THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA IRVINE GIFT OF Everett C. Edwards The D. Van Nostrand Company intend this booh to be sold to the Public at the advertised price, and supply it to the Trade on terms which will not allow of reduction HANDBOOK OF ROCKS FOR USE WITHOUT THE MICROSCOPE, BY JAMES FURMAN KEMP, A.B., E.M., PROFESSOR OF GEOLOGY IN THE SCHOOL OF MINES, COLUMBIA UNIVERSITY, NEW YORK. WITH A GLOSSARY OF THE NAMES OF ROCKS AND OF OTHER LITHOLOGICAL TERMS. FOURTH EDITION, REVISED. NEW YORK: D. VAN NOSTRAND COMPANY, 23 MURRAY AND 27 WARREN STREETS. 1908. 1 /03V COPYRIGHT 1896, BY J. F. KEMP. COPYRIGHT 1900, BY J. F. KEMP. COPYRIGHT 1904, BY J. F. KEMP. COPYRIGHT 1908, BY J. F. KEMP. THE NEW ERA PRINTING COMPANY LANCASTER, PA. PRINTERS, ELECTHOTVPER8, BINDERS PREFACE. The clear presentation of the subject of rocks to beginners is not an especially simple undertaking. The series of objects is extremely diverse, and many unrelated processes are involved in their production. In order not to confuse and bewilder students, the teacher must emphasize the intelligible points and the recog- nizable characters, avoiding alike distinctions that have their chief foundations in past misconceptions, such as the time element in the classification of igneous rocks, and that require microscopic study to substantiate them. In the following pages the attempt has been made to avoid these difficulties, and only to mention and emphasize the characters which a beginner, properly equipped with the necessary preliminary training in mineralogy, can observe and grasp. Some years of annually going over this ground have convinced the writer that for this purpose we are not likely to reach a more serviceable, fundamental classification than the time-honored one of Igneous, Aqueous (or Sedimentary) and Metamorphic rocks. They furnish not alone convenient central groups, but also pre- pare the student for subsequent geological reading. With the Aqueous have been placed the Eolian as a similar, although very minor division, so that fire, water and air, the ancient elementary agents, are emphasized in their work upon the earth, and the fun- damental classification is based, as it should be, on method of origin. The only illogical step involved is the placing of the breccias together with the sediments, but breccias are so subordi- nate and go so conveniently with conglomerates, that it has been done. The igneous rocks are the ones which present the greatest diffi- culties to the learner. In the following pages, after a preliminary exposition of principles, the very minor group of the volcanic glasses is first taken up, because it is the simplest and because it illustrates cooling from fusion most forcibly. Passing then through the felsitic and porphyritic to the granitoid textures, rocks of in- iii iv PREFACE. creasing complexity are one after another attacked. Analyses have been freely used to illustrate the chemical differences of mag- mas, because in no other way can the varieties be fundamentally described. Within fairly narrow limits the chemical composition of the magma establishes the mineralogy of the rock. The Aqueous and Eolian rocks are not difficult to understand. The metamorphic are in many respects the most obscure of all, but it is hoped that enough varieties have been selected and em- phasized to serve for field use and for the reasonably close deter- mination of the great majority of those that will be met in Nature. Many names will be encountered in geological reading that are not mentioned in the book proper. To explain them and to avoid confusing the main text with unessential matter, they have been compiled in a Glossary. Practically all the names for rocks will be found there, and some related, geological terms. The chief guide in its preparation has been the index of Zirkel's great Lehr- buch der Petrographie, but not a few American terms are intro- duced, which are not in it nor in Loewinson-Lessing's Petrograph- isches Lexikon, to which the writer is also greatly indebted. Other works, English, French and American, have likewise been at hand. One only needs to compile a glossary to appreciate what numbers of unnecessary and ill-advised names for rocks burden this unfortunate branch of science, and to convince one that the phi- lological petrographer comes near to being the enemy of his kind. So far as possible, technical words of classical derivation have been avoided in the main work in favor of simple English, and for the rocks described, American types have been especially sought with which to illustrate the different species, because they are more significant and accessible to readers on this side of the ocean. The text, except the glossary, appeared as a series of papers in the School of Mines Quarterly during 1895-96. J. F. K. AUGUST, 1896. NOTE TO THE SECOND EDITION. In the preparation of the second edition, but little change has been made in the main text. The Glossary has, however, been rewritten and brought up to date. J. F. K. DECEMBER, 1899. PREFACE TO THE THIRD EDITION. Several important changes have been introduced in the present edition, chiefly in connection with the igneous rocks. The com- position of the minerals entering into them has been more fully stated, graphic formulas being employed where they seemed de- sirable. The ingenious star-shaped diagrams which were first used by W. C. Broegger for single analyses and subsequently em- ployed by W. H. Hobbs for composite groups, have been adapted to the analyses here selected and have been given under each im- portant division of the igneous rocks. They present characteristic pictures of chemical composition which are well adapted to em- phasize this important feature for beginners. Whenever, in an analysis, ferrous iron has not been separately determined, it has been necessary to assume a value for it on the basis of related analyses, but experiments with varying values have shown that within the limits set by the total iron oxides, the variation in the general shape of the figure is scarcely appreciable. In the description of textures and their development much greater stress is laid than formerly upon the geological occurrence of the rock masses. The several forms, dikes, sheets, laccoliths, etc., have therefore been illustrated by cuts and in the table of classifi- cation, p. 23, they have been introduced in a separate column. The igneous rocks have also been treated in a slightly different way. Thus, having established a series of analyses characteristic of a certain group, as for instance the rhyolites and granites, this magma is followed through the several textures from the products of a quick chill to those of slow and deep-seated cooling. In nearly all cases four stages are emphasized and a uniform nomen- clature is employed. Thus we have the Rhyolites, Rhyolite- porphyries, Granite-porphyries, Granite, and similarly for all the others. The diabases present the one exception to this uniform treatment. In developing the above plan an old and widely employed nomenclature has been used, which experience of some vi PREFACE TO THIRD EDITION. years in the class-room and laboratory leads the writer to believe, has distinct advantages. The matter relating to the sedimentary and metamorphic rocks has not been essentially changed. A chapter has been added on the recasting of analyses of igneous rocks, which may serve as an introduction to the Quantitative Classification of Cross, Iddings, Pirsson and Washington, the latter being too complicated for an elementary book. Finally the Glossary has been brought up to date. The writer is greatly indebted to his colleagues, Dr. Charles P. Berkey for advice and assistance in editing and Professor A. W. Grabau for suggestions regarding the sedimentary rocks. J. F. K. APRIL, 1904. NOTE TO THE FOURTH EDITION. In the present edition the pages relating to the recasting of rock- analyses (155-158) have been somewhat amplified, and factors for turning molecular proportions into percentages are introduced (pp. 166-167). An appendix to the Glossary brings the rock- names up to 1908. T "F* "K APRIL, 1908. J< ABBREVIATIONS. A. A. A. S. , or Proc. Amer. Assoc. Adv. Sci. Proceedings of the American Association for the Advancement of Science. Amer. Geol., or A. G. American Geologist. Amer. Jour, of Sci., or A. J. S. American Journal of Science, some- times called Silliman's Journal. Bull. Geol. Soc. Amer. Bulletin of the Geological Society of America. Bull. Mus. Comp. Zool. Bulletin of the Museum of Comparative Zoology, Harvard University, Cambridge, Mass. Jahrb. d. k. k. g., Reichs. Jahrbuch der kaiserlichen, koniglichen Geologischen Reichsanstalt, Vienna, Austria. Jour, of Geol. Journal of Geology, published at the University of Chicago. Neues Jahrb., or N. J. Neues Jahrbuch fur Mineralogie, Geologic und Palaeontologie, Stuttgart, Germany. Quar. Jour. Geol. Soc., or Q. J. G. S. Quarterly Journal of the Geological Society of London. Tsch. Mitth. Tschermak's Mineralogische und Petrographische Mit- theilungen, Vienna, Austria. U. S. Geol. Surv. United States Geological Survey, Washington, The Publications are Bulletins, Monographs, Annual Reports. Folios and Professional Papers. Zeits. d. d. g. Ges. Zeitschrift der deutschen geologischen Gesell- schaft, Berlin, Germany. Zeits. f. Krys. Zestschrift fur Krystallographie, Munich, Germany. vii TABLE OF CONTENTS. Preface iii Abbreviations vi CHAPTER I. Introduction. Rock-forming Minerals. Prin- ciples of Classification I CHAPTER II. General Introduction to the Igneous Rocks. Classification 15 CHAPTER III. The Igneous Rocks, continued. The Glasses. The Rocks whose chief feldspar is orthoclase. The Phonolites and Nephelite-Syenites 25 CHAPTER IV. The Igneous Rocks, continued. The Dacite- Quartz-Diorite Series and the Andesite-Diorite Series . 52 CHAPTER V. The Igneous Rocks, continued. The Basalt- Gabbro Series. The Feldspar-free Basalts. The Pyroxe- nites and Peridotites. Ultra-basic Igneous Rocks . . 63 CHAPTER VI. Remarks in Review of the Igneous Rocks . 80 CHAPTER VII. The Aqueous and Eolian Rocks. Introduc- tion. The Breccias and Mechanical Sediments not Lime- stones 84 CHAPTER VIII. Limestones. Organic Remains not Lime- stones. Rocks Precipitated from Solution. Determina- tion of the Aqueous and Eolian Rocks 9^ CHAPTER IX. The Metamorphic Rocks. Introduction. The Rocks Produced by Contact Metamorphism . . . .112 CHAPTER X. The Metamorphic Rocks, continued. The Rocks Produced by Regional Metamorphism. Introduction. The Gneisses and Crystalline Schists 121 CHAPTER XI. The Metamorphic Rocks, continued. The Rocks Produced by Regional Metamorphism. The Quartzites and Slates. The Crystalline Limestones and Dolomites. The Ophicalcites, Serpentines and Soapstones 133 x TABLE OF CONTENTS. CHAPTER XII. The Metamorphic Rocks, concluded. The Rocks Produced by Atmospheric Weathering. The De- termination of the Metamorphic Rocks 144 CHAPTER XIII. The Recalculation of the Chemical Analyses of Rocks 149 Glossary 168 Appendix 237 Index 243 LIST OF ILLUSTRATIONS. FIGURES. 1. Dike of Andesite, Los Cerrillos, N. M. . . . . 15 2. Surface-flow, Leucite Hills, Wyo. . . . . . 15 3. Volcanic Neck, Boar's Tusk, Wyo. . . . 15 4. Cross-section of Laccolith, S. D. . . . . 15 5. Intruded Sheets, near New Madrid, N. M. . . . . 16 6. Laccolith, Ragged Top Mountain, S. D 16 7. Geological diagram of Fig. 6. . . . . 16 8 and 9. Diagrams Illustrating the Rhyolites. . . . 29 10 and 1 1. Diagrams Illustrating the Granites. ... 34 12 and 13. Diagrams Illustrating the Trachytes. . . . 39 14 and 15. Diagrams Illustrating the Syenites. . . . 43 1 6 and 17. Diagrams Illustrating the Phonolites. . . . 45 18. Platy phonolite, Sugar Loaf Mountain, S. D., facing. . . 46 19 and 20. Diagrams Illustrating the Nephelite-syenites. . . 50 21. Diagram Illustrating Leucite Rocks. . . . . . 51 22 and 23. Diagrams Illustrating the Dacites. . . . 53 24 and 25. Diagrams Illustrating the Andesites. . . . 57 26 and 27. Diagrams Illustrating the Diorites. . . . 61 28 and 29. Diagrams Illustrating the Basalts. .... 64 30 and 31. Diagrams Illustrating the Limburgites. ... 68 32. Diabasic Texture. . . . . . . . . 71 33 and 34. Diagrams Illustrating the Gabbros. . . . 73 35 and 36. Diagrams Illustrating the Pyroxenites . . . 76 37 and 38. Diagrams Illustrating the Peridotites. . . . 76 39 and 40. Diagrams Illustrating the Quantitative Mineralogy of the Igneous Rocks. ....... 80 41. The Formation of Limestones from Coral Reefs. . . 99 vi A HAND BOOK OF ROCKS. FOR USE WITHOUT THE MICROSCOPE. CHAPTER I. INTRODUCTION. ROCK-FORMING MINERALS. PRINCIPLES OF CLASSIFICATION. A rock may be best defined as any mineral or aggregate of min- erals that forms an essential part of the earth. The word mineral is used because this is our most general term for all inanimate nature, and while the lifeless remains of organisms often contribute in no small degree to rocks, no rock is made up of those which are still alive. In instances a single mineral forms a rock, but among minerals this is the exception. By far the greater number are in such small amount that they cannot properly be considered rocks. Rock-salt, ice, calcite, serpentine, cemented fragments of quartz, kaolin and a few others are in sufficient quantity, but the vast majority of rocks consist of two or more. The condition that a rock should form an essential part of the earth is introduced to bar out those minerals or aggregates, which, though important in them- selves, are none the less insignificant as entering into the mass of the globe. Thus the sulphide ores, while locally often in con- siderable quantity, when broadly viewed are practically neglectable. Yet this is somewhat arbitrary and there are single minerals and aggregates that may properly give rise to differences of opinion. The following pages err, if at all, on the side of demanding that the amount should be large. A rock must also have an individual character, sufficient to establish its identity with satisfactory sharp- ness. The species cannot be marked off with the same definition as in plants, animals, or minerals, and there is here again reason- able opportunity for differences of opinion as to the limits which should be set, some admitting of finer distinctions and greater multi- 2 A HAND BOOK OF ROCKS. plicity of species than others ; but, after all has been said, there should be a well-marked individuality to each rock species which any careful and qualified observer may readily recognize. Too great refinements and too minute subdivisions ought to be avoided. The determining conditions of species will be taken up at greater length, when the preliminaries of classification have been set forth, but it must be appreciated that the point of view is also a most important factor. Thus if one is studying the geology of a dis- trict with close accuracy, and is tracing out the history and development of its rocks with microscopic determinations and descriptions of minerals and structures which may be minute, finer distinctions will naturally be drawn than those that suggest them- selves to one who is engaged in ordinary field work or in mining or engineering enterprises. It is for the latter class that these pages are prepared and throughout the descriptions and classifica- tion here given, the necessary limitations and the practical needs of such observers are always kept in mind. Textural and min- eralogical distinctions are alone emphasized where easily visible on a specimen, although never made contradictory of principles of origin and classification which could be carried to greater length and subdivision. Rocks embrace matter in a great variety of structures and con- ditions. While in general we picture them to ourselves as solid, yet under the terms of our definition, we have no logical right to bar out liquids or even gases. The physical condition may vary with ordinary temperatures. Thus we cannot reject ice as an ex- tremely abundant and important rock, and yet its solid condition results from water with a moderate loss of heat, and at ordinary temperatures the same molecules may be in a liquid or gaseous state. All that we know of volcanoes indicates that liquid, molten magmas exist for long periods deep in the earth, yet they are none the less rocks because of their liquidity. In general, how- ever, rocks are solid, and gases or liquids (except water) de- serve no further attention. In texture rocks may be loose and in- coherent as in sand, gravel, volcanic dust and the like, or they may be extremely dense, hard and solid, as in countless familiar examples. This solidity or massiveness has its limitations, for all observation and experience show that what are apparently solid INTRODUCTION. 3 masses are really broken up by multitudes of cracks into pieces of varying size. All quarries a'nd mines have these, and they may aid or annoy the operators according to the purposes of excava- tion. They will again be referred to at length. Unless too deep within the earth, rocks are also in all cases permeated with minute pores and spaces that admit of the penetration of water and other liquids, especially if under pressure. These are important factors in terrestrial circulations. THE CHEMICAL ELEMENTS IMPORTANT IN ROCKS. The chemical elements really important in rocks are compara- tively few, and are those which are most widespread in nature. The best estimate which has been made is that of F. W. Clarke, in Bulletin 78, of the U. S. Geological Survey, pp. 3443. The crust to ten miles below sea level and the air and the ocean are embraced. The composition of the solid crust is reached by averaging up analyses of igneous and crystalline rocks, 880 in all; 321 from the United States, 75 from Europe, 486 from all quarters. Ig- neous rocks being the source of all the others, furnish the best data for the general chemistry of the globe. The composition of the ocean is then averaged in with that of the rocks, on the basis of 7 per cent, for the former and 93 per cent, for the latter, with a further addition of O.O2 per cent, for the nitrogen of the atmos- phere. Other ingredients, as the oxygen of the air, are less than o.oi per cent, and are neglected. o 49.98 Na 2.28 P 0.09 Si 25-3 K 2.23 Mn 0.07 Al 7.26 H 0.94 S 0.04 Fe 5-08 Ti 0.30 Ba 0.03 Ca 3-51 C 0.21 N O.O2 Mg 2.50 Cl, Br 0.15 Cr o.oi The remaining elements may be omitted in this connection, although, as a moment's reflection will show, they include all the common metals except iron and manganese. There is good ground for believing that toward the center of the earth the metallic elements become much more abundant, and that near the center some of the heaviest known are in excess, but these inferences, however well-based, concern materials far beyond 4 A HAND BOOK OF ROCKS. actual experience, and of no great moment in this connection. As regards rocks we have to deal with the outer portions of the globe, to which we are accustomed to refer as the crust. This term is not meant to indicate anything as to the condition of the interior, but merely the exterior as contrasted with the inner parts. The chemical elements above cited are combined, except per- haps in volcanic glasses, in the definite compounds which form mineral species. These compounds change, more or less, in the course of time, under the action of various natural agents, chief of which are water, carbonic acid and oxygen, but at any particular stage, however complex the rock may be, it is made up of definite chemical compounds, though we may not be able to recognize them all. The most important compounds are not numerous and are practically limited to the following : silicates, oxides, carbo- nates, sulphates, chlorides, and of far inferior moment phosphates, sulphides, and one native element graphite. As a broad conception in speaking of these compounds it is in many respects advantageous to have the igneous rocks pri- marily before our minds, because as stated above they are the sources of the others. In taking up the minerals the purpose here is to emphasize their chemical composition and relative im- portance, not to describe them as would be done in a text-book on mineralogy so as to enable a student to recognize them, for such preliminary knowledge is here assumed. Our purpose is to make prominent the chief chemical compounds entering into the earth, and to prepare the way for a true conception of the range and relations of its constituent rocks. THE SILICATES. THE SILICATES are grouped as follows : the feldspars and feld- spathoids; the pyroxenes; the amphiboles ; the micas; olivine. The last four groups are often collectively called the ferro-magne- sian silicates. Zircon and titanite conclude the list of those im- portant in igneous rocks. In addition there are a number of others that are especially characteristic of altered or metamor- phosed rocks, viz : epidote, scapolite, garnet, tourmaline, topaz andalusite, cyanite, fibrolite or sillimanite, and staurolite. Finally a few hydrated silicates complete the list. INTRODUCTION. 5 THE FELDSPARS and their related minerals are all double sili- cates of alumina and an alkali or an alkaline earth or both. We speak of them as alkali-feldspar, potash-feldspar, soda-feldspar, lime-soda feldspar, etc., based on this fact. They are generally grouped as orthoclase, representing monoclinic feldspar with its two cleavages at right angles (hence the name), and as plagioclase or triclinic feldspar, with oblique cleavages, and one striated cleavage plane. Orthoclase and albite are salts of H 4 Si 3 O 8 in which one monad element potassium or sodium and one triad, aluminum, satisfy the acid radicle. Anorthite, on the other hand, is a salt of 2(H 4 SiO 4 ), one dyad, calcium, and two triads, aluminum, making eight bonds of affinity in all being required to satisfy the acid radicles. Orthoclase is chiefly K 2 O,Al 2 O 3 ,6SiO 2 , which ex- panded form may be condensed to KAlSi 3 O 8 . Na,O replaces more or less of the K 2 O, without affecting the crystal system. Suf- ficient amounts of soda are however capable of changing the system to triclinic and the feldspar is called anorthoclase. Micro- cline is also a triclinic variety of potash feldspar, with a cleavage angle slightly less than a right angle, but with peculiar and char- acteristic optical properties, which are chiefly of moment in micro- scopic work. The clear, unclouded orthoclase of the later volcanic rocks is often called sanidine. It does not differ essentially from the orthoclase of the older rocks, and the distinction based on geological age is obsolete, but as the terms are still used in the literature of the subject it is well to understand them. The plagioclase feldspars embrace a practically unbroken series from pure soda-alumina silicate in albite, Na 2 O,Al 2 O 3 ,6SiO 2 or when condensed NaAlSi 3 O 8 , to pure lime-alumina silicate, anor- thite, CaO,Al 2 O 3 ,2SiO 2 or CaAl 2 Si 2 O 8 . Various mixtures of these two molecules give the intermediate species, but the two on which special stress is ordinarily placed are oligoclase, with soda in excess and hence called soda-lime feldspar, and labradorite with lime in excess and hence called lime-soda feldspar. If we rep- resent the orthoclase molecule, KAlSi 3 O 8 by Or ; the albite molecule, NaAlSi 3 O 8 by Ab ; and the anorthite, CaAl 2 Si 2 O 8 by An ; all the intermediate feldspars can be algebraically expressed. Thus anorthoclase lies between Ab 2 Orj, and Ab 4 5 Orj ; albite em- braces those from Ab through Ab 8 An t ; oligoclase, 6 A HAND BOOK OF ROCKS. through Ab 2 A ni (the intermediate mixtures Ab 3 An 2 through AbAn are called andesine); labradorite includes Ab^ through Ab\n bytownite A^An, Ab.An, ; anorthite Ab 1 An 8 to An. Thfa conception of feldspars as isomorphous mixtures of molecules is a very valuable one and by determining specific gravity, optical properties and chemical composition, one or all, the different members can be identified. Practically, however, in the ordinary determination of rocks, aside from microscopic work, we are forced by the difficulty of distinguishing the intermediate varieties, into the general use of orthoclase and plagioclase, and rely on the presence or absence of the striations peculiar to the basal cleavage of the latter in distinguishing between the two, but of course ex- perience and familiarity with the general characters and associa- tions of minerals in rocks often enables one to determine very closely the minor varieties. We would naturally look for ortho- clase, albite and oligoclase in acidic rocks or those high in silica, while in basic rocks we would expect those near the anorthite end. All the feldspars have very similar crystal forms when these are developed, as they occasionally are in rocks. When they are small and irregularly bounded, cleavage faces should be sought out and examined with a pocket lense. It is interesting to note that only in igneous rocks do we obtain crystals uniformly de- veloped on all sides, for only in a fused magma do they swim and grow without a fixed support. The word feldspar is spelled by English writers "felspar," but among Americans the more correct form, based on the etymology, is employed, following the German original " Feldspath." FELDSPATHOIDS. With fhe feldspars are placed two other im- portant and closely related minerals, nephelite and leucite, to which may also be added one that is quite rare, melilite. Nephe- lite is an hexagonal, soda-alumina silicate, 4Na 2 O,4Al 2 O 3 ,9SiO 2 , in which some of the Na 2 O is replaced by K 2 O and CaO. It appears in a subordinate series of igneous rocks that are rich in soda. Leucite is an isometric potash silicate, K 2 O,Al 2 O 3 ,4SiO 2 , with a little Na 2 O replacing part of the K 2 O. It is a salt of metasilicic acid, H 2 SiO 3 , like the pyroxenes and amphiboles, but because of the triad ele- ment, aluminum, it is necessary to have two of the acid radicles. Thus dividing the expanded formula given above, by 2 and con- INTRODUCTION, 7 densing we have KAlSi 2 O 6 . It appears as an important rock- making mineral in the igneous rocks of ten or fifteen localities the world over, and is therefore of very limited distribution. Melilite is an extremely basic, lime-alumina silicate, i2CaO,2A! 2 O 3 ,9SiO 2 , and appears in a few rare basalts. Reference may also be made to sodalite, noselite and haiiynite which are occasionally met, but which are chiefly of microscopic interest. The feldspars, together with the feldspathoids nephelite and leucite, are the most important of the rock-making minerals in their relations to the classification of rocks. In order to have a standard series of analysis with which to compare those of rocks later given, the following table is inserted of theoretical feldspars and feldspathoids. The relative amounts of the several oxides will suggest the extent to which the mole- cules are present in any rock whose analysis is known : KAlSi,O, NaAlSi.O. CaAl 2 Si.,0 8 Na.Al 8 Si 9 O sl KAlSi a O. Ca 12 AlSi 9 3 . Si0 2 64.7 68.6 43-1 45-0 55-0 38.1 Al,0, 18.4 19.6 36.8 34-3 23-5 H-5 K a O 16.9 21.5 Na 2 O 11. 8 20.7 CaO 20.1 47-4 Sp. Gr. 2.57 2.62 2. 75 2.58 2.48 2-93 Recalling what has been said about the replacement of the alka- lies by one another, and that we never meet any of these minerals chemically pure, according to the formulas above given, and making suitable allowance for this replacement, we may still appreciate that orthoclase and albite, being high in silica, favor acidic rocks, and the others being low in silica, basic ones ; that nepheline implies a magma rich in alumina and soda, leucite one rich in potash, and melilite one low in silica and alumina, but high in lime. THE PYROXENES and the AMPHIBOLES are best described to- gether. Each embraces a series of compounds of the same chemical composition, differing only in physical and optical properties. As the table shows, they vary from magnesia silicate through a series of lime and lime-alumina silicates, with an iron silicate generally present. They are all primarily salts of metasilicic acid, H 2 SiO 3 . The monad and dyad bases, sodium, calcium, magnesium and 8 A HAND BOOK OF ROCKS. ferrous iron make simple and easily understood compounds, such as Na 2 0, SiO 2 ; CaO, SiO 2 ; MgO, SiO 2 ; and FeO, SiO 2 . On analysis however the triad bases, ferric iron and aluminum are quite invariably found in augite and hornblende. This was not easy to understand until the following replacement was suggested. If we write the formula of the diopside molecule graphically it will appear as follows : Mg-O Si=O A i . Ca O Si=O Now in the lower line the calcium and silicon have together six bonds of affinity. So have two ferric irons, or two aluminums. If, therefore, we replace the calcium and silicon with the two irons we Mg-0-Si=0 i i This is a common molecule in augite. Condensed it will be MgFe 2 - SiO 6 . We also have MgAl 2 SiO 6 . The explanation will serve to make clear the condensed formulas of the several molecules as given in the table below, for otherwise the molecules with the triad elements and with only one silicon often prove very puzzling. The graphic formula for diopside will also show how two bases may enter a single molecule. All the pyroxenes have a prismatic cleavage of nearly 90 (87 10' or thereabouts), while the amphi- boles cleave along a prism of nearly 120 (124 u'). COMPOSITION. f MgOSi0 2 I (FeOSi0 2 J CaMgSi 2 O, CaMgSi 2 6 CaFeSi 2 O g CaMgSi 2 6 CaFeSijOg MgAl.SiO, MgFe a Si0 6 FeAl 2 SiO, NaFeSi,0 6 PYROXENE. Enstatite Bronzite Hypersthene Diopside Malacolite (Diallage) Augite Acmite ^Egirine AMPHIBOLE. Anthophyllite > Orthorhombic Tremolite Actinolite Hornblende Arfvedsonite Monoclinic INTRODUCTION. 9 Under the orthorhombic pyroxenes enstatite has least of the mole- cule FeO,SiO 2 , i. e., FeO less than 5 per cent. ; bronzite has FeO more than 5 and less than 14 per cent. ; while hypersthene has still higher percentages of FeO. The increase brings about a darker color and changed optical properties. The orthorhombic pyroxenes are much less frequent than the monoclinic, but are of wide distri- bution, especially hypersthene. The orthorhombic amphiboles are of minor importance and are but seldom met. The light-colored monoclinic pyroxenes are almost pure lime- magnesia silicates, and are called diopside. They are chiefly found in crystalline limestones. As iron increases, they pass into malaco- lite, which may also contain small amounts of the aluminous mole- cules. Neither of these pyroxenes is of special abundance as a rock-maker. When pinacoidal cleavages around the vertical axis appear in addition to the prismatic ones, in pyroxenes of the general composition of malacolite they are called diallage and are important in some igneous rocks. But the chief rock-making pyroxenes are the dark aluminous, ferruginous ones, which are called augite, and these are among the most important of all minerals in this con- nection. The igneous rocks rich in soda, in which nepheline is common, are the ones that contain acmite and aegirite, the soda- pyroxenes. The monoclinic amphiboles are closely parallel in their occur- rence and relations to the pyroxenes. Tremolite is met in crystal- line limestones. Actinolite may form schistose rocks by itself, but much the most important variety is hornblende, the aluminous variety corresponding to augite. The soda amphibole, arfvedson- ite, is rare. The pyroxenes and amphiboles are often collectively re- ferred to as the bisilicates, the oxygen of the base being to the oxygen of the silicon, as shown in the first two formulas, in the ratio of 1:2. It is also interesting to note that many blast furnace slags are calculated on the basis of the formulas for pyroxene. THE MICAS. All the micas are salts of orthosilicic acid, H 4 SiO 4 . This acid radicle will be satisfied by one monad base and one triad together, such as potassium and aluminum, but as dyad bases also occur with both these it is necessary to assume multiples of the I0 A HAND BOOK OF ROCKS. orthosilicic acid. Thus muscovite, the simplest of the common micas, has a graphic formula like this H O K O f-0- Al \ -O 1-0 Al J O 1-0-. For biotite which is more complex we need three molecules of the acid as follows : K O Si O "IE O O O -O -O Si H O Ferric iron may take the place of the aluminum, and various other variations may be made of this general formula. When con- densed, the formula for muscovite will be HKAl 2 Si 2 O 8 , and the variety of biotite given above will be HKMgFeAl 2 Si 3 O 12 . Biotite is the dark mica and is much the commonest of the group. It is very widespread, and is easily recognized by its cleavage even small crystals can be picked apart into leaves with the point of a knife. Biotite is often called magnesia-mica. It enters into the classification of igneous rocks in an important way- Phlogopite is a lighter-colored but closely related variety, which favors crystalline limestones. Muscovite, from its richness in pot- ash, is often called potash-mica. It is widespread in granites and schists. In composition it closely resembles orthoclase. OLIVINE, the unisilicate of magnesium and iron, 2(Mg, Fe)O, SiO 2 , completes the list of silicates which are of the first order of im- portance in igneous rocks. The above name is usually employed in preference to chrysolite. Olivine is practically limited to basic igneous rocks. Like the micas it is an orthosilicate. Zircon and titanite are interesting microscopic accessories, but as rock-making minerals they are seldom visible to the naked eye. INTRODUCTION. n Along the contacts of intrusions of heated igneous rocks, and in regions where the original sediments have undergone strong dy- namic disturbances, with oftentimes attendant circulations of waters more or less heated, a series of characteristic silicates is in each case developed. Garnet, tourmaline, topaz, andalusite, scapolite and biotite are especially characteristic of the former; garnet, cyanite, silllmanite, staurolite, biotite, and muscovite of the latter. Details of the development and associations of each of these groups are subsequently given under the metamorphic rocks. Epidote results when feldspars and the ferro-magnesian silicates undergo decay and alteration in proximity, so that the solutions afforded may react on one another. The hydrated silicates of chief importance include a magnesian series, embracing talc and serpentine, which result from the ferro- magnesian minerals ; a ferruginous aluminous series, with much iron oxide, usually collectively called "chlorite," and derived from the iron-alumina silicates ; and finally kaolin, the hydrated silicate of alumina that is chiefly yielded by feldspar. Zeolitic minerals are also often met, but rather as vein fillings and in amygdaloidal cavities than as important rock makers. The oxides include quartz and its related minerals chalcedony and opal, and the oxides of iron magnetite and hematite and the hydrated oxide, limonite. With these should be mentioned chro- mite and ilmenite (menaccanite), which are of minor importance. Quartz is found in all rocks high in silica. Magnetite and hema- tite are at times almost abundant enough to constitute rocks them- selves. They favor igneous and metamorphic varieties when pres- ent in a subordinate capacity. Magnetite is the most widespread of all the rock-making minerals. Limonite is an alteration prod- uct. Chromite is practically limited to the basic igneous rocks and their serpentinous derivatives. Ilmenite is a common accessory in many igneous rocks. The carbonates are calcite, dolomite and siderite, all three being really members of an unbroken series from pure carbonate of cal- cium, through admixtures of magnesium carbonate to pure magne- site on the one hand or with increasing carbonate of iron to pure siderite on the other. The sulphates of moment are anhydrite and gypsum, the latter the hydrous, the former the anhydrous salt of I2 A HAND BOOK OF ROCKS. lime. The one chloride is the sodium chloride, rock salt or halite, and the one phosphate is apatite, which is a phosphate and chlonde of lime. The two sulphides of iron, pyrite and pyrrhotite are the only ones sufficiently widespread to deserve mention, and graphite is the chief representative of the elementary substances, although native sulphur might perhaps with propriety be also men- tioned. 'We speak of minerals as essential and accessory, meaning by the former term those that constitute a large part of the rock, and that must be mentioned in the definition ; by the latter those that are present in small amounts or that are more or less fortuitous. Primary minerals are those that date back to the origin of the rock, as for instance the ones that crystallize out from a molten magma as it solidifies; secondary minerals are formed by the alteration of the primary. Feldspars, pyroxene and hornblende are good illustrations of the former; hydrated silicates of the latter. THE PRINCIPLES UNDERLYING THE CLASSIFICATION OF ROCKS. Rocks must of necessity be classified in order to place them in their natural relations so far as possible and to allow of their syste- matic study. At the same time they are so diverse in their nature and origin that the subject is not an easy one. They must however be grouped on the basis of their structures and textures ; or of their mineralogical composition ; or of their chemical composition ; or of their geological age ; or of their method of genesis. One or several of these principles enter into all schemes. On the basis of the first, rocks have been classified as massive and stratified ; as crystal- line and fragmental or clastic, each with subdivisions on one or more of the other principles. On the basis of the second we have had those with only one mineral (simple rocks) and those with several (complex rocks). The chemical composition as shown by a total analysis (bausch-analysis), without regard to special mineral components, is of almost universal application in a subordinate capacity. It must be regarded in the group of igneous rocks and in those that are deposited from solution, chiefly highly calcareous or highly siliceous rocks. The principle of geological age was formerly much valued in connection with the igneous rocks, but it INTRODUCTION. 13 is a thoroughly exploded one. The principle of origin or genesis is the most philosophical of all as a fundamental basis, but while in the greater number of cases it may be readily applied there are some puzzling members whose entire geological history is not well understood. Very early in the development of the subject it was appreciated that there were two great, sharply contrasted groups, according as the rocks had consolidated and crystallized from a molten condition or had been deposited in water either as mechan- ical fragments or as chemical precipitates. Widened observation, especially in arid and sandy regions, has added to these a less im- portant group of those whose particles have been heaped together by the wind. They are called the eolian rocks and will be taken up together with the aqueous, with which they have many points in common. Two grand divisions have therefore been established, the igneous, on the one hand, and the aqueous and eolian on the other. Even a limited field experience soon convinces the observer that many rocks are encountered which cannot be readily placed with either of the two great classes whose origin is comparatively simple. Rocks for instance are met having the minerals common to the igneous but with structures that resemble those of sediments in water. Great geological disturbances, especially if of the nature of a shearing stress, may so crush the minerals of any igneous rock and stretch them out in bands and layers as to closely imitate a recrystallized sediment. The baking action of igneous intrusions on fine sediments, such as clays and muds, makes it difficult for an observer, without the aid of thin sections and a microscope, to say where the former sediment ends and the chilled magma begins. Sediments buried at great depths and subjected to heat and hot water become recrystallized with their chemical elements in new combinations. These excessively altered rocks have been often grouped into a separate, so-called " metamorphic " division, which was a sort of " omnibus " of unsolved geological problems. This metamorphic group is useful, and the term is a common one in the science, but wherever possible it is well to appreciate the true affinities of its members which though altered are still referable to their originals. I 4 A HAND BOOK OF ROCKS. In the following pages these three divisions will be adopted, but the metamorphic group will be reduced to a minimum by remark- ing, in connection with descriptions of the unaltered rocks, the changes that igneous and aqueous undergo. We take up, therefore, in this order : A. The Igneous Rocks. B. The Aqueous and Eolian Rocks. C. The Metamorphic Rocks. FIG. I. Dike of andesite 15 ft. thick and 50 ft. high, cutting sandstones. Ortiz arroyo, near Los Cerrillos, N. M. D. W. Johnson, School of Mines Quarterly, July, 1903, 461. FIG. 2. Surface flow. Black Rock Mesa, Leucite Hills, Wyo. Kemp and Knight, Bulletin Geol. Soc. Amer., XIV., 323, 1903. FIG. 3. Volcanic Neck. The Boar's Tusk, Leucite Hills, Wyo. Kemp and Knight, Bull. Geol. Soc. Amer., XIV., 328, 1903. FIG. 4. Laccolith. Cross-section of the Ragged Top laccolith of phonolite, Black Hills, S. D. J. D. Irving, Annals N. Y. Acad. Sci., XII., Plate VI., 1899. CHAPTER II. GENERAL INTRODUCTION TO THE IGNEOUS ROCKS. CLASSIFICATION. The Igneous rocks are first treated because they have been the originals, according to our best light, from which all the others have been directly or indirectly derived, for either from the frag- ments, as afforded by their decay, or from the mineral solutions, yielded by their alteration, possibly in the primitive history of the globe, all the others have been produced. The igneous rocks occur in dikes, sheets, laccoliths, bosses and vast irregular bodies, for which we have no single term. Dikes (spelled also dykes) have penetrated fissures in other rocks, and have solidified in them. They therefore constitute elongated and relatively narrow bodies, of all sizes, from a fraction of an inch in thickness and a few feet in length, to others a thousand or more feet across and miles in length. Sheets are bodies of relatively great lateral or horizontal extent, compared with their thickness. They are either surface flows, which may be afterwards buried or else are intruded between other strata. In the last case, if len- ticular in shape, they are often called laccoliths. Roughly cylin- drical masses, such as might chill in the conduit of a volcano, are called necks. Irregular, projecting, rounded bodies are called bosses. The enormous masses of crystalline rocks like granite that often cover hundreds of square miles, and that frequently appear to have fused their way upward by melting overlying rocks into their substance, are called batholiths. They have in most if not all instances, only been uncovered by erosion, for the name means a rock belonging to the depths of the earth. It will be later brought out that the character of the occurrence, whether as dike, surface flow, intruded sheet, or batholith, has an important influence on the texture. Igneous rocks are characteristically massive, as contrasted with the stratified structure of the sedimentary, and the term massive is sometimes employed as a synonym of igneous. Other synony- mous terms are eruptive and anogene, both meaning that the rocks 15 i6 A HAND BOOK OF ROCKS. I have come up from below. Many S years ago the distinction was made ^ between those that have crystallized deep within the earth, the plutonic, ^ and those that have been poured out * on the surface, the volcanic. The "5 words intrusive and effusive or extru- c sive have been employed in much the d same way. Between surface flows and & deep-seated masses (batholiths) and j; their characteristic textures, every gra- | ^ dation is to be expected and is met, w * and an intermediate group has even ~ > tinction is not carried out here, the two J extremes being believed to illustrate a ^ O> the varieties satisfactorily when accom- S | panied by auxiliary remarks on the intermediate types. o ^ We are tending more and more to S>J employ the word structure for the "i larger features of a rock, as for instance ^ | a massive structure as against a strati- JE > fied, while the smaller features are de- 1^ scribed as textures, as for instance a Q glassy texture, a porphyritic or a gran- J itoid, terms that refer to characters which may be seen even on a small fragment. Glassy texture, as the name | implies, is that of glass or slag and has | no definite minerals. It results when a molten magma is so quickly chilled that the minerals have no opportunity H to form. Porphyritic implies larger x/> crystals, well formed or corroded and 52 rounded, embedded in a more finely IGNEOUS ROCKS. 17 crystalline, or even in a "glassy groundmass." There may be sev- eral sizes and kinds of these crystals, and because of their promi- nence in the rock they are called phenocrysts, i. e. y apparent cry- stals, but phanerocryst is better etymologically. If a magma cry- stallizes as a mass of very fine or microscopic crystals without phenocrysts, its texture is described as felsitic. A granitoid or granular texture has the component crystals all of about the same size, and very seldom possessing their own crystal boundaries. Strictly speaking, there is no groundmass in granitoid rocks. Sometimes from a local abundance of mineralizers (as later ex- plained), granitoid rocks have small cavities into which the com- ponent minerals project with well-bounded crystals. Such are called miarolitic. Textures in igneous rock are due to several factors that have in- fluenced the development of the magma during its consolidation. The most important are chemical composition, temperature, rate of cooling, pressure and the original presence of dissolved vapors called mineralizers. The fusibility varies with the chemical com- position. The most acid or siliceous magmas, i. e., those with 65 75 per cent. SiO 2 are least fusible. When molten they are viscid and ropy. The fusibility increases with the decrease of silica down to the basic rocks with 40 to 50 per cent. SiO 2 . The ultra-basic rocks which graduate into practically pure bases, as in some rare, igneous iron ores, are less fusible. This statement that acid rocks are least fusible often puzzles a student who is familiar with blast furnace practice and the composition of slags, in which the most siliceous are regarded as most fusible, but slags themselves, as a comparison of analyses will readily show, are to be paralleled with basic rocks. The importance of the fusibility as regards textures lies in the fact that the highly siliceous quickly chill, become ropy and freeze. They therefore especially yield glasses. The easily fusible remain fluid at lower temperatures, crystallize out as min- erals to a greater degree and seldom yield glasses. They flow farther from the vent and tend to develop the porphyritic or even a variety of granular texture. The influence of temperature has been partly outlined in speaking of composition, but it will readily appear that in its progress to the surface a basic magma might stand for a considerable period at a temperature of fluidity, whereas Ig A HAND BOOK OF ROCKS. an acid magma in the same situation would consolidate. The rate of cooling is important Cooling magmas tend to break up into minerals. As a general thing it requires a very quick chill to pre- vent their formation. Hence it is that even volcanic glasses which appear to be perfect glass to the eye are shown to be full of dusty, microscopic minerals under the microscope. Volcanic glasses are chiefly found on the outer portions of flows or dikes, but instances are known where sheets of them are very thick, as at Obsidian Cliff in the Yellowstone Park. The common experience with lavas is that certain crystals develop to notable size, it may be an inch or more in diameter, while the magma stands beneath the surface, in circumstances favorable to their formation. These are then caught up in the moving stream and brought to the surface or near it where the final consolidation takes place and fixes them in the so-called groundmass. A quick chill makes a fine-grained groundmass when not a glassy one, and slow cooling yields one more coarsely crystalline, but in the final cooling or consolidation at or near the surface, crystals are seldom if ever developed of a size commensurable with those formed in the depths. By this process of partial crystallization below and final consolidation on the surface, the porphyritic texture is almost always developed, but in strict accuracy it should be stated that cases are known where phenocrysts appear to have formed in lavas after coming to rest. Magmas also flow to the surface with no phenocrysts (or " intratelluric " crystallizations) and then consolidate not as glass, but as finely crystalline aggregates, practically all groundmass. The resulting texture is called felsitic. Pressure, such as is developed upon a magma deep within the earth or during its passage to the surface is thought to exert an in- fluence upon the formation of many phenocrysts and to be necessary for their development. Dissolved vapors, such as steam, hydrofluoric and boracic acids, are also important factors. Acidic magmas are more generally provided with them than basic, and where locally abundant they lead to variations both in the mineral composition and texture at different places in the consolidated rock. They may prevent the development of glass, and cause a sheet such as Obsi- dian Cliff, in the Yellowstone Park, to present alternations of glassy and stony layers, the latter being formed of microscopic crystals. IGNEOUS ROCKS. 19 A word should be added about the chemical composition of rocks and about the interpretation of analyses before the rocks themselves are taken up. The analyses are reported in percent- ages of oxides, for the most part, and these are arranged in the following series, SiO 2 , A1 2 O 3 , Fe 2 O 3 , FeO, CaO, MgO, Na 2 O, K 2 O, H 2 O. In order to have anhydrous materials, it is customary to ignite and determine loss on ignition. This loss includes both H 2 O and CO 2 and where large throws uncertainty over the relations of the elements left behind, because of the evident advance of decay. Small percentages of other oxides are quite invariably present and in refined work are determined. These are TiO 2 , MnO, NiO, BaO, SrO, S, Cl, P 2 O 5 , Li 2 O, and even rarer ones. They are how- ever always in very small quantity. We often recast an analysis, by dividing, as in the determination of a mineralogical formula, each percentage by the molecular weight. We thus get numerical molecular ratios which indicate the relative numbers of individual molecules and enable us to draw conclusions as to the way in which they are combined with one another in the component minerals of the rock. If we know the chemical formulas of the minerals we can sometimes calculate the percentage of each in the rock. In Chapter XIII. this subject is further treated with an illustrative example. The calculations cannot however be made when two or more bases appear together in two or more min- erals. Variations in chemical composition entail variations in resulting minerals, but it is also true that the same magma, if consolidating under different physical conditions of heat, pressure, etc., at different times may yield somewhat different minerals, for instance, hornblende instead of augite, or vice versa. A study of analyses soon makes one more or less familiar with the minerals that would necessarily result. The more important points are the amounts of silica, of the alkalies and alkaline earths, of iron oxides and of alumina. For instance, as a rule, only magmas high in SiO 2 yield quartz, for otherwise it would combine with the bases. Much K 2 O is necessary for an orthoclase or leucite rock, but much Na 2 O for one with nepheline. MgO in relatively large amount is required to yield olivine or an orthorhombic pyroxene, and when feldspars drop away and rocks become very basic we expect high CaO, MgO, FeO, Fe 2 O 3 , and low SiO 2 . In rocks tested for pur- 2O A HAND BOOK OF ROCKS. poses of building, the percentage of sulphur is important and very little should be present. It occurs in some form of pyrites, which by its decay generates sulphuric acid and destroys the stone or stains it with limonite. It should never reach I per cent Using the molecular ratios as the basis of plotting, extremely in- teresting and significant diagrams have been devised, first for indi- vidual cases by W. C. Brogger * and later of groups by W. H. Hobbs,f so that the latter's figures are like composite photographs of the chemical composition of the several groups of igneous rocks. Diagrams of this sort are subsequently given which will epitomize for the student the chemical characteristics of each of the groups. Various other devices have been suggested but the ingenious plan of Brogger is the best. The specific gravity or density of a rock is an important feature in its practical bearings. While it may in ice be less than I, and in coals and certain carbonaceous deposits may drop as low as 1.25, and in very porous sandstones reach 2.25, yet in the common rocks it is seldom below 2. 50, and ranges from this to over 3.00. Granites are usually about 2.65, but basic rocks, rich in iron, attain to the higher limits, even above 3.0. Determinations are important in those rocks used for building purposes, and are expressed in pounds per cubic foot. Of recent years we have come to regard molten magmas as es- sentially solutions of some compounds in others, and to appreciate that solutions do not cease to be such, even when the temperature is very high. It results from this that the crystallization of the minerals of an igneous rock takes place from the magma as this in its cooling successively reaches a point of saturation for the salt in question. The least soluble separate the earliest of all, and then the others in order ; but as the pressure under which they rest is also a factor, and this is subject to variation, as indeed is the temperature during movement to the surface, one mineral's period of formation may overlap another's more or less. The order of formation will be determined by the laws of thermo- dynamics and necessarily the mineral that develops the most heat in crystallizing will be the first to crystallize. As a general rule, * "Das Ganggefolge des Laurdalits," Kristiania, 1898, 255. t " Suggestions Regarding the Classification of the Igneous Rocks," Journal of Geology, VIII., i, 1900. IGNEOUS ROCKS. 21 the relations of the minerals in rocks show that the earliest to form are apatite ; the metallic oxides (magnetite, ilmenite, hematite) ; the sulphides (pyrite, pyrrhotite) ; zircon and titanite. These are often called the group of the ores. Next come the ferromagnesian sili- cates, olivine, biotite, the pyroxenes and hornblende. Next follow the feldspars and feldspathoids, nepheline and leucite, but their periods often begin well back in that of the ferromagnesian group. Last of all, if any excess of SiO 2 remains, it yields quartz. In the variation of the conditions of pressure and temperature just referred to, it may and does often happen that crystals are again redissolved in the magma, or are resorbed, as it is called ; and it may also happen that after one series of minerals, usually of large size and of intratelluric origin, has formed, the series is again re- peated on a small scale as far back as the ferromagnesian silicates. Minerals of a so-called second generation thus result, but they are always much smaller than the phenocrysts, and are characteristic of the groundmass. It follows from what has been stated that the residual magma is increasingly siliceous up to the final consolidation, for the earliest crystallizations are largely pure oxides. It is also a striking fact that the least fusible minerals, the feldspars and quartz, are the last to crystallize and therefore we must introduce the conception of solution in order to explain the process ; otherwise the minerals would inevitably form in the reverse order of their fusibilities, the most infusible leading off. The accompanying table of fusing points will be of interest in this connection. It must be borne in mind that no mineral can crystallize while the magma has a tem- perature above its fusing point for the conditions of pressure pre- vailing at the time. TABLE OF THE FUSING-POINTS OF THE COMMON ROCK-MAKING MINERALS. yEgirite 925 C. Microcline 1155 C. Hornblende 1025 101085 Orthoclase 1175 Nephelite 1080 101095 Magnetite 1185 Augite 1085 101095 Hypersthene 1185 Albite 1 1 10 Muscovite 1230 Oligoclase 1 120 Leucite 1300 Labradorite 1125 Olivine 1350 Biotite 1130 Bronzite 1400 Sanidine 1 130 Quartz fuses still higher. Anorthite 1132 22 A HAND BOOK OF ROCKS. TABLE OF THE FUSING-POINTS OF SOME OF THE COMMON IGNEOUS ROCKS. Granite "40 C. Basalt 1060" C. Monzonite "9O Limburgite 1050 Phonolite 1090 Lava (Etna) 1010 Lava (Vesuvius) 1080 These values are taken from an abstract of a paper by C. Doel- ter, reviewed in the Neues Jahrbuch, 1903, Volume II., page 60. From them, it is evident that while olivine, for example, melts, at 1350, limburgite, a rock containing large proportions of it, melts at 1050. Magnetite, the earliest to crystallize of all the minerals mentioned, fuses of itself more than a hundred degrees above many of the rocks containing it. In the matter of the study and determination of a rock species, es- pecially of an igneous rock, it is desirable to procure materials as fresh and unaltered as possible. If feldspars have all changed to kaolin and clay, and if ferromagnesian silicates are merely chlorite or ser- pentine, and if secondary quartz, calcite and the like have formed, it is very difficult if not impossible to draw correct or even well-grounded inferences. Rocks near ore bodies are very often of this character. Bearing in mind these differences of texture and the causes of them, it is possible to group igneous rocks in such arrangement that they can be intelligently studied, and identified with a reason- ably close approximation to the truth. It should be appreciated, however, that with finely crystalline rocks, whose components are too small for the unassisted eye, the microscope is the only re- source, and with this as an aid much greater subdivision can be attained. The object here in view is to limit the discussion purely to the study without the microscope. The scheme of classification of the igneous rocks has three principles underlying it, viz : texture, mineralogical composition and chemical composition. The textures are five : glassy, felsitic, porphyritic, granitoid and fragmental, and the table is arranged from top to bottom so that they come in this order. The arrange- ment is adopted because it brings the glassy which are the simplest of all rocks at the outset, where they can be best taken up by the beginner. From top to bottom after the glassy rocks, the surface flows with their peculiar species come next, and then we pass through those with increasing proportions of phenocrysts, to the thoroughly granitoid. The word porphyry as a suffix has been K *il if II II l O X I 3 JtMll !3#S jliH I 1 I? S3 . 3 8 2.71 5- 56.45 20.08 I-3I 4-39 2.14 0.63 7-13 5.61 77 6. 46.11 14-75 2.20 4-51 7.82 5-73 3-84 1.29 59 2.904 7- 46.73 10.05 3-53 8.20 13.22 9.68 3-76 1.81 .24 I. Fourche Mtn. near Little Rock, Ark., J. F. Williams ; Igneous Rocks of Ark., 88. 2. Plauen, near Dresden, F. Zirkel, Pogg. Ann., CXXIL, 622. 3. Custer Co., Colo. Cross, Proc. Colo. Sci. Soc., 1887, 240. 4. Biella, Piedmont, Cossa., Turin Acad., ii., XVIII., 28. 5. Sodalite-syenite, Highwood Mtns., Mont., W. Lindgren, A. J. S., Apr., 1893, 296. 6. Minette, Rhode Island, badly decomposed, contained CO 4 7.32, Pirsson, A. J. S., Nov., 93, 375. 7. Shonkinite, Highwood Mtns., Mont., Weed and Pirsson, Bull. Geol. Soc. Amer., VI., 414. Comments on the Analyses. The syenites mark a decrease in SiO 2 from the granites and a general increase in all the bases. The high percentage of alkalies is especially worthy of remark, and the notably large amounts of soda, showing the passage to the nephe- line syenites. The parallelism with the trachytes is close. The last two analyses exhibit excessively basic extremes, whose theo- retical significance is commented on in the next paragraph. Figs. 1 4 and 1 5 are based respectively upon the average molec- ular ratios and the average percentages of the above analyses ex- cept Nos. 6 and 7. They show interesting contrasts with Figs. I o and 1 1 of the granites, being shortened right and left, and length- ened above and below. The figures are almost the same as those of the trachytes. Mineralogical Composition, Varieties. The name syenite was suggested by Syene, now Assuan, an Egyptian locality, where a hornblende granite was formerly obtained for obelisks, and if its local significance were perpetuated, syenite as formerly should be applied to this rock. But Werner used it in the last century for IGNEOUS ROCKS. 43 the well-known typical rock from the Plauenschen Grund (see Analysis 2), near Dresden, that contains almost no quartz, and of recent years this has been its correct use. Typical syenites have orthoclase and hornblende; those with biotite are called mica- syenites. Some plagioclase is always present and magnetite, apatite and zircon are invariable. When the plagioclase becomes equal in amount with the orthoclase the rocks are called monzonites and they mark a transition to the diorites. Mica syenites in dikes, FlGS. 14 AND 15. Diagrams illustrating the chemical composition of the syenites whose analyses appear in the above table. Fig. 14 is based on molecular ratios ; Fig. 15 on percentages. basic and of dark color, have been called minette. Orthoclase and augite afford augite-syenite. An excessively basic one (Analysis 7), from the Highwood Mountains, Mont, has recently been de- scribed by Weed and Pirsson under the name Shonkinite. It is of great theoretical importance, as it shows that orthoclase is not lim- ited to acidic rocks, but may be the prevailing feldspar in very basic ones. Still more recently J. P. Iddings has noted others of similar character from the region of the Yellowstone Park. (Jour, of Geology, December, 1895, 935.) Basic nephelite-syenites have been earlier known. Still the table on page 23 expresses the gen- eral truth, the exceptions being excessively rare rocks so far as yet known. Syenites are themselves rare rocks. With high soda, 44 A HAND BOOK OF ROCKS. the mineral sodalite develops and yields sodalite syenites which are passage forms to nephelite syenites. Relationships. Syenites are most closely allied with nephelite- syenites, into which with increase of soda they readily pass. They also with increasing plagioclase shade into diorites and the augite- syenites are closely akin to gabbros. Geological Occurrence. Syenites form irregular masses and dikes, precisely as do granites. Alteration. There is little to be said that was not covered un- der granite. The rarity of syenite- makes it a much less serious factor. In metamorphism they pass into gneisses. Distribution. Syenites occur in the great igneous complex of the White Mountains. They form large knobs and dikes near Little Rock, Ark., and a dike is known in Custer County, Colo. One of the few American minettes yet discovered is a dike on Conanicut Island, R. I., described by Pirsson (see Analysis 7). Abroad, syenites are better known. The Plauenscher Grund, near Dresden, Biella in the Piedmont, and the vicinity of Christiania, Norway, are the best known. Minettes are especially famous in connection with the mining district about Freiberg, Saxony, and in the Vosges mountains. THE PHONOLITE-NEPHELITE-SYENITE SERIES. THE PHONOLITES. SiO, A1 3 O, Fe 2 O s FeO CaO MgO K 2 O Na,0 Loss. Sp. Gr. If 61.08 18.71 I.9I 0.63 1.58 O.o8 4.63 8.68 2.21 2.582 2. 60.02 20.98 ^2.21 0.51 1.18 tr. 5-72 8.83 O.7O 2.576 3- 59.46 23.00 S.S 2 I.OO 0.50 4.90 7-13 0.71 4- 59-17 19-74 3-39 ... 0.92 0.15 6-45 8.88 1.18 2.566 5- 56.43 22.25 2.66 0.97 1.41 tr. 2.77 II. 12 2.05 2-54 6. 49.18 20.65 5-97 2.43 0.29 6.88 9-72 1. 60 2-553 7- 45.18 23.3I 6. ii ... 4.62 1.45 5-94 11.17 1.14 8. 44-50 22.96 6.84 8.65 1.65 4-83 6.70 2.06 I. Mato Tepee or Devil's Tower, near Black Hills, Wyo., Pirsson, A. J. S., May, 1894, 344. 2. El Paso Co., Colo., Cross, Proc. Col. Sci. Soc., 1887, 169. 3. Island of Fernando de Noronha, Brazil, Giimbel, Tscher. Mitt., 1880, II., 188. 4. Near Zittau, Saxony, v. Rath, Z. d. d. g. G., VIII., 297. 5. Wolf Rock, Cornwall, Eng., Phillips, Geol. Mag., VIII., 249. 6. Leucite-phonolite, near Rieden, Germany, Zirkel, Lehrbuch, II., 465. 7. Eleolite-porphyry, Beemerville, N. J., J. F. Kemp, N. Y. Acad. Sci., XL, 69. 8. Eleolite-porphyry, Magnet Cove, Ark., J. F. Williams, Igneous Rocks of Ark., 261. IGNEOUS ROCKS. 45 Comments on the Analyses. It is at once apparent from the analyses that the range in silica, except in the last two, is much like that of the trachytes, but that the alumina goes higher, and that the alkalies are in extremely large amounts. No other rocks, except the corresponding granitoid types, reach these amounts in alkalies. The soda which is necessary for the formation of the nepheline is naturally in excess. The rare leucite-phonolites, as a general thing, are more basic and show comparatively high potash. The last two analyses of intrusive or dike members are abnormally basic for phonolitic rocks. sro FlGS. 16 AND 17. Diagram illustrating the chemical composition of the phonolites whose analyses appear in the above table. Fig. 1 6 is based on molecular ratios ; Fig. 17 on percentages. Figs. 1 6 and 17 are based respectively upon the average molec- ular ratios and the average percentages of the first five analyses in the above table. In analyses 3 and 4 it was however necessary to make an adjustment of the percentage of Fe 2 O 3 with the FeO which had not been determined separately. The amounts are, however, in any event, so small as not appreciably to affect the diagrams. The pronounced development of the alkalies and alumina below the horizontal line comes out forcibly and furnishes interesting contrasts with the rhyolites. 4 6 A HAND BOOK OF ROCKS. General Description. The Phonolite Series embraces a group of rocks not often easy of identification without the microscope. They are rare and are seldom met by the field geologist or engi- neer. When they are found, however, they afford exceptionally interesting material for detailed study, and, inasmuch as they have been discovered in more recent years in association with some of our most productive gold deposits, they possess an impor- tance for the mining engineer which they formerly lacked. The rocks of the phonolite series are usually dense and finely crystalr line ; they are very seldom vesicular or even glassy. Dull green and gray are the common colors, but as they approach the trachytes they become lighter in shade. The light-colored min- erals are in excess, orthoclase being the most important single component and the only one which is usually large enough to be recognized by the eye alone. The nephelite is almost always too small to be visible without the microscope. Its easy gelatinization, however, makes it possible for the observer often to detect it by simple chemical tests. Thus a small sample of the rock in ques- tion is finely powdered and gently warmed in very dilute nitric acid. The nephelite passes readily into solution and when the liquid is decanted from the undissolved grains and is boiled down well toward dryness, gelatinous silica results. No other, common, rock-making and gelatinizing mineral is so easily soluble as nephe- lite, olivine alone approaching it. The commonest dark silicate in the phonolites is augite and its little dark glistening prisms may occasionally be recognized. Hornblende is very rare, and biotite is almost never seen. All these minerals are only visible when present as phenocrysts ; the components of the groundmass cannot be resolved by the eye alone. The fusing point of the phonolites is less than that of the -trachytes, being somewhat under 2000 F. (1090 C). The Phonolites proper are felsitic or slightly porphyritic rocks, which are not always easily to be distinguished from felsitic varie- ties of trachytes and andesites. They are, however, characteristic- ally dense, and as the rocks often have a peculiar and marked ten- dency to break up into thin slabs or plates, which ring musically under the hammer, they sometimes reveal themselves in this way. Fig. 1 8 reproduces a very striking outcrop of phonolite which FIG. 1 8. View of an exposure of platy phonolite, Sugar Loaf Mountain, Black Hills, S. D. J. D. Irving, Annals N. Y. Acad. Sci., XIV., PI. IX., 1899. IGNEOUS ROCKS. 47 shows this property. The chemical test mentioned above should always be used in corroboration before the identification is positively made. The phonolites proper are found in surface flows and dikes. The Phonolite-porphyries result when the phenocrysts become notably abundant. The phenocrysts are then chiefly orthoclase with a few augites, and perhaps with an occasional titanite. Nephelite in porphyritic crystals is known from a few localities but is seldom seen. The phonolite-porphyries occur in dikes and in- truded sheets. When the phenocrysts constitute the greater part of the rock the Nephelite-syenite porphyries are developed. They are extremely rare rocks and mark the passage to the nephelite syenites. Synonyms and Relatives. The name phonolite is an old one. It was given by Klaproth in 1 80 1 to the rocks which had long been called clinkstone and was merely the Greek equivalent of this colloquial term. Phonolite was formerly restricted to Tertiary and later eruptives but no time distinction is longer implied when it is used, although as a matter of fact most of the known phonolites belong to this portion of geological time. The phonolites are closely related to the trachytes, but they have more soda and alumina and at the same time not enough silica to form albite. Thus as the silica rises there comes a time when albite can absorb all the soda and then nephelite becomes an impossibility. All the more can nephelite never appear with original quartz, because quartz itself is an impossibility until all the albite possible has been produced. The abundant soda in the phonolite magma occasions the frequent production of noselite and hauynite, but they can seldom be detected with the eye alone. For the same reason the dark green, acicular, soda pyroxene segirite frequently takes the place of the augite and in the groundmass may constitute a perfect felt of little needles. This variety of phonolite is called tinguaite, but it also cannot be readily determined by the unassisted eye. Apachite, gieseckite-porphyry, liebnerite-porphyry, and sussexite are rocks related to the phonolites and will be found defined in the Glossary. With the increase of orthoclase and the decrease of nephelite the phonolite series passes into the trachytes with which they are in all respects closely akin. With the increaae of plagioclase and 4 8 A HAND BOOK OF ROCKS. the dark silicates they pass in the opposite direction into certain basaltic rocks with nephelite. The leucite rocks of trachytic affinities constitute a rare and minor group of the phonolite series from which they might with propriety be separated to form a series of their own. They are, however, so rare that they are only mentioned here under the phonolites. When the potash in the magma becomes relatively rich and the silica so poor that there is more than enough of the former and too little of the latter to yield orthoclase, leucite be- comes a possibility. Hence it follows that leucite and orthoclase usually go together and that leucite is sometimes found with nephelite, but as soon as the silica becomes abundant enough to combine with all the potash and its attendant alumina, to yield orthoclase, leucite is an impossibility. All the more do we thus never find leucite with original quartz. Felsitic or porphyritic rocks with leucite, orthoclase and augite or some related dark silicate are usually called leucite trachyte. If to this aggregate nephelite be added leucite-phonolite results. The related rocks leucitophyre, orendite and wyomingite will be found defined in the Glossary. Alterations. The nephelite changes quite readily to natrolite and perhaps analcite, while leucite yields analcite. Metamorphic processes are yet to be studied. Distribution. The true volcanic phonolites are only known in a few localities in this country, such as the Black Hills, where they form dikes, sheets and isolated buttes (Devil's Tower), and the Cripple Creek mining district of Colorado, where the comparatively few dikes known have proved of great importance as associates of the ores. Nephelite- or eleolite-porphyries (tinguaites) are exceedingly rare rocks and have been found near Magnet Cove, Ark., and Beem- erville, N. J., associated with nephelite-syenite. Phonolites are much more abundant abroad, being well known in many parts of Germany. The varieties with leucite are especially familiar from the vicinity of Rieden, in the extinct volcanic district of the Eifel. A peculiar leucite rock, with abundant scales of phlogopite, gives the name to the Leucite Hills, two or three miles north of Point of Rocks, Wyo. Leucite tinguaites occur near Magnet Cove, Ark., in the High wood Mountains, Mont., and near Rio Janeiro, Brazil. IGNEOUS ROCKS. 49 Tuffs are known abroad but not in this country, and exhibit few features calling for special mention. THE NEPHELITE SYENITES. Sp. Gr. SiO, A1 2 O 3 Fe a O 3 FeO CaO MgO K,O Na,0 Loss. I. 60.39 22.51 0.42 2.26 0.32 0.13 4-77 8.44 o-57 2. 59-70 18.85 4.85 1-34 0.68 5-97 6.29 1.88 3- 59.01 18.18 I-6 3 3-65 2.40 1.05 5-34 7-03 0.50 4- 56.30 24.14 1.99 0.69 0.13 6.79 9.28 1.58 5- 54.20 21.74 0.46 2.36 i-95 0.52 6.97 8.6 9 6. 52-75 22.55 3.65 1-85 0.15 7-05 8.10 3.6o 7- 5I-90 22.54 4-03 3-5 3-" 1.97 4.72 8.18 0.22 8. 50.96 19.67 7.76 4-38 0.36 6.77 7.67 1.38 9- 50.36 19-34 6.94 3-43 7.17 7.64 3-51 10. 41-37 16.25 16.93 12.35 4-57 3.98 4.18 0-45 I. So-called Nephelite-syenite, or Litchfieldite, Litchfield, Me., W. S. Bayley, G. S. A., III., 241. 2. Nephelite-syenite, Fourche Mountains, Ark., J. F. Williams, Igneous Rocks of Ark., 88. 3. Nephelite-syenite, Red Mountains, N. H., W. S. Bayley, G. S. A., III., 250. 4. Ditroite, Hungary, Fellner, Neues Jahrb., 1868, 83. 5. Foyaite, Portugal, Jannasch, Neues Jahrb., II., II. 6. Nephelite-syenite, Sao Paulo, Brazil, Machado, Tsch. Mitt., IX., 1888, 334. 7. Laurdalite, variety of Nephelite-syenite. Lund, Norway, Brogger, Syenit-pegmatit-gange, 33. 8. Leucite- syenite, Arkansas, J. F. Williams. Igneous Rocks of Ark., 276. 9. Nephelite-syenite, Beemerville, N. J., F. W. Love for J. F. K., N. Y. Acad. Sci., XL, 66. 10. Basic Nephelite-syenite, Beemerville, N. J., J. F. Kemp., N. Y. Acad. Sci., XL, 86. Comments on the Analyses. A considerable range is shown in the SiO 2 , some analyses going below the usual percentages for syenites and the last analysis being abnormal. In general the amounts of alkalies are extremely high, with Na 2 O in excess, in which respect the phonolites are paralleled. Figs. 19 and 20 are based respectively upon the average molec- ular ratios and the average percentages of the first seven analyses in the above table. In Nos. 2, 4, and 6 it has been necessary to adjust the undetermined percentages of FeO, but the error, if one is introduced, is not great in any event. The figures closely re- semble those of the phonolites and present the same general pecu- liarities. Mineralogical Composition and Varieties. The minerals of neph- elite-syenite are in general the same as those of syenite proper, with the addition of nephelite, often sodalite, and several charac- teristic ones into which the rare earths enter as bases. Zircon is widespread and is often large enough to afford fine crystals. 4 50 A HAND BOOK OF ROCKS. For this reason the rocks were named zircon-syenite many years ago. The nephelite is often called eleolite (or elaeolite), from the former custom of speaking of this mineral in pre-Tertiary rocks as eleolite and in later ones as nephelite, just as we have had ortho- clase and sanidine, but the custom is gradually falling into disuse. Attempts have been made to give different names according to the dark silicate ; for instance, those with hornblende were called foyaite, from Foya, a Portuguese locality ; those with biotite, miascite from Miask, in the Urals. But both these minerals so often appear to- gether or with pyroxene that the practice is not generally observed, Ditroite is a variety rich in blue sodalite. The Litchfield, Maine, FIGS. 19 AND 20. Diagrams illustrating the chemical composition of the nephelite- syenites which appear in the above table. Fig. 19 is based on molecular ratios ; Fig. 20 on percentages. rock has been shown by Bayley to have as its feldspar albite al- most exclusively, and he, therefore, has called it litchfieldite. The texture of nephelite-syenites varies very much. At times it is very coarsely granitoid, and again it is what is called trachytic, i. e., with little rods of feldspar, more or less in flow lines, like a trachyte and marking a passage to the phonolites. Types have been based on these characters. Where at all finely crystalline, the determination of nephelite-syenites, as against true syenites, is a matter for the IGNEOUS ROCKS. 51 microscope. Nephelite-syenites are comparatively rare rocks. Corresponding rocks with leucite are as yet only known from Arkansas and Montana. Relationships. As already remarked, the nephelite-syenites are closely related to the true syenites, and to the phonolites. With certain basic plagioclase rocks with nephelite, called theralites, they are also of near kinship. Geological Occurrence, Alteration. The nephelite-syenites are specially prone to appear as dikes, often on a very large scale. Their alteration affords no special features, as distinguished from the syenites or granites, except as regards the secondary minerals from the nephelite. Natrolite, muscovite and kaolin are all known in this relation and the last two have been called liebenerite and gieseckite. Cancrinite also results from the alteration of nephelite. The rarity of the nephelite-syenites has prevented their playing an important role among metamorphosed rocks. Distribution. Nephelite-syenites are known in North America at Montreal and Dungannon, Ont. ; Litchfield, Me. ; Red Hill, N. H. ; Salem, Mass. ; Beemerville, N. J., where a superb dike is exposed ; near Little Rock, Ark., where the area is extensive ; in the San Carlos Mountains, Tamaulipas, Mexico, and at several less well known localities. Very interesting ones occur near Rio Janeiro, and in the State of Sao Paulo, Brazil. Abroad the Portu- guese locality, in the Monchique Mountains ; the one at Ditro, in Hungary, and the wonderful dikes near Christiania, in Norway, so prolific in rare minerals, are of especial interest. FIG. 21. Diagrams illustrating the chemical composition of Wyomingite, a leucite rock. The upper is based on molecular ratios ; the lower on percentages. CHAPTER IV. THE IGNEOUS ROCKS, CONTINUED. THE DACITE-QUARTZ-DIORITE SERIES AND THE ANDESITE-DIORITE SERIES. THE DACITE-QUARTZ-DIORITE SERIES. THE DACITES. Sp. Gr. SiO, A1 2 O, Fe,O, FeO CaO IgO Na,O K a O Loss. I. 69.96 15-79 2.50 1-73 < 3.64 3-80 4.12 i-53 2. 69.36 16.23 0.88 1-53 3-17 34 4.06 3-02 0.45 3- 67.49 16.18 1.30 1.22 2.68 34 4-37 2.40 2.69 4- 67.2 17.0 3-5 1.2 4.5 5 3-7 1.6 0.9 5- 67.03 16.27 3-97 3-42 19 2.71 3-50 1.56 6. 66.03 14-57 2-57 1.19 3.38 .89 3-7i 2.70 2.07 7- 63.36 16.35 2.12 3-05 4-79 . J.28 3-58 2.92 0.99 I. McClelland Peak, near Comstock Lode, Nev., F. A. Gooch, Bull. 17, U. S. G. S., 33. 2. Lassen's Peak, California, Hague and Iddings, A. J. S., Sept., 1883, 232. 3. Sepulchre Mountain, Yellowstone Park, J. P. Iddings, Phil. Soc. Wash., XL, 210. 4. Nagy-Sebes, Hungary, Doelter, Tscher. Min. Petr. Mitt., 1873, 93. 5. Eureka Dist., Nev. A. Hague, Mono. XX., U. S. G. S., 264. 6-7. Colombia, S. America, From Kiich's Petrographie of Colombian Volcanoes, quoted in Jour. Geol., I., 171. Comments on the Analyses. It appears at once from the analyses that the dacites are high in silica, in which they equal the lower ranges of rhyolites. As compared with the latter, soda is prevail- ingly in excess of potash, and as a rule the other bases run higher and especially the lime. The diagrams in Figs. 22 and 23, show considerable similarity with those of the rhyolites, but on close comparison it will appear that in the former, the soda is in marked excess over the potash and both the lime and the magnesia are represented by longer intercepts. General Description. The Dacites embrace a group of rocks which so strongly resemble the rhyolites as often to make it dif- ficult, if not impossible, for an observer to positively identify them as against the latter. The light-colored minerals are the ones which give character to the group. Quartz and feldspar are the prominent components and the prevailing feldspar is plagioclase, and one of the more acidic varieties. Biotite is perhaps the most 52 IGNEOUS ROCKS. 53 common of the dark silicates but both hornblende and augite are frequent. The minor accessories, apatite, zircon, magnetite, etc., are seldom visible to the eye. The prevailing colors are light FlGS. 22 AND 23. Diagrams illustrating the chemical composition of the dacites which are given in the above analyses. Fig. 22 is based on molecular ratios ; Fig. 23 on percentages. grays, yellows and pale reds. The fusing point is perhaps slightly less than that of the rhyolites. Glasses and cellular textures are not uncommon. The textures of the dacites range from felsitic to coarsely por- phyritic. The Dacites proper are felsitic to moderately porphyritic rocks, sometimes cellular from their crystallization as surface flows. When finely felsitic their components cannot be distinguished and recognized with the eye alone, and then the microscope is the sole resource for accurate determination. They can otherwise only be called felsites. When, however, the phenocrysts become prominent the only possible question is between dacites and rhyolites, for these are the only two with quartz in this relation. The observer must then study the cleavage faces of the feldspars with a good lense, and if the greater number of these display the striations peculiar to plagioclase the identification of the dacites can be satisfactorily made. When the phenocrysts become abundant the dacites proper pass into the Dacite-porphyries. The groundmass is, as a rule, felsitic. The cellular texture disappears entirely and the rocks become dense and characteristically porphyritic types. The interiors of 54 A HAND BOOK OF ROCKS. thick surface flows, the dikes, intrusive sheets and the outer parts of laccoliths are their special homes. When phenqcrysts are in marked excess over the groundmass and constitute the greater part of the rock, the Quartz-diorite porphyries result. The groundmass is rather coarsely felsitic and becomes increasingly coarse as the quartz diorites are approached. All the minerals forming the phenocrysts are now not difficult to recognize. The quartz-diorite porphyries are met in deep-seated dikes, thick intrusive sheets, and in the central parts of laccoliths. They mark a textural transition to the quartz diorites. Synonyms and Relatives. The name dacite was created in 1 863 by an Austrian geologist, G. Stache, who had been working upon the eruptives of the old Roman province of Dacia, now in the dis- trict of Hungary known as the Siebenbiirgen. Under it was em- braced a series of rocks somewhat indefinitely called by earlier lithologists andesitic quartz-trachytes, and other undesirable names. The name dacite has proved to be a useful one and is quite universally employed to-day. The dacites were originally considered to be necessarily Tertiary or later in geological age but now no time restriction is applied to them. Varieties are some- times made on the basis of the dark silicate present such as mica- dacite, hornblende-dacite or augite-dacite. The dacites are close relatives of the andesites into which they pass with increasing basicity, and with the disappearance of quartz. They are also very closely akin to the rhy elites and to those passage rocks from rhyolites to dacites, called pantellerites and keratophyres, which are defined in the Glossary. Quartz-porphyrite is an old synonym of dacite porphyry. Alteration, Metamorphism. The alteration of the dacites is practically like that of the rhyolites, but the greater abundance of soda-lime feldspar may yield a trifle more calcite. The light- colored silicates change to kaolin. In metamorphism the dacites yield siliceous schists especially when greatly mashed or sheared. Tuffs. The tuffs and breccias are essentially like those of the rhyolites. From them on account of the almost universal advance of alteration they cannot readily be distinguished without the microscope and even then the sharp determination may present great difficulties. IGNEOUS ROCKS. 55 Distribution. Dacites usually appear as subordinate members in eruptive regions where the andesites are the chief rocks. They are therefore widespread in the volcanic districts of the Cordilleran region and of Central and South America. THE QuARTZ-DlORITES. SiO, A1 3 O, Fe 3 3 FeO CaO MgO Na,0 K 2 Loss. I. 70.36 15-47 0.98 I.I7 3.18 0.87 4.91 1.71 1. 06 2. 67.54 17.02 2.97 0-34 2.94 !$ 4.62 2.28 0-55 3- 65.27 I5.76 1.36 3-44 2.14 4-57 3-97 ... 0.42 4- 63.97 I5-78 2-35 1.87 3.71 2.84 4.36 4.01 0.58 5- 62.43 17.88 1.78 3-53 3-43 4-50 3.10 2-75 1-37 I. Quartz-diorite, Enterprise, Butte Co., Calif., H. W. Turner, Anals. by W. F. Hillebrand, 14 Ann. Rep. U. S. Geol. Survey, 482, 1894. 2. Quartz-mica-diorite, Electric Peak, Yellowstone Park, J. P. Iddings, Anal, by Whitefield, Bull. Phil. Soc. of Washington, II., 206. 3. Quartz-augite-diorite, Watab, Minn., A. Streng, Neues Jahrbuch, 1877, 232. 4. Quartz-mica-diorite, Crandall Basin, Wyo., J. P. Iddings, Mono. 32, U. S. G. S., p. 261, W. H. Melville, Analyst. 5. Quartz-mica-diorite, Omeo, Viet, A. W. Howitt, Trans. Roy. Soc. Viet, XXII., 99. Comments on the Analyses. The quartz-diorites, although acidic rocks, do not have as high percentages of silica as the granites, but in lime and soda they range slightly higher on ac- count of the prevailing plagioclase. In general they strongly resemble the granites and diagrams based on the above analyses would hardly differ from those of the granites given earlier. Mineralogical Composition, Varieties. The quartz-diorites are granitoid rocks whose chief feldspar is plagioclase and which con- tain also quartz as an essential component. The dark silicates are hornblende and biotite, one or both. The light-colored minerals are in excess over the dark ones, but this relationship is less pro- nounced in the more basic varieties. The typical mineralogical aggregate contains hornblende. When biotite is the chief, dark silicate the rocks are called quartz-mica-diorites. The fusing point of the rocks is a shade less than that of the granites. From all other rocks except granites the quartz-diorites are distinguished by their granitoid texture and quartz. From the granites the prevalence of striated feldspar is the chief distinction. Relationships. The quartz-diorites are close relatives of the granites on the one hand and of the diorites on the other. To the former group an easy transition is afforded by the grano- 5 6 A HAND BOOK OF ROCKS. diorites, while the so-called quartz-monzonites mark a transition to the syenites. Tonalite and adamellite will be found denned in the Glossary. Geological Occurrence. The quartz-diorites form batholiths, dikes and local developments of diorites. Alteration. The alteration is in all essentials similar to that of granite. Distribution. Quartz-diorites occasionally appear in the eastern areas of crystalline rocks. A famous one with mica is an important member of the Cortlandt series of eruptives near Peekskill, N. Y. Others with hornblende are known in the Yellowstone Park and in the Sierras. THE ANDESITE-DIORITE SERIES. THE ANDESITES. SiO, A1 2 O S Fe,O s FeO CaO MgO Na 2 K 2 O Loss I. 67.83 15.02 ... 5.16 3-07 0.29 2.40 3.20 I. II 2. 65.50 14.94 1.72 2.27 2-33 2.97 5.46 2.76 1-37 3- 63-49 18.40 2.44 1.09 2.30 0.66 5-70 4.62 1.04 * 62.94 18.14 3-82 6.28 3-o6 3.83 .22 0.60 5. 61.62 1 6. 86 6.61 6-57 2.07 3-93 .66 6. 61.58 16.34 6.42 5-i3 2.85 2.69 . 65 0.64 7- 59.48 16.37 3 .2I 3-17 4.88 3-29 3-30 .81 2.02 8. 56.19 16. 12 4.92 4-43 6-99 4.60 2.96 37 1.03 9- 56.91 18.18 4-65 3-6i 7.11 3-49 4.02 .61 0.36 I. Hb. -mica-andesite, Eureka Dist., Nev , Mono. XX., U S. G. S., 264. 2. Hb.- mica-andesite, Sepulchre Mountain, Yellowstone Park, J. P. Iddings, Phil. Soc. Wash., XI , 210. 3. Mica-andesite, Rosita Hills, Colo., W. Cross, Colo. Sci. Soc., 1887, 250. 4. Lassen's Peak, Calif., Hague and Iddings, A. J. S., Sept., 1883,225. 5. Mt. Rainier. See last reference. 6. Pyroxene-andesite, Eureka Dist., Nev., Mono. XX., U. S. G. S., 264. 7. Hypersthene-andesite, near Red Bluff, Mont, G. P. Merrill, Proc. U. S. Nat'l Museum, XVII., 651. 8. Hypersthene- andesite, Buffalo Peaks, Colo., W. Cross, Bull. I., U. S. G. S., 26. 9. Colombia, S. America. Comments on the Analyses. It appears at once from the analyses that the andesites lap over the lower limits of the dacites and have much the same range in silica as the trachytes. All the bases reach notable percentages, but the alkalies recede as the others increase. As compared with both dacites and trachytes these contrasts are well, brought out in the diagrams, Figs. 24 and 25. The in- tercepts of silica shorten, whereas those of lime and magnesia not- ably lengthen. IGNEOUS ROCKS. 57 General Description. The Andesite series embraces a large and wide-spread group of rocks, which marks an important step from the more acidic to the more basic limits of the igneous types. Its members are emphatically rocks of medium acidity, with the light- colored minerals still in excess over the dark-colored ones. The feldspars are therefore the most prominent components but there is a marked increase in the ferromagnesian silicates as compared with the dacites. Quartz fails except as a rare and sporadic com- ponent. Hornblende and augite begin to take precedence over CaO Sid.,' 1 FIGS. 24 AND 25. Diagrams illustrating the chemical composition of the andesites, based on the above analyses. Fig. 24 refers to molecular ratios ; Fig. 25 to per- centages. biotite, but all three are common. The prevailing colors are grays or greens, mottled by the light and dark phenocrysts. The andesites have fusing points near 2000 F. (1100 C). They rarely afford large amounts of glasses. The textures of the Andesite series range from felsitic to coarsely porphyritic. The Andesites proper are felsitic or moderately por- phyritic rocks, sometimes cellular from their crystallization as surface flows. When finely felsitic they cannot be readily distin- guished from the trachytes and even from the dacites and rhyolites of the same texture, although they are usually provided with more of the dark silicates than are the last two. For sharp determina- tion recourse must be had to the microscope, but it is fair to men- 5 8 A HAND BOOK OF ROCKS. tion that andesites are much more abundant rocks in Nature than are trachytes, so that in doubtful cases the chances strongly favor the former. When examined with the microscope the finely crystalline groundmasses of the andesites are often found to be a fine felt of little rods of feldspar, giving a texture that is fairly characteristic of this group. With the development of phenocrysts the exact determination of the andesites becomes less difficult. Quartz fails and the feld- spars constitute the more prominent porphyritic crystals. The cleavage faces of the latter should then be examined with a lense and if the greater number exhibit the characteristic striations of plagioclase the andesites may be recognized as against the tra- chytes. This determination may be further fortified by the fre- quent greater prominence of the dark silicates. The andesites proper occur characteristically in surface flows. As the phenocrysts become abundant the andesites proper pass into the Andesite-porphyries. The groundmass is, as a rule, fel- sitic. The cellular texture disappears entirely and the rocks are dense and markedly porphyritic. The interiors of thick surface flows, the dikes and intrusive sheets and the outer parts of lacco- liths are their special home. When phenocrysts are in marked excess over the groundmass and constitute the greater part of the rock the Diorite-porphyries result. The groundmass is rather coarsely felsitic and becomes increasingly so as the diorites are approached. All the minerals forming the phenocrysts are now not difficult to recognize. The diorite-porphyries occur in deep-seated dikes, thick intrusive sheets and the central parts of laccoliths. They mark a textural transi- tion to the diorites. Synonyms and Relatives. The name andesite was first proposed by L. von Buch in 1835 for certain lavas from the Andes Moun- tains which consisted of albite and hornblende, and which there- fore differed from trachyte in the old sense. The name did not come into general use until 1858 since which time it has been quite universally employed for the porphyritic and felsitic plagio- clase-bearing eruptives of medium acidity. For a time it was re- stricted to the Tertiary and later rocks but this limitation is no longer current and textural features are alone emphasized. Ande- IGNEOUS ROCKS. 59 sites whose chief dark silicate is biotite are called mica-andesites ; those with hornblende, hornblende- or amphibole-andesites ; while those with augite are known as augite-andesites. Hypersthene- andesites are occasionally met and result when the magma is rich in magnesia. The augite-andesites differ from the olivine-free basalts in having the light-colored minerals in excess. While the time-distinction was still preserved in the classification of igneous rocks, the pre -Tertiary andesites were called by some porphyrite, to which name the several prefixes, mica, hornblende and augite were attached. Later porphyrite was employed for the deep-seated or intrusive andesites, which are here called andesite- porphyry and diorite -porphyry, but even this use is practically obsolete as it is certainly unnecessary. Other rock-names more or less closely related to andesite, such as asperite, propylite, volcanite and latite will be found defined in the Glossary. Andesites, with the increase of orthoclase and the corresponding decrease of plagioclase, pass into the trachytes ; and with the increase of the dark silicates and corresponding decrease of feldspar they shade into the basalts. The appearance of quart? in notable amounts marks a transition to the dacites. Practic- ally unbroken series can easily be selected to all these related groups. Alteration, Metamorphism. The andesites in decay afford kaoli- nized material and mixtures of this with chloritic products that are very difficult to identify. Thus the now famous andesitic breccia at Cripple Creek, Colo., can rarely be shown to the eye to be other than a white, kaolinized mass, and decomposed outcrops of massive flows are no less unsatisfactory. Where metamorphic processes affect older flows, felsitic and silicified forms result similar to those mentioned under rhyolites. The tracing of the history of the rock is then a matter for the microscope and chemical analysis when indeed it can be done. Tuffs. Andesitic tuffs and breccias (i. e., aggregates of angular, volcanic ejectments coarser than tuffs) are rather common in the western volcanic districts. With ordinary observation they can only be identified by finding fragments large and fresh enough to indicate the original. Such have proved of great economic im- portance at Cripple Creek, Colorado. 60 A HAND BOOK OF ROCKS. Distribution. Andesites are very wide-spread in the West. The vast laccoliths that form many of the peaks in Colorado are intruded andesite-porphyries of a rather acidic type, frequently with some orthoclase. In the Yellowstone Park they are impor- tant. In Nevada, as at Eureka and the Comstock lode, they have proved of great geological interest, and especially in and near the latter, with its many miles of drifts, shafts and tunnels, very impor- tant data for the study of rock masses have been afforded. The old cones along the Pacific, Mt. Hood, Mt Shasta, Mt. Rainier and others are chiefly andesite. The products of Mexican and South American volcanoes are also of this type, and indeed along the whole Pacific border the recent lavas have many features in common. Abroad andesites are seldom lacking in great volcanic districts. THE DIORITES. SiO, Al a 3 Fe,0. FeO CaO MgO Na,O K a O Loss. Sp. Gr. 1. 61.75 18.88 0.52 3.52 3.54 1.90 3.67 1.24 4.46 2.79 2. 58.05 18.00 2.49 4.56 6.17 3.55 3.64 2.18 086 3. 56.71 18.36 ... 6.45 6.ii 3.92 3.52 2.38 ... 2.86 4- 52.35 15-72 2.90 7.32 8.98 7.36 2.81 1.32 1.35 5. 50.47 18.73 -M9 4-9 2 8 - 82 3-48 4-62 3.56 0.58 2.87 6. 48.98 17.76 2.14 6.52 8.36 2.09 6.77 2.08 4.50 7. 48.19 16.79 18.37 ... 6.85 1.32 5.59 1. 1 1 2.31 I. Diorite. Pen-maen-mawr, Wales, J. A. Phillips, Q. J. G. S., XXXIII., 424, 1877. 2. Diorite, Electric Peak, Yellowstone Park, J. P. Iddings, Bull. Phil. Soc. Washington, II., 206. 3. Diorite (granitoid andesite?), Comstock Lode, Nev , R. W. Woodward, 4Oth Par Survey, I., opp. p. 676. 4. Augite-diorite, Little Falls, Minn., A. Streng, Neues Jahrb., 1877, 129. 5. Augite-diorite, Mt. Fairview, Custer Co., Colo., W. Cross, Anal, by Eakins, Col. Sci. Soc., 1887, 247. 6. Porphyritic-diorite, St. John, N. B., W. D. Matthew, Trans. N. Y. Acad. Sci., XIV., 213. 7. Diorite dike rich in magnetite, Forest of Dean Mine, N. Y., J. F. Kemp, A. J. S., Apr., 1888, 33 l. Comments on the Analyses. As regards silica the analyses be- gin where those of the quartz-diorites leave off, and extend to lower limits than those of the andesites. The last three are indeed more basic than is typical of the diorites. The bases, iron, lime and magnesia show a marked increase, but in the typical cases do not yet reach the figures of the basaltic rocks which follow. Soda is in excess over potash as would follow from the prevalence of plagioclase. Figs. 26 and 27 are of special interest when compared with those of the andesites which they greatly resemble. The diorites IGNEOUS ROCKS. 61 show a slight increase in the intercepts above the horizontal, a slight decrease in silica and in potash, but in other respects they are, as they ought to be, very much the same as Figs. 24 and 25. FIGS. 26 AND 27. Diagrams illustrating the chemical composition of the diorites, which are given in the above analyses. Fig. 26 is based on molecular ratios ; Fig. 27 on percentages. Mineralogical Composition. The diorites are granitoid igneous rocks, whose chief feldspar is plagioclase and whose chief dark silicate is either hornblende or biotite. Those with hornblende are called simply diorites ; those with biotite, mica-diorites. Some augite is often present, marking a passage to the gabbros and giv- ing the rock the name augite-diorite. It is however a matter of much difficulty to distinguish hornblende from augite with the eye alone, and unless the observer can make certain of the cleavages approximately 120 for hornblende, and 90 for augite doubt may arise. In the typical diorites the feldspars are in excess over the dark silicates and contrasts are thus afforded with the typical gabbros, but the name diorite in ordinary use is often applied to rocks with a decided excess of hornblende. Additional difficulty in the sharp application of the word arises because under the influ- ence of metamorphism original augite, as for instance in a gabbro, changes readily in whole or in part to hornblende, and a mineral- 62 A HAND BOOK OF ROCKS, ogical aggregate thus results which corresponds to diorite, yet which did not crystallize directly in this form. When working with the microscope the observer can follow out these changes, but when depending on the eye alone, it is necessary to base the determination on the minerals as we find them. Magnetite, titanite and apatite are almost always present as accessory minerals in the diorites, but are usually too small to be seen. Garnet is not infrequent, and pyrrhotite at times is in con- siderable amount. The name diorite was first applied in 1822 by the Abbe Haiiy. It is derived from the Greek verb, to distinguish, and was sug- gested by the fact that in the rocks first named the white feldspar could be easily distinguished from the black hornblende. In the course of time it became a very widely used field name among geologists and miners. Varieties. The varieties mica-diorite and augite-diorite have already been defined. A dioritic rock occurring in dikes and con- taining both hornblende and biotite has been named kersantite. Camptonite is applied to a rock, found in dikes which are often met in close association with nephelite-syenite and which have the composition of hornblende-diorite. Additional details regarding both these as well as augite-diorite and definitions of banatite, vogesite, and kersanton will be found in the Glossary. Alteration, Metamorphism. In ordinary alteration the feldspar of diorites kaolinizes and the hornblende changes to chlorite, affording one of the varieties of the so-called greenstones. Under shearing stresses in metamorphism the diorites pass into gneisses, and into hornblende schists or amphibolites. In many mining regions even decided schistose varieties are still called diorite. A final stage is chlorite-schist, wherein the hornblende has altered to chlorite. Distribution. True, original diorites are not very common rocks in America. In the Sudbury nickel district, north of Lake Huron, dense, dark diorites are the chief rock containing the ore, but there is always the possibility that the hornblende is altered augite. Mt. Davidson, above the Comstock Lode, is either a true diorite or a granitoid phase of andesite. Authorities differ as to its interpretation. Diorites are well known abroad and have been described from various places in Great Britain, Germany, France and Austria. CHAPTER V. THE IGNEOUS ROCKS, CONTINUED. THE BASALT-GABBRO SERIES. THE FELDSPAR-FREE BASALTS. THE PYROXENITES AND PERIDOTITES. THE ULTRA-BASIC ROCKS. THE BASALT-GABBRO SERIES. THE BASALTS. Basalts. SiO, A1 2 O 3 Fe 2 3 FeO CaO MgO Na 2 O K 2 O Loss. Sp. Gr. I. 57-25 16.45 1.67 1.77 7.65 6-74 3.00 1-57 0-45 2. 53-8i I3.48 3.02 7-39 10.34 6.46 3-23 0.64 0-57 2-75 3- 53-62 22.09 4.21 ... 6. 02 6.24 3-16 0-57 5-3 4- 52.27 17.68 2.51 5.00 8-39 6.05 4.19 1-58 0.82 5- 51-58 11.92 2. 9 6 13-05 8.52 4.09 o-95 0-34 1.52 2.989 6. 50.38 19-83 6.05 2.OO 10.03 5.36 2-15 1.76 1-37 7- 49-45 17.58 3-41 3-41 7.20 4.05 5-83 i-57 4-34 8. 49.04 1 8. ii 2.71 7.70 7.11 4.72 4.22 2. II 1.29 2.738 9- 48.40 17-95 2.28 8.85 10.05 6.99 2.86 1.03 0-34 2.8 10. 47-54 19-52 4.24 6-95 11.70 6.66 3-09 o. 16 2.981 ii. 46.43 17.10 ii. 16 10.38 9.78 2.50 2.65 I. Basalt with quartz, Cinder Cone, Calif., J. S. Diller, A. J. S., Jan., 1887, p. 49, Anal. Hillebrand. 2. Kilauea, Sandwich Is.; Cohen., Neues Jahrb., 1880, II., 41. 3. Iceland, Schirlitz, Tsch. Mitth., 1882, 440. 4. Rio Grande Canon, N. M., J. P. Iddings, A. J. S., Sept., 1888, 220, Anal. Eakins. 5. Dalles, Oregon, Lemberg, Z. d. d. g. G., XXXV., 1 1 6. 6. Richmond Mtn., Eureka Dist., Nev., A. Hague, Mono. XX., U. S. G. S., 264, Anal. Whitfield. 7. Point Bonita, Calif, F. L. Ransome, Bull. Geol. Dept. Univ. Calif., L, 106. 8. Buffalo Peaks, North Park, Colo., Woodward, 4Oth Parallel Surv., II., 126. 9. Shoshone Mesa, Nev., Woodward, 4Oth Par. Surv., II., 617. 10. Cascade Mts., Oregon, Jannasch, Tsch. Mitth., 1881, 102. ii. Glassy basalt, Edgecombe Island, near Sitka, Alaska, Lemberg, Z. d. d. g. G., XXXV., 570. Comments. The first analysis is very like the more basic ande- sites, except in its high percentage of MgO. It is of a curious and exceptional basalt. with quartz phenocrysts, regarding which men- tion is made later. In general, the others are notably high in the oxides of iron, in CaO and MgO. The alkalies wane because of the increasing inferiority of the feldspars, which give place to augite and olivine. The specific gravity is high. The diagrams, Figs. 28 and 29, present interesting contrasts with all which have preceded. The intercepts for silica have 63 6 4 A HAND BOOK OF ROCKS. drawn in, because of the increasing basicity. The alumina has de- cidedly shrunk as have the alkalies, indicating the waning amounts of the feldspars. The iron, lime and magnesia on the contrary are much greater, and thus emphasize the increase of the dark silicates. FIGS. 28 AND 29. Diagrams illustrating the chemical composition of the basalts, which are given in the above table of analyses. Fig. 28 is based on molecular ratios ; Fig. 29 on percentages. General Description. The Basalt series marks a decided step toward the basic extreme of the igneous rocks. The dark-colored silicates are now in excess and give the chief characters to the rocks. Augite and olivine are the ones of greatest importance, hornblende being very rare, and biotite scarcely known. The feldspars appear as the more basic plagioclases, labradorite to anorthite. Magnetite is a prominent component although usually too small to be recognized by the eye alone. The prevailing colors of the basalts are dark grays and blacks. When red or green the color is due to the advance of alteration. The fusing points are comparatively low, being in the neighborhood of 1940 F. (1060 C). Glassy varieties are rare, although slaggy crusts and scorias are not unusual. The tendency to crystallize has been marked and irresistible during the process of cooling. IGNEOUS ROCKS. 65 The textures are much more commonly felsitic than with the other groups, but porphyritic varieties are often met. The Basalts proper are felsitic, or, rather rarely, somewhat porphyritic rocks, and often cellular because of their crystallization as surface flows. The crusts upon these streams of basic lava may be very scori- aceous because of the free expansion of the dissolved vapors but from a point not far below the surface and thence to the interior, the rock becomes increasingly dense. Such cellular development as has taken place is then manifested in scattered, rounded or almond-shaped cavities called amygdaloids from the Greek word for an almond, whose outlines they closely resemble. These cavities frequently become filled with secondary calcite, quartz or chlorite, but the rounded masses must not be mistaken for pheno- crysts. The under as well as the upper portions of basaltic flows are provided with them and in the copper region of Lake Superior they attain decided importance, because they have occasionally furnished a place of deposition for copper. Basalts are frequently dense and felsitic and exhibit no pheno- crysts whatever. They must then be recognized by their dark colors, and high specific gravity. When the phenocrysts manifest themselves they are most commonly olivine with which augite may at times be recognized. If studied with the microscope the felsitic varieties are resolved into a finely crystalline mass of small augites, plagioclase rods and magnetites. A little glass may occasionally be detected. With the development of phenocrysts in moderate numbers the basalts proper pass into the Basalt-porphyries, often called dolerites. Phenocrysts of augite and olivine become prominent features of the rock, but visible plagioclases are few. The cellular texture disap- pears and the rocks are dense and solid. The basalt-porphyries constitute the interiors of thick flows and often form dikes. Yet it happens much more frequently with these basaltic rocks than with those more acidic, that the interiors of thick flows, instead of be- coming porphyritic, develop coarser and coarser, even textured varieties which must be treated with the granitoid members. When phenocrysts are in marked excess over the groundmass and constitute the greater part of the rock, the Gabbro-porphyries result The groundmass is then coarsely felsitic and becomes in- 5 66 A HAND BOOK OF ROCKS. creasingly coarse as the gabbros are approached. These rocks occur in deep-seated dikes and in the outer parts of laccoliths and thick intrusive sheets. Synonyms and Relatives. The name basalt is a very ancient term and has been explained in several ways. Many regard it as a corruption of basanites, which was used by Pliny, although it is uncertain to what rock he applied it The Greek word for the black touchstone or Lydian stone used by the ancient jewellers is similar to this last form. Others refer it to Basan or Bashan, the kingdom of Og, as mentioned in the Old Testament, Deuteronomy III., i. Again an Ethiopian word "basal," used by Pliny for an iron-bearing rock, has been suggested. Agricola in the sixteenth century gave it 'its present signification. The basalts almost always have olivine as an essential compo- nent, but there are certain rather uncommon varieties which lack it, but which still have the dark silicate, augite, in excess over the plagioclose, and which therefore differ from the andesites. They are called olivine-free basalts, and mark a transition to the augite- andesites. Very rarely indeed, hornblende replaces the augite in basalts and then the rocks are called kulaites from the occurrence of this variety in the Kula basin, Lydia, Asia Minor, where they have been discovered and studied by H. S. Washington. While the time-distinction was esteemed of weight by the students of rocks, the name basalt was restricted to those of Terti- ary or later age. The pre-Tertiary representatives were then called augite-porphyrites if they lacked olivine, and melaphyre if they possessed it. Melaphyre still survives as a much used term but the time distinction no longer obtains recognition. In rare instances nephelite and leucite are found in basalts when studied with the microscope. The two feldspathoids may appear, each by itself or both together and they may replace the plagioclase in part or in whole. These varieties cannot be distinguished from normal basalts without microscopic study. They have been named tephrite, leucite-tephrite, basanite, leucite basanite, nephelinite, leucitite, nephelite-basalt and leucite-basalt, all of which will be found defined in the Glossary. It is fruitless to attempt their study without the microscope. The most experienced observers might easily confound them with ordinary basalts. IGNEOUS ROCKS. 67 At several localities in America and abroad a very extraordinary and abnormal variety of basalt has been met which has quartz, even as a visible phenocryst. This mineral has resulted either from some extraordinary circumstances attending early crystalliza- tion, so that quartz developed as a phenocryst, or else because with a percentage of silica at the upper ranges of the basalts, the ferromagnesian bases were in such amount as to leave an excess of silica after their basic compounds were formed. This residual silica then crystallized as quartz since it had no alternative. These chemical relations are however extremely unusual, and the rocks are exceptional in the highest degree. Two other varieties of basalts may be mentioned at this point and at somewhat greater length because of their special minera- logical and chemical interest and their relations to corresponding granitoid types. Neither of them can be recognized with the eye alone as differing in any respect from the ordinary basalts, but when studied with the microscope they present great contrasts. In both, feldspars and feldspathoids practically or absolutely fail. We have left then the augitites, which consist of augite in a glassy groundmass, and the limburgites which have both augite and olivine in a similar groundmass. As the analyses will show these mineralogical results become possible when, with very low silica, the alkalies and alumina so far fail that they are taken up in the dark silicates or glassy base. AUGITITES. SiO 2 A1 2 O, Fe 2 O 3 FeO CaO MgO Na 2 6 K 2 O Loss. Sp. Gr. 1. 44.17 11.24 9-97 6.22 10.77 655 3.04 1.97 2.31 2. 43.35 11.46 11.98 2.26 7.76 11.69 3-88 0.99 3.00 2.974 LlMBURGITE. 3. 46.90 10.17 l - 22 5- l 7 6.20 20.98 1.16 2.04 5.42 2.86 4. 40.22 14.41 7.42 2.36 11.53 7-29 3-94 1-90 1. 10 2.89 I. Augitite, Mariupol, Russia, J. Morozewicz, Neues Jahrb., 1900, I., 394. Also TiO, 2.83. 2. Augitite, Hutberg, Bohemia, J. E. Hibsch, Tsch. Mitth., XIV., no, 1894. Also TiOj 2.43, Pj,O 5 1.54. 3. Limburgite, Bozeman, Mont., G. P. Merrill, Proc., U. S. Natl. Mus., XVII., 640, Anal. Chatard. 4. Limburgite, Palma., L. Van Werveke, Neues Jahrb., 1879, 485. Figs. 30 and 31 are of special interest when compared with those of the basalts, Figs. 28 and 29. The great diminution in the intercept of alumina and the great increase in lime and mag- 68 A HAND BOOK OF ROCKS. nesia are very marked. The diagrams strongly resemble those of the pyroxenites and peridotites, given subsequently under Figs. 35-38. There is some question in the minds of many observers as to whether the so-called glassy base of the augitites and limburgites really is a glass or whether it is not some isotropic mineral such as analcite. The basic magmas crystallize so readily as to make so much glass improbable. Analcite is the mineral usually thought of in this connection. It even gives a name to certain analcite-basalts, in which its presence has been positively shown. FIGS. 30 AND 31. Diagrams illustrating the chemical composition of the lim- burgites which are given in the above table. Fig. 30 is based on molecular ratios ; Fig- 31 on percentages. The related rocks monchiquite, fourchite and ouachitite, will be found denned in the Glossary. A very rare basaltic rock has the feldspathoid melilite as its chief feldspathic component. The magma is high in lime. The rock can only be identified with the microscope. Its related type alnoite is defined in the Glossary. Alteration, Metamorphism. The olivine of basaltic rocks is the first mineral to alter, and it soon becomes a network of serpentine IGNEOUS ROCKS. 69 veinlets enclosing unchanged nuclei. The augite also passes readily into chlorite and finally the feldspar kaolinizes. The preva- lence of green, chloritic products suggested the name greenstone for the old basaltic rocks. The basaltic rocks are extremely im- portant in connection with metamorphism, and the iron-mining regions around Lake Superior present superb illustrations of the process. The augite has the greatest tendency to pass into green hornblende, by what- is called a " paramorphic " change, i. e., a change in the mineral without change in the chemical composition and without, as in pseudomorphs, preserving the original form. Under shearing stresses and movements, accompanied by this paramorphic change, the basaltic rocks pass into hornblende- schists, and even chlorite-schists or green-schists, losing their mas- sive structure entirely and becoming a very different rock, and one that can be traced to its original with great difficulty. The wide- spread Catoctin schists of Virginia were derived in this way. The secondary hornblendic rocks are also called amphibolites. Tuffs. Basaltic tuffs, agglomerates, breccias, etc., are well known and often accompany the massive flows. They mark an explosive stage of eruption before or after the actual outpouring of lava. Distribution. Balsaltic rocks are enormously developed in this country. The oldest strata are penetrated by numerous black, igneous dikes, in practically all their exposures. The New Eng- land seacoast is especially seamed by them, and hundreds may be met in a short distance. The Adirondacks and the White Moun- tains, the Highlands of New York and New Jersey, have many. In the East are the intruded sheets of Triassic basaltic rocks, largely diabases and described below. They may reach 500 feet in thickness, and form many of the most prominent landmarks, such as Cape Blomidon, N. S.; Mts. Tom and Holyoke, Mass.; East and West Rock, near New Haven, Conn., the Palisades on the Hudson, and many dikes in the Richmond, Va., and Deep River, N. C., coal fields. Around Lake Superior, both in the iron and in the copper regions, are still greater sheets, for many thousands of feet of basalt (diabase) are present on Keweenaw Point. On the north shore near Port Arthur, the head-lands of Thunder Bay exhibit superb examples. The iron-bearing strata 7 o A HAND BOOK OF ROCKS. are penetrated by innumerable dikes. The greatest of all the Amer- ican basaltic areas is, however, met in the Snake River region of southern Idaho and extends into eastern Oregon and Washington. Many thousands of square miles are covered with the dark lava and are locally called the " Lava Beds." In Colorado, as at the Table Mountains, near Golden and Fisher's Peak, near Trinidad, there are prominent sheets, and the same is true of many other points in this State. In New Mexico, Arizona and Texas they are also met. The volcanoes of the Sandwich Islands are basaltic. Basaltic rocks with nephelite are scarcely known in the United States. Some minor dikes in the East, a volcanic neck at Pilot Knob, near Austin, Texas, dikes and sheets in Uvalde Co., Texas, and a few dikes at Cripple Creek, Colorado, are practically the only localities yet identified. Leucitic rocks, more phono- litic than basaltic, are known in the Leucite Hills, Wyo., and in Arkansas. Of basaltic affinities they occur in New Jersey, but these and the nepheline rocks are of small practical moment, although of great scientific interest. Basalts have quite as great development abroad as here. The islands off the north coast of Scotland are famous localities, and many of the volcanic regions of the continent are no less well provided. The lavas of Etna are chiefly basaltic, and those of Vesuvius are remarkable for their richness in leucite. In India are the great basalt fields of the Deccan, which are comparable in extent with those of the Snake River region of the West. THE DIABASES. Si0 2 A1 2 S Fe 2 s FeO CaO MgO Na 2 O K 2 O Loss. Sp. Gr. I. 54.52 19.10 2.83 5.89 7.25 3.92 3.73 2.30 0.59 2.7 2- 53-13 13-74 I-o8 9-10 9.47 8.58 2.30 1.03 0.90 2.96 3. 49.28 15.92 1.91 10.20 7.44 5.99 3.40 0.72 3.90 2.86 4. 48.75 17.17 0.41 13.62 8.82 3.37 1.63 2.40 ... 2.985 5. 46.28 12.96 4.67 6.06 10.12 8.71 3.75 3.34 2.921 6. 45.46 19.94 15-36 ... 8.32 2.95 2.12 3.21 2.30 2.945 I. Diabase Hills, Nev., Woodward, 4Oth Parallel Surv., I., Table opposite p. 676. 2. Penn. R. R. cut, Jersey City, N. J., G. W. Hawes, A. J. S., iii., IX., 186. 3. Lake Saltonstall, Conn., Ibid. 4. Dike near Boston, Mass., W. H. Hobbs, Bull. Mus. Comp. Zool., XVI., i. 5. Point Bonita, Calif., F. L. Ransome, Bull. Geol. Dept. Univ. Calif., I., 106. 6. Dike at Palmer Hill, Ausable Forks, N. Y., J. F. Kemp, Bull. 107, U. S. G. S., 26. IGNEOUS ROCKS. 71 The diabases constitute a transitional group as regards texture from the felsitic basalts to the granitoid gabbros. The low fusing points and the great tendency to crystallize, which are possessed by the basaltic rocks, lead to the development of rather finely yet entirely and visibly crystallized aggregates of plagioclase, augite and often olivine, which in the refinements of texture differ from the true gabbros. The plagioclase is in elongated and sharply rectangular rods, especially when viewed with the microscope. The augite and olivine, in irregular development, are packed in between these interlacing rods. It is evident that the plagioclase, FlG. 32. Diabasic texture, drawn from a microscopic slide of a diabase from Pig- eon Pt., Minn. The actual field was three eighths inch (l cm.). The rods of plagio- clase are shown enclosing the augite olivine and magnetite. The shaded mineral with light borders is augite ; the shaded mineral with heavy borders is olivine. The black mineral is magnetite. contrary to the usual order of crystallization, has abnormally fin- ished its period before the ferro-magnesian silicates were much advanced, and that the latter have been forced to adapt themselves to whatever space remained. This result has often been reached in dikes and in the interiors of thick surface flows and intruded sheets, whose outer parts are characteristic basalts or even amygda- loidal varieties. The diabases are thus a peculiar phase of the granitoid texture as here defined, and their texture is called the diabasic or diabasic granular. They differ from the typical gab- 72 A HAND BOOK OF ROCKS. bros in that the feldspars of the latter are about as broad as long, and the succession of generations in the crystallization of the com- ponent minerals of the gabbros has been normal. Yet intermediate textures are known and gabbros are sometimes described as dia- basic. When in the true diabases the augites become so abundant, large and coarsely crystalline, as to include the rectangular rods of plagioclase in a matrix of pyroxene, the texture is called ophitic. The ophitic texture is usually a matter for the microscope, but the diabasic can often be detected by the eye, and the feldspar rods may, in extreme cases, be an inch or more in length. THE GABBROS. SiO, Al,0, Fe 4 O, FeO CaO MgO Na 3 O K 2 O Loss. Sp. Gr. I. 59-55 25.62 0.75 7-73 tr. 5-09 0.96 0-45 2.66 2. 55-34 16.37 0-77 7-54 7-Si 5-05 4.06 2.03 0.58 3- 54-72 17.79 2.08 6.03 6.84 5-85 3.02 3.01 ... 2.928 4- 54-47 26.45 1.3 0.67 10.80 0.69 4-37 0.92 0-53 2.72 5- 53-43 28.01 0-75 11.24 0.63 4-85 0.96 tr. 2.67 6. 52.14 29.17 3-26 10.81 0.76 3.02 0.98 0.58 7- 49-15 21.90 6.60 4-54 8.22 3-03 3-83 1.61 1.92 8. 48.02 I7-50 1. 80 7.83 I3.I6 10.21 1.48 tr. 0.79 9- 46.85 19.72 3.22 7-99 13-10 7-75 1-56 0.09 0.56 10. 46.85 18.00 6.16 8.76 10.17 8-43 2.19 0.09 0.30 3.097 n. 45-66 16.44 0.66 13.90 7-23 "57 2.13 0.41 0.07 I. Anorthosite, Chateau Richer, Canada, T. S. Hunt, Geology of Canada, 1863. 2. Norite, Cortlandt Series, Montrose Point, Hudson River, Anals. by Munn, for J. D. Dana, A. J. S., Aug., 1881, p. 104. 3. Gabbro, near Cornell Dam, Croton River, H. T. Vulte, for J. F. Kemp, unpublished. 4. Anorthosite, Summit of Mt. Marcy, Adirondacks, A. R. Leeds, 3Oth Ann. Rep. N. Y. State Museum, reprint, p. 14, 1876. 5. Anorthosite, Nain, Labrador, A. Wichmann, Z. d. d. g. Ges., 1884. 6. Gabbro, Iron Mtn., Wyo., 4Oth Parallel Surv., II., 14. 7. Gabbro, near Duluth, Minn., Streng, Neues Jahrb., 1876, 117. 8. Gabbro-diorite, Baltimore, Md., average of seventeen samples, Mackay for G. H. Williams, U. S. G. S., Bull. XXVIII., 39. 9. Gabbro, Baltimore average of twenty-three samples, ibid. 10. Gabbro, Southwest Adirondacks, C. H. Smyth, Jr., A. J. S., July, 1894, 61. n. Gabbro, Northwest Minn., W. S. Bayley, Anals. by Stokes, Jour. Geol., I., 712. Comments on the Analyses. The range in composition pre- sented by the gabbros is in many respects the same as that of the basalts, just as we would naturally expect ; but there is one analysis (No. i) which goes higher in silica. The rock is, how- ever, a variant from the typical gabbros, as may be seen from the lack of iron and magnesia, leaving little else than the necessary elements for plagioclase. The same is true of Nos. 4 and 5. The IGNEOUS ROCKS. 73 other analyses, however, exhibit very characteristic percentages, and upon studying them with care the reader will see that the aggregate of plagioclase, augite and often olivine, which is what constitutes the typical gabbro, must result from their crystallization. The percentages in silica range from 55 to 45. The alumina is in general quite high. In No. 6 it reaches a maximum for all the igneous rocks given. Except in i, 4, 5 and 6, the bases iron, magnesia, lime and soda are quite high. Potash naturally is low, since orthoclase is usually present in small amount. c.o FIGS. 33 AND 34. Diagrams illustrating the chemical composition of the gabbros which are given in the above analyses. Fig. 33 is based on molecular ratios ; Fig. 34 on percentages. Figs. 33 and 34 are very like those of the basalts, Figs. 28 and 29. Lime, however, is relatively much greater than magnesia, owing in part to the fact that analyses I, 4, 5 and 6 are included in the general average. The FeO is greater than the Fe 2 O 3 , just re- versing the relations shown in the diagram of the basalts, and doubtless due to these particular selections of analyses. There 74 A HAND BOOK OF ROCKS. seems to be no fundamental reason why the basalts should differ from the gabbros in these respects. Mineralogical Composition, Varieties. The name gabbro is of Italian origin, and has been applied in recent years, and with growing favor to the great group of granitoid rocks which consists, in the typical cases, of plagioclase and pyroxene. The diabases, as was explained above, are texturally and mineralogically transitions from the basalts to the true gabbros. The so-called gabbro group is a very large and characteristically variable one. Originally the name gabbro was only applied to a mixture of plagioclase and the variety of monoclinic pyroxene called diallage, that has pinacoidal as well as prismatic cleavages, but of late years all granitoid, plu- tonic, pyroxene-plagioclase rocks are collectively spoken of as the gabbro group. In the typical gabbros the dark silicates predomi- nate over the light-colored ones, but rocks are included in the general group, to which this restriction does not apply. In this particular the facts of field occurrence and natural relationship have broken down the sharpness of mineralogical definitions. At the acidic extreme we have in Canada and the Adirondacks enormous masses of rock that are practically pure, coarsely crystalline labra- dorite. Pyroxene is the dark silicate when any is present, but often it is insignificant. These pure feldspar rocks are best called anorthosites, from the French word for triclinic feldspar, but the word is not to be confused with anorthite, the lime feldspar, with which it has no special connection. An old and obsolete synonym of anorthosite is labradorite-rock, of interest because widely used in early reports on the Adirondacks. As monoclinic pyroxene in- creases the rocks pass into gabbros proper. More or less biotite and hornblende may also be present. If the pyroxene is ortho- rhombic we call the rock norite. Varieties with olivine are frequent, giving olivine-gabbro and olivine-norite. Gabbros and norites are not readily distinguished without the microscope, unless the bronzy appearance of hypersthene can be recognized. In the former case, gabbro is a good collective term. Norites were formerly called hypersthene rock, or hypersthene-fels, both of which are undesirable rock names. Gabbro intrusions of not too great extent for careful study have been observed to grow more basic toward the outer margins. IGNEOUS ROCKS. 75 THE PYROXENITES AND PERIDOTITES. Pyrox. SiO, A1 2 O 3 Fe.,O 3 FeO CaO MgO Na a O K a O Loss. Sp. Gr. 1. 55.14 0.25 3.48 4.73 8.39 26.66 0.30 2. 53.98 1.32 I. 4 I 3.90 15.47 22.59 0.83 3.301 3. 44.01 11.76 15.01 ... 4.06 25.25 Peridotite. 4. 47.41 6.39 7.06 4.80 14.32 15.34 0.69 1.40 2.10 3.30 5- 46.03 9.27 2.72 9.94 3.53 25.04 1.48 0.87 0.64 3.228 6. 41.00 7.58 ... 5.99 10.08 23.59 0.52 ... 4.73 2.989 7. 36.80 4.16 ... 8.33 8.63 25.98 0.17 2.48 0.51 8- 33- 8 4 5-88 7.04 5.16 9.46 22.96 0.33 2.04 7.50 9. 29.81 2.01 5.16 4.35 7.69 32.41 O.U 0.20 8.92 2.78 I. Pyrox enite, var. Websterite, Webster, N. C., E. A. Schneider for Geo. H. Wil- liams, Amer. Geol., July, 1890, p. 41. 2. Pyroxenite, Baltimore, Md., T. M. Chatard for G. H. Williams, ibid. 3. Pyroxenite, Meadow Creek, Mont., Geo. P. Merrill, Proc. U. S. Natl. Mus., XVII., 658. 4. Peridotite, Cortland Series, Montrose Pt., N. Y., Emerson for G. H. Williams, A. J. S., Jan., 1886, 40. 5. Peridotite, Custer Co., Colo., L. G. EakinsforW. Cross, Proc. Colo. Sci. Soc., 1887, 245. 6. Peridotite, Baltimore, Md., L. Mackay for G. H. Williams, Amer. Geol., July, 1890, 39. 7. Peridotite, Dewitt, N. Y., H. S. Stokes for Darton and Kemp, Amer. Jour. Sci., June, 1895, 456. 8. Mica Peridotite, Crittenden Co., Ky., W. F. Hillebrand for J. S. Dil- ler, A. J. S., Oct., 1892, 288. 9. Peridotite, Elliott Co., Ky., J. S. Diller, Bull. 38, U. S. G. S., p. 24. Comments on the Analyses. As compared with the gabbros the pyroxenites are characterized by the falling off in A1 2 O 3 , due to the disappearance of feldspar, and by the increase in CaO and MgO necessitated by the predominance of the pyroxenes. The peridotites reach a lower percentage of silica than any other igneous rocks so far cited, but if this is accompanied by high H 2 O, allowance must be made for the relative decrease of the original SiO 2 because the rock has obviously changed to serpentine. The great percentages of MgO are very notable, and are due to the presence of much olivine, magnesian pyroxene and, in instances, biotite. Chromic oxide is also always present in small amounts, and oxides of nickel and cobalt are usually in perceptible quantity. The diagrams illustrating the pyroxenites and peridotites are of much interest when compared with those of the gabbros. The intercepts for silica have drawn in ; those for alumina and the alkalies have almost disappeared. The iron and lime have not changed very greatly, but the magnesia has expanded enormously, and has afforded a very significant and interesting set of figures. A HAND BOOK OF ROCKS. FIGS. 35 AND 36. Diagrams illustrating the chemical composition of the pyroxenites, which are given in the above analyses. Fig. 35 is based on molecular ratios ; Fig. 36 on percentages. FIGS. 37 AND 38. Diagrams illustrating the chemical composition of the peridotites, which are given in the above analyses. Fig. 37 is based on molecular ratios ; Fig. 38 on percentages. IGNEOUS ROCKS. 77 The resemblance to the figures of the limburgites given above (Figs. 30 and 31) is close. Mineralogical Composition, Varieties. The gabbros pass in- sensibly, by the decrease of plagioclase, into the pyroxenites and peridotites, and in any great gabbro area all these are usually present, but they may occur also as independent masses. The pyroxenites are practically pyroxene, with little if any other min- erals. There is some variety, according as the rock contains one or several of the following : enstatite, bronzite, hypersthene, dial- lage or augite ; but with the unassisted eye, it is seldom that one can be sure of these distinctions, except as the orthorhombic pyroxenes exhibit a bronze luster. Hornblende, magnetite and pyrrhotite may also be present. With the accession of olivine, peridotite results, so named from the French word, peridot, for olivine, and a number of varieties have been made according as the olivine is associated with one or more of the minerals cited for pyroxenites. The list is given under Peridotite in the Glossary. The distinctions are however hardly possible without microscopic aid. As the extreme of peridotites we have a nearly pure olivine rock, called dunite, important in North Carolina. Much magnetite may be associated with peridotite ; indeed at Cumberland Hill, R. I., there is enough to almost make the rock an ore. Chromite, too, is a frequent associate. As peridotites shade into a porphyritic texture, especially in dikes, they have been called picrites, and even additional varieties, such as kimberlite, have been made. Black hornblende, which is brown in thin sections, is frequent in both pyroxenites and peridotites, and may even form a rock itself, hornblendite. Dark brown biotite is also often present in con- siderable amount. Some writers have regarded the pyroxenites and peridotites as of doubtful igneous origin and have placed them with metamorphic rocks, but from their frequent association with gabbro, and from their independent occurrence in dikes, there is no good reason to doubt their true, igneous nature. A very rare granitoid rock, consisting of plagioclase, nepheline and ferro-magnesian silicates has been called theralite from the Greek verb to seek eagerly, because its discovery was anticipated by H. Rosenbusch before it was actually found by J. E. Wolff in the Crazy 7 8 A HAND BOOK OF ROCKS. Mountains, Montana. It is an extremely rare combination of min- erals, but of special scientific interest because it corresponds among the granitoid rocks to the tephrites and basanites of the porphyritic. Alteration, Metamorphism. The gabbros alter chiefly by the formation of serpentine and chlorite from the dark silicates. The pyroxenites and peridotites change readily into serpentine, often with an intermediate stage as hornblende-schist. Under dynamic stresses, especially shearing, anorthosites and gabbros pass into gneissoid types, and in the process much garnet may be developed. This is especially true in the Adirondacks. The larger feldspars may be left in the gneisses as " eyes," or, to adopt the German term, as "Augen," affording Augen-gneiss, i. e., gneisses with comparatively large lenticular feldspars. Much hornblende, espe- cially in true gabbros, is often developed in the process. The basic members, the pyroxenites and peridotites develop into amphibolites or hornblende- schists, which latter often furnish very puzzling geological problems. Distribution. The anorthosites occur in several Canadian areas, as at the headwaters of the Saguenay River, and again north of Montreal ; in the higher peaks of the Adirondacks and some of their outliers such as Mt. Marcy and its neighbors ; and to the northeast of Laramie, Wyo., in the Laramie range. Gabbros are also present in vast quantity in the Adirondacks and are likewise well known in the White Mountains, in the famous Cortlandt series, near Peekskill, on the Hudson, and in the vicinity of Balti- more. Around Lake Superior gabbros are of great importance. The basal members of the Keweenawan system and other older intrusions are largely formed of them. Fine specimens can be had at Duluth. Gabbro is a characteristic wall rock of titaniferous magnetite. Pyroxenites occur as subordinate members of the gabbro areas, especially near Baltimore. Peridotites are in the same relations in the Cortlandt series, in the Baltimore area and in North Carolina. They are also known on Little Deer Island, Me.; at Cumberland Hill, R. I.; in dikes near Syracuse, N. Y.; at Presqu' Isle, near Marquette, Mich.; in Kentucky; in Cali- fornia and elsewhere in the West. When outlying dikes are met, far from any visible, parent mass of igneous rocks and in sedi- mentary walls, they are very frequently peridotite. IGNEOUS ROCKS. 79 Abroad, anorthosites and gabbros are abundant in the Scandi- navian peninsula, whose geology is in many respects like that of Canada and the Adirondacks. In the north of Scotland gabbros are of especial interest because they have been shown by Judd to be the deep-seated representatives of the surface basalts. On the continent they are important rocks in many localities. The same is true of Australia and such other parts of the world as have been studied. Of especial interest are the peridotite dikes in South Africa that have proved to be the matrix of the diamond. ULTRA-BASIC IGNEOUS ROCKS. METEORITES. A few ultra-basic igneous rocks are known in which the silica decreases almost to -nil, and in which the bases, especially iron, are correspondingly high. They are in general rather to be con- sidered as basic segregations in a cooling and crystallizing magma than as individual intrusions. The Cumberland Hill, R. I., so- called peridotite, cited above, has very little silica. Titaniferous ores have almost none, but they are often exceptionally rich in alumina. In a few cases metallic iron has been detected in basic igneous rocks, suggesting analogies with meteorities. Meteorites are rare and only of scientific interest, but it is extremely suggestive that such silicates as are met in them are chiefly olivine and enstatite, minerals rather characteristic of very basic rocks. The commoner meteorites are an alloy of metallic iron and nickel, but some rare sulphides are occasionally present. As filling out the theoretical series we cannot bar out water and ice. There is no reason why they should not be considered igneous rocks of extremely low fusing point, but they are so familiar that a simple reference to them is sufficient. CHAPTER VI. REMARKS IN REVIEW OF THE IGNEOUS ROCKS. Chemical Composition. Igneous magmas vary from about 80 per cent, silica as a maximum to practically none as a minimum, but important rocks rarely drop below 40 per cent. Alumina is highest in the anorthosites or feldspathic gabbros, where it may exceed 25 per cent. It is lowest in the pyroxenites and may be less than I per cent. The oxides of iron are almost lacking in the highly siliceous, but may reach beyond 20 per cent, in the basaltic RHYOLITES AND GRANITES TRACHYTES-SYENITES RARE BASIC PHONOLITES NEPH. SYEN'T'S ORTHOCLASE ROCKS DACITES ANDESITES BASALT GROUP Q'TZ DIOR1TES DIORITES GABBROS PYROXENITES PERIDOTITES ORES FIGS. 39 AND 40. Diagram intended to illustrate graphically the mineralogical composition of the igneous rocks. The numbers indicate percentages in silica. The upper diagram, Fig. 39, includes the orthoclase rocks, the lower, Fig. 40, the plagio- clase and non-feldspathic ones. Q is quartz ; O, orthoclase ; NL, nephelite and leu- cite ; P, plagioclase ; M, muscovite ; B, biotite ; H, hornblende ; A, augite ; Ol, olivine ; the black area, magnetite, pyrrhotite, and other metallic minerals. rocks, and with TiO 2 may be nearly 100 per cent, in some igneous iron ores. Lime attains its maximum of 12-15 P er cent - m the gabbros and pyroxenites, while magnesia in the pyroxenites and peridotites may even surpass 25 per cent. Potash is most abun- dant in the orthoclase and leucite rocks ; soda in those with nephe- 80 IGNEOUS ROCKS. 81 lite. Combined alkalies may reach 1 5 per cent, in the phonolites and nephelite syenites. In general they are, however, about 410, and may practically fail. Water in quantities over i per cent, as a rule is an indication of decay, but in the pitchstones this is not positively true, for the water reaches close to 10 per cent, and the rocks are apparently unaltered. Texture. All of the typical textures are easily recognized in characteristic development, but the glassy shade insensibly into the felsitic, the felsitic into the porphyritic, and the porphyritic into the granitoid. There are, therefore, intermediate forms that are diffi- cult to classify. Yet, on the whole, the four textures are the most satisfactory basis for classification, and as a guide, in accordance with which to study. Chemical composition being the same, texture is a result of the physical conditions surrounding the magma at the time of crystallization and of the presence of mineralizers. Mineralogy. The above diagrams, with a reasonable approxi- mation to the truth, illustrate the quantitative mineralogy of the igneous rocks. A section cut through the charts at any one point expresses the relative amounts as well as kinds of the several min- erals in the rocks whose names are along the top lines, and whose percentages in silica are approximately shown. No mention is made of texture. In the orthoclase rocks quartz disappears at about 65^) SiO 2 , while orthoclase continues to the end; plagio- clase in small amount is quite constantly present throughout the series. Nephelite and leucite come in as indicated. Muscovite appears only in the more acidic granites. Biotite and hornblende vary in relative quantity, but toward the basic end both yield to augite. The rocks at the basic end are chiefly those recently dis- covered by Weed and Pirsson in Montana, by Iddings in the Yellowstone Park and by Lawson in the Rainy Lake region. In the plagioclase and non-feldspathic rocks quartz and orthoclase soon run out, so far as any notable or regular amount is concerned. Plagioclase holds along to about 45^0 SiO 2 , and at about 55^) SiO 2 may in the anorthosites be the only mineral present. Biotite and hornblende are present all the way through, but toward the basic end they tend to yield in importance to augite, which latter in some pyroxenites, at about 49^ SiO 2 , may be the only silicate present. Olivine begins to appear at $$fi and steadily increases with occa- 6 82 A HAND BOOK OF ROCKS. sional lapses almost to the end, where it may be the chief mineral. The ores, as the extreme case, and without regard to silica, increase so as to be the only minerals in the rock, forming thus the theo- retical, basic limit. The diagrams also emphasize the fact that igneous rocks shade into one another by imperceptible gradations, and this is true of the orthoclase and plagioclase groups them- selves, although not suggested by the separation of the two in the drawings. The continuation of the orthoclase series to a basic extreme is a fact that we have only appreciated in very recent years. A careful scrutiny of analyses and mineralogical composition leads to the conclusion that practically the same magma may, under different physical conditions of crystallization, afford miner- alogical aggregates that vary considerably in the proportions of the several minerals now yielding more hornblende, again more augite, and even affording quartz in a basalt. Hence, analyses in different groups overlap more or less, and the difficulty of drawing sharp lines of distinction is increased. Yet, allowing for this vari- ation chemical composition determines the resulting mineralogical aggregate and is fairly characteristic. Determination of Igneous Rocks. In determining an igneous rock, the texture should first be regarded, next the feldspars. If orthoclase prevails, the presence or absence of quartz establishes the rock. If plagioclase prevails, we look for biotite, hornblende, pyroxene and olivine. If no feldspar is present, we look for the presence or absence of olivine. On this basis the table on page 23 is to be used there are, however, many finely crystalline rocks which elude the power of the unassisted eye. If of light shades, they can generally be referred with reasonable correctness to the rhyolites, trachytes, felsites or andesites. If dark, they are prob- ably basaltic in their nature, and the name " trap " is a very useful and sufficiently non-committal term. Care should be exercised not to confuse the porphyritic rocks having angular phenocrysts with the amygdaloids, or the rocks whose almond-shaped cavities, produced by expanding steam, have been filled by later introduced calcite, or quartz, or other secondary minerals. While books are of great assistance, really the only way to become properly familiar with rocks is to use the books in connection with correctly labeled and sufficiently complete study collections. IGNEOUS ROCKS. 83 Field Observations. A rock is not to be considered by any in- telligent observer as a dead, inert mass in nature, but as an im- portant participant in the ceaseless round of changes which confront us on every side. Familiarity with specimens and varieties in collections ought always to be followed by observation in the field. We have all grown to believe that in limited areas igneous rocks, however varied they may be, are yet intimately related in their origin, or are bound together by ties of kinship, " consanguinity," as Iddings has called it. Some regions like eastern Montana and the Black Hills have especial richness of high soda or potash magmas, giving rise to nephelite and leucite rocks, and sodalite syenites ; Colorado, Utah and New Mexico have wonderful and enormous laccolites of andesites (porphy rites). The Pacific coast in South America has andesites in vast extent from active vol- canoes, and in North America from extinct cones. Idaho, Ore- gon and Washington are marked by basalts. The Atlantic coast region has a long series of very ancient volcanoes, that preceded the early fossiliferous strata from Newfoundland to North Carolina and that yielded nearly the entire series of the volcanic rocks. In the Adirondacks, on the Hudson near Peekskill, near Baltimore, and around Lake Superior we find the members of the gabbro family ; while near tidewater along the Atlantic seaboard we have granites, almost all with biotite. Such facts as these suggested the creation of the term " petrographic provinces," to J. W. Judd, in the endeavor to suggest these kinships of magmas in certain limited districts. There are many others even in North America that could be cited, but the above will suffice to remind the reader that these broader relationships should be always before him while extending his acquaintance with rocks as they occur in the natural world about him. CHAPTER VII. THE AQUEOUS AND EOLIAN ROCKS. INTRODUCTION. THE BREC- CIAS AND MECHANICAL SEDIMENTS NOT LIMESTONES. The members of this, the second grand division, are much simpler, and, as a general thing, much easier to identify and to understand than are the igneous. No single term is comprehensive enough to include them all, and even the double one selected above in the endeavor to embrace as many as possible, and to avoid the multiplication of grand divisions, still falls short of including several. Nevertheless, those not embraced (the breccias) are of limited distribution, and, for many reasons, go best with the other fragmental rocks, even if, strictly speaking, they are neither aqueous nor eolian in origin. Sedimentary is the most useful term, and is universally applied as a partial synonym of the above, for it fairly includes the most important members of the series, but the rocks deposited from solution and the eolian rocks can hardly be under- stood by it. The rocks will be taken up under the following groups : I. Breccias and Mechanical Sediments, not Limestones. II. Limestones. III. Organic Remains, not Limestones. IV. Precipitates from Solution. The limestones are reserved for a special group, although they be- long in instances to each of the other three. They form, however, such an important series in their scientific and practical relations, that it is in many respects advantageous to take them all up together. I. BRECCIAS AND MECHANICAL SEDIMENTS, NOT LIMESTONES. Group I. is described in order from coarse to fine according to the following series, minor varieties not cited in the table being mentioned in the text under their nearest relatives. COARSE TO FINB. BRECCIA. GRAVEL AND CONGLOMERATE. SAND AND SANDSTONE. ARGILLACEOUS SANDSTONE. CALCAREOUS SANDSTONE. SILT AND SHALE. CALCAREOUS SHALE. CLAY. MARL. 84 THE AQUEOUS AND EOLIAN ROCKS. 85 BRECCIAS. The word breccia is of Italian origin and is used to describe ag- gregates of angular fragments cemented together into a coherent mass. The breccias cannot all be properly considered to be either aqueous or eolian, and some have already been referred to under the fragmental igneous rocks. Oftentimes they resemble con- glomerates, but, unless formed of fragments of some soluble rock, whose edges have become rounded by solution, there is no diffi- culty in distinguishing them. Breccias, as regards their angular fragments and interstitial filling, may be of the same materials or of different ones. We may distinguish Friction breccias (Fault breccias), Talus breccias, and, for the sake of completeness, may also mention here Eruptive breccias. Friction breccias are caused during earth-movements by the rubbing of the walls of a fault on each other, and by the conse- quent crushing of the rock. The crushed material of finest grade fills in the interstices between the coarser angular fragments, and all the aggregate is soon cemented together by circulating, mineral waters. Such breccias occur in all rocks and are a frequent source of ores, which are introduced into the interstices by infiltrating solutions. Quartz and calcite are the commonest cements. Talus breccias are formed by the angular debris that falls at the foot of cliffs and that becomes cemented together by circulating waters, chiefly those charged with lime. Eruptive breccias may be produced either by the consolidation of coarse and fine, fragmental ejectments like tuffs, or by an erupting sheet or dike that gathers in from the wall rock sufficient fragments as inclusions to make up the greater part of its substance. These are finally cemented together by the igneous rock itself and afford curious and interesting aggregates, oftentimes representing all the rocks through which the dike has forced its way to the surface. A crust may also chill on a lava stream, and when an added im- pulse starts anew the flowing, the crust may be shattered into an eruptive breccia of a still different type. We often speak of breccias as " brecciated limestone," "brecciated gneiss," or some other rock, thus making prominent the character of the original. When the fragments and the cement are con- 86 A HAND BOOK OF ROCKS. trasted in color, very beautiful ornamental stones result, which may be susceptible of a high polish. A moment's consideration of the above methods of origin will convince the reader that breccias, except as formed of loose, vol- canic ejectamenta, are of very limited occurrence. Although deeply buried rocks that share in profound earth movements often suffer crushing and brecciation on a large scale, the effects are chiefly detected by microscopical study. GENERALITIES REGARDING SEDIMENTATION. In the production of the rocks next taken up, moving water plays so prominent a part that its general laws are described by way of necessary introduction. All streams or currents charged with suspended materials exercise a sorting action during the deposition of their loads. With materials of the same density the sorting will grade the deposit according to the sizes of the particles. With materials of different densities, smaller particles of heavier substances will be mixed with larger particles of lighter ones. Assuming a swift current, we readily see that, when it slows up, the large and heavy fragments drop first of all ; then the smaller fragments of the heavier materials and the larger ones of those successively lighter, until at last the smallest particles of the light- est alone remain in suspension. It is also important to bear in mind that, the density being the same, the diameter of the trans- portable particle increases with the sixth power of the velocity. Thus, if we have a current of the proper velocity it will be able to lift a grain of quartz a sixteenth of an inch in diameter ; but if the velocity is doubled, the transportable particle will be four inches in diameter. An appreciation of this law makes the size of boulders moved by many streams, in times of flood, less surprising. On the other hand, when the suspended material becomes excessively fine, the ratio of its surface to its volume is so extremely high that adhesion, or chemical action akin to hydration, or some other influence not well understood, operates in pure, fresh waters, so as to practically render sedimentation impossible, even if the medium be perfectly quiet. W. M. Brewer has shown by a series of ex- periments with all sorts of clays, lasting over many years, that if we introduce into such an emulsion a mineral acid or a solution THE AQUEOUS AND EOLIAN ROCKS. 87 of salt or of some alkali, the turbidity clears with remarkable quickness. When, therefore, sediment-laden streams flow into the ocean, or into salt lakes, even the finest part of their load speedily settles out. While we may state thus simply the laws of sedimentation, we must not expect in Nature such well-sorted and differentiated results as would at first thought appear to be the rule. Of rivers and shore currents the two great transporting agents the former are subject to floods and freshets, giving enormously in- creased efficiency for limited periods, and again to droughts, with the same at a minimum. Hence varying sediments overlap and are involved together. Eddies and quiet portions in the streams themselves contribute further confusion, and an intermingling of coarse and fine materials. Shore currents have parallel increases of violence in times of high wind and storms, and sink again in times of calm. Eolian deposits are subject to even greater fluctuation, and their irregularities are more pronounced than those of the true Aqueous. Both these classes of rocks are marked by a more or less perfect arrangement of their materials in layers. The layers give rise to regular beds in deposits from quiet and uniform currents, and, although in those from swift ones they are very irregular, as explained above, nevertheless, bedding, or stratification, is in the highest degree characteristic of the Aqueous and Eolian grand division. When in the presence of these sedimentary rocks in the field, the observer should always appreciate that they reproduce past conditions, and that they indicate the former presence of water, either in a state of agitation and with high transporting power for the coarse varieties, or as quiet reaches in which were laid down the finer deposits. Rightly approaching and interpreting them, we may see that the ocean has advanced across the land in times of submergence, leaving behind its widening trail of shore gravels, now conglomerates ; that these have been followed up and buried first by fine offshore sediments, and later by the remains of organ- isms now appearing as limestones, until succeeding elevation causes the waters again to retreat and prepare the way for another " cycle of deposition." 88 A HAND BOOK OF ROCKS. GRAVELS AND CONGLOMERATES. Loose aggregates of rounded and water-worn pebbles and bould- ers are called gravels, and when they become cemented together into coherent rocks they form conglomerates. Sand almost always fills the interstices. Silica, calcite and limonite are the commonest cements. The component pebbles are of all sorts of rock depend- ing on the ledges that have supplied them, hard rocks of course predominating. Rounded fragments of vein quartz are especially frequent. Gravels and conglomerates, if of limited extent, indicate the former presence of swift streams ; if of wide area they suggest the former existence of sea beaches and the advance of the sea over the land. Component pebbles are of course older than the conglomerate itself, and if igneous, they may establish the age of the intrusion as older than the conglomerate. Fossiliferous boul- ders prove the age of the conglomerate as later than their parent strata. Under favorable circumstances gravels may be cemented to conglomerates in a comparatively few years. Conglomerates are exclusively aqueous. Gravels and conglomerates graduate by imperceptible stages into pebbly sands and sandstones, and these into typical sands and sandstones. Notably unsorted aggregates of relatively large and more or less angular boulders in fine sands or clay indicate glacial action. Metamorphism. Under dynamic stresses, especially in the nature of pressure and shearing, the pebbles of a conglomerate may be flattened and rolled out into lenses, and these are often observed. If the pebbles are feldspathic as is the case in those from granite ledges, and if the interstitial filling is aluminous and not purely quartzose as in the commonest cases, conglomerates, when recrystallized, may pass into augen-gneisses with their char- acteristic "augen," or " eyes " of feldspar and quartz that but faintly suggest their original character. Excessive metamorphism may further develop types closely simulating granite, forming thus the so-called " recomposed granite" of the Lake Superior regions. Occurrence. Gravels are too familiar to require further refer- ence. Conglomerates are met in all extended sedimentary series. Our greatest one lies at the base of the productive Coal Measures of Pennsylvania and adjacent States. It is properly called the THE AQUEOUS AND EOLIAN ROCKS. 89 " Great Conglomerate." Remarkable ones with squeezed pebbles are met in the Marquette iron region of Michigan. In Central Massachusetts there is an augen-gneiss that has been derived from a Cambrian conglomerate. It is quarried at Munson, and sold as granite, and is a widely known building stone. Around Narragan- sett Bay, R. I., are conglomerates, in part at least of Carboniferous age, in all stages in the progress to gneiss. SANDS AND SANDSTONES. SiO, 99.78 A1 2 3 O.22 FeO Fe a 3 MnO CaO MgO K 2 Na 2 O Loss. Sp. Gr. 2. 98.84 . o. 17 -34 tr. tr. tr O 23 3. 99.62 0.13 0.7 J* 4- 99-47 0.17 0. 12 0.90 0.50 0.07 0.15 0.12 2.647 5- 95-85 2, 64 ... 0.81 0.08 ... ... 0-45 (2.245) 6. 94-73 0.36 2.64 0.38 0.36 ... ... 0.83 7- 91.67 6.92 tr. 0.28 o-34 ... ... I.I7 2.240 8. 82.52 7.07 3-55 1.42 1-83 tr. tr. tr. 3-61 9- 69.94 13- 1 5 2.48 0.70 309 tr. 3-3 5-43 I.OI (2-36) I. Sand from Cambrian Quartzite, Chesire, Mass., S. Dana Hayes, Mineral Re- sources, 1883-' 84, p. 962. 2. Oriskany Sandstone, Juniata Valley, Penna. A. S. McCreath, Idem. 3. Siluro-Cambrian saccharoidal sandstone, Crystal City, Mo. An- alyzed by Glass Co., 0.22 not determined, Idem. 4. Novaculite, Rockport, Ark. R. N. Brackett, for L. S. Griswold, Geol. Ark., 1890, III., 161. 5. Salmon-red Triassic Sandstone, Glencoe, Colo. Quoted by G. P. Merrill, "Stones for Building and Deco- ration," p. 420. The Sp. Gr. is of one from Ralston, near by. 6. Cambrian red Sandstone, Portage Lake, Mich., Idem. 7 Light-gray sub-carboniferous sandstone, near Cleveland, Ohio, Idem. 8. Olive-green carboniferous sandstone, Dorchester, N. B., Idem. 9. Red triassic sandstone (brownstone, arkose), Portland, Conn., Idem. Comments on the Analyses. The first three illustrate the purity of the sand in exceptional cases. We may properly infer that the sediments were derived either from preexisting sandstones, that had already been once sorted and separated from their aluminous ingredients, or from excessively weathered and kaolinized quartzose rocks, such that the feldspar had entirely passed into clay, and had been eliminated in deposition. No. 4 is a novaculite, and is an ex- cessively fine, fragmental deposit. Nos. 5, 6 and 9 are red sand- stones, and indicate the comparatively small percentage of iron oxides that may cause a deep coloration. No. 7 is free from iron, but has some aluminous material, evidently a very pure clay, from the lack of iron. No. 8 has its iron as protoxide, for the rock is a green variety. Its manganese oxide is worthy of remark. No. 9 o A HAND BOOK OF ROCKS. 9 is a feldspathic sandstone, or arkose, whose analysis, except that the A1 2 O 3 is low and the CaO rather high, might answer for a granite. The specific gravity of sandstones varies widely. Quartz itself is 2.6-2.66, and specially dense sandstones reach 2.5, but, being characteristically porous rocks, the usual range is 2.2-2.4. They often go lower and many even reach 1.8. Mineralogical Composition, Varieties. The mechanical sedi- ments whose predominant particles are finer than pebbles, and yet, in most cases, of notable size, are grouped under this head. They are found in all stages of coherence, from loose sands to exces- sively hard, metamorphic rocks called quartzites. Quartz is much the commonest mineral that contributes the grains, as it is the most resistant of the common rock -making minerals. In river sands the grains are angular, but in those continually washed together on a sea beach, they become more or less rounded. Garnets, magne- tite, zircon and other hard and resistant minerals are widely dis- tributed in small quantities. Feldspathic sands also occur, and when they are compacted into firm rock they are called arkose. As in the conglomerates, the cementing materials of sandstones are silica, calcite and limonite, but in many the character or cause of the bond is rather obscure. Those with siliceous cement yield the most durable stone for structural purposes ; those with ferruginous afford the greatest range of colors, such as olive-green, yellow, brown, and red. Calcareous cements may be detected by their feeble effervescence. Sandstones entirely formed of calcareous frag- ments are known, but are described under limestone. A curious and exceptional rock is the novaculite, that is exten- sively developed in Arkansas. It was long thought to be allied to the cherts, which it much resembles, but microscopic investigation has led Griswold to determine it to be a finely fragmental deposit of quartz grains, practically a siliceous ooze. In fineness it is parallel with the clays, but it contains little else than silica. Aqueous sandstones generally exhibit well-marked bedding planes, although cases are familiar in which the bedding is exces- sively coarse and the layers are extremely thick. Swirling eddies in the original stream or currents give rise to cross-bedding and various irregularities. In fact, all the phenomena of beaches and THE AQUEOUS AND EOLIAN ROCKS. 9 i stream-bottoms, such as ripple-marks, worm-borings, shells, etc. are preserved in sandstones. Eolian sands are usually of aqueous deposition in their origina? condition, but they are afterwards taken up by the wind and driven along as dunes and dust into more or less remote districts. When they finally reach a state of rest and consolidate, they have very irregular stratification, cross-bedding, swirling curves, pinching and swelling layers and other characteristic phenomena. Finer varieties afford a surface deposit that is generally called "loess," and thaf may lack all stratification. More or less water-transported mate- rial is also intermingled, making the term one of not particularly sharp definition. This mixed character has made the loess of many localities a rather puzzling geological problem. It is alwayr loosely textured and is important in its relations to agriculture. Sands and sandstones pass by insensible gradations into the varieties in the upper line of the series shown in the tabulation on p. 84 by the increasing admixture of clayey or argillaceous ma- terials. The base is kaolin, A1 2 O 3 , 2SiO 2 , 2H 2 O, a mineral that forms microscopic, scaly crystals and that has the property of plas- ticity, and this property it lends to the last members of the series, which in exceptional cases may contain little else. The lower series passes gradually into the fragmental limestones, by the increasing admixture of calcite. Metamorphism. The purer sandstones in metamorphism yield quartzites which are denser and harder than their originals, because by deposition of cementing quartz, the fragmental grains are very firmly bound together. The later deposited quartz often con- forms to the optical and crystallographic properties of the grain around which it crystallizes. No sharp line divides sandstones and quartzites ; they shade imperceptibly into one another. Less pure sandstones, if crushed and sheared in the metamorphic process, yield siliceous or quartz schists from the development of micaceous scales between the grains. Flexible sandstone or itacolumite has been thought to owe to them its property of bending, but it is now generally attributed to the interlocking of grains. Occurrence. Sandstones are so common in all extended geolog- ical sections as to deserve slight special mention. Next to lime- stones they are the most widely used of building stones in quantity, 9 2 A HAND BOOK OF ROCKS. although the money value of the annual output of granite is greater. The Potsdam sandstone of the Cambrian in New York and on the south shore of Lake Superior is extensively quarried. Other prominent sandstones are the Medina of New York, the Berea grit of the Subcarboniferous of Ohio ; and the red and brown Triassic sandstones both of the Atlantic seaboard and the Rocky Mountains. ARGILLACEOUS SANDSTONE, SHALE, CLAY. (a) SiO 2 (b) SiO, A1 3 O S FeO Fe 2 0, CaO MgO K 2 O, Na 2 O (a) H 3 O (b) H 2 O Shale. I. 69.92 23-46 O.2O 0. 4 8 0.40 1-43 3-84 2. 67.29 15-85 6.16 0-95 0.19 8.71 3- 64.37 19-73 9.07 0.82 2.32 3-78 4- 62.86 20.65 9 .2I 0.48 0-34 6.26 5- 58.45 21.96 8-43 1.05 i-57 4.00 6.51 6. 43-13 40.87 3-44 8.90 5-32 2.42 0.20 Brick Clay. 7- 8l.7I 9.81 3-8o 0.48 0.26 ... 3-91 8. 75.88 11.22 5-04 0.48 0-35 6.76 9- 65.14 I3-38 7-65 2.18 2.36 8.51 10. 62.OO I8.IO 9.11 5*66 u. 57.80 22.6O 1.85 1.07 12.68 12. 53-77 20.49 9-23 2.04 4.22 9.60 13- 45-73 29.69 6.86 0.44 1. 01 3-42 12.86 Potter's Clay. 14. 27.68 36.58 22.95 1.28 0-45 0-37 1.96 6-74 2.05 15. 42.28 18.02 24.12 1.46 0-59 0.68 2.42 7-77 0.86 Fire Clay. 16. 61.60 28.38 0.52 0.46 0.36 5-08 17. 38.10 12.70 3 I -53 0.92 tr. 0.40 11.30 2.50 1 8. 45-29 40.07 1.07 0.26 0.08 0.48 13.18 Residual Clay. 19. 55-42 22.17 8.30 0.15 1-45 2.49 9.86 20. 33-55 30.18 1.98 3-89 0.26 i-57 10.72 Kaolin. 21. 46.50 39-57 ... 13-93 NOTE. Where two values of SiO 2 are given, the first is the combined silica, i. e. , chiefly in kaolin, and the second the free silica, which is practically comminuted quartz. Under H,O, where two values are given, the first is combined water, likewise chiefly in kaolin, the second is the free water, which has simply soaked in. I. Haydensville, Hocking Co., O. Quoted by H. Ries, XVI. Ann. Rep. Director U. S. Geol. Survey, Part IV., p. 572. 2. Hornellsville, Steuben Co., N. Y., Ibid., 572. 3. Kansas City, Mo., Ibid., 570. 4. Red Shale, Sharon, Mercer Co., Pa., Ibid., 572. 5. Leavenworth, Kan., Ibid., 570. 6. Clinton, Vermilion Co., Ind., Ibid., 570. 7. Washington, Davies Co., Ind., Idem, 566. 8. Salem, Washington Co., Ind., Idem, 566. 9. Red Clay, Plattsburg, Clinton Co., N. Y., Ibid., 568. . THE AQUEOUS AND EOLIAN ROCKS. 93 10. Red Clay, Lasalle, 111., Ibid., 564. II. Rondout, N. Y., Ibid., 568. 12. Brown Clay, Fisher's Is., N. Y., Ibid., 568. 13. Hooversville, Somerset Co., Pa., Ibid., 568. 14. Akron, O., Ibid., 562. 15. East Liverpool, O., Ibid., 562, alsoTiO 2 , 1.20. 16. Woodbridge, N. J., Ibid., 556. 17. Cheltenham, Mo., Ibid., 556. 18. Wood- land, Pa., Ibid., 556. 19. Morrisville, Calhoun Co., Ala., Ibid., 574. 20. Near Batesville, Ark., Ibid., 574, also P 2 O 5 , 2.53. 21. Pure Kaolin A1 2 O 3 2SiO 2 , 2H 2 O. Comments on the Analyses. The analyses are significant when compared with those of the sandstones on p. 89. It appears at once that there is a great decrease in silica, and a great increase in alumina, and, as a rule, in all the other bases and water. Among themselves 'there is wide variation, but by using No. 2 1 , as indi- cating pure kaolin, it is possible to infer how much quartz sand is mingled with the clay, due allowance being made for the fragments of unaltered feldspar, as shown by the alkalies, for silicates, hydrous and anhydrous, involving iron, lime and magnesia, and for carbo- nates of lime, magnesia and iron. Shales and brick clays are shown to be comparatively impure admixtures of kaolin and quartz ; potter's clay is much less so, and fire-clay is little else than these two. No. 1 8 is practically pure kaolin. Mineralogical Composition, Varieties. The argillaceous sand- stones have a finer grain than the sandstones proper, and tend to form thin but tough beds. They find their best examples in the flagstones of our eastern cities. Shales lack this coherence and break readily into irregular slabs and wedge-shaped fragments of no notable size. As sands give rise to sandstones, so on harden- ing and drying, muds and silts yield shales. Shales show all grades from gritty and coarse varieties to fine and even ones approximat- ing clays. The finer shales when ground have the same plasticity as clay, and are often moulded and baked into brick, especially of the vitrified kinds for paving. Shales may be black from bitumi- nous matter in them, and are then described as "bituminous." They grade into cannel coals, but great areas of them such as the Genesee Shale of New York and the Huron Shale of Ohio, have as much as 8 to 20 % hydrocarbons and yield quite copious prod- ucts on distillation. As the particles of quartz become finer and finer and not too abundant, the plasticity of the kaolin presently asserts itself so that the shales pass into clays. In the most even and homogeneous grades, they show but slight grit to the teeth, but in coarser 94 A HAND BOOK OF ROCKS. varieties they are decidedly gritty even to the fingers. They are often separated into thin beds by layers of sand that mark the times of freshets during their formation and the attendant deposi- tion of coarse material. Clays of earlier geological date are hard and dense rocks and must be ground before use. Such are the fire-clays immediately beneath Carboniferous coal-seams. Clays are blue, red and brown according to the state of the iron oxide, whether ferrous or ferric, or they may be nearly white when it fails. The less pure brick clays as shown by the analyses contain oxides of iron, calcium, magnesium and of the alkalies in quantity, but fire-clays practically lack these. As contrasted with the transported or sedimentary clays just mentioned, there are residual clays that result from the decay of impure limestones and that are found on weathered outcrops. They are very impure and variable in composition, but they are markedly plastic. Metamorphism. In metamorphic processes shales become com- pacted and oftentimes silicified. Their lack of homogeneity causes them to yield irregularly breaking and very tough rocks called graywackes, which differ only in greater hardness from their unaltered originals. Excessively silicified shales are called phtha- nites and are important in the Coast Range of California. Shales also under shearing stresses and attendant mineralogical reorgani- zation pass into schists of various kinds, such as quartz-schist, mica-schist and possibly hornblende-schist. G. F. Becker even mentions rocks derived from them that are mineralogically like diabases and diorites, but their recognition is a matter for micro- scopic study. Clays under shearing stresses develop new cleavages without regard to their original bedding and from the homogeneous character of the original and the perfection of the cleavage, slates result, which are of great practical importance. Occurrence. Shales and clays are such common members of extended geological sections as to deserve no special mention. They are often a thousand feet or more in thickness and cover great areas. THE AQUEOUS AND EOLIAN ROCKS. 95 CALCAREOUS SANDSTONES, MARLS. Calc. Sandst . SiO 2 A1 S 3 FeO Fe 8 0, CaO MgO K 2 O Na 2 O C0 2 H Loss r I. 79.19 3-75 7.76 3-20 3.26 2. 38.41 5-77 1.79 20.08 8.82 0. 12 O.29 Calc. Shales. 3- 39-70 26.83 19.28 2.43 5-" 4- 28-35 12.37 21.47 8.24 5-73 Marls. 5- 43-70 25.00 8.85 2-33 ... 5.40 9.21 6. 38.70 IO.20 18.63 9.07 1.50 3-65 6.14 IO.OO 7- 28.78 11.63 2. 9 6 24.50 2.91 2.12 22.66 4-18 I. Calcareous sandstone, Flagstaff, Ariz. Quoted by G. P. Merrill, Stones for Building and Decoration, 420. 2. Calcareous sandstone, Jordan, Minn., Idem. 3. Genesee Shale, Mt. Morris, N. Y., supplied by H. Ries. 4. Niagara Shale, Rochester, N. Y., H. T. Vult6, analyst. Supplied by H. Ries. 5. Cretaceous Marl, Hop Brook, N. J., Geol. of N. J., 1868, 419; also P 2 O 5 , 2.18. 6. Cretaceous Marl, Red Bank, N. J., Idem, 418; also P.,O 5 , 1.14, SO 8 0.14. 7. Subcarboniferous marl, Bowling Green, Ky., Ky. Geol. Surv., Chem. Analyses A, Part 3, 90; also, P 2 O 5 , 0.25. Comments on the Analyses. The analyses illustrate in a very suggestive way the passage of these mechanical sediments into im- pure limestones. The gradual intermingling of more and more of shells and other remains of organisms brings it about. The high P 2 O 5 of the marls, as cited under the references, is worthy of re- mark. It is to be appreciated that the lime and magnesia and some of the iron of the analyses are to be combined with CO 2 , even though the CO 2 is not mentioned. Mineralogical Composition, Varieties. Calcareous sandstones are practically sandstones with rich calcareous cement, or with a large amount of organic fragments intermingled with the prevailing quartz sand. They are passage forms to the fragmental limestones. Calcareous shales derive their lime partly from the fine organic sediment that is deposited with the siliceous and aluminous particles and partly from contained fossils. Beds of these rocks are partic- ularly favorable layers for the discovery of the latter, and often break the monotonous barrenness of a geological section composed of ordinary shales. Marls, strictly speaking, are calcareous clays, and originate in typical cases by the deposit of limy slimes along with the aluminous. The lime destroys the plasticity of the clay and yields a crumbling rock, often richly provided with fossils and of value as a fertilizer. Grains of glauconite, the green silicate of potash and iron, are at times present, and characterize the so-called 96 A HAND BOOK OF ROCKS. " green sands " which are valuable as fertilizers. The term marl is somewhat loosely used in its applications, and moderately coarse calcareous sands, and even beds which show but small percentages of lime on analysis are designated by it in the States along the Atlantic seaboard from New York south. It is clear that marls are intermediate rocks between clays and impure earthy limestones. Metamorphism. The rocks of this group are altered in meta- morphic processes to schistose forms, not so essentially different from those resulting from the common aluminous shales and clays, except that the richness in lime facilitates the production of minerals requiring it. The marls, when high in lime, behave like impure limestones, and are prolific sources of silicates. Marls are, how- ever, much more common in later and unmetamorphosed formations than in older ones, although it may be that in the latter they have yielded some schistose derivatives not readily traceable back to them. Occurrence. Calcareous sandstones and shales are met as occa- sional beds in series of the more abundant, distinctively aluminous varieties. Marls are chiefly developed in the Cretaceous and Tertiary strata of the Atlantic seaboard and around the Gulf of Mexico. Freshwater ones are not lacking in the Tertiary lake basins of the West. CHAPTER VIII. LIMESTONES ; ORGANIC REMAINS NOT LIMESTONES ; ROCKS PRECIPITATED FROM SOLUTION. DETERMINATION OF THE AQUEOUS AND EOLIAN ROCKS. II. LIMESTONES. SiO, A1 2 O, Fe 2 3 FeO CaO MgO CO, H 2 O Insol. CaC0 3 MgCO, Living Organisms. I. (Coral) 54-57 2-54 97.46 2. (Reef- rock) 53-82 I.OI 96.11 2.13 3. ( Lagoon Sed.) 54-58 0.85 97-47 1.79 4. (Coral) 44.96 3-87 80.29 8.14 5. (Oyster Shells) 44.4 i-3 35-4 14-5 (79-28) (2-73) Calcite. 6. Pure Mineral. 56. 44- 100. Dolomite. 7. Pure Mineral. 3-43 21.72 54-35 45.65 Marine Limestones. 8. 0.63 0-55 55-6 0.23 99-30 0.49 9- 1. 06 53-78 0-34 0.90 1.13 96.04 0.72 10. 1.25 53-89 o. 10 96.24 O.2I ii. 1.84 0.64 1.82 51.40 2.23 41.19 0.27 91.80 4.68 12. 12.34 7.00 44.41 0.44 79-30 0.92 13. 3.77 0.08 6.80 33-79 15-32 42.21 60.35 32.61 14. o-55 29-54 21. 08 1.82 0.60 52.75 44.28 Waterlime. IS- 18.34 7-49 37.60 I. 4 8 3-94 67.14 2.90 16. 15-37 11.38 25.70 12.44 1.20 45.91 26.14 Siliceous. 17- 1.20 17.69 10.59 1.24 43.72 31.60 22.24 Freshwater Limestone. 1 8. 0-37 54- 16 0.15 43.68 1.49 96.71 0.31 19. 1.83 O.22 34-20 O. II 26.79 4-64 31-28 61.07 0.23 Travertine. 20. 0.08 0.15 53.83 0.90 41.79 1.43 94-97 0-43 I. Stag's horn coral {Millepora alcicornis}, S. P. Sharpless, Amer. Jour. Sci., Feb., 1871, 1 68. 2. Bermuda coral reef rock. A. G. Hogbom, Neues Jahrb., 1894, I., 269. 3. Bermuda coarse lagoon sediment, Idem. 4. Average of 14 analyses of the coral Lithothamnium from localities the world over, Idem, 272. 5. Oyster shells, Geol. of New Jersey, 1868, 405. 6. Calculated from CaCO s . 7. Calculated from CaCO 3 MgCOj. 8. Crystalline Siluro-Camb. limestone, Adams, Mass., E. E. Olcott for Marble Co. 9. Limestone, Bedford limestone, Ind. Quoted by T. C. Hopkins, Mineral In- dustry, 1894, 505. 10. Solenhofen lithographic stone. Quoted by G. P. Merrill, 7 97 98 A HAND BOOK OF ROCKS. Stones for Building and Decoration, 415. 1 1. Limestone, Hudson, N. Y., Th. Egleston. 12. Trenton limestone, Point Pleasant, Ohio, vide No. 10. 13. Surface Rock, Bonne Terre, Mo., J. T. Monell, unpublished. 14. Limestone, Chicago, T. C. Hopkins, Mineral Industry, 1895, 508. 15. Hydraulic limestone, Coplay, Penn. Quoted by W. A. Smith, Mineral Industry, 1893, 49. 16. Hydraulic limestone, Rosendale, N. Y., Idem. 17. Siliceous limestone, Chicago, 111., vide No. 14. 18. Miocene lime- stone, Chalk Bluffs, Wyo., R. W. Woodward, 4Oth Parallel Surv., I., 542. 19. Eocene limestone, Henry's Forks, Wyo., B. E. Brewster, Idem. 20. Travertine, below Hotel Terrace, Yellowstone Park, J. E. Whitfield, for W. H. Weed, 9th Ann. Rep. Dir. U. S. Geol. Surv., 646. Comments on the Analyses. The first three analyses and the fifth indicate that the calcareous parts of living organisms are quite pure calcium carbonate. The fourth analysis is of that species of coral which, so far as we know, is highest in magnesia. Small amounts of calcium phosphate are often present as well, some shells being richer than others. Nos. 6 and 7 are introduced so as to give a basis for estimating the purity of the following limestones : Nos. 8, 9 and 10 are extremely pure varieties, and from these, as a starting point, the other components increase in one analysis and another. No. 14 is a nearly typical dolomite. Nos. 12 and 17 are highly siliceous, and Nos. 1 5 and 1 6 are both strongly argillaceous. The last two are closely parallel in composition with marine varie- ties. An analysis of a travertine is given in No. 20. It at once appears that Nos. 13, 14, 16 and 17 are far higher in magnesia than any known living organism, and it is evident that an original organic deposit must have undergone an enrichment in magnesium carbonate to bring them about. Dana suggested many years ago that coral or other organic sand, when agitated in sea- water, probably exchanges a part of its calcium for magnesium, and there is much reason to think that it does. Otherwise, the change must have been brought about by magnesian solutions per- colating through the rock and altering it by the replacement proc- ess called dolomitization, or dolomization. Much of the silica, no doubt, results from radiolarians and sponge spicules, but much also, together with the alumina, from fine fragmental sediments. Origin. Much the greater number of the important limestones are of marine origin, but in certain geological formations fresh- water ones are well developed. The calcareous remains of organ- isms have been their principal source, and of these the forami- nifera, the corals, and the molluscs have been the chief contributors. 7 HE AQUEOUS AND EOLIAN ROCKS. 99 Their shells have often become thoroughly comminuted to a calcare- ous slime before final deposition, so that the resulting rock affords no trace of organic structure. The solubility of the carbonate of lime aids in the cementation of the slime to rock and tends to efface the organic characters. Limestones pass by insensible gradations through more and more impure varieties into calcareous shales and marls, but, as a rule, they are deposited in deeper water than the true shales and sandstones. This conception must not be applied too strictly, because, beyond question, a depth of a few feet has often sufficed, and too much emphasis has often been placed upon the depth regarded as necessary for limestones. Coral sands ac- cumulate on or near the immediate shore, and may even be heaped up by the wind. The general geological relations involved in the deposition of limestones are well illustrated in the accompanying Fig. 41. The FIG. 41. Cross-section of a fossil-coral reef at Alpena, Mich., showing the reef- coral in the fan-shaped pattern ; the coarse coral-sand in the shaded part ; and the fine sand shading into slimes farther away. After A. \V. Grabau, Annual Kept. Mich. State Geologist, 1901, 176. reef of coral grows constantly and from the action of the breaking waves is partially comminuted to sand, which settles on the flanks and furnishes a place of residence for various mollusca whose hard parts contribute also to the growing limestone. The finer material is transported to a greater distance and gradually settles out as slimes which afford dense and often thin-bedded varieties. The conditions for the deposition of the latter have often been unfavorable for organic life, and it frequently happens that the resulting limestones are devoid of fossils except in the vicinity of the old reef. Based upon the varieties just outlined, A. W. Grabau has suggested the following varieties of limestones ; organic limestones, such as would be afforded by the reef itself; coarse, clastic limestones or calci- rudites (t. e., lime-rubbles) ; sand limestones or calcarenites (/. e., ioo A HAND BOOK OF ROCKS. lime-sands) ; and mud limestones or calcilutites (i. e., lime-muds) (Bulletin Geol. Soc. Amer., XIV., 348-352.) In confined estuaries of sea water subjected to evaporation, enough carbonate of lime is precipitated directly from solution, to yield important strata, which are often met in a series of beds associated with rock salt and other precipitated rocks as later set forth. Calcareous deposits from limy springs may also almost reach the dignity of rocks, and when abundant are called travertine or calcareous tufa. If particles of dust, etc., are suspended in limy springs or in concentrated estuarine waters, they gather concentric shells of the carbonate and may yield oolitic deposits from the co- alescence of the concretions. Some algae likewise secrete oolitic calcite and contribute extensively to rocks. Mineral Composition, Varieties. Calcite is the chief mineral of limestones, and when thin sections are magnified it exhibits its characteristic cleavages. Dolomite and siderite accompany it fre- quently, and their molecules also replace the calcium carbonate, in a greater or less degree, so as to form double carbonates. An un- broken series can readily be traced from pure calcium carbonate, through more and more magnesian forms, to true dolomite. Those with over 5 per cent. MgO are usually described as magnesian limestone, and when the MgO mounts well toward the 21.72 per cent, in the mineral dolomite, we use the latter name. In the same way, a series of ferruginous varieties may be established toward the clay ironstone and black band ores, and a siliceous series toward the flints and cherts. Cherty limestones are a very common variety, and are referred to again in connection with chert. When the argillaceous or clayey intermixtures enter, argillaceous or hydraulic varieties result that are generally drab and close-grained, and are useful in the manufacture of cement. Bituminous matter may be present, making the limestones black, and this, in the form of asphalt, may yield asphaltic varieties. Besides these varieties established on the basis of chemical com- position, special names may be given because of structure. Thus earthy limestones tend to crumble to dirt ; oolitic limestones re- semble the roe of a fish ; pisolitic varieties consist of concretions of size comparable with peas ; and other terms are employed, that are self-explanatory. Prominent fossils suggest names, such as THE AQUEOUS AND EOLIAN ROCKS. 101 crinoidal, from fossil crinoids ; coraline, foraminiferal and many more of local or stratigraphic significance. Practical applications play a part in nomenclature, supplying " waterlime," " cement- rock," " lithographic limestones," etc. Metamorphism. Limestones feel the effect of metamorphism with exceptional readiness and under deforming stresses, probably accompanied by elevation of temperature, and in the presence of water, or along the contacts with intruded dikes and sheets of igneous rocks, they lose their sedimentary characteristics, such as bedding-planes and fossils, and change into crystalline marbles. The contained bituminous matter becomes graphite ; the alumina and silica unite with the lime, magnesia and iron to give various silicates. Other oxides together with the bituminous ingredients contribute to the various colorations. Mechanical effects are manifested in flow lines, brecciation and other familiar features of many that are cut and polished for ornamental stones. Impure limestones which undergo these metamorphic changes are the most prolific of all rocks in variety and beauty of minerals. Arendal, Norway, and the crystalline limestone belt from Sparta, N. J., north through Franklin Furnace are good illustrations. The crys- talline limestones will be again mentioned under the metamorphic rocks. Occurrence. Limestones are too common to deserve special mention as regards occurrence. They are frequently met in all parts' of the country, but the Trenton limestone of the Ordovician, the Niagara of the Silurian and the Subcarboniferous limestones of the Mississippi Valley are specially worthy of note. III. REMAINS OF ORGANISMS NOT LIMESTONES. Calcareous remains are much the most important of the contribu- tions made by organisms to rocks, but there are others, respectively siliceous, ferruginous and carbonaceous, which deserve mention. SILICEOUS ORGANIC ROCKS. The principal members of this group are infusorial or diatoma- ceous earths ; siliceous sinters ; and cherts, hornstones or flints, the three last names being practically synonymous. Infusorial earths consist of the abandoned frustules of diatoms, which are micro- 102 A HAND BOOK OF ROCKS. scopic organisms belonging to the vegetable kingdom. Though not a common rock, they yet are met in series of sedimentary strata, both freshwater and marine, with sufficient frequency to justify their mention. Some foreign earthy materials are unavoid- ably deposited with them. The siliceous sinters are extracted from hot springs by algae which, as shown by W. H. Weed, are capable of living and secreting silica in waters up to 185 F. They are far less important geologically than the infusorial earths. Chert is a rock consisting of chalcedonic and opaline silica, one or both. It possesses homogeneous texture and is usually associated with limestones, either as entire beds, or as isolated, included masses. It often has druses of quartz crystals in cavities, and in thin sections under the microscope it sometimes exhibits sponge spicules. Cherts not provided with these organic remains may be regarded with great reason as chemical precipitates, and as American varie- ties in the great majority of cases lack them the cherts receive more extended mention under the chemical precipitates. SiO 2 A1 7 O 3 Fe 2 O 3 FeO CaO MgO Na 2 O K 2 O H 2 O Infus. Earths. 1. 91.43 2.89 0.66 0.36 0.25 0.63 0.32 3.8 2. 86.90 4.09 1.26 0.14 0.51 0.77 0.41 5.99 3. 75.86 9.88 2.92 0.29 0.69 0.08 0.02 8.37 Silic. Sinter. 4. 89.54 2.12 tr. 1.71 tr. 1. 12 0.30 5.13 Chert. . ' CaCOj. MgCO 3 . v ' 5. 34-0 0.80 63.4 1.5 0.3 I. Miocene, Little Truckee River, Nev., R. W. Woodward, 4Oth Parallel Survey, I., opposite p. 542. 2. Fossil Hill, Nev., Idem. 3. Richmond, Va., M. J. Cabell, Mineral Resources, 1883-84, p. 721. 4. Deposit from Old Faithful, Yellowstone Park, J. E. Whitfield, for W. H. Weed, 9th Ann. Rep. Dir. U. S. Geol. Sur., 670. 5. Cretaceous chert, England, Jukes-Brown and Hill, Quar. Jour. Geol. Soc., Aug., 1889. Comments on the Analyses. The infusorial earths are fairly high in water, and this is the main cause of low silica, but, as stated above, their growth and accumulation in water make it unavoid- able that more or less clay and other sediments should mingle with them. In these and the other members of the series, it is important to understand that much of the silica is opaline, or amor- phous, hydrated silica, and not quartz or chalcedony. Tests of the amounts soluble and insoluble in caustic alkali are usually made to determine the proportions of the two, for, while it is not THE AQUEOUS AND EOLIAN ROCKS. 103 an accurate separation quartz and chalcedony being themselves somewhat soluble it gives an approximate idea. No. 4 is a de- posit separated from the geysers by algae and evaporation. No. 5 is largely due to sponge spicules, mixed in with chalk, and there- fore is high in calcic carbonate. Miner alogical Composition, Varieties. The mineralogy of the infusorial earths can be stated less definitely than the chemical com- position. The individual diatoms are very minute, but the analyses indicate both opaline and chalcedonic silica as being present. In the sinters and cherts, when the latter can be shown to be organic, the same two varieties are recognizable, and with them are varying amounts of calcite. The infusorial earths are fine, powdery de- posits, resembling white or gray, dried clays, but they lack plas- ticity and are best recognized with the microscope. Siliceous sin- ters, often called geyserite, are cellular crusts and fancifully shaped masses that closely resemble calcareous tufas, but that are readily distinguished by their lack of effervescence. Chert is dense, hard and homogeneous, and of white, gray or black color. It readily strikes fire with steel, and when it breaks has a splintery or con- choidal fracture. It is often decomposed to powdery silica on the outside, and in extreme cases may yield rather large deposits of this powder, which are called " tripoli," and are used for various practical purposes. Mention may again be made of the cherts that seem best explained by chemical precipitation. Metamorphism. The cherts alone of these rocks are of suffi- cient importance to attract attention in this connection, and their metamorphism is briefly referred to on page 109. Occurrence. Infusorial earths are abundant near Richmond, Va., and on Chesapeake Bay, at Dunkirk, and Pope's Mills, Md. Beds deposited in evanescent ponds or lakes are also well known in States farther north. In the West, the Tertiary strata have yielded them in Nevada. In California and Oregon great areas are re- ported by Diller. Siliceous sinters produced by algae are quite extensive in the Yellowstone Park, and similar deposits, perhaps caused by the same agent, are found in many regions of hot springs. Sinters chemically precipitated also occur. The most important occurrences of chert are all mentioned together on page 1 10. I04 A HAND BOOK OF ROCKS. FERRUGINOUS ORGANIC ROCKS. It is a question whether these deserve the dignity of rocks, for they may with great propriety be classed with the minerals, dis- tinctively so called. It will therefore only be mentioned that many have attributed the formation of beds of limonite to the separation of iron hydroxide by low forms of organisms. Even granting this, it is still true that such limonites are insignificant when compared with those that result by purely inorganic re- actions in the decay of rocks. Important strata of cherty car- bonates of iron are present in the iron mining districts around Lake Superior and have been, no doubt, the principal source of the hematites. Van Hise regards them as probably of organic origin, but the evidence is not decisive and they may be chemical precipitates. Clay-ironstone and black-band ores that is, argil- laceous and bituminous ferrous carbonate sometimes form con- tinuous beds instead of the usual isolated lenses, but when they do, they are not organic in origin, although decaying organic matter may be instrumental in preserving the reducing conditions that are necessary to the formation of the ferrous salt CARBONACEOUS ORGANIC ROCKS. When plant tissue accumulates in damp places and under a pro- tecting layer of water which prevents too rapid oxidation, new accessions may more than compensate for loss by decay so that ex- tensive deposits may result. These become progressively rich in carbon by the loss of their other elements and yield beds of con- siderable geological, but much greater practical importance. The course of the changes and the several stages are indicated in the following table : C. H. O. N. Total. Woody Tissue 50 6 43 i 100 Peat 59 6 33 2 100 Lignite 69 5.5 25 0.8 100.3 Bituminous Coal 82 5 13 0.8 100.8 Anthracite 95 2.5 2.5 trace. loo The changes are in the nature of loss of oxygen and hydrogen, and also of carbon, but the decrease of the first two is relatively so much greater, that the carbon actually is enriched. The table is theoretical in that no account is taken of the more or less fortuitous THE AQUEOUS AND EOLIAN ROCKS. 105 mineral matter which forms the ash together with a small percentage of incombustibles in the vegetable tissue itself. Peat is a more or less incoherent mass of twigs and stems, decidedly carbonized and darkened, but with the original structures, as a general rule, still well preserved and recognizable. By gradual stages it passes into lignite, which is still further compacted, and which exhibits the original structures more faintly. In bituminous coal, they are seldom recognizable, and the aggregate is compact and black. In anthracite the coal is dense, amorphous and lustrous. The oxi- dation necessary to the later varieties may have been largely per- formed before actual burial in other rocks, but the changes are continuous and progressive in all. Other organic derivatives, such as asphalt, petroleum, etc., are not considered to be of sufficient abundance to rate as rocks. Metamorphism. Anthracite is locally produced from bituminous coal near igneous intrusions, and by regional metamorphism, as later explained. The chemical changes are the same as those progressive ones above outlined, but are doubtless more rapidly brought about. Anthracites become graphitic, and, as a theoret- ical extreme, pass into graphite. Natural cokes are also produced along intruded dikes. Occurrence. Peat favors cool and moist latitudes in all parts of the world, and is chiefly of fresh water origin. Lignites and coals are best developed in the Carboniferous and Cretaceous strata, and where the former occur in the East and the latter in the West, they often contain coal seams. IV. PRECIPITATES FROM SOLUTION. The name of this group indicates the character of the rocks that comprise it. Bearing in mind the condition established at the outset, p. I, that a rock should form an essential part of the earth, it is evident that water is the only natural solvent abundant enough to yield such rocks, and that only the most widespread compounds which are notably soluble in it, or in its common solutions of other more soluble salts, can meet this requirement. The rocks may be conveniently taken up under the following heads, i. Precipitates involving the alkaline earths and alkalies. 2. Siliceous precipitates. 3. Ferruginous precipitates. io6 A HAND BOOK OF ROCKS. PRECIPITATES INVOLVING THE ALKALINE EARTHS AND ALKALIES. The carbonate of lime in stalactites, stalagmites and crusts on the walls and floors of caves in limestone or in the surface deposits from limy springs, affords a rock of this character. It is a form of limestone, from pure varieties of which it does not differ in com- position, although its banded structure and rings of growth, which we may describe by Posepny's useful word " crustification," in a measure distinguish it. Naturally such deposits are often beauti- fully crystalline, free from admixture except of associated dissolved materials and as a rule purer than sedimentary limestones. They yield our well-known onyx marbles. Some regularly stratified de- posits of limestones that are associated with the precipitated rocks next discussed have doubtless originated together with the latter. Gypsum and rock salt are the chief members of this subgroup. They occur quite invariably in association, and have resulted alike from the evaporation of sea-water and from the drying up of lakes, originally fresh. Both are mixed more or less with dust and other mechanical sediments washed or blown into the evaporating res- ervoir, or are interbedded with other salts which were present in a minor capacity in the mother liquor, but instances of thick beds, especially of rock salt of surprising purity, are well known. When these attain several hundred or even a thousand feet, it is evident that more than twenty-five times this depth of salt water, on the basis of the known composition of the sea, would have to be evap- orated, and this is a practical absurdity for any conceivable con- fined body, even with occasional renewals from breaches of the bar- rier. It would be necessary to assume wide stretches of shallows which were practically evaporated to dryness, while at the same time subsidence of the coast was progressing at just about the necessary rate to keep pace with the growth of the salt. The recent ex- planation, however, advanced as the " Bar theory," by Ochsenius,* clears it up. We need only to assume a relatively deep and nearly land-locked estuary, with a shallow bar between it and the sea. Evaporation continually concentrates the confined salt water and especially the portion on the shallow bar. This, becoming rich in mineral matter and of high specific gravity, flows inward and * Zeitschrift f. Praktische Geologic, May and June, 1893. An excellent abstract by L. L. Hubbard appears in the Geol. of Michigan, V., Part II., p. ix. THE AQUEOUS AND EOLIAN ROCKS. 107 down the slope of the bar to the bottom of the estuary. In the course of time, and allowing for the influence of pressure in the depths and of temperature, conditions favorable to precipitation, first, of the insoluble gypsum, later of the more soluble common salt will be reached, and in varying and alternating layers they will be built up indefinitely, or until some upheaval or subsidence alters the relations of the estuary to the sea. More or less anhydrite is also deposited, and is later found in extended cross-sections of salt-bearing strata. The most soluble ingredients, such as KC1, MgCl 2 , MgSO 4 , etc., become continually richer in the mother liquor, and unless this is also finally evaporated, they escape and are not found in the series. So far as we know, the Stassfurt dis- trict, in Germany, is almost the only place where this escape has been prevented on a large scale, although rock salt is of world- wide distribution. Gypsum forms at times gray or black earthy beds, that look very much like limestone, but of course do not effervesce. Again, it is in white, cream-colored or more deeply tinted layers, yielding ala- baster. Minor portions are in condition of selenite, the clear, trans- parent variety, and thin coats of native sulphur are seldom lacking. Rock salt forms crystalline beds, often stained red or brown, by iron oxide. Both gypsum and salt may impregnate associated sediments more or less, yielding gypseous or saline shales and marls. In many localities gypsum deposits have undergone a complex series of chemical changes in the general nature of deoxidation from car- bonaceous matter present, so as to yield native sulphur in large amounts. Metamorphism. None of the above rocks are worthy of men- tion as regards metamorphism. Occurrence. In America, gypsum is found especially in the Upper Silurian of New York ; the Lower Carboniferous of Michi- gan and Nova Scotia ; the Triassic in the states of the Great Plains such as Kansas and Texas ; in undetermined Mesozoic in Iowa ; and in the Jura-Trias or in undetermined strata in Colorado, Utah and the West. Rock salt occurs in the Upper Silurian of southern New York ; in the Triassic of Kansas ; in the Quaternary (?) of Petite Anse, La., and at many places of recent geological age in the West. io8 A HAND BOOK OF ROCKS. SILICEOUS PRECIPITATES. K 2 O (a)SiO 8 (b)SiO 2 Als,O 3 Fe 3 O, CaO MgO Geyserite. I. 81.95 6.49 tr. 0.56 0.15 Cherts. 2. 99.46 0.29 0.4 tr. 3- 3-35 95-78 o. 16 tr. O.OI 4- 4.5293-65 0.83 0.05 O.OI 5. 98.10 0.24 0.27 0.18 6. 94.91 2.85 0.42 tr. Sil. Oolite. 7- 95-83 2.03 1-93 tr. CaCO s MgC0 3 8. 56.50 1.50 16.84 2.60 9- 3-7 1.42 88.71 8.09 Cherty iron carbonates. CaO MgO 10. 58.23 0.06 5.01 0.38 9-59 II. 46.46 0.24 0.64 1.87 3.10 12. 28.86 1.29 i. 01 0.74 3-64 H,Oor Na 2 O Loss. Sp. Gr. 0.65 2.56 7.50 o-34 0.20 0.78 0.23 1.16 FeO 18.41 26.28 37-37 12.54 MnO 0.25 2.08 O.2I 1.22 0.97 0.68 2.63 2.688 2.654 CO, 5-22 19.96 25.21 NOTE, (a) SiO 2 means silica soluble in caustic alkali ; (b) SiO 2 silica insoluble in the same. I. Geyserite, Splendid Geyser, Yellowstone Park, J. E. Whitfield for W. H. Weed, gth Ann. Rep. U. S. Geol. Sur., 670. 2. Gray unaltered chert, Joplin, Mo. Anal- ysis made by U. S. Geol. Surv. Quoted in Ann. Rep. Geol. Sur. Ark., 1896, III., 161. 3. White altered chert, Galena, Kan., Idem. 4. Unaltered chert, Bellville, Mo., Idem. 5. Decomposed chert, or Tripoli, Seneca, Mo., W. H. Seamon. Quoted by E. O. Hovey, Amer. Jour. Sci., Nov., 1894, 406. 6. Chert, Roaring Springs, Newton Co., Mo., J. D. Robertson, for E. O. Hovey, Idem. 7. Siliceous oolite, Center Co., Penn., Barbour and Torrey, Amer. Jour. Sci., Sept., 1890, 249. 8. Silica-lime oolite, Idem. 9. Lime-silica oSlite on same specimen as No. 8, Idem. 10, II. Cherty iron carbonates, N. E. Minn., T. M. Chatard, for C. R. Van Hise, Monograph XIX., U. S. Geol. Survey, 192. 12. Cherty iron carbonate, Sunday Lake, Gogebic Range, Mich., W. F. Hillebrand, Idem. Comments on the Analyses. The first seven are high in silica, some approximating chemical purity. No. I has admixtures of mud thrown out by the geyser from its walls. The five cherts, 2-6 inclusive, have but slight amounts of alumina, iron and lime, and low percentages of water. Nos. 3 and 4, by the determina- tions of soluble silica give us some idea of the amount of the opaline form that is present. The three analyses 7, 8 and 9 are a most instructive series, passing as they do from nearly pure silica into a moderately siliceous, magnesian limestone, from which the first two are thought to have been derived by replacement. Nos. 10, 1 1 and 12 are the curious cherty carbonates of iron from which THE AQUEOUS AND EOLIAN ROCKS. 109 the Lake Superior iron ores have been formed by subaerial decay. Their richness in magnesia as compared with lime is noteworthy. Mineralogical Composition, Varieties. Cherts are so exceed- ingly fine grained that they give no indication of their constituent minerals to the unaided eye. The microscope shows, however, that they are chiefly chalcedony in excessively minute crystals, with which are associated varying amounts of opaline silica, quartz crystals, calcite or dolomite rhombs and du^ty particles of iron oxide. In foreign cherts as stated above on p. 102, sponge spic- ules have been met, but not in the important American varieties. Cherts often have an outer powdery crust, due to alteration, and while as shown by analysis 5, this may not mean any notable chemical change, it may penetrate whole beds and leave only a white, incoherent mass called " tripoli," that is used for a polishing powder and for various other purposes. Cherts have spherulites occasionally and are still more often oolitic. The cherty or sili- ceous rocks of the formations containing the Lake Superior iron ores are mixtures of chalcedonic silica and carbonate of iron in varying proportions, and in their alteration they afford more or less sharply differentiated jaspers and hematites. Three analyses of varying composition are given above, Nos. 10, n and 12. Cherts vary in color from black through gray to creamy white. As stated earlier, cherts are intermingled in all proportions with limestones. They are very puzzling problems as regards origin. Where devoid of organisms, the majority of observers regard them as in some way precipitated chemically from sea water, possibly as gelatinous silica. They may also result by replacement of limestone. Their structure and relations give us few definite clues on which to base a firm conclusion. As earlier stated, others regard them as derived from siliceous remains of organisms,such as sponges, radio- larians and the like, which may have been redissolved and worked over into chalcedony, making them practically precipitates. Cherts are also called hornstone and flint. Metamorphism. Purely siliceous cherts are unpromising sub- jects for metamorphism, except as they yield silica for the produc- tion of 'silicates from cherty limestones. The ferruginous cherts of Lake Superior pass into actinolitic and magnetitic slates, a most interesting change, especially in the former case. The lime, maer- no A HAND BOOK OF ROCKS. nesia and iron are combined with silica under the metamorphosing influences so as to yield the variety of actinolite called grunerite. Occurrence. The abundance of cherts or related rocks in the region of Lake Superior, either associated with limestone or in the cherty carbonates described above, is remarkable. In their eco- nomic products, they are the most important strata present. The Siluro-Cambrian limestones are often cherty both east and west, and in the New York and Ohio Devonian, the so-called " Cornifer- ous " limestone was named from its richness in " hornstone." In the Mississippi Valley the lower Carboniferous strata are particu- larly prolific in cherts. Fractured cherts are the chief gangue of the zinc ores of southwest Missouri. FERRUGINOUS PRECIPITATES. Some iron ores doubtless originate in this way, and the processes by which the soluble proto-salts are oxidized and precipitated as the insoluble ferric hydroxide are well understood. But they may be considered rather as minerals than as rocks. The cherty iron carbonates of the preceding section have already been cited, and the clay ironstones and black-band iron ores are omitted from further mention for the same reasons as are the limonites. THE DETERMINATION OF THE AQUEOUS AND EOLIAN ROCKS. The members of this series are much easier to recognize than are the igneous. Breccias, conglomerates and sandstones are at once apparent from their fragmental character. Breccias differ from conglomerates in the angular shape of their component frag- ments. As the sandstones become finer, the argillaceous varieties may be distinguished by the peculiar odors emitted by all clays and clayey rocks when breathed upon. The calcareous sandstones and marls betray themselves by effervescence with acid. All lime- stones, unless too rich in magnesia, effervesce in cold acid, and the more readily if first scraped up into a little heap of powder with a knife. Dolomites effervesce much less readily, and warm acid may be necessary. Infusorial earth may need the microscope for its certain identification, and then the abundance of the little organisms is very apparent. The cherts are so characteristic in appearance as to admit of little uncertainty, except as compared with the silici- THE AQUEOUS AND EOLIAN ROCKS. in fied tuffs and excessively fine felsites, called petrosilex, in which case geological surroundings or the microscope are the only resources. The ferruginous rocks, if such be allowed, are self-evident, as are the carbonaceous. Gypsum is easily recognized when in the crystal- line form, but when black and earthy, the observer may be forced to determine its lack of effervescence, and to make a sulphur test with the blowpipe. Nevertheless with these rocks as with the igneous, although to a less degree, it is very advisable to gain ex- perience with correctly labeled study collections or with the syste- matic exhibits of a museum, so that the student may have a fund of personal observation back of him from which to draw, and on which to depend when a rock comes up for determination. For field work and travel, it is well to appreciate that a few dry crystals of citric acid, that can be dissolved in a little water as needed, serve very well for tests of effervescence. They are more safely carried than are liquid mineral acids. CHAPTER IX. THE METAMORPHIC ROCKS. INTRODUCTION. THE ROCKS PRO- DUCED BY CONTACT METAMORPHISM. The word metamorphism was first introduced into geological literature by Lyell in 1832, and was used to describe the processes by which rocks undergo alteration. It was particularly applied by him to those stratified rocks that, from deep burial in the earth, and from the consequent heat and pressure to which they have been subjected, have assumed structures and textures resembling those of the unstratified primary or plutonic. In this sense it has been generally employed since, and it implies an increase in crys- tallization, hardness and those attributes, which are especially associated with the crystalline schists, as contrasted with the un- altered sediments. The literal meaning of the phrase " the processes by which rocks undergo alteration " may, nevertheless, be somewhat more com- prehensive than this, and may be made to include the changes produced by atmospheric agents, which we ordinarily describe by the term weathering, and in the following pages the products of this latter form of alteration will be briefly considered as a third and concluding group. The metamorphic rocks will therefore be taken up under the following three classes : I. Rocks reduced by Contact Metamorphism. II. Rocks produced by Regional Metamorphism. III. Rocks produced by Atmospheric Weathering. By contact metamorphism is meant the series of changes that are effected by an igneous intrusion, such as a dike or a laccolith upon the rocks through which it is intruded. These changes are often profound, and are brought about by the heat of the intrusion as well as by vapors and hot solutions which it may likewise give forth. The wall-rock may be itself igneous or sedimentary, or even metamorphic. This form of metamorphism is sometimes called "local" as contrasted with "regional." THE METAMORPHIC ROCKS. 113 By regional metamorphism we describe the series of changes which are produced in the rocks of wide areas or "regions" by deep burial, mountain-making upheavals, and by heat and pres- sure. Although Lyell had stratified rocks before him as the chief materials on which these agents acted, yet it is well recognized to- day that igneous rocks are no less profoundly affected, and indeed that the results of their alteration may be almost or quite indistin- guishable from those derived from sediments. But there is great uncertainty as to the original condition of many regionally meta- morphosed rocks, and although the endeavor has been made in previous pages to throw as much light on them as possible, by systematically referring to the alteration and metamorphism of simple types, nevertheless, many are obscure, and in their history are involved some of the profoundest problems of geology. By atmospheric weathering is meant the series of changes wrought in rocks at or near the surface of the earth, by the ordinary atmos- pheric agents, water, oxygen, carbonic acid and the like. The changes are chiefly in the nature of disintegration, loss of soluble ingredients and decomposition, and in general they produce a marked shrink- age of bulk. It is important to appreciate that under whatever form the meta- morphic rocks are met, they are of necessity alteration products of the two grand divisions over which we have already passed. GENERALITIES REGARDING CONTACT METAMORPHISM. Widening observation has shown that contact metamorphism is produced by all varieties of igneous rocks and that it may be broadly stated to be independent of the kind of rock forming the intrusion. Granites, syenites, nephelite-syenites, diorites, gabbros and even peridotites have in one place and another proved to be efficient agents. Yet the following statements may be said to hold good. 1. Plutonic rocks are more favorable to it than volcanic. This follows because plutonic rocks cool slowly at considerable depths and stand therefore at high temperatures for long periods next their walls. 2. Magmas rich in mineralizers are much more favorable than are those poor in them. This naturally follows from the powerful ii 4 A HAND BOOK OF ROCKS. influence exerted by escaping vapors. It is tantamount to saying that acidic rocks are in general more efficient than basic ones, because experiment shows, and field observation indicates, that abundant absorbed vapors accompany and facilitate the fusion of the rocks high in silica, whereas basic rocks are much more largely the results of dry fusion. Granites, for instance, are the com- monest and most effective agents of contact metamorphism. 3. As regards the walls, sedimentary rocks possess varying sus- ceptibilities. Highly siliceous sandstones and conglomerates, for example, are stubborn subjects, and manifest but slight altera- tion ; but highly aluminous or calcareous beds are favorable to recrystallization, because they contain the alumina, iron, lime, magnesia and the alkalies which will combine with silica, under metamorphosing influences, to yield copious contact minerals. Of all rocks, impure limestones yield the most varied and interesting results. 4. With a favorable intrusion, the apparent distance to which the metamorphosing influence penetrates, depends on the angle of emergence of the intrusion. If it comes up at a low angle it may lie but a short distance below the surface for a considerable stretch on one side of the outcrop, so that the metamorphosed area may apparently extend to a great distance, although at no point far from the source of heat. Around a vertical dike the distance would naturally be less. Again, the alterations progress much less readily across the bedding of stratified rocks than along it. Hence, an intrusion that cuts across the bedding produces more wide- spread effects than does one parallel with it. 5. It is believed by many, especially among English and Ger- man observers, that there is very slight migration of material dur- ing metamorphism, and therefore that the contact minerals have resulted from the silica and the bases which were practically in the same places before the intrusion as after it. It follows that there has been no chemical introduction or substitution, but only rear- rangement of molecules during the process. An analysis, there- fore, of a reasonably large-sized sample would indicate the compo- sition of the original rock, except so far as water, carbonic acid and other volatile ingredients have been driven off. From obser- vations upon an intrusion of granite in Westmoreland, England, THE METAMORPHIC ROCKS. 115 which cuts a decomposed, basic, amygdaloidal lava, Alfred Harker concluded that the migration had not exceeded one twentieth of an inch. But among the French much greater power of chemically affecting the walls is attributed to intrusions, and in instances it certainly seems as if, in addition to the fluorine and boron which we all know penetrate into wall rocks during the escape of mineralizers, hydro-fluosilicic acid might impart silica and that some of the bases, and especially the alkalies, might migrate in heated solutions, to a moderate distance. 6. Notwithstanding the truth of the foregoing generalities, it is a curious fact that contact effects are sometimes strangely lacking where we would naturally expect them, and they are often of varying intensity and irregular distribution, where they do occur. These anomalies can in part be explained by the general principles already cited, of which no doubt the presence or absence of min- eralizers, the superheated or relatively cold condition of the intru- sion are chief. But every observer of wide experience is some- times much puzzled by what he meets in Nature. I. THE ROCKS PRODUCED BY CONTACT METAMORPHISM. Although the principal results of contact metamorphism are manifested in the walls of the intrusion, the igneous rock is itself influenced. It is therefore necessary to note both the internal and the external effects, or those upon the intrusion and those upon the walls. The area over which the latter are manifested is often called the aureole, and the concentric rings of decreasing alteration as one passes outward from the intrusion are called zones. Internal Effects. The igneous rock suffers a relatively rapid loss of heat in its marginal portions as compared with its interior, and as a result it very commonly assumes a porphyritic, felsitic or even, just as the contact, a glassy texture, although it may be granitoid within. Where these textures are well developed the passage from one to the other is extremely gradual, and if the wall rock has been originally a shale or a clay that has been baked to a dense mass, one may need microscopic examination to determine where the intrusion ends and the wall rock begins. The changes in texture in the intrusion are accompanied more or less by changes in chemical composition and in not a few cases progres- n6 A HAND BOOK OF ROCKS. sive analyses have shown the margins to be much more basic than the interior of the intrusion. The chilling of the former has thus produced chemical rearrangements in the magma previous to con- solidation. External Effects. Recalling the statement earlier made that within the limits already set forth the character of the intrusion is immaterial, the most convenient and intelligible method of treat- ment will be to briefly outline several typical cases wherein the commoner sedimentaiy rocks are known to have been affected, and then to refer to one or two instances wherein igneous or regionally metamorphic ones have suffered alteration. The same order will be preserved for the sediments as appears under Chapters VII. and VIII. Breccias are too limited in distribution to be a serious factor. Conglomerates and sandstones so generally consist of silica, that they supply but little raw materials of a favorable kind. The small amounts of alumina present may combine with the silica to afford sillimanite (A1 2 O 3 SiO 2 ) and stimulated circulations of hot water may cause added deposition of quartz around the grains so as to develop increased hardness. With shales and clay rocks, even if in the form of slate (see later, p. 1 34), the effects are more pronounced ; and around intrusions in them well-marked and well -identified zones have been described. At the contact of the igneous rock with the sediment a breccia or " mixed zone " of intrusive and fragments of wall-rock is some- times, although not always, met. More commonly the shales, slates, clay or their kindred rocks are baked and altered to a dense flinty product known as a hornfels or hornstone, which latter name in this sense is, however, not to be confused with its use for flints and cherts. It breaks in irregular, angular masses and has a very close resemblance to dense trap. Its mineralogy is, as a general thing, a subject for microscopic study, but it may be said that biotite in small scales is rather the most widespread mineral present, and that andalusite, garnet, cyanite, staurolite, tourmaline, ottrelite, rutile, hornblende, feldspars and other minerals more or less char- acteristic of such surroundings frequently appear. They may be of considerable size and the prisms of andalusite of the variety chiastolite, with the light and dark maltese crosses showing in their THE METAMORPHIC ROCKS. 117 cross-sections, are especially frequent. As the contact is left the hornfels often passes into mica schist. Farther out the mineralog- ical changes become less marked ; the andalusite and other crystals are less and less well developed and finally shade into mere dark spots or aggregates of biotite, magnetite and bituminous matter. When even these fade out the unchanged sediment is met. In some localities it has therefore been possible to establish three zones, which are, in the reverse order of the above succession, the knotty or spotted slates, the knotty mica schists, and the hornfels, usually with andalusite. By knotty is meant the aspect given by the larger contact minerals in the midst of finer aggregates. These are the names adopted for a well-known contact studied by the emi- nent German petrographer, Rosenbusch, in the Vosges Mountains. At a famous American locality in the Crawford Notch of the White Mountains, on the slopes of Mt Willard and not far from the Crawford House, the granite has penetrated an argillitic mica schist or micaceous slate, and the zones are somewhat differ- ent. G. W. Hawes in 1881 established the following seven: i. The argillitic mica schist (chloritic) ; 2. Mica schist (biotitic) ; 3. Tourmaline hornstone ; 4. Tourmaline veinstone (a small con- tact band, rich in tourmaline); 5. Mixed schists and granite; 6. Granite porphyry (biotitic) ; 7. Granite (hornblendic). This is one of the most complete and best-exposed contacts known, and illustrates both external and internal effects.* The succession illustrates the alteration of chlorite to biotite by the granite, and then near the contact the development of tourmaline from the boracic and fluoric emanations which were afforded by it. On the southeast corner of Conanicut Island, in Narragansett Bay, granite has penetrated Carboniferous shales, as described by L. V. Pirsson,f and has baked them to compact hornfels near the contact. Spotted slates are likewise met resembling those de- scribed above. Immediately beneath the diabase of the Palisade ridge at Hoboken, N. J., the Triassic shales are baked to a compact hornfels with abundant tourmalines and near Beemerville, N. J.,J * Hawes' paper is in the American Journal of Science, January, 1881, p. 21. |L. V. Pirsson, On the Geology and Petrography of Conanicut Island, R. I. American Journal of Science, Nov., 1893, p. 363. % J. F. Kemp, Trans. New York Acad. Set., XL, p. 60. n8 A HAND BOOK OF ROCKS. a great dike of nepheline-syenite has come up through Ordo- vician shales and has altered them in places to remarkably dense, black hornfels. Near Crugers, on the Hudson River, mica-diorites have penetrated mica schists and have developed in them a con- siderable number of characteristic, contact minerals, but the changes in the schists are not specially apparent to the eye.* As western and other eastern areas are further studied, no doubt additional cases will be fully described. Many are known and await careful field work. The contact effects on limestones furnish extremely interesting phenomena, involving a series of minerals somewhat different from those just described. On account of the general lack of migration of material the elements of the minerals must be present in the unaltered rock. Pure limestones therefore merely crystallize into equally pure marbles. Siliceous and argillaceous ones become thickly charged with biotite, garnet, vesuvianite, scapolite, pyrox- enes and amphiboles, tourmaline, spinel, and not a few more. Garnet and vesuvianite are especially characteristic. Good con- tacts have been met at several American localities. Near St. John, N. B., t granite has penetrated Laurentian limestone and has developed a garnet zone, with more or less pyroxene. Diorites cutting or including limestone in the Cortlandt series J have caused the formation of pyroxene, scapolite, hornblende and other minerals. In the valley extending from Warwick, N. Y., southwest to Sparta, N. J., are most instructive exhibitions, and rich mineral localities are based on them. Granite is the principal intrusive. The western Adirondack region of New York contains many more where gabbro and limestone come together, and where the well- known mineral localities occur. C. H. Smyth, Jr., has lately identified their contact nature and will in time describe them. Abroad, the region about Christiania in Norway has proved to be classic ground for these phenomena, and a great contact of diorite on Triassic limestone at Predazzo in the Tyrolese Alps has pro- duced the characteristic zones on a grand scale. *G. H. Williams, Amer. Jour. Set., Oct., 1888, 265. t\V. D. Matthew, Trans. N. Y. Acad. Set., XIII., 194. JG. H. Williams, Amer. Jour. Set., Oct., 1888, 267. J. F. Kemp and Arthur Hollick, Annals N. Y. Acad. Sci., VII., 644. THE METAMORPHIC ROCKS. 1 19 Increasing experience, in the West and in Mexico, has shown that copper ores are often deposited along the contacts of eruptives and limestone. Thus in the Seven Devils district, in western Idaho, bornite occurs between diorite and white marble, and is mixed with epidote and garnet as a gangue, both being minerals characteristically developed in these surroundings. The inclusions of wall rock caught up by an advancing intru- sion on its way to the surface are instructive examples, and often are afterwards found entombed in the igneous rock and more or less altered. The lava flows of Vesuvius and the ejected bombs have been of extraordinary interest in this respect. Limestones are frequent among them and they exhibit the same zones as the larger occurrences. Vesuvianite, in fact, received its name from this association. Of the remaining members of the grand division of the Aqueous rocks, the Carbonaceous are the principal ones deserving mention. Coal seams of the normal bituminous variety have been cut in not a few places by igneous dikes, and display in a marked degree the metamorphosing effect. The volatile hydrocarbons have been driven off and the coal has become an impure coke. The Triassic coal basins of Virginia and North Carolina exhibit many instances where diabase dikes have wrought the change, and in the region of Puget Sound basalt intrusions have effected similar results. In Colorado and New Mexico, the near approach of an igneous sheet has brought about the formation of anthracite, and in fact all grades of coal can be detected from rich bituminous to hard an- thracite, according to the nearness of the dike or laccolite. Reference may also be made to the hills of soft magnetite, near Cornwall, Pa., where a great dike of diabase has altered limonite to this more crystalline and thoroughly anhydrous mineral. Where intrusions cut other igneous or metamorphic rocks the effects are much less apparent, because the walls are resistant to change, being themselves already crystalline. Around granites, however, even in these conditions, great pegmatite dikes and veins are copiously produced, which seems to be in large part brought about by escaping heated vapors and solutions. Remarkable cases of contact metamorphism are, however, cer- tainly caused by these last named agents. As rocks they are not 120 A HAND BOOK OF ROCKS. specially abundant, although of great scientific interest. Some in- trusions have emitted copious emanations of hydrofluoric and bora- cic acid in conjunction with superheated steam. These vigorous reagents have attacked the wall rocks, when originally formed of crystalline silicates, making them porous and cellular from the de- struction of feldspars, and have often caused the crystallization of quartz, tourmaline, topaz, fluoric micas, fluorite, apatite and other characteristic minerals of which cassiterite is of much economic importance. Such metamorphic products when essentially consist- ing of quartz and mica are called greisen. Tourmaline granites likewise result. It is not to be overlooked, however, that mineral- izers have also played a large part in the cases earlier cited, nor should the remark be omitted in conclusion that they and similar agents have been of very great importance in the formation of ores. CHAPTER X. THE METAMORPHIC ROCKS, CONTINUED. THE ROCKS PRODUCED BY REGIONAL METAMORPHISM. INTRODUCTION. THE GNEISSES AND CRYSTALLINE SCHISTS. INTRODUCTION. This subdivision embraces rocks which differ widely among them- selves, but which have nevertheless important features in common. The following generalities are applicable in a large way and will serve to emphasize some of the most important points. 1. Regionally metamorphosed rocks are all more or less per- fectly crystalline. This is least developed in the slates. 2. They are all more or less decidedly laminated or foliated, although some amphibolites, marbles and serpentines are quite massive. The laminations are due to the arrangement of the con- stituent minerals, and especially the dark -colored ones, in parallel alignment, so that light and dark layers stand out conspicuously. The terms bedded and stratified should never be applied to them because the banding is largely due to dynamical processes, and has no necessary connection with original sedimentation. 3. They are of ancient geological age or else are in greatly dis- turbed districts. It is important in connection with these rocks to distinguish be- tween the effects produced by heat or thermal metamorphism and the effects produced by mechanical forces or dynamic metamor- phism. By thermal metamorphism we understand the alterations caused by heat not necessarily accompanied by the mechanical effects such as shearing, crushing and the like, that are compre- hended under dynamic metamorphism. Contact metamorphism is of course a variety of the former which, however, is also brought into play alike when rocks are so deeply buried that they come within the sphere of influence of the earth's interior heat, and when from dynamic stresses, they are crushed so that their particles move or slide under great pressure on one another and develop heat by friction. If we imagine for a moment great bodies of I2 2 A HAND BOOK OF ROCKS. rocks which have definite crushing resistances, buried under a load of overlying strata, so deep within the earth that their limits of re- sistance are exceeded, yet so confined that they cannot fly apart, we perceive that they must yield by internal crushing, and if the upheaval of a mountain range eases the strain, that they must flow as a mass. It is to this flow, accompanied by shearing, that the lamination of metamorphic rocks is largely due. Prominent or conspicuous minerals are strung out in parallel line s, often- times with wavy folds and curves, and in the end a foliated or laminated structure is superinduced that suggested the bedding of sediments to the early geologists. It is not to be denied, however, that the laminations do at times correspond to original bedding, because where the contrasts in chemical and mineralogical compo- sition among the layers are pronounced, they doubtless mark such correspondence, but cases are well known of old conglomerate beds passing directly across the prevailing schistosity of a gneissic district. During these shearing and flow movements large crystals, such as the feldspars of porphyries, and the larger uncrushed nuclei of minerals in a general pulp are squeezed and stretched into lenses, and remain like eyes between eyebrows, so that they are called " Augen " from the German word for eyes. Swirling curves and eddies in the laminations are also familiar phenomena and cannot be explained in any other way. These changes may take place without mineralogical alteration, as when granitoid rocks pass into gneisses which contain simply the crushed fragments of the originals, but as a general thing new com- binations are formed in the metamorphosed rock. Pyroxene passes into hornblende ; soda-lime feldspars become scapolite or saussurite, and other changes ensue which are best detected with the microscope. Sedimentary rocks suffer entire recrystallization, and sometimes so thoroughly lose their original characters that no clue is afforded as to their history. In regional metamorphism precisely as in the case of the contact metamorphic rocks, it is generally believed that there is no change in composition, except perhaps by the loss of volatilizable ingredients, but only rearrangement of elements. A gross analysis of a reasonably large sample will therefore give a clue to the composition of the original. Heated waters, generally THE METAMORPHIC ROCKS. 123 charged with mineral matter and steam, have no doubt contributed largely in bringing about the final results. The Regionally Metamorphosed rocks will be described under the following heads : 1. The Gneisses and Crystalline Schists. 2. The Quartzites and Slates. 3. The Crystalline Limestones and Dolomites: The Ophical- cites, Serpentines and Soapstones. THE GNEISSES. Introductory. Gneiss is an old word which originated among the early German miners in the Saxon districts. It was especially ap- plied by them to laminated rocks of the mineralogical composition of granite, and in this sense it is quite widely employed to-day. But there are many important gneisses which correspond in min- eralogy to the other plutonic rocks, and which are quite as properly designated by this name, so that gneiss has come to be a term that is of loose geological significance very much as is trap, but that is none the less useful for this reason. We may therefore define gneiss as a laminated, metamorphic rock which usually corresponds in mineralogy to some one of the plutonic types. Gneisses differ from schists in the coarseness of the laminations, but as these become finer they pass into schists by insensible gradations. Varieties are sometimes indicated by prefixing the name of the most prominent silicate, usually one of the ferro-magnesian group, thus hornblende-gneiss, biotite -gneiss, pyroxene-gneiss, but we also often speak of quartz-gneiss, orthoclase-gneiss, plagioclase- gneiss, garnet -gneiss and the like. It is evident at once that the above names are incomplete. Hornblende-gneiss, for instance, does not indicate whether the rock contains orthoclase or plagioclase, quartz or no quartz, and the other ones cited are open to the same or similar objections, and if in the endeavor to embody fuller descriptions we string together the names of all the minerals in the rock, we employ an objection- able and awkward method of coining words. A system has, how- ever, been suggested by C. H. Gordon,* in a recent paper that obviates many of these objections and that is adopted below with * Bulletin of the Geological Society of America, VII., 122. 124 A HAND BOOK OF ROCKS. some abbreviation to make it suitable for an elementary book. It is based on the parallelism which exists between the mineralogy of gneisses and that of the massive plutonic rocks, and it avails itself of the short names of the latter, which indicate in each case, a definite combination of minerals, to describe the aggregates present in the former. Two sedimentary terms are also added. Massive Type. Gneiss of Correspond- ing Mineralogy. Sedimentary Type. Derived Gneiss. Granite Granitic Gneiss Conglomerate Conglomerate Gneiss Syenite Syenitic Gneiss Sandstone Quartzite Gneiss Diorite Dioritic Gneiss Gabbro Gabbroic Gneiss Pyroxenite Pyroxenitic Gneiss Peridotite Peridotitic Gneiss Dr. Gordon also suggests that, when gneisses are evidently dynamic derivatives from a massive rock, this relationship be in- dicated by using the terms granite-gneiss, syenite-gneiss and so on. If, however, differentiations in the magma before crystallizing have given rise to laminations, he advocates that such be distin- guished by the adjective gneissoid, as gneissoid gabbros. Gneisses are occasionally met which do not exactly correspond to any of the above names. Chlorite, for example, is a not un- common mineral, and while it is evidently an alteration product from pyroxene, hornblende or biotite, the original mineral is not at once apparent, and some such name as chlorite -gneiss must be used. In the same way cordierite-gneiss describes those rare varieties containing cordierite (iolite and dichroite are synonyms of cordierite) ; sillimanite-gneiss, garnet-gneiss, epidote-gneiss and others convey in their names their characteristic features. ANALYSES OF GNEISSES. Chemical analyses often enable us to trace back gneisses to their original rocks, whether igneous or sedimentary, but it requires careful study of correct type analyses and some familiarity with their ranges in composition to do it. So far as their number admits the analyses quoted on earlier pages will be found sug- gestive : THE METAMORPHIC ROCKS. 125 SiO, Al a O, Fe 2 O s FeO CaO MgO K;O Na a O Loss or H a O I. 76.61 12-45 i-33 0.84 5-42 3.12 0-53 2. 74-95 9.42 7-47 ... 1-65 0.13 2. 02 4-05 1.02 3- 73-47 15.07 1.15 4.48 0.12 0-38 5-59 4- 71.46 15.06 ... 2.43 1.40 O.42 5-17 3-23 0.83 5- 69-35 18.83 2.00 5-94 3-78 6. 69.94 14.85 7.62 2.10 0.97 4-33 4-30 0.70 7- 61.96 19-73 4.60 0-35 1.81 2.50 0.79 1.82 8. 57-66 22.83 7-74 1.16 3-56 5-72 0.60 1.50 9- 57-20 19-57 9-52 0-59 5-73 4.40 0.28 2.13 0.88 10. 54-89 I3-67 i-35 5-63 4.70 8-34 i-95 2.76 I. Granite gneiss, west side of Black Hills, 4Oth Par. Survey, I., p. no. R. W. Woodward, Anal. 2. Called a dioritic gneiss in reference, contains hornblende, quartz, plagioclase, orthoclase. Idem, R. W. Bunsen, Anal. 3. Conglomerate gneiss, so-called granite ; Munson, Mass. Quoted by G. P. Merrill, Stones for Building and Decoration, p. 418. 4. Granite gneiss, Iron Mountain, Wyo., R. W. Woodward, Anal. See under No. I. 5- Dark variety of No. 3. 6. Granite gneiss, derived from a hornblende granite, Trembling Mountain, Quebec, Fundamental Laurentian of Logan. F. D. Adams, Amer. Jour. Sci., July, 1895, p. 67. W. C. Adams, Anal. 7. Quartzitic gneiss, with garnet, sillimanite, graphite and pyrite ; St. Jean de Matha, Quebec, Idem, N. N. Evans, Anal. 8. Granite gneiss, probably a metamorphosed clay or slate. Trembling Lake, Quebec. Contains garnets and sillimanite, F. D. Adams, Amer. Jour. Sci., July, 1895, p. 67, W. C. Adams, Anal. 9. Dioritic gneiss, New York City, P. Schweitzer, Amer. Chemist, VI., 457, 1876. 10. Gneiss containing malacolite, scapolite, orthoclase, graphite, pyrite. Rawdon, Quebec. See under No. 8. Comments on the Analyses. Nos. i, 4 and 6 are clearly derived from granites, presumably by dynamic metamorphism. The analyses correspond closely in their general features with those given on p. 33 except that the A1 2 O 3 of No. i is a trifle low, and the Fe 2 O 3 of No. 6 a trifle high. Nos. 3 and 5 are now known to be metamorphosed Cambrian conglomerate, although so thor- oughly recrystallized as to be a well-known, commercial granite. The conglomerate must have come from granitic or dioritic orig- inal rocks. Nos. 7 and 8 correspond to the analyses of slates as noted by F. D. Adams in the original reference (see also under slates, p. 135). No. 10 as noted by Adams is of doubtful interpre- tation. The high alkalies, lime, magnesia and the moderate silica suggest a basic syenite or trachyte, but the alumina is exception- ally low for these. It may be a tuff or a slightly altered sediment from these originals. No. 2 is a very anomalous rock, and it is difficult to refer it to an original diorite, it is so high in silica and so low in alumina. The iron is very large for so acidic a rock. 126 A HAND BOOK OF ROCKS. No. 9 is undoubtedly an altered sediment as indicated by the local geology. Nothwithstanding the anomalies of composition, chem- ical analyses supply one of the surest clues to the geological his- tory of gneisses and it is to be hoped that they will be multiplied in America. At present but few are available, far fewer than of igneous rocks. Alteration. The alteration of gneisses is similar in all respects to that of their corresponding massive types. The feldspars alter to kaolin, the micas and hornblende to chlorite and the rock softens down to loose aggregates that contribute heavily to the sedimen- tary rocks. Distribution. Gneisses are abundant in ancient, geological for- mations. The early Archean is their especial home, and they form the largest part of its vast areas in Canada, around the Great Lakes, along the Appalachians and in the Cordilleran region. But no single division of geological time monopolizes them any more than such an one does plutonic rocks. There are Cambrian and even Carboniferous gneisses in New England, and dynamic metamor- phism may produce them from massive rocks of almost any age. The later geological formations are, however, seldom buried suf- ficiently deep to be in favorable situations. Much the same holds true of Europe and the rest of the world. The gray and red gneisses of the mining districts about Freiberg, in Saxony, those of the Highlands of Scotland, those in Scandinavia, and the won- derful exhibitions of dynamic metamorphism in the Alps are to be cited as of unusual historic and scientific interest. Granulite. Granulite is a word that has possessed somewhat contrasted meanings according as it has been used in Germany, France or England. In Germany as first employed it was applied to a finely gneissoid rock that consists chiefly of feldspar, quartz and garnets. These original granulites have other minerals more or less prominently developed, of which cyanite, augite, biotiteand horn- blende are chief. The texture of the rock is extremely dense, and except for the garnets, cyanite or augite, the individual minerals are hardly discernible. Among French and English speaking peoples the name granulite has been applied to granitic rocks that appear to the eye to be chiefly quartz and feldspar, although the microscope may show muscovite. They are practically binary granites, or THE METAMORPHIC ROCKS. 127 rich quartz and feldspar gneisses. The name has also been used for coarse plutonic rocks that have been crushed down by dyna- mic metamorphism into a finely granular and homogeneous aggre- gate. But so far as metamorphic rocks have been met in America, cases are very rare which cannot be satisfactorily described without the use of this word, which has been so perverted from its original application as to be practically valueless without an accompanying explanation. THE CRYSTALLINE SCHISTS. The crystalline schists have finer laminations than the gneisses, but in other respects the mineralogy is often much the same, and as already stated no very sharp line can be drawn between them. It is important to note that the words " schiste " of the French and " Schiefer " of the Germans are applied to shales, slates and meta- morphic schists indiscriminately, but in English schist is only used for metamorphic rocks. The more important schists are broadly classified, according to the principal ferro-magnesian silicate that is present, into the following three groups under which they will be taken up. (a) Mica-schists. () Hornblende-schists or Amphibolites. (<:) Various Minor Schists. THE MICA-SCHISTS. SiO, A1 2 O 3 Fe 2 O, FeO CaO MgO K 2 O Na 2 O H 2 O I. 82.38 11.84 2.28 I.OO 0.83 0.38 0.77 2. 79-5 13-36 2.87 0.71 o-95 4.69 0.36 0.78 3- 69-4S 14.24 6-54 2.66 i-35 2.52 4.02 0.52 4- 66.21 18.60 5-34 0.44 1.24 3-80 2.16 2.04 5- 62.98 16.88 2. 4 8 5.00 tr. 1.58 7-45 3.02 6. 6i.57 19-53 5-44 2.61 tr. 1.90 2.14 3-48 7- 60.49 19-35 0.48 5-98 1. 08 2.89 3-44 2-55 3-66 8. 57-67 17.92 9.00 3-19 3-29 3-86 1.0 9 3-19 9- 55-12 24.32 6.1 3 4-99 tr. tr. 2.83 2. 7 I ... 10. 49.00 23-65 8.07 0.63 0.94 9.11 i-75 3-4i I. Mica-schist, rich in quartz, Monte Rosa, Switzerland, Zulkowsky, Sitz. Wiener Akad., XXXIV., 41, 1895. 2. Mica-schist, with quartz and green mica, Zermatt, Switzerland. Bunsen in Roth's Tabellen, 1862. 3. Garnetiferous mica-schist with feldspar, Brixen. Tyrol. Schonfeld and Roscoe, Ann. der. Chem. u. Phar., XCI., 1854, 305. 4. Mica-schist, near Meissen, Saxony, Hilger quoted in Roth's Tabellen, 1879. 5. Mica-schist, Crugers, N. Y., contains quartz, orthoclase, biotite, muscovite, a little oligoclase,etc. F. L. Nason for G. H. Williams, Amer. Jour. Sci., Oct., 1888. 128 A HAND BOOK OF ROCKS. 259. 6. Crumpled garnetiferous mica-schists, Idem. 7. Argillitic mica-schist, G. W. Hawes, Geology of New Hampshire, Part III., 219. 8. Mica-schist near Messina, Sicily, Ricciardi, quoted in Roth's Tabellen, 1884, p. ix. 9. Staurolite mica-schist, with biotite, muscovite, quartz, sillimanite, garnet. See under No. 6. 10. Sericite schist, Wisconsin, Wis. Geol. Surv., I., 304. Comments on the Analyses. Like the majority of gneisses the mica-schists are more or less closely parallel with the granites in chemical composition because the constituent minerals are so largely the same in both. But where they have been formed from metamorphosed sediments such as shales, clays, and the like, the alkalies are often lower than in the case of siliceous igneous rocks, and, what is still more characteristic of sediments as contrasted with highly siliceous igneous rocks, the magnesia is in excess of the lime. A comparison of the above analyses with those of the rhyolites, trachytes, granites and syenites earlier given will forcibly bring this out. The local geology as well as the analyses, indicate that there is little doubt that Nos. 5, 6, 7 and 9 are altered sedi- ments, and the presumption is strong that almost all the others are also. Mineralogical Composition, Varieties. The most prominent and abundant minerals in the mica-schists are quartz, muscovite and biotite. While they are more or less interleaved together, yet close examination of the coarser varieties shows that they are in layers irregularly parallel and to a large extent distinct. The minerals are in all degrees of relative abundance, quartz sometimes largely predominating and marking a passage to the quartzites, while again the micas may be in great excess. Both muscovite and biotite are met, the former being, perhaps, rather the more abundant. With these chief minerals are almost always associated very considerable amounts of feldspar, both orthoclase and plagio- clase, and variable proportions of garnet, staurolite, cyanite, silli- manite, tourmaline, apatite, pyrite and magnetite. The garnet and staurolite may exhibit surprisingly well devel- oped crystals and illustrate the extraordinary power of certain compounds to crystallize under circumstances apparently ill-adapted to their perfect development. Mica-schists embrace a series from rather coarsely crystalline varieties to others that are excessively fine-grained and that are near relatives of the slates. The minerals of the latter may be of THE METAMORPHIC ROCKS. 129 microscopic dimensions, and only the aggregate of shining scales reveals them as mica. Such aggregates, of a silvery white color but of composition essentially the same as normal muscovite, are called sericite, and the corresponding schists, sericite-schists. A soda-mica (muscovite and its relatives are potash micas) is called paragonite. Hydromica is a name applied many years ago by Dana to sericite, paragonite, and perhaps others resembling them, so that for these finely micaceous schists, especially in our eastern states, hydromica schist is a field name that has been largely used in practice and in geological reports. These fine-grained mica- schists that approximate slates are also made a special group by many, under the name phyllite, a very useful term and one to be strongly commended. Mica-schists - are also met that are high in lime and that mark transitions to the crystalline limestones. The abundance of calcite or dolomite betrays them, and to such the names calcareous schist or calc-schist are applied. Mica-schists result from the thorough metamorphism or recrys- tallization of sandstones, shales and clays, and also from the crush- ing and excessive shearing of igneous rocks, granitoid and por- phyritic alike. A possible origin from ancient volcanic tuffs is always to be considered in the study of a district, but the questions of origin are obscure and are subjects for thorough chemical and microscopical investigation. Alteration. The mica-schists are rather resistant to alteration and often appear on mountain tops. When alteration does prevail, they soften to masses of quartz sand, chlorite scales and kaolin. Distribution. The mica-schists form the country rock over vast areas in New England and to the south along the eastern Appa- lachians. Although long regarded as of uncertain or obscure geo- logical relations they are now recognized as being in large part at least metamorphosed Cambrian and Ordovician shales or related sediments. Around Lake Superior and in the regionally meta- morphosed areas of the West they are not lacking. TMF. HORNBLENDE SCHISTS OR AMPHIBOLITES. Introductory. Under dynamic metamorphism the basic igneous rocks whose chief bisilicate is pyroxene, pass very readily into hornblendic rocks, with a greater or less development of schistosity. 1 3 o A HAND BOOK OF ROCKS. On account of the prevailing parallel arrangement of the prismatic crystals of hornblende, schistosity is seldom entirely lacking, but where less distinct the name amphibolite has proved to be a useful alternative, and indeed is of wide general application. Sedimentary rocks are also known in rarer instances to yield similar results. SiO, A1.0, Fe 2 s FeO CaO MgO K 3 O Na,O H 2 O I. 52.39 16.13 1.6 4 1.44 8.76 4.70 1.42 2-59 0.17 2. 50-44 8.18 1. 06 6.28 "55 I7-63 0.50 2. 9 8 0.98 3- 49.19 18.71 5-3 4.04 5-92 7.98 0.77 1.44 5-05 4- 46.31 11.14 21.69 9.68 tr. 6.91 4-44 5- 44-49 16.37 5-07 5-5 7-94 7-50 0.56 2-59 4-99 I. Hornblende-schist, Grand Rapids, Wis., Geology of Wis., IV., 629. Also, Fe in pyrite, 0.34 ; S, 0.39 ; P 2 O 5 , 0.28 ; Ca in apatite 0.815. 2. Pseudo-diorite of Becker, Knoxville, Calif., Monograph XIII., U. S. Geol. Surv., 101, W. H. Melville, Anal. Also, MnO 0.213, Cr.;O 3 0.480. 3. Hornblende-schist derived from gabbro, Lower Quinnesec Basin, Wis. R. B. Riggs for G. H. Williams, Bull. 62, U. S. Geol. Surv., p. 89. Also, CO 2 1.82. 4. Hornblende-schist near Cleveland Mine, Mich., Foster and Whitney, Rept. on the Iron Lands of Lake Superior, p. 92. 5. Hornblende- schist, Lower Quinnesec Falls, Wis., R. B. Riggs for G. H. Williams, Bull. 62, U. S. Geol. Surv., p. 91. Also CO 2 5.38. Comments on the Analyses. The analyses indicate basic rocks, of decidedly variable composition. Nos. 3 and 5 are certainly sheared igneous rocks. No. 2 is regarded by Becker as a meta- morphosed sediment. It is quite different from the others in its low alumina, and its great excess of magnesia over lime. No. I appears to be an altered igneous rock and No. 4 is probably the same. Aside from exhibiting the composition of these rocks, the analyses are interesting when compared with those of the basic diorites (p. 60) and the gabbros and pyroxenites (pp. 72, 75). Mineralogical Composition, Varieties. The most abundant min- eral in these rocks is naturally hornblende. With it are associated oftentimes biotite, augite, plagioclase, garnet, magnetite, pyrite and pyrrhotite ; but quartz, except as forming veinlets, is not often met nor is it to be expected in such basic rocks. The commonest va- riety of hornblende is black to the eye, but is green in thin section. It forms prismatic crystals from moderately coarse to microscopi- cally fine. The prisms are interlaced so as to make a very tough aggregate and one that breaks with difficulty under the hammer. Light green actinolite may also form schists. Black scales of bio- tite appear interlaminated with the hornblende. The augite is not THE METAMORPHIC ROCKS. 131 readily distinguished from the hornblende with the eye alone. It is in large degree the remnants of original pyroxenes that have partially passed into hornblende during the metamorphic process. The plagioclase also represents to a great extent the feldspar that was in the original gabbro or other igneous rock from which the amphibolite has been derived. The plagioclase is often replaced by secondary products, such as epidote, calcite, scapolite and others, which together make up the aggregate formerly called saussurite, and regarded as an individual mineral. The minor accessories, magnetite, pyrite, pyrrhotite and garnet deserve no special mention. Except magnetite, which never fails, they are of more or less irregular occurrence. Alteration. The hornblende passes readily into chlorite and softens to a scaly mass with the separation of much limonite that yields a characteristic, rusty outcrop. If any pyrite or pyrrhotite is present it greatly expedites the alteration by its contribution of sulphuric acid. The feldspars yield calcite and kaolin and the whole mass becomes a rusty clay or soil. Occurrence. The hornblende-schists constitute individual belts in schistose regions in the midst of other metamorphic rocks and also great areas by themselves. Dikes and sheets of diabase and plutonic masses of gabbro in districts that have been subjected to violent dynamic upheavals readily pass into them. The same areas in the Eastern States that were cited under gabbro contain them, and they are minor members in the schistose districts of New England. Around Lake Superior they form a most important part of the geology of the iron ore regions, and in the Black Hills, the Rocky Mountains and the ranges of California they are often met. VARIOUS MINOR SCHISTS. Under this collective term are assembled a series of minor rocks, no one of which compares in importance with the schists already mentioned, but all of which may be met as subordinate members of metamorphic districts. There are also others in considerable variety which are esteemed too unimportant for an elementary book. SiO a A1 S O, Fe s O 3 FeO CaO MgO K 3 O Na,O H 2 O Chlorite schist. 1. 49.18 15-09 12.90 10.59 5.22 1.51 3.64 1.87 2. 47-10 2.14 44.33 0.36 0.13 S.I9 i 3 2 A HAND BOOK OF ROCKS. scWs! SiO a A1 2 3 F C2 3 FeO CaO MgO K 3 O Na,O H 2 O 3. 58.66 9.26 4.42 0.94 22.78 4.09 4. 50.81 4-53 3-5 2 4-26 31-55 4-42 Epidote schist. 5. 41.28 18.48 9.44 8.20 7.04 7.48 2.21 3.52 2.74 Eclogite. 6. 48.89 14.46 2.00 7.15 13.76 12.21 0.17 1.75 0.40 Glaucophane schist. 7. 47.84 16.88 4.99 5.56 11.15 7-89 0.46 3.20 1.98 I. Chlorite schist, Klippe, Sweden, Cronqvist for Tornebohn. Quoted by Roth, Gesteinsanalysen, 1884, p. viii. 2. Chlorite schist, Foster Mine, Mich., C. F. Chandler, Geol. of Mich., I., 91. 3. Talc schist, Fahlun, Sweden, Uhde quoted by Roth, Gesteinsanalysen, 1861, 56. 4. Talc schist, Gastein, Austria, R. Richter. Idem. 5. Epidote schist from diabase, South Mountain, Pa., C. H. Henderson, Trans. Amer. Inst. Min. Eng., XII., 82. 6. Eclogite, Altenburg, Austria, Schuster, Tscher. Mitt., 1878, 368. 7. Glaucophane schist, Monte Diablo, Calif., W. H. Mel- ville, Bull. Geol. Soc. Amer., II., 413. Comments on the Analyses. These analyses are too variable to admit of much in the way of comparative remarks, for the rocks are so totally unlike. No. I suggests an original diabase or some such rock. No. 2 is abnormally rich in iron, doubtless in large part from magnetite or hematite. The high magnesia in Nos. 3 and 4 is characteristic and indicates their close relations with ser- pentines. No. 5 is an altered diabase. No. 6 is of a rock variable in its mineralogy and obscure in its history. No. 7 is practically a hornblende-schist with glaucophane, an amphibole that is high in soda, instead of common hornblende. Mineralogical Composition, Varieties. The chlorite schists are marked by the presence of this green micaceous mineral in large amount. More or less quartz is also generally present, and not in- frequently plagioclase, talc, epidote and magnetite. The schistose texture is pronounced. The chlorite-schists are manifestly altera- tion products from some rock, with abundant, anhydrous, iron- alumina silicates. Hornblende-schists, presumably from basic igneous rocks are the general source. Certain chlorite-schists are often called " green schists." Talc-schists are characterized by sufficient talc to make this mineral prominent and in addition they have quartz as the next most abundant constituent. Feldspar may at times be noted, and some micaceous mineral is not rare. Care is necessary not to con- fuse fine scales of the last named with talc itself. Various accessory THE METAMORPHIC ROCKS. 133 minerals likewise occur, and the magnesian carbonates, dolomite and magnesite are often present. Obviously the talc-schists have resulted from the alteration of some rock with one or more anhy- drous, magnesian silicates that lacked iron. Tremolite and ensta- tite are the most available, but the original sources of these are often obscure. Siliceous dolomites or intrusive pyroxenites at once suggest themselves, but the iron must of necessity have been low, so as not to yield serpentines. Epidote-schists result when the ferro-magnesian silicates and the plagioclases are so favorably situated with reference to each other as to establish mutual reactions. They especially arise as phases in the metamorphism of pyroxenic or hornblendic rocks, such as diabase, hornblende-schists and the like. Eclogite is a rock scarcely known in America, having, as yet, only been noted near the Wash- ington Mine, Marquette District, Mich. (Geol. of Wis., III., 649). It is a well recognized variety, however, in Europe. It consists of bright green amphiboles and pyroxene, of garnet and of a variety of minor minerals. In ordinary determination it would not be distinguished from a garnetiferous, actinolite schist. GLaucophane is a blue soda amphibole that is rare in America, except in the Coast range of California, where it characterizes certain important schists. The rocks have a pronounced blue shade, and contain in addition quartz and feldspar. In California they certainly are al- tered shales. Graphite appears quite commonly as a characteristic mineral of certain schists, and may justify the use of the name graphite schist. More or less mica, and always quartz and feld- spar are associated. Distribution. Chlorite-schist and talc-schist are not uncommon members of our larger metamorphic series, especially along the Appalachians in New England and around Lake Superior. Epidote-schist is less common in the same relations. The occur- rence of eclogite and glaucophane-schist has already been cited. Graphite-schist is not infrequent in the metamorphosed Paleozoic strata of the East. CHAPTER XL THE METAMORPHIC ROCKS, CONTINUED. THE ROCKS PRODUCED BY REGIONAL METAMORPHISM. THE QUARTZITES AND SLATES. THE CRYSTALLINE LIMESTONES AND DOLOMITES, OPHICALCITES, SERPEN- TINES AND SOAPSTONES. THE QUARTZITES. SiO, A1 2 O 3 Fe a O s FeO CaO MgO K,O Na a O H 2 O Sp. Gr. 1. 97.1 1.39 1.25 0.18 0.13 2. 96.44 1.74 0-33 0.17 0.22 0.13 O.ig 0.90 3. 84.52 12.33 2.12 0.31 tr. o.i I 0.34 2.31 2.74 I. Quartzite, Chickies Station, Perm., Penn. Geol. Surv. Rep. M., p. 91. 2. Sandstone partly altered to Quartzite, Quarry Mtn., Ark., R. N. Brackett for L. S. Griswold, Geol. of Ark., 1890, III., 140, 161. 3. Quartzite, Pipestone, Minn., W. A. Noyes in Minn. Geol. Surv., I.,. 198. Comments on the Analyses. There is no essential difference in the analyses of quartzites and sandstones, as the few quoted above will show, but doubtless the resulting quartzite is somewhat richer in silica than the original sandstone. Comparatively few analyses of quartzites have been made in America. Mineralogical Composition, Varieties. The quartzites are meta- morphosed sandstones, and differ from the latter principally in their greater hardness, and to a certain extent in their fairly pronounced crystalline character. These qualities are due to the presence of an abundant siliceous cement that is crystalline quartz, and that is often deposited around the grains of quartz of the original sand- stone, so as to continue their physical and optical properties. The original grains have, therefore, had the power of controlling the orientation of the molecules of the new silica as it crystallized. When the original sandstone has been argillaceous the resulting quartzite contains mica and especially muscovite, and with increase of the mica, such quartzites pass through the intermediate varieties of quartz-schist into mica-schists. A very curious and more or less micaceous variety is the so-called flexible sandstone or itacol- umite, whose grains have the power of slight movement on one another from their loosely interlocked arrangement, so that thin 134 THE METAMORPHIC ROCKS. 135 slabs may be bent through a considerable arc. Quartzites also result from pebbly sandstones and conglomerates, and the pebbles of these latter are often flattened by the dynamic movements with which the metamorphism is at times associated. There is no sharp line of demarcation between quartzites and sandstones, and while the extremes of soft sandstones and hard quartzites are entirely dif- ferent, the determination of intermediate varieties is more or less arbitrary. Alteration. Quartzites sometimes soften to sand on their out- crops, and in the process, almost the last vestiges of alumina or lime may be removed. In this way the sands in analysis No. I, p. 89, were formed. In general, however, they are excessively resistant rocks, and tend to form prominent ledges. Distribution. Quartzites occur in almost all series of metamor- phosed sediments, and as these are best developed in the later Archean (Huronian, Algonkian) strata, they especially characterize them. In the metamorphic belt in New England and along the Appalachians, they are frequent, as well as in the Huronian, around Lake Superior and Lake Huron and in the similar areas of the West. THE SLATES. Si0 2 A1 2 O S Fe 2 O, FeO CaO MgO K S O Na 2 O H 2 O I. 66.45 3-38 I.7I 1.41 2.86 6.28 0.05 0.90 4.03 2. 66.00 24.60 tr. tr. 3-67 2.22 3-oo 3- 65.85 16.65 5-31 0-59 2.95 3-74 I-3I 3.10 4- 64-57 I7-30 7.46 1.16 2.60 1.99 4.62 5- 63-3I 16.16 3-79 0.15 4-44 7-56 1.54 2.65 6. 60.50 19.70 7-83 1. 12 2. 2O 3-i8 2. 2O 3-3 7- 60.32 23.10 7.05 0.87 3-83 0.49 4.08 8. 57.00 20. 10 10.98 1.23 3-39 i-73 1.30 4.40 9- 55-88 21.85 9-3 o. 16 1.49 3-64 0.46 3-39 10. 54.80 23-I5 9.58 i. 06 2.16 3-37 2.22 3-90 I. Slate, Llanberis, Wales. Quoted by G. P. Merrill, Stones for Building and Decoration, p. 421, also MnO, 0.91, CO 2 , 1.30. 2. Slate, Etchemin Riv., N. B., T. S. Hunt, Phil. Mag. (4), VII., 237, 1854. 3. Roofing slate, Westbury, Can., Idem. 4. Roofing slate, Lehesten, Germany. Frick, quoted by Roth, Gesteinsanalysen, 1861, p. 57. 5. Damourite slate, Hensingerville, Pa., Geol. of Penn., Rep. M., 91. 6. Roofing slate, Wales, T. S. Hunt as under No. 2. 7. Slate, Lancaster Co., Penn., also FeS 2 , 0.09. See under No. I. 8. Roofing slate, Angers, France, T. S. Hunt, as under No. 2. 9. Blue black carbonaceous slate, Peach Bottom slate, York Co., Penn., also MnO 0.586, CoO tr., C 1.974, FeS, 0.51, SO 3 0.022. See under No. I. IO. Roofing slate, Kingsey, Quebec, T. S. Hunt, as under No. 2. I 3 6 A HAND BOOK OF ROCKS. Comments on the Analyses. The analyses are especially signifi- cant when compared with those of the shales and clays, p. 92, and with those of the mica-schists, p. 1 26, with which latter they are closely parallel. Two features at once impress the observer, the excess of magnesia over lime, and the excess of potash over soda. The former stamps their origin as from sediments rather than from igneous rocks of these percentages in silica, because this relative excess of magnesia as noted under the mica-schists is rather characteristic of sediments. Miner alogical Composition, Varieties. As the sandstones during metamorphism pass into quartzites, so the shales and clays become slates, when not so thoroughly recrystallized as to yield mica- schists or phyllites. The more sandy shales afford varieties that break irregularly and that lack homogeneity, but tough and even slates result from homogeneous clays and are among the most remarkable of rocks. The distinctive feature of slates as against shales is the possession of a new cleavage that may lie at any angle with the original bedding of the rock, and that has no defi- nite relation to it. The cleavage has been developed by dynamic strains that have, beyond question, involved a shearing stress and some differential movement among the layers, though it may have been microscopic. As a matter of observation the component grains of slates have become flattened and lie parallel with the new cleavage, and any mica flakes or hornblende needles that may be present lie along it. Various explanations have been advanced for slaty cleavage, and its artificial production in different substances has occupied several investigators. Based principally upon experiments performed by Professor John Tyndall, over forty years ago, it has been usually referred to a compressive force at right angles to its plane. Tyndall subjected blocks of wax to pressure, using wet glass plates as his buttress of resistance. The blocks were of course greatly reduced in thickness and were forced to spread or bulge laterally. Shortly afterward H. C. Sorby, partly on the basis of the flattening of the component grains, and the alignment of mica scales, explained the cleavage as due to planes of weakness caused by this new arrange- ment. Recently, G. F. Becker of the U. S. Geological Survey has repeated the experiments of Tyndall with modifications. So long THE METAMORPHIC ROCKS. 137 as the resisting glass plates were wet with water the slaty cleavage was developed, but when they were smeared with a heavy lubricat- ing oil, although there was lateral expansion during compression, no bulging took place and no cleavage was developed. Manifestly therefore the frictional drag of the plates enters into the problem, and although the resolution of the forces involved is somewhat complex, a shearing stress results that is a strong factor in pro- ducing the cleavage.* In the case of the large beds or strata which are metamorphosed into slate in Nature, the case is even less simple, and the contrasts in rigidity, between the beds that yield slates, and their enclosing strata, are less pronounced than in the experiment, but there is little doubt that the compression and slight lateral flow which occasion a flattening of the grains and an alignment of the scaly minerals across the direction of application of the force in this way produce the cleavage. All slates have cross-cleavages, or, it may be, joints, more or less well developed, and one of these may even be perfect enough in connection with the regular cleavage, to cause the slate to break into small prisms available for slate pencils, for which in earlier years they were em- ployed. All slate quarries also show curly slates, where quartz- veins or sandy and harder streaks in the original sediment have caused imperfections in the cleavage. It has been noted that in some quarries the available plates appear to become thicker in depth, as if the surface weathering had been a factor in developing the cleavages. Though commonly drab to black, they may be red, green or purple. Slates pass by all intermediate gradations into phyllites and mica-schists. The word slate is also loosely used for shales that have never had any secondary cleavage induced in them, and this is especially true of the black, bituminous shales that occur with coal seams, but in strict, geological use, the new cleavage and metamorphism should be essentials of a true slate. Alteration. Slates are exceedingly resistant as is shown by their use in thin slabs for roofs, and they often constitute prominent ledges or even peaks. They soften down to a clay in the last stages of alteration, but always on the outcrop are more tender than in * G. F. Becker, Finite Homogeneous Strain, Flow and Rupture in Rocks, Bull. Geol. Soc. Amer., IV., 82, 1893. I 3 8 A HAND BOOK OF ROCKS. depth, so that much dead work is unavoidable in opening quarries. Distribution. Our most prominent slates are Cambrian or Or- dovican in age. Along the Green Mountains and especially in northern Vermont they are strongly developed. Again in eastern Pennsylvania, in Virginia and in Georgia they are met in great areas. On the south shore of Lake Superior merchantable grades have been somewhat developed. Along the western slopes of the Sierra Nevada Mountains they are very important rocks. THE CRYSTALLINE LIMESTONES AND DOLOMITES. Loss FejO. or C.CO, MgCO, SiO, Al.O, FeO Insol. H a O 1. 99.51 0.29 0.20 2. 99.24 0.28 ' ' 3- 98.43 0.30 0.31 0.38 0.15 4- 98.21 2.35 0.15 0.35 5. 98.00 0.57 1.63 6. 96.82 1.89 0.10 2.12 7. 92.42 6.47 0.35 0.95 8. 70.1 25.40 2.40 9. 54-62 45.04 o. 10 0.7 10. 54.25 44.45 0.60 I. Statuary Marble, Brandon, Vt. Quoted by G. P. Merrill, Stones for Building and Decoration, 417. 2. Marble, Carrara, Italy, Idem. 3. Marble, Knoxville, Tenn., Idem, also S, 0.014, Organic Matter, 0.068. 4. Cross-grained black and white mot- tled Marble, Pickens Co., Ga., locally called Creole ; Geol. Surv. Ga., Bulletin I., 87. 5. White Marble, Rutland, Vt., see under No. i. 6. Coarsely crystalline white Mar- ble, Cherokee Quarry, Pickens Co., Ga., see under No. 4. 7. White Crystalline Limestone, Franklin Furnace, N. J., Geo. C. Stone, unpublished. 8. Crystalline Magnesian Limestone, Tuckahoe, N. Y., H. L., Bowker for Lime Co. 9. Crystalline Dolomite, so-called " Snowflake Marble," Pleasantville, N. Y., l6th Ann. Rep. Dir. U. S. Geol. Survey, Part IV., p. 468. 10. Crystalline Dolomite, white marble, Inyo Co., Calif., Ann. Rep. Calif. State Mineralogist, I |SiO 2 giving in this form a molecular weight of 1 1 8, by which factor the total molecular proportion of the CaO obtained for this mineral in recasting may be immediately multiplied in order to get the per- centage of the mineral itselC In a similar way the molecules of the other minerals cited have been treated. While these recalculations are simple in themselves yet they have an element of the obscure and the elusive. Prob- ably all teachers have discovered the ease with which the inex- perienced become confused. Having determined the percentages by weight of the several component minerals in a rock, it is sometimes desirable to express their percentages by volumes. This may be done if the weight- percentage of each mineral is divided by its specific gravity. The several quotients must then be added up to a total, which when divided into each of the quotients in turn will give its volume-per- centage. In such recasting it is somewhat surprising to the inex- perienced to note what a relatively small percentage by volume, a i 5 8 A HAND BOOK OF ROCKS. relatively high percentage by weight of a heavy mineral involves. Calculations along these lines are sometimes of much importance in the investigations of lean or disseminated ores. Thus those lean magnetites which can only be treated by crushing and mag- netic concentration can be expressed in terms of much greater significance than the mere chemical analyses. While ores of this type only involve normal rock-making minerals, yet galena and blende in limestone, or any others regarding whose composition and specific gravity we have the necessary data can be treated in a precisely similar way. When the recasting of analyses can be carried out it forms a good check on the accuracy of the analytical work, because if the per- centage results do not actually or approximately afford some rea- sonable combination of minerals, errors have obviously been made. By a series of assumptions regarding the composition of some of the minerals which make trouble in the recasting, Messrs. Cross, Iddings, Pirsson and Washington have developed a series of so-called " standard " minerals into which for purposes of quan- titative classification any analysis of an igneous rock can be broken up. This computed mineralogical aggregate is called the " norm." It may or may not differ seriously from the actual composition called the " mode," but once the norm is calculated, any rock of which a good analysis has been made, can be quickly placed in the quantitative scheme of classification. The methods and the scheme itself are too complicated to be described here and reference should be made to the original works.* While admirably adapted for the purposes of the investigator into the chemical relations of rocks and magmas, yet for the reasons that it makes texture play the most subordinate part of all in its determinations ; that it requires a chemical analysis for its application in new cases ; and that it deals with assumed and often non-existent minerals instead of those really present ; it cannot be used by the ordinary observer or the field geologist. * Cross, Iddings, Pirsson and Washington, Quantitative Classification of Igneous Rocks, Chicago, 1903. H. S. Washington, Chemical Analyses of Igneous Rocks, Professional Paper No. 14, U. S. Geological Survey. J. P. Iddings, Chemical Composi- tion of Igneous Rocks, expressed by means of diagrams, etc. Professional Paper No. 18, do., do. CHEMICAL ANALYSES OF ROCKS. 159 Silica, Si0 2 . Molec. Weight 60. Log. 1.778151. o I 2 3 4 5 6 7 8 9 30 3i .500 .516 .501 .518 503 520 505 -521 .506 523 .508 525 .510 .526 5" .528 513 53 515 53 1 32 533 535 536 538 540 -541 543 545 -546 -548 33 550 551 553 555 -556 -558 .560 -561 563 -565 34 .566 .568 57 571 573 575 576 -578 .581 1 .583 .600 .585 .601 .586 .603 .588 .605 -590 .606 591 .608 593 .610 595 .611 '596 .613 -598 615 .616 633 .618 635 .620 .636 .621 .638 .623 .640 .625 .641 .626 643 .628 .645 .630 .646 -63? .648 39 .650 .651 653 655 .656 .658 .660 .661 -663 -665 4 o .666 .668 .670 .671 673 -675 .676 .678 .680 .681 41 683 .685 .686 .688 .690 .691 693 695 .696 .698 42 .700 .701 703 70S .706 .708 .710 .711 713 .715 43 .716 .718 .720 721 723 725 .726 .728 730 73 1 44 733 735 736 738 .740 .741 743 745 .746 748 45 750 751 753 755 756 -758 .760 .761 .763 .765 46 .766 .768 .770 771 773 775 .776 .778 .780 .781 47 48 783 .800 .785 .801 .786 .803 .788 .805 .790 .806 .791 .808 793 .810 795 .811 .796 .813 .798 .815 49 .816 .818 .820 .821 .823 .825 .826 .828 .830 -831 50 833 835 .836 .838 .840 .841 -843 845 .846 .848 5' .851 853 855 .856 .858 .860 .861 .863 .865 52 .866 .868 .870 .871 873 .875 .876 .878 .880 .881 53 .883 885 .886 .888 .890 .891 -893 895 .896 .898 54 .900 .901 903 905 .906 .908 .910 .911 913 9*5 it .916 933 .918 935 .920 936 .921 .938 923 940 925 .941 .926 -943 .928 945 930 .946 931 .948 57 950 .951 953 955 956 958 .960 .961 -963 965 58 .966 .968 970 .971 -973 975 .976 .978 .980 .981 59 983 985 .986 .988 .990 .991 -993 995 .996 .998 60 I.OOO I.OOI .003 1.005 1. 006 1.008 .010 I.OII 1.013 1.015 61 1.016 1.018 .020 I.O2I 1.023 1025 .026 1.028 1.030 1.031 62 1.033 1-035 .036 1.038 1.040 1.041 -043 045 1.046 1.048 63 .051 053 1-055 1.056 1.058 .060 .061 .063 1.065 64 .066 .068 .OJO I.O7I 1-073 1-075 .076 .078 .080 .081 .083 .085 .086 1. 088 .090 1.091 093 095 .096 .098 66 .100 .101 .103 105 .106 i 108 .no .in "3 "5 67 .116 .118 .120 .121 .123 1.125 .126 .128 .130 131 68 133 135 .136 .138 .140 1.141 -143 -145 .146 .148 69 .150 151 153 -155 .156 1.158 .160 .161 163 .165 70 .166 .168 .170 .171 '73 f I 75 .176 .178 .180 .181 .183 .185 .186 .188 .190 1.191 193 195 .196 .198 72 .200 .201 .203 .205 .206 1.208 .210 .211 .213 215 73 .216 .218 .220 .221 .223 1.225 .226 .228 .230 231 74 233 235 .236 .238 .240 1.241 243 245 .246 .248 .250 251 253 255 .256 1.258 .260 .261 .263 .265 76 .266 .268 .270 .271 273 1-275 .276 .278 .280 .281 77 .283 .285 .286 288 .290 1.291 293 -295 .296 .298 78 .300 .301 303 35 .306 1.308 .310 I.3II .315 79 .316 318 .320 3 2I 1-323 I-325 .326 1-328 1-33 331 i6o A HAND BOOK OF ROCKS. Alumina, A1 2 3 , Molec. Weight 102. Log. 2.008600. i 2 3 4 5 6 7 8 9 .000 .001 .002 .003 .004 .005 .006 .007 .008 .009 I .010 .on .012 .012 .013 .014 .015 .016 .017 .018 2 .020 .021 .021 .022 .023 .024 .025 .026 .027 .028 3 .030 .031 .031 .032 033 .034 35 .036 37 .038 4 .040 .041 .041 .042 043 .044 .045 .046 .047 .048 1 .050 059 .051 .060 .051 .06l .052 .062 053 .063 .054 .064 .055 .065 .056 .066 .057 .067 058 .068 7 .069 .070 .071 .071 .072 073 .074 075 .076 .077 8 .078 .080 .080 .081 .082 .083 .084 .085 .086 .087 9 .088 .089 .090 .091 .092 93 094 095 .096 .097 10 .098 .099 .IOO .101 .102 .103 .104 105 .106 .107 n .108 .109 .no .III .112 .112 "3 .114 "5 .116 12 .117 .118 .119 .I2O .121 .122 .123 .124 .125 .126 13 .127 .128 .129 .130 .13! .132 133 134 135 .136 14 137 .138 139 .140 .141 .142 143 .144 145 .146 11 .147 .157 .148 .158 .149 .159 .ISO .I60 I5 1 .161 '.\62 153 .163 - 1 ! 4 .164 '55 .165 .156 .166 17 .167 .168 .169 .170 .170 .171 .172 173 .174 175 18 .176 .177 .178 .179 .180 .181 .182 .183 .184 .185 19 .186 .187 .188 .189 .190 .191 .192 193 .194 195 20 .196 .197 .198 .199 .200 .201 .202 .203 .204 205 21 .206 .207 .208 .209 .210 .211 .212 .213 .214 .215 22 .216 .217 .218 .219 .220 .221 .222 .223 .224 .225 23 .226 .227 .228 .229 .230 .230 .231 .232 233 234 24 .235 .236 237 .238 239 .240 .241 .242 243 .244 25 .245 .246 .247 .2 4 8 .249 .250 .251 .252 253 254 26 27 2 |5 .265 :& 257 .267 .259 .269 .260 .270 .261 .271 .262 .272 .263 273 .264 273 28 274 .275 .276 .277 .278 .279 .280 .281 .282 283 29 .284 .285 .286 .287 .288 .28 9 .290 .291 .292 293 Ferric Oxide, Fe,0 s , Molec. Weigbt 160. Log. 2.204120. i 2 3 4 5 6 7 8 9 o .000 .000 .001 .002 .002 .003 .003 .004 .005 .005 I .006 .007 .007 .008 .009 .009 .010 .010 .on .012 2 .013 .013 .OI4 .014 .015 .015 .016 .017 .017 .018 3 .019 .020 .O2O .020 .021 .022 .022 .023 .024 .024 4 .025 .025 .026 .027 .027 .028 .029 .029 .030 .030 5 .031 .032 .032 033 034 034 35 035 .036 037 6 037 .038 039 0S9 .040 .040 .041 .042 .042 043 7 .044 .044 .045 .045 .046 .047 .048 .049 .049 .050 8 .050 .050 .051 .052 .052 S3 .054 .054 .055 055 9 .056 .057 057 .058 059 059 .060 .060 .061 .062 10 .062 .063 .064 .064 .065 .065 .066 .067 .067 .068 n .069 .069 .070 .070 .071 .072 .072 .073 .074 .074 12 075 .075 .076 .077 .077 .078 .079 .079 .080 .080 13 .081 .082 .082 .083 .084 .084 .085 .085 .086 .087 4 .087 .088 .089 .089 .090 .090 .091 .092 .092 093 IS .094 .094 .0 9 5 .095 .096 .097 097 .098 .099 .099 CHEMICAL ANALYSES OF ROCKS. 161 Ferrous Oxide, FeO, Molec. Weight 72. Log. 1.851333. o i 2 3 4 5 6 7 8 9 o .000 .001 .003 .004 .005 .007 .008 .010 .on .012 I .014 .015 .017 .018 .019 .021 .022 .024 .025 .027 2 .028 .030 .030 .032 .033 035 .036 .038 039 .040 3 4 .042 .056 043 .057 .044 S 8 .046 .060 .048 .061 .049 .062 050 .064 .051 .065 '.067 3* 5 1070 .071 .072 .074 075 .076 .078 .079 Io8o .082 6 .083 .085 .086 .088 .089 .090 .092 093 .094 .096 7 .097 .099 .100 .101 .103 .104 .106 .107 .108 .no 8 .in .112 .114 "5 .117 .118 .I2O .121 .122 .124 9 125 .126 .128 .129 .131 133 134 .136 137 138 10 .140 .140 .141 143 .144 .146 147 .149 ISO 151 n .153 154 156 157 .158 .160 .161 .162 .I6 4 .165 12 .167 .168 .170 .171 .172 .174 175 .176 .178 .179 U .180 .182 .183 185 .186 .188 .189 .190 .192 193 .194 .196 .197 .199 .200 .201 .203 .204 .206 .207 15 .208 .210 .211 .212 .214 .215 .217 .218 .220 .221 Magnesia, MgO, Molec. Weight 40. Log. 1.602060. O i 2 3 4 5 6 7 8 9 O .OOO .002 .005 .007 .010 .012 .015 .017 .020 .022 I .025 .027 .030 .032 035 037 .040 .042 045 .047 2 .050 .052 055 057 .060 .062 .065 .067 .070 .072 3 075 .077 .080 .082 085 .087 .090 .092 095 .097 4 .IOO .IO2 105 .107 .no .112 .115 .117 .120 .122 5 125 .127 .130 .132 .135 137 .140 .142 145 .147 6 .150 .152 155 157 .160 .162 .165 .167 .170 .172 7 175 .177 .180 .182 .185 .187 .190 .192 .195 .197 8 .2OO .202 205 .207 .210 .212 .215 .217 .220 .222 9 .225 .227 .230 .232 235 237 .240 .242 .245 .247 10 .250 .252 255 257 .260 .262 .265 .267 .270 .272 n 275 .277 .280 .282 .285 .287 .290 .292 295 297 12 .300 .302 .305 307 .310 .312 315 317 .320 .322 '3 325 327 330 332 337 340 342 345 347 350 352 .360 .362 365 .367 370 372 IS 375 377 .380 .382 .385 387 390 392 395 397 1 6 .400 .402 .405 .407 .410 .412 415 .417 .420 .422 17 425 427 430 432 435 437 .440 .442 445 447 18 45 452 455 457 .460 .462 .465 467 .470 .472 9 475 477 .480 .482 485 .487 .490 .492 495 497 20 .500 502 505 57 .510 512 515 517 .520 .522 21 525 527 530 532 535 537 540 542 545 547 22 55 552 555 557 .560 .562 .565 .567 .570 572 23 24 575 .600 .12 .580 .605 .582 .607 .585 .610 -587 .612 59 .615 592 .617 595 .620 597 .622 25 .625 .627 .630 .632 635 637 .640 .642 .645 647 162 A HAND BOOK OF ROCKS. Lime, CaO, Molec. Weight 56. Log. 1.748188. o i 2 3 4 5 6 7 8 9 .000 .002 .003 .005 .007 .009 .010 .012 .014 .016 I .018 .020 .021 .023 .025 .027 .029 .030 .032 034 2 .036 .038 39 .041 043 045 .047 .048 .050 .051 3 53 S5 057 059 .060 .062 .064 .066 .068 .070 4 .071 73 075 .077 .078 .080 .082 .084 .086 .087 5 .089 .091 093 094 .096 .098 .100 .IO2 .103 .105 6 .107 .109 .no .112 ."4 .116 .118 .120 .121 .123 7 125 .127 .128 .130 132 134 135 137 39 .141 8 143 .144 .146 . I4 8 .150 .151 153 155 157 159 9 .160 .161 .164 .166 .168 .169 .171 173 175 .177 10 .178 .180 .182 .184 .185 .187 .189 .191 193 .194 II .196 .198 .200 .201 .203 .205 .207 .209 .2IO .212 12 .214 .216 .218 .219 .221 .223 .225 .226 .228 .230 13 .232 .234 .235 237 239 .241 243 .244 .246 .2 4 8 H .250 251 253 255 257 259 .260 .262 .26 4 .266 Soda, \a A Molec. Weight 62. Log. 1.792392. O i 2 3 4 5 6 7 8 9 .000 .001 .003 .005 .006 .008 .009 .Oil .013 .014 I .016 .018 .019 .021 .022 .024 .026 .027 .029 .030 2 .032 .034 035 037 39 .040 .042 .043 045 .047 3 .048 .050 .051 053 055 .056 .058 059 .061 .063 4 .064 .066 .068 .06 9 .071 .072 .074 .076 .077 .079 5 .080 .082 .084 .085 .087 .089 .090 .092 093 095 6 .097 .098 .IOO .101 .103 .105 .106 .108 .109 .ill 7 113 .114 .116 .118 .119 .121 .122 .124 .126 .127 8 .129^ .130 .132 .134 135 137 139 .140 .142 143 9 145 .147 .I 4 8 .150 151 153 155 .156 .158 159 10 .161 .163 .I6 4 .166 .168 .169 .171 .172 .174 .176 ii .177 .179 .ISO .182 .184 .185 .I8 7 .189 .190 .192 12 193 .195 .197 .198 .200 .2OI .203 .205 .206 .208 13 .209 .211 .212 .214 .215 .217 .219 .221 .222 .224 4 .226 .227 .229 .230 .232 234 235 237 239 .240 15 .242 243 245 .247 .248 .250 .251 253 255 .256 CHEMICAL ANALYSES OF ROCKS. 163 Potash, K 2 0, Molec. Weight 94. log. 1.9*3128. o I 2 8 9 o .000 .001 .OO2 .003 .OO4 .005 .006 .007 .008 .009 I .010 .012 013 .014 015 .Ol6 .017 .018 .019 .020 2 .021 .022 023 .024 .024 .026 .027 .029 .030 .031 3 .032 033 034 035 .036 037 .038 039 .040 .041 4 .042 043 .045 .046 .047 .048 .049 .050 051 5 054 055 .056 057 .058 59 .060 .061 .062 .063 6 .064 06 5 .066 .067 .068 .069 .070 .071 .072 73 7 .074 075 .076 .078 .079 .080 .081 .082 .083 .084 8 .08| .086 .087 .088 .08 9 .090 .091 .092 093 .094 9 .096 .097 .098 .099 .IOO .101 .102 .103 .104 .105 10 .106 .107 .108 .109 .no .112 "3 .114 .115 .116 ii .117 .118 .119 .I2O .121 .122 .123 .124 I2 5 .126 12 .127 .129 .130 .131 .132 133 134 J 35 .136 1 37 13 .138 139 .140 .141 .142 143 .144 145 .147 .148 14 .149 .150 151 152 53 154 155 .156 .158 15 159 .160 .162 I6 3 .164 .165 .166 .167 ]i68 .169 Water, H 2 0, Molec. Weight 18. Log. 1.255273. i 2 3 4 5 6 7 8 9 o .000 .005 .Oil .016 .022 .028 33 .040 .044 .050 I 055 .061 .066 .072 .080 083 .090 .094 .100 .105 2 .in .116 .122 .128 133 139 .144 150 155 .161 3 .166 .172 .178 183 .I8 9 .194 .200 .205 .211 .216 4 .222 .228 233 239 .244 .250 255 .261 .267 .272 5 .278 .283 .28 9 .294 .300 35 3" 316 .322 .328 6 333 339 344 350 355 .361 .367 372 378 .383 7 .389 394 .400 405 .411 .417 .422 .428 433 439 8 444 450 455 .461 .467 472 478 483 489 494 9 .500 .505 5" 517 522 .528 533 539 544 550 Carhonic Acid, C0 2 , Molec. Weight 44. Log. 1.643453. o i 2 3 4 5 6 7 8 9 .000 .002 .004 .007 .009 .Oil .014 .016 .018 .020 I .023 .025 .027 .029 .032 034 .036 039 .041 043 2 045 .0 4 8 .050 052 055 057 59 .061 .064 .066 3 .068 .O7O 073 075 .077 .080 .082 .084 .086 .089 4 .091 093 95 .098 .100 .102 .104 .107 .109 .in 5 "3 .116 .118 .120 .123 125 .127 .129 .132 134 6 .136 139 .141 143 .145 .148 .150 152 154 157 7 159 ,i6i .163 .166 .168 .170 173 175 .177 .179 8 .182 .184 .186 .189 .191 193 195 .198 .200 .202 9 .204 .207 .20 9 .211 .2 .216 .218 .220 .223 .225 164 A HAND BOOK OF ROCKS. Titanic Acid, T10 2 , Molec. Weight 82. Log. 1.913814. o i 2 3 4 5 6 7 8 9 o .000 .001 .002 .003 .005 .006 .007 .008 .010 .on I .012 .013 .014 .015 .017 .018 .019 .020 .022 .023 2 .024 .025 .026 .028 .029 .030 .031 033 034 035 3 .036 .038 039 .040 .041 -043 .044 045 .046 .047 4 .049 .050 .051 .052 053 .055 .050 .057 .058 059 c .061 .062 .063 .064 .066 .067 .068 .069 .070 .072 6 73 .074 .075 .077 .078 .079 .080 .081 .083 .084 7 .o8 S .086 .088 .089 .090 .091 .092 .094 095 .096 8 .097 .098 .IOO .101 .102 103 .105 .106 .107 .108 9 .109 .in .112 "3 .114 .116 .117 .118 .119 .120 Zirconla, Zr0 2 , Molec. Weight 122. Log. 2.086360. .001 2 o .000 .001 .002 .003 .004 .005 .006 .006 .007 Phosphoric pentoxlde, P,0 5 Molec. Weight 142. Log. 2.152288. I .000 .008 .015 2 3 .002 .009 .016 4 5 6 7 8 9 o I 2 .000 .007 .014 .OOI .008 .015 .003 .010 .017 .003 .010 .017 .004 .Oil .018 .005 .012 .019 .005 .013 .020 .006 .013 .020 Sulphuric anhydride, 80 3 , Molec. Weight 80. Log. 1.903090. I 2 3 4 5 6 7 8 9 o .000 .012 .001 .014 .OO2 .015 .004 .010 .005 .017 .006 .019 .007 .020 .009 .021 .010 .022 .Oil .024 Chlorine, Cl, Atomic Weight 35.5. Log. 1.550228. I 2 3 4 5 6 7 8 9 o .000 .002 .006 .008 .on .014 .017 .020 .022 .025 Fluorine, F, Atomic Weight 19. Log. 1.278754. I 005 2 3 4 5 .026 6 .031 7 8 9 .047 .000 .OIO .016 .021 037 .042 Sulphur, 8, Atomic Weight 32. Log. 1.505150. O I 2 3 4 5 6 7 8 9 o .OOO .003 .006 .009 .012 .016 .019 .022 .025 .028 I .031 .034 037 .040 .044 .047 .050 053 .OS6 .059 2 .062 .066 .06 9 .072 075 .078 .081 .084 .087 .091 CHEMICAL ANALYSES OF ROCKS. Chromic oxide, Cr 2 3 Molec. Weight 152.8. Log. 2.184123. 165 o X 2 3 4 5 6 7 8 9 o .000 .000 .001 .002 .002 .003 .004 .004 .005 .006 I .006 .007 .008 .008 .009 .010 .010 .on .012 .012 2 .013 .014 .014 .015 .016 .016 .017 .018 .018 .OI9 3 .020 .020 .021 .022 .022 .023 .024 .024 .025 .026 4 .026 .027 .028 .028 .029 .030 030 031 .032 .032 5 033 .034 .034 035 .036 .036 037 .038 .038 039 Nickel oxide, NiO, Molec. Weight 15. Log. 1.875061. Cobalt oxide, CoO, Molec. Weight 15. Log. 1.875061. O I 2 o .000 .001 .002 .00 4 .005 .006 .008 .OIO .on .012 I .013 .015 .016 .017 .019 .O2O .O2I .023 .024 .025 2 .027 .028 .030 .031 .032 033 35 .036 037 039 Cnpric oxide, CuO, Molec. Weight 79.1. Log. 1.898176. o I 2 3 4 5 6 7 8 9 o .000 .001 .002 .004 .005 .006 .007 .009 .010 .on Manganons oxide, MnO, Molec. Weight 71. Log. 1.851258. o I 2 3 4 5 6 7 8 9 .000 .001 .003 .004 .005 .007 .008 .010 .Oil .012 I .014 015 .017 .018 .020 .021 .022 .024 .025 .027 2 .028 .030 .031 .032 034 035 .036 .038 39 .041 3 .042 043 .045 .046 .048 .049 .050 .052 053 055 4 .056 .osS 059 .060 .062 .06} .obs .066 .068 .069 5 .070 .072 073 .074 .076 .077 .079 .080 .081 .083 Baryta, BaO, Molec. Weight 152.8. Log. 2.184123. o i 2 3 4 5 6 7 8 9 .000 .000 .OOI .002 .002 .003 .004 .004 .005 .006 I .oc6 .007 008 .008 .009 .010 .010 .on .012 .012 Strontia, SrO, Molec. Weight 103.5. Log. 2.014940. o I 2 3 4 5 6 7 8 9 .000 .000 .002 .003 .004 .005 .006 .007 .008 .009 I .010 .on .on .012 .013 .014 .015 .016 .017 .018 Lithia, Li,0, Molec. Weight 30. Log. 1.477121. o I 2 3 4 5 6 7 8 9 .000 .003 .006 .010 .013 .016 .020 .023 .026 .030 I .033 .036 .040 043 .046 .050 053 .056 .060 .063 1 66 A HAND BOOK OF ROCKS. FACTORS FOR TURNING MOLECULAR PROPOR- TIONS INTO PERCENTAGES. In each case multiply the molecular proportion of the given constituent oxide or oxides by factor cited below, which is usually the molecular weight of the compound. (For explanation see p. I57-) Mineral. Composition. Oxide Used. Facto, Quartz, Si0 2 Si0 2 60 Orthoclase, K 2 O,Al 2 O 3 ,6SiO 2 K 2 556 Albite, Na 2 0,Al 2 3 ,6SiO s Na 2 524 Anorthite, CaO,Al 2 3 ,2Si0 2 CaO 278 Nephelite, Na 2 O,Al 2 O 3 ,2SiO 2 Na 2 284 Leucite, K 2 0,Al 2 O s , 4 Si0 2 K 2 436 Melilite, i2CaO,2A! 2 O s ,9SiO 2 CaO 118 Analcite, Na 2 0,Al 2 3 , 4 Si0 2 2H 2 Na 2 440 Sodalite, 3(Na 2 0,Al 2 3 ,2Si0 2 ) + 2 NaCl NajO 323 Hauynite, 3 (Na 2 0,Al 2 3 ,2Si0 2 ) + 2CaSO 4 Na 2 375 Noselite, 3 (Na 2 0,Al 2 8 ,2Si0 2 ) + 2Na s SO 4 Na,0 379 Muscovite, K 2 0,Al 2 O s ,2Si0 2 K 2 3 l6 H 2 0,Al 2 O s ,2Si0 2 240 Biotite, 2(K 2 0,Al 2 3 ,2Si0 2 ) K 2 3l6 2 K 2 0,Fe 2 0,, 2 Si0 2 ) 2(H 2 0,Al 2 O s ,2Si0 2 ) 2(H 2 O,Fe 2 O 3 ,2SiO 2 ) 2MgO,SiO 2 K 2 H 2 H 2 Si0 2 374 240 298 140 2FeO,SiO, SiO, 204 Amphiboles and Pyroxenes, MgO,Si0 2 FeO,Si0 2 MgO FeO 100 132 CaO,SiO 2 MnO,Si0 2 MgO,Al 2 3 ,Si0 2 MgO,Fe 2 3 ,Si0 2 FeO,Al 2 O s ,Si0 2 Na 2 0,Fe 2 3 , 4 Si0 4 CaO MnO MgO MgO FeO Na 2 iiti 131 202 260 234 462 Olivine, 2MgO,Si0 2 Si0 2 140 2FeO,SiO 2 Si0 2 204 Magnetite, FeO,Fe 2 8 Fe 2 8 232 Ilmenite, FeO,TiO 2 Ti0 2 154 Apatite, 9CaO, 3 P 2 5 ,CaCl 2 CaO 116 gCaO, 3 P-jOg, CaF 2 CaO 112 Kaolin, Al 2 O s ,2Si0 2 ,2H 2 0, A1 2 3 258 Serpentine, 3MgO,2Si0 2 ,2H 2 MgO 9 2 Calcite, CaO,CO 2 CaO 100 Magnesite, MgO,C0 2 MgO 84 GLOSSARY. NOTE. In the following definitions, when fuller explanations are to be found in preceding pages, references are given to them and they should be consulted. No attempt has been made to unnecessarily repeat previous statements. In an appendix will be found new names given 1904-1908. Aa, a Hawaian word specially introduced into American usage by Maj. C. E. Button, and employed to describe jagged, scoriaceous, lava flows. It is contrasted with pahoehoe. 4th Ann. Rep. U. S. Geol. Survey, 95. Ablation, a name applied to the process whereby residual deposits are formed by the washing away of loose or soluble materials. Absarokite, a general name given by Iddings to a group of igneous rocks in the Absaroka range, in the eastern portion of the Yellowstone Park. They have porphyritic texture with phenocrysts of olivine and augite in a groundmass, either glassy or containing leucite, orthoclase or plagioclase, one or several. They range chemically, SiO,, 46-52 ; A1,O S , 9-12 ; MgO, 8-13 ; alkalies, 5-6.3, with potash in excess. The name is of greatest significance when taken in connection with shosho- nite and banakite. Jour, of Geol., III., 936. Abyssal-rocks, a synonym of plutonic rocks as used in preceding pages. The word has been suggested and especially used by W. C. Brogger. Accessory components or minerals in rocks are those of minor im- portance or of rare occurrence, whose presence is not called for by the definition of the species. Acidic, a descriptive term applied to those igneous rocks that con- tain more than 65 per cent. SiO 2 , as contrasted with the medium of 65 per cent, to 55 per cent, and the basic at less that 55 per cent. ; still the limits are somewhat elastic. Acmite-trachyte, a trachyte whose pyroxene is acmite or segirite and whose feldspar is anorthoclase. It therefore differs from normal trachyte in its prevailing soda instead of potash, as is shown by the acmite, a soda-pyroxene, and the anorthoclase, a soda-feldspar. The acmite-trachytes are intermediate between the true trachytes and the 1 68 GLOSSARY. 169 phonolites. They were first described from the Azores (Mugge, Neues Jahrbuch, 1883, II., 189) and have also been found in the Crazy Mountains, Mont. ; see p. 38, Anals. 4 and 5. Adamellite, a name proposed by Cathrein as a substitute for tonalite, on the ground that tonalite means a hornblende-biotite granite, rich in plagioclase, whereas adamellite, which better describes the rocks at the Tyrolese locality, means a quartz-hornblende-mica-diorite with granitic affinities. Adamellite emphasizes the dioritic characters ; tonalite, the granitic. The name is derived from Monte Adamello, near Meran, Tyrol, the locality of tonalite. Neues Jahrb., 1890, I., 75. Brogger uses it for acidic quartz-monzonite. Eruptions-folge bei Predazzo, 6 1 . Adinole, a name for dense felsitic rocks, composed chiefly of an aggregate of excessively fine quartz and albite crystals, such that on analysis the percentage of soda may reach 10. Actinolite and other minerals are subordinate. Adinoles occur as contact rocks, associated with diabase intrusions and are produced by them from schists (com- pare spilosite and desmite). They also constitute individual beds in metamorphic series. (Compare porphyroid, halleflinta. ) The name was first given by Beudant, but has been especially revived by Lessen. Zeits. d. d. Geol. Ges., XIX., 572, 1867. jEgirite, the name of this soda-pyroxene is often prefixed to nor- mal rock-names because of its presence, as for instance, aegirite-granite, aegirite-trachyte. Microscopic study has shown that the mineral is much more widely distributed than was formerly appreciated. Aerolite, a synonym of meteorite. Agglomerate, a special name for volcanic breccias as distinguished from other breccias and from conglomerates. Ailsyte, a name derived from Ailsa Craig, Scotland, and suggested for a micro-granite with considerable riebeckite, which occurs there. M. F. Heddle, Trans. Edinburgh Geol. Soc., VII. , 265, 1897. Akerite, a special name coined by Brogger for a variety of syenite at Aker, Norway, that is a granitoid rock consisting of orthoclase, consider- able plagioclase, biotite, augite and some quartz. (W. C. Brogger, Zeitsch. f. Krys., 1890, 43.) Alaskite, a name proposed by J. E. Spurr as a general term for all rocks consisting essentially of quartz and alkali feldspar, without regard to texture. Those with xenomorphic or hypautomorphic textures are alaskites; those with panautomorphic textures alaskite-aplite ; those with porphyritic texture involving a fine-grained or aphanitic ground- mass, tordrillites (which see). Amer. Geol., XXV., 231, 2oth Ann. Rep. U. S. G. S. Part 7, 189, 195. i;o A HAND BOOK OF ROCKS. Albite, the name of the mineral is sometimes prefixed to normal rock names, because of its presence in the rocks; as for instance albite- diorite, albite-porphyrite. Albitophyre, a name given by A. Michel-Levy to a dike rock, in which are developed very large, polysynthetic phenocrysts of albite. In the groundmass are microlites of the same mineral, together with chlorite and limonite. Comptes rendus, CXXIL, 265, 1896. Alboranite, a variety of hypersthene-andesite, poor in soda, from the island of Alboran, east of the Straits of Gibraltar, and 80 km. south from Spain. The recasting of a typical analysis gave plagioclase, (Ab t An 4 . 5 ) 41.5; hypersthene, 5; augite, 20; magnetite, 9; basis, 24.5; total loo. The rocks are porphyritic with plagioclase pheno- crysts. F. Becke, Tschermaks Mittheilungen, XVIII., 553, 1899. Aleutite, a name proposed by J. E. Spurr for those members of his belugites (which see) having a porphyritic texture with an aphanitic or finely crystalline groundmass. Amer. Geol., XXV., 233, 1900. 2oth Ann. Rep. U. S. G. S., Part 7, 209. Algovite, a name proposed by Winkler, for a group of rocks, practi- cally diabases, or porphyritic phases of the same, in the Algauer Alps. They also embrace gabbros according to Roth, and are doubtless various textural varieties of an augite-plagioclase magma. Neues Jahrbuch, 1895, 641. Allalinite, a name derived from Allalin mountain in the Pennine Alps, and applied by H. Rosenbusch to an actinolite-saussurite rock, which had been derived from gabbro without losing the characteristic texture of the latter. That is, the allalinites are not sheared and crushed as in the flaser- gabbros and forellensteins. Massige Gest. , 328, 1895. Allotriomorphic, an adjective coined by Rosenbusch in 1887 to describe those minerals in an igneous rock which do not possess their own crystal faces or boundaries, but which have their outlines impressed on them by their neighbors. They result when a number of minerals crystallize at once so as to interfere with one another. They are espe- cially characteristic of granitoid textures. The word was unnecessary, as xenomorphic had been earlier suggested for the same thing, but it is in more general use than xenomorphic. See also anhedron. Alluvium, Lyells' name for the deposit of loose gravel, sand and mud that usually intervenes in every district between the superficial covering of vegetable mould and the subjacent rock. The name is derived from the Latin word for an inundation (Elements of Geol., 6th Ed., N. Y., 1859, p. 79). It was employed by Naumann as a general term for sedi- ments in water as contrasted with eolian rocks. It is generally used GLOSSARY. 171 to-day for " the earthy deposit made by running streams or lakes, espe- cially during times of flood." (Dana's Manual, 1895, P- 8l -) In a stratigraphical sense it was formerly employed for the more recent water- sorted sediments, as contrasted with " diluvium," or the stratified and unstratified deposits from the melting of the continental glacier of the Glacial Period. This use, with fuller study of the Glacial deposits, is practically obsolete. Alnoite, a very rare rock with the composition of a melilite basalt, that was first discovered in dikes on the island of Alno, off the coast of eastern Sweden. The special name was given it by Rosenbusch to em- phasize its occurrence in dikes and its association as a very basic rock, with nepheline syenite. Alnoite has been discovered near Montreal by F. D. Adams (Amer. Jour. Sci., April, 1892, p. 269) and at Man- heim Bridge, N. Y., by C. H. Smyth, Jr. (Amer. Jour. Sci., Aug., 1893, 104). Alsbachite, a name given by Chelius to a variety of granite-porphyry, forming dykes in Mt. Melibocus, and containing large mica crystals and rose-red garnets. Notizbl. Ver. Erdk. zu Darmstadt, 1892, Heft. 13, i. Alum-shales, shales charged with alum, which in favorable localities may be commercially leached out and crystallized. The alum results from the decomposition of pyrites, because the sulphuric acid, thus pro- duced, reacts on the alumina present, yielding the double sulphate that is alum. Ampelite, a name, specially current among the French, for shales, charged with pyrite and carbonaceous matter, which may yield alum- shales. Amphibole, the generic name for the group of bisilicate minerals whose chief rock-making member is hornblende. It is often prefixed to those rocks which have hornblende as a prominent constituent, as am- phibole-andesite, amphibole-gabbro, amphibole-granite, etc. Amphibolite, a metamorphic rock consisting chiefly of hornblende, or of some member of the amphibole group. It is as a rule a synonym of hornblende-schists, but is preferable to the latter, when the schistosity is not marked. See p. 130. Amygdaloids are cellular lavas, whose cavities, caused by expanding steam-bubbles, resemble an almond in size and shape. Basaltic rocks are most prone to develop them. The term is used in the form of the adjective, amygdaloidal, and properly should be limited to this. As a noun it is also employed for secondary fillings of the cavities, which are usually calcite, quartz or some member of the zeolite group. Amygda- loidal rocks are of chief interest in America, because certain basaltic 172 A HAND BOOK OF ROCKS. lava sheets on Keweenaw Point, Lake Superior, have their amygdules filled with native copper and are important sources of the metal. Amyg- daloidal cavities are limited to the upper and lower portions of lava sheets. The name is derived from the Greek word for almond. Analcite-basalt, a variety of basalt whose feldspar is more or less re- placed by analcite. The analcite is at times in such relations as to give reason for thinking it an original mineral and not an alteration product from feldspar. Analcite-basalts occur in the Highwood Mountains, Mont. (See W. Lindgren, zoth Census, XV., 727, Proc. Calif. Acad. Sci., Ser. II., Vol. III., p. 51. Comptes Rendus, Fifth Internal. Geol. Cong., 364). Analcite-diabase has been met in California. (H. W. Fairbanks, Bull. Dept. Geol. Univ. of Calif., I., 173.) See also in this connection teschenite. Analcite-tinguaite, tinguaite (which see) with considerable analcite. Analcitite, Pirsson's name for the olivine-free analcite-basalts. Jour. Geol., IV., 690, 1896. Anamesite, an old name suggested by von Leonhard, in 1832, for those finely crystalline basalts, which texturally stand between the dense typical basalt, and the coarser dolerites. The name is from the Greek for "in the middle." Andalusite-hornstone, a compact contact rock containing andalusite. It is usually produced from shales or slates by intrusions of granite. Andendiorite, a tertiary, quartz -augite-diorite, which occurs in areas like islands in the midst of the volcanic rocks of the Chilean Andes. The quartzes are remarkable for their inclusions of glass and of fluid with salt crystals. A. W. Stelzner, Beitrage Geol. d. Argent. Republik, 212, 1885. Andengranite, a biotite bearing hornblende-granite, similar in occur- rence and microscopic features to the andendiorite, 1. c., 208. Andesite, volcanic rocks of porphyritic or felsitic texture, whose crystallized minerals are plagioclase and one or more of the following : biotite, hornblende and augite. The name was suggested by L. von Buch in 1836, for certain rocks from the Andes, resembling trachytes, but whose feldspar was at first thought to be albite, and later oligoclase. See p. 58. Anhedron, a name proposed by L. V. Pirsson for the individual, min- eral components of an igneous rock, that lack crystal boundaries, and that cannot therefore be properly called crystals according to the older and most generally accepted conception of a crystal. Xenomorphic and allotriomorphic are adjectives implying the same conception. The name means without planes. Bulletin Geol. Society of America, Vol. VII., p. 492, 1895. GLOSSARY. 173 Anogene, a general name for rocks that have come up from below ; /. 73- Diorite-porphyrite, a prophyrite whose groundmass is a finely crys- talline diorite, and whose phenocrysts are prevailingly plagioclase. It is contrasted with hornblende-porphyrite, whose phenocrysts are pre- vailingly hornblende. Dipyr, a variety of scapolite, often used as a prefix to the names of rocks that contain the mineral. Disthene, synonym of cyanite, sometimes used as a prefix in rock names. Ditroite, a nephelite syenite from Ditro in Hungary, especially rich in blue sodalite. See p. 50. Dolerite, coarsely crystalline basalts. The word has had a somewhat variable meaning during its history and among different peoples. The English use it interchangeably with diabase ; indeed the definitions given here justify this usage, except that the characteristic texture of diabase is not essential to this definition of dolerite. But the diabasic texture is more of a microscopic feature than a megascopic. Dolerite is from the Greek for deceptive, and was given by Hauy in allusion to its appli- cation to later rocks, that could not be distinguished from older green- GLOSSARY. 187 stones. The name is a long standing indictment of the time element in the classification of igneous rocks. Dolomite, is applied to those rocks that approximate the mineral do- lomite in composition. Named by Saussure, after Dolomieu, an early French geologist. See p. 100. Dolomitization or Dolomization, the process whereby limestone be- comes dolomite by the substitution of magnesian carbonate for a portion of the original calcium carbonate. If the MgCO s approximates the 45.65 per cent, of the mineral dolomite, there is great shrinkage in bulk, leading to the development of porosity and cavities up to 1 1 per cent, of the original rock. Domite, a more or less decomposed trachyte from the Puy de Dome in the French volcanic district of the Auvergne. The typical domite contains oligoclase and is impregnated with hematite. Drift, a general name for the unsorted deposits of the glacial period. Where subsequently worked over by water they are called modified drift. Dunite, a member of the peridotite group that consists essentially of olivine and chromite. It was named from the Dun Mountains in New Zealand, the original locality, but it also occurs in North Carolina. The name was given by v. Hochstetter in 1859. Geol. v. Neu Seeland, 218, 1864. Durbachite, a name given to a basic development at the outer border of a granite intrusion in Baden. It has the general composition of mica syenite. The name was given by Sauer, Mitth. d. grossh. bad. Geol. Landesanstaldt, II., 233. Dykes, see dikes. Dynamometamorphism, a general term for those metamorphic changes in rocks that are produced by mechanical as distinguished from chemical processes, but the former are seldom unattended by the latter. See p. 121. Dysyntribite, a name given by C. U. Shepard, Amer. Assoc. Adv. Sci., 1851, 311, to a mineral or rock in St. Lawrence Co., N. Y., which is a hydrated silicate of aluminium and potassium, and is related to pinite ; the same means hard to crush. Compare parophite. See also, Smith and Brush, Amer. Jour. Sci., ii., XVI., 50, and C. H. Smyth, Jour, of Geol., II., 678, 1894. K Eclogite, a more or less schistose metamorphic rock, consisting of a light-green pyroxene (omphacite), actinolite (var. smaragdite) and garnet. Scarcely known in America. See p. 133 and anal. 6, p. 132. The name is from the Greek to select, in reference to its attractive ap- pearance. 1 88 A HAND BOOK OF ROCKS. Effusive, a name applied to those rocks that have poured out in a molten state on the surface and have there crystallized, /. e., volcanic rocks. See p. 16. Elaeolite or Eleolite, a name formerly current for the nephelite of pretertiary rocks. It is best known in the rock-name eleolite-syenite, a synonym of nephelite-syenite, but the latter is preferable. See nephe- lite-syenite. Elvan, Cornish name for dikes of quartz-porphyry or of granite-por- phyry. Endomorphic, used as a descriptive adjective for those phases of contact-metamorphism that are developed in the intrusion itself. It is synonymous with internal as used on p. 115. Enstatite, the variety of orthorhombic pyroxene with less than 5 per cent. FeO. It is largely used as a prefix to the names of rocks that con- tain the mineral. Eorhyolite, eobasalt, etc., a series of names proposed by O. Norden- skjoeld for the older equivalents of the rhyolites, basalts, etc. The terms are practically equivalent to aporhyolite, apobasalt, etc., but the latter have priority. Bull. Geol. Inst. Univ. ofUpsala, I., 292, 1893. Epidiabase, a name proposed by Issel as a substitute for epidiorite because believed to be more appropriate. Liguria geologica, I., 324, 1892. Cf. epidiorite. Epidiorite, a name applied to dikes of diabase, whose augite is in part altered to green hornblende. The name was coined before it was understood that the hornblende was secondary in this way. It was first applied by Gumbel in 1879 to a series of narrow dikes that cut Cambrian and Ordovician strata in the Fichtelgebirge. The name emphasizes their later age than the typical pre-Cambrian diorites, but its significance has been expanded in later years. Epidosite, rocks largely formed of epidote. The epidote seems gen- erally to be produced by the reactions of feldspars and bisilicates upon each other during alteration. Epidote, the name of the mineral is often prefixed to the names of rocks containing it. As a rule, the presence of epidote indicates the advance of alteration. Erlan or Erlanfels, a name proposed by Breithaupt for metamor- phic rocks, which consist essentially of augite, /. e., augite schists. The name is derived from the iron-furnace at Erla, near Crandorf, Saxony. Erosion, geological term for the process of the removal of loose mate- rials in suspension in running water or in wind. GLOSSARY. 189 Eruptive, the name ought properly to be only applied to effusive or volcanic rocks, but it is often used as a synonym of igneous. Essexite, a name derived from Essex County, Mass., and applied by J. H. Sears to a peculiar rock, occurring with the nephelite-syenite of Salem Neck. It is an intermediate rock between the nephelite-syenites, the diorites, and the gabbros, and contains labradorite, orthoclase, and more or less nephelite or sodalite, together with augite, biotite, bar- kevicite, olivine, and apatite. Bulletin Essex Institute, XXIII., 146, 1891, H. S. Washington, Journal of Geology, VII., 53, 1899. Esterellite, a name given by A. Michel-Levy to a variety of diorite- porphyry from Esterel, France. The rock shows some peculiarities of chemical composition which have given it soecial interest in discussions relating to differentiation. Bull. Service Carte geol. de la France, LVIL, 21, 1897, Bull. Soc. Geol. de la France (3), XXIV., 123. Eucrite, a name given by G. Rose to rocks and meteorites that con- sist essentially of anorthite and augite. The term is practically obsolete. Pogg. Annalen, 1835, I., i. Eudyalite, the name of the mineral is sometimes prefixed to the rare nephelite-syenites that contain it. Euktolite, a name derived from the Greek words for "desired rock " and given by H. Rosenbusch to one which filled a gap in his classifica- tion of rocks. Sitzungsber. der k. p. Akad. Wissensch., Berlin, VII., no, 1899. The same rock had been previously named venanzite (which see). Cf. Amer. Jour. Sci., May, 1899, 399. Eulysite, a name given by Erdmann to rocks interlaminated with the gneisses of Sweden, and consisting of olivine, green pyroxene and gar- net. Neues Jahrb., 1849, 837. Euphotide, the name chiefly used among the French for gabbro. It was given by Hauy, and is derived from the Greek words for well, and light, in allusion to its pleasing combination of white and green. Eurite, used among the French as a synonym of felsite, but also ap- plied to compact rocks chiefly feldspar and quartz, such as some granu- lites. The name was first given by Daubisson to the groundmass of por- phyries, because of their easy fusibility compared with hornstone or flint. Eutaxitic, a general name for banded volcanic rocks. The banding is due to the parallel arrangement of portions of the rock that are con- trasted either in mineralogy or texture. Exomorphic, a descriptive term for those changes which are produced by contact-metamorphism in the wall rock of the intrusion ; the antithesis of endomorphic. It is synonymous with external as used in p. 115. Extrusive, synonym of effusive, much used in America. 190 A HAND BOOK OF ROCKS. Farrisite, a name derived from Lake Farris in Norway, and applied by Brogger to a very peculiar rock, which is as yet known only in one small dike. The rock is finely granular in texture and consists of some soda-bearing, but not sharply identified, tetragonal mineral related to melilite, together with barkevicite, colorless pyroxene, biotite, serpen- tinous pseudomorphs after olivine, magnetite and apatite. Das Gang- gefolge des Laurdalits, 66, 1898. Feldspar, the name of the mineral is often prefixed to the names of those rocks that contain it, such as feldspar-porphyry, feldspar- basalt, etc. Feldspathoids, silicates of alumina and an alkali or alkaline earth, that are practically equivalent to feldspars in their relations in rocks. The principal ones are nephelite, leucite and melilite, but sodalite, nosean, hauyne and analcite could perhaps be also considered such, although their composition varies from the above. Felsite, the word was first applied in 1814 by Gerhard, an early geol- ogist, to the fine groundmasses of porphyries. These were recognized to be fusible as distinguished from hornstone, which they resembled (com- pare eurite) . Felsite is now especially used for those finely crystalline varieties of quartz-porphyries, porphyries or porphyrites that have few or no phenocrysts, and that, therefore, give but slight indications to the unaided eye of their actual mineralogical composition. The microscope has shown them to be made up of microscopic feldspars, quartzes and glass. Petrosilex has been used as a synonym. Seep. 17. Felsitic has been employed as a megascopic term in the preceding pages to describe those textures which are characteristic of felsites, *. t., micro-crystalline, but without phenocrysts. Seep. 17. It is often used also to describe the groundmasses of truly porphyritic rocks, that are micro-crystalline, but clearly not glassy. In this sense we have felsite- porphyry, felso-liparite, felso-dacite, etc. Felsophyre, a contraction for felsite-porphyry. Felspar, the current spelling of feldspar among the English. It is based on an old typographical error in Kirwan's Mineralogy, I., 317, 1794, now, however, firmly established in general usage. Ferrite, microscopic crystals of iron oxide. Ferrolite, Wadsworth's name for rocks composed of iron ores. Rept. State Geol. Mich., 1891-92, p. 92. Fibrolite, synonym of sillimanite, and sometimes used as a prefix to rock names. Fiorite, siliceous sinter, named from Mt. Santa Fiora, in Tuscany. GLOSSARY. 191 Fim, Swiss name for the granular, loose or consolidated snow of the high altitudes before it forms glacial ice below. Flaser-structure, a structure developed in granitoid rocks and espe- cially in gabbros by dynamic metamorphism. Small lenses of granular texture are set in a scaly aggregate that fills the interstices between them. It appears to have been caused by shearing that has crushed some por- tions more than others, and that has developed a kind of rude flow- structure. Flint, a compact and crypto-crystalline aggregate of chalcedonic and opaline silica. Chert and hornstone are synonyms. See pp. 108, 109. Float, a term much used among Western miners for loose, surface deposits, which are usually somewhere near their parent ledges. Flow-Structure, a structure due to the alignment of the minerals or inclusions of an igneous rock so as to suggest the swirling curves, eddies and wavy motions of a flowing stream. It is caused by the chilling of a flowing, lava current. Fluxion-structure is synonymous. Foliation, the banding or lamination of metamorphic rocks as dis- tinguished from the stratification of sediments. Forellenstein, a variety of olivine-gabbro, consisting of plagioclase, olivine and more or less pyroxene. The dark silicates are so arranged in the lighter feldspar as to suggest the markings of a trout. (German, Forelle.) Formation, as defined and used by the U. S. Geological Survey, is a large and persistent stratum of some one kind of rock. It is also loosely employed for any local and more or less related group of rocks. In Dana's Geology it is applied to the groups of related strata that were formed in a geological period. Fourchite, a name proposed by J. Francis Williams for those basic dike rocks that consist essentially of augite in a glassy groundmass, /. Jahresber. iiber die Fortschrit. Chemie u. Mineralogie, 1844, 262.) It was soon shown by the micro- scope to be an aggregate. Krassyk, a local name for a decomposed ferruginous schist in the Beresov gold-mining district of the Urals. Archiv fur practische Geologic, II., 537. GLOSSARY. 201 Kugel, the German word for ball or sphere and often prefixed to those igneous rocks that show a spheroidal development, such as cor- site, orbicular granite, etc. Kulaite, a name derived from the Kula basin in Lydia, Asia Minor, proposed by H. S. Washington, for those rare basalts (there abundant) in which hornblende surpasses augite in amount. " The Volcanoes of the Kula Basin." Privately printed. New York, 1894, Amer. Jour. Sci., Feb., 1894, p. 115. Kullaite, a name derived from the Swedish locality Kullen, and ap- plied by A. Hennig to a dike-rock which is regarded as an intermediate type between the diabases and the granites. In a feldspathic ground- mass of ophitic (diabasic ?) texture, are red phenocrysts of plagioclase and microcline. The groundmass has rods of oligoclase-andesine with augite, orthoclase and titaniferous magnetite. See Review in Neues Jahrbuch, 1901, II., 59. Kuskite, a name derived from the Kuskokwim river, Alaska, and ap- plied by J. E. Spurr to certain porphyritic dikes, which cut Cretaceous shales, and which have phenocrysts of quartz, scapolite, and probably basic plagioclase (the last now represented by alteration products), in a groundmass of quartz, orthoclase and muscovite. Compare yentnite, Amer. Jour. Sci., Oct., 1900, 311 and 315. Kyschtymite, a name derived from the Kyschtym mining district of the Urals, and given by J. Morozewicz to a rock consisting chiefly of anorthite and corundum, with which are associated biotite, spinel, zir- con, apatite and, as secondary minerals, muscovite, chlorite, kaolin and chromite, Tsch. Mitth., XVIII., 212, 1898. Labradorite, the name of the feldspar is prefixed to many rock names. Labradorite rock was formerly much used for anorthosite, which see. Laccolite, a name based on the Greek word for cistern and suggested by G. K. Gilbert for those intrusions of igneous rock that spread out laterally between sedimentary beds like a huge lens, and that never reach the surface unless exposed by erosion. See p. 15 ; also Geology of the Henry Mountains, Utah, p. 19. Lamprophyre, a general term, now used in a somewhat wider sense than as originally proposed by Giimbel, who suggested it. Rosenbusch, in the Massigen Gesteine, gave it its present significance. Lamprophyres are dike rocks of porphyritic texture, whose predominant phenocrysts are the dark silicates, augite, hornblende or biotite. They are practi- cally basic dikes. The word means a shining rock, and was first applied 202 A HAND BOOK OF ROCKS. in 1874 to small dikes in the Fichtelgebirge that were rich in biotite. In a somewhat modified sense it has recently been employed by L. V. Pirsson, as single term for the basic ''complementary rocks" (see Complementary Rocks), and as the antithesis of oxyphyre, which applies to the acidic complementary rocks of an eruptive area. Lapilli, volcanic dust and small ejectments, the results of explosive eruptions. Lassenite, Wadsworth's name for unaltered, glassy trachytes. Rept. State Geol. Mich., 1891-92, p. 97. The name is derived from Las- sen's Peak, Cal. Laterite, a name derived from the Latin word for brick earth, and applied many years ago to the red, residual soils or surface products, that have originated in situ from the atmospheric weathering of rocks. They are especially characteristic of the tropics. Though first applied to altered, basaltic rocks in India, laterite has had in later years a general application without regard to the character of the original rock. Compare saprolite. See pp. 144, 145. Latite, a name suggested by F. L. Ransome, for the rocks that are intermediate between the trachytes and andesites. Latite is meant to be a broad family name and to include the effusive representatives of the plutonic monzonites. Plagioclase and orthoclase are both present ; augite, hornblende, biotite and olivine vary in relative amounts. The textures may be glassy, felsitic or porphyritic. The name is derived from the Italian province of Latium but was suggested by studies on Table Mtn., Tuolumne Co., Calif., Bull. 89, U. S. Geol. Survey. Com- pare trachydolerite, ciminite, vulsinite, monzonite. Laurdalite, a name given by Brogger to a coarsely crystalline variety of nephelite-syenite, that is abnormal in having for its feldspar natron- orthoclase, rarely natron -microcline, instead of the normal potash ortho- clase. The dark silicates are biotite, diallage and olivine. Zeitsch. f. Kryst., XVI., 28, 1890. Laurvikite, a name applied by Brogger to a Norwegian variety of augite-syenite that contains natron -orthoclase as its chief feldspar and most abundant mineral. The other components are rare plagioclase, pyroxene, biotite, barkevicite or arfvedsonite, olivine and magnetite. Besides microscopic accessories, nephelite is occasionally met. Zeitsch. f. Kryst., XVI., 29, 1890. Compare pulaskite. Lava, a general name for the molten outpourings of volcanoes. Laxite, Wadsworth's name for the fragmental or mechanical rocks, especially when unconsolidated. Rept. of State Geol. of Mich., 1891- 92, p. 98. GLOSSARY. 203 Lentils, a short name for lenticular beds in a stratified series. Leopardite, a siliceous rock from North Carolina, spotted with stains of manganese oxide. It is usually considered to be a quartz-porphyry. Leopard rock, a local name in Canada, applied to pegmatitic rocks which are associated with the apatite veins of Ontario and Quebec. See C. H. Gordon, Bulletin Geolog. Society of America, VII. , 122. Leptinite or Leptynite, the French synonym of granulite as used among the Germans. See granulites. Leptomorphic, a term suggested by Giimbel for crystallized substances which lack definite crystalline borders, as the nephelite in many ground- masses. Fichtelgebirge, 1879, 240. Lestiwarite, a name proposed by Rosenbusch for the aplitic dike- rocks that accompany nephelite-syenites in Norway and Finland. They are chiefly or almost entirely alkali feldspar, with very subordinate pyroxene or amphibole. They had been previously called syenite- aplites by W. C. Brogger. Lestiwarite is derived from the Finnish locality Lestiware. Massige Gesteine, II. , 464. Das Ganggefolge des Laurdalits, 207. Leucite, the name of the mineral is prefixed to names of many rocks which contain it, as, leucite-absarokite, leucite-syenite, etc. Leucite-basalt, basaltic rocks with olivine, in which leucite replaces plagioclase. See p. 66. Leucite-basanite, basaltic rocks that contain both leucite and pla- gioclase. As contrasted with leucite-tephrites, they contain olivine. See p. 66. Leucitite, basaltic rocks without olivine in which leucite replaces plagioclase. Compare leucite-basalt. Leucitophyre, a name formerly used as a general one for the leucite rocks, but now by common consent restricted to those phonolites that contain both leucite and nephelite. Leucite-tephrite, basaltic rocks without olivine, that contain both plagioclase and leucite. Compare leucite-basanite. Leucocratic, a descriptive term, suggested by W. C. Brogger for those eruptive rocks in which the light-colored minerals, /. P and 2? oo, so that they resemble a rhombohedron. The orthoclase is rich in soda. Rhyolite, volcanic rocks, of porphyritic or felsitic texture, whose phenocrysts are prevailingly orthoclase and quartz, less abundantly bio- tite, hornblende or pyroxene, and whose groundmass is crystalline, glassy, or both. The name is from the Greek to flow, and refers to the frequent flow structure. Rhyolite is current in America, whereas liparite and quartz-trachyte are more used abroad. The name was given in 1860 by v. Richthofen. (Jahrb. d. k. k. Reichsanst, XL, 153, 1860.) Rill-marks, small depressions in sandstones, produced by the eddying of a retreating wave on a seabeach under the lee of some small obstruc- tion, such as a shell or pebble. Ripple-marks, corrugations in sandstones produced by the agitation of waves or winds when the rock was being deposited. Rockallite, a name proposed by J. W. Judd for a rock from Rockall Island, a small reef in the North Atlantic, 240 miles west of Ireland. Rockallite is a granitoid rock, consisting of quartz, albite and aegirite, in proportions respectively of 38 : 23 : 39, in the specimen investigated. Trans. Royal Irish Academy, XXXI. , Part III., 39 ; Amer. Jour. Sci., March, 1899, 241. Rock-flour, a general name for very finely pulverized rocks or min- erals which lack kaolin and, therefore, the plasticity of clay, and which are much finer than sand. Rock-flour, which is largely pulverized quartz, may be separated from most clays. Saccharoidal, a term applied to sandstones whose texture resembles that of old-fashioned loaves of sugar. Sagvandite, a curious rock from near Lake Sagvand, Norway, that is mainly bronzite and magnesite. A little colorless mica, and more or less chromite and pyrite are also present. The name was given by Petterson. Neues Jahrb., 1883, II., 247. Sahlite, a variety of pyroxene, sometimes prefixed to rock names. Salband, a term current among miners for the parts of a vein or dike next to the country rock. Sand, incoherent fragment of minerals or rocks of moderate size, say one-quarter of an inch (6 mm.) and less in diameter. Quartz is much the commonest mineral present. See p. 89. Sandstone, consolidated sands. See p. 89. GLOSSARY. 22 $ Sanidinite, a name applied especially to certain trachytic bombs that occur in tuffs in the extinct volcanic district of the Laacher See, Ger- many. Recently it has been suggested by Weed and Pirsson for the extreme case of feldspathic syenites, in which all other minerals except orthoclase practically fail. They establish a series as follows : All orthoclase, no augite Sanidinite. Orthoclase exceeds augite Augite-syenite. Orthoclase equals augite Yogoite. Augite exceeds orthoclase Shonkinite. All augite, no orthoclase Pyroxenites of various types. Amer. Jour. Sci., Dec., 1895, p. 479. Subsequently yogoite was withdrawn in favor of monzonite, which has priority. Idem, May, '896, 357, 358. Santorinite, a name proposed by H. S. Washington for those excep- tional andesitic or basaltic rocks, which, with a high percentage of silica (65-69), yet have basic plagioclases, of the labradorite-anorthite series. The name was suggested by the volcano Santorini. (Journal of Geology, V., May-June, 1897, 368.) See also Fouque, Santorini et ses Eruptions, Paris, 1879, and Etude des Feldspaths, 317-320. The prevailing bisilicate at Santorini is pyroxene. Sanukite, Weinschenk's name for a glassy phase of andesite that contains bronzite, augite, magnetite, and a few large plagioclases and garnets. The rock is related to the andesites as are the limburgites to the basalts. Neues Jahrb. Beilageband, VII., 148, 1891. Saussurite-gabbro, gabbro whose feldspar is altered to saussurite. See p. 131. Saxonite, Wadsworth's name for peridotites consisting of enstatite or bronzite and olivine. It is a synonym of harzburgite, but saxonite has priority. Lithological Studies, 1884, p. 85. Schalstein, an old name for a more or less metamorphosed diabasetuff. Schiller-fels, enstatite or bronzite peridotite with poicilitic pyrox- enes. Orthorhombic pyroxenes possess the poicilitic texture to a pecu- liar degree, and especially when more or less altered to bastite, and the term schiller, which expresses this, is especially applied to them. Schillerisation, Judd's name for the process of producing poicilitic texture by the development of inclusions and cavities along particular crystal planes. The cavities are largely produced by solution, some- what as are etch figures, and are afterwards filled by infiltration. Quart. Jour. Geol. Soc., 1885, 383; 1886, 82. Schist, thinly laminated, metamorphic rocks which split more or less readily along certain planes approximately parallel. Seep. 127. 15 226 A HAND BOOK OF ROCKS. Schlieren, a useful German term, largely adopted into English, for those smaller portions of many igneous rocks, which are strongly con- trasted with the general mass, but which shade insensibly into it. Thus portions of granite are met, much richer in biotite and hornblende than the normal rock, or much more coarsely crystalline. Pegmatite streaks occur and other differentiations of the original magma. Several differ- ent varieties may be made, for a discussion of which see Zirkel's Lehr- buch der Petrographie, I., 787, 1893. Schorl, an old name for tourmaline, still sometimes used in names of rocks. Scoria, coarse, cellular lava, usually of basic varieties. Scyelite, Judd's name for a rock, related to the peridotites, that oc- curs near Loch Skye, in Scotland. Its principal mineral is green horn- blende, presumably secondary after augite ; with it are bleached biotites and serpentine, supposed to be derived from olivine. See Quar. Jour. Geol. Soc., 1885, 401. Secondary, a term used both for rocks and minerals, that are derived from other rocks and minerals, such as sandstone, clay, or other sedi- ments ; chlorite from augite, etc. See the contrasted word primary. Sedimentary, rocks whose components have been deposited from suspension in water. See p. 84. Selagite, a name of Hauy's for a rock consisting of mica, dissemi- nated through an intimate mixture of amphibole and feldspar, but it has been since applied to so many different rocks as to be valueless. Selenolite, Wadsworth's name for rocks composed of gypsum or an- hydrite. Kept. State Geol. of Mich., 1891-92, p. 93. Septaria, literally little walls, a name applied to concretions, largely of argillaceous material, which are traversed by cracks. The cracks are filled as a rule with calcite or quartz, affording an intersecting network from which weathering may have removed the original, included, argil- laceous matter. Sericite-schist, mica-schist whose mica is sericite. See p. 129. Sericite is also used as a prefix to many names of metamorphic rocks containing the mineral. Serpentine, a metamorphic rock consisting chiefly of the mineral serpentine. See p. 140. Shastalite, Wadsworth's name for unaltered, glassy forms of ande- site. Rept. of Mich. State Geol., 1891-92, p. 97. Shonkinite, a name given by Weed and Pirsson to a rock from the Highwood Mountains, Mont., which they define as "a granular, plu- tonic rock consisting of essential augite and orthoclase, and thereby GLOSSARY. 22? related to the syenite family. It may be with or without olivine, and accessory nepheline, sodalite, etc., may be present in small quantities." Bull. Geol. Soc. Amer., VI., 415, 1895. See Anal. 7, p. 42. Later they state that augite should exceed orthoclase. Amer. Jour. Sci., Dec., 1895, p. 479. Shoshonite, a general name proposed by Iddings for a group of igne- ous rocks in the eastern portion of the Yellowstone Park. They are porphyritic in texture, with phenocrysts of labradorite, augite and oli- vine, in a groundmass that is glassy or crystalline ; in the latter case orthoclase and leucite, alone or together, are developed. Chemically they range: SiO 2 , 50-56; A1,O 8 , 17-1 9. 7; CaO, 8-4.3; MgO, 4.4- 2.5 ; Na 2 O, 3-3.9 ; K 2 O, 3.4-4-4- The rocks are to be considered in connection with absarokite and banakite. Jour, of Geol., III., 937. Siderolite, as used by Fletcher and generally in English, is a name for meteorites that are partly metallic iron and partly silicates. As used by others it is applied to more purely metallic ones. Sideromelane, von Waltershausen's name for a basaltic glass from the palagonite tuffs of Sicily. Vulk. Gest. v. Sicilien und Island, 202, Silicalite, Wadsworth's name for rocks composed of silica, such as diatomaceous earth, tripoli, quartz, lydite, jasper, etc. Kept. State Geol. Mich., 1891-92, p. 92. Silicification, the entire or partial replacement of rocks and fossils with silica, either as quartz, chalcedony or opal. Sillite, Giimbel's name for a rock from Sillberg, in the Bavarian Alps, variously referred by others to gabbro, diabase, mica-syenite and mica- diorite. Beschr. der bay. Alpen, 184, 1861. Sills, an English name for an intruded sheet of igneous rock. Silt, a general name for the muddy deposit of fine sediment in bays or harbors, and one much employed in connection with engineering enterprises. Sinaite, an alliterative substitute for syenite proposed by Rozieres be- cause on Mt. Sinai true, quartzless syenites occur, whereas at Syene the rock is a hornblende-granite. Slickensides, polished surfaces along faults, or fractures produced by the rubbing of the walls upon each other during movement. Soapstone, metamorphic rocks, consisting chiefly of talc. See p. 143. Soda-granite, granites especially rich in soda, or whose soda exceeds the potash. Compare analyses, p. 33. See natron-granite. Sodalite-syenites, syenites rich in sodalite ; close relatives of nephe- lite syenites. See anal. 5, p. 42. Sodalite-trachytes also occur. 228 A HAND BOOK OF ROCKS. Soggendalite, a name proposed by C. F. Kolderup for a variety of diabase that is especially rich in pyroxene, and that is intermediate be- tween true diabases and pyroxenites. The type rock forms a dike near Soggendal, Norway. Bergens Museums Aarbog, 1896, 159. Soil, surface earth mixed with the results of the decay of vegetable or animal matter, so as usually to have a dark color. Sblvsbergite, Brogger's name for quartzless or quartz-poor grorudites ; that is, medium to finely crystalline, dike rocks, with prevailing alkali - feldspar (mostly albite and microcline) with segirine, or in the basic varieties with hornblende (kataforite), sometimes also with a peculiar mica. In the most basic members quartz entirely fails and nephelite appears. (Die Eruptivgesteine des Kristianiagebietes, I., 67.) Sondalite, a name proposed by Stache and von John for a meta- morphic rock consisting of cordierite, quartz, garnet, tourmaline and cyanite. Jahrb. d. k. k. g. Reichsanst, 1877, 194. Sordawalite, an old name for the glassy salbands of small diabase dikes. The sordawalite was regarded as a mineral. It is derived from Sordawalar, a locality in Finland. Compare wichtisite. Spessartite, a name proposed by Rosenbuch, for those dike rocks, which, whether porphyritic or granitoid in texture, consist of prevailing plagioclase, hornblende and diopside. Orthoclase and olivine oc- casionally appear. Massige Gesteine, 532, 1896. The name is de- rived from Spessart, a group of mountains in the extreme northwest of Bavaria, but as it has already been used for a variety of garnet, it is a very unfortunate selection. Spheroidal, a descriptive term applied to igneous rocks that break up on cooling into spheroidal masses analogous to basaltic columns ; also used as a synonym of orbicular as applied to certain granites. Spherulites, rounded aggregates or rosettes, large or small, of acicular crystals that radiate from a center. They are chiefly met in the microscopic study of acidic, volcanic rocks and commonly consist of feldspars and quartz. When of one mineral they are called by Rosenbusch sphere-crys- tals. Theymay reach large size, though mostly microscopic. See p. 26. Spilite, an early French name for dense, amygdaloidal varieties of diabase. Spilosite, a spotted, contact rock produced from shales and slates by intrusions of diabase. It corresponds to the hornfels of granite con- tacts. Zincken in Karsten und v. Dechen's Archiv, 1854, 584. Stalactite, depending, columnar deposits, generally of calcite, formed on the roof of a cavity by the drip of mineral solutions. Compare stalagmite. GLOSSARY. 229 Stalagmite, uprising, columnar deposits, generally of calcite, formed on the floor of a cavity by the drip of mineral solutions from the roof. Compare stalactite. Steatite, soapstone, talc rocks. Structure, used generally in America for the larger physical features of rocks, as against texture, which is applied to the smaller ones. See p. 1 6. Many, however, employ them interchangeably. Compare also petrical and lithical. Stylolite, small, columnar developments in limestones or other cal- careous rocks that run across the stratification. They appear to have been caused by some unequal distribution of pressure in consolidation, or by a capping fossil, as against the surrounding rock. Subsoil, the layer of more or less decomposed and loose fragments of country rock that lies between the soil and the bed rock in regions not covered by transported soils. Suldenite, a name given by Stache and von John to gray, acidic, andesitic porphyrites in the Eastern Alps. They range from 54-62 SiO 2 and have, in the prevailing gray groundmass, phenocrysts of horn- blende, plagioclase, a little orthoclase and accessory augite, biotite and quartz. Compare ortlerite. Surficial, a general name, lately introduced by the U. S. Geological Survey, for the untransported surface, alteration products of igneous rocks. Sussexite, a special name suggested by Brogger for the eleolite por- phyry, originally described by Kemp, from Beemerville, Sussex Co., N. J. Die Eruptivgesteine des Kristianiagebietes, 1895. The name was, however, applied years ago to a hydrated borate of manganese and magnesia, from Franklin Furnace, N. J. Syenite, granitoid rocks consisting in typical instances of orthoclase and hornblende. In mica-syenites, biotite replaces hornblende. In augite-syenites augite does the same. For etymology and history see p. 42. Compare also laurvikite, monzonite, nordmarkite, pulaskite, sanidinite, shonkinite, yogoite. Syssiderite, Daubree's name for those meteorites which consist of silicates cemented together by metallic iron. T Tachylyte, Breithaupt's name for a basaltic glass. It was originally regarded as a mineral and was named from two Greek words suggested by its quick and easy fusibility. See analyses 15, p. 25, and descrip- 23 o A HAND BOOK OF ROCKS. tion, p. 26. Kastner's Archiv fur die gesammte Naturlehre, VII., 112, 1826. Compare hyalomelane. >- Taconyte, a name proposed by H. V. Winchell for the cherty or jas- pery, but at times calcareous or more or less quartzitic rock, that encloses the soft hematites of the Mesabi Range, Minn. Taconytes are regarded as in large part altered greensands by J. E. Spurr. The term is current in the Mesabi iron range. XX. Ann. Rep. Minn. Geol. Survey, 124. The name is derived from Taconic, E. Emmons' rejected geological system. Taimyrite, an acidic trachyte rich in soda, and regarded as the effusive equivalent of nordmarkite. Chrustschoff ; Melanges geol. et paleontol. Acad. Sci. St. Petersburg, I., 153, 1892. The original locality is in Siberia. Talc-schist, schistose rocks consisting chiefly of talc and quartz. See p. 132. Talc is also prefixed to several other rock names. Taurite, a name given by A. Lagorio, to a variety of rhyolite, with granophyric or spherulitic texture, rich in soda, and containing segirite. Guide to the Excursions, 7th International Geol. Congress, XXXIII. , 27, 1897, St. Petersburg. Tawite, a name given by W. Ramsey to a very peculiar rock of both granitoid and porphyritic texture and consisting of pyroxene and sodalite. It occurs in the nephelite-syenite area of Kola in Finland and is derived from Tawajok, a local geographical term. Fennia, XI., 2, 1894. Taxite, Loewinson-Lessing's name for lavas, that, on crystallizing, have broken up into contrasted aggregates of minerals so as to present an apparent clastic texture either banded, /'. Amer. Jour. Sci., May, 1899, 399. Verite, a name derived from the Spanish locality Vera, near Cabo de Gata, and given by Osann to a post -Pliocene glassy rock, with pheno- crysts of biotite and microscopic crystals of olivine and augite and some- times plagioclase, all of which seldom form half the mass of the rock. It is a glassy variety of the mica-andesites with exceptional olivine. Z. d. d. g. G., XLL, 311, 1889. Vintlite, a quartz-porphyrite occurring in dikes near Unter-Vintl, in the Tyrol. Compare toellite from the same region. Pichler, Neues Jahrb., 1871, 262. Viridite, a microscopic name suggested by Vogelsang and formerly used for the small, green, chloride scales often met in thin sections. As their true nature has now been determined, they are generally called chlorite. Vitro, a prefix meaning glassy and used before many rock names, as vitrophyre, in order to indicate a glassy texture. Vitrophyre, Vogelsang's name for quartz -porphyries and porphyries with glassy groundmass. Vogesite, Rosenbusch' s name for syenitic dikes, in which the dark hornblendes or augites are in excess over the light colored feldspars. Mass. Gest., 1887, 319. The name is derived from Vogesen, the Ger- man form of Vosges. Volcanic, surface flows of lava as distinguished from plutonic rocks, see p. i 6. Volcanite, a name proposed by W. H. Hobbs, for an anorthoclase- augite lava with the chemical composition of dacite. Bull. Geol. Soc. Amer., V., 598. The name was suggested by the original occurrence GLOSSARY. 235 on the island of Volcano, one of the Lipari group, where the rock is met as cellular bombs. Volhynite, a porphyrite containing plagioclase, hornblende and bio- te phenocrysts in a holocrystalline groundmass of feldspar and chlorite. The name was given by Ossovsky, and it is based on the original occur- rence in Volhynia. See Chrustschoff, Bull. Soc. Min. France i88c 441. Vulsinite, a name suggested by H. S. Washington for a group of rocks intermediate between trachytes and andesites. They contain much labradorite in addition to the usual minerals of trachyte. The name is derived from the Vulsinii, an ancient Etruscan tribe inhabiting the region where the type specimens were obtained. Journal of Geology, IV., 547. Compare latite and trachydolerite. w Wacke, an old name for the surficial, clayey products of the altera- tion of basalt. The syllables are still current in graywacke. Wash, a miner's term in the West for loose, surface deposits of sand, gravel, boulders, etc. Websterite, a name proposed by G. H. Williams for the pyroxenites near Webster, N. C., that consist of diopside and bronzite, with the latter porphyritically developed. Amer. GeoL, VI., 35, 1890. The name websterite had been previously used by A. Brogniart in 1822 for aluminite. Hauy's Mineralogie, II., 125. Wehrlite, a name originally suggested by von Kobell for what was supposed to be a simple mineral, but which proved to be a peridotite consisting of olivine and diallage. Weiselbergite, Rosenbusch's name for those augite-porphyrites whose groundmass consists of a second and sometimes third generation of plagioclase rods and augites, arranged in flow lines in a glassy basis. Mass. Gest, 501, 1887. Wadsworth uses the name for an altered andesite glass. Kept, of State Geol. of Mich., 1891-92, p. 97. Whinstone, a Scotch name for basaltic rocks. Wichtisite, a glassy phase of diabase, named from a Finland locality, Wichtis. Compare sordavalite. Wyomingite, a name suggested by Whitman Cross, for the variety of rock from the Leucite Hills, Wyoming, which consists almost entirely of leucite and phlogopite. Small, acicular crystals of diopside are very subordinate, and apatite is also present. Amer. Jour. Sci., Aug., 1897, 120. This is the rock described by Zirkel in 1876 and was the first 236 A HAND BOOK OF ROCKS. known occurrence of leucite in America. Fortieth Parallel Survey, VI., 259. X Xenogenites, Posepny's term for mineral deposits of later origin than the wall rock. The name means foreigners, and refers to their later introduction. Compare idiogenites. Trans. Amer. Inst. Min. Eng., XXIII., 205, 1893. Xenolith, a term proposed by W. J. Sollas, for included masses of rock, caught up in an igneous intrusion. The term means foreign rock. Xenomorphic, Rohrbach's textural name for those minerals in an igneous rock, whose boundaries are determined by their neighbors. Its antithesis isautomorphic, which see. Xenomorphic is synonymous with allotriomorphic, over which it has priority. Tsch. Mitt., 1886, 18. Y Yentnite, a name derived from the Yentna River, Alaska, and sug- gested by J. E. Spurr for certain granitoid rocks, consisting of oligoclase, scapolite and biotite, with a few zircons. The scapolite is believed to be an original mineral. Amer. Jour. Sci., Oct., 1900, 310. Yogoite, a name suggested by Weed and Pirsson from Yogo peak, one of the Little Belt Mountains, Mont., for a syenitic rock composed of orthoclase and augite in about equal amounts. See also sanidinite and shonkinite. Amer. Jour. Sci., Dec., 1895, 473-479. z, Zircon-syenite, a name originally given by Hausmann to certain Nor- wegian nephelite-syenites which were rich in zircons. Later it was practically used as a synonym of nephelite-syenite, but is now obsolete. Zirkelite, a name proposed by Wadsworth in 1887 to designate altered, basaltic glasses, in distinction from their unaltered or tachylitic state. Geol. Surv. Minn. Bull., 2, 1887, p. 30. Zobtenite, Roth's name for metamorphic rocks with the composition of gabbros, t. e. , rocks not certainly igneous. The name is derived from the Zobtenberg, a Silesian mountain. Sitz. Berl. Akad., 1887, 611. Zonal-structure, a term especially used in microscopic work to de- scribe those minerals whose cross-sections show their successive, con- centric layers of growth. Zwitter, a Saxon miner's term for a variety of greisen. Only of sig- nificance in connection with tin ores. APPENDIX. Rock-names given 1904-1908, or if employed earlier, overlooked in the preceding Glossary. * Albitite, a name applied by H. W. Turner to granitoid rocks, con- sisting essentially of albite. The original occurrence is a series of dikes cutting serpentine, near Meadow valley, Plumas Co., Calif., but under the name soda-syenite, similar rocks have been described from various places on the Pacific coast. (Amer. Geologist, June, 1896, 378-380.) Allochetite, a name derived from the Allochet valley on the eastern side of the Monzoni region of the Austrian Tyrol and applied by J. A. Ippen to a dike rock related to the tinguaites. The allochetite is por- phyriticin texture and contains phenocrysts of plagi oclase (labradorite), orthoclase, titanaugite, nephelite and magnetite, in a groundmass of augite, magnetite, hornblende, nephelite, orthoclase and occasional biotite. (Verh. der k. k. geol. Reichsanstalt, Vienna, 1903, 132.) Ariegite, a name derived from the French departement of Ariege, on the border with Spain, and suggested by A. Lacroix to especially designate a variety of pyroxenite, abnormally high in alumina (A1 2 O 3 17-20 per cent.). The constituent minerals are diallage, bronzite, brown hornblende, green spinel, occasionally olivine, garnet, biotite, and andesine. (Cited in Rosenbusch, Massige Gesteine, 481, 1907.) Bekinkinite, a name derived from the Bekinka mountain in Madagas- car and coined by H. Rosenbusch (Massige Gesteine, 4th ed., p. 441) for a variety of ijolite originally described by A. Lacroix. The rock consists of about 75 per cent, titanaugite, with nephelite, as the other chief constituent. There is some anorthoclase, and accessory olivine, apatite and leucoxene. Bekinkinite is believed to correspond to a deep- seated nephelite -basalt. Blairmorite, an analcite-trachyte named by Cyril Knight after Blair- more, a town in southwestern Alberta, Can., near the Crows Nest Pass coal fields. Although only found as yet in tuffs, the rock is recognizable as a new species. Phenocrysts of orthoclase and analcite (at first con- sidered leucite) are set in a groundmass of predominant orthoclase rods, some plagioclase, analcite and titanite. (Cyril Knight, Canadian Record of Science, IX., 275, 1905.) Blavierite, a peculiar contact rock occurring at several places in the 237 238 A HAND BOOK OF ROCKS. ancient massifs of Mayenne and of the Pyrenees, in France. It results from the action of intrusive rnicrogranites, upon sericite schists. While preserving the schistose structure, it has in addition to the fine micaceous components of the schist, dihexahedral quartzes with orthoclase and oligoclase, apparently referable to the intrusive. (L. Bergeron, Bull. Soc. geol. de France, 1888 (3), XVII., 58.) Brotocrystals, etc. A. C. Lane has suggested the following five varieties of phenocrysts : ( i ) Brotocrystals, those phenocrysts whose corroded or embayed outlines prove that they were formed at a period antedating the eruptive stage. The name is from the Greek and means "eaten" or ''gnawed" crystal. (2) Rhyocrystal, those phenocrysts which have sharp, crystallographic outlines, and which are arranged in flow-lines, so that they are clearly products of the effusive period. The name, suggested by F. E. Wright, means a "flow- crystal." (3) Eocrystals, those phenocrysts which are developed under the conditions prevailing in the magma after it has come to rest. They increase uni- formly in size from the border or most rapidly chilled portion, to the center or most slowly cooled portion. The name means ' ' early crystal. ' ' (4) Oriocrystals, those phenocrysts whose conditions of formation lie midway between the great heat of the original intrusive and the relative coldness of the walls. As the former cools and the latter heats up, con- ditions are established at the border which very slowly change and which permit the growth of large individuals. The word means "border crystal." (5) Metacrystals, relatively large crystals in metamor- phosed sedimentary and igneous rocks, such as staurolite, garnet, anda- lusite, etc. The word means metamorphic crystal. (Bull. Geol. Soc. Amer., XIV., 386-388, 1902.) Cascadite, an adaptation by Rosenbusch into the forms of the custom- ary nomenclature of Pirsson's term cascadose, of the quantitative sys- tem. The name is derived from Cascade Co., Mont., where the rock, which is a variety of minette, occurs. (Bull. U. S. Geol. Survey, 237, p. 149. Rosenbusch, Mikros. Phys., 4th ed., II., 698.) Chibinite, a name derived from the Russian designation of the Finnish locality, better known among petrographers as Umptek, and applied by W. Ramsay to a variety of nephelite-syenite. It is a coarsely crystalline aggregate of microperthite, nephelite, aegirite, titanite, eudi- alyte, lamprophyllite and rarer minerals. (Rosenbusch, Mikros. Physi- ographic, 4th ed., II., 231.) Dahamite, a dike-rock occurring at Dahamis, on the island of Sokotra, in the Indian Ocean, east of Cape Guardafui, Africa. The texture is porphyritic. The phenocrysts are albite, a little orthoclase and some dark silicate, too badly altered for recognition. The brown APPENDIX. 239 groundmass consists of predominant albite, some quartz and unusual amounts of riebeckite. (A. Pelikan, Denkschr. math, -naturwis. Klasse, Ak. d. Wiss., Vienna, LXXL, 77, 1907.) Ehrwaldite, a basic dike-rock from Ehrwald, consisting of pheno- crysts of altered olivine, biotite, barkevicite and titanaugite, in a groundmass of augite-microlites and altered glass. (Rosenbusch, Mikr. Phys., II., 701. The name was given by Pichler. ) Ekerite, a name given by W. C. Brogger to a granite, rich in alkalies and having arfvedsonite as its prevailing dark silicate. ( Nyt. Mag. f. Naturvid., XLIV., 114, 1906.) Additional details are given by H. Rosenbusch. (Mikros. Phys., 4th ed., II., 525.) Eocrystal, see brotocrystal. Fergusite, a name derived from Fergus Co., Montana, and coined by L. V. Pirsson to describe a granitoid intrusive rock consisting of dom- inant leucite, now represented by pseudoleucite and subordinate augite. The accessories are apatite, iron ores, biotite, and sporadic olivine. Fergusite is the deep-seated representative of the leucitites. The typical locality is the Arnoux stock of the Highwood mountains. (Bulletin 237, U. S. Geol. Survey, 89, 1905.) Garganite, a name applied by Viola and de Stefani, to a dike-rock of composition varying from an olivine-kersantite, rich in biotite and hornblende, which forms the borders, to an augite-amphibole vogesite in the centre. (Cited by Rosenbusch, Mikros. Phys., 4th ed., II., 681, from Boll. Roy. Com. geol. d'ltalia, 1893, 129.) Giumarrite, a dike-rock, described as a hornblende bearing augitite, and occurring near Giumarra, Sicily. (C. Viola, Boll. Roy. Com. geol. d'ltalia, 1901.) Garevaite, a porphyritic and very basic dike-rock, from the northern Urals. Phenocrysts of diopside are found in a ground mass, chiefly olivine, magnetite and chromite. Minor components are pyroxene and labradorite. (L. Duparc and F. Pearce, Comptes Rendus, CXXXIX., 154, 1894.) Gladkaite, a dike -rock in the dunite of the Gladkaia Sopka, northern Urals. It has a finely crystalline texture and consists of acidic plagio- clase, quartz, hornblende, biotite, apatite, magnetite and secondary epidote and muscovite. (L. Duparc and F. Pearce, Comptes Rendus, CXL., 1614, 1905. See also Nature, June 22, 1905, p. 192.) Heptorite, a variety of monchiquite, containing in a colorless iso- tropic groundmass, assumed to be a glass, basaltic augite, acicular horn- blende and hauynite. It occurs as a finely crystalline, narrow dike on the contact of trachyte and graywackes in the Siebengebirge or Seven Mountains of the Rhine valley. The name is given by K. Busz and is 240 A HAND BOOK OF ROCKS. based on the Greek equivalent of Siebengebirge. (Neues Jahrbuch, 1904, II., 86.) Holyokeite, a name derived from Mt. Holyoke, Mass, and suggested by B. K. Emerson for a purely feldspathic phase of the Triassic diabase, found in fragments in a volcanic breccia. The rock is 70 per cent, albite, with calcite (16.42), orthoclase, (9.41), and ilmenite (1.63), as the principal remaining constituents. (Journal of Geology, X., 508, 1902.) Imandrite, a name suggested by W. Ramsay for a contact rock, be- lieved to have been produced from graywacke by the neighboring nephelite-syenite of Umptek. The original feldspars are now largely silicified. (Cited by Rosenbusch, Mikr. Phys., 4th ed., II., 251.) Katzenbuckelite, a name suggested by A. Osann for the famous nephelite-porphyry or biotite-tinguaite porphyry of Katzenbuckel, Baden. Phenocrysts of nephelite, biotite, olivine, noselite and magnetite are set in a coarser or finer groundmass of nephelite, biotite and sometimes aegirite and amphibole. (Rosenbusch, Mikros. Phys., 4th ed., II. , 632.) The rock is almost the same thing as the nephelite-porphyry or sussexite (which see) of Beemerville, N. J. Kodurite, a name derived from the Kodur manganese mine near Vizagapatam, in the northeastern portion of the Madras Presidency, India. It was given by L. L. Fremor to a rock consisting of potash feldspar, manganese garnet, and apatite. It is usually of granitoid tex- ture, with medium coarseness of grain, but it may be pegmatitic. (Geol. Survey of India, XXXV.) Koellite, a name suggested by W. C. Brogger for a basic dike-rock, consisting of olivine, lepidomelane, barkevicite, apatite, magnetite, anorthoclase and nephelite. (H. Rosenbusch, Mikros. Phys., 4th ed., II., 705.) Krageroite, a gabbroic rock, consisting of plagioclase and rutile and occurring at Kragero, Norway. (H. Rosenbusch, Mikr. Phys., 4th ed., II., 354.) Mangerite, a name based upon Manger, a Norwegian locality, by C. F. Kolderup and applied to granitoid rocks consisting essentially of microperthite and augite. By dynamic metamorphism the augite may pass into hornblende and biotite. Gneissoid structures are also induced. Quartz-mangerites represent acid facies. The rocks are associates of the anorthosites of Norway. (Die Labradorfelsen des westlichen Nor- wegens, Bergens Museums Aarbog, 1903, 102.) The same rocks are abundant in the Adirondacks where they are commonly called syenites. Marscoite, an intermediate contact rock, produced by the action of granite during deep-seated stages, upon included fragments of gabbro. APPENDIX. 241 Various new minerals result and old ones have new physical properties. (A. Harker, Tertiary Igneous Rocks of Skye, Mem. Geol. Survey, United Kingdom, 1904, 175, 192.) Metacrystal, see brotocrystal. Monmouthite, a basic, granitoid rock whose essential minerals are nephelite and hornblende, and whose more frequent accessories are plagioclase, cancrinite, and calcite, with sodalite, apatite, sphene, bio- tite, pyrite and iron ores in extremely small amount. The monmouthite appears as bands produced by magmatic differentiation in an albitic nephelite syenite (litchfieldite) along its contact with limestone. On analysis the chief constituents of monmouthite were the following: SiO, 39.74, A1 2 3 30.59, FeO 2.19, CaO 5.75, K a O 3.88, Na,O 13.25, CO, 2.17, H 2 O i.oo; all the rest 1.29, no one being over .60, total 99.86. The name was given by F. D. Adams and is derived from the township of Monmouth, Ontario. (Amer. Jour. Sci., April, 1904, 269.) Oriocrystal, see brotocrystal. Plagiaplite, an aplitic dike rock, consisting of acidic plagioclase, quartz and a little hornblende. (L. Duparc and S. Jerchoff, Arch. Sci. phys. et nat. Geneva, Feb., 1902, cited by Rosenbusch, Mikr. Phys., 4th ed., II., 590.) Plumasite, a dike-rock consisting essentially of oligoclase and corun- dum. It cuts peridotite near the Diadem mine, Plumas Co., California, and was first described and named by A. C. Lawson, Bull. Dept. Geol- ogy, Univ. of Calif., III., 219, 1903. Rhyocrystal, see brotocrystal. Rizzonite, a name derived from a locality, in the Monzoni region, and suggested by Doelter and Went for a limburgite dike. (Sitzungsber. Wiener Akad., Jan. 15, 1903.) '_', Routivarite, a name dervied from the famous Swedish locality Routi- vara, where the titaniferous iron ores, with abundant spinel, occur. It was applied by H. Sjogren to a phase of rock bordering on the ore and consisting of striated and unstriated feldspar, quartz and garnet. (Geol. For. i. Stockholm. Forh., XV., 62, 1893.) Schriesheimite, a dike-rock of the composition of amphibole-perido- tite and having a marked poikilitic texture. It was named from the Schriesheim valley near Heidelberg. (H. Rosenbusch, Mikros. Phys., 4 thed., II., 458-) Sommaite, a name derived from Monte Somma (Vesuvius) and sug- gested by A. Lacroix for blocks having the composition of a leucite-oli- vine-monzonite. (Cited by H. Rosenbusch, Mikros. Phys., 4th ed., II., 169.) Stubachite, a name suggested by E. Weinschenk for a more or less 242 A HAND BOOK OF ROCKS. serpentinized variety of dunite, having also diallage, tremolite, talc, magnetite, pyrite and breunnerite. (Cited by H. Rosenbusch, Mikros. Phys., 4th ed., II., 476.) Taspinite, a granitic rock enclosing an intrusive mass of more or less metamorphosed granite-porphyry in the Rofna valley of the Upper Rhine in Switzerland. (Cited by H. Rosenbusch, Mikros. Phys., 4th ed., II., 517.) Tilaite, a name derived from a locality in the northern Urals and suggested by L. Duparc and F. Pearce for a variety of olivine gabbro, exceptionally rich in diopside. (Cited by H. Rosenbusch, Mikros. Phys., 4 th ed., II., 353.) Tjosite, a name suggested by W. C. Brogger for a dike-rock, consist- ing of prevailing pyroxene, abundant apatite and magnetite, with grains of olivine, all set in a paste of anorthoclase rods. (H. Rosenbusch, Mikros. Phys., 4th Ed., II., 705.) Unakite, a peculiar granite consisting essentially of epidote, pink feldspar and quartz. The name is derived from the Unaka range of mountains along the border of North Carolina and Tennessee, and was first given by F. H. Bradley in 1874. (Amer. Jour. Sci., May, 1874, 519.) Other localities have since been noted. (See T. L. Watson, Idem, Sept., 1906, 248.) Valbellite, a name derived from the Valbella valley of Piedmont, and applied by R. W. Schafer to a dike of amphibole-peridotite, con- sisting of olivine, brown hornblende, bronzite, pyrrhotite, spinel and magnetite. (Cited by H. Rosenbusch, Mikros. Phys. , 4th ed. , II. , 462. ) Vaugnerite, a name derived from Vaugneray near Lyons, and applied by Fournet in 1836 to a dike-rock, which is now shown by Michel-Levy and Lacroix to be an amphibole -granite. They advise dropping the special name. (Bull. Soc. mineral de France, 1887, X., 27.) Windsorite, a name derived from Windsor, Vt., and applied by R. A. Daly to a dike rock "leucocratic, hypidiormorphic-granular, com- posed essentially of alkaline feldspar (microperthite and orthoclase), basic oligoclase, quartz and biotite, and characterized by high alkalies, (potash slightly in excess of soda), relatively low lime, contained es- sentially in the plagioclase) , low iron and low magnesia. " (Bull. 209 U. S. Geol. Survey, 48, 1903.) INDEX. NOTE. The index only concerns the main portion of the book and not the Glos- sary. Attention may be called to the latter as embracing many rocks not otherwise mentioned. Accessory minerals, 12 Acmite in trachyte, 41 Acmite-trachyte, 41 Actinolite, 9 Actinolitic slate, 109 ./Egirite, 9 Alabaster, 107 Alumina, molecular ratios, 158 Amphibole-andesite, 59 Amphiboles, 7 Amphibolites, 129-131 Analyses of rocks, 19 Andalusite, 116 Andesine, 6 Andesite-porphyries, 58 Andesites, 56 analyses, 56 description, 57 varieties, 58 synonyms and relatives, 58 alteration and metamorphism, 59 andesitic tuffs, 59 Anhydrite, n Anogene, 15 Anorthite, 5 Anorthoclase, 5 Anorthosites, 74 Anthophyllite, 8 Anthracite, 104-105 Apachite, 47 Apatite, 12 Aplite, 34 Apobsidian, 27 Aporhyolites, 31 Aqueous and Eolian rocks, Introduction, 84 Arendal, Norway, 101 Arfvedsonite, 8, 9 Argillaceous limestone, 100 Arkose, 90 Atmospheric weathering, denned, 112, 113 rocks produced by, 144-147 Augen in gneisses, 122 Augite, 8, 9 defined, 9 Augite-andesite, 59 -porphyrites, 66 243 Augitites, 67 Aureole, 115 Auvergne, trachytes, 41-42 Bar theory, 106 Baryta, molecular ratios, 163 Basalt-porphyries, 65 Basalts, 63 alteration and metamorphism, 68 analyses, 63 basalt tuffs, 69 description, 64 distribution, 69 synonyms and relatives, 66 varieties, 65 Basanite, 66 Batholiths, 15 Becker, G. F., cited, 136, 143, 145 Beemerville, N. J., 117 nephelite syenite, 48 Biella, syenite analysis, 42 Binary granite, 35 Biotite, 10 Bituminous coal, 104105 shales, 93 Blackband iron ore, 104, no Black Hills, S. D., phonolites, 48 trachyte, 41 Bosses, 15 Bostonite, 41 Brazil, weathered rocks of, 144 Breccias, 85, 116 eruptive breccias, 85 friction breccias, 85 .lus breccias, 85 Broegger, W. C., cited, 150 Bronzite, 9 Bytownite, 6 C Calcareous sandstones and marls, 95 analysis, 95 metamorphism of, 96 mineralogical composition, 95 occurrence, 96 varieties, 95 Calcarenites, 99 Calcareous shales, 99 244 INDEX. Calcilutites, IOO Calcirudites, 99 Calcite, II Calc-schist, 129 Camptonite, 62 Cancrinite, 51 Carbonaceous sediments, 104 Carbonates, rock- form ing, II Carbonic acid, molecular ratios, 161 Cement-rock, 101 Chalcedony, 101, IO2 Chemical analysis, calculations from, 19 recasting, 149 composition, diagrams of, 20 influence of, 17 elements important in rocks, 3 Chert, 101-103, 108-110 Cherty iron carbonates, 108-110 limestone, loo Chiastolite, 116 Chlorides, mineral, 12 Chlorine, molecular ratios, 162 Chlorite, n schist, I3I-I33 Christiania, Norway, 118 syenites, 44 Chromic oxide, molecular ratios, 163 Chromite, II Citric acid, 1 1 1 Clarke, F. W., cited, 3 Classification scheme, igneous rocks, 22, 23 of rocks, principles of, 12 Clay, analysis, 92 description, 94 ironstone, 104, no Cobalt oxide, molecular ratios, 163 Conanicut, R. I., 117 minette, 44 Conglomerates, 116 Consanguinity, 83 Contact metamorphism, 112-115 external effects, 113 internal effects, 112 zones, 113 Coral limestone, 99 Corniferous limestone, 1 10 Cornwall, Pa., 119 Cortlandt series, quartz-diorite, 56 series, contacts, 118 Crawford Notch, N. H., 117 Crinoidal limestone, 101 Cripple Creek, Col., phonolites, 48 Cross, Whitman, cited, 158 Crugers, N. Y., 118 Crustification, 106 Crystalline limestone, 138140 schists, 127 Crystallization, order of, 20, 21 Cupric oxide, molecular ratios, 163 Custer Co., Col., syenite, 44 trachyte, 41 Cyanite, n Cycle of deposition, 87 D Dacite-porphyry, 53 Dacites, 52-54 Dana, J. D., cited, 129 Degeneration of rocks, 145 Derby, O. A., cited, 144 Diabases, 70 analyses, 70 texture of, 71 Diallage, 9 Diatomaceous earth, 101, 103 Dikes, 15 Diopside, 9 Diorite-porphyries, 58 Diorites, 60 alteration and metamorphism, 62 analyses, 60 distribution, 62 mineralogical composition, 6 1 varieties, 62 Dissolved vapors, 18 Ditroite, 50 Dolerite, 65 Dolomite, II, 100 crystalline, 138-140 Drachenfels-trachyte, 42 Dunkirk, Md., 103 Dykes, see dikes Dynamic metamorphism, 122 Earth, composition of the crust, 3 Eclogite, 132, 133 Effusive rocks, 16 Eleolite, see nephelite syenite, see nephelite syenite Enstatite, 9 Eolian sands, 41 Epidote, n schist, 132, 133 Essential minerals, 12 External contact metamorphism, 116 Extrusive rocks, 1 6 Feldspars, 5 Feldspathoids, 6 Felsite, 30 Felsitic texture, 17 Ferric oxide, molecular ratios, 158 Ferromagnesian silicates, 4 Ferrous oxide, molecular ratios, 159 Ferruginous organic rocks, 104 Flexible sandstone, 134 Flint, 109 Fluorine molecular ratios, 162 Foyaite, 50 INDEX. 245 Franklin Furnace, N. J., 101 H: Freiberg minettes, 44 Saxony, 126 Freshwater limestone analyses, 97 Friction breccia, 85 Halite, 12 Harker, Alfred, cited, 115 Hawes, G. W., cited, 117 Fusibility of magmas, 17 Fusing points of minerals, 21 of rocks, 22 Hematite, n High wood Mtns., Mont., 48 Hillebrand, W. F., 151 Hoboken, N. J., 117 Hollick, A, cited, 118 Hornblende, 8, 9 Gabbro, 72 Hornblende-schists, 129-131 alterations and metamorphism, 78 Hornblendite, 77 analyses, 72 Hornfels, 116, 117 distribution, 78 Hornstone, 109 mineralogical composition, 74 Hyalomelane, 26 varieties, 74 Hydraulic limestone, loo Gabbro-porphyries, 65 Hydromica-schist, 129 Garnet, II Hypersthene, 9 Generations of minerals, 21 Hypersthene-andesite, 59 Geyserite, 102, 103, 108-110 fels, 74 Gieseckite-porphyry, 47 Glasses, 25 , analyses, 25 distribution of, 27 geological occurrence, 27 Iddings, J. P., cited, 158 Igneous rocks, 1 5 review of, 80 relationships, 27 determination of, 82 varieties, 26 field observations, 83 Glassy texture, 16 mineralogy of, 81 Glauconite, 95 Glaucophane schist, 132, 133 range of composition, 80 typical textures, 8 1 Gneisses, 123-126 table of, 23 Gordon, C. H., cited, 123 Ilmenite, n Grabau, A. W., cited on limestones, Infusorial earth, 101-103 99 Granite-porphyries, 31 Internal contact metamorphism, 115 Intratelluric, 1 8 Granites, 33 Iron Mtn., Mo., trachyte-porphyry, 41 analysis of, 33 Itacolumite, 134 distribution, 38 metamorphism of, 37 K mineralogy of, 34, 35 Kaolin, n occurrence, 36 anals., 92 relationships, 36 Kemp, J. F., cited, 117, 118 uses of, 37 Keratophyre, 41-54 varieties, 34 Kersanite, 62 Granitoid texture, 17 Kimberlite, 77 Grano-diorites, 36 Knotty schists, 1 1 7 diorite, 55, 56 slates, 117 Granulite, 126, 127 Kulaites, 66 Graphic granite, 35 T Graphite, 12 Graphite schist, 133 Gravels and conglomerates, 88 metamorphism of, 88 occurrence, 88 Labradorite, 6 Labradorite-rock, 74 Laccoliths, 15 Lake Champlain trachyte, A: Greensands, 96 Laterite, 144 Green schists, 132 Greisen, 120 Latite, 41 Laurdalite, anals., 49 Groundmass, 17 Grorudite, 32 Grunerite, no Gypsum, n, 106, 107 Leucite, 6 Leucite-basalt, 66 Leucite-basanite, 66 Leucite Hills, Wyo., 48 246 INDEX. Leucite-phonolite, anals., Rieden, Ger- many, 44 defined, 48 Leucite rocks, 48 syenite anals., 49 tephrite, 66 Leucitophyre, 48 Liebenerite-porphyry, 47 Lignite, 104, 105 Limburgites, 67 Lime, molecular ratios, 160 Limestone, crystalline, 138-140 alteration, 140 composition, 138 occurrence, 140 varieties, 139 Limestones, 97 analyses, 97 at igneous contacts, 118 origin of, 98 Limonite, II Liparite, defined, 31 Litchfieldite, anals., 49 defined, 50 Lithia, molecular ratios, 163 Lithographic limestone, 101 Lithophysse, 27 Living organisms, analyses of calcareous parts, 97 Local metamorphism, 112 Loess, 91 Luxullianite, 35 Lyell, 112 Nf Magnesia, molecular ratios, 159 Magnet Cove, Ark. Nephelite-syenite, 48 Magnetite, II Magnesian limestone, 100 Malacolite, 9 Manganous oxide, molecular ratios, 163 Marble, see crystalline limestone Marls, analyses, 95 Matthew, W. D., cited, 118 Mechanical sediments, 84 Melaphyre, 66 Melilite, 7 basalt, 68 Merrill, G. P., cited, 145 Metamorphic rocks, determination of, 147* 148 Metamorphism, 1 12 of limestones, 101 Meteorites, 79 Miarolitic, 17 Mica-andesite, 59 Micas, 9 Mica-schists, 127, 129 Mica-syenites, 43 Microcline, 5 Micro-granites, 36 117 Mineralizers, 17, 18 Minerals, accessory, 12 essential, 12 primary, 12 secondary, 12 Minette, 43 Minor schists, 131-133 Mixed zone, III Molecular proportions, 150 ratios, 150 Molten magmas, nature of, 20 Monzonites, 43 Mt. Willard, N. H., Muscovite, 10 N Necks, defined, 15 Nephelite, 6 Nephelite-basalt, anals., 66 Nephelite-syenite, analyses, 49 description, 49-51 porphyry, 47 Nevadite, 31 Nickel oxide, molecular ratios, 163 Norite, 74 Novaculite, analysis, 89 defined, 90 o Obsidian, 26 cliff, cited, 27 Ochsenius, 106 Oligoclase, 5 Olivine, 10 Olivine-free basalts, 66 Onyx marble, 106 Ophicalcites, 140-143 alteration, 143 composition, 140 distribution, 143 varieties, 142 Ophiolite, see ophicalcite Oolitic limestones, 100 Orbicular granite, 36 Orendite, 48 Organic remains not limestone, 101 Orthoclase, 5 Oxides, common in rocks, 1 1 Oxygen quotient, 149, 150 JP Paisanite, 32 Pantellerite, 41, 54 Paramorphism, 69 Peat, 104-105 Pegmatite, 35 Pele's hair, anals., 14 Peridotites, 75 Perlite, 26 Petrographic provinces, 83 Petrosilex, in Petite Anse, La., 107 INDEX. 247 Phanerocryst, 17 Phenocrysts, 17 Phlogopite, IO Phonolite-obsidian, anals., 25 Phonolite-porphyry, 47 Phonolites, analyses, 44 description, 45-49 Phosphates, mineral, 12 Phosphoric pentoxide, molecular ratios, 162 Phthanites, 94 Phyllite, 129 Pilot Knob, Mo., trachyte-porphyry, 41 Pirsson, L. V., cited, 117, 151, 158 Pisolitic limestone, loo Pitchstone, 26 Plagioclase feldspars, 5 Plauenschen Grund syenite, 43 Plutonic rocks, 1 6 Popes Mills, Md., 103 Porphyrite, 59 Porphyritic texture, 16 Porphyry, a pre-tertiary trachyte, 41 Potash molecular ratios, 161 Precipitates from solution, 105 Predazzo, Tyrol, 118 Pressure, influence of, 18 Primary minerals, 12 Principles of rock classification, 12 Propylite, 59 Pumice, 26 Pyrite, 12 Pyroxenes, ^ Pyroxenites, 7$ Pyrrhotite, 12 Q Quartz, II basalt, 67 Quartz-diorite, 55 porphyry, 54 Quartzites, 134-1 35 Quartz-keratophyres, 32 pantellerites, 31 porphyries, 31 porphyrite, 54 trachyte, 31 Rate of cooling, influence of, 1 8 Regional metamorphism, 112-113 Regionally metamorphosed rocks, 121-123 Residual magma, 21 Rhyolite granite series, 28 porphyries, 31 Rhyolites, 28 alteration of, 32 analyses of, 28 distribution, 32 general description, 30 relationships, 32 synonyms, 31 Rhyolite, alteration, 143 composition, 141 distribution, 143 varieties, 143 Rhyolite tuffs, 33 Richmond, Va., 103 Rieden, Germany, leucite rocks, 48 Rock, definition of, I magmas, 17 salt, 12, 106-107 Rocks, chemical composition of, 3 physical range of, 2 principles of classification, 12 the three great classes of, 13, 14 Rosenbusch, H., cited, 117 St. John, N. B., 118 Sands and sandstones, 89 analyses, 89 metamorphism of 91 mineralogical composition, 90 occurrence, 91 varieties, 90 Sandstones, 116 Sanidine, 5 Saprolite, 145 Scapolite, n Schists, 127 Scorias, 26 Secondary minerals, 12 Sedimentation, 86 agents of, 87 Selenite, 107 Serpentines, 140-143 alteration, 143 composition, 140 distribution, 143 varieties, 142 Seven Devils, Idaho, 119 Shales, 116 and clays, 92 analyses, 92 metamorphism of, 94 mineralogical composition, 93 occurrence, 94 varieties, 93 Sheets, 15 Shonkinite, 43 Siderite, II Silica, molecular ratios, 157 Silicates, 4 Siliceous limestones, 97 oolite, 108-110 sinter, 101-103 Sillimanite, II Slates, 1 06, I35-I3 8 alteration, 137 composition, 135 development, 135, 136 distribution, 138 2 4 8 INDEX. Slates, varieties, 135 Slaty cleavage, 137 Smyth, C. H., Jr., cited, 118, 143 Soda, molecular ratios, 160 Soapstone, 140-143 Sodalite-syenite, 44 Sorby, H. C., cited, 136 Sparta, N. J., 101 Specific gravity, 20 Spheroidal granite, 36 Spherulites, 26 Stalactites, 106 Stalagmites, 1 06 Standard minerals, 158 Stassfurt, 107 Staurolite, II Steatite (seesoapstone). Stratification, 87 Strontia, molecular ratios, 163 Structure, significance of term, 1 6 Sulphates, mineral, II Sulphides, important, 12 Sulphur, molecular ratios, 162 Sulphuric anhydride, molecular ratios, 162 Surficial, defined, 145 rocks, 145 Sussexite, 47 Syene, Egypt, 42 Syenite-porphyries, 40 Syenite recalculated, 151 Syenites, analyses, 42 description, 42, 43 Tabulation of igneous rocks, 23 Tachylyte, anals., 25 Talc in soapstone, 143 schist, 132 Temperature, influence of, 17 Tephrite, 66 Texture, meaning of term, 16 Textures, factors governing, 17, 18, 24 of rocks, 1 6, 17 Theralite, 77 Tinguaite, 47 Titanic acid, molecular ratios, 162 Titanite, 10 Tonalite, 56 Topaz, ii Tourmaline, II granite, 35 Trachyandesite, 41 Trachydolerite, 41 Trachyte-porphyries, 40 Trachyte-syenite series, 38 Trachytes, 38 analyses, 38 description, 39 mineralogy of, 39 synonyms, 40 textures of, 40 Trachyte texture, 40 Travertine, loo Tremolite in marble, 139 Tripoli, 103, 109 Tyndall, John, cited, 136 u Ultra-basic igneous rocks, 79 Vesuvianite, 119 Volcanic rocks, 16 Volcanite, 41 Vosges Mtns., 117 Vulsinite, 41 w Wadsworth, M. E., cited, 147 Washington, H. S., cited, 156, 158 Water, molecular ratios, 161 Waterlime, 101 Weathering, 112, 144 Websterite, analysis, 75 Weed, W. H., cited, 151 Westmoreland, Eng., 114 White Mtns., N. H., syenite, 44 Williams, G. H., cited, 118 Wyomingite, 48 z Zeolites, n Zircon, IO Zirconia, molecular ratios, 162 DATE DUE JUN 2 3 R6CD JUL 1973 UC SOUTHERN REGIONAL A 000826405 3