin section. Index of re- fraction, moderate (1*65). Double refraction, very strong, between that of muscovite and talc. Occurs in association with zeolites and calcite in the arnygdaloidal cavities of basalt ; also in zones of contact-metamorphism. Datolite may be distin- guished from topaz, andalusite and wollastonite by its very strong double refraction. Corundum. Alumina: A1 2 O 3 . Hexagonal. In hexagonal prisms terminated by the basal plane or in steep Rgxagonal bi-pyramids. Parting or pseudo- cleavage, parallel to the basal plane. Fracture, uneven to conchoidal. Index of refraction, high. Double refraction, the same as that of quartz. Often colour- less in thin section. When coloured, the colour is unevenly distributed in spots or streaks. The blue variety is sapphire ; the red, ruby. With strong coloura- tion, pleochroism is observable. Corundum occurs in rocks rich in alumina in granite, syenite, nepheline-syenite, etc. ; also in 150 CONSTITUENT MINERALS [PART n pegmatite. It is developed in zones of contact- metamorphism especially in argillaceous limestones. Corundum may be distinguished from quartz, nepheline and apatite, which it resembles in regard to double refraction, by its higher index of refraction. PART III. THE CLASSIFICATION OF THE IGNEOUS ROCKS. CHAPTER I. THE PLUTONIC ROCKS. To produce a satisfactory classification of the igneous rocks, a classification which shall be at once simple and comprehensive, is one of the most difficult problems the petrologist can attack. The various types are so intimately related that any attempt at rigid and systematic arrangement is not likely to meet with a great measure of success. But some method of grouping rocks is a necessity that must be faced. As explained on pages 5 and 6, the most convenient primary division of the igneous rocks is into plutonic, hypabyssal and volcanic. -The further classification adopted here is one based mainly on chemical considerations. It has been found that the best results are obtained by arranging the rocks in the order of their silica contents. With this arrangement should be com- bined a division into series according to the alkali and calc-alkali contents, as shown by the constituent felspars and felspathoids. 152 CLASSIFICATION [PART m Arranged in the order of their silica contents, the plufeoriic rocks can be divided into three groups: 1. Acid, with silica contents above 66 per cent. 2. Intermediate, with silica contents between 66 and 52 per cent. ; and 3. Basic, with silica contents below 52. These limits are, of course, quite arbitrary, but it will be seen later that they permit a convenient separation in accordance with existing records of rock types. At the extreme basic end there is a small assemblage of rocks which contain very little or no felspar. These rocks (the picrites, peridotites, hornblendites and pyroxenites) consist of ferro- magnesian minerals and iron-ores. They form a sub-group generally known as the ultrabasic rocks. They need not be considered here in the general scheme of classification. The division according to alkali and calc-alkali contents is accomplished by means of the felspars and felspathoids, the variation in which enables the formation of a number of series, each series being characterised by its dominant felspar or felspathoid, and each comprising acid, intermediate and basic rocks. Thus, a series can be formed which is characterised by the predominance of the lime- bearing felspars (oligoclase to anorthite). This may be conveniently termed the calc-alkali series. It comprises rocks ranging from granodiorite through tonalite and diorite to gabbro, according to the per- centage of silica. In contrast with this is the alkali series, in which there is a dominant alkali-felspar (orthoclase, microperthite, soda-orthoclase, microcline, CHAP. l] THE PLUTONIC ROOKS 153 anorthoclase, or albite, etc.), or by a felspathoid (nepheline, sodalite, etc.). Occupying a middle position between the calc-alkali and the alkali series is the monzonite series, which comprises rock types characterised by the presence of both orthoclase and a lime-bearing plagioclase without either being greatly predominant. Although in Nature there is a perfect gradation in each of these series from a most acid to a most basic type, yet for classificatory purposes it is convenient to break them up into three divisions corresponding to the Acid, Intermediate, and Basic groups as above defined. By doing this the following scheme of classi- fication is obtained : SCHEME OF CLASSIFICATION OF THE PLUTONIC EOCKS. Alkali Series. Monzonite Series. Calc -alkali Series. it tj GRANITE ADAMELLITE GRANODIORITE sL 1 1 SYENITE MONZONITE DIORITE if ALKALI-GABT3RO KENTALLENITE GABBRO The alkali series may be further sub - divided into: 1. A potash series, with predominant potash -felspar 154 CLASSIFICATION [PART m (orthoclase, microcline) e.g., potash -granite, potash- syenite, shonkinite. 2. A soda series, with a predominant soda-felspar (anorthoclase, soda-orthoclase, albite) ; or with a soda- lime felspar in combination with nepheline, and soda- pyroxenes and soda-amphiboles e.g., soda-granite, soda-syenite, essexite. 3. A felspathoid series, in which nepheline, sodalite, or allied felspathoid, is largely developed, and felspar subordinate e.g., nepheline-syenite, nepheline-gabbro. It will be seen by reference to the table that there is continuity of types both vertically (in series) and horizontally (from series to series). For example, not only do the granodiorites pass by decreasing silica- content into diorites and thence into gabbros, but they pass also, by decreasing lime and increasing alkalies, into the adamellites and thence into the alkali-granites or granites proper : and similarly for each series and for each group. The divisions are adopted for convenience, but they do not represent actual hard and fast lines existing in Nature. The affinities of the various members of a continuous series, or members of different series in the same group can be shown graphically by Iddings' l method, in which the molecular proportions of the silica are represented by abscissae and those of the bases by ordinates (see Fig. 78), or by Brogger's 2 modification of Michel-Levy's 3 method, by which the molecular ratios 1 Iddings : Bull. Phil. Soc. Washington, vol. xi., pp. 191-220. 2 Brogger: Das Ganggefolge des Laurdalits, Vidensk. Skrifter I. Math.-natur. Klasse, 1897, No. 6., p. 257. 3 Michel-Levy : Bull. d. serv. de la carte geol. de la France, No. 57, vol. ix. (1897). CHAP, i] THE PLUTONIC ROCKS 155 are plotted so as to give a characteristic picture of the rock type. In Brogger's modification the molecular ratios are plotted on axes radiating from a central point : four of these are formed by a vertical and a horizontal line, and four by lines forming an angle of 120 with the vertical and horizontal lines. On the horizontal line the molecular proportion of the silica is laid off 1 half to the right and half to the left. On the vertical line the alumina is laid off downward, the lime upward. The ratios for the iron (FeO) and magnesia are laid off on the oblique axes above, and the soda and potash below the horizontal line. By joining the points thus obtained, a picture of the rock is produced, the shape of which at once shows the chemical character of the rock. An example will make the matter clear. Suppose a diorite to have the percentage composition represented by the figures in column I. of the following table, then the molecular proportions, obtained by dividing per- centage by molecular weight, will be as shown in column II., and the lengths to be laid off in millimetres as shown in column III. I. II. III. SiO a 56-52 942 47 A1 2 3 16-31 16 16 Fe 2 0, 4-28 FeO 5-92 082 8-2 MgO 4-32 108 10-8 CaO 6-94 124 12-4 Na 2 3-43 055 5-5 K 2 1-44 015 1-5 H 2 1-03 1 A convenient scale is 100 millimetres to the unit. 2 The ferric oxide (Fe 2 O 3 ) can be plotted on the FeO line and beyond the FeO ; but this is not essential to the present purpose. 156 CLASSIFICATION [PART in Plotting these results in the manner described, the diagram marked II in figure 73 is obtained : FIG. 73. Diagrams for three typical members of the Calc-alkali Series. I. Granodiorite ; II. Diorite ; III. Gabbro. In the same figure the molecular proportions of a granodiorite and a gabbro are shown, and an inter- esting comparison may be made. The increase in lime and magnesia, and decrease in alkalies with increasing basicity, are well shown. In figure 74 are similar plots of the molecular proportions of a CHAP, i] THE PLUTONIC ROCKS 157 diorite, a monzonite, and a soda-syenite. These illustrate the variation from series to series in the same (in this case, the intermediate) group. The figures show the comparatively high lime, magnesia and iron in the representative of the M FIG. 74. Diagrams for three typical members of the Intermediate Group. I. Diorite ; II. Monzonite ; III. Soda-Syenite. calc-alkali series, the greater proportion of alkalies without diminution of lime in the monzonite series, and the greatly increased alkalies with diminished lime, magnesia and iron in the representative of the alkali series. 158 CLASSIFICATION [PART m The division of the igneous rocks into series is justified by the fact that although the assemblage of rocks in any given area of the earth's surface may range from quite basic to very acid types, they usually have chemical characters in common which place them in a definite series. Thus, for example, the whole chain of the Andes in South America, the corresponding coast- ranges (the Sierra Nevada, etc.) and the main chain of the Rocky Mountains in the Western States of North America, are characterized by the occurrence of rocks belonging to the calc-alkali series, namely, granodiorites and diorites among the plutonic rocks, and their corres- ponding volcanic equivalents the andesites ; while the hill ranges east of the Rockies (in Montana and other Central States) 1 and of the Andes (e.g., in the Argentine and Paraguay) present types progressively richer in alkalies. Again, over large areas of the eastern portion of the African Continent and in the adjacent islands, the prevailing types belong to the alkali series. 2 In smaller areas, well-named by Professor Judd, 3 petrographical provinces, the prevalence of types belonging to a 1 Pirsson : Petrographical Province of Central Montana Amer. Journ. of Sci., vol. xx., 1905, p. 36. 2 Lacroix : Nouv. Arch. d. Museum, 4me. ser., vols., 1902, 1903. See also an interesting paper on this subject by G. T. Prior : Comparison of Volcanic Rocks from the Great Rift Valley with rocks from Pantell eda, the Canary Islands, Ascension, St. Helena, Aden and Abyssinia, Journ. of the Min. Soc., vol. xiii., 1903, p. 228. a J. W. Judd : On the Ancient Volcano of the district of Schemnitz, Quart. Journ. Geol. Soc., vol. xxxii., 1876, p. 292. CHAP, i] THE PLUTONIC ROCKS 159 definite series is also shown. Thus the Kola Penin- sula 1 and New England 2 may be quoted as examples of alkali provinces. On the other hand, the occurrence in a limited area of members of the calc-alkali series is exemplified by the plutonic complex of Garabal Hill } near Loch Lomond, 3 which comprises rocks ranging from hornblende-biotite granite through torialite and diorite to peridotites. As a whole the British Isles constitute a calc-alkali province ; but there are certain limited areas in which alkali rocks predominate (e.g., Cornwall, Leinster, and the neighbourhood of Loch Borolan, in Scotland). The rocks composing the Brocken granite massif in the Harz, 4 (hornblende- biotite -granite, tonalite, diorite, and gabbro), are another example. It was Eosenbusch 5 who, by extending Durocher's 6 hypothesis of the liquidation of the original earth magma into two zones consisting of an upper acid and a lower basic magma, first formulated the theory that the plutonic rocks have been produced from a number of partial magmas which themselves were differentiated from an originally homogeneous earth- magma (Urmagma). From the magma-basins in 1 W. Ramsay and V. Hackman : Das Nephelin-syenit-gebiet auf Kola, Fennia, vol. xi., No. 2, Helsingfors, 1894. 2 H. S. Washington : Journ. of Geol. , 1898. 3 Dakyns and Teall : Quart. Journ. Geol. Soc., vol. xlviii., 1892, p. 104. 4 Lessen: Zeits. D. Geol. GeselL, vol. xxxii., 1880, p. 206. 5 H. Rosenbusch: Tschermak's Min. Pet. Mitth., vol. xi., 1890, p. 144. 6 J. Durocher : Ann. d. Mines, ser. 5, vol. xi., 1857, p. 217. 160 CLASSIFICATION [PART m which these earliest separated products had ac- cumulated, later differentiations also arose, perhaps under the influence of the pressure due to earth movements in a sinking area, as postulated by Brb'gger for the Christiania district. 1 According to Rosenbusch's theory, the first step in rock evolution consisted in the differentiation of the original earth-magma into two classes of secondary magmas: (1) Those rich in alkalies which he designated the nepheline-syenite (foyaitische) and theralite magmas, and (2) those poorer in alkalies, but richer in lime and magnesia, which he called the granito-diorite and gabbro-peridotite magmas. This division corresponds almost exactly to the alkali and calc-alkali rock series, and Rosenbusch adopts these names for the magmas in the last edition of his Mikroskopische Physiographic (vol. ii., 1907). The recognition of the existence of an intermediary or connecting magma giving rise to the monzonite rock series, the members of which present affinities sometimes to the alkali, and sometimes to the calc- alkali series, we owe to Brogger's classic work in Southern Tyrol and in the Christiania district. THE GRANITE FAMILY. The granite 2 family is by far the largest division of the pi u tonic rocks, for it comprises all rocks of 1 Die Triadische Eruptionsfolge bei Predazzo: Vidensk. Skrtfter I. Math.-natur. Klasse, 1895, p. 177, and The Basic Eruptive Rocks of Gran, Quart. Journ. Geol. Soc., vol. 1., 1894, p. 15. 2 The word granite was first used by Csesalpinus in the year 1596. CHAP, i] THE PLUTONIC ROCKS 161 deep-seated origin with over 66 per cent, of silica. In general terms, the members of this family may be characterized as consisting of a holoc-rystalline, and as a rule coarsely crystalline, aggregate of quartz, two or more felspars, and one or more varieties of mica, hornblende or augite. Accessory minerals are numer- ous : apatite, zircon, sphene, orthite, rutile, garnet, topaz, cordierite (pinite), cassiterite, tourmaline, fluorspar, molybdenite, hsematite, magnetite and iron- pyrites being among them. Resulting from the de- composition of the original constituents (and especially of the felspar) are sericite, talc, kaolin, epidote, calcite, chlorite and serpentine. The essential minerals can be easily distinguished in the hand-specimen : the quartz by its vitreous lustre, pellucidity, conchoidal fracture and absence of cleavage ; the felspars by their large and lustrous cleavage surfaces, their twinning and their opaque white or pink colour; and the micas by their brilliant lustre, platy cleavage, and characteristic colour, silvery white for muscovite, brownish-black for biotite. Hornblende, when present, is a black or dark green variety, with the usual prismatic cleavage, the cleaved surfaces meeting at an angle of 124 degrees. With regard to structure, the granitic structure is typically one in which the component minerals are equidimensional and devoid of crystal faces, each grain having common boundaries with its neighbours, as if formed by simultaneous crystallization. Frequently, however, some of the felspar occurs as large, well- shaped phenocrysts embedded in a holocr^ystalline groundrnass of quartz, felspar and mica (povphyritic, 162 CLASSIFICATION [PAUT m granite). The size of the individual grain varies considerably. Often a granite is coarse-grained in the interior of the mass, and fine-grained in its marginal portions, where cooling has progressed more rapidly. A parallel arrangement of the constituents, especially when the felspars are idiomorphic, or are developed porphyritically, betrays flow-movements in the partially consolidated granite magma, and is sometimes accompanied by protoclastic granulation ; but such flow-structures must be distinguished from the foliation produced by differential movements due to earth stresses after consolidation. In such foliated granites (gneissose granites, augen-gneiss), "nuclei" of the original quartz or felspar are surrounded by granular aggregates of cataclastic quartz and felspar, the whole forming eye-like masses wrapped round by fibrous and ribbon-like laminae of white mica. A common feature is the presence of dark patches or "heathen," as they are termed by the quarrymen. These patches, which are mostly of irregular shape, are composed of ferromagnesian minerals, iron-ores, and a little plagioclase felspar. Their origin has been the subject of some controversy. J. A. Phillips 1 con- sidered them to have arisen by a segregation of the more basic constituents of the granite ; but it is pro- bable that in the majority of cases they are fragments (xenoliths) of pre-existing rocks caught up and en- closed by the granite magma. The spheroidal aggre- gations of plagioclase, dark mica, hornblende and 1 Quart. Journ. Geol. Soc., vol. xxxvi. (1880 , p. 1, and vol. xxxviii. (1882), p. 216. CHAP, i] THE PLUTONIC ROCKS 163 magnetite round a central nucleus, with both radial and concentric structure, have, however, undoubtedly been formed by concretion during the consolidation of the granite magma. The spheroid-granite of Mul- laghderg in County Donegal is a well-known example. 1 Granite occurs as bathyliths, laccoliths, and stocks, often forming the central core of a mountain range, and surrrounded by an aureole of rocks which pre- sent unmistakable evidence of the metamorphism produced by heat, or in some cases by the fluoric and boric emanations accompanying the intrusion. With- in this contact zone, which, in the case of a large granite massif intruded into folded rocks, may have the width of a mile or more, the rocks are crumpled, baked and altered into hornstone. In slates charac- teristic contact-minerals are developed biotite, mus- covite, chlorite, ottrelite, tourmaline, chiastolite, anda- lusite, corundum, kyanite, cordierite, sillimanite, and staurolite. The development of these minerals in the portion of the contact zone furthest removed from the the granite often assumes the character of spots and knots in the slates. The metamorphism round the Leinster, Cornish and Skiddaw granites, and that pro- duced by the intrusion of the newer granites of the Highlands may be cited. In the case of calcareous rocks, limestones are converted into marble, with development of lime-silicates, as for example, the minerals wollastonite, idocrase, diopside arid lime garnet ; while calcareous shales yield lime-silicate 1 Hatch : Quart. Journ. Geol Soc., vol. xliv., 1888, p. 548, with a summary of the literature on spheroid-bearing granites. 164 CLASSIFICATION [PART in rocks (the kalksilikat-hornfels of the Germans). The Coniston limestone, where it is in contact with the Shap granite at Wasdale Head, affords a good example of this kind of contact-rnetamorphism. Similar phenomena may be seen around the Dart- moor, St. Austell and Bodmin Moor granites. THE CLASSIFICATION OF THE GRANITES. The granites are usually divided according to the nature of their ferromagnesian minerals into : granite proper, with both light and dark mica ; muscovite-granite, with white mica alone ; biotite- granite, with dark mica alone ; hornblende-biotite- granite, with hornblende in addition to dark mica ; hornblende-granite, with hornblende alone ; augite- granite, with augite in addition to biotite or horn- blende. Since the number of combinations of these minerals is very great, the classification is a clumsy one, and it is much more convenient to classify the granites according to the felspar content. As already explained, the granites may be divided, according to the relative proportion of alkali and soda- lime fel- spars, into an alkali- family, with predominating alkali felspars, a cole-alkali family (the grano- diorites), with predominating lime-bearing plagioclase, and, forming a connecting link between these two extremes, a family in which neither alkali nor soda- lime felspar greatly predominate (the adamettite family}. The dividing lines between these three families must of necessity be drawn arbitrarily ; but practical limits may be fixed by assigning those CHAP, i] THE PLUTONIC ROCKS 165 granites, in which less than two-thirds and more than one-third of the total felspar content is alkali-felspar, to the adamellite family ; while those with alkali-felspar, amounting to more than two- thirds of the total felspar, rank with the alkali- granites, and those with less than one-third with the granodiorites. THE ALKALI-GRANITE SUB-FAMILY. The alkali-granites may be again sub-divided according to the nature of the alkali-felspar into the potash-granites, in which the predominating felspar is a potash felspar (orthoclase, perthite or microcline), and the soda-granites l in which the predominating felspar is a soda-felspar (soda-orthoclase, anorthoclase, or albite). The potash-granites contain abundant muscovite, either alone or in association with dark mica. The former occurs in small lustrous flakes without crystal contours. The dark .mica is biotite or more rarely a lithia-bearing mica (lithionite). It is strongly pleochroic, in brown or reddish-brown to pale yellow colours, and often contains* numerous enclosed zircons with pleochroic halos. The dominant mineral is the potash-felspar, sometimes in large phenocrysts, usually twinned on the Carlsbad type. It may be orthoclase, perthite or microcline. The latter is recognisable under the microscope by its 1 The name soda-yranite was first used by Prof. Haughton in describing some of the Leinster granites. Trans. Hoy. Irish Acad., vol. xxiii. (1859), p. 608. 166 CLASSIFICATION [PART m characteristic cross-hatching. Enclosures of other minerals, black mica, zircon, apatite, etc., are common; and alteration products, such as muscovite and kaolin, are usual. A soda-lime felspar (usually oligocla.se) accompanies, but is subordinate to, the potash felspar. Its twin-lamellation between crossed nicols affords a ready distinction. The true soda-granites are comparatively rare. They are characterized by the predominance of soda silicates. Soda-orthoclase and anorthoclase or albite are the dominating felspars ; and with these are associ- ated various soda-amphiboles (arf vedsonite, riebeckite, glaucophane and hastingsite) and soda-pyroxenes (segirine, ssgirine-augite and acmite). Different types are usually distinguished by prefixing the name of the dominant amphibole or pyroxene thus: riebeckite- granite, segirine-granite, etc. With decreasing silica the soda-granites grade into the soda-syenites, or, if nepheline be developed, into nepheline-syenites. For rocks which consist of quartz and alkali-felspar without an appreciable quantity of ferro-magnesian minerals, Spurr has suggested the name alaskite. 1 The chemical composition of some typical alkali- granites is given on page 167. THE ADAMELLITE SUB -FAMILY. Here are included the granites in which the lime- bearing plagioclase felspar either equals the alkali- felspar in quantity, or is not greatly subordinate thereto. The potash-mica (muscovite) is usually 1 Spurr : Amer. GeoL, April, 1900, p. 230. CHAP. l] THE PLUTONIC ROCKS 167 - c 10 T|- ON O O 00 O VO . 2 rt vO O t- CO ^ ^ CO ON M > &0 c M row M* J * J> O COM o II t^* M o M .3 ,11 lOOO LO C^ 1 W rt |-OO 1 xo ^ < ^~ ^ cJ o d *~o co *l 0) Is ONM vOoOOOOO^rO OO Mi&J vOM .*-<. ^WM^ON vo 3 |J t^. M PH la Mt-^- vo^nr^voir, O^ H! jf c* vo -o M ' ci io 6 |3 o '5 C rt 2 j=!i ,_; ex tyo^ rf 2 . r* (N, N ? 1 1 CO tO ^ 1 I O N O ' ' O to O ' OO M o o o .tj c o H- in ro t^ 10 TJ-VO o^ ro a !>. - LO HH "o"O > ls OS vO^O" NM ^i-row VO M O O M V -ill to Th rOOO ^ t- vo LOVO OO ON vOrOON^NOOOOOCOW fO ON ON| si p CO M3 HH M CO Tf M VO M ON ON 2 - c ^^j-MMNNMVOON M -V >|g <3 vot^-MMMWro-^- VO M O^ ON 4) M ^-vo . t^^o vo M rj- CO 00 OrO^O r^OO^^CCO <3- U*5 N > E"5j 43 <^ cx5 r^- rO c5 a ^* VO M M 4j ..'|& IOMVO . ^-ooo^-^t^ "2 iOMw|O^OM^^ rf CO 6.3 . ^ COvOM MMrfrf^jj.j ON ON ti ~iJ t^OCN lONVO^OO 1 OsOO N H OO -^ 10 t^ | W t^. 11 00 ^- CO M" tO W ' JT O VO M ON ON ll M TJ- t- toco 10 fo ^oo O l>-ONO 'J* . O 1 * t" O* "* VO O 11 ON t* N M -H TO ro & C* U , & C r^ X ^ ii I v. 6 O d d > > d ri 172 CLASSIFICATION [PART in FIG. 76. Hornblende-granite (granodiorite). f. Felspar (chiefly plagioclase). q. Quartz. m. Mica. h. Hornblende, with granules of Epidote. THE SYENITE 1 FAMILY. This large family of intermediate rocks is char- acterized by the presence of one or more of the alkali-felspars (orthoclase, perthite, soda-orthoclase, anorthoclase, microcline), or of the felspathoids (neph- eline, leucite, and sodalite), together with a subordi- 1 The term syenite was used by Pliny to designate a rock occurring at Syene (Assouan), in Egypt. This rock is a horn- blende-biotite-graiiite. Werner applied it to all granular igneous rocks containing dominant felspars, with subordinate amphibole, pyroxene and other minerals, and without noticeable quartz. CHAP, i] THE PLUTONIC ROCKS 173 nate soda-lime-felspar, and one or more of the following ferro-magnesian constituents: amphibole (hornblende, arfvedsonite, barkevikite), pyroxene (augite, segirine), and mica (biotite, lepidomelane, etc.). Rarer types are characterized by the presence of cancrinite, zircon, eudialyte, katapleiite, or melanite. Among accessory minerals, sphene, apatite, zircon, and the usual iron- ores (magnetite, ilmenite, and haematite), deserve especial mention. A rarer accessory is perovskite. The typical structure of the syenites is the granitic. When quartz is present it occasionally occurs in micro- graphic intergrowth with orthoclase. The nepheline- syenites show a tendency to the development of the porphyritic structure, with large phenocrysts of nepheline. THE POTASH-SYENITE SUB-FAMILY. The original type selected by Werner for the syenites is the orthoclase-hornblende-rock of Plauen, near Dresden. Werner's object was to establish a type of felspathic igneous rock having a granitic structure, but distinguished from granite by the absence of noticeable quartz. The rock of Syene (Assouan), in Egypt, from which Pliny derived the word syenite, contains quartz, and is in fact a hornblende-biotite- granite. The Plauen rock (plauenite of Brogger), on account of its dominant hornblende, may be con- veniently termed a hornblende-syenite, and the terms augite- or biotite-syenite may be used for syenites characterized by the combination orthoclase-augite and orthoclase-biotite. In these rocks there is always present, in sub- 174 CLASSIFICATION [PART in ordination to orthoclase, a member of the soda-lime series (oligoclase or andesine). Quartz is not entirely absent, as the microscopic examination of thin sections shows. By the increase in this mineral the rocks pass through quartz-syenite into potash-granite. The dark- coloured minerals are often present in considerable quantity, and then impart a dark tint to the rock. The hornblende, which, with rare exceptions, is a green variety, may be recognized by its pronounced pleochroism, its well-marked cleavage, its prism angle ANALYSES OF POTASH- SYENITES. i. Eeichenstein. ii. Plauen. in. Biella. Average. Si0 2 62-51 59-83 59-37 60-57 A1 2 3 12-78 16-85 17-92 15-85 Fe 2 3 2-56 6-77 ) 8-23 FeO 4-70 7.01 2-02 1 MgO 3-3:3 2.61 1-83 2-59 CaO 4-76 4.43 4-16 4-44 Na 2 2-71 2-44 1-24 2-13 K 2 4-81 6-57 6-68 6-02 H 2 1-53 1-2D 0-38 1-06 Ti0 2 0-81 0-26 0-53 P 2 5 0-58 )-58) 100-56 101-03 101-21 I. Augite-syenite, Reichenstein, Silesia. Traube : Neues Jahrbuch, 1890., I, p. 206. II. Hornblende-syenite, Plauen, near Dresden. F. Zirkel : Fogg. Ann. B., 122, 1864, p. 622. III. Hornblende-syenite, Biella, Piedmont. Cossa : Mem. Acad. d. Sc. di Torino. (2) B. VIII., p. 28. CHAP, i] THE PLUTONIC ROCKS 175 of 124, its small extinction angle, etc. The augite is more variable in character : sometimes it is almost colourless or pale-green ; at other times it partakes more of the nature of diallage, having then a violet- brown tint. The mica is always a brown biotite. It is very liable to chloritization, turning green in the first stage of the process. Muscovite is never present except as a secondary constituent. The foregoing analyses of typical potash -syenites were selected by Brogger l for the calculation of the average composition of potash-syenites. THE SODA-SYENITE SUB-FAMILY. In the soda-syenites the dominant felspar is either soda-orthoclase or anorthoclase, although albite and orthoclase may also be present without being inter- grown to form perthite. The pyroxenes and am phi- boles are represented by the soda-silicates segirine, arfvedsonite, barkevikite, riebeckite, etc. In conse- quence of the subordination of dark-coloured mineral the soda-syenites have in general a lighter colour than the potash-syenites. They grade by increasing quartz into the soda-granites, and by the appearance and in- crease of nepheline into nepheline-syenites. The following are the chief types : Pulaskite. This type was first described by J. F. Williams 2 from the Fourche Mountains in Arkansas as consisting essentially of a soda-orthoclase (micro- 1 Die Triadische Eruptionsfolge bei Predazzo. Vidensk. Skrifter I. Math.-natur. Klasse, 1895, No. 7, p. 31. 2 J. F. Williams : The Igneous Rocks of Arkansas. Ann. Rep. Geol. Survey of Arkansas, 1890, p. 74. 176 CLASSIFICATION [PART in perthite) together with biotite and a brown or' green hornblende (barkevikite or arfvedsonite). Plagioclase and quartz are, as a rule, absent. The occasional presence of nepheline and sodalite indicates a passage to the nepheline-syenites. MAP OF THE CHRISTIANA REGION. Quartz- Syenite. ( Nordma rkite). \ Granitite. Rhomben-porphyrie \&-A itgite-porphyrite {Basic Rocks. \ Devonian & \Siliirian. I A rcluean. Scale. 1:1,250,000 = 05068 inch to the mile, or about 19-73 niles to the inch. Typo; Etching a FIG. 77. Map of the Christiania District, showing the distribution of the Nordmarkite and other members of that petrographical province. (In reproducing this map from the Quart. Joiirn. of the Geol. Soc. (vol. I., 1894, p. 16), I have at Prof. Brogger's request interchanged the references to the Nordmarkite and the Granitite, which had been incorrectly given.) Closely related to pulaskite is the nordraarkite type, described by Brogger 1 as covering a large area 1 Brogger : Die post-silurischen Eruptivgesteine des Chris- tiania-gebietes. Zeitsch. fitr Kryst., vol. xvi., 1890, p. 55. CHAP, i] TliE PLUTONIC ROCKS 177 in the Christiania district, except that it contains quartz, and thus constitutes a passage to the soda- granites. The felspar is a red micropertbitic ortho- clase, veined with albite. Some oligoclase is also present; and the coloured minerals, which are present in subordinate quantity, consist of segirine and arfved- sonite. The Laurvikite l type of Brogger 2 is characterized chiefly by its rhomb-shaped felspars, which consist of eryptoperthite or anorthoclase, or of both. Plagio- clase is absent. The coloured minerals comprise green segirine, hornblende (barkevikite) and biotite (lepido- melane). Accessory constituents are olivine, apatite and zircon, and occasionally a little nepheline and sodalite. Rosenbusch 3 proposes to embrace the alkali- arnphibole syenites under the name Umptekite, which was given by Ramsay* to a type occurring on the margin of the nepheline-syenite massif of the Kola Peninsula. The Akerite type, described 'by Brogger 5 from Aker in /the Christiania district, is a quartz-bearing augite-syenite which, besides alkali-felspar (a perthitic orthoclase), contains much plagioclase, and is therefore closely allied to the monzonite family (see p. 183). The augite of this rock is a green idiomorphic diopsied. 1 Name derived from Laurvik in Southern Norway. 2 Brogger: Zeitsch. fur Kryst. , vol. xvi., 1890, p. 30. 3 MikrosTcopische Physiographic, 1907, p. 151. 4 Ramsay : Fennia II. No. 2. Helsingfors, 1894. 6 Brogger : loc. cit., p. 45. M 178 CLASSIFICATION [PART HI The soda-syenites may also be classified according to their pyroxenes and amphiboles into riebeckite- syenite, arfvedsonite-syenite, cegirine-syenite, etc. The following are analyses of the chief types : ANALYSES OF SODA-SYENITES. I. n. IIL IV. v. Nordmarkite Umptekite. Pulaskite. Laurvikite, Akerite. Si0 2 64-04 6371 60-03 5685 5i'79 A1 2 3 17-92 16-59 20-76 21-56 16-64 Fe 2 3 FeO 0-96 2-08 2-92 0-66 4-01 0-75 3-44 1-14 i 9-58 MgO 059 0-90 0-80 0-85 2-63 CaO 1-00 3-11 2 62 5-26 5 12 Na 2 6-67 8-26 5-96 6-07 630 K 2 6-08 2-79 5-48 3-66 2-55 H 2 1-18 0-19 0-59 0-52 1-27 Ti0 2 0-62 0-86 Other constits. 23 0-20 0-07 101-37 100-19 101-07 99-35 99-88 I. Nordmarkite, Tonsenas, Christiania. P. Janasch, quoted by Brogger : Zeitsch. fur Kryst., vol. xvi., 1890, p. 54. II. Umptekite, Umpjaur, Kola Peninsula, quoted from Rosenbusch : Gesteindehre, p. 112. III. Pulaskite, Fourche Mts., Arkansas, -ibid. IV. Laurvikite, red variety, Nottero, Christiania. G. Forsberg in Brogger : Zeitsch. fur Kryst., vol. xvi., 1890, p. 30. V. Akerite, Vettakollen, near Christiania. Th. Kjerulf, quoted by Brogger : Die Triadische Eruptionsfolge bei Predazzo, Vidensk. Skrifter I., 1895, No. 7, p. 33. THE FELSPATHOID-SYENITE SUB-FAMILY. We come now to the consideration of the important division of the syenite family which in addition to an CHAP, i] THE PLUTONIC ROCKS 170 alkali-felspar (orthoclase, rnicrocline, micro-perthitc, soda-orthoclase, or albite) contains one or more of the felspathoid minerals (nephe'ine, leucite, sodalitc, etc). The pyroxenes and amphiboles are of great variety, and occur either independently or intergrown. Among the former, diopside, segirine-augite and aegirino (acmite), and among the latter, brownish-green horn- blende, barkevikite, arfvedsonite, and hastingsite are known ; while the biotite is usually the variety known as lepidomelane. The m.embers of this sub-family are usually classified according to the nature of the felspathoid constituent into nepheline-syenite, leucite- syenite, sodalite-syenite, etc. Nepheline-Syenite. A great number of types of nepheline-syenite have been distinguished. Of these the most widely distributed is the Foya type (foyaite), 1 which was first described by Blum from the Foya Hills, in the Serra de Monchique of Southern Portugal. In this type potash-felspar (orthoclase) and nepheline are present in approximately equal proportions, while the coloured minerals are subordinate. According as the latter are represented by hornblende, augite, or biotite, the varieties hornblende- foya/ite, augite- foyaite and biotite- foyaite can be distinguished. In some nepheline-syenites, a soda-felspar accom- panies the potash mineral. Thus litchfieldite is the 1 It should be noted that Brogger (Zeitsch. fur Kryst., vol. xvi., 1890, p. 39) has adopted the term foyaite for nepheline-syenites having a trachytic structure (i.e. with tabular felspars in parallel arrangement) in contradistinction to ditroite, which he uses for those having a granitic structure. 180 CLASSIFICATION [PART in name given by Bay ley 1 to a type in which orthoclase is subordinate to albite ; and in the mariupolite of Morozewicz 2 albite is alone present. Ditroite (from Ditr6 in Transylvania) is a type characterized by abundant sodalite and subordinate coloured minerals (chiefly biotite, seldom augite). In miaskite (so named by G. Rose from Miask in the Ilmen Mountains of Russia) both sodalite and cancrinite are present, and biotite is abundant. The laurdalite of Brogger 3 is a characteristic type developed typically at Laurdal in Southern Norway. It is a coarse-grained rock containing both nepheline and sodalite, the former often in large phenocrysts. The felspar, which may be either a soda-orthoclase (cryptoperthite) or a soda-microcline (anorthoclase), occurs in rhomb-shaped crystals, a character which allies laurdalite to laurvikite. Olivine is an accessory constituent. Eudialyte-syenite, a rock consisting of alkali- felspars, nepheline, aegirine and eudialyte, has been described by W. Ramsay from the Kola Peninsula. The kadapleiite-syenite of Tornebohm 4 contains, besides alkali-felspars, nepheline and segirine, eudia- lyte and katapleiite. Gancrinite-syenite, also described by Tornebohm, consists of albite, a little orthoclase, nepheline, segirine and cancrinite. All the types of nepheline-syenite described above contain felspar in greater or less quantity. There 1 Bull Geol. Soc. America, 1882, III., p. 231. 2 Tscherm. Min. u. Pet. Mitt., vol. xxi. (1902), p. 238. 3 Zeitsch. fur Kryst., vol. xvi. 1890, p. 32. 4 Sveriges Geol. Under sokningar, 1906. CHAP, i] THE PLUTONIC ROCKS 181 exists, however, a type in which the felspathic con- stituent is absent, the rock then consisting solely of nepbeline with a ferromagnesian constituent. Thus Ramsay l has described as urtite a type which consists solely of nepheline and segirine, and Adams 2 has described as monmouthite a similar rock, con- sisting essentially of nepheline and black hornblende, which occurs in the township of Monmouth in Ontario. Leucite-syenite. The leucite-syenites are far less abundant than the nepheline-syenites. The first example of this rock was described by J. F. Williams 3 from Magnet Cove in Arkansas. The rock consists of porphyritic crystals of pseudo-leucite in a ground- mass of nepheline, orthoclase and melanite. A similar rock was described by Hussak 4 from Serra dos Pogos de Caldas in Brazil. Sodalite-syenite. A type of syenite in which sodalite predominates has been described by Ramsay from the Kola Peninsula as tawite. It is composed of sodalite and segirine. A type consisting solely of sodalite appears to exist in Greenland. It may be described as sodalite-rock. 5 The average chemical composition of the nepheline- syenites has been calculated by Brogger 6 from thirteen 1 Geol. Foren. i. Stockh. Forh., vol. xviii. (1896), p. 459. 2 Amer. Journ. of Sci., vol. xvii. (1904), p. 269. 3 Ann. Eep. Geol Surv. Arkansas, for 1890 and 1891. 4 Neues Jahrb., 1892, ii. p. 146. 5 Rosen busch : Mikroskopische Physiographic, 1907, p. 240. 6 Brogger : Die Eruptionsfolge der Triadischen Eruptivgesteine bei Predazzo in Siidtyrol. Vidensk. Skrifter L, 1895, No. 7, p. 29. 182 CLASSIFICATION [PART in w CO w 4) .- >, ~^ S O rhfOlOt^'t-'OOMN N 00 >'S| 3 J* M M'^-tO'-irOOOTl'O S . 5 .11 t* O to t> M T^- t->. ON o O w g j *-A U i en tn '3 S g . 1 fO O O to N O N t^vo N O IO N R c ^c E^^| o^ ^'- IM o JS^^^-^g;-, m . ? ^Sr? J^-C'rt^^ ^11 PS ^ 2,2'S j! C fa 02 IO M 8 M ^S1 S c oT C c 53 S 6? V = 2,- O O ON to ONOO ONOQ tO M ONH*VONf^lO VO 00 j| gp.srg)^ Itii^ a |6S vO Tt" "-I O O ON VO HH IO N M 1NS1| lf-s|l s " 2 'a KHXI t * M ONO ONNl>.t^iO, t^. rj- -f M N tOOO ^ 1 vO 0> to ^ N t^ LO O" 10 0* o ON 3 ^ll3 "ttT cTfil r% & s< M "W'2 . i-^00 IO a* 5 G fe h3 > I W ^^S^ S D -US fc '" 13 r. *0 ON (U >> CHAP, i] THE PLUTONIC ROCKS 183 analyses of typical occurrences to be Si0 2 = 55'78, Ti0 2 = 0'55, A1 2 O 3 = 21-34, Fe 2 O 3 = 5\35, MgO = 0'78, CaO - 1-93, Na a O = 7-82, K 2 O = 5-83, H 2 O = 1-29. Ratio of the bases CaO : Na 2 O : K 2 O = 1:4:2. The analyses given in the table on p. 182 are of well- known types. THE MONZONITE FAMILY. Occupying a middle position in the Intermediate Group between the syenites and the diorites are the monzonites ; 1 that is to say they are rocks in which * * FIG. 78. Diagram showing the relation of rocks of the Monzonite Series. 1 This name was given by de Lapparent in 1864 to the rocks of Monzoni in the Tyrol (Ann. d. Mines. (6), vi., 1864, p. 259), and has been used with varying significance by different authors. It is used here in the sense defined by Brogger : " Das wirklich charakterische bei diesen Gesteinen ist class sie in der Regel Orthoklas und Plagioklas fiihren ungefahr gleich reichlich, oder jedenfalls beide reichlich fiihren." Die Triadische Eruptions- folge bei Predazzo, loc. cit. p. 21. 1F4 CLASSIFICATION [PART m the soda-lime felspars (plagioclase) play a greater role than in the syenites, but not so great as in the diorites. Since the two kinds of felspar may be associated in every proportion, it is necessary to adopt some arbitrary dividing lines between the monzonites and the syenites on the one side, and between the monzonites and the diorites on the other. The suggestion made by Lindgrcn l is adopted here, namely, that rocks having alkali-felspar to the extent of more than two-thirds of the total felspar are classed with the. syenites, and those with less than one-third with the diorites. Consequent!} 7 the mon- zonites embrace all those intermediate plutonic rocks in which the ratio of alkali-felspar to soda-lime felspar is less j than two-thirds and more than one-third. With regard to silica the monzonites have a percentage range between 52 and 66. With more than 66 per cent, they pass into the adamellite division of the granites, and with less than 52 per cent., into olivine rocks of the kentallenite type. 2 The most acid division of the family, corre- sponding to the tonalites in the diorite family and to the quartz-syenites in the syenite faniily, are the banatites. The range of silica in this sub-division is fixed by Brogger at between 63 and 66 per cent. The monzonites are characterized by a fairly high percentage of alumina, which averages about 17 per 1 Amer. Journ. of ~8ci. , vol. ix., 1900, p. 269. 2 Brogger: Die Triadische Eruptionsfolge bei Predazzo, loc. p. 61. CHAP. l] THE PLUTONIC ROCKS 185 cent. ; lime is from 6 to 8 per cent. ; while potash and soda are present in roughly equal percentages. The soda-lime felspar may vary from andesine to labra- dorite. Quartz is absent, or present only in small quantities in the most acid members (the banatites). The dark minerals pyroxene (both augite and hy- FIG. 79. _V : Mcmzonite, Monzoni, Tyrol. (Between crossed nicols.) Or. Orthoclase. PL Plagioclase. P. Pyroxene. H. Hornblende. O. Quartz. B. Biotite. (After Brogger.) persthene), hornblende and biotite, are variable in quantity, but always sufficiently developed to deter- mine a dark colouration of the rock. Nepheline and leucite occur occasionally, thus affording a passage to the nepheline-syenites. Oiivine only appears in the 186 CLASSIFICATION [PART in more basic members of the family. Accessory minerals are sphene, zircon, apatite, and the usual iron-ores. The mean chemical composition, as cal- culated by Brogger from a number of analyses of typical monzonites, is given in the following table: ANALYSES OF MONZONITES. i. ii. in. IV. Banatite Monzonite Monzonite Monzonite Chamwood Stratherric. Monzoni. Brb'icger's Forest. Average. Si0 2 64-30 59'34 54-20 54-90 A1 2 3 17-89 18-91 15-73 17-44 Fe 2 3 FeO ) 4-75 6-22 95 3-67 5-40 ) 9-64 MgO 1-12 2-f>5 3-40 3-26 CaO 3-33 3-97 8-50 6-96 Na 2 3-84 2-61 3-07 3-50 K 2 3-37 291 4-42 3-51 H 2 1-60 1-43 0-50 1-23 Ti0 2 72 0-40 71 Other constits. 39 1-21 10020 100-00 100-50 I. Banatite, Croft Hill, Charnwood Forest. (E. E. Berry). II. Monzonite, Stratherric, Glen Liath, Inverness-shire (W. Mackie, Trans. GeoL Soc. Edin., vol. viii., 1905, p. 54.) I'll. Monzonite, Monzoni, Tyrol. (M. Schmelck). IV. Brogger's average of fourteen typical Monzonites. Die Triadische Eruptionsfolge bei Predazzo, loc. cii., p. 63. THE DIOR1TE FAMILY. The diorite family is that division of the intermediate gjroup of plutonic rocks in which soda-lime felspar CHAP, i] THE PLUTONIC ROCKS 187 largely predominates over alkali -felspar. According to the rule we have adopted, the ratio of ortho- clase to plagioclase must not exceed one-third to two- thirds. The dominant felspar is a soda-lime felspar ranging from oligoclase to labradorite ; but some orthoclase is notably present in all diorites. With regard to silica, the limits are those of the intermediate group, namely 52 to G6 per cent. With more than 66 per cent, the diorites pass into the granodiorites, and with less than 52 per cent., into the gabbros. The more acid diorites contain a fan- amount of quartz, and it is customary to distinguish these rocks as quartz -diorites or tonalites. The original Tonale type is a quartz-diorite containing abundant dark mica, together with hornblende. The tonalites are allied to the banatites, the difference between them consisting in the relative proportion of orthoclase and plagioclase. The most common ferro- magnesian constituent in the diorites is green or brown hornblende, 1 in crystals or allotriomorphic grains; but brown biotite, pale green or colourless augite (salite or malacolite) and rhombic pyroxenes also occur. Among accessory minerals the iron -ores (magnetite, ilmenite, pyrites), apatite, sphene and zircon are the most frequent. Secondary minerals are chlorite, epidote, limonite and calcite. Typical diorites have a granitic structure, but with a texture less coarse than in the granites. Frequently 1 The name diorite (Or. diopifa) was given by Haiiy on account of the marked contrast between the hornblendic and the felspatohic constituent. (Traite de Min., 1822, p. 540.) 188 CLASSIFICATION [PART m the grains are allotriornorphic and equi-dimensional. Occasionally, however, there is a tendency towards porphyritic structure, and then the plagioclase, and more rarely the hornblende^ is idiomorphic. Ortho- clase and quartz occur interstitially, with a tendency towards micrographic intergrowth. Spheroidal structure, similar to that described when dealing with the granites, is well developed in the ball-diorite or napoleonite of Corsica, where the spheroids consist of hornblende and felspar, and show a well- developed radial and concentric arrange- ment. A usual classification of the diorites based on the ferromagnesian minerals is: diorite ( = hornblende- diorite), mica-diorite, augite-diorite and hypersthene- or enstatite-diorite. Some selected analyses of typical occurrences are given in the table on p. 189. THE ALKALI-GABBRO FAMILY. The family of the alkali-gabbros comprises plu tonic rocks of granitic structure, with less than 52 per cent, of silica, and containing a considerable proportion of alkali -felspar. It includes basic representatives of the felspathoid, the pota&h, the soda and the mon-zonite series. These are distinguished here as the nepheline-gabbro, the shonkinite, the essexite, and the keritallenite sub-families, all of which are closely allied and grade into one another by varia- tions in their felspabhic and fels-pathoid elements. CHAP. l] THE PLUTONIC ROCKS 189 C rt .2 o IO N O W vO CO W CSI tO M K" 0) p N to N *> t> OO N M M o }l IO M , M rh t^OO CO Tj- ON IO M OO N ^- tO to CO ON 1 CQ "If co^ ONvo' OO CO M to 1-1 8 M rC ON M CO CO to CO t^OO vO - ^ O f^* co LO co w ON to ON CO S-2-3 TJ- vo ON to 00 ** N ON 10 1-1 s Q 0) .:g | M "-i CO N CM rJ-CO^CO ^O rj- tOCOCO ONCOON-^-^-O N IO 00 ON ^ P2 !-< vo" vo* ^" to ^vo" CO M M . 1 IO M o Q l> rt ON ONCO N t^* OO ^" O CO "3 "^ O OO Tf O ON M M tOVO I t^ M c g cood co N M vo co M 6 1 ^a M , ! i 11111 o ro "> ^ OJ^piHpqSU^MWHQ ^ W -J2 o od 06 "^- M M o rood co M M o J ; o r\ ONOO CO ^ N t~ IO Tj" HI HI Q\ U <* M Q^ E > I CO O ON HI vo *O ^T O ON vo ON CO ON ^_, TT "- 1 ON ^ ^ ! .-..;* I f ^j -q- n HI ON w 8 . I o 10 NOOCOVO^OON ON ^o K VO t^ N l>- CO t^VO N ' N O\ PL) TJ- M ON _aj 'S tOiOCOOOOMMVOrJ-OO CO CO ON O T M M _A_ O O2 o'l CHAP. l] THE PLUTONIC ROCKS 201 THE GABBRO FAMILY. This family comprises the basic members of the calc-alkali series of plutonic rocks ; in other words, the gabbro are rocks of granitic habit with less than 52 per cent, of silica, and composed essentially of a lime-bearing plagioclase, together with one or more ferro-magriesian constituents. M+MUff -04Z - FIG. 86. Diagram showing; the relations of Gabbro to the Calc-alkali Series. The word gabbro is derived from an Italian place- name, and originally signified a serpentine containing diallage. Von Buch first gave the type more pre- cision by defining it as a rock composed essentially of plagioclase felspar and diallagic augite. 1 1 Hatty's euphotide (TraiU de Mm., 1822, p. 535) was used in this sense. 202 CLASSIFICATION [PART in The felspar is a basic plagioclase generally labra- dorite, but occasionally bytownite or anorthite. Some oligoclase or andesine may be present in subordinate quantity. Orthoclase is absent except in rare instances, when it is associated with quartz in micrographic intergrowth. Chemically the gabbros are characterized by high percentages of lime, magnesia, and ferrous oxide, FIG. 87. Olivine-Gabfcro, Sgurr nan Gillean, Cuillin Hills, Skye. F\ Plagioclase. A. Augite. 0. OH vine. (From a thotograibh by K. H. Rastail.} together with low alkalies ; potash is practically absent. There is a natural passage to diorites ; and plagioclase-augite rocks with more than 52 per cent, silica may appropriately be termed augite-diorites. Rocks on the border line are sometimes termed gabbro-diorite or diorite-gabbro. CHAP i.] THE PLUTONIC ROCKS 203 According to the nature of the ferro-magnesian constituents occurring in conjunction with the plagio- clase, we may distinguish the following types : Gabbro = labradorite + augite. Olivine-gabbro = labradorite + augite + oli vine. Norite = labradorite + augite + hypersthene. Olivine-norite = labradorite + augite + hypersthene + oli vine. Troctolite i = labradorite + olivine. Eucrite = anorthite + augite + olivine. AUivalite 2 = anorthite + olivine. Anorthite-rOCk== anorthite alone. FIG. 88. Olivine-Gabbro, Sgurr nan Gillean, Cuillin Hills, Skye. PL Plagioclase. A. Augite. O. Olivine. (From a photograph by R. H. RastalL} 1 So named by von Lasaulx from its spotted appearance, due to the contrast of the dark olivine grains, which are full of magnetite, to the paler felspar crystals, (cf. the German name Forellenstein, anglice, troutstone). 2 Harker: Summary of Progress, GeoL Surv., 1903, p. 56. The name is derived from Allival, a mountain in the Isle of Rum. 204 CLASSIFICATION [PART m By a diminution in the proportion of felspar and corresponding increase in the pyroxenic constituent, the gabbros pass into the pyroxenites ; and similarly if the olivine be increased at the expense of the felspar, they pass into the peridotites. The felspar of the gabbros occurs in allotriomorphic, or in partially idiomorphic grains. It is twinned on the Carlsbad, albite and pericline types, often in com- bination. The well-marked albite twin-lamellation gives high extinction angles in properly oriented sections. When zonal structure is observed, the outer- most zone is less basic than the interior, as shown by the variations in the extinction angles. Thus labradorite may be bordered by andesine. Though often perfectly fresh, the felspar is sometimes weathered into an opaque white substance known as saussurite, which can be resolved by microscopic examination into a confused and intimate aggregate of granules and fibres of zoisite, epidote and secondary felspar (albite), the last-named being of water-clear and glassy habit. l^ o 2 M rf w + CJN M 1 * a > O J ChMCOvCiOi-iONONr^co co co 6 s 1 K | > w l> M ^3> x-v05 a^ ^ rt S O a; II K/3> e CO t*** O VO CO CO vO ^O CO ON Tf O ^ ONCO -^-CO co u j? > ^2 'S ^ v < w . m c |l *i3 %* ON v i-A w ^ r.-s - & ^s -g . M ON ON XH > OS 'nj O ^isi i **H * i. Soda-felsite Arran. c^ vO ^^ ro w O t"** CO M M ^ 6 r^ O i-5 VO 6 o i:-6 M i- 5IJIJ ? 5 5 T o> '3 stgS'S S i i i i i i i i i i i i i i i i OcT/-v O/^C.-s^ o3'-S > 218 CLASSIFICATION [PART m obsidians only by containing some 6 to 8 per cent, of water. By processes of devitrification they assume a stony or felsitic character (felsitic pitchstones) ; and it is probable that many of the compact flinty felsites have been produced from pitchstones by subsequent alteration. The chemical composition of some typical British quartz-porphyries, etc., is shown in the table on p. 217. THE PORPHYRY FAMILY. This family includes hypabyssal equivalents of the syenites, nepheline-syenites and monzonites, on the one hand, and of the phonolites and trachytes on the other. The various types may be distinguished by reference to the plutonic rocks with which they are allied, e.g., syenite-porphyry, nepheline-syenite- porphyry, monzonite-porphyry, or by reference to the nature of the phenocrysts, e.g., felspar-porphyry (orthophyre), nepheline-porphyry, etc. In the monzonite-porphyries, plagioclase is associated with orthoclase among the phenocrysts. The porphyries are all of intermediate chemical composition, and quartz rarely occurs in the form of phenocrysts, although it is often present to a small extent in the groundmass between the felspar prisms. When it is associated with the felspar in the form of micropegmatite, a type allied to the more acid granophyres is developed ; and transitional forms between acid granophyres and granophyric porphyries of intermediate composition no doubt exist. CHAP, n] THE HYP ABYSSAL ROCKS 219 THE PORPHYRITE l FAMILY. This family embraces the hypabyssal equivalents of the plutonic diorite* on the one hand, and of the volcanic andesites 01. the other. The porphyrites occur as dykes, sills, and small laccoliths. The dominant porphyritic constituent is a lime-bearing plagioclase felspar. Ferro-magnesian minerals (augite, hypersthene, hornblende or biotite, occur either as phenocrysts or as constituents of the groundmass. The latter consists chiefly of felspar prisms with the occasional addition of interstitial quartz. Interstitial quartz occurs also intergrown with felspar as micro- pegmatite, in a granophyric type. The granophyric porphyrites, or dioritic granophyres, as they may be also termed since they are not markedly porphyritic, constitute a passage between the true granophyres and the dolerites. The rock of Markfield in the Charnwood Forest, or that of Penmaenmawr, which is classed by some as a hypersthene-diorite and by others as a hypersthene-dolerite, may be taken as a type of these rocks. 2 The chemical composition of some typical British porphyrites is shown in the table on p. 220. 1 The former use of the word "porphyrite" for altered andesitic lavas, has been definitely abandoned in this country. As now used the distinction lies in the intrusive character of the porphyrites as opposed to the extrusive nature of the andesites. 2 Since a specific name is desirable, that of markjieldite, from the Charnwood Forest occurrence, will be used for this type in this book. 220 CLASSIFICATION OO "t fOUD in y$ M to N N t^. o o M N OO CT\ O O u Q 2 ON Ou, <-x M u-> o M t^ t-i COvO M M t-OMD M vo oo N oo ON O ^ >-i I I CHAP, n] THE HYPABYSSAL ROCKS 221 THE DOLERITE FAMILY. The dolerites 1 are dark-coloured rocks occurring in the form of small intrusive bosses, small laccoliths, sills and dykes. They are essentially holocrystalline aggregates of plagioclase and monoclinic augite ; but various mineralogical types are furnished by the additional associations of a rhombic pyroxene, horn- blende, mica, olivine or quartz, as for example hypersthene dolerite, hornblende dolerite, mica- dolerite, olivine - dolerite, quartz - dolerite. A variety consisting almost entirely of plagioclase has been termed leucophyre. Many dolerites have a green colour due to the presence of a member of the chlorite family, which is developed at the expense of the augite. Hence the name greenstone? which is still retained as a useful field-term without regard to composition. The term diabase* has been used instead of dolerite ; but its use is not to be commended, since in Germany it has been applied to the more altered (chloritized) types, including those of volcanic origin. 1 This name was introduced by Haiiy in 1822. Traite de Min., p. 573. 2 De la Beche, in his " Report on the geology of Cornwall, Devon and West Somerset '' (p. 27), defines " greenstone " as a combination of hornblende and felspar. Microscopic examina- tion showed, however, that the ferro-magnesian constituent of these rocks is essentially augite, the hornblende being mainly of secondary origin. 3 This word was introduced in 1813 by Alexandre Brongniart for rocks composed of felspar and hornblende ; but the same mineral association was often designated by Haiiy's term, diorite. The term was revived by Hausmann in 1842, who applied it to rocks composed of labradorite, pyroxene and chlorite. 222 CLASSIFICATION [PART in The plagioclase of the dolerites is labradorite or anorthite; ii may be quite fresh, or much decomposed, altering in the latter case to an aggregate of minute granules of epidote, calcite, and occasionally of quartz. In thin section the augite is usually of a pale violet- brown tint. It mostly occurs in large ophitic masses, enclosing lath-shaped crystals and microlites of felspar. In altered rocks the augite is partially, sometimes FIG. 93. Olivine-dolerite, Derbyshire. PI. Plagioclase. A. Augite. O. Olivine. (From a photograph by R. H. Rastall.} even completely, replaced by a scaly and fibrous aggregate of some chloritic mineral. The original hornblende of the hornblende-dolerites is usually the common brown variety. In the so-called epidiorites, however, it is of a pale green colour (uralite, actinolite, smaragdite), and is produced by paramorphism of the augite. The olivine of the oli vine-bearing dolerites is CHAP, n] THE HYPABYSSAL ROCKS 223 rarely quite fresh, being stained brown with limonite, blackened with magnetite-dust, or completely replaced by serpentine or calcite. When quartz occurs in the dolerites it usually forms a micrographic intergrowth with felspar, and the resulting micropegmatite is interstitial between the other constitutents (e.g. in dolerites of the Ratho type among the Carboniferous Rocks of Scotland). Such rocks are intimately related to tne dioritic granophyres or markfieldites. (See p. 219). Among accessory minerals the iron-ores, magnetite and ilmenite are always more or less abundantly present, the latter giving rise, when altered, to leucoxene (an amorphous variety of sphene). Pyrites is sometimes present in disseminated granules, and apatite in six-sided needles. Quartz, calcite, epidote, chlorite and serpentine are common secondary miner- als ; and these minerals are often distributed in veins and nests through the rocks. The structure of the dolerites is characteristically ophitic ; but in some cases a granular structure prevails. This is due to the occurrence of the augite in round granules, instead of in large ophitic plates enveloping the lath-shaped felspars. Where porphyrit'ic structure is developed, phenocrysts of felspars, and sometimes of olivine and augite, are embedded in a groundmass of felspar microlites and granular augite. Interstitial glassy matter is only found as a marginal phenomenon. The Carboniferous intrusions of the Midland Valley of Scotland afford excellent examples of nearly all the varied types of structure that have been met with in the dolerites. (See p. 338), 224 CLASSIFICATION [PART IH w C J> ^ CO CO "st" CO vO CO M CO N CO ON ' *-H o 1* TJ- 1-4 ON P ^ C ^ C/ rt rri a o b w ^T^ ^ ex rt rt c 4; J^ ^ . . Ci O covo M ON M VO to COOO ^f J U 5 >> ^"o^ rf M ON ON *^'_r -~ Q Q I C/2 _ . > . J> o 2 2^ .55 8 5 C CO to ON t^ 1000 to CO co^O ON CO IJ^W; C/3 i-I c tOO^ONONONtO t^CO CO cj > rt W 1 "5 P ^? f|S| 3W r^. l.fllj o O cot^Tj-cON lOrf-^-ON O\ '"" O ^ J; W N O M ^* ^" CO ^" CO M C 1 -! h^ C^ ON oj O r _, 11 IO M ll/^ I 1 I 1 ^ c/: , , . C 3 o O O^P-i Q ' ^ ^ > > CHAP, n] THE HYPABYSSAL ROCKS 225 The intrusive character of the dolerites is especially shown by their contact phenomena : baking or " porcellanization " of shales: formation of banded and spotted rocks (the so-called desmoisites and spilosites), and of hornstone-like rocks rich in alkalies (adinoles). 1 The chemical composition of some typical British dolerites is shown in the table on p. 224 FIG. 94. Teschenite, Lamlash, Arran. PI. Plagioclase. A. Augite. O. Olivine. (from a photograph by R. H. Rastall.) THE MUGEARITE-TESCHENITE FAMILY. The rocks comprised in this family may be regarded as the hypabyssal equivalents of the alkali-gabbros the mugearites being allied to the essexites, and the 1 Lessen : Ueber den Spilosifc und Desmoisit Zincken's, Z&its. Deuinh. Geol Gesett., vol. xviv. (1872), pp. 701-786. 226 CLASSIFICATION [PART m teschenites to the nepheline-gabbros. The mugearite type is described by Harker 1 as characterized by high alkalies and low magnesia and lime. Felspars rich in alkalies make up 70 per cent, of the rock. They appear under the microscope as lath-shaped crystals imparting a trachytic character to the rock. The felspars consist chiefly of oligoclase with subordinate orthoclase. The remaining constituents are olivine, augite, iron-ore, and apatite. The teschenites or analcime-dolerites are dyke rocks essentially composed of plagioclase felspar, augite and analcime. Olivine is also a frequent constituent. With regard to the analcime, it is an open question whether this mineral is an original constituent, or produced by the decomposition of nepheline, as is considered to be the case in the typical teschenites of Silesia or Moravia. 2 In the figure on page 225 the analcime cannot be dis- tinguished, as it occurs in small interstitial patches. The following table gives some analyses of typical teschenites and mu^earites: 1 The Tertiary Igneous Rocks of Skye, Mem. Geol. Sure., 1904, p. 265. 2 Lindgren : Proc. Calif. Acad. 3d., vol. iii. (1890), p. 29. Pirsson : Journ. of Geol., vol. iv., 1896, p. 679. Cross : Journ. of Geol. vol. v. (1897), p. 684. Pelikan : Tschermak's Min. u. Pat. Mitt., vol. xxv., 1900 p. 113. CHAP, n] THE HYPABYSSAL ROCKS 227 ANALYSES OF TESCHENITES AND MUGEARITES. i. n. in. IV. Mugearite, Teschenite. Teschenite, Teschenite, Skye. Silesia. N. Berwick. Bathgate. Si0 2 49-24 48-18 47-03 45-71 A1 2 3 15-84 11-80 15-36 15-23 Fe 2 3 6-09 9-79 3-38 2-84 FeO 7-18 5-90 7-35 6.93 MgO 3-02 6.05 5-10 8-11 CaO 5-26 7-50 8-47 7-34 Na 2 5-21 3-46 4.32 3-96 K 2 2-10 1-57 3-00 1-31 H 2 2-69 3-20 2-82 6-24 Ti0 2 1-84 2.64 1-64 P 2 5 1-47 0-49 73 47 Other constits. 59 71 47 73 100-53 98-65 100-67 100-51 I. Mugearite, Druim na Criche, 5 miles S.S.W. of Portree, Skye (W. Pollard). The Tertiary Igneous Rocks of Skye. Mem. Geol. Suro., 1904, p. 263. II. Teschenite, Boguschowitz, Silesia. Rosenbusch : Gesteins- lehre, p. 176. III. Teschenite, Craigleith Island, N. Berwick (W. Pollard). Summary of Progress, Geol. Swrv., 1906, p. 75. IV. Teschenite, Sill at Blackburn, Bathgate (W. Pollard). Summary of Progress, Geol. Surv.^ 1906, p. 74. THE APLITE-PEGMATITE FAMILY. The aplites are the most acid differentiation pro- ducts of the granite magma. They are found in association with minettes, to which they are com- plementary in Brogger's sense, in narrow dykes in the neighbourhood of, or penetrating granitic intrusions. 228 CLASSIFICATION [PART m Besides their high silica content, they have a considerable proportion of alkalies and a sub- ordinate quantity of ferro-magnesian constituents. In consequence they are characterized by a light colour white to buff. In the hand-specimens they present a remarkably even and fine-grained, or saccharoidal texture ; while under the microscope the constituents show a remarkable degree of idiomorphism. The dominant constituent is felspar a potash, soda or lime-bearing variety, according to the special type. The proportion of quartz varies. According to the nature of the felspathic constituent and the proportion of quartz, the aplites may be sub-divided as follows: Burite 1 or Granite-aplite = orthoclase + quartz. Soda-aplite = soda-felspar (anorthoclase, albite, etc.) + quartz. Lestiwarite 2 or Syenite-aplite= orthoclase with little or no quartz. Malchite 3 or Diorite-aplite = lime-bearing plagioclase with little or no quartz. Mica (muscovite) occurs in small quantities in most aplites. In the malchites the coloured mineral is usually hornblende. Common accessory constituents are garnet, zircon, tourmaline, and apatite. Pegmatites. The members of this peculiar group are allied to the aplites in composition, but differ 1 Daubuisson (1819). 2 Brogger. Die Grorudit-Tinguait Serie, Zoc. cit., p. 312. 3 Osann : Mitth. d. Grossh. Bad. Geol. Landesanstalt, 1892, ii. p. 380. CHAP, n] THE HYPABYSSAL ROCKS 229 from them in possessing an extremely coarse-grained texture, and in the prevalence of graphic structure ; l also in the variety of accessory minerals they contain. They are now generally considered to be true igneous products, and to represent one of the last phases of consolidation of the plutonic magmas. The super-heated water that was set free during the first consolidation of the magma, doubtless played an important r61e in their formation ; and it is to the concentration in this water of the rarer elements and compounds from the magma, that the crystalliza- tion of many of the rarer accessory minerals in the drusy cavities of the pegmatites, is to be attributed. Consisting essentially of quartz, felspar (orthoclase or microcline) and mica (muscovite or biotite) the peg- matites are the home of a great number of accessory minerals, among which may be mentioned garnet, tourmaline, datolite, zircon, eudialyte, apatite, epidote, orthite, topaz, cassiterite, corundum, and fluorspar. The pegmatite veins associated with the syenites of Norway are remarkable for an abundance of minerals containing rare earths, 2 while in those associated with gabbro masses, apatite is often a, dominant important constituent. THE BOSTONITE FAMILY. The bostonites are the acid differentiation pro- ducts of a magma allied in composition to nepheline- 1 The name has reference to this structure : Gr. Tr^y/ta (Haiiy : Traite de Min., 1822, p. 536. 2 Brogger : Zeitsch. fur Kryst., vol. xvi., 1890, passim. 230 CLASSIFICATION [PART m syenite, or to the essexites ; the complementary basic rocks being the camptonites. They are fine-grained to compact light-coloured rocks, occurring in dykes or in small intrusions. The essential component is an alkali-felspar (inicrocline, anorthoclase, perthite), but a very small quantity of biotite and hornblende may be present. The micro- structure is trachytic a struc- ture characterized by the occurrence of the felspars in small rod-like microlites, forming a groundmass in which phenocrysts of felspar may or may not be present, A parallel arrangement of the microlites is common. The more acid members of the bostonite family, which contain free quartz, are termed quartz- bostonites. The type described by Professor Brogger 1 as lindoite is closely related to the bostonites. THE TINGUAITE-SOLVSBEROITE FAMILY. The solvsbergites and tinguaites are allied to the bostonites. Occurring as dykes in association with, or as marginal modifications of, nepheline- syenite intrusions, they are undoubtedly differenti- ation products of the magma which formed the main intrusion. Consequently they are rich in soda, but poor in lime and magnesia. The essential con- stituents of the solvsbergite type are potash-felspar (orthoclase or microcline), soda-felspar (anorthoclase or albite), segirine or a soda-hornblende. The same mineral combination, with the addition of nepheline, 1 Die Grorudit-Tinguait Serie, loc. cit. CHAP. Il] THE HYPABYSSAL ROCKS 231 fl CO ONCO VO co O M CO t^ I r-i LOCO ^ tO M M t^> ^* l>* | ^d" CO oo w "11 VOONCOOOt-'Otoi-i o 1O M 1-1 ON ON .8 f 1 ON COVO CO N N vO ON to | rf i ^11 3 ; o 02 co rt-vo CO M VO -<* VO M ON ON rt ^ 'U H ! . rt rt 3^ 4 O to cooo vO O t- ^- N w O 1 N VO to NNVOVOMr-iMM | vO - U V c 1 S 1 s-S fl 1O 04 M to Tj- !> M 8 M ** rt ^ *- . S ' ^ t/1 v (J ^ 1^ ri cor-toco toONioco 1 I to G *s 2 S s > -g G - 6 "i Tf N vo ONVO co M -^- t^ co O N t^ t 'H vo t^CO CN LOCO ON N vO O CO co > G M VO M IO *J^ COLOMMMCJ^-O o *6 CO M MM '3 j M 10 t^> N O CO 10 i-OVO I vo co CO LO ^"uJ* CJ.NMNNN COO 6 Tf M M I-J M S o ,~ X coco coco^t^t^cor^M M N LO O a? - cO co M ^ ON 00 O "^ "^t" 1i ONM t.COCX) M o M il C-l M rtOOOO OVO M\O ONt^-ON c^ vo t^ M vO CO ^" ^" vO ^" ^-^ LO M B|| NO-^-voooThWMMNcoM M 2 . ts *- OJ CJ vO co vO CO r ^ vo VO O ON ci M 10 >o LO co t^- co coco M LO ON ? " 5.S Cl CO N Tf VO ^O M LO M M CO IO M o * H m M co N rfco N t^. 10 - N ON ON ^ c 5 s-s 10 M ON o CO 1 1 1 1 1 1 1 1 1 1 1 5 o Q" rro 9/-P s o 1 o ^ si c^ o bo J3 ^.s a " .^j VJ \j> r^ ^ i_ ra y i-^i aj o ^ >, ^ G J ^ tn J ^ g 2 1 4 ^ Is tti-s -" Sw o ^ < ^' ^ d > > ^' 238 CLASSIFICATION [PART m and composition to the augitite and limburgite flows. The olivine occurs in phenocrysts, the augite generally in small crystals in the groundmass. According as the mineral associated with the olivine and augite is biotite or hornblende, Rosenbusch sub-divides the rocks into hornblende- or biotite-monchiquite. The glassy base is often decomposed, and then gives rise to analcime. Possibly the analcime may also occur as an original constituent, as has been maintained by Pirsson and others. 1 The alndites are a variety of biotite-monchiquite in which melilite and perovskite are largely developed in the groundmass. The chemical composition of some typical melano- cratic rocks is shown in the table on page 237. THE PERKNITE 2 FAMILY. These rocks are composed of various combinations of hornblende, augite, and rhombic pyroxenes, together with accessory biotite, olivine and iron-ores. They are heavy, dark- coloured rocks, and occur as deep- seated masses or as dykes. They are usually divided into pyroxenites and hornblendites, according as the dominant constituent is a pyroxene or a hornblende. The following types have been established. Pyroxenite 3 with dominant monoclinic pyroxene. Diallagite with dominant diallage. Websterite or diallage-hypersthene rock. Hypersthenite with dominant rhombic pyroxene. 1 Pirsson : Journ. of Geol. (1896), iv. pp. 679-690. Flett : Trans. Roy. Soc., Edin., vol. xxxix. (1900), p. 889. 2 Trepicvos dark. Turner : Journ. of Geol, vol. ix., 1901, p. 507. 8 Sterry Hunt (1862). CHAP. Il] THE HYPABYSSAL ROCKS 239 Hornblendite l with dominant hornblende. Hornblende-pyroxene-rock. Eclogite a variety of hornblende-pyroxene rock in which the hornblende is emerald-green (smaragdite), the augite, grass-green (omphacite). A third con- stituent is red garnet. When typically developed, the members of the perknite family contain neither felspar nor olivine : by the addition of the former they pass into the norites and gabbros, and by the addition of the latter into the peridotites. The chemical composition of some types is given in the following table. ANALYSES OF PERKN-ITES. I. ii. in. IV. Websterite. Hornblende- Pyroxenite. Hornblendite. Pyroxene rock. Si0 2 53-25 48-04 45-05 37-90 A1 2 3 2-80 7-82 6-50 13-17 Fe 2 3 69 2-01 3-83 8-83 FeO 5-93 9-32 ' 7-69 8-37 MgO 19-91 13-33 12-07 9-50 CaO 16-22 13-01 18-66 10-75 Na 2 19 69 94 . 2-35 K 2 48 78 2-12 H 2 29 3.07 2-40 1-40 Ti0 2 1-16 2-65 5.30 Other constits. 70 2-03 31 99-98 100-96 10088 99-69 1. Websterite, Mt. Diablo W. A. Melville : Butt. U.S. Geol. tiurv., No. 168, p. 213. 1 J. D. Dana (1880). 240 CLASSIFICATION [PART m II. Hornblende-pyroxene rock, Mariposa County, California W. F. Hillebrand : Journ. ofGeol, vol. ix., 1901, p. 509. III. Pyroxenite, Brandbergefc, Gran, Norway Von Schmelck : Quart. Journ. Geol. Soc., vol. 1., 1894, p. 31. IV. Hornblendite, Brandberget, Gran, Norway Von Schmelck : Quoted from Washington's Analyses, p. 345. THE PICRITE FAMILY. Picrites * were first described by Tschermak, as olivine-augite rocks intrusive in the Cretaceous and FIG. 98. Enstatite - picrite, Lochinver, Sutherland. E. Enstatite. O. Olivine. / Felspar. (From a photograph by R. H. Rastall.} Eocene formations of the highlands between Netitit- schein, Teschen and Bielitz in Moravia and Silesia. Similar rocks have since been noticed by Gtimbel 1 Name given by Tschermak (in 1866) on account of the high percentage of magnesia (bitter-earth) in these rocks (Gr. 7n/c/>6s). CHAP, n] THE HYPABYSSAL ROCKS 241 in the Fichtelgebirge (Palceopikrit), and other ob- servers have recorded their occurrence in the Lower Devonian slates of the Rhine valley (Nassau, Ruhr- thai, Wetzlar, etc.). The picrites are dark, heavy rocks, composed mainly of olivine and augite, with a variable but always small amount of felspar. In addition to these minerals, brown biotite is sometimes present; while bright- green chloritic and serpentinous products, probably also a secondary hornblende (pilite), are usually developed at the expense of the ferro-magnesian minerals. Hornblende-picrite 1 is a similar rock, in which brown hornblende is present instead of the usual augite. A colourless augite (malacolite) is, however, sometimes associated with the hornblende. A characteristic feature of the picrites is the lustre- mottling, seen in hand-specimens. This phenomenon is produced by the interrupted reflection of light from the cleavage-surfaces of the augite or hornblende, as the case may be, the interruption being due to a poeci- litic inclusion of grains of olivine. Chemically these rocks contain little silica, a small proportion of alumina (10 per cent, or less), and a very large amount of magnesia, ranging from 15 to 30 per cent. ; while the total alkalies are usually below 2 per cent. The specific gravity lies between 2'8 and <3. In the following table are some analyses of types. 1 Bonney (Quart. Journ. Geol. Soc., 1881, p. 454). Q L'42 CLASSIFICATION ANALYSES OF PICRITES. PAKT III i. n. nr. IV. Picrite, Picrite, Picrite, Picrite, Little Assynt. Blickburn. Anglesey. Giimbelberg Si0 2 46-23 44-73 42-94 40-79 A1 2 3 6-30 11-89 10-87 10-41 Fe 2 3 4-30 4-85 3-47 3-52 FeO 7-07 6-61 10 14 6-39 MgO 25-13 10-77 16-32 2334 CaO 6-08 7-69 9-07 8-48 Na 2 1-08 2-77 90 1-71 K 2 33 89 15 0-71 H 2 3-78 7-64 6-09 4-04 Ti0 2 1-53 Other constits. 33 1-06 100-63 100-43 99-95 99-39 T. Picrite, Little Assynt, Scotland (Grant Wilson). II. Sill at Blackburn, Bathgate Hills (W. Pollard). III. Picrite, Anglesey (J. H. Phillips). IV. Picrite, Giimbelberg (Tschermak). THE PERIDOTITE FAMILY. The peridotites are ultrabasic rocks without felspar and composed mainly of olivine, the remain- ing constituents being one or more members of the pyroxene-hornblende group together with iron - ore, spinel or garnet. The rocks occur chiefly as dykes. They are especially remarkable for their excessively basic character and high specific gravity: SiO 2 = 35 45%; Al 2 3 = 0-6%; FeO = 8-10%; jCaO -0-2% ; M0 = 35-48%. Alkalies are present in small quan- CHAP, n] THE HYPABYSSAL ROCKS 243 titles only. Sp. G. 3'0-3'3. By decomposition they give rise to serpentinous rocks. The following names have been applied to some of the combinations : Dunite l mainly composed of olivine, together with chromite and other spinels, and sometimes garnet (pyrope). Wehrlite 2 = olivine + augite (diallage). Saxonite or Harzburgite 8 = olivine + rhombic pyroxene. Lherzolite 4 = olivine +augite (diallage) + hypersthene. Oortlandtite 5 = olivine + hornblende + hypersthene. Scyelite 6 = olivine + biotite + hornblende. Eulysite 7 = olivine + augite (diallage) + garnet ; a type analogous to eclogite. Since the number of mineral combinations is very large whilst the occurrences are limited, the divisions in the above table are unnecessarily numerous for such a small family. A simpler classification is by the ferro- magnesian constituents, as follows : Dunite olivine rock. 1 So named from the Dun Mountains in New Zealand, where rocks of this type were discovered by Hochstetter in 1859. 2 Von Kobell (1838). 3 The former name was proposed by Wadsworth in 1884, on account of the occurrence of these rocks in Saxony ; the latter by Rosenbusch in 1885, in reference to the well-known Harzburg occurrence. 4 Named by Delame"therie after Lake Lherz in the Pyrenees. 5 Name suggested by G. H. Williams for a type occurring in the " Cortlandt Series " of Peekskill-on the Hudson River, N.Y. 6 J. W. Judd : Quart. Journ. Geol Soc., vol. xli. (1885). 7 Name given by A. Erdmann (1849). 244 CLASSIFICATION [PART in Hornblende-peridotite = olivine + hornblende. Pyroxene-peridotite including wehrlite, saxonite and Iherzolite. Hornblende pyroxene-peridotite including cort- landtite and eulysite. Hornblende-biotite-peridotite including scyelite. ANALYSES OF PERIDOTITES. i. ii. Dunite, Wehrlite, New Zealand. Loch Garabal. Si0 2 42'8 38.6 A1 2 3 3-7 Fe 2 3 ' ' 7-6 FeO 9-4 7-8 MgO 47-4 27.7 CaO 7-7 Na 2 K 2 2 Loss on ignition 6 6-4 100-2 99-7 I. Dunite, Dun Mountains, New Zealand (Reuter). II. Wehrlite, Loch Garabal (J. H. Player). TABLE SHOWING THE MUTUAL RELATION OF THE FELSPAR-FREE ROCKS. Olivine-free rocks (Perknites). Hornblendite Diallagite Hypersthenite Websterite Hornblend e-py rox- ene-rock Eclogite Olivine-bearing rocks (Peridotites). Hornblende- peridotite Wehrlite Saxonite Lherzolite Rocks with a little felspar (Picrites). Hornblende- picrite ( Picrite proper or I pyroxene picrite Cortlandtite, etc. Hornblende- pyroxene-picrite Eulysite CHAP, n] THE HYPABYSSAL ROCKS 245 SERPENTINES. By alteration, under the influence of surface agencies, the peridotites give rise to serpentinous rocks ; and there is no doubt that a great number of serpentines have originated in this way. On the other hand, some have been shown to be derived from augitic rocks, such as gabbro, diabase, etc. (antigorite- serpentine)', and others, from hornblendic rocks, as in the case of the Rauenthai serpentine, described by Weigand. Fragments of unaltered olivine, diallage, and bastite (the latter derived from a rhombic pyroxene) are often found embedded in serpentine, thus furnishing a proof of the mode of the origin of these rocks. Again, their mode of occurrence, in dykes and bosses, leads to the same conclusion. 1 The chemical changes necessitated by the conversion of olivine and augite to serpentine are illustrated by the analyses given on page 247. The chemical changes illustrated by these analyses can be expressed by the following equations : (1) Alteration of olivine to serpentine. 2[2MgO.SiOJ + C0 2 + 2H 2 = MgC0 3 + 3Mg0.2Si0 2 ,2H 2 (2) Alteration of enstatite to serpentine. 4[MgO.SiOJ + C0 2 + 2 FT 2 = MgC0 3 + 2Si0 2 + 3Mg0.2Si0 2 2H 2 1 See a paper by Prof. Bonney and Miss Raisin on " The "microscopic structure of minerals forming serpentine, and their relation to its history." Quart. Journ. Geol. Soc , vol. Ixi. (1905), p. 690. 246 CLASSIFICATION [PART m Serpentines are dull-green and red, often mottled rocks. They are so soft that they can be easily scratched, or even cut with the pen-knife. Veins of fibrous chrysotile and steatite frequently traverse them. In some cases the serpentinous mineral is disseminated through limestone, as in the rocks known as ophioalcite. CHAP, ll] THE HYPABYSSAL ROCKS 247 ,- < 1 1 rN 02 w ~ - > OJ . N M rt" M 10 N N 00 *^ M 1 IO ^ li'j 4- d d 6 ON TJ- d ' ON o w fcS CO O o i-a'^i i> O W 10 co >J ^j- O-< ^3 ^ "^ NO N N CO ** ^ Tj- M O O sl ( 1 s |f M CO ON ON O LO CO N I CO\O 1 N O HH O 1 N TJ- ' 66" ^ ID CO CO ON tti B 02 1 1 i i t i i i i i ^ {2; o:q 0oqo OQ *^ f^H &H ^ O I^H ^ CHAPTER III THE VOLCANIC ROCKS. THE volcanic rocks can be classified in the same manner as the plutonic rocks. The same limits of 66 and 52 per cent, of silica separate the acid from the intermediate and the intermediate from the basic groups respectively, the small ultra-basic sub-group being represented by the rocks known as limburgites and augitites. The calc-alkali series is represented by the dacites, andesites and basalts ; the alkali series by the rhyolites, trachytes and alkali-basalts; while the middle or monzonite series is represented by the trachy-andesite series. This classification is shown in the following scheme, where for comparison the corresponding plutonic types are inserted in square brackets. CLASSIFICATION OF THE VOLCANIC ROCKS. Alkali Series. Trachy-andesite Series. Calc-alkali Series. rs| 8 ? ^ BHYOLITB [Granite] TOSCANITE [Adamellite] DACITE [Granodiorite] ^4 TRACHYTE [Syenite'] TRACHY-ANDESITE [Monzonite] ANDESITE [Diorite] 8 < 11 ALKALI-BASALT [Alkali-gabbro] ClMINITE x [Kentallenite] BASALT \Gabbro~] 1 The Ciminites range in silica-content from 54 to 57 per cent. 248 CHAP, in] THE VOLCANIC ROCKS 249 The ratio of alkali-felspar and calc-alkali-felspar is a variable quantity in these rocks; when the alkali- felspar is balanced by the calc-alkali-felspar the rocks belong to the trachy-andesite series, but a preponder- ance of the alkali-felspar would bring them into the alkali series. As pointed out when dealing with the plutonic rocks, every gradation between the types established by the scheme of classification exists not only vertically, by variation in silica content, but also laterally by variation in the ratio of the alkali and calc-alkali molecules. As with the plutonic rocks also the alkali-series may be further divided into sub-series, in which felspathoids, soda-felspars, and potash-felspars respectively predominate. Thus the phonolites are nepheline-bearing trachytes and both soda -trachytes and potash- trachytes exist. Again, the tepJirites and basanites, the leucite- and melilite- basalts are felspathoid- bearing alkali-basalts. THE EHYOLITE FAMILY. The rhyolites 1 are the volcanic equivalents of the granites, or, in other words, rocks produced by the consolidation of a granite magma under vol- canic conditions. They are highly acid, containing, in most cases, free silica in the form of quartz. 1 The name rhyolite was given to these rocks by Richthofen (1860) on account of the fluidal structure frequently shown by them (Gr. pva%). They have also been termed quartz-trachytes; and again liparites (Roth. 1860), this designation having been given them on account of their occurrence in the Lipari Islands. A crystalline and granitoid variety of rhyolite has been described under the name of nevadite by Richthofen. 250 CLASSIFICATION [PART in Having been poured out as subaerial or submarine lavas, they usually present marked fluidal, spherulitic and perlitic structures. Occasionally they are com- pact, but more frequently they have a porphyritic structure. The phenocrysts are quartz and felspar, imbedded in a light-coloured trachytic groundmass, which is mainly composed of microlites of felspar. Prisms of augite and hornblende, plates of biotite and granules of magnetite also occur. A glassy base is present in variable proportion, according to the conditions that prevailed during cooling. If the proportion of glass is so large that the crystals are subordinate, the rocks are known as obsidians. Accessory minerals are magnetite, tridymite, apatite, cordierite, garnet, sphene and allanite. The quartz is sometimes bi-pyramidal, but oftener without definite shape. Though usually pellucid and colourless, it has frequently a dark-coloured or smoky hue. In some rhyolites the conditions of eruption have been such as to prevent the separation of quartz ; but the high percentage of silica shown by chemical analysis facilitates the correct diagnosis of such rocks. The felspar is usually sanidine a glassy variety of orthoclase. It occurs frequently in large tabular crystals, which are glassy, clear and much fissured ; but also in smaller and less regular grains. In addition to the orthoclase, there is often present a triclinic felspar. This is sometimes oligoclase, but more frequently albite, soda-mierocline or anortho- clase. In some rhyolites the sole porphyritic con- stituent is a soda-felspar, having the same glassy CHAP, m] THE VOLCANIC ROCKS 251 habit as the potash -felspar (sanidine). According to the predominance of potash-felspar or soda-felspar, the rhyolites may be divided into potash-rhyolites and soda-rhyolites. The latter comprise the more acid pantellerites. (See page 256.) Fluxion -structure is often shown in the ground- mass by the beautiful wavy lines of flow which sweep round the larger embedded grains. The same struc- ture is also occasionally brought out under the microscope by the strict parallelism of the felspar- microlites, or by bands of crystallites and trichites in the glassy rhyolites and obsidians. The identity of many so-called felsites with the rhyolites has been demonstrated by the researches of Rutley and Allport. 1 Many of these rocks, consisting of a cryptocrystalline aggregate of quartz and felspar (felsitic matter), in which porphyritic crystals of quartz and felspar are often embedded, were found to possess traces of perlitic, spherulitic and fluidal structures. (See Fig. 21.) These structures, being characteristic of rocks that have consolidated in a vitreous or semi- vitreous condition, indicated the true nature of the rocks that possess them. They are, in fact, ancient flows of rhyolitic lava that have sub- sequently acquired a cryptocrystalline structure by a gradual process of devitrification. Like the rhyo- lites, they contain porphyritic crystals of both * Rutley : Quart. Journ. Geol. Soc., vol. xxxvii. (1881), p. 403, and vol. xliv. (1888), p. 74 ; ^Allporb : ibid., vol. xxxiii. (1877), p. 449. It was, however, Clifton Ward who first showed that the Welsh felsites were acid volcanic rocks: Quart. Jourfi. Geol. Soc., vol. xxxi. (1875), p. 388. 252 CLASSIFICATION [PART m unstriped and striped felspar, and their chemical composition shows that they may be separated into a potash type and a soda type. These lavas are found interbedded with sedimentary strata, in associa- tion with tuffs and breccias, which were doubtless a product of the same volcanic activity. A nodular character is very common in rhyolitic lavas. The nodules vary in size from a small marble to a man's fist ; in some cases they are even larger. Probably in many instances the nodules are enlarged spherulites ; but they have also been ascribed to contraction on cooling as in true perlitic structure, and to the infilling of original vesicles with quartz and agate by infiltration (liihophysce of Richthofen. The chemical composition of the chief types is given in the following table : ANALYSES OF RHYOLITES. i. Soda- Rhyolite, Brittas. ii. Potash- Rhyolite, Tardree. in. Potash- Rhyolite, Wrekin. IV. Potash-Soda Rhyolite, Fishguard. v. Acid Pantellerite, Mt. St. Elmo. Si0 2 77-29 76-4 75-4 68-01 67-48 A1 2 3 Fe 2 3 14-62 ) 14-2 } 1-6 13.2 8 17-48 0-41 9-70 7-42 FeO 6 2-21 MgO 38 2 0-46 0-77 CaO tr. 6 4 tr. 1-45 Na 2 7-60 1-8 1-1 577 7-21 K 2 16 4-2 7-0 7-08 2-94 H 2 57 1-5 7 0-73 0-96 100-62 100-3 994 99-94 100-14 CHAP, m] THE VOLCANIC ROCKS 253 I. Soda-Rhyolite. Brittas Bridge, Co. Wicklow, Ireland (F. H. Hatch). II. Potash-Rhyolite. Tardree, Co. Antrim, Ireland (J. H. Player). III. Potash-Rhyolite. Lea Rock, near the Wrekin (J. H. Player). IV. Potash-Soda-Rhyolite. Goodwick, Fishguard (Tadman). V. Acid Pantellerite. Monte St. Elmo, Pantellaria (Fb'rst- ner). THE TRACHYTE FAMILY. This great family embraces all the members of the alkali series of volcanic rocks of intermediate com- position, and thus corresponds to the syenite family among the plutonic rocks. The name trachyte was originally applied to all volcanic lavas which, owing to their vesicular texture, 1 are rough to the touch. The use of the term, however, has been gradually limited. First, it was restricted to rocks of intermediate com- position ; and then these were further subdivided according to the nature of the dominant felspar. This mineral, which occurs in glassy porphyritic crystals, is either the potash variety (orthoclase), a soda felspar (anorthoclase), or a soda-lime plagioclase (oligoclase, andesine). The term trachyte is now applied solely to rocks containing dominant alkali- felspars; while rocks with dominant lime-bearing plagioclase are now termed andesites, on account of their abundant occurrence in the Andes of S. America. The trachytes are divisible into three sub-families : 1 Gr. rpaxte, rough (Haiiy : Traite de Min., 1822, p. 579). 254 CLASSIFICATION [PART in 1. The Phonolites with dominant nepheline or other felspathoid. 2. The Soda-Trachytes or Pantellerites with dominant soda-felspar (anorthoclase). 3. The Potash-Trachytes or Trachytes proper with dominant potash-felspar (sanidine or Sfiassy orthoclase). FIG. 99, Phonolite. n. Nosean. s. Sanidine. k. Hornblende. The micrplites are Sanidine, and the square and hexagonal sections, Nepheline. (After Fouque and Levy. ) The phonolites 1 are the volcanic equivalents of the nepheline-syenites. They are rocks composed essentially of sanidine, nepheline and a ferro- magnesian constituent. (See Fig. 99.) Sometimes 1 Name (from Gr. wrj') introduced by Klaproth. CHAP, in] THE VOLCANIC ROCKS 255 the place of the nepheline is taken by leucite, and then the rock is termed leucite-phonolite. Leucito- phyre is the name given by Rosenbusch to a type containing both nepheline and leucite together with sanidine. 1 Associated with the nepheline or leucite are sodalite, nosean and hatiyne. The ferro-magnesian constituent is generally a green augite, which is some- times a soda variety with low extinction angles (segir- FIG. IOO. Trachyte, Peppercraig, Garlton Hills, near Haddington. (Between crossed nicols.) Or. Orthoclase (Sanidine) with glomero-porphyritic structure. A. Augite. The microlites are felspar. ine). Brown mica and hornblende occur as accessory constituents. Other accessories are apatite, iron-ore (magnetite), sphene and melanite (black garnet). 1 The term was used originally by Coquand for leucite rocks generally. 256 CLASSIFICATION [PART in Natrolite and other zeolitic minerals result from the alteration of the nepheline. The phonolites are usually very compact rocks of a greyish-green colour, with a spotted appearance. They ring under the hammer, and hence have been termed clinkstone. A platy and spheroidal mode of weathering is a characteristic feature, the decom- posing rock having a concentric shelly structure like that of an onion. The soda- trachytes or pantellerites, first de- scribed by Foi stner * and so-named by him because of their occurrence on the island of Pantelleria, off the coast of Sicily, are a peculiar group of rocks in which the dominant felspar is a member of the triclinic soda-potash series (anorthoclase). Chemi- cally these rocks are characterised by a high alkali- content, with dominant soda. 2 Beside anorthoclase, segirine-augite, diopside, cossyrite, and more rarely hornblende, may be present. The groundmass may be glassy or trachytic with abundant felspar microlites. The potash-trachytes, or trachytes proper, are light-coloured, rather porous rocks consisting mainly of felspar, ferro-magnesian minerals forming only a small proportion of the whole. The phenocrysts of orthoclase have a glassy habit (sanidine). They occur either as tabular crystals twinned on the Carlsbad law, or as irregular grains. In the ground- l Zeitsch. fur Kryst., vol. v. (1881), p. 348, and vol. viii. (1884), p. 125. 2 The high silica-content of many of the original pantellerites described by Forstner, points to a rhyolite rather than to a trachyte type. See analysis on p. 250. CHAP, in] THE VOLCANIC ROCKS 257 mass, however, the felspar appears in long needles and microlites, which are unstriated, and doubtless consist largely of orthoclase. A parallel arrange- ment, due to flow, is often exhibited and this consti- tutes the typical trachytic structure. Interstitial matter, when present, is usually felsitic, a true glassy base being rare. The ferro-magnesian minerals are brown mica, green or blue amphibole FIG. 101. Trachyte, Bass Rock. (Between crossed nicols. ) Composed almost entirely of lath-shaped crystals of orthoclase (sanidine). and colourless or pale-green pyroxene (malacolite). According to the nature of the dominant species, the trachytes may be divided into mica-trachytes amphibole-trachytes, and pyroxene-trachytes. Accessory minerals are abundant : namely, iron- 258 CLASSIFICATION [PART ni ores, zircon, apatite, sphene, hatiyne and nosean. Quartz is occasionally present in small quantity, indicating thus a passage to the rhyolites. On the other hand, some trachytes have had their silica percentage increased, subsequently to consoli- dation, by hydro-thermal impregnation with secondary silica (chalcedony, hyalite, opal, etc., or quartz). Trachytes occur in flows, dome-shaped masses, and dykes, in numerous volcanic districts: Auvergne, Siebengebirge, Henry Mountains in America, etc. The chemical composition of some British types is shown in the table on page 259. THE TRACHY-ANDESITE FAMILY. The trachy-andesites are effusive rocks corres- ponding to the monzonites of the plutonic class. They are rocks of intermediate composition, in which orthoclase and lime-bearing plagioclase are equally developed. They thus occupy a middle position between the trachytes and the andesites, as the monzonites do between the syenites and the diorites. Rocks of this type are of limited occurrence, but, according to Washington, they occur among the volcanic districts of Italy, in the Euganean Hills, and among the Siebengebirge lavas. 1 The toscanite family embraces the more acid 1 H. S. Washington: Journ. of Geol., vol. iv. (1896), pp. 547 and 881. Washington distinguishes a type with alkali- felspar and acid plagioclase (andesine to oligoclase), which he terms domite, from a type with alkali-felspar and basic plagio- clase (labradorite to anorthite), which he styles wdsinite. CHAP. Ill] THE VOLCANIC ROCKS ON Q *"* t^ *O IH to oo 00 $ s ~y 00 f* 10 M tooo M M ON rt-VO N O . O I 1 ^9 to N IOOQ Tj- ON W O O -OOO N H rt'g M O I-H COI>.N O tOOO H^IVO I . J M 00 g 00 ^_1 3 C5 o !lo e :e PC 2 ^ c y s K M ' d S > > ^ 260 CLASSIFICATION [PART in members of the trachy-andesite series i.e., those with a silica-content of from 66 to 73 per cent. It corre- sponds to the adamellite family among the plutonic rocks. The toscanites contain phenocrysts of both potash-felspar and plagioclase (andesine to labra- dorite), together with quartz and a ferro-magnesian mineral (biotite, augite, or hypersthene), embedded in a glassy groundmass. According to Washington, rocks of this type are found at Bracciano, Cerveteri, Tolfa, Monte Amiata, in Central Italy ; and they are probably represented in the oceanic islands of the Atlantic basin (at the Azores, Madeira, Teneriffe, and Ascension). 1 The small sub-family of Oiminites is the most basic division of the trachy-andesite series. It corresponds to the basic monzonite, or kentallenite, division of the plutonic rocks. The name is derived by H. S. Washington 2 from Monte Cimini, in the Viterbo district of Italy, where the type rock occurs. It consists of phenocrysts of augite, olivine and felspar in a groundmass of felspar microlites, augite granules, magnetite and apatite. The felspars consist of alkali- felspar (orthoclase) and a basic plagioclase. The silica percentage varies from 54 to 57 per cent. Allied types are the shoshonite and absarokite of Iddings. 3 1 H. S. Washington : Journ of Geol., vol. v. (1897), p. 349. 2 Journ. of GeoL, vol. v. (1897), p. 349. 8 Journ. of Geol. t vol. iii. (1895), p. 935. CHAP, in] THE VOLCANIC ROCKS ANALYSES OF TRACHY-ANDESITES. 261 i. n. HI. Toscanite, Monte Cucco. Trachy-Andesite, Bolsena. Ciminite, Monte Cimini. Si0 2 66-24 58-21 55-46 Al a 3 15-64 19-90 15-36 Fe 2 3 1-16 4-07 1-34 FeO 2-19 0-87 4-50 MgO 0-89 0-98 7-90 CaO 2-17 3-58 6-69 Na 2 2-05 2-57 1-79 K 2 6-60 9-17 6-63 H 2 3-25 0-74 0-38 100-19 100-09 100-05 I. Toscanite, Monte Cucco, Cerveteri, Italy (H. S. Wash- ington). II. Trachy-andesite (Vulsinite), Bolsena, near Orvieto, Italy (H. S. Washington). III. Ciminite, Fontana di Fiesoli, Monte Cimini, near Viterbo, Italy (H. S. Washington). THE ANDESITE FAMILY. This large family includes volcanic rocks of inter- mediate composition belonging to the . calc-alkali series. It corresponds to the diorite family of the plutonic rocks. The andesites are dark-coloured, compact or vesicular, semi-vitreous volcanic rocks occurring mainly as lava-flows, and more rarely as dykes. They are composed essentially of a glassy plagio- clase felspar and a ferro-magnesian constituent, together with a glassy base. According to the nature 262 CLASSIFICATION [PART in of the ferro-magnesian constituent, they may be divided into hornblende - andesites, biotite- andesites and pyroxene-andesites ; and interme- diate varieties are produced by the various combin- ations of these minerals. (See Figs. 102 and 103.) The hornblende is the common brown variety, occurring in well-formed lozenge-shaped crystals, FIG. 102. Andesite (Mica-andesite). Porphyritic crystals of Plagioclase Felspar (;) and Mica (m) imbedded in microlitic ground-mass. {Aftet Fouque and Levy.} which often have a dark border, due to the corrosive action of the molten magma in which they floated when first formed. The monoclinic augite is of a pale brown colour and is non-pleochroic. The hyper- sthene is pleochroic, in pale green and reddish tints. CHAP. Ill] THE VOLCANIC ROCKS 263 Both occur in well-formed crystals, giving square sections with truncated corners. The felspar occurs in good-sized crystals, producing a marked porphy- ritic structure. In its glassy character it resembles sanldine, but it is distinguished therefrom by its twin-striation. Accessory minerals are magnetite, apatite, garnet, sphene, olivine, allanite, and cor- V FIG. 103. Hornblende- Andesite. Porphyritic crystals of Plagioclase Felspar (5) and Hornblende (h) t in a microlitic ground-mass. (After Fouqut and Levy.} dierite. Quartz is sometimes present. With an in- creasing proportion of this mineral the andesites pass into dacites. Some rocks, however, that from chemical analysis might be considered as dacites, 264 CLASSIFICATION [PART in owe their acidity to secondary impregnation, by hydro-thermal agencies (warm springs containing dissolved silica), with chalcedonic and opaline silica. The groundmass of the andesites is usually microlitic, being composed of needles of felspar and an abundant glassy base. The latter is generally of a pale brown colour and either clear or partially devitrified. By an increase in the proportion of this base, the rocks gradually pass into true andesitic glasses. The andesites become much altered by atmospheric decomposition : the felspar, originally fresh and glassy, becomes turbid ; the augite, partially or completely converted to chlorite ; the hypersthene, to fibrous bastite; and the magnetite, to red oxide of iron. The vitreous base gives place to a dull stony mass ; and the rocks become tinged red by much disseminated oxide of iron, or green by diffused chlorite ; these minerals, together with zeolites and carbonates, also filling the vesicular cavities. Altered andesites have been termed porphyrites and propylites by some authors. Modern English writers, however, use the word porphyrite in a different sense. (See page 219.) The family of the dacites 1 comprises the most acid members of the calc-alkali series of volcanic rocks, and as such corresponds to the granodiorites in the plutonic class. With diminishing silica percent- age they pass into andesites, from which they are mineralogically distinguished by the presence of 1 Name introduced by Stache for the quartz-andesites of Transylvania. CHAP. Ill] THE VOLCANIC ROCKS 265 quartz. Phenocrysts of this mineral occur in cor- roded pyramids, with inlets of the glassy ground- mass. The remaining constituents (plagioclase and one or more of the ferro-magnesian minerals horn- blende, augite, hypersthene or biotite), possess the same general characteristics as those of the andesite family. (See page 262.) ANALYSES OF ANDESITES. i. n. in. Dacite, Hornblende- Andesite, Lassen's Peak, andesite, Blackford Hills, California. Mount Shasta. Scotland. Si0 2 68-72 61-58 57-53 A1 2 3 15-15 16-96 16-47 Fe 2 3 1-16 1-75 4-42 FeO 1-76 2-85 3-23 MgO 1-28 3-67 1-32 CaO 3-30 6-28 6-16 Na 2 4-26 3-94 4-13 K 2 2-78 1-28 2-09 H 2 74 1-30 1-02 Ti0 2 31 49 91 Other constits. (chiefly C0 2 ) 30 25 3-27 99-76 100-35 100-55 I. Dacite, East End of Chaos, Lassen's Peak, California (W. F. Hillebrand). II. Hornblende-andesite, Mount Shasta, California (H. N. Stokes). III. Andesite, Blackford Hills, near Edinburgh (E. G. Radley) 266 CLASSIFICATION [PART in THE BASALT 1 FAMILY. The basalts are dark -coloured lavas of basic com- position and high specific gravity, representing the extrusive or volcanic type of the plutonic gabbros. When fresh, they are characterized by their black FIG. 104. Crystal of Fused Hornblende in basalt. colour, and minutely crystalline to compact texture and high density. When altered or decomposed by the action of the weather, they assume a reddish or greenish hue, and are much veined by decomposition products. A further stage of decomposition converts the basalt into a loose crumbling material known as 1 The word " basalt " is one of the oldest in petrography. It is said to be derived from the Ethiopian word signifying an iron- bearing stone. According to Pliny, "invenit eamdem JEgyptus in Ethiopia, quern vocant basaltem, ferrei coloris et duritiae? unde et nomen ei dedit." CHAP. Ill] THE VOLCANIC ROCKS 267 wacke, which is but little removed from clay in com- position. Microscopic examination of a fresh basalt shows it to be composed essentially of plagioclase FIG. 105. Olivine-Basalt. Porphyritic crystals of Olivine (o) and smaller ones of Augite (a) imbedded in a microlitic groundmass. The clear spaces (c) are cavities. (After Fouqut and Lhry.) (labradorite or anorthite) and augite. When olivine is present, the rock is termed olivine-basalt. (See Fig. 105.) In addition to the augite there are often present partially fused fragments of brown mica and of hornblende. (See Fig. 105.) Adopting a mineralogical classification, the different types of basalt may be summarized thus : a. Basalt proper = plagioclase + augite. b. Olivine-basalt = plagioclase + augite + olivine. c. Hornblende-basalt = plagioclase + augite H- hornblende. 268 CLASSIFICATION [PART in Accessory minerals present in the basalts are ilmen- ite and magnetic iron-ore, in black opaque grains ; apatite in colourless needles; and zircon, in conspicuous red grains. Isolated and much-corroded quartz grains occur ; but it is an open question whether they are indigenous to the basalt or are xenocrysts caught up during its eruption. Chlorite, calcite, and epidote, either disseminated or filling amygdaloid al cavities, FIG. 106. Olivine-basalt (Markle Type), Markle Quarry, Garlton Hills. (Between crossed nicols.) PL Plagioclase. O. Olivine. The black specks are magnetite; the microlites, felspar. Augite is represented by decomposition products only. occur abundantly in basalts that have undergone alteration. The chlorite is derived mainly from the decomposition of the augite ; the calcite and epidote, from the felspar ; while the olivines are mostly CHAP. Ill] THE VOLCANIC ROCKS 269 entirely replaced by serpentine, limonite, magnetite, or carbonates of lime and magnesia. Zeolites (natrolite, phillipsite, scolecite) and agates are fre- quently the material composing the amygdales of the basalts. The following are chemical analyses of typical basalts : ANALYSES OF BASALTS. I. ii. in. IV. v. Basalt, Basalt, Basalt, Basalt, Basalt Markle. Hailes Castle Skye. Kippie Law. Average. Si0 2 49-54 49-07 46-61 46-01 48-78 A1 2 3 22-23 19-43 15-22 19-19 15-85 Fe 2 0, 9-55 10-58 3-49 5-91 5-37 FeO 1-12 2-35 7-71 6-75 6-34 MgO 2-80 4-36 8-66 6-81 6-03 CaO 7-19 7-87 10-08 8-68 8-91 Na 2 4-56 3-31 2-43 3-27 3-18 K 2 1*81 98 0-67 1-20 1-63 H 2 2 -42 2-26 3-17 3-07 1-76 Ti0 2 1-81 1-39 PA o-io 0-47 MnO 08 32 0-13 19 0-29 101-30 100-53 100-08 101-08 100-00 I. Carboniferous Olivine-Basalt. Markle Quarry, Garlton Hills, Haddingtonshire (J. S. Grant- Wilson). II. Carboniferous Olivine-Basalt. Harles Castle, Garlton Hills, Haddingtonshire (J. S. Grant- Wilson). III. Tertiary Olivine-Basalt. Allt Fionnfhuachd, Drynoch, Skye (W. Pollard). IV. Carboniferous Olivine-Basalt. Kippie Law, Garlton Hills, Haddingtonshire (J. S. Grant- Wilson). V. The average of 161 analyses of typical basalts (largely olivine-bearing) : R. A. Daly, Journ. of Geol., vol. xvi., 1980, p. 409. 270 CLASSIFICATION [PART m With regard to structure, the basalts are very variable. They may contain no phenocrysts, or be crowded with large crystals of the different con- stituents. They may be holocrystalline or semi- vitreous. They may be very felspathic, passing then into the andesites, or nearly free from felspar, grading then into limburgites and augitites. In one common type porphyritic olivines and augites are knbedded in a groundmass composed of lath-shaped felspars and microlitic or granular augite. Such rocks are characteristically developed on the right bank of the Rhine (Siebeagebirge, etc.), and in the Midland Valley of Scotland. In some basalts, e.g., the Tertiary lavas of the west of Scotland, there is a tendency towards the ophitic structure ; and in these rocks a gradual passage to the ophitic dolerites can be traced. The andesites, into which the basalts also pass by every gradation, are in general distinguished from them by an abundance of felspar phenoorysts, while augite occurs in smaller and less frequent crystals, and olivine only as a rare accessory. Basalt occurs in "flows" or beds of great extent and variable thickness, forming characteristic plateaux and terraced hills (Snake River Plains of Oregon, the Deccan in India, the Western Isles of Scotland and the North of Ireland); also in dykes which represent, in many cases, the fissures through which the molten lava welled out. The fact that in these vast plateaux there is no thickening toward particular centres whence eruption might have been supposed to take place, and no accumulation of pyroclastic material, is CHAP, in] THE VOLCANIC ROCKS 271 the main evidence on which the "fissure eruption" theory is based. The " bedded " basalt is slaggy and amygdaloidal on the outer surface. Where basalt has been quickly cooled, by contact with cold rock, a thin layer or selvage of black opaque glass is invariably FIG. 107. Synthetically prepared Olivine-basalt. (After Fouqul and L/vy.) present. This basaltic glass is known as tachylyte. It decomposes into a yellowish substance known as palagonite. 272 CLASSIFICATION [PART in Basalt is very hard and tough, and constitutes an excellent material for paving and road-metalling. Olivine-basalt has been artificially produced by fusing the necessary ingredients, and allowing the fused mass to cool slowly. Fig. 107 shows the appearance under the microscope of a thin section of an olivme-basalt synthetically prepared by Messrs. Fouque' and LeVy. THE ALKALI-BASALT FAMILY. This family comprises all those basic rocks in which in addition to, or in place of, the lime-bearing plagioclase there is present either an alkali-felspar (orthoclase), or a felspathoid (nepheline, leucite or melilite). It thus corresponds to the alkali-gabbro family in the plutonic rocks. The tephrites x and basanites 2 form the basic end of the felspathoid series, and only differ from the phonolites by containing plagioclase instead of ortho- clase felspar. The tephrite type may be defined, there- fore, as essentially composed of plagioclase, augite and either nepheline or leucite, while basanite is a similar type with the addition of olivine. In the nepheline-, leucite- and melilite-basalts, felspar is not present as an essential constituent. Otherwise the composi- tion is similar to that of the tephrites and basanites. The nepheline of these rocks is sometimes present 1 Name introduced by Delametherie and Cordier for the olivine-free plagioclase-nepheline basalts. 2 Name introduced by A. Brongniart for the divine-bearing variety. CHAP. Ill] THE VOLCANIC ROCKS 273 in recognizable six-sided crystals ; more often, how- ever, in small disseminated granules, and then it can only be proved by chemical tests. Leucite, on the other hand, is easily known by its characteristic polygonal shape, zonal inclusions and optical be- haviour. (See Fig. 108.) Melilite occurs in colourless or yellow granules, in poecilitic plates, or in idio- FIG. IDS Leucite-Tephrite of Vesuvius. Crystals of Leucite (/) and Augite (a) imbedded in a microlitic groundmass. The irregular clear spaces (c) are cavities. (After Fouqut and Ltvy. ) morphic crystals, giving straight extinction and low double refraction. Common accessory constituents are haiiyne, nosean, melanite (black garnet) and perovskite. 274 CLASSIFICATION [PART ITI The alkali-basalts may be summarized thus : i ET / f -nepheline \ , 1t 1. Felspar \ _ le ^ ite ^basalts: a. Tephrite = plagioclase+ { "^euche I + au g ite ' gVta^^ 2. Basalts without felspar : a. Nepheline-basalt 1 = nepheline + augite. b. Leucite-basalt = leucite + augite. c. Melilite-basalt = melilite + augite. The following table gives some analyses of typical alkali-basalts. ANALYSES OF ALKALI-BASALTS. i. n. in. IV. v. Tephrite, Basanite, Leucite- Nepheline- Melilite- Kaiserstuhl, Mount Inge, Basalt, Basalt, Basalt, Baden. Texas. Montana. Texas. Cape Colony. Si0 2 50-08 48-13 47-98 40-32 36-15 A1 2 3 18-87 18-44 13-34 9-46 15-18 Fe 2 3 3-48 3-41 4-09 4-75 4-87 FeO 3-49 4-30 4-24 7-48 9-11 MgO 2-14 3-06 7-01 18-12 13-63 CaO 6-70 5-89 9-32 10-55 11-40 Na 2 4-10 8-00 3-51 2-62 2-42 K 2 4-58 3-80 5-00 1*10 1-81 H 2 4-17 1-77 2-10 1-82 2-32 Ti0 2 1-39 1-74 0-58 2-66 2-30 Other constits. 1-16 1-30 3-12 1-21 1-24 100-16 99-84 100-29 100-09 100-43 i Roseubusch (Physiographic, p. 1431 ) calls these rocks nephelinite and hucitite when they are free from olivine, but nepheline-basalt and leucite-basalt when they contain that mineral. CHAP III.] THE VOLCANIC ROCKS 275 T. Tephrite, Mondhalde, Kaiserstuhl, Baden (F. Graeff). II. Basanite, Mount Inge, Uvalde County, Texas (W. F. Hillebrand). III. Leucite - basalt, near High wood Peak, High wood Mountains, Montana (L.' V. Pirsson). IV. Nepheline-basalt, Tom Munn's Hill, Uvalde County, Texas (W. F. Hillebrand). V. Melilite-basalt, Spiegel River, near Heidelberg, Cape Colony (J. Lewis). THE LIMBURGITE-AUGITITE FAMILY. The so-called magma-basalts 1 are rocks con- sisting solely of ferro-magnesian minerals and iron- PIG. 109. Magma-basalt, composed of crystals of Magnvtite and Augite imbedded in a brown glassy base. In the centre is a patch of trichites. ore imbedded in an abundant glassy base. Augite of the common brown type is the most abundant constituent ; it occurs in prismatic crystals, giving 8-sided cross-sections, and in microlites Iron- 1 Boricky : Petrog. Studien an den Basaltgest. Bohmens. 276 CLASSIFICATION [PART in ore (magnetite) is always present, both in granules and trichites (see Fig. 86). Those rocks which, in addition to augite, contain olivine, are termed limburgites ; * while those without olivine, are known as augitites. 8 Another type in which FIG. IIO. Limburgite, Whitelaw Hill, Garlton Hills. O. Olivine. A. Augite. Gl. Glassy base. phenocryats of hornblende occur in addition to those of auwite and olivine, has been described by H. S. Washington 8 wader the name of kulaite from Kula in Irdia. The following table gives some analyses of typical limburgites, augitites and kulaites. 1 Rosenbusch : Neues Jahrb., 1872, p. 35. 2 Doelter (1882). 3 Washington : Amer. Jurn. Sci, vol. xlvii., 1894, p. 144 ; and Journ. of G-eol., vol. viii., 1900, p. 44. CHAP, in] THE VOLCANIC ROCKS 277 ANALYSES OF LIMBURGITES, AUGITITES AND KULAITES. i. ii. in. IV. Kulaite. Limburgite. Augitite. Limburgite. SiO a 46-71 42-78 41-05 40-2 A1 2 3 15-59 8-66 24-19 12-8 Fe 2 3 8-27 9-51 4-0 FeO 1-40 17-96 10-4 MgO 4-90 10-06 5-11 11-9 CaO 6-41 12-29 10-99 10-4 Na 2 4-97 2-31 5-69 2-7 K 2 2-85 0-62 1-89 0-8 H 2 4-24 3-96 1-62 3-4 Ti0 2 2-40 28 2-9 Other constits. 2-14 0-95 99-88 99-87 100-05 99-5 I. Kulaite, Blackie Heugh, Haddingtonshire (W. Pollard). II. Limburgite, Limburg, Kaiserstuhl, Baden (quoted from Rosenbusch's Gesteinslehre). III. Augitite, Madeiral, Sao Vicente, Cape Verd Islands (Doelter). IV. Limburgite, Whitelaw Hill, Garlton Hills, Haddington- shire (J. H. Player). The structure of the limburgites and augitites is usually porphyritic, the porphyritic constituents being olivine and augite. A second generation of augite microlites is very common ; but the olivine is less frequently found in two generations. In 278 CLASSIFICATION [PART in. some types no porphyritic constituent was formed, and the minerals occur only as small grains and microlites embedded in a copious glassy groundmass. In such cases the rock was no doubt erupted before any portion of the magma had crystallized. PART IV. DISTRIBUTION OF THE IGNEOUS ROCKS AS ILLUSTRATED BY THE BRITISH ISLES. CHAPTER I. DISTRIBUTION OF THE PLUTONIC ROCKS IT Ls difficult to avoid repetition in dealing with the distribution of the plutomc rocks, for their intrusions rarely consist of a single petrographical unit. They are rather complexes of units which were, as a rule, intruded in the order of decreasing basicity. The large acid intrusions of Cornwall and Devon may appear to constitute an exception to this rule, since the earliest intrusions were already very acid ; but even these comprise intrusions of slightly different composition. DISTRIBUTION OF BRITISH GRANITES. West of England. Devon and Cornwall. The five largest masses of granite in these counties have been respectively designated the Dartmoor, Brown Willy (or Bodmin Moor), St. Austell, Falmouth (or Carn Menelez), and Land's End masses. The 279 280 DISTRIBUTION IN BRITISH ISLES Scilly Islands are the highest points of a sixth large mass. Besides these larger protrusions there are many smaller bosses e.g., those of St. Michael's Mount, Godolphin Hills, Cam Brea and Cam Marth, CHAP, i] DISTRIBUTION PLUTONIC ROCKS 281 Belovely Beacon, Kit Hill and Kingston Down, and Lundy Island in the Bristol Channel. Some of these are connected, under the killas covering, with the main granite mass near which they lie (e.g., at Carn Brea). Others are independent intrusions from a common magma basin. The rocks have usually a grey colour, and are porphyritic with large phenocrysts of white alkali-felspar, of which the parallel arrangement evidences flow movements in the partially consolidated magma. They belong to the alkali division of the granites, inclining rather to the potash series, (See analyses on p. 167.) A finer-grained more acid granite is occasionally found penetrating the granite -porphyry as in the Scilly Isles, on Bodmin Moor (where three such intrusions of a later granite have been observed), 1 and in the Carn Menelez mass, between Bolitho and Boswyn, and at Praze. 2 Midland Counties. Malvern Hills (Worcester- s/are).--Gneissose granite occurs at Great Malvern (North Hill), and hornblende-granite to the north of the Wych. Leicestershire At Mount Sorrel, in the Charnwood Forest, the granite contains much plagioclase, subordinate orthoclase, together with quartz, biotite and hornblende. It belongs, there- fore, to the adamellite division, but appears to grade into granodiorite. Lake District. There are three principal masses 1 Summary of Progress, Geol. Surv., 1906, p. 38, and Barrow : " The Geology of the Scilly Isles," Mem. Geol. Surv., 1906, p. 4. ? Summary of Progress, Geol. Surv., 1903, p. 25. CHAP. I] DISTRIBUTION -PLUTONIC ROCKS 283 of granite in this district : Shap Fells in Westmor- land, and Skiddaw and Eskdale in Cumberland. The Shap rock consists of orthoclase (occurring both as large phenocrysts and in the groundmass, oligo- clase, quartz, biotite and sphene. The orthoclase phenocrysts constitute about one-tenth of the rock. Excluding these, plagioclase is the dominant felspar of the ground-mass. 1 From these data it would appear that this granite belongs to the adamellite division. The Skiddaw rock is a white or grey rock, cropping out on the north-west side of Saddleback, in the Caldew valley. There it is surrounded by a contact-zone of chiastolite, andalusite, and cordierite rocks. It contains a good deal of plagioclase, but the dominant felspar is an alkaM-felspar (perthite). The dominant mica is biotite, but some muscovite also occurs. The Eskdale rock is a large irregular mass extending from Wastwater and Eskdale Fell, through Eskdale and Muncaster, southwards to Bootle an area of roughly 35 square miles. It includes a variety of types, the most usual of which is a granitic rock of coarse texture but porphyritic habit; but granophyric types also occur. The dominant felspar is perthite, although both ortho- clase and oligoclase occur independently. Quartz is plentifully present, and both muscovite and biotite are constituents. The accompanying siHs and dykes contain felspar phenocrysts up to two inches in length. North Wales. The most important mass of granite is that of the Sarn district in the Lleyn 1 Harker and Marr : Quart. Journ. Geol. $oc., vol. xlvii. (1891), p. 279. 284 DISTRIBUTION IN BRITISH ISLES [PART iv peninsula (S.W. extremity of Caernarvonshire). It is a biotite-granite, consisting of crystals of orthoclase (somewhat altered), oligoclase and biotite, with abundant granular quartz. In addition to these con- stituents there is a little iron-ore, apatite and zircon. It is intrusive in Ordovician slates, and builds the round hill of Mynydd Cefn Amlwch, N.W. of Sarn, and extends in an elongated patch, four miles long, by Bryn-Croes to beyond Meillionydd. See Fig. 123, p. 136. An outlier of the same rock occurs at Pyllau- giach. Most of the acid intrusives of North Wales and Anglesey, however, belong to the hypabyssal group of granophyres and microgranites, and will be referred to on p. 314. Isle of Man. There are two important granite masses in the island, the Foxdale granite and the Dhoon granite. The former appears as the top of a hidden dome. There is a larger and a smaller exposure, separated by almost half a mile of the slate series. The intrusion was subsequent to the primary folding and crushing of the " Manx slates." * The Dhoon granite appears as an ovoid patch occupying an area of about half a square mile. The intrusion apparently descends into the slates with very steep sides. 2 From the petrographical description published by the Geological Survey, both these granites must be of the adamellite type, since plagioclase (oligoclase) appears to play quite as important a part in them as 1 Eep. Geol. Surv., 1896, p. 32. 2 Eep. Geol. Surv., 1895, p. 6. CHAP. I] DISTRIBUTION-PLUTONIC ROCKS 285 do the alkali-felspars (orthoclase or microcline). The dominant mica is muscovite, although biotite also occurs. The Dhoon rock is more porphyritic than that of Foxdale. 1 A third and much smaller boss consisting of grano- fo/atoftyre 'ho Id He ad DOUGLAS . I $H Tnas E'jltj Carboniferous I 3 J Mint Stetes Cambria F fox (Safe Gran/tp ff/ioon ffrjn/te Ontlar}{/& firan/te Jcid ' ff/fres from feurda/e Gran/fie ^ Pyfres from " Ghoon Granite FIG. Geological Map of the Isle of Man. Scale : I inch = 10 miles. (After Lamplugk.) 1 Watts : Geology of the Isle of Man, Mem. Geol. Surv., 1903, pp. 312 and 318. 286 DISTRIBUTION IN BRITISH ISLES [PART iv diorite in association with basic rocks, occurs in the Oatland complex, near Santon. South- West Scotland. There are three principal masses of granite intrusive in the Silurian and Ordovician rocks of Kirkcudbrightshire. They are collectively known as the Galloway granites, but may for convenience be distinguished as the massifs of (1) Criffel, (2) Cairnsmore of Fleet, and (3) Loch Dee. In all of them there is a considerable variation in composition, and the dominant character of each mass is different. Thus the prevailing rock in the Criffel and Loch Dee masses is a granodiorite, in which the bulk of the felspar is a lime-bearing plagioclase (oligoclase), and the alkali-felspar occurs, together with quartz, as interstitial matter. 1 In the Cairnsmore of Fleet massif, on the other hand, alkali-felspar (micro- cline) plays a more important part, and the dominant type is adamellite. The granodiorites of the Criffel and Loch Dee masses also pass, by an increase in the amount of alkali-felspar and quartz, into the adamellite type, 2 and by a decrease in the amount of quartz into tonalite or quartz-diorite ; but the granodiorite and tonalite types have not been observed in the Cairns- more of Fleet massif. The ferro-magnesian minerals 1 Teall (The Silurian Rocks of Scotland, Mem. Geol. Surv. 1899, p. 600) writes : " The principal portion of the Criffel mass is a tonalite rather than a true granite, but in view of the fact that it is identical in general appearance with a typical grey granite, it has been considered undesirable to change the name by which it has always been known." 2 This is especially true of the Loch Dee mass, which thus appears to occupy a position intermediate between the Criffel and Cairnsmore of Fleet masses. CHAP. 1] DISTRIBUTION-PLUTONIC ROCKS 287 are biotite and hornblende, either alone or in association. Augite also occurs. The hornblende is ar"?* Carsphatrn \ &7 o / FIG. 114. ' Map of the Galloway District, showing the distribution of the granite complexes. Scale : I inch = 13^ miles. especially abundant in the granodiorites. A clove- coloured sphene distinguishes the Criftel rock. 288 DISTRIBUTION IN BRITISH ISLES [PART iv The variation in the composition of these granite masses is attributed by Teall to the differentiation of an original magma, rather than to the assimilation of the sediments into which the granite rocks intrude. The difference in the average composition of the three great masses is on this theory due to differ- entiation in a deep-seated basin ; while the local differences of an individual mass are ascribed partly to the intrusion of heterogeneous magma, partly to successive intrusions of already differentiated magmas, and partly to further differentiation in situ. 1 Smaller granite masses also occur in the Galloway district : at the Mull of Galloway ; on the east side of the Cree, south of Creetown ; Cairn smore of Carsphairn ; Spango Water ; and south of New Cumnock. All these rocks are allied in composition, and appear to belong to the same petrographical pro- vince as the larger masses above described. They are chiefly granodiorites, and are associated with tonalites and quartz-norites (" hyperite"). The con- tact metamorphism of the Galloway granite has been described by Miss Gardiner 2 and by Teall. 8 South-East and Central Highlands. This dis- trict is remarkable not only for a great development of granitic rocks, but also for the remarkable range ' Teall : Mem. Geol. Snrv., Scotland. Sheet 5 (1896), 41-43 ; The Silurian Rocks of Scotland, Mem. Geol. Surv., vol. i. (1899), p. 623 ; and Rep. Geol. Surv., 1896, p. 43. 2 Quart. Journ. Geol. Soc., vol. xlvi. (1890), p. 569, 8 The Silurian Rocks of Scotland, Mem. Geol. Surv. (1899), pp. 631-645. CHAP, i] DISTRIBUTION PLUTONIC ROOKS 289 in composition and variation in the mode of occurrence of the latter. The granites are broadly divisible into two : an older and a newer group, the former being in Mr. Barrow's opinion of pre- Torridonian age and the latter of post-Silurian age. 1 These two groups differ fundamentally in their mode of occurrence; for, while the newer granites occur usually in large coherent masses, the older are more commonly in comparatively small patches, in small sills permeating the invaded rocks (injection lit par lit), or are represented by veins of pegmatite which, near the parent granite, bulk almost as large as the rocks into which they are intruded, but diminish gradually as the parent rock is left. The area of pegmatite is in one case more than 30 miles long ; its breadth, however, is usually only a few miles. The characteristic feature of the more acid types of the older granites is their tendency to sub-division into an infinite number of small intrusions, which often permeate large areas and are largely responsible for the high degree of crystallization of the schists and gneisses into which they have been injected. 'As a whole they are later than the main folding movements; although the greater part of the 1 Boulders of some of the newer granites (e.g., the Kincardine- shire mass) are found in the Lower Old Red Sandstone conglom- erate . (See Summary of Progress, Geol . Surv. , 1907 , p . Ill) ; but that the intrusion of granite material was continued to a later period, is shown by the fact, that, in the West Highlands, intrusions of granite (Ben Cruachan) have produced contact metamorphism in the Glencoe Volcanic Series, which has been found to be of Lower Old Red Sandstone age. (Summary of Progress, Geol. Surv., 1905, p. 95 ; and Q. J. Geol. Soc., vol. Ixv., 1909, p. 611.) CHAP, i] DISTRIBUTION PLUTONIC ROCKS 291 intrusions took place when the region was still under the influence of great earth-stresses, with the result that the material of the earlier,, has often been separated from that of the later, consolidation. The latter appears either in the form of minute sills or threads of granite intruded along the divisional planes of the metamorphosed sediments, or as pegmatite in sills or veins. The granite and pegmatite sills are as a rule foliated ; while the coarser vein material is more often unfoliated. Probably the intrusions took the form of sills while the district was still affected by the great earth stresses, and that of veins as these stresses gradually ceased. Even single small intrusions show the result of magmatie differentiation. The central core is often a slightly foliated granite ; on the margin a rock of a more alkaline and foliated type predominates ; while the fringing veins pass into aplite. Each portion is slightly abnormal in composition, but were all parts mingled, the whole would make up a granite of normal composition. The intrusions can be separated into two great groups, by means of the dominant felspar ; in the first (the alkali-granite type) alkali felspar (orthoclase and microcline) predominates ; in the second (the granodiorite type) the alkali felspar is greatly subordinate to a lime-bearing plagioclase (oligoclase). The two groups may be further sub- divided according to the micas present. In the following table Mr. Barrow gives a general view of the whole series of older granites. 1 1 Compare Summary of Progress, GeoL Surv., 1902, p. 94. 292 DISTRIBUTION IN BRITISH ISLES [PART iv ALKALI-GRANITE TYPE. GRANODIORITE TYPE. (A. 1.) Brown mica only present. (0.1.) Oligoclase-biotite gneiss Biotite gneiss of Ben Glen Muick, Glen Doll. Vuroch, Glen Tilt, etc. ( A. 2. ) Biotite exceeds muscovite. (0. 2. ) Oligoclase-biotite-musco- Cairnshee granite of vite gneiss. The Duchray Lower Beeside. Dyke- Hill gneiss, veins in the western area. (A.3.) Muscovite exceeds biotite. ( O.3. ) Oligoclase-muscovite Muscovite-biotite granite gneiss. Present only in and gneiss of Forfar. small amount as veins. Pegmatite veins specially abundant. Not only are the newer .granites far larger and more coherent than the older, but they also tend to occur in complex masses of widely varying composition. 1 Where the mutual relations of the rocks forming the complex are clear, the law of de- creasing basicity has been found to hold good. The most basic is the oldest, and the most acid the youngest of the series of intrusions. Differentiation in situ may have taken place to a small extent ; but the more acid rocks can be generally proved to be intrusive in the more basic. This rule, it should be noted, is applicable to the granite masses only ; in the case of the dykes the rule may be reversed. The most abundant constituent of the great co- herent masses is a granite that contains brown mica, but no white ; such as the Kincardineshire mass, Cairngorm, Ben Avon, Hill of Fare, Peterhead, and 1 Compare Teall's description of the Garabal Hill Complex. Quart. Journ. Geol. 8oc. t vol. xlviii., 1892, p. 104. CHAP. I] DISTRIBUTION PLUTONIC ROCKS 293 Glen Tilt. The Kincardineshire mass is the least variable, and it contains the largest amount of plagioclase (oligoclase), and biotite ; but according to Mr. Barrow the plagioclase is not largely in excess of orthoclase, so that the rock must be referred to the adamellite rather than to the grano- diorite division. A feature of this intrusion are the apophyses thrown out on its south-western margin. These rapidly pass to quartz-porphyries, in which there is no porphyritic orthoclase, showing that this mineral was the last to separate out. The granites of the Hill of Fare and of Peterhead have a slightly higher silica percentage than the Kincardineshire mass ; while the Ben Avon and Cairngorm, the Lochnagar and the Glen Tilt masses, are the most acid of all. A feature of these granites is the frequent occur- rence on one side or the other of more or less complex masses of slightly older and more basic material (diorite, etc.), into which the granite often sends veins. This is well illustrated by the Glen Tilt mass, which has a dioritic complex on its southern and eastern margins. A similar phenomenon is seen on the eastern margin of the Lochnagar mass, on both sides of the Dee. Besides the occurrences mentioned above there are, further north, several intrusions of the newer granites, for instance, that of Ben Rinnes in Elgin- shire, and Loch Moy * in the Findhorn area. North of the Moray Firth, large granitic intrusions of the J Summary o/ Progress, Geol. Surv., 1904, p. 82. 294 DISTRIBUTION IN BRITISH ISLES [PART iv same age occur at Ben Loyal, Strath Halladale, Ord of Caithness, and Strath of Fleet (Lairg). Since most of these granites contain a large proportion of plagioclase, they belong chiefly to the adarnellite division, in some cases approximating even to granodiorites. South- West Highlands. In this district there are the large intrusions of the Moor of Rannoch, Ben Cruachan, and Glen Fyne, and the somewhat smaller intrusions of Ben Nevis and Ballachulish near Loch Linnhe. As far as can be gathered from the published descriptions of these granite complexes, many of them appear to be largely made up of a basic type of granite in which plagioclase is equal to, or even considerably in excess of orthoclase : they would therefore be referable to the adamellites, or to the granodiorites, as the case may be. Thus a common type is that which forms an important part of the Ben Cruachan mass, namely, a granodiorite with predominant plagioclase and subordinate orthoclase, together with quartz, biotite and hornblende. 1 Some- times a basic rock is penetrated by a more acid type : thus the more acid Starav granite (adamellite) appears to be intrusive in the Ben Cruachan type (grano- diorite), see Fig. 130, p. 170. 2 Similarly the grey foliated rock consisting of quartz and oligoclase, with subordinate orthoclase, biotite and hornblende, which forms a considerable proportion of 1 Summary of Progress, Geol. 8urv., 1905, p. 99. 2 Summary of Progress, Geol. Surv., 1898, p. 79. CHAP. I] DISTRIBUTION PLUTONIC ROCKS 295 the Moor of Rannoch mass, is also a granodiorite although it passes near the margin into a pink granite richer in orthoclase and without hornblende (adamellite). 1 The Ben Nevis and Ballachulish rocks are sphene- bearing granodiorites, resembling the Criffel granite. 2 Associated with the granite masses of the South- West Highlands, are differentiation products of a still more basic type. Thus Teall and Dakyns 3 have described a complex of tonalite, diorite, hyperite (norite and hypersthene-diorite) at Meall Breac and Garabal Hill (near the head of Loch Lomond) in association with the porphyritic granite of Glen Fyne ; and J. B. Hill 4 mentions that there are many small protrusions of monzonite, diorite, etc., between the granite masses of Glen Fyne and Ben Cruachan, as at Beinn Buidhe, in the neighbourhood of Kilmelfort, which are intimately related to the larger intrusions Western Isles of Scotland. The granite rocks of the Western Isles are mainly of Tertiary age. They are divided by Harker 6 into two sub-groups, according to their acidity. In the first the percentage of silica varies from 75 to 77, and the ferro-magnesian con- stituent, which is sparsely represented, is biotite. This type occurs at St. Kilda, at Beinn an Dubhaich in Skye, and constitutes the main mass of Arran. (It 1 Summary of Progress, Geol. Surv., 1906, p. 80. 2 Teall : Report Geol. Surv. , 1896, p. 21. 3 Quart. Journ. Geol. Soc., vol. xlviii. (1892), p. 104. * Quart. Journ. Geol. Soc., vol. Iv. (1899), p. 478. 5 Harker : The Tertiary Igneous Rocks of Skye, Mem. Geol. Surv., 1904, p. 153. 296 DISTRIBUTION IN BRITISH ISLES [PART iv is also represented in the Mourne Mountains of Ireland.) The second group has silica varying from 70 to 72 per cent. The ferro-rnagnesian constituent is more abundant, and consists of hornblende and augite. There is also a tendency towards the development of granophyric structure, especially in augite-granites. This type constitutes the mass of the Red Hills of Skye. It occurs also in Mull, and in Rum (Orval). The Ross of Mull granite appears to be of newer Palaeozoic age. 1 It is a type characterized by the presence of microcline-perthite. The Goatfell granite in Arran is a similar rock. South-East Ireland. The largest mass of granite in the British Isles is that of the Mount Leinster range in the south-east of Ireland. It is usually a coarse-grained rock, composed of potash-felspar (microcline), plagioclase (varying from albite to oligo- clase), quartz, muscovite, and biotite. The stone, which varies considerably in texture and quality, is quarried at Killiney and elsewhere for building and paving purposes. The main intrusion is an alkali- granite, in which potash dominates over soda ; but in some of the subsidiary intrusions, which are doubtless differential products of the same great magma basin, soda predominates ; thus a soda-granite occurs at Aughrim. 2 (See Fig. 116 and analysis on p. 167.) Another forms the summit of Croghan Kinshela. (See Fig. 116 and analysis on p. 167.) This consists mainly of a brilliant white soda-felspar and grey 1 Judd : Quart. Journ. GeoL Soc., vol. xxx., 1874, p. 244. See also Summary of Progress, GeoL Surv., 1907, p. 66. 2 Sollas : Trans. Roy. Irish Acad., vol. xxix., p. 471. CHAP, i] DISTRIBUTION PLUTONIC ROCKS 297 quartz. 1 As far as the age of the intrusion of the Mount Leinster granite can be ascertained it is post- Silurian and pre-Carboniferous : the Silurian rocks are metamorphosed at the contact, while the con- glomerate at the base of the Carboniferous contains Wow Wcxfordffan 2.8 mx/teJ to a*v incA/ FIG. 1 1 6. Sketch-map of S.E. of Ireland, showing the Mount Leinster granite. boulders of the granite. The minerals of the contact- zone include andalusite, chiastolite, staurolite, garnet and mica. 1 S. Haughton : Trans. Eoy. Irish Acad., vol. xxiii., 1859, p. 563. 298 DISTRIBUTION IN BRITISH ISLES [PART iv North-East Ireland. In the north-east of Ireland there are three principal occurrences : namely, those of the Mourne Mountains, Carlingford and Newry. The Mourne Mountain granite is a rock of rather peculiar type consisting of orthoclase, albite, and green mica. It is remarkable for the idiomorphic character FIG. 117. Map of the Carlingford and Mourne Mountains District, N.B. Ireland, showing the distribution of the granites and gabbrofc Scale i inch = 20 miles. of its quartz and the variety of minerals found in its cavities. The age of this intrusion is certainty post- Carboniferous, and may be Tertiary. The Carlingford CHAP, i] DISTRIBUTION PLUTONIC ROCKS 299 rock is a granophyric granite, probably of Tertiary age. The Newry granite, which extends southward from Slieve Croob to Newry, and thence to Slieve Gullion and Forkill, is intrusive in Ordovician rocks. It includes both a potash-granite and an adamellite type, and the coloured minerals are biotite and horn- blende. North- West Ireland. In the provinces of Ulster and Connaught there are large granite complexes comprising many different types, often foliated ; as examples those of Barnesbeg, Mullaghderg, Gweebara, Glen ties, Barnesmore, Blacksod Bay, the Ox Moun- tains, Corvock brack, Island of Omey, Learn, Round- stone, and Galway may be quoted. 1 Channel Islands, etc. Granite with two micas occurs in the Scilly Islands ; and biotite-granite is quarried in the Channel Islands, for instance, at St. Brelade in Jersey. DISTRIBUTION OF SYENITES. North Wales. A coarse-grained potash-syenite, consisting of large crystals of black hornblende and pink orthoclase together with subordinate quartz, augite and some accessory minerals, occurs in intrusive masses of the nature of laccoliths at Llanfaglen and Glan-y-mor, 1J mile south-west of Caernarvon. North- West Highlands of Scotland. Quartz- syenite (nordmarkite), syenite, and nepheline-syt Aite, all types belonging to the soda series, occur in the 1 Watts : Guide to the Rock Collections, Geological Survey of Ireland, 1895, passim. 300 DISTRIBUTION IN BRITISH ISLES [PART iv plutonic complex of Ledbeg, north of Loch Borolan. In this complex, which is of later date than the Cambrian, but older than the post-Cambrian move- ments, the chief mass, forming the hill of Cnoc-na- Sroine, is composed of soda-syenite and nordmarkite. The varieties free from quartz sometimes contain pseudomorphs after nepheline, and accessory melanite. Comparatively unaltered nepheline-syenite occurs on the north side of Cnoc-na-Sroine at the base of the hill, and on the south side of Ledbeg River. This rock is composed of nepheline and alkali-felspar (perthite) in nearly equal proportions, with a greenish biotite and melanite as accessories. 1 Channel Islands. Syenites occur in the Channel Islands, as, for instance, at St. Helier in Jersey. DISTRIBUTION OF MONZONITES. North of England. Some of the augite-granites described by Teall 2 in the Cheviot Hills (e.g., at Hedgehope, Linhope Burn, Staindrop Rigg) would appear to be monzonites : their silica-content is low, and they contain approximately equal proportions of a lime-bearing plagioclase (oligoclase-andesine) and alkali-felspar (perthite). Wales. It is probable that monzonites occur among the augite-diorites of St. David's Head, some of which are reported to contain a considerable proportion of orthoclase. 1 Teall : Geological Structure of the North-West Highlands, Mem. Oeol. Surv., 1907, p. 446. * Teall : Geol. Mag., 1885, p. 106. CHAP, i] DISTRIBUTION PLUTONIC ROCKS 301 Scotland. Monzonites occur in association with adamellite in the plutonic complexes of the South- West Highlands. 1 They have been described as occurring in many small protrusions in the neigh- bourhood of the granite-masses of Glen Fyne and Ben Cruachan. The rock of Eilean Balnagowan, Loch Linnhe, described by Teall 2 as a diorite containing an exceptional amount of orthoclase and possessing affinities with the syenites, must be a monzonite. They also occur in the Find horn area (see p. 304). DISTRIBUTION OF DIO RITES. England. Scarcely any diorites of the true plutonic type have been described as occurring in England. The so-called diorites of the Charnwood Forest and of North and South Wales are por- phyrites of the Markfield type (with or without hypersthene). Those of Nuneaton are a more basic type of hypabyssal rock, and must be classed with the dolerites. In the Charnwood Forest area a type that appears to fall within the accepted definition of a diorite is the rock of Brazil Wood, near Mount Sorrel ; and recently Dr. Flett 3 has described as.tonalite or quartz-diorite the so-called " syenite " of Hestercomb, near Taunton in Somerset. This is a granitic to porphyritic rock, composed of oligoclase, biotite and quartz. 1 J. B. Hill: Quart. Journ. Geol. Soc., vol. lv., 1899, p. 478, and Summary of Progress, Geol. Surv., 1897, p. 48. 2 Teall : Rep. of Geol. Surv., 1896, p. 21. 3 Flett : The Geology of the Quantock Hills, Mem. Geol. Suw. t 1908, p. 29. 302 DISTRIBUTION IN BRITISH ISLES [PART iv Scotland. Diorite and tonalite are very common members of the newer granitic complexes of the Southern Uplands, and of the Eastern, Central and South- West Highlands. Most of the granite masses have, on one side or the other, complex masses in which diorite plays the chief r61e. As a rule they are slightly older than the more acid granite, since the latter sends veins into them. Glen Tilt and Loch- nagar may be mentioned as examples in the Central Highlands, and Criffel, Loch Dee, and Cairnsmore of Carsphairn in the Southern Uplands. Diorites also occur in small bosses south of Glen Luce, and three miles north of Kirkcowan. 1 In the South - West Highlands a large mass of diorite is found on the south and south-eastern side of the Ben Cruachan granite, and reaches Its maximum development on Ben-a-Bruidh. This rock consists of idiomorphic plagioclase, brown hornblende, biotite, and a small quantity of interstitial quartz and micropegmatite. Magnetite and apatite occur as accessories. 2 Tonalite, augite-diorite, and mica-diorite are also found in the plutonic complex on the south-east side of the Loch Fyne granite at Meall Breac and Garabal Hill, near the head of Loch Lomond. 3 Several small intrusions of basic material occur near the granite masses of Ben Nevis and Ballachulish in the neighbourhood of Loch Linnhe ; for instance, the quartz-augite-diorite of Rudha Mhor, and the augite-diorite of Ardshiel 1 Rep. Geol 8urv., 1896, p. 43. 2 Summary of Progress, Geol. Surv. , 1898, p. 77. 3 Dakyns and Teall : Quart. Journ. Geol. Soc., vol. xlviii., 1892. p. 104. CHAP, i] DISTRIBUTION PLUTONIC ROCKS 303 Hill. These rocks contain interstitial quartz and orthoclase, and are no doubt basic modifications of the granite magma. 1 There are also many intrusions of diorite or tonalite in the area south of Loch Melfort, Argyllshire, which probably are off-shoots FIG. Il8. Map of the Meall Breac and Garabal Hill area, showing the Glen Fyne Granite and its associated plutonic complex. (After Dakyns and Teall.) from a large underlying mass. These rocks contain plagioclase, hornblende, and biotite, with interstitial quartz and felspar. 2 1 Teall : Rep, Geol. Surv., 1896, p. 21. * Kynaston : Summary oj Progress, Geol. Surv., 1902, p. 80. 304 DISTRIBUTION IN BRITISH ISLES [PART iv Masses of diorite, which do not appear to be directly associated with granitic intrusions, are also found in the Central Highlands e.g., the diorites of Glen Shee and Glen Doll. The Glen Doll complex ranges in composition from gabbro, through diorite to tonalite. Tonalite and diorite also occur as basic modifica- tions of the many granites in the Findhorn area ; for instance, the diorite patch north and south of Tomatin, and other occurrences, in the valley of the Findhorn. 1 Diorite occurs at Netherly, Stob Hill, and Danda- leith 2 as small intrusions, which are probably geneti- cally connected with the granite complex of Ben Rinnes. The Netherly diorite is rather coarse- grained, and consists of hornblende, biotite, greyish quartz, and felspar. The latter is mainly plagioclase, but some orthoclase is also present. In addition to these minerals there is a little grey-green pyroxene, sphene and apatite. The Dandaleith rock is a dark- coloured quartz-diorite, composed of green hornblende, plagioclase felspar, and varying amounts of quartz. Orthoclase is always present and when it becomes of equal importance with the plagioclase the rock passes into a monzonite. Ireland. In the south-east of Ireland, diorites are intrusive in Ordovician sediments. They consist of quartz-mica-diorite or tonalite (as in the irregular boss extending from Carrigmore to West Aston, four miles east of Rathdrum), of quartz-diorite (as at Bologh Lower, three miles east of Rathdrum 3 ), and 1 Summary of Progress, Geol. Surv., 1904, p. 83. 2 Flett: Lower Strathspey, Mem. Geol. Surv,, 1902, p. 36. 3 Hatch : Geol. Mag., 1889, pp. 262, 263. CHAP, i] DISTRIBUTION PLUTONIC ROCKS 305 augite-diorite (as in the neighbourhood of Kilpatrick House, north of Arklow). Channel Islands. Diorites compose the whole of the northern part of Guernsey ; and occur in Jersey in St. Aubyn and St. Clement Bays ; also in Alderney, as at Fort Touraille. These rocks often contain augite in addition to hornblende. DISTRIBUTION OF ALKALI-GABBROS. Although this important division of the plutonic rocks is no doubt well represented among the basic members of the igneous complexes of Scotland, very few examples have, as yet, been described. South- West Highlands. The type occurrence of kentallenite is at Kentallen Quarry, half a mile west of the Ballachulish granite, near Loch Linnhe It is locally known as " black granite," and is remarkable for its " lustre mottling." Orthoclase and plagioclase are present in approximately equal proportions, and the remaining constituents are olivine, augite, and biotite. The silica percentage is about 48. 1 A similar rock occurs as a small boss near the head of Gleann Leac-na-Muidhe. Other intrusions of the same rock occur in Glen Creran. 2 Other intrusions of this rock have been met with near Beinn Bhuidhe (Brannie Burn, and Allt an Sithein, a tributary of the Shira River). 3 These are assigned to the same period of 1 Teall : Eep. Geol. Swn>., 1896, p. 23. 2 Summary of Progress, Geol. Surv., 1904, pp. 69 and 70 ; and Hill and Kynaston, Quart. Journ. Geol. Soc., vol. Ivi., 1900, pp. 631-540. 3 Summary of Progress, Geol. Sure. , 1899, p. 50. TT 306 DISTRIBUTION IN BRITISH ISLES [PART iv intrusion as the Ben Cruachan granite, and conse- quently are of later date than the Lower Old Red Sandstone lavas of Argyllshire. 1 North- West Highlands. The type locality for borolanite is north of Loch Borolan in South Assynt, where the rock occurs in the neighbourhood of the granitic mass of Cnoc-na-Sroine, near Ledbeg. Typical borolanite is found in the small burn named Allt- a'-Mhuilinn on the six-inch map of the Ordnance Survey. This burn crosses the road about one and a quarter mile east of the inn at Aultnacallagach. Outside the plutonic area of Cnoc-na-Sroine the only other place in the North- West Highlands where rocks allied to borolanite have been found is the Coigach district of West Ross-shire, where they occur as dykes intrusive in the Torridon Sandstone. 2 DISTRIBUTION OF GABBEOS. Devon and Cornwall. Typical diallagic gabbros (with and without olivine) occur in the Lizard district (Karakclews and Coverack, Pen Voose, and Kildown near Cadgwith). 8 These rocks are much foliated in places, the augite having been replaced by fibrous hornblende (uralite or actinolite), and the felspar altered into saussurite. Lake District. Gabbro builds a portion of Carrock Fell, north-east of Keswick, where it is 1 Kynaston : The Geology of Mid- Argyll, Mem. Geol. Surv. 1905, p. 102. 2 Teall : Geological Structure of the North- West Highlands, Mem, Geol. Surv., 1907, p. 444. See also Trans. Roy. Soc. Edin., 1892, p. 171. 3 Teall : Geol Mag., 1886, p. 483. CHAP, i] DISTRIBUTION-PLUTONIC ROCKS 307 associated with granopbyre. The felspar of the gabbro is chiefly labradorite. The ferro-magnesian constituent is chiefly augite, but enstatite is some- times intergrown with the augite. In one type brown mica is an abundant constituent ; ilmenite predominates in marginal portions ; while in the innermost portions quartz is found intergrown with felspar (in part orthoclase) as interstitial micro- pegmatite. 1 Another but smaller mass occurs in the Cross Fell range at Guns Fell. This rock con- sists of plagioclase (labradorite to andesine) and augite, together with secondary hornblende. North Wales. Two small bosses of gabbro break through the granite of Sarn in the Lleyn peninsula (at Craig-y-fael, two miles south-west of Sarn, and Plas Llangwnadl). Harker 2 describes these occurrences as partially amphibolized gabbro, consisting, at Craig-y- fael, of " augite and diallage, hornblende and actinolite, felspar and opaque iron-ore, with some pale-green decomposition-product which has the character of a chloritoid." The hornblende-plagioclase rocks of the Lleyn peninsula often have a gabbro-like structure as at Penarfynydd. (See p. 336.) South Wales. A good type of gabbro or norite occurs at St. David's Head in Pembrokeshire. It contains enstatite as well as augite, and some biotite. Quartz is sparingly present. With increased silica percentage the rock passes into enstatite-diorite. 3 A 1 Harker: Quart. Journ. Geol. Soc., vol. 1. (1894), p. 316. 2 Harker : The Bala Volcanic Series of Caernarnon shire, loc. cit., p. 89. 3 J. V. Elsden : Quart. Journ. Geol. Soc., vol. hd., 1905, p. 584 308 DISTRIBUTION IN BRITISH ISLES [PART iv diallage-gabbro with labradorite and olivine, forms the summit of Hanter Hill in Herefordshire. South Scotland. Gabbros which closely resemble those of the Lizard district, occur in Ayrshire, in the Carrick district, at Lendalfoot, and at Colmonell. Norites, or " hyperites " dark-coloured, medium to coarse-grained rocks, composed of plagioclase, hypersthene, augite and biotite form a part of the granite complexes of Loch Dee, Galloway area. 1 Western Isles of Scotland. Some of the most rugged hills of these islands are built up of intrusive masses of gabbro. The largest area of gabbro is in Skye, where it appears as laccoliths in the Cuillin Hills, and as bosses in the Broadford district. Both olivine and oli vine-free gabbros occur, and the two types are intimately associated. The felspar is usually labradorite ; but bytownite and anorthite occur in the more basic types. The pyroxene is usually augite, but occasionally the rhombic pyrox- ene is associated with the monoclinic variety. A large proportion of the mountains of the southern half of Rum consists of basic and ultra-basic plutonic rocks. According to Harker, three partial magmas were formed by deep-seated differentiation, and in- truded as laccoliths in the following order: (1) ultra- basic rocks; (2) eucrites and gabbros ; (3) granites and granophyres. The ultra-basic rocks sometimes con- sist of dunite (olivine-rock), but a more common type is a rock consisting of olivine with some anorthite, enstatite, augite, and chromite. There are many sheets 1 Teall: Hep. Geol Surv., 1896. t>. 42. CHAP, i] DISTRIBUTION PLUTONIC ROCKS 309 of pale and spotted rocks composed of anorthite and oli vine in practically equal proportions (allivalite). The basic rocks are principally norites or eucrites, consisting of hypersthene, augite, olivine, and anor- FIG. 119. Map of a portion of the Isle of Skye, showing the distribution of the gabbro and granite and of the multiple basic dykes and composite sills. (After A. Barker.} thite or bytownite. In the olivine-gabbros the felspar is labradorite, and rhombic pyroxene is uncommon. 1 1 Harker : The Geology of the Small Isles of Inverness-shire. Mem. Geol. 8urv., 1908, and Quart. Journ. Geol. Soc., vol. lix., 1903, p. 208. 310 DISTRIBUTION IN BRITISH ISLES [PART iv In Mull the gabbro occurs mainly in numerous sheets intruded into the bedded basalts. The promontory of Ardnamurchan consists largely of gabbro. Central and Eastern Highlands. Gabbros and hornblende-gabbros (without olivine) form a great mass at Portsoy, near Banff. Another mass occurs further to the south, a little north of the Dee (Aberdeenshire). Another crops out in the fork of the rivers Dee and Muick, south-east of Balmoral ; and several small bosses are exposed between this point and Glen Fernate in Perthshire. Similar small intrusive masses occur along a line drawn in a south- westerly direction from Ben Bhrackie, near Pitlochry, through Ben Lawers to the west coast. These coarse gabbros were, in Mr. Barrow's opinion, intruded into the Highland sediments while the latter were still practically horizontal, from Loch Fyne on the west coast to Portsoy on the east coast. Their position is shown on the map on page 290. In addition to the dominant chain there are subordinate intrusions along other lines, a good example of which occurs in Glen Fernate, 1 where the gabbro cores trend north-west and south-east. In the movements that led to the present structure of the Highlands, these chains of gabbro intrusions formed, according to Mr. Barrow's view, a line of resistance against which the old Highland sediments were squeezed and folded. Although the gabbros have been involved in the crushing, numerous cores, and at times con- siderable masses have preserved their original granitic and ophitic structures. But the pyroxene has been 1 Sheet 56 of the Geological Survey of Scotland. CHAP, i] DISTRIBUTION PLUTONIC ROCKS 311 replaced by actinolite, and the ilinenite by granular sphene or rutile. These gabbros were the feeders of a number of intrusive sills, now for the most part greatly altered to actinolite-schists, some of which are very persistent; that at the base of the Loch Tay limestone is probably the best known, though it is not the most important. The troctolite type of gabbro is found as a basic differentiation of some of the plutonic complexes of post-Silurian age, as in the Belhelvie complex. North -West Highlands. Rocks composed of plagioclase (labradorite) and pyroxene (hypersthene, augite) occur as bands in the fundamental complex or Lewisian gneiss of Sutherland. According as they contain garnet or not, they may be divided into a pyroxene-granulite and a gabbro or norite type. Associated with the pale-green augite is a green hornblende, and accessory biotite occurs in scales. The garnet of the pyroxene-granulites is of a deep red colour. By a diminution in the quantity of felspar the rocks pass over into pyroxenites. 1 Ireland. Gabbros of the eucrite type occur in the Carlingford district, in association with grano- phyre. The micropegmatite which occurs in the more basic portions of these gabbros (as at Barna- vave), is considered by Sollas to owe its origin to an injection of the gabbro by the granophyre magma. 2 1 Teall : Geological Structure of the North-West Highlands. Mem. Geol. Surv., 1907, p. 50. 2 Sollas: Trans. Roy. Irish Acad., vol. xxx, (1894), pp, 477-512. CHAPTER II. DISTRIBUTION OF THE HYPABYSSAL ROCKS. DISTRIBUTION OF QUARTZ-PORPHYRIES. Devon and Cornwall. Acid dykes, locally known as "elvans," are abundant in these counties. They traverse both the Devonian slates (killas) and the granite bosses, and are no doubt genetically connected with the intrusion of the latter. In petro- graphical character they vary from compact felsites to microgranites and porphyries, containing large crystals of felspar and quartz, such as occur near St. Hilary, at Sydney Cove, at Croft Michel near Crowan, and at Grenofen near Tavistock. Some are micro- crystalline in the central part of the dyke and porphyritic near the margin (e.g., the Perranporth el van). Occasionally the chilled margins show flow structure. Lake District. The so-called ''syenite" of Butter- mere, which extends from Buttennere to Ennerdale and Wastwater, is a typical granophyre. The ferro- magnesian constituent of this rock is chiefly biotite, or chlorite derived from it. Augite is subordinate. 1 1 Rastall : Quart. Journ. Geol Soc., vol. Ixii. (1906), p. 253. '' '.. ' : '. '.': /MECKLI j : : ': :: ' ; : .' .- ' :''.'.. WOOD Alluvium Skiddaw Slates Borrpwdale Volcanic Series Eskdale Granite Granophyre Basic Modifications of Granophyre FIG, 120. ap showing the geological relations of the Ennerdale Granophyre, Lake District. tr R. H. RastalL} 314 DISTRIBUTION IN BRITISH ISLES [PART iv The Armboth dyke is a spherulitic or granophyric quartz-porphyry, composed of bright-red felspar (orthoclase) and dark bi-pyramidal quartz grains, scattered eventy through a dun-coloured ground- mass, containing garnets. Granite-porphyry and microgranite occur as bosses and dykes : thus the two laccoliths of St. John's Vale, which are intruded at the junction of the Borrowdale volcanic series and the Skiddaw slates, and one of which is quarried near Threlkeld station, are granite-porphyries con- taining phenocrysts of orthoclase and small garnets ; and dykes of a similar rock, with or without por- phyritic crystals of quartz and felspar, occur, for instance, at the following localities : east side of Hawes Water, Kirk Fell, Ulpha near Broughton, and Black Combe. In the Wastwater district there are innumerable dykes and sills connected with the Eskdale granite, some of which show felspar pheno- crysts up to two inches in length. A remarkable granite-porphyry, containing, besides phenocrysts of a red orthoclase, plagioclase and quartz, large plates of muscovite and small flakes of biotite, occurs at Dufton Pike in the Cross Fell Inlier, and is known locally as the Dufton granite. A dyke of spheru- litic felsite traverses the rocks of High Fell in Cumberland. North Wales. Intrusive bosses of granite-por- phyry, microgranite, and granophyre are abundant in Caernarvonshire, Anglesey, and Merionethshire. According to Harker, 1 the granophyres are character- 1 The Bala Volcanic Series of Caernarvonshire, loc. cit., p. 49, and Tertiary Igneous Rocks of Skye, Mem. Geol, Surv., p. 156. 316 DISTRIBUTION IN BRITISH ISLES [PART iv ized by the presence of augite, while the microgranites have both this mineral and biotite. Only in two localities is the place of these minerals taken by a peculiar blue soda-hornblende (riebeckite) (Paisani type of Mynydd Mawr 1 and Bwlch Cywion). A good type of microgranite (" Nevin type " of Harker) is found at Trefor and Nant Gwrtheyrn in Yr Eifl, and at Mynydd Nevin. Microgranite (eurite of Messrs. Cole and Jennings) 2 forms the ridge of Cader Idris, south-west of Dolgelly, and another ridge of the same rock runs due north to the east of Rhobell Fawr (Allt Llwyd and Ddualt). At Tan-y-Grisiau, 2 miles north of Ffestiniog, there is an intrusive mass of microgranite (sometimes granophyric) 2J miles long. The date of the intrusion was subsequent to the hardening and cleavage of the Tremadoc rocks and possibly later than the consolidation of the Llandeilo beds. The rock is characterized by abundant quartz and poorness in ferro-magnesian minerals. The fel- spar appears to be largely a triclinic variety, possibly soda-orthoclase. 3 Granophyres occur in Caernarvon- shire in the Moel Perfedd laccolith, at Y Drosgl and Craig -dol- Owen, at Pen-y-gar (2J miles east of Llanaelhaiarn, between Pistyll and Nevin), 4 at Llanbedrog and Mynydd-tir-y-cwmmwd. 1 Mynydd Mawr is a boss-like mass three miles west of Snowdon. According to Harker it is probably the plug of a volcanic vent of Bala age, marking the source of some of the lavas of Snowdon, Moel Hebog and Llwyd Mawr. 2 Quart. Journ. Ged. oc., vol. xlv., 1889, p. 422. 3 Jennings and Williams, Quart. Journ. Geol. Soc., vol. xlvii. (1891), p. 368. 4 Hnrker, Ivc. tit., p. 55. CHAP, n] DISTRIBUTION HYPABYSSAL ROCKS 317 South Wales. Spberulitic or granophyric quartz- porphyries and micrograiiites occur at St David's. The main mass of granophyre (Dimetian of Dr. Hicks) is probably a laccolithic intrusion in the Pebidian volcanic series. The intrusion, however, was pre-Cambrian. 1 The marginal portion of the rock is porphyritic. A dyke of granophyre traverses the ridge of the Stanner Rock, near Old Radnor. Isle of Man. Numerous acid dykes of pre- Carboniferous age traverse the Manx slates on the southern margin of the Foxdale granite. They con- sist of small phenocrysts of plagioclase, orthoclase, and hexagonal crystals of muscovite in a micro- crystalline groundmass. One of them (the Crosby dyke), has been analysed, and the result shows that the soda-alkali preponderates. 2 Similar dykes occur on the west coast north of Bradda Head, and on the east coast north of Maughold Head, and these have also been traced to the Foxdale granite. Acid dykes also spring from the eastern margin of the Dhoon granite. They have a platy structure due to shearing, and contain small grains of opalescent blue quartz. 3 South Scotland. Quartz -porphyry dykes are abundantly developed in connection with the granitic intrusions of the Southern Uplands. They consist of ij. F. N. Green: Quart. Journ. Geol. Soc., vol. Ixiv., 1908, p. 363. 2 Quart. Journ. Geol. Soc., vol. xlvii., 1891, p. 439. 3 The Geology of the Isle of Man, Mem. Geol. Surv., 1903, pp. 313-315. 318 DISTRIBUTION IN BRITISH ISLES [PART iv quartz-porphyries and microgranites, as at Dalbeattie, Castle Douglas, Carsphairn, and of felsites, as at Bennan Hill, west of Loch Ken, and at the head of Loch Doon. A few intrusive bosses of quartz-porphyry occur in the Midland Valley. That of Tirito Hill may be quoted as an example. A riebeckite-rock of the Paisani type occurs at Ailsa Craig. 1 South- West Highlands. An enormous number of intrusions of quartz-porphyry, associated with sills of porphyrite, occur between Loch Awe and Loch Fyne, in the Loch Awe series, of which they form a considerable proportion. They are probably connected genetically with the plutonic masses of Ben Cruachan and Garabal, 2 which they are also found traversing. Central Highlands. Dykes, sills and small bosses of quartz-porphyry occur sporadically in the Central Highlands : thus their occurrence has been noted in the mountainous district between Braemar and Glen Ey. 8 The so-called "porphyrite" of Lintrathen in Forfarshire is a quartz-porphyry containing bi- pyramids of quartz and a black wide-angled potash inica, set in a groundmass of orthoclase. Western Isles. The granophyres and micro- granites of Mull, Skye, Eigg and Rum form a charac- teristic feature of the Tertiary volcanic geology of these islands. They occur in intrusive bosses, sills : Min. Mag., vol. ix., 1891, p. 219. 2 Rep. Geol. Surv. for 1895, p. 25 ; and Summary of Progress, 1897, p. 86. 3 Eep. Gtol. Surv. for 1896, p. .19. CHAP, ii] DISTRIBUTION HYPABYSSAL ROCKS 319 and dykes. Typical masses are those of Loch Ba' and Glen More in Mull. Three small bosses occur in the island of Eigg. In Rum the same rocks build the summit of Orval. In Skye they occur in the Strath Valley, at Beinn-na-Cro between Strathmore and Strathbeg, and in the southern part of the island they extend from Loch Sligachan into the heart of the Cuillin Hills. The apophyses of the larger granite masses of the Red Hills (see p. 296) are granophyres. Pitchstone in veins and dykes is also common in the Western Isles. Thus it occurs in Skye, at Glamaig, Glas Beinn Bheagin Strathbeg, and Beinn-na-Caillich ; in Rum, at Orval ; in Eigg, at Rudh an Tangairt near Laig, and at the harbour; in Arran, at Brodick, Corrie- gills, Caisteal Abhail, Tormore; and in Ardnamurchan, at Ben Hiant. At the Scuir of Eigg there is remark- able pitchstone, formerly regarded as a superficial flow which filled an ancient valley in the plateau basalts. Recent investigations appear to show that it is an intrusive sill. 1 Ireland. Quartz-porphyries,, microgranites and felsites occur as dykes in the crystalline area of Clifden, Northern Galway. 2 Pitchstones and fel- sites occur as- dykes in the north-eastern portion of the island; e.g., the pitchstone cutting the granite of the Barnesmore range in County Donegal. Felsite dykes and sills, which were intruded at some period intermediate between the Bala group and 1 Barker : Quart. Journ. Geol. Soc., vol. Ixii. , 1906, p. 40. 2 Watts : Guide to the Rock Collection, Geol. Surv. of Ireland, 1895, p. 63. 320 DISTRIBUTION IN BRITISH ISLES [PART iv the Upper Old Red Sandstone occur on the coast of Waterford. These rocks often resemble agglomerate or volcanic breccia. They consist, however, of intru- sive felsite crowded with innumerable fragments of black shale and various igneous rocks. 1 Chemical analysis shows that these felsites are divisible into potash, potash-soda, and soda types. 2 (See page 214.) The following examples will suffice : potash-felsite at one mile west of Great New town Head and at Garrarus Strand, potash-soda-felsite at Bunmahon and Annes- town, soda-felsite at Knockmahon and Ballydouane. Microgranites, quartz-porphyries, and felsites occur as marginal modifications of the Mount Leinster granite. These rocks occur as sills in the Ordovician and Silurian sediments that flank the granite massif. Like the granite, they are of pre-Carboniferous age. Rocks rich in soda (soda-felsites) are frequent amongst them. DISTRIBUTION OF PORPHYRIES. Representatives of this family are rare in the British Isles, or have escaped notice. Possibly rocks that have been classed as quartz-felsites would, if analysed, be found to have an intermediate rather than an acid composition. On the other hand, some rocks classed as porphyrite possibly contain sufficient orthoclase in addition to their plagioclase to warrant their being termed monzonite-porphyry. The "Lamb- stone porphyry," a sill intrusive in the Arenig rocks near Llangynog, 6 miles south-west of Caermarthen, 1 A. Geikie: Quart. Journ. Oeol. Soc., vol. Ivi. (1900), p. 693. 2 Hatch : Geol. Mag., 1889, p. 545. CHAP, n] DISTRIBUTION HYPABYSSAL ROCKS 321 would, from its description by Messrs. Can trill and Thomas, appear to be a monzonite-porpbyry. Set in a fine felsitic groundmass are idiomorphic crystals of orthoclase and plagioclase (oligoclase), the latter somewhat bigger than the former. There are also minute but well-shaped crystals of hornblende. 1 DISTRIBUTION OF PORPHYRITES. Leicestershire. Rocks that have been described as syenites and as diorites 2 occur as small intrusions in pre-Cambrian rocks at Groby, Markfield, Cliff Hill, Bawdon Castle, Bradgate, and Hammer Cliff in the Charnwood Forest. Their mode of occurrence and micro-structure show, however, that they are hypa- byssal rocks. A microscopic examination of the rocks of Markfield and Groby reveals large idiomorphic crystals of plagioclase, which, although rather decom- posed, plainly show their twin-striation, and green hornblende, which is partly original and partly secondary after augite, both minerals being embedded in a groundmass of micropegmatite. These rocks are dioritic granophyres. It is suggested on -p. 219 to term them markfieldite, after the type occurrence at Markfield. Similar rocks are found in South Leicester- shire, at Enderby, Narborough, Croft, Earls Shilton, 1 Cantrill and Thomas: Quart. Journ. Geol. Soc., vol. brii. (1906), p. 245. 2 Bonney : Quart. Journ. Geol. Soc., vol., xxxiii. (1877) p. 754; xxxiv. (1878), p. 199; xxxvi. (1880), p. 313; and Rutley : Geol. Mag., Dec. 3, vol. iii., p. 557. X 322 DISTRIBUTION IN BRITISH ISLES [PART iv C H JL R N~W O D JF O JZ JE S T Groby Jiaron far* Earls tSJiilbai-l- JBcu-row 'HiU> 1 Scale of Jfrtes. of M.i Sketch of the igneous rocks of Charnwood Forest. (After Bonncy.) Horizontal lines = slaty rocks ( = ash beds). Cross-hatching = igneous intrusions. CHAP ii.] DISTRIBUTION- HYPABYSSAL ROCKS 323 and Stony Stanton (see map on p. 322), where their mode of occurrence suggests that they are part of a buried mountain range, the peaks of which project through the Triassic plain. North of England and South of Scotland. Some of the so-called "basalt" or "andesite" dykes of the South of Scotland must be classed with the porphyrites : e.g., the Eskdale Muir and Wat Carrick dykes. The " augite-granophyre " of Carrock Fell in the Lake District is a plagioclase-augite-rock, with a small amount of orthoclase and quartz, present as micropegmatite. It is a dioritic granophyre or mark- fieldite. Many of the so-called quartz-dolerites of the Lake District also belong here. Wales. Rocks of the Markfield type, but contain- ing hypersthene in addition to augite and without hornblende, occur in both North and South Wales. In South Wales they occur near St. David's Head, 1 at Carnedd Givian, Carnedd Sleithe, Carn Perfedd, Carn Ffold, Carn Trelwyd, and Penberry Hill. In these rocks orthoclase and quartz are present in variable quantity, mainly as interstitial micropegma- tite. In the Carn Trelwyd rock, however, the proportion of orthoclase is so high as to suggest a passage to monzonite-porphyrite. With decreasing silica they pass into hypersthene-dolerites (Carn Hen, Carn Llidi), in which the interstitial micropegmatite is rare or absent. In North Wales the best example of the Markfield type is the hypersthene rock of 1 Elsden : Quart. Journ. Geol. 8oc. t vol. Ixi. (1905), p. 588, and ibid., vol. Ixiv. (1908), p. 273. 324 DISTRIBUTION IN BRITISH ISLES [PART iv Penmaenmawr, which occurs as an intrusion in Orclo- vician strata on the north coast of Caernarvonshire, between Conway and Bangor. This rock consists of lath-shaped plagioclase and granular to hypidiomor- phic augite and hypersthene, in a groundmass of micropegmatite. Hypersthene-porphyrites, without granophyric structure, occur at Carn Boduan, one mile south of Nevin; and at Yr Eifl, in the Lleyn Peninsula. The Carn Boduan rock consists of phenocrysts of plagioclase, augite and hypersthene, embedded in a ground mass of felspar- microlites. The Yr Eifl rock is of similar nature, but it contains brown mica in addition to the above-named constituents, and the groundmass is crypto-crystalline to micro-crystalline. A porphy rite, described by Elsden as lime-bostonite, 1 occurs near Abercastle on the coast of Pembrokeshire, where it is intrusive in Ordovician rocks. It consists of a few felspar phenocrysts in a felspathic ground- mass, the felspar being oligoclase. No ferro-magnesian minerals are visible, but chlorite and calcite, which are present in small quantities, may be derived from their decomposition. North- West Highlands of Scotland. Porphy- rites are found south of Loch Assynt, as sills in the Torridon Sandstone, on Canisp and on Suilven, and near Inchnadamff. They were intruded before the Elsden: Quart. Journ. Geol Soc. 9 vol. Ixi. (1905), p. 594. According to the classification adopted in this book the name bostonite is limited to rocks which are associated with " com- plementary" lamprophyres, and clearly owe their origin to differentiation. CHAP, n] DISTRIBUTION-HYPABYSSAL ROCKS 325 beginning of the post-Cambrian movements, which they are found to have shared. The well-known " Canisp porphyry " may be taken as a type. It is composed of large tabular crystals of oligoclase-albite embedded in a groimdmass of quartz and turbid felspar. Brick-red crystals of orthoclase are sporadic among the phenocrysts. ^Egirine-augite and biotite are the ferro-magnesian constituents. Central Highlands. Dykes and sills of porphy- rite, which are genetically related to the newer granite intrusions, occur on the banks of the Garry and near Strowan. The Beinn a' Chuallaich sill, with its plagioclase phenocrysts half an inch in length, may be taken as a type. Quartz occurs in doubly terminated pyramids, and hornblende and biotite are both present. 1 South- West Highlands. Porphyrites occur as dykes and sills in Lome, between Loch Etive and Loch Awe. They strike, in a north-east and south- west direction, toward the granite of Ben Cruachan, which they are found likewise to traverse. 2 The type is a hornblende-porphyrite, consisting of phenocrysts of plagioclase, hornblende and biotite, in a micro- or cry pto-crystal line groimdmass of quartz and felspar. They appear to be connected genetically with the Ben Cruachan granite, from which they differ but little in chemical composition. Similar rocks are found further south in the Cowal district. They occur as 1 Flett : Geology of Blair Atholl, etc., Mem. Geol. Surv., 1905, p. 116. a Pep. Geol Surv. , 1895, p. 29, and 1896, p. 25 ; and Summary of Progress, 1897, p. 86. 326 DISTRIBUTION IN BRITISH ISLES [PART iv sills and dykes, and probably have a genetic con- nection with the granitic rocks of Loch Fyne. 1 South- West Scotland. For phy rites occur in the Galloway district, among the dykes associated with the Critfel and Loch Dee granite masses. Plagioclase, hornblende, biotite and occasionally quartz occur as phenocrysts in a compact ground- mass. The colour varies from grey to red. The microscopic structure may be crypto-crystalline, micro-poecilitic, micropegmatitic, or even spherulitic. 2 Midland Valley. A dioritic granophyre with hypersthene (hypersthene-markfieldite) occurs at Tillicoultry, north of the Forth. It closely resembles the Penmaenmawr rock, described on p. 324. Ireland. Augite-porphyrites, which were intruded between the Bala beds and the Upper Old Ked Sand- stone, occur on the coast of Waterford, as at Knock- ma.hon, Buiimahon, and Baity douane Bay. 3 On Lambay Island, off the east coast of County Dublin, an augite-porphyrite occurs as intrusions in Bala rocks at Pilot's Hill and the Flint Rock ; this, the well-known " Lambay porphyry," resembles the porfido verde of the ancients. It consists of a green decomposed groundmass, composed mainly of lath- shaped felspars and augite granules, but containing also calcite, epidote, chlorite and magnetite. In this 1 The Geology of Cowal, Mem. Geol. Surv., 1897, pp. 96, 298 ; The Geology of Mid-Argyll, Mem. Geol. Surv., 1905, p. 113. 2 Teall : The Silurian Rocks of Scotland, Mem. Geol. Surv. (1899), p. 627. 3 Cowper Reed : Quart. Journ. Geol. Soc., vol. Ivi., 1900, p. 688. CHAP. II DISTRIBUTION HYPABYSSAL ROCKS 327 groundmass are embedded idiomorphic phenocrysts of labradorite. 1 DISTRIBUTION OF THE MUGEARITES AND TESGHEN1TES. The type locality for mugearite is in Skye at Druim na Criche, a flat ridge 2 miles north-east of Roineval and 5 miles south-south-west of Portree. It is so named from Mugeary, a crofter village lying a short distance to the south. Mugearites identical with the typical rock of Skye occur in Eigg and in Muck. 2 Rocks of similar chemical and mineral composition occur among the Carboniferous rocks of Midlothian (e.g., Whinny Hill, Arthur's Seat, and Corston Hill, 12 miles west-south-west of Edinburgh) and East Lothian. Most of them have been mapped as lavas, but it is possible that they are sills. 8 A teschenite, described by Corstorphine 4 as an olivine-bearing analcime-diabase from a large sill in the south-east corner of Arran (Lamlash), consists of olivine, plagioclase, augite, and analcime, together with apatite and black iron-ore. Teschenites have also been recorded by the Geological Survey among the basic intrusions in the Carboniferous rocks of Midlothian, for instance, at Lasaulx: TschermaVs Min. u. Pet. Mitth., 1878, p. 419 ; and Gardiner and Reynolds : Quart. Journ. Geol. Soc., vol. liv., 1898, p. 135, 2 Harker : Tertiary Igneous Rocks of Skye, loc. cit. p. 265 ; and Geology of the Small Isles, Mem. Geol. Surv., 1908, p. 126. 3 Flett : Summary of Progress, Geol. Surv., 1907, p. 120. 4 Corstorphine : Min. u. Pet. Mitth., vol. xiv., 1895, p. 463. 328 DISTRIBUTION IN BRITISH ISLES [PART iv Whitehouse Point, Dalmeny, near Edinburgh. The last-named occurrence is rich in analcime and brown hornblende. It also contains plagioclase, augite, and serpentinized olivine. A similar rock occurs at Wester Ochiltree, 2J miles south-east of Linlitbgow, and at Blackburn, Bathgate. 1 Other localities are Salisbury Crags; Inchcolm; Craigleith Island, North Berwick; Carcraig and Gullane Hill 2 in the Firth of Forth. The analcime -bearing dolerite intrusions, which have recently been described by Dr. Flett as essexites, appear to belong rather to the teschenite family on account of their hypabyssal character. The occurrences mentioned are: a dyke of great thickness, intruded in the Carnpsie lavas, one mile north of Lennoxtown, and a sill at Lochend, near Edinburgh. 8 The Bartestree dolerite, according to Reynolds, 4 and the Glee Hill dolerite, according to Barker, 5 contain analcime, and these should therefore be included in the teschenite family. A typical teschenite occurs at Whitwick in Leicestershire. 6 DISTRIBUTION OF DOLERITES. Devonshire and Cornwall. Numerous small bosses, laccoliths and sills of much-altered dolerite, locally known as greenstone, protrude through the 1 Summary of Progress, Geol. Surv., 1907, p. 54. 2 Teall : British Petrography, 1888, p. 191 . 3 Summary of Proqress, Geol. Sum., 1907, p. 54, 4 Reynolds : Quart. Journ. Geol. Soc., vol. Ixiv., 1908, p. 501. 5 Harker : Petrology for Students, 1908, p. 148. 6 Leicestershire Coalfield, Mem. Geol. Surv. p. 33. CHAP. 11] DISTRIBUTION HYP ABYSSAL ROCK8 329 Devonian slates and the Culm-measures of these counties. Thus they occur abundantly on the margin of the Dartmoor granite ; in the Teign Valley, north-west of Chudleigh and north of Newton Abbot ; also in the neighbourhood of Tavis- tock, where they swell out into large irregular and lenticular masses. Again, numerous sills are de- veloped around Padstow Harbour ; in the neigh- bourhood of Endellion ; between Davidstow and St. Clether, north-east of Camelford ; and between Lis- keard and Saltash. In texture these rocks vary from very compact dolerites, as at Gurnards Head, Zennor, and Harlyn, near Padstow, to coarser ophitic rocks, as at Temple ton, Cornwall ; Yarner Beacon, Dartington ; Ashton, Christen, and Cro- combe, north-west of Chudleigh ; the Botter Rock, near Hennock ; Smear Ridge, near Tavistock. Mica- dolerite occurs at Saltash, near Plymouth. Midland Counties. Warwickshire. Fine-grained, rather felspathic dolerites occur as sills and dykes in the Cambrian and pre-Cambrian rocks of Nuneaton, as at Hartshill, Marston Jabet, Bedworth, Atherstone, Chilvers Coton, etc. These intrusions were described as diorites by Allport l and Rutley ; but their mode of occurrence and structure show them to be hypabyssal rocks, while their mineral composition suggests a basic rather than an intermediate type ; unfortunately no analyses appear to have been made of these rocks. Under the microscope a type occurring at Griff Farm, two miles south of Nuneaton, and at 1 Allport : Quart. Journ. Geol Soc., vol. xxxv., 1879, pi C37. 330 DISTRIBUTION IN BRITISH ISLES [PART iv Chilvers Coton, is seen to be largely composed of lath- shaped and prismatic crystals of plagioclase. Wedged in between the felspars are patches of chlorite and calcite, which no doubt mostly represent augite. In other types hornblende plays a more prominent rdle: thus, at Marston Jabet it occurs in long idio- morphic crystals, and at Tuttle Hill in long needles visible in the hand specimen. Such types suggest affinities to the lamprophyres. 1 In the rock occurring at Atherstone and Griff Hollow the hornblende occurs in large plates ophitically enclosing the felspar laths. Such rocks may be appropriately termed hornblende- dolerite. In some occurrences pseudomorphs after olivine occur ; and occasionally the increase of olivine and corresponding diminution of the felspar content, indicate affinities to hornblende-picrites. Precisely similar rocks occur in the Malvern range, as at the Lower Wych Quarry. Watts 2 mentions rocks of similar type as intrusive in the Archaean rocks of the Lickey, in the Shineton shales of the Wrekin, in the rocks of the Longmynd and its northern continuation at Bayston Hill, near Shrewsbury. Olivine-dolerites, intrusive in Carboniferous strata, occur at Pouk Hill, near Walsall ; Swinnerton Park, 8 miles east of Stafford ; Rowley Regis, near Birmingham ; Barrow Hill, west of Dudley ; Tansley Hill, near Dudley ; and many other places. 3 They are composed of plagioclase, brown augite, iron- 1 Watts : Proc. Geol. Assoc., vol. xv., 1897, pp. 394-396. 2 Loc. cit., p. 395. 3 Allport : Quart. Journ. Geol. Soc., vol. xxx., 1874, p. 520. CHAP, n] DISTRIBUTION HYP ABYSSAL ROCKS 331 stained olivine, and iron ore, and vary in structure from granular to ophitic. Frequently the ophitic structure is developed in the central portion of the dyke or sill, while near the margin the augite assumes a granular structure. Derbyshire. Dr. Arnold-Bemrose 1 gives the following localities for sills of olivine-dolerite in the Carboniferous Limestone of Derbyshire : Peak Forest, Potluck, Water Swallows, Tides well Dale, New Bridge, Ible, Bonsall, High Peak Tavern, and Low Farm. North of England. Many of the dykes which traverse the northern counties are dolerites (without olivine). The Great Whin Sill, which runs from Cross Fell to Dunstanburgh Head, a distance of about 70 miles, is a sub-ophitic dolerite, containing enstatite in places. It also contains quartz in micro- graphic intergrowth with felspar. At Tyne Head, and at Cauldron Snout in Teesdale, it forms a rather coarsely crystalline variety, with long blades of augite. Its age is uncertain, but it is intrusive in, and therefore later than, the Carboniferous Limestone. The Cleveland dyke, which cuts Secondary strata (Lias at Grosmont, Yorkshire) is probably an older Tertiary intrusion. It appears at the surface at intervals for a distance of ninety miles. Its ground- mass consists of lath-shaped felspars, granular augite, iron-ore and interstitial matter, and contains pheno- crysts of labradorite. It is, therefore, a porphyritic dolerite. The Acklington dyke, which runs from 1 Quart. Journ. Geol Soc., vol. Ixiii. (1907), p. 278. 332 DISTRIBUTION IN BRITISH ISLES [PART iv Bondicar, on the coast of Northumberland, into the Cheviot district, is a rock of the same type. Other well-known dykes occur in Durham and Northumber- land : Tudhoe and Hett ; Hebburn ; Tynemouth and Coley Hill (with large porphyritic crystals of anorth- ite); High Green, Seaton, and Hartley. These may all be classed as dolerites without olivine. A dyke of olivine-dolerite occurs, however, near Morpeth. 1 Lake District. Dolerites are not quite so abundant in the Cumbrian as in the Cambrian areas. The sills are best developed south of the Borrowdale Fells (in Langstrath, on Stonethwaite Fells, and crossing Greenup Gill to Ullscarf). A well-marked boss is formed by the dolerite of Castle Head, near Keswick. Similar rocks occur at Wythop Fells; and at Swirrel Edge, near the summit of Hel- vellyn. Shropshire, Montgomeryshire, Radnorshire, Herefordshire, and Worcestershire. Basic in- trusions of at least two ages occur in these counties : namely, those that pierce Cambrian, Ordovician, and the lowest beds of the Silurian formations, and those that pierce the Old Red Sandstone and Carboniferous formations. The latter contain olivine, while as a rule the former are free from that mineral. Hypersthene-dolerites of the older series are intrusive in the Longmyndian sediments at Rat- linghope. They are coarse-grained and ophitic, and do not contain olivine. The Ordovician beds of the Shelve area (10 miles west of Church Stretton) are 1 Teall : Quart. Journ. Geol. Foe., vol. xl. (1884), p. 209. CHAP, n] DISTRIBUTION HYPABYSSAL ROCKS 333 pierced by a vast number of intrusive dykes and sills of this rock. They are of post-Llandovery age. 1 At Corndon and at Pitchfolds the laccolithic character of these sills can be seen. At the former locality the main intrusion rests conformably on the bedding planes of altered Hope shales, and has arched up the overlying Stapeley ash-beds. 2 In the Breidden Hills (12 miles west of Shrewsbury), hypersthene- dolerites containing both hypersthene and augite, and often with ophitic structure, occur at Rodney Pillar, Criggon, Belau Bank, and Trewern. According to Watts a they are probably of post-Silurian age. The dolerites of the Carneddau range, near Builth in south-west Radnorshire, cut Ordovician (Llandeilo) rocks, but are not found piercing the Llandovery beds; they are therefore considered by Woods 4 to be of post-Llandeilo and pre-Silurian age. Mica-dolerites have been described from Stanner and Hanter Hill, near Old Radnor. A distinct type occurs, as sills in the Cambrian beds of the Malvern range, on the borders of Herefordshire and Worcestershire. These rocks are ophitic and porphyritic olivine-dolerites. According to Groom 6 their intrusion took place after the deposition of the Tremadoc beds, and before that of the Upper May Hill Sandstone (Llandovery). Younger olivine-dolerites are intrusive in the Old Red Sandstone at Bartestree, near Hereford, and at 1 Watts : Proc. Geol. Assoc., vol. xiii., 1893-94, p. 297. 2 Watts : Rep. Brit. Assoc., 1886, p. 670. 3 Watts : Quart. Journ. Geol. Soc., vol. xli. (1885), p. 537. 4 H. Woods : Quart. Journ. Geol. Soc., vol. 1. (1894), p. 587. 5 Groom : Quart. Journ. Geol. Soc., vol. Ivii. (1901), p. 183. 334 DISTRIBUTION IN BRITISH ISLES [PART iv Brock Hill, near Shelsley Beaucharap in Worcester- shire. They are probably of post-Carboniferous age. The Bartestree dyke, which has a thickness of 35 feet, is a multiple dyke made up of successive injections of basic material. It strikes in an east-north-east direction through Old Red marls and sandstones, which are strongly metamorphosed by it for ten feet from the contact. 1 (See also p. 328.) Post-Carboniferous olivine-dolerites occur, as intru- sive sheets in the Coal measures, at the Clee Hills, five miles north-east of Ludlow ; and a few miles further to the north-east at Kinlet, near the Wrekin, they are intrusive in the Millstone Grit and Carbon- iferous Limestone. There, in common with many other dolerites intrusive in Carboniferous i^ocks described by Allport, 2 they contain idiomorpbic plagioclase, brown augite, divine, and abundant iron- ore. Portions of both the Bartestree and the Clee Hill intrusions contain analcime, a fact which indi- cates a relationship to the teschenites. North Wales. The dolerites in the Ordovician rocks of Caernarvonshire and Merionethshire occur usually in the form of sheets (sills), intruded along i>he bedding planes ; but occasionally they swell out to irregular or lenticular masses (bosses and lacco- liths). In composition they are chiefly ophitic plagioclase-augite rocks, containing no olivine, but with much secondary hornblende, chlorite, calcite and epidote. 1 Reynolds : Quart. Journ. Qeol. Soc., vol. Ixiv., 108, p. 501. 2 Allport : Quart. Journ. Geol. Soc. t vol. xxx. (1874), p. 529. CHAP, n] DISTRIBUTION-HYPABYSSAL ROCKS 335 A good representative of the Caernarvonshire dolerites is the " Gimlet Rock," which is quarried for paving-setts and road-metal, at Pwllheli in the Lleyn peninsula. In Merionethshire the dolerite intrusions are especially developed in the neighbour- hood of Dolgelly. Thus, the mass forming the ridge of Mynydd-y-gader, north of Cader Idris, consists of a medium-grained ophitic dolerite. A similar rock occurs at Bwlchau-yr-Figen, three miles north-west of Dinas Mowddwy. A more granitoid type is developed at B \vlch-yr-Hendref, near Capel Arthog, six miles south-west of Dolgelly. A hornblendic type is represented in the Lleyn peninsula. These hornblende-dolerites are composed of brown hornblende in ophitic plates moulding plagioclase felspar, together with a pale augite and iron ores. Olivine, partially altered to serpentine, occurs as an accessory constituent. In the Lleyn they constitute three oval masses (laccoliths ?), two of which build the hills Mynydd Penarfynydd, 1 Careg- llefain and Mynydd-y-graig, the third lying to the east of Rhiw. The Penarfynydd rock, though coarse- grained, presents a marked ophitic structure. Besides the Ordovician intrusions, certain dolerite dykes occurring in the Lleyn and in Anglesey (Menai) are considered by Harker to be of post-Carboniferous age. 2 1 Underlying the hornblende-dolerite is the beautiful horn- blende-picrite of Mynydd Penarfynydd. 2 Harker : Geol Mag., 1887, p. 409, and 1888, p. 267. See also a discussion of the age of the Llyn Padarn dykes by J. V, Elsden : Quart. Journ. Geol. 6oc., vol.: Ix. (1904), p. 372. FIG. 123. Sketch-map of the Sarn District, JJ Granite. ^1 Gabbro and g Diorite. " r Dolerite. I Hornblende- dolerite. Hornblende- picrite. Dips. Probable faults marked . NAMES OF PLACES. B. Bryn-croes. M. JMeillionydd. P^ Penarfynydd. 8. Barn. R. Rhiw. Cf. Oraig-y-fael. 336 CHAP, n] DISTRIBUTION HYPABYSSAL ROCKS 337 South Wales. Numerous sills and bosses of dolerite occur in the Ordovician rocks of Pem- brokeshire, along the coast, from St. David's Head to Strumble Head. These sills run out to sea in great numbers, in many cases forming headlands and promontories. A larger mass of typical ophitic dolerite, without olivine, occurs near Llanwnda, 2 miles north-west of Fishguard. Between Fisliguard and Newport the sills are greatly developed. They run north-east and south-west, parallel to the strike of the rocks into which they are intrusive, and are continued in a southerly direction into the heart of Pembrokeshire. 1 Hypersthene-dolerites, containing labradorite, both monoclinic and rhombic pyroxenes, a little biotite and abundant ilmenite, have been described by Elsden 2 (as biotite -norite) from several localities near St. David's Head. Isle of Man. Numerous dykes and small bosses of altered dolerite occur in the Manx slates. They are earlier than the acid intrusions (e.g., those of the Dhoon granite), by which they are cut. Newer (Tertiary ?) olivine-dolerite dykes occur at many places on the coast, as on the southern side of Maughold Head. They are especially numerous in the Carboniferous rocks of the southern part of the island. 8 1 See descriptions by Cowper Reed (Quart. Journ. Geol. Soc., vol. xli., 1895, p. 149), and Elsden (ibid. vol. Ixi., 1905, p. 579, and vol. Ixiv., 1908, p. 273). 2 Elsden : Quart. Journ. Geol: xSfoc., vol. Ixiv: (1908), p. 278. 3 Lamplugh : The Geology of the Isle of Man, Mem. Geol. Surv. (1903), p. 327. Y 338 DISTRIBUTION IN BRITISH ISLES [PART iv Midland Valley of Scotland. Numerous in- trusions of dolerite occur as bosses, sills, and dykes in the Carboniferous rocks of the Midland Valley of Scotland. They are especially numerous in Mid- lothian and Linlithgowshire, in the neighbourhood of Edinburgh (Heriot Mount, the Dasses, etc.), and to the west and north-west of that town (Corstorphine Hill, Dalmahoy Hill, Ratho, Cramond, and Craigie Hill), and in Fifeshire (Burntisland, Cleish Hills, and near Ceres). These rocks are remarkable for a variety of composition and structure, depending on the nature of the phenocrysts and the relation of the latter to the groundmass. A prime division is into olivine and olivine-free dolerites. The latter generally con- tain quartz in the form of interstitial micropegmatite. In the following classification, 1 according to structure, the first three are olivine-free types, and the last an olivine-bearing type : (a) The Ratho Type. Plagioclase and augite in sub- ophitic intergrowth, with a small amount of interstitial micropegmatite. Idiomorphic hypersthene is fre- quently present. Examples : Ratho, 8 miles west of Edinburgh ; Glenboig, near Glasgow ; and Stirling Castle. (b) Bowden Hill Type. Plagioclase and augite in sub- ophitic intergrowth, but with occasional large patches of groundmass, in contact with which the augite is idiomorphic. Idiomorphic hypersthene is also present. There is more interstitial micropegmatite than in the 1 Hatch: Hep. of the Geol. Surv., 1896, p. 64, and The Geology of Central and Western Fife, Mem. Geol. Surv., 1900, p. 81. CHAP. II] DISTRIBUTION HYPABYSSAL ROCKS 339 Ratho type. Examples : Bowden Hill in Linlithgow- shire, and the sill running northward from North Queensferry. (c) Tholeiite Type. Felspar phenocrysts, incon- spicuous and small. Mainly a mesh of fine felspar laths or microlites with granular augite, magnetite and unindividualised matter. This type occurs in the chilled margins of sills, and as small dykes. (d) Gallaston Type. 1 Phenocrysts of olivine in C 3 / c / f e r o a s 5 3 n ^ ' s t one f Carboniferous) FIG. 124. Map showing Distribution of Intrusive Dolerites in the neighbour hood of Edinburgh. Scale I inch =4 miles. a sub-ophitic groundmass of plagioclase and augite. Example : Gallaston quarry at Kirkcaldy. Olivine-dolerites occur in Forfarshire (south of Glamis) and in Roxburghshire (Dunian and Black Law). They also form a part of Corstorphine Hill and Dalmahoy Hill, near Edinburgh. 1 Compare the Jedburgh and Kilsyth types of basalt described oc pages 377 and 378. 340 DISTRIBUTION IN BRITISH ISLES [PART iv Western Isles of Scotland. Ophitic olivine- dolerites of Tertiary age are intrusive mainly as sills between the bedding planes of the Tertiary basic lavas, and of the Jurassic strata, of Skye, Mull, and the small islands lying to the south of Skye : Rum, Eigg, Muck, etc. They are also represented on the mainland, especially in Ardnaniurchan in Argyllshire. South- West Scotland. Ophitic dolerites and epidiorites occur as intrusions in Ordovician rocks on the Ayrshire coast near Ballantrae. South- West Highlands. Numerous sills of " epidiorite " or dolerites, in which the augite has been changed to hornblende, extend over a tract of country, some 5 miles in breadth, through Argyllshire for example, between the Pass of Brander and the main road from Taycreggan to Glenhaut ; in the neighbourhood of Loch Awe ; and in the Kilmartin district. They also occur as well-marked dykes, filling fissures in the quartzite of the Isles of Scarba and Lunga. While the sills have been folded and cleaved with the sediments into which they are intruded, the dykes have escaped a part of the folding movement, and therefore are of slightly later date. They are cut, however, by the porphyrite and lamprophyre dykes, and by the Tertiary dolerites. 1 The most prevalent type is a felspar-hornblende-schist with scattered phenocrysts of plagioclase felspar, but every grada- tion exists from a coarse dolerite type to the finest- grained schist. 5 * Besides plagioclase and green horn- 1 Summary of Progress, Geol. Surv,, 1902, p. 75. 2 J. B. Hill : Summary of Progress, Geol. Surv., 1900, p. 41. CHAP, n] DISTRIBUTION HYPABYSSAL ROCKS 341 blende, the rocks contain chlorite, a pale greenish biotite, epidote, leucoxene after titaniferous iron-ore, and carbonates. North-West Highlands. Dolerite, partly altered to epidiorite and to hornblende-schist, occurs in numberless dykes, having a north-west and south- east strike, in the Lewisian gneiss between Loch Laxford and Enard Bay. The dyke-like character of the intrusions is obvious, but in the southern area, between Loch Maree and the Islands of Rona and Raasay, it is not so apparent, although even there the evidence for the intrusive nature of the basic rocks is not wanting. 1 They are dark in colour, and medium to coarse in grain. The structure is hypidiomorphic- granular rather than ophitic, although occasionally lath-shaped crystals of labradorite may be observed penetrating grains of augite. Amongst them are enstatite- and hornblende-dolerites, besides the usual type in which augite is the dominant ferro-magnesian constituent. Olivine is in most cases absent. South-East of Ireland. Dolerites passing into epidiorites are intrusive in the Ordovician rocks of County Wicklow (e.g., at Arklow Head and in the Aughrim Valley). 2 In the east of Waterford there are numerous intrusions of dolerite dykes and sills of pre-Upper Old Red Sandstone age. Some show true ophitic structure, as at Tramore Bay ; in others the ophitic structure is less developed, as at Little Island, Great Newtown Head ; while in yet a third division iThe Geological Structure of the North- West Highlands, Mem. Geol. Surv., 1007, p. 89. 2 Hatch': Geol Mag., 1889, p. 261. 342 DISTRIBUTION IN BRITISH ISLES [PART iv the structure is allotriomorphic-granular, as at Gar- rarus, Tramore. 1 North-East of Ireland. Tertiary dolerites occur in the basalt plateau of Antrim (e.g., at Portrush ; Scawt Hill, 4 miles south-south-east of Glenarm ; and at Fair Head). DISTRIBUTION OF APLITES. As the most acid differentiation product of the granite magma, aplites are invariably associated with granite occurrences. It will therefore suffice to give a few examples. Devon and Cornwall. Dykes of aplite occur in Devonshire in connection with the Dartmoor granite. Thus at Meldon there is an aplite dyke, containing accessor topaz and a pale-coloured tourmaline. Other occurrences on Dartmoor are at Row Tor and Scarey Tor. The Cornish granites are also frequently varied with aplite. They are especially numerous in the central portion of the Carn Menelez mass. Green tourmaline characterises an aplite vein at Lanner. 2 Wales. Veins of soda-aplite penetrate the dioritic granophyre of Carn Llidi at St. David's Head. This type is composed mainly of albite felspar. 3 Scotland. Aplite veins are found, in association with lamprophyres, in the neighbourhood of the Criffel and Loch Dee granites in south-west Scotland. 4 They Reed : Quart. Journ. Geol. Soc., vol. IvL, 1900, p. 684. 2 Summary of Progress, Geol. Surv., 1903, p. 26. 3 Elsden : Quart. Journ. Geol. Soc., vol. Ixiv., 1908, p. 284. 4 Rep. Geol. Surv., 1896, p. 44. CHAP, n] DISTRIBUTION -HYPABYSSAL ROCKS 343 also occur in the Cairnsmore of Fleet mass, and are there thickly studded with small garnets. 1 An occur- rence of malchite or diorite-aplite has been recorded, by the Geological Survey, in Glen Finnan, Inverness- shire. It is a grey, fine-grained rock, composed essentially of plagioclase and hornblende, bat contain- ing also a little quartz, orthoclase, and biotite. The microscopic structure is panidiomorphic. 2 Ireland. Veins and dykes of a white, fine-grained garnetiferous aplite traverse the Mount Leinster granite. They can be well seen in the quarries on Killiney Hill, and in the railway cuttings below. DISTRIBUTION OF PEGMATITES. Cornwall. In Cornwall and in the Scilly Isles pegmatites are found penetrating the granite or the adjacent killas. At Tresayes Downs in the St. Austell district there is a vein of very coarse pegmatite, 150 feet wide, in the killas, a few yards from the granite margin. It consists mainly of orthoclase felspar, with some quartz, white mica, tourmaline, and fluorspar. Occasionally the mica predominates. Thus, on St. Denis Downs there is a vein of dark mica associated with a graphic aggregate of quartz and felspar. 3 Scotland. Pegmatites are also associated with aplites in many of the south of Scotland granite areas; and fine-grained aplites even appear to pass in some cases into coarse-grained pegmatitic rocks 1 Miss Gardiner : Quart. Journ. Geol. Soc., vol. xlvi., 1890, p 569. 2 Flett : Summary of Progress, Geol Sum., 1901, p. 70. 3 Summary of Progress, Gtol. Surv., 1906, p. 37. 344 DISTRIBUTION IN BRITISH ISLES [PART iv with graphic structure, as at Knocknairling Hill in the Galloway district. 1 A similar association of fine- grained aplite and coarse-grained pegmatite may be seen in Cornwall, as at Trewavas Head. 2 Pegmatites of intrusive origin are abundant in many portions of the Lewisian gneiss ; for instance, in the neighbourhood of Laxford, where they occur in association with granites with which they are no doubt genetically connected. The commonest type is one in which microcline and quartz are associated with pronounced graphic structure. When foliated, the microcline individuals form " eyes." A good example of this structure occurs near Shieldaig on the south side of Loch Torridon. 3 In the Central Highlands veins of pegmatite and aplite fringe the granitic areas, and Barrow has explained their occurrence as due to a squeezing out, under pressure, of the residual granite magma. 4 DISTRIBUTION OF BOSTONITES. Scotland The pale-brown coloured dykes, de- scribed by the Geological Survey as trachytes, which are associated with camptonites in the Cowal district of Argyllshire, 5 belong probably to the bostonite family. The following are the localities 1 Miss Gardiner : Quart. Journ. Geol. 6oc., vol. xlvi. (1890), p. 569. 4 Teall : The Silurian Rocks of Scotland, Mem. Geol. Surv., (1899), p. 62. * North- West Highlands, Mem. Geol. Surv., 1907, p. 258. 4 Barrow: Quart. Jourti. Geol. Soc., vol. xlix. (1893), pp. 330-386. 6 Geology of Cowal, Mem. Geol. Surv., 1897, p. 166. CHAP, n] DISTRIBUTION-HYPABYSSAL ROCKS 345 mentioned : Cruach Mhor, Craigendaive, Dum Mor, and Dunans. Some of these dykes have a glassy selvage in which spherulitic structure is occasion- ally developed. The central, well-crystallised portion consists of small lath-shaped crystals of orthoclase, together with scales of brown mica and a few prisms of pale augite. For chemical analysis see page 231. A series of pale-coloured felsites, which occur associated with lamprophyres in innumerable sills and dykes of post-Cambrian age in the Assynt district, belong to this division of the differentiated dyke rocks. Their distribution is well shown in the one-inch sheets, 101 and 107, of the Geological Survey, and Inchnadamff is a convenient centre for their examination. A typical example occurring on the north shoulder of Cnoc an Droighinn, one mile north-east of the hotel, consists of phenocrysts of alkali-felspar (cryptoperthite) in a felspathic matrix crowded with acicular microlites of aegirine. There is thus a resemblance to Brogger's typical grorudite. Another example, but with less segirine, and therefore more resembling Brogger's lindoite, occurs at Sgonnan Mor. 1 These rocks contain a fairly high percentage of silica, and must therefore be termed quartz-bostonites. An occurrence of bostonite also in association with lamprophyres, has been described by Flett in the Orkney Islands. 2 For chemical analysis see page 231. 1 Teall : Geological Structure North- West Highlands, Mem. GeoL Surv., 1907, pp. 449 and 450. See also Geol. Mag., 1900, p. 391. 2 Trans. Eoy. Soc. Edin., vol. xxxix. (1900), p. 887. 346 DISTRIBUTION IN BRITISH ISLES [PART iv DISTRIBUTION OF THE LAMPROPHYRES. North of England. The' North Country lampro- phyres occur as small intrusions (chiefly narrow dykes) over an area extending from Teesdale to Furness, and from Bassenthwaite to Ingleton, a circular area having a diameter of 50 miles. In the centre of this area lies the Shap granite, and in consequence of the common age of the intrusions (post-Silurian but pre- Carboniferous), and the radial grouping of the dykes, a genetic connection between the granite and the lamprophyre dykes has been urged by Messrs. Harker and Marr. 1 It would appear as if the lam- prophyres were a special basal modification of the granite magma, in which, with a greatly diminished percentage of silica, the total alkalies remain un- changed, while the potash is increased at the expense of the soda. 2 It has been noticed that the basic patches in the Shap granite bear a strong resemblance to some of the lamprophyres. Associated with the basic dykes are acid intrusions (aplites, etc.) which belong to a slightly earlier period of intrusion, as they are cut by the lamprophyres. In composition they belong to the group of mica-lamprophyres, being in part minettes, in part kersantites : but the distinction is not easy to make, on account of the felspar being mainly confined to the groundmass and obscured by secondary products. The following are some occurrences that have 1 Barker and Marr : Quart. Journ. Geol. Soc., vol. xlvii. (1891), p. 285. 2 Harker : Geol. Mag., 1892, p. 199. CHAP, n] DISTRIBUTION HYPABYSSAL ROCKS 347 been described in the Sedbergh district ; l Cross Haw Beck ; railway cutting, Wiudermere ; Kendal road ; railway, Docker Garth ; south of Haygarth ; Helm Gill ; Taith's Gill ; in the Rawthey, at Ward's Intake ; in Backside Beck ; Holbeck Gill ; Uldale Head; Watley Gill; and Westerdale. There are several localities in the Cross Fell range, of which Crowd undle Beck, and Swindale Beck may be quoted as examples. Lampropbyres of a variable composition occur also in the Lower Palaeozoic inliers of Ingleton and Teesdale. Devon and Cornwall. In Cornwall there is a dyke of minette at Trelissick Creek near Falmouth, and one of kersantite between Rosecreage Beacon and Watergate Bay on the west coast. One of a group of mica-lamprophyres occurring between Newquay and Holywell Bay, contains, in addition to large plates of dark brown mica, crystals of pale -green augite and dark-brown hornblende, in a matrix of panidiomorphic felspar and interstitial quartz. 2 Excellent examples of mica-lamprophyre occur in Devonshire, at Rose Ash, and in other localities. Isle of Man. Lamprophyres, both the hornblende and mica varieties, occur in numerous dykes traversing the Manx slates. They are of pre-Carboniferous age. Although they also occur inland, they are mostly known in the cliff sections on the east coast, as at Langness ; on the west coast, north of Peel, and at Contrary Head. 1 Hatch : Eep. Brit. Assoc. for 1890, p. 814 ; and Geology of Mallerstang, Mem. Geol. Surv., 1891, passim. 2 Summary of Progress, Geol. Surv., 1903, p. 162. 343 DISTRIBUTION IN BRITISH ISLES [PART iv South- West Scotland. Lamprophyres occur in considerable variety, cutting Silurian rocks in Kirkcudbrightshire. Thus they are strongly de- veloped on the Colvend shore, and at Craigneuk Point, five to six miles south of Dalbeattie. Both micaceous and hornblendic types are found. Mica- lamprophyres (minettes and kersantites) occur at Greenhill, east of Micklewood, 4 miles north of Kirkcudbright ; at the Bridge of Dee, 2J miles south-west of Castle Douglas ; and at Glenwharfen Burn, Scarwater, Dumfriesshire. Lamprophyre dykes appear in great abundance in the neighbourhood of the granite complexes, and are probably connected with them in origin. Thus in the Galloway district they are especially abundant in the neighbourhood of the Criffel and Loch Dee granite masses, of which they doubtless represent the basic differentiation products, the complementary acid products being aplites. 1 North- West Highlands. Lamprophyres allied to minctte, but containing microcline instead of ortho- clase, occur among the basic dykes in the Lewisian gneiss. The rock of Creag Tombaca, Glen Dhu, 1J miles east-south-east of Kylesku, may be taken as a type. 2 Hornblendic lamprophyres (vogesites and spessartites) of post-Cambrian age occur abundantly as dark basic sills in the dolomites of the Assynt district, 3 where they are associated with bostonites. (See p. 345.) 1 Silurian Rocks of Scotland, Geol. Surv. Mem., 1899, p. 627. 2 North- West Highlands, Mem. Geol. Surv., 1907, p. 98. s North-West Highlands, Mem. Geol. Surv., 1907, p. 452. CHAP, n] DISTRIBUTION HYPABYSSAL ROCKS 349 Central Highlands. Dykes of lamprophyre are abundant in this district. The Kenmore kersantite and the Carn Dearg vogesite are typical. Minettes and spessartites are rare. 1 South -West Highlands and Western Isles. Lamprophyres, both of the hornblendic and of the micaceous type (kersantites, vogesites, and campton- ites), occur as numerous sills and dykes in the metamorphic rocks of the north and east parts of Cowal, and they are especially developed in Glen Shira and Brannie Burn. The dominant felspar is plagioclase. Phenocrysts of augite, biotite and horn- blende occur, according to the special type. The augite is a colourless variety (in thin section). The hornblende occurs frequently in long brown prisms, or in brownish -green hypidiomorphic crystals. 2 The rock of Glen Catacol, Isle of Arran, described as a syenite, 3 is more correctly a vogesite ; for it occurs as a dyke, and consists of idiomorphic brown hornblende in an aggregate of orthoclase and oligoclase. Ireland. Mica-lamprophyre dykes, intrusive in Silurian rocks, occur in County Louth ,at 1 mile north-west of Devlin's Bridge, at Starinagh, and at Oriel Demesne. Similar dykes are found at Clogher Head. Both hornblendic and micaceous lampro- phyres appear to be of widespread occurrence in the north of Ireland. They occur as thin dykes 1 Geology of Blair Atholl etc., Geol. Surv. Mem., 1905, p. 119. 2 Geology of Cowal, Geol. Surv. Mem., 1897, p. 106; and Geology of Mid- Argyll, ibid., 1905, p. 108. 3 Geology of Arran, Mem. Geol. Surv., 1903, p. 109. 350 DISTRIBUTION IN BRITISH ISLES [PART iv in Ordovician rocks on the coast of County Down. Kersantites occur 3 miles north-west of Down- patrick, and at Bally water ; while a camptonite has been recorded at Dillon, south-east of Downpatrick. 1 A dyke on the shore of South Bay, 3 miles east of Portaferry, contains a blue soda-amphibole (riebeckite or arfvedsonite.) 2 A vogesite with acicular pheno- crysts of hornblende in a groundmass of ortho- clase, hornblende and interstitial quartz, has been recorded at Clondermot, in the Raphoe area of County Donegal 3 ; a camptonite with long needles of brown idiomorphic hornblende, at Lottery in Galway ; and a camptonite with both hornblende and biotite, at Rue Bane Point in Antrim. 4 Professor Reynolds has described the occurrence of a hornblende-lampro- phyre, with idiomorphic green hornblende, in small intrusions near Tourmakeady, County Mayo. 6 Channel Islands. Dykes of kersantite occur in Guernsey (e.g., at Moulin Huet and Bee du Nez), and in Jersey (e.g., at St. Helier). DISTRIBUTION OF MONGHIQUITES. An interesting series of monchiquites, in association with camptonites and bostonites, has been described by Flett 6 in the Orkneys. One of the chief types 1 Watts : Guide, etc., p. 75. 2 Seymour : Summary of Progress, Geol. Surv., 1899 3 p. 181. 8 Watts: Guide, etc., p. 73. 4 Watts : Guide, etc., p. 77. 5 Reynolds : Brit. Assoc. Rep., 1908. Flett: Trans. Royal Soc. Ediri., vol. xxix., 1900, pp. 887-896. CHAP, n] DISTRIBUTION HYPABYSSAL ROCKS 351 is a biotite-monchiquite with abundant phenoerysts of olivine and large plates of biotite. Augite occurs in small crystals of the second generation. The ground mass is decomposed to calcite and fibrous zeolites, so that it is impossible to determine whether it represents an originally glassy residuum, or a final crystallization of nepheline. Another type in the Orkneys is melilite-monchiquite, in which the groundmass consists wholly or in part of melilite in plates of irregular shape. Thin dykes of monchiquite occur also in the Upper Old Red Sandstone of Dunnet Head in Caithness. 1 The rock described as a lim- burgite 2 from Whitelaw Hill (Chester's Quarry), in Haddingtonshire, appears on further investigation to be probably intrusive in Upper Old Red Sandstone, 3 in which case it is related to the monchiquites. The occurrence of monchiquite has also been recorded among the Tertiary dykes of Argyllshire. Thus on the west side of Camas an Fhais, half a mile north- east of Rudha Fionn Aird, there is a monchiquite containing phenoerysts of anorthoclase. 4 DISTRIBUTION OF PICRITES, England. The following are the localities recorded for picrite : Clicker Tor, and Molenicks, in Cornwall ; Shelve Pool and Cwmmawr, north-west of Hyssington, 1 Summary of Progress, Geol. Surv., 1905, p. 115. 2 Hatch : Trans. Royal Soc. Edin., 1892, pp. 116-117. 3 Summary of Progress, Geol. Surv., 1904, p. 97. 4 Flett : Summary of Progress, Geol. Surv., 1907, p. 56. 5 Ussher : Geology of Plymouth, Mem. Geol. Surv., 1907, p. 101. 352 DISTRIBUTION IN BRITISH ISLES [PART iv in Shropshire ; and Little Knott and Great Cockup, near Bassenthwaite, in the Lake District. The latter occurrence is a hornblende-picrite. North Wales. A beautiful type of hornblende- picrite occurs at Mynydd Penarfynydd in the Lleyn peninsula in Caernarvonshire, where it is intrusive in rocks of Arenig age. It consists of brown hornblende, olivine and felspar, together with a pale augite, brown mica, magnetite and secondary products. The shining cleavage-surfaces of the horn- blende are interrupted by dull spots of serpentinized olivine, and thus produce a characteristic lustre- mottling. 1 Another Caernarvonshire locality is Pen-y- rhiwiau, near Clynog-fawr. Similar rocks occur in Anglesey ; viz., north-east of the town of Llanerchy- medd, Pengorphwysfa, near Amlwch, and at Ty- Croes. The Llanerchymedd rock is black and coarse- grained, with large crystals of hornblende, showing lustre-mottling. According to Harker, it forms laccolithic intrusions in Arenig rocks. The Pen- gorphwysfa occurrence is a large, irregularly-shaped mass on the western slope of Llaneilian Mountain. Midland Valley of Scotland. Picrites of well- marked character occur among the Carboniferous rocks of the basin of the Forth, as at Blackburn, near Bath- gate ; Barnton Railway Station ; 2 and on the island of Inchcolm. The Inchcolm rock is a singularly beautiful type : besides olivine, augite and felspar, it contains brown hornblende, biotite, iron-ore, apatite, 1 Barker : Quart. Journ. Geol Soc., vol. xliv. (1888), p. 454. 2 Monckton : Quart. Journ. Geol. Soc. t vol. 1. (1894), 39. CHAP, n] DISTRIBUTION-HYPABYSSAL ROCKS 353 and a vivid bluish-green serpentinous mineral derived from the alteration of the olivine. A fibrous green hornblende (pilite) is also developed to some extent. 1 A very similar rock has recently been discovered at Lethan Hill, Waterside, Ayrshire ; another occurs at Turnberry Castle, 6 miles north of Girvan, on the Ayrshire coast. Western Isles and Highlands of Scotland. Judd 2 has recorded the association of picrites with the Tertiary gabbros and dolerites of Rum, the West of Skye, and the Shiant Isles. Hornblende-picrites also occur in many of the igneous complexes of the Central Highlands and South-West Highlands. Typ'ical picrites occur as dykes in the Lewisian gneiss, e.g., on the south side and near the outlet of Loch Assynt, about 2 miles on the road from Lochinver to Inchnadamff. These dykes show pas- sages to hornblende-schist, and some are much serpentinized. DISTRIBUTION OF PEEKNITES. Various members of this family are represented as bands in the Lewisian gneiss of Sutherland (e.g., at Scourie), and in other areas. The occurrence in Ross-shire 3 of small lenticular sills of hornblendite containing garnet, has been noted by the Geological Survey. An eclogite composed of omphacite and garnet with accessory rutile, has been recorded 1 A. Geikie : Trans. Hoy. Soc. Edin., vol. xxix. (1880), p. 504. 2 Jucid : Quart. Journ. Geol. Soc., vol. xli, (1885), p. 393. 3 Summary of Progress, G-eol. Surv., 1901, p. 109. Z 354 DISTRIBUTION IN BRITISH ISLES [PART iv at Beinn a Chapuill in the Glenelg district. 1 A scapolite-bearing perknite has been described by Flett 2 from Badcall, in Sutherland. It consists chiefly of dark green or brownish-green hornblende, pale green augite and scapolite. Hypersthene also occurs, but less abundantly. DISTRIBUTION OF PERIDOTITES. Peridotites occur as the most basic products of differentiation of deep-seated magmas, in association with picrites, pyroxenites, hornblendites, gabbros, etc. Thus they are found among the Tertiary plutonic rocks of Skye and of Rum ; 3 and appear as dykes in the fundamental complex of the North-West Highlands, 4 and of Connemara in Ireland. Wehrlite and other peridotites occur as basic differentiations in the plutonic complex of Garabal Hill, near the head of Loch Lomond, in the Southern Highlands. Scyelite or biotite-hornblende-peridotite has been described by Judd 6 from the western border of Caithness (Loch Scye), and the same type has been recorded in Ross- shire at Cam Cas nan Gabhar. 6 A peridotite of remarkable freshness (cortlandtite) consisting of hypersthene, amphibole, and olivine 1 Summary of Progress, Geol. Surv., 1899," p. 176. 2 Summary of Progress, Geol. Surv., 1906, p. 124. 3 Barker : The Tertiary Igneous Rocks of Skye, Mem. Geol. Surv., 1906, p. 374, and The Small Isles, ibid., 1908. 4 Geology of the North-West Highlands, Mem. Geol. Surv., 1907, p. 44. 5 Judd : Quart. Journ. Geol. Soc., vol. xli., 1885, p. 393. 6 Flett : Summary of Progress, Geol. Surv., 1901, p. 109. CHAP, n] DISTRIBUTION HYPABYSSAL ROOKS 355 with magnetite, has been recorded by Sollas at Derreennagusfoor, four miles south - east of Maum Hotel, in County Gal way. 1 Hornblende-peridotites associated with amphibolites, occur in small intrusions and dykes in Ordovician strata at Glendalough and Greystones, in the eastern part of County Wicklow. 2 DISTRIBUTION OF SERPENTINES. Serpentine occurs in many localities in Cornwall (Lizard district), Anglesey, Ayrshire (Lendalfoot, Ballantrae, and Balhamie Hill near Colmonell), Aber- deenshire, and Banf&hire (Portsoy). In Forfarshire and Perthshire there are dykes of serpentine ; and as a derivative from an olivine-enstatite rock it forms the Coyles, conspicuous hills lying some 4 miles west of Ballater, in the Central Highlands. As an alteration product of peridotites it also forms a part of the igneous complex of Garabal Hill, near the head of Loch Lomond, in the Southern Highlands. In the west of Ireland serpentines occupy a con- siderable area in Northern Galway and Sligo. The famous green " ophicalcite " of Connemara is chiefly from quarries near Clifden and at Lissoughter. Many of these rocks are probably derived from the altera- tion of olivine-bronzite rocks like saxonite, as they contain numerous crystals of bastite. 1 Rep. Geol. Surv., 1895, p. 38. 2 J. A Thomson : Quart. Joum. Geol. Soc., vol. Ixiv., 1908, p. 475. CHAPTER III. DISTRIBUTION OF THE VOLCANIC ROCKS. DISTRIBUTION OF EHYOLITES. Midland Counties, England. Rhyolitic lavas, wh ; ch appear to be of pre-Cambrian age, occur in Shropshire, at the Wrekin near Wellington, at Caer- Caradoc near Church Stretton, near Wrockwardine, and at Pontest'ord Hill on the Longrnynd. 1 The Lea rock at Overlay Hill, near Wellington, consists of a beautiful red spherulitic rhyolite, showing a well- marked flow-structure ; and the Wrockwardine and Pontesford Hill rocks contain spherulitic nodules, similar to the well-known ones of Boulay Bay in Jersey. 2 Lake District. Rhyolites form a part of the Borrowdale volcanic series. They are felsitic rocks, presenting strong flow structures, and occasionally containing garnets (as at Illgill Head). They occur in Langdale (Dungeon Ghyll), on Crinkle Crags 1 Professor Lapworth informs me that there can be no doubt of the. pre-Cambrian age of these lavas, since there are pebbles of the rhyolites in the basal conglomerate of the Cambrian quartzite at Caer-Caradoc. 2 Parkinson: Quart. Journ. Geol. Soc., vol. Ivii., 1901, p. 220 ; and W. S. Boulton : Quart. Journ. Geol. Soc., vol. lx., 1904, p, 450. 356 CHAP, in] DISTRIBUTION VOLCANIC EOCKS 357 (Gunson Knott) and Great Gable (Sty Head), near Stickle Tarn, in the Copper Mine Valley, near Grize- dale Tarn, in Long Sleddale, and near Great Yarlside. A nodular variety occurs between Shap Wells and Stockdale. North Wales. Very acid rhyolites (composed of porphyritic crystals of quartz and felspar in a felsitic base) form the Llyn Padarn ridge, and crop out near Bangor. They are held to be of pre-Cambrian age. The great majority of the rhyolitic lavas of Caernarvonshire are, however, of Ordovician (Bala) age. They are grouped by Harker 1 in four series, namely : 1. Dwygyfylchi and Y Drosgl series. 2. Pen-yr-Oleu-wen and Carnedd Llewellyn series. 3. Y Glyder-fach, Capel Curig and Conwy Mountain series. 4. Snowdonian series. The Snowdonian lavas can be studied on Snowdon, Moel Hebog, and in the Gwynant and Glaslyn valleys. They form the plateau of Llwyd-mawr, and occur in numerous outliers ; e.g. y west of Dolwyddelen, Moel Siabod, Llyn Lledr, and near Bettws-y,-Coed ; also west and east of Pwllheli, and near Llanbedrog, in the Lleyn peninsula. These rocks consist of a compact felsitic base con- taining scattered crystals of quartz and felspar. When fresh they are dark-coloured ; but when weathered they are lighter in colour and possess a 1 The Bala Volcanic Series of Caernarvonshire, Sedgwick Prize Essay for 1888, 1889, p. 9. 358 DISTRIBUTION IN BRITISH ISLES [PART iv A & "" \s*<: ' x \ / i ir V FIG. 125. Sketch-map of North Wales, showing the distribution of the intrusive rocks and lavas. (After A. Harker.) CHAP, in] DISTRIBUTION-VOLCANIC ROCKS 359 laminated character, due to original flow- structure (Clogwyn-dur-Arddu). Streaks and lenticles of quartz sometimes follow the lines of flow. Ferro- rnagnesian minerals are subordinately developed, and are invariably represented by a pale chlorite. The rocks are frequently nodular, and often present spherulitic and perlitic structures. Some of the best localities for the nodular variety are : Conwy Falls, Digoed and other places near Bettws-y-Coed ; near Capel Curig, Glyder-fawr, Esgair-felen, at the head of Llanberis Pass ; and in the Lleyn peninsula at Pwllheli and Pen-y-chain. Rhyolites are represented in the Coomb volcanic series (Arenig) at Llangynog, 6 miles south-west of Caermarthen. They are pale grey to blue, and consist of phenocrysts of orthoclase and plagioclase set in a silicified groundmass showing flow-structure. The acid lavas were extruded between two eruptions of andesite. 1 Rhyolites of Arenig age are also developed at Mynydd y Gader, Dolgelly. South Wales. In the Fishguard district rhyolitic lavas of Llandeilo and Bala age occur. They are characterised by a high percentage of silica, combined with high alkalies, both potash and soda. 2 Speaking generally, they consist of phenocrysts of quartz and orthoclase in a microfelsitic groundmass ; but the structure of the latter is variable. Similar lavas (the Llanrian series) occur at Abereiddy Bay. 3 Rhyolites 1 Cantrill and Thomas : Q. J. G. S., vol. Ixii. (1906), p. 223. 2 Cowper Reed: Q. J. G. S., vol. li. (1895), p. 177. 3 Elsden : Q. J. G. S., vol. Ixi. (1905), p. 580. 360 DISTRIBUTION IN BRITISH ISLES [PART iv also occur in Ramsey and Skomer Islands, off the coast of Pembrokeshire. The Skomer Island rocks are nodular. Scotland. Rhyolitic lavas of Lower Old Red Sand- stone age occur in the Glencoe volcanic series of the Western Highlands. 1 (See Fig. 130, p. 371.) They are only sparsely developed in the Tertiary volcanic area of the Western Isles : Harker 2 describes a small group, in association with trachytes, on the northern border of the Cuillin Hills, in Skye. They resemble the rhyolites of Iceland. Ireland. A great series of red rhyolitic flows of Lower Llandeilo age occurs in the Tourmakeady dis- trict of County Mayo. 3 Nodular and banded rhyolites are associated with Wenlock and Llandovery beds at Clogher Head, in County Kerry. 4 A spherulitic rhyolite occurs at Lough Guitane among the Dingle beds (Old Red Sandstone) of Killarney. 6 Fine rhyolites occur in the Tertiary volcanic area of County Antrim at Templepatrick, Tardree and Ballyrnena. These were erupted in Eocene times, between the outpouring of the lower and the upper basalts. They are light-coloured rocks, containing phenocrysts of quartz and sanidine in a microlitic groundmass. In structure they vary from com- pact and lithoidal to crystalline and porphyritic. The Templepatrick rock shows flow-structure, and a glassy 1 Summary of Progress, Geol. Surv., 1905, pp. 95 and 98. 2 The Tertiary Igneous Rocks of Skye, Mem. Geol. Surv., 1904, p. 61. 3 Reynolds and Gardiner : Rep. Brit. Assoc., 1908. 4 Reynolds and Gardiner : Q. J. G. S., 1902, p. 226. 6 Watts : Guide, etc., p. 92. CHAP, in] DISTRIBUTION VOLCANIC ROCKS 361 type (obsidian) occurs at Sandy Braes. In this rock a perlitic structure is developed, which not only affects the glass, but extends to the enclosed quartz crystals.' 1 Jersey. A well-known spherulitic rhyolite occurs in Northern Jersey at Boulay Bay. DISTRIBUTION OF PHONOLITE8. England. The phonolite of the Wolf Rock, off the coast of Cornwall, is the only representative of the nepheline rocks in England. It consists of sanidine, nepheline, nosean, and aegirine. Scotland. A dark-brown to grey phonolite occurs at Traprain Law in Haddingtonshire. This is a vent from which doubtless lavas were emitted in Lower Carboniferous times ; but these have been completely removed by denudation. The rock consists of sparsely-developed nepheline, a bright-green soda- augite (segirine or segirine-augite), and sanidine fel- spar, the latter in small, lath-shaped c^stals with parallel arrangement producing a marked flow-struc- ture. Phenocrysts are rare. 2 Similar rocks have been described from Skelfield Pen, 8 miles south of Hawick, and Pikethaw Hill, Teviot Head, 13J miles south of Hawick. 3 DISTRIBUTION OF TRACHYTES. Devonshire. Trachytes are represented in the Permian volcanic series near Exeter : thus biotite- 1 Watts : Quart. Journ. Geol, Soc., vol. 1. (1894), p. 367. 2 Hatch : Trans. Royal Soc, Edin.< vol. xxxvii. (1892\ pp. 115 and 116. 3 H. J. Seymour: Summary of Progress, Geol. Surv., 1900, p. 164. 362 DISTRIBUTION IN BRITISH ISLES [PART iv trachyte occurs at Kellerton Park, near Exeter ; at Knowle, Spencecomb, near Grediton ; and at Copple- stone, near Knowle Hill. The rock quarried at Pocombe appears to be a representative of the ciminite sub-family (see p. 260), since it contains both plagioclase and orthoclase in lath-shaped micro- lites, together with numerous red pseudomorphs after olivine, and augite in sparsely-scattered grains, often altered to carbonates. 1 Scotland. In the Garlton Hills of Haddington- shire the uppermost division of the Carboniferous lavas consists of trachytes : among them is a well- developed porphyritic type composed of large pheno- crysts of sanidine felspar (with which oligoclase is occasionally associated), imbedded in a groundmass of lath-shaped felspars. The phenocrysts are packed in glomero-porphyritic fashion. Such rocks are well seen in the Peppercraig quarries near Haddington, at Dirleton Craig, at Kae Heughs, under the Hopetoun Monument, in the Phantassie and Bangley quarries, and at Skid Hill. A non-porphyritic type of the trachyte occurs at Score Hill, Lock Pit Hill, Craigie Hill, and Pencraig. 2 These are pale-coloured rocks of buff, pink, and mauve tints. They consist of a mass of closely-interlaced felspars of tabular habit, and in consequence have a somewhat glistening appearance. A marked microfluidal structure is oc- casionally evidenced by the parallel arrangement of the microlites. The vents from which these trachytes 'Teall : Summary of Progress, Q-eol. Surv., 1899, p. 171. 2 Hatch : Trans. Boy. Soc. Edin., 1892, p. 121. CHAP, in] DISTRIBUTION- VOLCANIC ROCKS 363 flowed are represented by the trachyte domes of North Berwick Law and the Bass Rock. Trachytes also occur in association with the Old Red Sandstone andesitic lavas of the Pentland and the Braid Hills (e.g., at Morton Hall in the Braid Hills, and at Woodhouselee, Pentland Hills). A trachyte occurs at the Eildon Hills, near Melrose. It consists essentially of short prisms of alkali-felspar and soda-bearing ferro-magnesian minerals. At one place (Mid-Eildon Hill), the trachyte contains riebeckite. Soda-trachytes of Upper Bala age occur at Hamilton iSC Upper Old Red Sandstone. pjjjjflj Trachyte with ?23J~ Silurian. Dyke Feeders. FIG. 127. Section through the Eildon Hills. (After B. N. Peach.} Hill, north-east of Peebles ; and at Wrae Quarry and Glencotho Quarry, Holmswater, near Broughton. They consist mainly of alkali-felspar (anorthoclase) in small phenocrysts set in a microlitic groundmass. 1 A few trachytic flows have been recorded among the Tertiary volcanic rocks of Skye, as at Fionn- Choire, on the northern side of the Cuillin Hills. 2 Ireland. Trachyte occurs in volcanic vents near the edge of the Limerick volcanic basin. 3 1 Teall: Silurian Rocks, Oeol. Surv. Mem., vol. i., 1899, p. 88. 2 Harker : Tertiary Igneous Rocks of Skye, Mem. OeoL Surv., 1904, p. 88. 3 Watts : Guide, etc., p. 93. 364 DISTRIBUTION IN BRITISH ISLES [PART iv DISTRIBUTION OF DAGITES. Dacites have been recorded by Hill and Bonney in Charnwood Forest at Peldar Tor, High Sharpley and Bardon (Maplewell Series). These lavas are associated with tuffs. They are certainly of pre- Carboniferous, and probably of Archaean age. Flett has described dacites of .Old Eed Sandstone age at Leuchars and Wormit Bay, on the south side of the Firth of Tay, in Eastern Fife. 1 DISTRIBUTION OF ANDESITES. Devon and Cornwall. The Permian volcanic series of Exeter contains, besides the trachytes, some andesites, as at Ide and Kellerton Park. Bristol District. In the neighbourhood of Tort- worth (e.g., at Charfield Green, Damery Bridge, Avening Green, Horsley Quarry, and Daniel's Wood) pyroxene- andesites associated with tuffs, occur on two horizons : the one overlaid by beds of Upper Llandovery age, and the other by beds of Lower Wenlock age. They consist of phenocrysts of labradorite and andesine in a groundmass of felspar laths and needles. The pyroxene is altered to bastite, or serpentine. 2 In the Eastern Mendips there is, north of Shepton Mallet, an interesting inlier of Silurian rocks, with which andesite lavas are associated, as at Moons Hill and Beacon Hill. These rocks contain conspicuous phenocrysts of plagio- clase and bastite pseudomorphs after enstatite. 3 1 The Geology of East Fife, Mem. Geol. Surv. (1902), p. 387. See also Judd : Quart. Journ. Geol. Soc., vol. xlii., 1886, p. 425. 2 Lloyd Morgan and S. H. Reynolds : Quart. Journ. Geol. Soc. t vol. Ivii., 1901, p. 267. 3 Reynolds : Q. J. G. S., 1907, p. 218. CHAP, in] DISTRIBUTION VOLCANIC ROCKS 365 Lake District. The vast accumulation of volcanic material known as the Borrowdale Volcanic Series, FIG. 128. Sketch-map of the Lake District. is mainly made up of andesitic lavas and tuffs. As a rule the lavas are compact, dark-coloured rocks, com- 366 DISTRIBUTION IN BRITISH ISLES [PART iv posed of numerous slender laths of felspar, with occasional large phenocrysts of plagioclase, and a glassy brownish base. The augitic constituent is generally replaced by chlorite. Rocks of this char- acter are well developed to the south of Keswick and in Borrowdale (e.g., Brown Knotts, Bleaberry Fell, Iron Crag, High Rigg); and in Langdale, Westmorland (Lingmoor, Side Pike, Chapel Sfcile). A more por- phyritic type occurs on Great Gable. Andesitic lavas belonging to the Borrowdale Series also occur at Bootle in Cumberland, and at Shap Fell, Westmorland. The Eycott lavas, which occur at Eycott Hill, one mile from Troutbeck station, near Keswick, and in the Cross Fell range, constitute a more basic type, probably more correctly to be classified with the basalts (see p. 374). Some of the members of this series are very porphyritic, containing fine large crystals of a striped plagioclase felspar (bytownite to anorthite). There are crystals of idiomorphic hypersthene altered to bastite. Northumberland. A series of andesitic lava- flows occurs in the Carboniferous Limestone Series of the river Tweed, near Carham Hall. Cheviot District. Andesites (augite- and en- statite-andesite) of Old Red Sandstone age occur in great abundance in the Cheviot Hills (Northumber- land and Roxburghshire). In texture they show considerable variety, being fine-grained or por- phyritic, felsitic or glassy, compact or vesicular. A glassy type with streaky flow-structure occurs at Yetholm. A glassy and perlitic type occurs at Buck- ham's Wall Burn (Northumberland), and at the CHAP, in] DISTRIBUTION VOLCANIC ROCKS 367 Curr, 4| miles east of Morebattle (Roxburghshire). Occasionally the rocks assume a more trachytic character, with development of sanidine instead of a striped felspar (e.g., near Jedburgh). Wales. The Ordovician rocks of Merionethshire and Caernarvonshire consist largely of andesitic lavas and their tuffs, building up a considerable portion of the mountain ranges of Cader Idris, the Arans, Arenig Fawr, Arenig Fach, Manod and Moelwyn. These hills form a semi-circle round the Cambrian rocks of the Harlech dome, extending from Barmouth through Dolgelly to Ffestiniog. In the Lleyn Peninsula Harker has described augite-andesites from Penmaen Castle, 1 mile west of Pwllheli, and Y Foel Fawr, 1J miles west of Llanbedrog. Ordovician andesites occur in the hills of the Shelve district, near the boundary between Shrop- shire and Montgomeryshire. Here lavas of Arenig age form the Stapeley Hills, from Llanfawr to Symonds Castle, attaining their greatest thickness and extent on Todleth. These lavas contain pheno- crysts of plagioclase, augite, and hypersthene in a tine microlitic groundmass. Often the phenocrysts are aggregated in glomero-porphyritic fashion. 1 In the same area, thick sheets of ash of similar com- position are intercalated in the Upper Arenig, and in the Bala rocks. Precisely similar hypersthene- andesites occur amongst the Bala ashes of the Breidden Hills (& in relation t,o the Etive granite complex. The Highland schists are left white. Faults are shown by heavy black lines. (After C lough, Maufe, and Bailey: The Cauldron-subsidence of Glencoe. ) 372 DISTRIBUTION IN BRITISH ISLES [PART iv DISTRIBUTION OF BASALTS. Devon and Cornwall. Vesicular basic lavas associated with breccias and tuffs, occur interbedded with the Devonian rocks of these counties. Thus in Cornwall, well-defined lavas occur at Endellion, St. Minver, St. Clether and near Padstow. In Devonshire the Middle Devonian 1 volcanic series of Ashprington and Totnes, described by Messrs. Champernowne and Ussher, consists of basaltic lavas much chloritized and impregnated with carbonates. They are often weathered to a rusty-red colour, and, together with the associated tuffs, have been fre- quently sheared into schistose rocks (Schalstein). In the lavas the flattened chloritic amygdales can sometimes still be recognized. A typical specimen from Sharpham Barton consists of isolated pheno- crysts of plagioclase set in an altered groundmass of augite, felspar, iron-ore and chlorite. 2 Seoriaceous basic lavas, together with tuffs and breccias, occur at Brent Tor, near Tavistock. They were probably erupted from a volcanic vent, of which Brent Tor itself represents the denuded stump. 8 Olivioie-basalts of Permian age are found in associ- ation with trachytic rocks in the neighbourhood of Ide and Dunchideock, near Exeter. They are much 1 According to Ussher the eruption of basio lavas appears to have commenced in the Lower Devonian and continued to as late as the base of the Upper Devonian. Mem. Geol. Surv., Sheet 850, 1903, p. 9; 2 Hatch : Mem. Geol. Surv. England, Sheet 850, 1903, p. 79, * Rutley : Brent Tor, Mem. Geol. Surv. (1878). CHAP, in] DISTRIBUTION VOLCANIC ROCKS 373 altered, the olivine being replaced by a red decom- position product. A characteristic feature is the presence of corroded grains of quartz. 1 Derbyshire. Contemporaneous flows of basaltic lava occur in the Carboniferous Limestone of this county. These rocks, locally known as " toadstone," 2 are strongly amygdaloidal, and are often much decomposed. They are olivine- basalts, composed of phenocrysts of plagioclase, olivine and augite, in a groundmass of lath-shaped felspars, with inter- spersed granules of augite and magnetite, or some glassy base. A variation of type is produced by the subordination of one or other of the porphy- ritic constituents. The olivine is usually altered to serpentine and s of felspar laths, granular augite, magnetite, and occasionally glassy interstitial matter. Examples : Lion's Haunch, Arthur's Seat, and Calton Hill. (2) Craiglockhart Type. Phenocrysts of olivine 1 Geol. Mag., 1890, p. 447, and ibid., 1893, p. 337. 2 It is difficult in the case of these rocks to separate those that were actually erupted at the surface, from those that are found in necks and pipes representing the vents through which flowed the lavas that reached the surface. Some of the types mentioned here occur also as such intrusions. 3 F. H. Hatch and W. W. Watts : Rep. Geol. Surv. for 1896, p. 64, and The Geology of Central and East Fife. Mem. Geol. Surv., 1900, p. 53. CHAP, m] DISTRIBUTION VOLCANIC ROCKS 377 and augite in a groundmass of felspar laths and granular augite. Examples : Oaiglockhart Hill, Edinburgh, and in the Kilpatrick Hills, north-east of Glasgow. (3) The Dalmeny Type. Abundant phenocrysts of olivine, less of augite and none of felspar, embedded in a mesh of slender lath-shaped felspars, the inter- spaces being filled with microlitic augite and minutely granular magnetite. Examples : Dalmeny ; in the Bathgate Hills of Linlithgowshire (Hillhouse and Kirton West Quarry) ; in Fife, between Burntisland and Kirkcaldy, and along the coast from Pettycur to Kinghorn. (4) Kippie Law Type. Phenocrysts of olivine and felspar in a groundmass of lath-shaped felspars, granular olivine and magnetite, and microlitic augite. Examples: Kippie Law in the Garlton Hills of Haddingtonshire ; and in the Kilsyth Hills. (5) Marlde Hill Type. A macro-porphyritic basalt, consisting of large phenocrysts of labradorite and small sporadic grains of olivine in a groundmass of felspar microlites and magnetite granules. Examples : Garlton Hills (Markle Quarry and Hailes Castle), Arthur's Seat, Edinburgh ; in the Kilsyth Hills, and on the Campsie Fells. (6) Jedburgh Type. 1 A micro-porphyritic basalt, with small phenocrysts of olivine and less of felspar in a groundmass of augite and felspar in sub-ophitic intergrowth. Examples : Near Jedburgh, and on the Campsie Fells. 1 Compare the Gallaston type described on p. 339. 378 DISTRIBUTION IN BRITISH ISLES [PART rv (7) Kilsyth Type. 1 Strongly porphyritic basalt, with large phenocrysts of felspar and smaller ones of olivine ; groundmass sub-ophitic Kilsyth Hills. These basalts are associated in the Garlton Hills with more basic rocks such as picrite and a limburgite or monchiquite type, as well as with more acid lavas trachytes, and phonolites. 2 In the Campsie Fell area, Mr. Bailey 8 of the Geo- logical Survey has recently worked out the following succession for the Carboniferous volcanic rocks north of Kilsyth ; the .horizon upon which rest the volcanic rocks is lower than in the type section of Bal lagan, to the west, or of Fintry, to the north. The Cementstone group is almost unrepresented, and the volcanic beds succeed the Cornstone group of the Upper Old Red Sandstone. The succession also is different. In the Ballagan section it is : (d) Upper group with several basalts of the Markle type and mugearites. (c) Lower group consisting of basalts of the Jedburgh type. (6) Cementstone group. (a) Cornstone group. Whereas, north of Kilsyth, the succession is : (d'} Upper group characterized by basalts of the Markle type. (c') Lower group of basalts of the Jedburgh type. (&') Still lower group characterized by several basalts of the Markle type. (d ) Cornstone group. - 1 Compare the Gailaston type described on p. 339. 2 Hatch : Trans. Roy. Soc. Edin. , vol. xxxvii. , 1892, p. 115. 3 Summary of Progress, Geol. Surv., 1907, p. 93. CHAP, in] DISTRIBUTION VOLCANIC ROCKS 379 FIG. I3T. Map of the Tertiary Igneous Rocks of the Western Isles of Scotland. Scale : I inch ~ 20 miles. 380 DISTRIBUTION IN BRITISH ISLES [PART iv c The Jedburgh group, which extends to the west for a dozen miles, thins away to the east, and the two Markle series come together. It appears that a volcanic centre furnished basalts of the Markle type in the neighbourhood of Kilsyth, at a time when the shales and cementstones of the west were being laid down without volcanic emission. Then were poured forth, from vents between Dumgoyn and Fintry, basalts of the Jedburgh type; and a wide plateau built up, the base of which rests in the west upon the Spout of Ballagan sandstone, while in the east it extends on to the earlier lavas. This complex group was, in turn, covered by fresh lava-flows, chiefly of the Markle type, emitted probably from the old Kilsyth focus of vulcanicity. The Tertiary basalts have a more marked ophitic structure than those of Carboniferous age. They occur abundantly in horizontal beds forming terraced hills in Arran, Mull, Rum, Skye, Raasay, etc. Dykes of the same rocks are abundant in Arran, Mull, Eigg, Skye, and Raasay. In numerous cases the dykes are bordered by a narrow selvage of dark-coloured glass or tachylyte. South- West Scotland. Basic lavas of Arenig age, associated with pyroclastic rocks, occur in the neighbourhood of Ballantrae in the south-west of Ayrshire, and in the Sanquhar district of Nithsdale. They are dark green or purple rocks. There are two common types : the one compact and non-porphyritic, with small spherical amygrlales, the other porphyritic, with large phenocrysts of plagioclase in a dark-green matrix. The green colour is due to the abundant CHAP, in] DISTRIBUTION VOLCANIC ROCKS 381 presence of chlorite, which has no doubt replaced the original augite of the groundmass. There are no phenocrysts of augite or hornblende ; but the tuffs of Bail Hill, near Sanquhar, contain large crystals of these minerals, derived probably from the breaking up of a magma in which these minerals had separated out. 1 " Pillow structure " is developed both in the Ballantrae and in the Sanquhar lavas. Olivine-basalts occur among the Lower Old Red Sandstone andesitic lavas (see page 370) in the Lome area, as at Melfort. 2 Ireland. Basalts of Bala age are associated with andesites in the Kildare inlier (see p. 370) at Grange Hill and the Hill of Allen. They constitute a porphyritic type containing phenocrysts of labra- dorite in a groundmass of felspar needles, augite and magnetite. Some serpentinous matter possibly represents olivine. 8 Carboniferous olivine-basalts occur in the Limerick district of the south- west of Ireland. Watts describes them as consist- ing of granular augite and magnetite set in a more or less devitrified glass, with microlites of felspar and porphyritic plagioclase, serpentinized olivine, and some well-marked augite. 4 The Tertiary basalts of Antrim belong to the same petrographicai province as the Western Isles of 1 Teall : Silurian Rocks of Scotland, Mem. Geol. Surv., vol. i., 1899, p. 84. 2 Summary of Progress, Geol. Surv., 1902, p. 27- 3 Reynolds and Gardiner : Quart. Journ. Geol. $oc., vol. lii. r , 1896, p. 287. 4 Watts : Guide to the Collection of Rocks, etc., Geol. Surv. Ireland, 1895, pp. 93 and 94. 382 DISTRIBUTION IN BRITISH ISLES [PART rv Scotland, and the rocks show no distinctive features. There were two epochs of eruption, both in Eocene times. The upper basalts are separated from the lower by a red lateritic zone derived from the decay of the lower basalts. The volcanic sheets afford in places magnificent examples of columnar structure, as at the famous Giant's Causeway. Isle of Man. Porphyritic olivine-basalts of Carboniferous age form the Stack of Scarlet, on the south coast of the Isle of Man. They are decomposed rocks consisting of phenocrysts of plagioclase in a ground mass of felspar needles, together with chlorite, calcite, and abundant iron-ore. Olivine is represented by chlorite replacements. 1 DISTRIBUTION OF ALKALI-BASALTS. Basic volcanic rocks with a felspathoid constituent have scarcely been recorded in the British Isles ; but Prior has described an analcime-bearing basalt from Rathjordan in County Limerick. This rock, which is probably a leucite-basalt in which the leucite has been changed to analcime, occurs as a lava interbedded with tuffs. 2 DISTRIBUTION OF LIMBURGITES. Limburgites have been described from the Whitelaw Hill, Haddingtonshire 3 (see p. 351); and from the necks or volcanic necks of Eastern Fife, as at Lath- 1 Geology of the Isle of Man, Mem. Geol. Surv., 1900, p. 324; and Hobson : Quart. Journ. Geol. Soc., vol. xlvii., 1891, p. 441. 2 G. T. Prior : Min. Mag., vol. xv. (1910), p. 315. 3 Hatch: Trans. Roy. Soc. Edin. vol. xxxvii., 1892, p. 116. CHAP, in] DISTRIBUTION VOLCANIC ROCKS 383 all an, near Largo, at Elie Ness, and at Dunino, 4 miles south-east of St. Andrews. 1 In Ireland limburgite lavas of Carboniferous age have been described in the Limerick area, as at Nicker Hill ; and Knockseefin. The type consists of phenocrysts of augite and olivine set in a plexus of augite microlites. 2 The silica percentage is 38. Similar rocks have also been described from Philips- town, Queen's County. 3 1 Flett and Seymour: Geology of East Fife, Mem. Geol. Surv., 1902, pp. 399-403. 2 Watts : Guide, etc., Oeol. Surv., Ireland, p. 94. 3 Watts: Guide, etc., Geol. Surv., Ireland, p. 38. APPENDIX TABLE FOR THE DETERMINATION OF THE CHIEF MINERALS OF IGNEOUS ROCKS IN THIN SECTION. BY R. H. RAS TALL AND J. ROMANES. EXPLANATION OF TABLE. Method of Working. Starting with division 1, two alternatives present themselves. Opposite each is a number referring to a subsequent division. Select the alternative suitable to the mineral under observation, and proceed to the division corresponding to the refer- ence number, where other sub-divisions will be found. Kepeat the process until the name of the mineral sought is ultimately reached. The bi-refringence is expressed as " very strong," " strong," " moderate," and " weak " j the following minerals being typical for each division : 1 Tr a . ( Calcite, ( Hypersthene. Very Strong Moderate ^ JF . , ^A\ -{ Zircon. ,. -4 Ouartz. (above -04) | (-007 -015) 1 2 . [ Muscovite. '. ( Orthoclase. Strong \ H 6 ' W& / f P h f ne ' (015-04) **; , (below -007) 1 *** ' [ Hornblende. ' [ Chlorite. 1 See page 80 for an explanation of these expressions. APPENDIX 387 / ' 1 C 2. Colourless Coloured - Reference Number 2. 13. Isotropic - ... 3. Doubly refracting - 4. Refractive index, very high - Garnet. Refractive index, very low; occa- sionally faintly blue; in inter- mediate rocks - Sodalite. Refractive index, very low; in basic rocks - Analcime. Refractive index, very low ; poly- gonal crystals with zonary structure Double refraction, very strong strong - moderate weak - Nosean. 5. 6. 7. 11. No cleavage ; prismatic form ; re- fractive index, very high ; crystals usually small - - Zircon. One cleavage; refractive index, moderate - Muscovite. Fine cleavage lines intersecting at an acute angle; refractive index, variable, according to position of section - - Oalcite. Usually in rounded grains; no cleavage ; conspicuous irregular cracks ; refractive index, high Olivine. Extinction oblique, maximum 38 Diopside. 15 Tremolite. 388 Divisions. 7, APPENDIX Refractive index, high low Reference Number. - Enstatite. 8. 9. 10, 11. 12. Mesh or bladed structure - - Serpentine. Cross-hatched twinning - - Microcline. Lamellar twinning - 9. , confined to lenticular patches - - Perthite. Carlsbad twin or none - - Orthoclase. Water clear, no twins * * Quartz. Maximum extinction angle 6-20 10. less than 6 Oligoclase. 20-35 Labradorite. over 35 Anorthite Refractive index lower than that of quartz - Albite. Refractive index higher than that of quartz - Andesine. Refractive index high ; hexagonal prisms ; transverse cracks ; ex- tinction straight Apatite. Refractive index high ; lath-shaped sections, with striations or peg- structure - Melilite. Refractive index low - 12. No twins; often lines of bright inclusions Nepheline. Rounded crystals, often with com- plex twinning- - - Leucite. APPENDIX 389 Divisions. 13. 15. 16. 17. 18. {Opaque Transparent Reference Number. 14. 15. I Black, silvery by reflected light - < I Yellow by reflected light - - -1 Magnetite. Ilmenite. Iron Pyrites. Isotropic; brown ; pink - ; green ; traces of pale blue Doubly refracting Double refraction very strong strong moderate M weak; colour very pale green Melanite Garnet. Pyrope Garnet ( Pleonaste ( Spinel. Haiiyne. 16. 17. 18. 19. Chlorite. Brown ; pleochroism weak ; small diamond-shaped crystals ; ex- tinction straight Sphene. Brown or green ; pleochroism strong; one cleavage; extinc- tion straight Biotite. Yellow; weak pleochroism; ex- tinction straight - Epidote. No cleavage; colour blue or brown; pleochroic; extinction straight Tourmaline. Two cleavages at 124; pleochroic 20. 87 21. One cleavage only visible; re- fractive index high - - 22. 390 APPENDIX Divisions. Reference Number. Refractive index high ; pleochro- ism slight, in green and pink ; 19. <| extinction straight - - Hypersthene. Refractive index low; colour very _ pale green - - Serpentine. {Brown or green - - Hornblende. Light blue - - Arfvedsonite. Deep blue - - Riebeckite. Pale green, pale brown or purplish brown - Augite. Emerald green ; slightly pleo- chroic --.",-. - JEgirine. Maximum extinction angle not more than 24; brown, green or blue ; pleochroic - 23. Maximum extinction angle 45; 22. { pale brown or green ; not pleo- chroic - - Augite. Maximum extinction angle 5; emerald green - ^girine. Extinction straight '''." - Hypersthene. Maximum extinction angle 20 to 24; brown or green- -" Hornblende. 1 Maximum extinction angle 14; pale blue Arfvedsonite. Maximum extinction angle 6; deep blue - - - * Riebeckite. INDEX Abercastle, 220, 324 Abereiddy Bay, 359 Aberlour, 171 Abriachan, 171 ABSAROKITE, 249, 260 Accessory minerals, 67 Acid rocks, 51, 152 Acklington dyke, 323, 331 ACMITE, 117 ACTINOLITE, 107 ADAMELLITE, 166 Adamellites, analyses of, 171 Aden, 158 ADINOLE, 225 ADTJLARIA, 88 ^EGIRINE, 117 ^EGIBINE-AUGITE, 113 ^EGIKINE-SYENITE, 178 Ailsa Craig, 109, 318 Aker, 177 AKERITE, 177, 178 ALASKITE, 166 ALBITE, 95 Albite type of twinning, 95 Alderney, 305 Alferric minerals, 56 Alkali series, 152 ALKALI-BASALT, 272 Alkali-basalts, distribution of, 382 ALKALI-FELSPARS, 86 ALKALI-GABBROS, 188, 200 Alkali-gabbros, distribution o/,305 Alkali-granites, analyses of, 167 ALLAJSTITE, 129 Allen, Hill of, 381 Allival, 203, 205, 207 ALLIVALITE, 203, 205, 207 Allotriomorphic crystals, 44 Allotriomorphic granular struc- ture, 45 Allt an Sithein, 305 Allt Fionnfhuachd, 269 Allt Llwyd, 316 Allt na Lairige, 171 391 Allt-a'-Mhuilinn, 306 ALNOITE, 238 Alumina in rocks, 52 Am Meallan, 200 AMBLYSTEGITE, 111 AMESITE, 141 AMETHYST, 82 Amiata, Monte, 260 Amlwch, 144 AMPHIBOLE, 104 AMPHIBOLE-TRACHYTE, 257 Amygdale, 27 Amygdaloidal structure, 27 ANALCIME, 140 ANALCIME-DOLERITE, 226 ANATASE, 137 ANDALTJSITE, 145 ANDESINE, 95 ANDESITE, 261 Andesites, analyses of, 265 Andesites, distribution of, 364 Anglesey, 109, 242, 355 Annestown, 217, 320 ANORTHITE, 95 ANORTHITE-ROCK, 203 ANORTHOCLASE, 93 ANTHOPHYLLITE, 109 ANTIGORITE, 143 ANTIGORITE-SERPENTINE, 245 Antrim, 342, 381 APATITE, 129 APLITE, 227 _ Aplites, distribution of, 342 APOPHYLLITE, 140 Apophysis, 13 ARAGONITE, 139 Arans, the, 367 Ardmuchnish, 237 Ardnamurchan, 310, 319, 340 Ardshiel Hill, 302 Arenig Fach, 367 Arenig Fawr, 367 ARFVEDSONITE, 108 ARFVEDSONITE-SYENITE, 178 392 INDEX ARKITE, 194 Arklow, 305 Arklow Head, 341 ARKOSE, 89 Armboth dyke, 314 Arran, 12, 28, 33, 216, 295, 296, 319, 327, 349, 380 Arthur's Seat, 327, 376, 377 ASBESTOS, 108 Ascension Id., 158, 260 Ash-beds, volcanic, 21 Ashprington, 372 Ashton, 329 Ashy Limestones, 21 Ashy Shales, 21 Assynt, 220, 231, 242, 348 Asterism, 79 Atherstone, 329 Atmospheric weathering, 67 AUGEN-GNEISS, 162 Aughrim, 296, 341 AUGITE, 113 AUGITE-DIORITE, 188 AUGITE-FOYAITE, 179 AUGITE-LAMPROPHYRE, 234 AUGITE-PORPHYRITE, 220 AUGITITE, 276 Aultnacallagach, 306 Automorphic, 44 Auvergne, 19, 258 Avening Green, 364 AXINITE, 148 Azores, 260 Backside Beck, 347 Badcall, 354 Bail Hill, 381 Balhamie Hill, 355 Ballachulish, 294, 302 Ball-and-socket joints, 27 Ballantrae, 355, 380 Ballybrood, 382 Ballydouane, 320, 326 Ballymena, 360 Ballywater, 350 BANATITE, 184 Bangley, 362 Bangor, 357 Bardon, 364 BARKEVIKITE, 108 Barnavave, 63, 311 Barnesbeg, 299 Barnesmore, 299, 319 Barnton, 352 Barrow Hill, 330 Bartestree, 328, 333, 383 BASALT, 266 Basalt, distribution of, 372 BASANITE, 272 Basic rocks, 51, 152 Bass Rock, 14, 257, 363 BASTITE, 113 Bathgate, 227 Bathgate Hills, 242, 377 Bathylith, 9 BAUXITE, 66 Baveno twinning, 87 Bawdon Castle, 323 Bayston Hill, 330 Beacon Hill, 364 Bee du Nez, 350 Bedworth, 329 Beinn a'Chairn, 217 Beinn a'Chapuill, 354 Beinn a'Chuallaich. 325 Beinn Buidlie, 295, 305 Beinn Dubhaich, 295 Beinn-na-Caillich, 319 Beinn-na-Cro, 319 Belau Bank, 384 Belhelvie,311 Belonites, 32 Belovely Beacon, 281 Ben Avon, 292 Ben Bhrackie, 310 Ben Cruachan, 289, 294, 301, 302, 318, 325 Ben Hiant, 319 Ben Lawers, 310 Ben Loyal, 294 Ben Nevis, 294, 302 Ben Rinnes, 293 Ben Vuroch, 292 Ben-a-Bruidh, 302 Bennan Hill, 318 Berkum, 61 Bettws-y-Coed, 357 Bielitz, 240 Biella, 174 BIOTITE, 122 BlOTITE-ANDESITE, 262 BlOTITE-FOYAITE, 179 Birefringence, maximum of, 80 Black Combe, 314 Black Law, 339 Blackburn, 227, 242, 352 Blackford Hills, 265 Blackie Heugh, 277 INDEX 393 Blacksod Bay, 299 Bleaberry Fell, 366 Bodmin Moor, 279 Boguschowitz, 227 BOLE, 66 Bologh Lower, 304 Bolsena, 261 Bombs, 21 Bondicar, 332 Bonsai, 331 Bootle, 283, 366 BOROLANITE, 192, 193, 200 Borrowdale, 62 Borrowdale series, 365 Boss, 9 Boston, U.S., 159 BOSTONITE, 229 Bostonites, distribution of, 344 Botallack, 167 Botter Rock, 329 Boulay Bay, 356, 361 Bowden Hill, 339 Bracciano, 260 Bradda Head, 317 Bradgate, 321 Braid Hills, 259, 368 Brandberget, 240 Brannie Burn, 305, 349 Bratholmen, 182 Brazil Wood, 301 Breccia, volcanic, 21 Breidden Hills, 367 Brent Tor, 372 Bridge of Dee, 348 Brittas Bridge, 252 Broadford, 308 Brock Hill, 334 Brocken Hill, 159 Brodick, 319 BBONZITE, 111 Broughton, 314 Brown Knotts, 366 Brown Willy, 279 Bryn Croes, 284 Bubbles in minerals, 77 Buchaille Etive Mor, 370 Buckham's Wall Burn, 366 Builth, 368 Bxmmahon, 320, 326 Burntisland, 338, 377 Burtness Combe, 332 Bute, Island of, 368 Buttermere, 312 Bwlch Cywion, 316 Bwlch-yr-Henclref, 335 Bwlchau-yr-Figen, 335 Bysmaliih, 17 BYTOWNITE, 95 Cader Idris, 316, 367 Caer-Caradoc, 356 Caernarvon, 299 Cairngorm, 82, 167, 292 Cairnshee, 292 Cairnsmore of Carsphairn, 302 Cairnsmore of Fleet, 286, 343 Caisteal Abhail, 319 Gale-alkali series, 152 CALCITE, 318 Calton Hill, 374, 376 Camas an Fhais, 351 Campsie Hills, 368, 377, 378 Campton Falls, 234 CAMPTONITE, 232, 234, 237 Canary Islands, 158 CANCBINITE, 99 CANCBINITE-SYENITE, 180 Canisp, 325 Canner, 224 Capel Curig, 357, 359 CARBONATES, 138 Carcraig, 328 Caregllefain, 335 Carham Hall, 366 Carinthia, 189 Carlingford, 298, 311 Carlsbad twinning, 87 Cam Boduan, 324 Cam Brea, 280 Cam Cas nan Gabhar, 354 Cam Dearg, ,349 Cam Ffold, 323 Cam Hen, 323 Cam Llidi, 323, 342 Cam Marth, 280 Cam Menelez, 279, 342 Cam Perfedd, 323 Cam Trelwyd, 323 Carnedd Givian, 323 Carnedd Llewellyn, 357 Carnedd Sleithe, 323 Carneddau range, 333, 368 Carrigmore, 304 Carrock Fell, 63, 306, 323 Carsphairn, 318 Castle Douglas, 318 Castle Head, 332 Castleton, 374 394 INDEX Cauldron Snout, 224, 331 Central Highlands, map, 290 Ceres, 338 Cerveteri, 260 CHALCEDONY, 84 CHALYBITE, 139 Chaos, 265 Chapel Stile, 366 Charneld Green, 364 Charnwood Forest, 186, 219, 301, 364 Charnwood Forest, map, 322 Chatoyancy, 79 Cheese Grotto, 27 Cheese wring, 146 Chester's Quarry, 351 Cheviots, 20, 300, 366, 375 CHIASTOLITE, 146 Chilvers Coton, 329 CHIOR- APATITE, 129 CHLORITE, 141 CHLORITOID, 143 Christiania District, 10, 15, 135, 160, 178, 328 Christiania District, map, 176 Christen, 329 CHROME-DIOPSIDE, 116 CHROMITE, 133 CHRYSOTILE, 108, 143 Cimini, Monte, 261 CIMINITE, 260 Cinders, 21 CiPOLLnsro, 108 Cleavage of minerals, 75 Clee Hill, 224, 32P, 334 Cleish Hills, 338 Cleveland dyke, 323, 331 Clicker Tor, 135 Clifden, 319, 355 Cliff Hill, 321 CLINKSTONE, 256 CLINOCHLORE, 141 Clogher Head, 349, 360 Clogwyn-dur-Arddu, 359 Clondermot, 350 Cnoc an Droighinn, 231, 345 Cnoc-na-Sroine, 193, 300, 306 COCCOLITE, 116 Coigach, 306 Coley Hill, 332 Colmonell, 308, 355 Colour in thin section, 69 Columnar structure, 25 Colvend, 348 Complementary rocks, 20 Coniston Limestone, 164 Connemara, 354, 355 Contact metamorphism, 61 Contact minerals, 71, 145 Contour of minerals, 73 Contrary Head, 348 Conwy Falls, 359 Conwy Mt., 357 Coomb, 359, 368 Coomb volcanic series, 359 Copper Mine Valley, 357 Copplestone, 362 CORDIERITE, 147 Core, granite, 10 Core, mountain, 10 Corndon, 333 Cornwall, map, 280 Corriegills, 217, 319 Corrosion of crystals, 39 Corrosion, zones of, 74 Corsica, 188 Corston Hill, 327 Corstorphine Hill, 338, 339, 352 CORTLANDTITE, 243 CORUNDOPHILITE, 141 CORUNDUM, 149 Corvockbrack, 299 COSSYRITE, 109 Cottonshope Burn, 375 Coverack, 207, 306 COVITE, 192, 200 COUSERANITE, 124 Cowal, 231, 325, 344, 349 Coyles, 355 Cracknowl, 374 Craig-dol-Owen, 316 Craigendaive, 345 Craigie Hill, 338, 362 Craigleith Island, 227, 328 Craiglockhart, 376 Craigneuck Point, 348 Craig-y-fael, 307 Crakelow, 374 Cramond, 338 Crazy Mts., 190 Creag Tombaca, 348 Creetown, 288 Criffel, 286, 302, 326, 342, 348 Criggon, 333 CROCIDOLITE, 109 Crocombe, 329 Croft, 321 Croft Hill, 186 INDEX 395 Croft Michel, 312 Croghan Kinshela, 167, 296 Crosby, 317 Cross Fell, 307 Cross Haw Beck, 34,7 Crowdundle Beck, 347 Cruach Mhor, 345 Cryptocrystalline, 44 CRYPTOPERTHITE, 93 Crystalline structure, 44 Crystallites, 31 Crystallization, intra-telluric, 40 Crystallization, order of, 35 Cucco, Monte, 261 Cuillin Hills, 49, 202, 207, 308, 319, 360, 363 Culzean, 368 Cuns Fell, 307 Curr, the, 367 Cwmmawr, 351 DACITE, 264 Dacites, distribution of, 364 Dalbeattie, 318 Dalmahoy Hill, 339 Dalmeny, 328, 377 .Damery Bridge, 364 Damhain, 231 Dandaleith, 189, 304 Daniel's Wood, 364 Dartmoor, 279, 329, 342 Dasses, the, 338 DATOLITE, 149 Davidstow, 329 Ddualt, 316 Deccan traps, 20, 270 Dee, River, 310, 349 Derreennagusfoor, 355 DESMOISITE, 225 Devlin's Bridge, 349 Dhoon, 284, 317 DIABASE, 221 Diablo, Mt., 239 DIALLAGE, 115 DlALLAGITE, 238 Digoed, 359 Dillon, 350 DlOPSIDE, 116 DIOIUTE, 186 DlORITE-APLITE, 228 Diorites, analyses, 189 Diorites, distribution of, 301 DIPYRE, 124 Dirleton Craig, 362 Ditro, 180, 182 DITROITE, 179, 180 Docker Garth, 347 DOLERITE, 221 Dolerites, -analyses of, 224 Dolerites, distribution of, 328 DOLOMITE, 139 Dolwyddelen, 357 Dome, 9 DOMITE, 158 Double refraction, 79 Downpatrick, 350 Drakensberg, 27 Druim na Criche, 227, 327 Drusy structure, 28 Drynoch, 269 Duchray Hill, 292 Dufton Pike, 314 Dum Mor, 345 Dunans, 231, 345 Dunchideock, 372 Dungeon Ghyll, 356 Dunian, 339 Dunino, 382 DUNITE, 243 Dunnet Head, 351 Dunstanburgh Head, 331 Dwygyfylchi, 357 Dyke, 11 Dyke, composite, 12 Dyke, multiple, 12 Dynamic metamorphism, 64 Ealaist Canner, 224 Earls Shilton, 321 ECLOGITE, 239 Eden Side, 233 Edinburgh neighbourhood, map, 339 Eigg, 215, 319, 327, 340, 380 Eildon Hills, 109, 363 Eilean Balnagowan, 301 ELJEOLITE, 99 Elie, 27, 382 EL VAN, 213 Ember Lane, 374 Enard Bay, 341 Endellion, 329, 372 Enderby, 321 Ennerdale granophyre, map, 313 ENSTATITE, 111 ENSTATITE-DIORITE, 188 ENSTATITE-PICRITE, 240 EPIDIORITE, 205 396 INDEX EPIDOTE, 128 Esgair-felen, 359 Eskdale, 283 Eskdale Muir dyke, 323 ESMARKITE, 148 Essential minerals, 67 Essex County, 328 ESSEXITE, 197, 200 EUCOLITE, 135 EUCKITE, 203, 207 EUDIALYTE, 135 EUDIALYTE-SYENITE, 180 Euganean Hills, 258 EULYSITE, 243 EUPHOTIDE, 201 EURITE, 228 Eutectic ratios, 37 Exeter, 361, 364 Extrusive rocks, 8 Eycott Hill, 366, 375 Eyes, 65 Fabric, 46 Fair Head, 342 Falmouth, 279 FAYALITE, 118 FELSITE, 214, 251 FELSITIC PITCHSTONES, 218 Felsitic structure, 213 FELSOPHYRE, 214 FELSPAR-PORPHYRY, 218 FELSPARS, ALKALI, 86 FELSPARS, MONOCLJNIC, 85 FELSPARS, TRICLINIC, 85 Felspathoid series, 154 FELSPATHOIDS, 98 Femic Minerals, 56 FIBROLITE, 146 Findhorn, 293, 304 Finkenberg, 64 Fionn-Choire, 363 Fishguard, 252, 337, 359 Fissure eruption, 21 Flint Rock, 326 Flockenbach, 237 Flow structure, 25, 27, 32 FLUOR- APATITE, 129 FLUORSPAR, 131 Fluxion structure, 32 Fontana di Fiesoli, 261 Forfar, 292, 339, 355 Forkill, 299 Form of minerals, 73 FORSTERITE, 118 Fort Touraille, 305 Fourche Mts., 175, 178 Foxdale, 284, 317 Foya Hills, 179 FOYAITE, 179, 182 Fumaroles, 58 GABBRO, 201 Gabbros, analyses of, 207 Gabbros, distribution of. 306 Gallaston, 339 Galloway district, map, 287 Galway, 299 Garabal Hill, 159, 295, 302, 318, 354, 355 Garabal, Loch, 231, 244 Garlton Hills, 255, 259. 268, 269, 277, 362, 377, 378 GARNETS, 124 Garrams, 320, 342 Garry, 325 Gas-inclusions in minerals, 76 GASTALDITE, 109 GEYSERITE, 84 Giant's Causeway, 26, 382 GlESECKITE, 99 GlGANTOLITE, 148 Gimlet Rock, 335 Glamaig, 319 Glamis, 339 Glan-y-mor, 299 Glas Beinn Bheag, 319 Glaslyn, 357 Glass -inclusions in minerals, 78 Glassy base, 40 GLAUCOPHANE, 108 Gleann Leac-na-Muidhe. 305 Glen Catacol, 349 Glen Creran, 305 Glen Dhu, 348 Glen Doll, 292, 304 Glen Etive, 371 Glen Ey, 318 Glen Fernate, 310 Glen Finnan, 343 Glen Fyne, 294, 301 Glen Fyne granite, map, 303 Glen Luce, 302 Glen More, 319 Glen Muick, 292 Glen Shee, 304 Glen Shira, 349 Glen Tilt, 292, 293, 302 Glenboig, 338 INDEX 397 Glencoe, 370 Glencoe volcanic series, 359, 360, 370 Glencotho Quarry, 363 Glendalough, 355 Glenties, 299 Glenwharfen Burn, 348 Globulites, 32 Glomero-porphyritic structure, 48 Glyder-fawr, 359 Gneisses, foliated, 65 Gneissic structure, 49 Goatfell, 296 Goblin Combe, 374 Godolphin Hills, 280 Goodwick, 253 Gordon's Butte, 190, 200 Grainbank, 237 Grange Hill, 381 Grangemill, 374 GRANITE, 160, 164 GRANITE, GNEISSOSE, 162 Granite, metamorphism round, 163 GRANITE, PORPHYRITIC, 161 GRANITE, SPHEROIDAL, 162 GRANITE-APLITE, 228 GRANITE -PORPHYRY, 212 Granites, distribution of, 279 Granitic structure, 45 GRANODIORITE, 169 Granodiorites, analyses of, 171 GRANOPHYRE, 214, 217 GRANOPHYRE, DIORITIC, 220 Granophyric structure, 46 Granulation, 64 Granulitic structure, 206 Graphic structure, 46 Great Cockup, 352 Great Gable, 357, 366 Great Newtown Head, 217, 320, 341 Great Yarlside, 357 Greenhill, 348 Greenland, 181 GREENSTONE, 221 Greenup Gill, 332 GREISEN, 58 Grenofen, 312 Greystones, 355 Griff Farm, 329 Griff Hollow, 330 Grizedale Tarn, 357 Groby, 321 GRORUDITE, 213 Grosmont, 331 Groundmass, 40 Guernsey, 305, 350 Giimbelberg, 242 Gunson Knott, 357 Gurnards Head, 329 Gweebara, 299 Gwynant, 357 Hailes Castle, 269, 377 Hamilton Hill, 259, 363 Hammer Cliff, 323 Hanter Hill, 308, 333 Hardness, scale of, 81 Harlyn, 329 Hartley, 332 Hartshill, 329 Harz, 62, 159 HARZBURGITE, 243 HAUYNE, 103 Hawes Water, 314 Haygarth, 347 Heathen structure, 28, 1 62 Hebburn, 332 Hedgehope, 300 Hedrum, 231 Heidelberg, 275 Helm Gill, 347 HELMINTH, 141 Helvellyn, 332 Henry Mts., 15, 258 HERCYNITE, 133 Heriot Mount, 338 Hestercomb, 301 Heterogenetic rocks, 63 Hett, 332 HEULANDITE, 140 High Green, 332 High Fell, 314 High Peak Tavern, 331 High Rigg, 366 High Sharpley, 364 Highwood Peak, 275 Hill of Fare, 293 Hillhouse, 377 Kingston Down, 281 Holbeck Gill, 347 Holmswater, 363 Holocrystalline, 30, 40 Holywell Bay, 347 Hopetoun Monument, 362 Hopton, 374 HORNBLENDE, 106 HORNBLENDE-ANDESITES, 262 398 INDEX HORNBLENDE-DIORITE, 188 HORNBLENDE-DOLERITE, 221 HORNBLENDE-FOYAITE, 179 HORNBLENDE-LAMPROPHYRE, 234 HORNBLENDE-PERIDOTITE, 244 HOBNBLENDE-PICBITE, 241 HOBNBLENDE-POBPHYKITE, 220 HORNBLENDE-PYROXENE ROCK, 239 Horsley Quarry, 364 Hougnatten, 167 Hoxa, 237 HUMBOLDTILITE, 102 Hungry Law, 375 HYALITE, 84 Hybrid rock, 63 HYDRO-NEPHELINITE, 99 Hydrothermal metamorphism, 60 Hypdbyssal rocks, 8 Hypabyssal rocks, distribution of, 312 HYPERSTHENE, 111 HYPERSTHENE-DIORITE, 188 HYPERSTHENE-DOLERITE, 221 HYPERSTHENITE, 238 Hypocrystalline rocks, 30 Ible, 331 Iceland, 19, 21 Ide, 364, 372 Idiomorphic Crystals, 44 IDOCRASE, 148 Igneous rocks, 2, 5 IJOLITE, 194 Illgill Head, 356 ILMENITE, 132 Inchcolm, 328, 352 Inchnadamff, 231, 236, 324 Inclusions in minerals, 76 Indian Valley, 171 Inge, Mount, 275 Ingleton, 347 Intermediate rocks, 152 Intrusive rocks, 8 Ion, 39 Iron Crag, 366 IRON ORES, 131 Iron oxides in rocks, 52 Isle of Man, map, 285 JAOUPIRANGITE, 194 Jedburgh, 367, 377 Jersey, 299, 300, 305, 350, 356, 361 Joints, 23 Kae Heughs, 259, 362 Kaiserstuhl, 275 Kangerdluarsuk, 135 Kaolinisation, 59, 88 KAOLINITE, 144 Karackclews, 306 KATAPHORITE, 108 KATAPLEIITE-SYENITE, 180 Kellerton Park, 362, 364 Kendal Road, 347 Kentallen Quarry, 305 KENTALLENITE, 198, 200 KERATOPHYRE, 214 KERSANTITE, 232, 237 Kildare, 370 Killdown, 306 Killiney, 296, 343 Kilmartin, 340 Kilpatrick Hills, 367, 377 Kilpatrick House, 305 Kilsyth Hills, 377, 378 KlMBERLITE, 64 Kincardineshire, 289, 292 Kinghorn, 377 Kinlet, 334 Kippie Law, 269, 377 Kirk Fell, 314 Kirkcowan, 302 Kirton, 377 Kit Hill, 281 Kniveton Cottage, 374 Knockmahon, 217, 320, 326 Knocknairling Hill, 344 Knockseefin, 382 Knots, lenticular, 65 Knowle, 362 Kola Peninsula, 135, 159, 177, 178, 180 KUGELPORPHYRE. 216 KULAITE, 276 KYANITE, 146 Labile solution, 42 LABRADORITE, 95 Laccolith, 14 Laccoliths, cedar-tree, 15 Laig, 319 Lairg, 294 Lake District, map, 365 Lake Laach, 99 Lambay Island, 326, 370 Lambstone porphyry, 320 INDEX 399 Lamlash, 225, 327 Lamorna, 167 LAMPROPHYRE, 232 Lamprophyres, distribution of, 346 Land's End, 59, 62, 64, 279 Lands berg, 171 Langness, 347 Lanner, 342 Lapilli, 21 Lassen's Peak, 265 LATERITE, 66 LAUMONTITE, 140 Laurdal, 180 LAURDALITE, 180, 182 LATJBVIKITE, 177, 178 Lava-beds, contemporaneous, 19 Lava- flows, 18 Laxford, 344 Lea Rock, 253, 356 Learn, 299 Ledbeg, 300 Lendalfoot, 308, 355 Lennoxtown, 328 Leonhardskopf, 237 LEPIDOLITE, 122 LEPIDOMELANE, 123 LEPTOCHLORITE, 141 LESTIWARITE, 228 Lethan Hill, 353 Lettery, 350 Leuchars, 364 LETJCITE, 100 LETJCITE-BASALT, 272 LEUCITE-PHONOLJTE, 255 LEUCITE-SYENITE, 181 LEUCITE-TEPHRITE, 273 LEUCITITE, 275 LEUCITOPHYRE, 255 Leucocratic rocks, 56, 209 LEUCOPHYRE, 221 LEUCOXENE, 132, 136 LHERZOLITE, 243 Lickey, the, 330 Limburg, 277 LIMBURGITE, 276 Limburgites, distribution of, 382 Lime in rocks, 52 Limerick, 363, 369, 381, 382 Lime-soda felspars, 94 LINDOITE, 230 Lingmoor, 366 Linhope Burn, 300 Lintrathen, 318 Lion's Haunch, 376 LIPARITE, 249 Liquid - inclusions in minerals, 77 Lissoughter, 355 Litchfield, 182 LlTCHFIELDITE, 179 LlTHOMARGE, 66 Lithophysce, 252 Lit-par-lit intrusion, 18, 19 Little Falls, 189 Little Island, 341 Little Knott, 352 Lizard, 306, 355 Llanalhaiarn, 316 Llanbedrog, 316, 357, 367 Llanberis Pass, 359 Llaneillian Mountain, 352 Llanerchymedd, 352 Llanfaglen, 299 Llanfawr, 370 Llangynog, 320, 359, 368 Llanrian series, 360 Llanwnda, 337 LlwdMawr, 316, 357 Llyn Lledr, 357 Llyn Padarn, 357 Loch Awe, 318, 340 Loch Ba', 319 Loch Borolan, 159, 192, 306 Loch Dee, 286, 302, 308, 318, 326, 342, 348 Loch Ken, 318 Loch Laxford, 341 Loch Maree, 341 Loch Melfort, 303 Lock Moy, 293 Loch Scye, 354 Loch Tay, 311 Lochend, 328 Lochinver, 240 Lochnagar, 293, 302 Lock Pit Hill, 362 Long Sleddale, 357 Longmynd, 330, 375 Longstrath, 332 Lome, 325, 370, 381 Lough Guitane, 360 Low Farm, 331 Lowenburg, 198 Lower Wych Quarry, 330 Lvinde im Lougenthal, 182 Lundy Island, 28J Lunga, 340 400 INDEX Lustre-mottling, 47, 241 LUXULLYANITE, 128 Madeira, 260, 277 Madeiral, 277 Magellan, Straits of, 170 MAGMA-BASALT, 275 Magnesia in rocks, 52 Magnet Cove, 135, 181, 192, 200 MAGNETITE, 131 MALACOLITE, 116 MALCHITE, 228 Malutiberg, 27 Malvern Hills, 281, 330, 333 Manebach twinning, 87 Manod, 367 Margarite, 122 MARIALITE, 124 Mariposa Co., 240 MARIUPOLITE, 180 Markfield, 219, 323 MARKFIELDITE, 219, 323 Markle Quarry, 268, 269, 37T MARSCOITE, 64 Marston Jabet, 329 Massif, 10 Matlock, 373 Maughold Head, 317, 337 Maum Hotel, 355 Meall Breac, 295, 302 Meeting of the Three Waters, 128 Meillionydd, 284 MEIONITE, 124 Meissen, 62 Melanocratic rocks, 56, 209 Meldon, 342 Melfort, 381 MELILITE, 102 MELILITE-BASALT, 272 Menai, 335 Mendips, 374 Metals in igneous rocks, 54 Metamorphic rocks, 2 Metamorphism, 60 Metasomatic changes, 57 Metastable solution, 42 Meteoric weathering, 65 Miarolitic structure^ 28 Miask, 54, 180 MICA, BLACK, 122 MICA, WHITE, 121 MicA-moRiTE, 188 MlCA-DOLERITE, 221 MlCA-LAMPROPHYRE, 232 MICA-TRACHYTE, 257 MICROCLINE, 90 Micro-crystalline, 44 MlCROGRANITE, 212 Micrographic structure, 46 Microlites, 32, 33 Micropegmatite, 43, 46 MlCROPERTHITE, 92 Microscope, minerals under, 67 Micro-structure of minerals, 75 Middle Hope, 374 Midland Valley, Scotland, map, 369 Miller's Dale, 373 Mineral-inclusions, 78 Miner alisers, 54 Minerals, formation of, 35 MINETTE, 232, 237 MISSOURITE, 194 MIZZONITE, 124 Moel Hebog, 316, 357 Moel Perfedd, 316 Moel Siabod, 357 Moelwyn, 367 Moel-y-Golfa, 367 Mohs 1 scale of Hardness, 81 MONAZITE, 130 MONCHIQUITE, 237 Monchiquites, distribution of, 350 Mondhalde, 275 Monksdale, 374 Monmouth, Ontario, 181 MONMOUTHITE, 181 Monte Amiata, 260 Monte Cimini, 261 Monte Cucco, 261 Monte St. Elmo, 252 Monte Somma, 99 Monteregian Hills, 198 MONTI CELLITE, 118 Montville, 247 Monzoni, 183, 186 MONZONITE, 183 MONZONITE-PORPHYRY, 218 Monzonites, analyses of, 186 Monzonites, distribution of, 300 Moon's Hill Quarry, 364 Moor of Rannoch, 294 Morozewicz, 180 Morpeth, 332 Morton Hall, 259, 363 Moulin Huet, 350 Mount Diablo, 239 Mount Inge, 275 INDEX 401 Mount Leinster, 163, 296, 320, 343 Mount Leinster, map, 297 Mount Shasta, 265 Mount Sorrel, 281 Mourne Mts., 28, 296 Mourne Mts.,. map, 298 Muck, 327, 340 MUGEARITE, 226 Mugearites, distribution of, 327 Mugeary, 327 Mull, 296, 310, 319, 340, 380 Mull of Galloway, 288 Mull of Kintyre, 368 Mullaghderg, 163, 299 Muncaster, 283 Mural jointing, 23 MUSCOVITE, 121 MYLONITE, 65 Mynydd Cefn Amlwch, 284 Mynydd Mawr, 109, 316 Mynydd Nevin, 316 Mynydd Penarfynydd; 335, 352 Mynydd- tir-y-cwmmwd, 316 Mynydd-y-gader, 335, 359 Mynydd-y-graig, 335 Myrmekite structure, 46 Nant Gwrtheyrn, 316 NAPOLEONITE, 188 Narborough, 321 National Bell Mine, 144 NATROLITE, 140 NATRON-GRANITE, 167 Navendale, 237 Necks, 14 NEPHELINE, 98 NEPHELTNE-BASALT, 272 NEPHELINE-GABBROS, 190 NEPHELINE-PORPHYRY, 218 NEPHELINE-SYENITE, 179 Nepheline- syenites, analyses of, 182 NEPHELINITE, 275 NEPHRITE, 107 Nernst's law, 39 Nether ly, 189, 304 Neubrun, 237 Neutitschein, 240 Nevada City, 171 NEVADITE, 249 New Bridge, 331 New Cumnock, 288 Newlyu, 220 2 C Newry, 298 Nicker Hill, 382 NORDMARKITE, 176, 178 NORITE, 203 North Berwick, 227 North Berwick Law, 14, 363 North Queensferry, 339 NOSEAN, 103 Nottero, 178 Nuneaton, 301, 329 Oatland, 286 OBSIDIAN, 250 Ochils, 368 OCHRE, 66 Odenwald, 237 OLIGOCLASE, 95 OLIGOCLASE-PORPHYRITE, 220 OLIVINE, 118 OLIVINE -BASALT, 267, 271 OLIVINE-DOLERITE, 221 OLIVINE-GABBRO, 203, 267 OLIVINE-NORITE, 203 Omey Id., 299 OMPHACITE, 116 OPAL, 83 OPHICALCITE, 246 Ophitic structure, 47 Orbicular structure, 48 Ord of Caithness, 294 Oriel Demesne, 349 Original minerals, 67 Orkney Islands, 231, 237, 345, 350 ORTHITE, 129 ORTHOCHLORITE, 141 ORTHOCLASE, 86 ORTHOPHYRE, 218 Orval, 296, 319 Orvieto, 261 OTTRELITE, 143 Overlay Hill, 356 Ox Mts., 299 Padstow, 371 PAL^OIIKRITE, 241 PALAGONITE, 65, 271 Pan-idiomorphic granular, 46 Pantelleria, 54, 109, 158 PANTELLERITE, 109, 251, 256 PARAGONITE, 122 Pa^s of Brandar, 340 Peak Forest, 331 Peel, 347 402 INDEX Peg structure, 103 PEGMATITE, 228 Pegmatites, distribution of, 343 Peldar Tor, 364 Pen Voose, 306 Penarfynydd, 307 Penberry Hill, 323 Pencraig, 362 Pengorphwysfa, 352 Penniaen Castle, 367 Penmaenmawr, 219, 324 PENNINE, 141 Pentlands, 368 Pen-y-chain, 359 Pen-y-gar, 316 Pen-yr-oleu-wen, 357 Pen-y-rhiwiau, 352 Peppercraig, 48, 255, 259, 362 Pericline type of twinning, 95 PEBIDOTITE, 242 Peridotites, distribution of, 354 PEBKNITE, 238 Perknites, distribution of, 353 Perlitic structure, 25 PEBOVSKITE, 136 Perranporth, 312 PEBTHITE, 91 Peterhead, 293 Petrographical Provinces, 158 Phantassie, 362 Phenocrysts, 40 Philipstown, 383 Phillipsite, 140 PHLOGOPITE, 123 PHONOLITE, 254 Phonolite, structure of, 24 Phonalites, analyses of, 259 Phonolites, distribution of, 361 Phosphoric acid in rocks, 53 PlCOTITE, 133 PlCBITE, 240 Picrites, distribution of, 351 PlEDMONTITE, 128 Pikethaw Hill, 361 PILITE, 108 Pilot's Hill, 326 FINITE, 148 Pipe -amygdaloid, 27 Pitchfolds, 333 PITCHSTONE, 33, 215, 217" PLAGIOCLASE, 94 Plas Llangwnadl, 307 Platy structure, 23 PLAUENITE, 173 PLEONASTE, 133 PLINTHITE, 66 Plombieres, 61 Plug, 17 Plutonic rocks, 1 Plutonic rocks, distribution of, 279 Pneumatolytic changes, 57 Pocombe, 362 Poecilitic structure, 47 Poly synthetic twinning, 94 Pontesford Hill, 356, 375 POBFIDO Rosso ANTICO, 129 Porphyries, distribution of, 320 POBPHYBITE, 219, 264 Porphyrites, distribution of, 321 Porphyritic crystals, 40 Porphyritic structure, 48 POBPHYBY, 40, 218 Portaferry, 350 Forth Hagog, 376 Portraine, 370 Portrush, '342 Portsoy, 310, 355 Potash in rocks, 53 Potash series, 153 POTASH-FELSITE, 217 POTASH-GBANITE, 165 POTASH-BHYOLITE, 251 POTASH-SODA FELSITE, 217 POTASH-SYENITE, 173 Potash-syenites, analyses of, 174 POTASH-TRACHYTE, 256 Potluck, 331 Pouk Hill, 330 PBASEOLITE, 148 Praze, 281 PROCHLOBITE, 141 PROPYLITE, 264 PSEUD O-LEUCITE. 101 PSEUDOPHITE, 143 Pseudo-Spherulites, 46 PULASKITE, 175, 178 PUMICE, 27 Pwllheli, 335, 357, 359, 367 Pyllaugiach, 284 PYBITES, 133 PYBOMEBIDE, _16 PYBOXENE, 110 PYBOXENE-ANDESITE, 262 PYBOXENE-GBANULITE, 206 PYBOXENE-PEBIDOTITE, 244 PYROXENE-TBACHYTE, 257 PYBOXENITE, 238 INDEX 403 QUARTZ, 81 QUARTZ-BOSTONITE, 230 QUARTZ-DIORITE, 187 QUARTZ-DOLERITE, 221 Quartz- porphyries, distribution of, 312 QUARTZ-PORPHYRY, 211, 213 QUARTZ-TRACHYTES, 249 Raasay, 341, 380 Rainy Lake District, 11 Ramsey Island, 360, 372 Ramshope Burn, 375 Rathdrum, 304 Rathjordan, 382 Ratho, 223, 338 Rattlinghope, 332 Rauenthal, 245 Rawthey, 347 Red Hills, 296, 309 Refraction, double, 79 Refraction, index of, 74 Reichenstein, 174 Renfrewshire Hills, 3o8 Resorption, zones of, 40 Rhobell Fawr, 316, 375 RHYOLITE, 249 Rhyolites, distribution of, 356 RlEBECKITE, 109 RlEBECKITE-SYENITE, 178 RlPIDOLITE, 141 Rock, 1 Rodney Pillar, 333 Rongstock, 198 Roscreage Beacon, 347 Rose Ash, 348 Ross of Mull, 296 Roundstone, 299 Row Tor, 342 Rowley Rag, 224 Rowley Regis, 330 RUBELLAN, 123 Rubislaw, 171 Rudh an Tangairt, 319 Rudha Fionn Aird, 351 Budha Mhor, 302 Rue Bane Point, 350 Rum, Isle of, 36, 49, 207, 2! TESCHENITE, 225, 226 Teschenites, distribution of, 327 THERALITE, 190, 200 Thermal metamorphism, 61 THOLEIITE, 339 Threlkeld, 314 Thiiringer Wald, 237 Tideswell Dale, 331 Tillicoultry, 326 TINGUAITE, 230 Tinto Hill, 318 Tissington, 373 Titanic Acid in rocks, 53 Titaniferous minerals, 135 TJTANOLIVINE, 118 TOADSTONE, 373 Todleth, 367 Tolfa, 260 Tom Munn's Hill, 275 Tomatin, 304 TONALITE, 187 Tongues, 13 Tonsenas, 178 TOPAZ, 148 Tormore, 12, 319 Tortworth, 364 TOSOANITE, 258 Totnes, 372 Tourmakeady, 350, 360 TOURMALINE, 126 TBACHY-ANDESITE, 258 TRACHYTE, 253 Trachytes, analyses of, 259 Trachytes, distribution of, 301 Tramore Bay, 341 Traprain Law, 259, 361 Trefor, 316 Trelissick Creek, 347 Tresayes Downs, 343 Trewavas Head, 344 Trewern, 333 Trichites, 32 TRIDYMITE, 83 TROCTOLITE, 203, 207 Troutbeck, 366 Tudhoe, 332 Tuffs, volcanic, 21 Turnberry Castle, 353 Turnberry Point, 368 Tuttle Hill, 330 Ty-Canol, 376 Ty-Croes, 352 Tyne Head, 331 Tynemouth dyke, 48, 332 Uldale Head, 347 Ullscarf, 332 Ulpha, 314 Ultrdbasic rocks, 152 Umpjaur, 178 UMPTEKITE, 177, 178 URALITE, 107 UBTITE, 181 Uvalde Co., 275 VEIN-QUABTZ, 83 Veins, 13 Vesicular structure, 27 VESUVIANITE, 148 Vettakollen, 178 Vitreous rocks, 30 VlTROPHYRE, 215 VOGESITE, 232 Volcanic rocks, 7 Volcanic rocks, distribution of, 356 Volcano, section of, 20 VTJLSINTTE, 258 WACKE, 66, 267 Wastwater, 314 Wat Carrick dyke, 323 Water Swallows, 331 Waterford, 217, 320, : 26. 341 Watergate Bay, 347 Watley Gill, 347 Weathering, changes due to, 65 WEBSTERTTE, 238 WEHRLITE, 243 WERNERITE, 124 West Aston, 304 Wester Ochiltree, 323 Westerdale, 347 406 INDEX Western Isles, Scotland, map, 379 Weston-super-Mare, 374 Whin Sill, 19, 224, 331 Whinny Hill, 327 Whitehouae Point, 32$ Whitelaw Hills, 277, 351, 382 Whitwick Colliery, 330 , Windermere, 347 Wistra, 189 WlTHAMITE, 128 Wolf Rock, 259, 361 WOLLASTONITE, 116 Woodeaves, 374 Woodhouselee, 363 Wormit Bay, 364 Wrae Quarry, 363 Wrekin, 252, 330, 356 Wrockwardine, 356 Wythop Fells, 332 Xenocryst, 28 Xenolith, 28 Xenomorphic, 44 XENOTIME, 130 Y Drosgl, 316, 357 Y Foel Fawr, 367 Y Glyder-fach, 357 Yarlside, 357 Yarner Beacon, 329 Yellowstone Park, 17 Yetholm, 366 Yr Eifl, 316, 324 Zennor, 329 ZEOLITES, 140 ZlNNWALDITE, 122 ZIRCON, 134 ZOISITE, 129 Zwartkoppie Range, 182 Some pinions. " This new edition of a well-known text-book tor students marks a decided advance on its predecessors. It is clearly written, well illustrated, and has, as a rule, been brought up-to-date. The work may be safely recommended as a text-book for students." Nature. "Dr. Hatch's text-book has been some eighteen years in existence, and has long been recognised as one of the best and most complete elementary works on the subject extant. The fact that it has been frequently reprinted shows its popularity. The present edition has been revised and rewritten to bring it up-to-date as regards later discoveries and theories." Literary World. " Has established itself upon a firm basis as a students' text-book, . . . the most recent and up-to-date exposition of the science of Petrology that exists in the English language. ... In our opinion no better work exists for the guidance of the student through one of the most intricate branches of geological science." The Quarry. "An excellent guide to the structure and classification of igneous rocks, whether plutonic, hypabyssal, or volcanic." Architect. "The fact that this well-known text-book has reached a fifth edition shows that it has been of value to students of Petrology. The several sketch-maps illustrating the distribution of British igneous rocks are of especial interest." Mineralogical Magazine. ' ' A useful introduction to the study of the igneous rocks. The author concludes with abstracts of most of the classical descriptions of igneous rocks in the United Kingdom, accompanied by repro- ductions of the original maps by which they were illustrated. This portion of the work should prove of special value to the student.' 1 Science Progress. "Mr. Hatch's text-book has been entirely rewritten, and it is to all purposes a new work of much wider compass, which, notwith- standing the admitted excellence of the original book, represents a great advance. In method and arrangement it exactly meets the need of the student, to whom the clearness of the author's style will appeal. As an introduction to the serious study of rocks the book ranks high." Mining Journal. " One of the very best small petrological text- books that I know.'' Prof. W. G. Brogger. " A perfect boon to students." J. E. Marr. "The best we have, not only for students' use but for handy reference and bibliography." J. Joly UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY Return to desk from which borrowed. This book is DUE on the last date stamped below. LD21- YB S YB 27333 UNIVERSITY OF CALIFORNIA LIBRARY