UC-NRLF 7-RA/NITES G-REE/NSTOMES A Series of Tables c*nd /Notes for Students ot Petrology LV f^UTLEY, f.G.S Lecturer fin Mineralogy^ Rf>yaJ C, " J egp of Science, London [ALL RH;HTS RESERVED] THOMAS MURBY 3 LUDGATE CIRCUS BUILDINGS, E.C 1894 ; BERKELEY IBRARY INIVERSITY OF CALIFORNIA EARTH SCIENCES LIBRARY UNIVERSITY OF CALIFORNIA. FROM THE LIBRARY OF DR. JOSEPH LeCONTE. GIFT OF MRS. LECONTE. No. GKA/NITES 7VND G"REE/NSTO/NES A Series of Tables and /Notes for Students of Petrology BY f^UTLEY, F-G-S \\ Lecturer on Mineralogy, Royal College of Science, London [ALL RIGHTS RESERVED] )on THOMAS MURBY 3 LUDGATE CIRCUS BUILDINGS, E.C 1894 QEtfol EARTH SCIENCES LIBRARY BUTLBB & TANNKH, THK SKLWOOD PBINTING WORKS, FROMB, AND LONDON. CONTENTS PAGE EXTRACTS FROM WORKS PUBLISHED BETWEEN 1820 AND 1860 ... 4 PREFATORY NOTE 5 INDEX OF SYMBOLS 7 TABULAR CLASSIFICATION OF ERUPTIVE ROCKS A. ULTRA-BASIC 8 B. BASIC .... 9 C. AND D. INTERMEDIATE 10, 11 E. ACID . . . . . . . . . . . .12 STRUCTURES DEFINED 13 MINERAL CONSTITUTION DEFINED 15 EXPLANATORY NOTES ON DETERMINATIVE TABLES 26 . DETERMINATIVE TABLES, I. TO VIII 29 INDEX TO THE DETERMINATIVE TABLES 46 NOTE ON THE MICROSCOPE 47 GENERAL INDEX 47 101309 EXTRACTS PROM WORKS PUBLISHED BETWEEN THE YEARS 1820 AND 1860. " Various classifications of volcanic substances have been proposed, among which the division into Trachytic and Basaltic seems to be that most commonly adopted." Geological Manual, p. 137. 1833. DE LA BECHE. " Trap. Tabular greenstone and basaltic rocks, from their rising up in step-like masses, were originally so termed ; but the name is now ex- tended to all igneous rocks which are not either strictly Granitic or decidedly Volcanic." Handbook of Geological Terms, 1859. PAGE. "Diabas. Syn. Griinstein z. Th. Diorit z. Th. Hyperit z. Th." Handbuch der Lithologie. 1860. BLUM. " The principle of separating from the trap or basalt the "greenstone protrusions," is in part correct; these intrusions consist of two kinds, namely, whin dykes or basaltic veins, and huge amorphous masses of greenstone, or greenstone- porphyry, which occasionally form large moun- tains, and which among igneous rocks are only secondary to granite in point of extent and im- portance." Eeport on the Geology of Londonderry and parts of Tyrone and Fermanagh, p. 77. 1843. POETLOCK. " When these granite- veins are of a large size, they are termed elvan-courses. ... In com- position, these elvans are either shorl rock, eurite, felsparite, or even varieties of fine-grained granite." A Treatise on Primary Geology, p. 22. 1834. BOASE. " J'ai et6 le premier a donner le nom scienti- fique et general de Diabase a la roche d6crite et caract6risee sous le nom de Griinstein par les min6ralogistes allemands." Classification et Caracteres Mineralogiques des Eoches, p. 80. 1827. BEONGNIAHT. " By the term granite is here understood every compound rock which is found in irregular masses beneath the lowest strata ; including, further, those veins which proceed from them so as to traverse the adjoining rocks . . ." A Geological Classification of Eocks, p. 225. 1821. MAC CULLOCH. " Granites no doubt vary in their chemical composition, and so do greenstones, yet they always so differ from each other as masses of matter that the one can never become the other from mere differences in cooling." Researches in Theoretical Geology, p. 379. 1834. DE LA BECHE. PREFATORY NOTE THE title " Granites and Greenstones " has been chosen because it is a com- prehensive one. The relation between granites and syenites and between rhyolites and trachytes induced the geologist of former days to look upon these rocks, with their associated dykes, as members of one great group, the most important and characteristic being GRANITE. In like manner he broadly treated the diorites, gabbros, etc. (together with the basalts, dolerites, and lamprophyres), as representatives of another great group, to which the name GREENSTONE was given. No distinction was drawn between trachytes and andesites; the dyke rocks of the greenstone group were termed traps, while those of the granite group were called elvans. There were" thus two great groups of eruptive rocks, Granites and Greenstones, and the geologist of the present day will do well to retain these time-honoured names for use in the field. They may not express very much, but what significance they have is broad, and they are far more valuable for use on a working map than the more precise names with which modern petrography is encumbered. The tables, placed at the beginning of these notes, indicate the mutual relations of the principal eruptive rocks. Above the name of each rock, symbols are placed, denoting its essential constituents. The symbol consists in most cases of the first three letters of the name of the mineral. In a few instances but one letter is used, sometimes with, at others without smaller qualifying letters or symbols. This plan of abbreviating the names is so simple that the meaning of each symbol is at once apparent ; but, to prevent PREFATORY NOTE possible mistakes, the tables are preceded by an index, giving the names in full. After the tables, come definitions of the terms used to denote the most important microscopic structures. These are succeeded by notes on the mineral constitution of the eruptive rocks, and here lists of the accessory and secondary minerals are given. The tables showing the classification of the eruptive rocks are divided by horizontal lines into three zones, the volcanic rocks, or lavas, being placed in the uppermost, the dyke-rocks in the middle, and the plutonic rocks in the lowest zone. It should, however, be remembered that there is, in reality, no sharp demarcation between these rocks. On the contrary, the dykes may, in a certain sense, be regarded as constituting a connection between the plutonic masses and their volcanic representatives.* The names of altered and pyroclastic rocks are printed in italics. The notes on mineral constitution are followed by a series of determina- tive tables, which differ in some respects from those hitherto published, being to a certain extent simplified, in order to meet the general requirements of students. Chemical formulae and specific gravities are not given, since they can be found in any good text-book of mineralogy, and the tables are cleared, as far as possible, of matter which does not relate to simple microscopic investigation. Explanatory notes precede the tables, which are accompanied by an index, reference to the latter at once showing upon which table the required mineral will be found. The notes are necessarily brief. For further information, the student may be referred to Teall's British Petrography, and to the works of Levy and Lacroix, Rosenbusch, Zirkel, v. Lasaulx, Kalkowsky, Gumbel and Jannettaz. * Veins, dykes and the infillings of volcanic fissures and pipes appear, indeed, to be merely progressive phases of vertical intrusion, while sills may be looked upon as the homologues of lava- flows, especially of those from fissure-eruptions, and laccolites as those of domes. In the latter case the homology is supported by the circumstance that both laccolites and domes are, as a rule, formed from highly viscous magmas which solidified as trachyte, while those sills which are intruded for long distances are generally basic in composition. The difference of the conditions under which on the one hand lava-flows and domes, on the other sills and laccolites are formed, is a sufficient rea- son why the latter are unaccompanied by tuffs, since the explosions of steam necessary for their production could not take place under the pressure of a great thickness of overlying rock. TABULAE CLASSIFICATION OF ERUPTIVE ROCKS, THE ESSENTIAL CONSTITUENTS BEING DENOTED BY SYMBOLS. INDEX OF SYMBOLS. Arf. Arfvedsonite. Gran. Granular ground- 0. Orthoclase. Aug. Augite. mass. O'f Anorthoclase. Bio. Biotite. Haiiy. Haiiyne. 0? Sanidine. Bro. Bronzite. Hor. Hornblende. Oliv. Olivine. Can. Cancrinite. Hyp. Hypersthene. Omp. Omphacite. Chro. Chromite. Lep. Lepidolite. P. Plagioclase. Cos. Cossyrite. Leu. Leucite. Alb. P. Albite. Dia. Diallage. Mag. Magnetite. -Ano. Ens. Enstatite. Melan. Melanite. P. Anorthite. Eud. Eudialyte. Melil. Melilite. Pero. Perowskite. Pel. Felsitic, Microfelsi- Micro. Microcline. Pic. Picotite. tic, Microcrystal- Mus. Muscovite. Q. Quartz. line or Crypto- Nep. Nepheline. Serp. Serpentine. j crystalline- N. Nepheline Sma. Smaragdite. groundmass. (decomposition Sod. Sodalite. Gar. Garnet. products after) . Vit. Vitreous matter. N Elseolite. A symbol in italics indicates either a groundmass or else residual, interstitial matter. A line over a symbol indicates that the mineral is partly or wholly represented by decomposition products. When a mineral is only present in small quantity, its symbol is given without a capital, e.g. bio. = Biotite, in small quantity. For accessory and secondary constituents see p. 15 et seq. ULTRA. BASIC SERIES. Silica = 39 to 45 % NON-FELSPATHIC. TABLE A. Picrite-porphyrites. Kimberlite. Peridotite Dykes. p. 25, p. 1 . 25, PYROXENITE GROUP. Hor. Aug. HOBNBLENDITE Sma. Omp. Gar. ECLOGITE HTPERSTHENITE Bro. BKONZITITE Ens. Dia. WEBSTERITE Dia. DlALLAGITE PERIDOTITE RO UP. Oliv. Ang. PlCRITE Oliv. Hor. HORNBLENDE -PlCRITE Serpentines. Oliv. Dia. WEHRLITE Oliv. Aug. Gar. EULTSITE Oliv. Dia. Bro. Pic. LHERZOLITE Serp. Bio. Pero. MiCA- PEKIDOTITE P-; 23. Oliv. Hor. Serp. Hor. Hyp. Mica. CORTLANDITE SCYELITE Oliv. Bro. SAXONITE Oliv. Cbro. DUKITE Serpentines, p. 25. BASIC SERIES. Silica=45 to 55 % WITH TABLE B. NON-FELSPATHIC PLAGIOCLASTIC FELSPARS. but potentially felspathic, nephelitic or leucitic, when not actually so (see p. 20). NEPHELINE SERIES. Basalt Tuffs. BASALT GROUP. Aug. Mag. Fi*. AUGITITE Palagonite HYALO- BASALT Oliv. Aug. Mag. Fit. LlMBDRGITE P. Leu. Aug. LECCITE- TEPHRITE M e I a p h y res. Leu. Aug. LEUCITITK Nep. Aug. NEPHELINITE P. Nep. Aug. TEPHRITE Leu. Aug. Oliv. LEUCITE- BASALT Nep. Aug. Oliv. NEPHELINE- BASALT P. Nep. Aug. Oliv. BASANITE P. Aug. Oliv. OLIVINE- BASALT P. Aug. Hor. HORNBLENDE -BASALT P. Aug. BASALT P. Aug. Oliv. Hyp. Fit. HYPERSTHENE -BASALT P. Aug. Bio. MICA- BASALT P.Q. Oliv. Aug. QUARTZ- BASALT Melil. Aug. MELILITE -BASALT DOLERITE GROUP. Ophites, Epidiorites, Diabases. Melil. Mag. Bio. Oliv. Aug. ALNOITE Aug. Hor. Bio. Fit. FOURCHITE Aug. Hor. Bio. Oliv. Kit. MONCHIQUITE P. Aug. Oliv. OLIVINE- DOLERITE P. Aug. Hor. HORNBLENDE -DOLERITE TACHYLYTE P. aug. q. LEUCOPHYRE P. Aug. DOLERITE P. Aug. Ens. ENSTATITE- DOLERITE P. Aug. Bio. MlCA- Do LERITE or PROTEROBASE G A B B R GROUP. Euphotides. Nep. hor.(or bio.) IJOLITE P. Nep. Aug. THERALITE P. Aug. Hor. Oliv. OLIVINE- GABBRO P. Aug. Hor. HORNBLENDE -GrABBRO P. Aug. GABBRO P. Hyp. (or Ens.) NORITE P. Hyp. QUARTZ- NORITE P. Oliv. TROCTOLITE P. Aug. Hor. Bio. TESCHENITE Ano. P. Aug. EUCRITE P. Hyp. (or Ens.) Oliv. OLIVINE- NORITE p. 20. p, 21. p. 22. Melaphyres, p. 24. Diabases, p. 24. Ophites, p. 25. Epidiorites, p. 25. Euphotides, p. 25. 10 INTERMEDIATE SERIES Silica =55 to 66 % TABLE C. PLAGIOCLASTIC FELSPAES. NEPHELINE SERIES. Andesite T uff s . ANDESITE GROUP. Pdlagonite HYALO-ANDESITE P. Hor. Aug. Hatty (nep ?) HAUYNE- ANDESITE HYALO-DACITE P. Hor. Fit. | HORNBLENDE- ; ANDESITE Pro For P. Aug. Fit. AUGITE- ANDESITE p y I i p h </ r i P. Hyp. Fit. HYPERSTHENE- ANDESITE e s . P. Bio. Fit. MiCA- ANDESITE P. Hor. Q. Fit. DACITE LAMPEOPHYRE GROUP. P. Bio. KERSANTITE P. Aug. Hor. Bio. Fit. CAMPTONITE P. Bio. Hor. Q. Gran. MALCHITE D I R I T E G R UP. Alb. P. Nep. LlTCHFIELDITE P. Hor. DlORITE P. Hor. CORSITE P. Aug. AUGITE- DlORITE P. Ens. ENSTATITE- DIORITE P. Bio. MlCA- DlORITE P. Hor. Q. QUARTZ- DlORITE P. Hor. Bio. Q. TONALITE Porphyrites, p. 24. Propylites, p. 24. 11 INTERMEDIATE SERIES. TABLE D. Silica=55 to 66 % WITH OETHOCLASTIC FELSPARS. NEPHELINE SERIES. Haiiyne and Nosean often present. Trachyte Tuffs. TRACHYTE GROUP. HYALO-PHONOLITE (The felspars are 1 he dominant constituents present. Grouudmass of HYALO- TRACHYTE . Plagioclase, us ten microlitic.) ually Oligoclaae, O? Nep. Aug. Vit. PHONO LITE O? Leu. Aug. Vit. LEUCITE- PHONOLITE 0? Leu. "Nep. Aug. Fit. LEUCITOPHYRE O? Hor. Pel. HORNBLENDE- TRACHYTE O? Aug. Pel. AUGITE- TRACHYTE O? Bio. Pel. MICA- TRACHYTE O A Aug. Cos. Fit. PANTELLERITE LAMPROPHYRE GROUP. O. Bio. Hor. HORNBLENDE- MlNETTE O. Hor. HORNBLENDE- VOGESITE O. Bio. Aug. AUGITE- MlNETTE 0. Aug. AUGITE- VOGESITE O. Bio. MINETTE O. Nep. Aug. Bio. Melan. BOROLANITE O. N? Aeg. Gran. TlNGUAITE 0. Of Q. BOSTONITE O. N? Aug. FOYAITE O. N? Hor. EL<EOLITE- SYENITE 8 Y (Plagioclase generall O. Hor. Bio. SYENITE E N 1 T E y present, except in augit O. Aug. Bio. AUGITE- SYENITE G R U e-syenites contaii O. Bio. MICA- SYENITE P . ling anorthoclase.) O. NK Hor. Zir. ZIRCON- SYENITE O. N? Can. Sod. Hor. DITROITE O. N? Arf. Sod. Eud. EUDIALYTE- SYENITE O. N?Bio. Q. MIASCITE p. 17. p. 17. p. 17. 12 ACID SERIES. Silica=over 66 % TABLE E. Felspars mostly plagioclase or anorthoclase. WITH ORTHOCLASTIC FELSPAES. R h y oli t e T u ff s , Pumice. RHYOLITE GROUP. (Flag ioclase often present.) Felnite pt. P.'Q. Pel. & Fit. SODA-RHYOLITE HYALO-BHYOLITE including OBSIDIAN, PEBLITIC OBSIDIAN and PITCHSTONE Of P. Q. hor. Fit. & Pel. DACITE pt. O? Q. Fel. & Fit. RHYOLITE or LIPARITE O* Aug. Cos. Fit. PANTELLERITE pt. E L V A N GROUP. PITCHSTONE DYKES P. Fel. KERATOPHYRE O. Fel. FELSPAE-POKPHYKY O. Q. Felsite pt. MICRO-GRANITK HAPLITE O. Q. Mus. PEGMATITE P. Q. Fel. QUARTZ-KERATOPHYRE O. Q. mus. bio. Fel. QUARTZ-FELSITE or QUARTZ-PORPHYRY GRANITE GROUP. (P lagioclase almost O. Q. Hor. Bio. HORNBLENDE- GRANITE always present. '. O. Q. Aug. Bio. A0GITE- GRANITE Slicrocline frequent Micro. Q. Hyp. HYPERSTHENE- GBANITE ) Arkose. Kaolin. O. Q. Mus. Bio. GRANITE O. Q. Mus. MUSCOVITE- GRANITE O. Q. Bio. BIOTITE- GHANITE or GRANITITE Felsites, p. 24. Arkose, p. 24. STRUCTURES DEFINED Idiomorphic. When the mineral constituents of a rock have attained the condition of fully developed crystals. Allotriomorphic. When the minerals occur in crystalline grains and not in perfectly formed crystals. Panidiomorphic. When all the constituents of a rock are idiomorphic. Hypidiomorphic. When some of the constituents are idiomorphic while others are allotriomorphic. Porphyritic. When some of the crystals are idiomorphic, while others have crystallised subsequently in a more rapid manner, less perfectly, or on a much smaller scale ; or when the residue of the magma has solidified as glass, the larger idiomorphic crystals are said to occur porphyritically. Such crystals have been termed " phenocrysts " by Prof. Iddings. Groundmass. That portion of a rock which constitutes the matrix in which porphyritic crystals are embedded. Microlitic. When the groundmass consists of minute crystalline bodies or microlites. Microcrystalline. When a rock, or the groundmass of a rock, consists of an aggregate of very diminutive, allotriomorphic, crystalline grains, the boundaries of which are recognisable under the microscope. Cryptocrystalline. When the minute allotriomorphic grains constituting a rock, or its groundmass, show no distinct boundaries under the microscope but appear to run irregularly one into another. Glassy base. The residual portion of a magma which has solidified rapidly as glass, constituting a vitreous groundmass, in which a variable quantity of micro- scopic bodies (microlites, trichites, globulites, etc.) have usually been developed. The vitreous matter in some rocks may be very small in amount, in others it may constitute the entire rock. Devitrification. The change caused by the development of crystalline structure in vitreous matter. A glassy rock when devitrified loses its glassy lustre and translucence, thus an obsidian may, through devitrification, assume a lithoidal character and pass into a felsite. The change may result from the development of globulites or trichites, or from the setting up of a microcrystalline or crypto- crystalline structure, and occasionally, to some extent, from the formation of spherulites. Perlitic. Essentially vitreous lavas and dykes frequently show what is termed perlitic structure, which consists in the development of small, very commonly microscopic, ellipsoidal or spheroidal cracks which are seldom quite continuous, but die out and are succeeded by others, the general arrangement being rudely concentric. These perlitic cracks are usually packed between irregularly-disposed rectilinear cracks. 13 14 GRANITES AND GREENSTONES Microfelsitic. This term is applied to a substance which in some respects closely resembles vitreous matter, but differs from it in being less translucent and in consisting of hazily-defined scales and fibres. When these are irregularly dis- posed the microfelsitic substance exerts no appreciable action upon polarised light and appears to be isotropic, but when there is either a parallel or a radial arrange- ment of the fibres the substance exhibits feeble double refraction. Felsitic. A micro- or crypto-crystalline aggregate of quartz and felspar is known as felsitic matter. The terms felsitic and microfelsitic have, therefore, widely different significations. The symbol Fel. employed in these tables includes, however, both felsitic and microfelsitic matter. Hyalopilitic is a term applied to the vitreous matter of a groundmass, when in great part filled with or replaced by diminutive crystals or microlites, which form a more or less densely felted mass. Pilotaxitic is applied to the felted structure above described, when no vitreous matter is present. Pilotaxitic structure is by no means rare in some of the basalts and melaphyres. Ophitic. When a crystal of one mineral is penetrated in different directions by elongated or lath-shaped crystals of another mineral (e.g. augite penetrated by felspars). The penetrated crystal thus appears to be broken up into a number of irregularly shaped and detached pieces, but these are seen in polarised light to be parts of the same crystal since they undergo simultaneous extinction. Olomero-porphyritic. When a rock is rendered porphyritio by crystalline- granular aggregates. Granular. Composed of irregularly-shaped crystalline grains, as in granites. In sedimentary rocks the grains are usually more or less rounded, as in sandstones, etc. Fluxion Structure is shown either by approximately parallel streams of micro- lites or by differences in texture, sometimes in colour, of the vitreous or other matter of which the rock is composed. When porphyritic crystals are present the streams are seen to sweep around them. Occasionally the fluxion-banding appears much contorted, like the mottling on a Damascus blade. These structures are common in rhy elites. Migration Structure resembles fluxion structure, but is assumed to result from the decomposition of original constituent minerals, and the migration and recry- stallisation of the products in fresh positions. Miarolitic Structure. When small, irregular cavities occur into which the constituent crystals of the rock project. Spherulitic Structure consists in the development of spherical or approximately spherical bodies, usually, but not always, consisting of radiating crystalline fibres, sometimes with interstitial glassy or microfelsitic matter. Much elongated spheru- litic bodies are termed axiolites. The large, hollow, spherulitic bodies occurring in obsidians, felsites, and rhyolites are called litkophyses. Spheroidal Structure appears to originate in several ways : (1) from weathering, blocks of rock bounded by intersecting joint planes becoming rounded more and more as decomposition proceeds ; (2) from mineral segregation ; (3) from contraction. Columnar Structure results from the contraction of a rock-mass on cooling. The shrinkage planes or joints, which by their intersection give rise to polygonal columns, are traversed by cross joints, and between these intersecting joint planes spheroidal structure is frequently developed. ACID SEEIES 15 Platy Structure. A few eruptive rocks, notably phonolites, have contracted on cooling in such a manner that they have become traversed by a series of parallel and often but slightly separated joints, so that the rock may be readily split into slabs. The phonolite of the Tuilliere, near Mont Dore, Auvergne, affords a good example of this structure, the slabs when quarried being sufficiently thin for roofing purposes. Holocrystalline. When a rock consists wholly of crystalline matter. HypocrystalUne. When vitreous or microfelsitic matter is associated with the crystalline material of a rock. Since the crystalline schists are not treated of in these notes, such terms as mylonitic structure, flaser structure, etc., are not defined. MINERAL CONSTITUTION DEFINED. 1Ens. Con. = Essential constituents. Ace. Con. = Accessory constituents. Sec. Con. = Secondary constituents. 1. RHYOLITE GROUP. OBSIDIANS. Essentially vitreous rocks, containing either no water or generally less than 3 per cent. Crystallites present, often in profusion. Sometimes with por- phyritic crystals or with microlites of sanidine, biotite, augite, etc. Gas inclusions frequently present. PUMICE. A frothy or vesicular condition of obsidian or of any vitreous rock, due to the inclusion of gas or steam in bubbles, which are usually drawn out into thread-like cavities or tubes. PEELITIC OBSIDIANS contain from 1 to 3 per cent, of water. Lustre vitreous, occasionally enamel-like. Perlitic structure present, also banding or fluxion struc- ture, streams of microlites and spherulites either isolated or coalescing in bands. PITCHSTONES. From 3 to over 9 per cent, of water usually present. Often densely packed with microlites. Frequently porphyritic with sanidine, quartz, plagioclase, etc. Augite, hornblende, and magnetite frequently present. Perlitic and spherulitic structures common.* Felsites (Halleflinta, Felstone, Petrosilex, Eurite pt.). Obsidians and pitch- stones which have undergone alteration from a vitreous to a lithoidal condition. The obsidians and pitchstones of the Archaean, Cambrian and Silurian periods are commonly met with in this devitrified slate. The change is due to the development of a crypto-crystalline or micro-crystalline structure in the once vitreous material. It does not, however, as a rule, suffice to mask perlitic and other structures which these rocks may have originally possessed. Epidote sometimes present. Felsites are not exclusively devitrified rocks, in some cases they occur as dykes and then approximate to the micro-granites or granophyres, but all felsites are, if the felspar be orthoclase, essentially crypto-crystalline or micro-crystalline aggre- gates of that mineral and quartz. SODA-FELSITES (Keratophyres, Eurites pt.) differ from the preceding in the felspathic constituent being partly or wholly a felspar rich in soda. Augite, horn- * Pitchstones occasionally occur as flows, but commonly as dykes. In the latter case they may be referred to the Elvan Group. 16 ACID SERIES blende, biotite, etc., sometimes present. The keratophyres approximate to fine- grained granitic and syenitic rocks. In the latter case they may be regarded as felsites of the trachyte group. RHYOLITES. Highly acid lavas, generally with a porphyritic structure. Crystals of sanidine and quartz in a microfelsitic groundmass, often with a variable amount of vitreous matter. Ace. Con. Oligoclase, biotite, hornblende, augite, apatite, sphene, dichroite, garnet, orthite, magnetite, specular-iron, tridymite. Sec. Con. Chlorite, epidote, pyrites, calcite, quartz, chalcedony, opal. The rhyolites commonly show fluxion- and spherulitic-structures. The pyromerides are coarsely spherulitic rhyolites. SOD A- RHYOLITES differ from the ordinary or potash rhyolites in the felspar being oligoclase, soda-microcline or albite. They form a connecting link between the rhyolites and the pantellerites. PANTELLERITES consist of a groundmass (often partly vitreous) of felspar and augite, with porphyritic crystals of anorthoclase, augite and cossyrite (triclinic hornblende). They form a link between the soda-rhyolites and the dacites. 2. ELVAN GROUP (Apophyses of deep-seated granitic masses). PITCHSTONE DYKES (see p. 15). MICROGRANITES. Fine crystalline aggregates of quartz and felspar with little or no mica. In the latter case they approximate to felsites. Garnet, schorl, etc., sometimes present. HAPLITES (Aplites). Similar to the preceding ; poor in mica, not porphyritic. QuARTZ-PoEPHYRiES (Qnartz-felsite). Porphyritic crystals of quartz, sometimes also of orthoclase, in a micro-crystalline or crypto-crystalline groundmass of quartz and felspar. A little mica often present, also schorl. At times the groundmass is micro-pegmatitic,and when this is the case the rock is sometimes termed GRANOPHYRE. Rosenbusch, mainly adopting Vogelsang's classification, divides the rocks which he includes under the head of Quartz-porphyry into Microgranite and Granophyre (with crystalline granular groundmass), Felsophyre (with felsitic groundmass), and Vitrophyre (with vitreous groundmass), the Felsophyres embracing devitrified rhyolites, pitchstones, and obsidians, and the Vitrophyres including pitchstones in their comparatively unaltered condition. FELSPAR-PORPHYRIES. The only porphyritic constituent is felspar in a micro- or crypto-crystalline groundmass of quartz and felspar. PEGMATITE. Coarsely crystalline granite occurring in segregation-veins. Graphic granite is a variety in which the felspar and quartz form a regular inter- growth, the two minerals having a definite mutual crystallographic orientation. Micro-pegmatite is a similar arrangement of quartz and felspar on a purely micro- scopic scale. GREISEN. Essential constituents quartz and mica, usually a lithia mica. Topaz and tinstone often present. The stockwerksporphyr of German miners contains black mica and more or less chlorite. Accession of orthoclase causes greisen to pass over into granite. 3. GRANITE GROUP (Holocrystalline-granular rocks). GRANITES. Sss. Con. Orthoclase (often microcline, almost invariably some plagioclase), mica (muscovite, biotite), quartz. INTERMEDIATE SERIES 17 Ace. Con. Apatite, sphene, zircon, schorl, topaz, andalusite, dichroite, garnet, or- thite, triphyline, monazite, pyrites, tinstone, wolfram, oligoclase, magnetite, ilmenite. In those granites which carry tinstone, schorl and topaz are very generally present. Sec. Con. Chlorite, epidote, calcite, talc, kaolin, hematite. Typical or NORMAL GRANITE contains both muscovite and biotite. In BIOTITE- GRANITE or GRANITITE the only mica is biotite. Other micas, e.g. lepidolite, lepi- domelane, etc., are sometimes present in granites. HORNBLENDE-GRANITES contain black or green hornblende ; AUGITE-GRANITES, pale green augite (malacolite). HYPERSTHENE-GRANITE, consisting of potash felspar, chiefly microcline, hyper- sthene, blue quartz, and a small amount of garnet, has been found by Mr. T. H. Holland in the Madras Presidency. He proposes the name CHARNOCKITE for this rock. 4. TRACHYTE GROUP. HYALO-TRACHYTES. Vitreous rocks, of restricted occurrence, with porphyritic crystals of sanidine, augite, and biotite. Glass generally brown or yellow, some- times assuming a microfelsitic character. Magnetite usually present. TRACHYTES. Holocrystalline-porphyritic or hypocrystalline-porphyritic rocks with microlitic, sometimes partly vitreous groundmass. The latter generally con- sists, however, of microlites of sanidine. Hornblende, augite (malacolite), bronzite, hypersthene or biotite may be present. Ace. Con. Apatite, sphene, zircon, dichroite, haiiyne, nosean, olivine, spinel, lavenite, rinkite, quai'tz, tridymite, magnetite, ilmenite. Sec. Con. Chlorite, alunite, chalcedony, hyalite, opal, pyrites. In HORNBLENDE- TRACHYTES the amphibole may be ordinary green hornblende or actinolite. In AUGITE- TRACHYTES the pyroxene may be malacolite or the soda varieties, acmite and aegirine. The MICA-TRACHYTES contain biotite. The trachytes are characterised by presence of sanidine and absence of quartz. When quartz is present the rocks approximate to rhyolites. 5. LAMPROPHYRE GROUP (in part). This group embraces the rocks generally known as mica-traps, including the minettes (related to the mica syenites), the kersantites (related to the mica-diorites), and the vogesites. MINETTES. Essentially dyke-rocks of holocrystalline-porphyritic character, consisting mainly of orthoclase and biotite. The felspar is sometimes partly plagio- clastic, as in some of the mica traps of Westmorland, in which case the rock occupies a position between minette and kersantite. HORNBLENDE VOGESITES consist of orthoclase and hornblende. AUGITE VOGESITES consist of orthoclase and augite. These rocks also contain magnetite, apatite, epidote, chlorite, calcite, and occasionally a little biotite and olivine. In structure the vogesites are granular and sometimes porphyritic. BOSTONITES are dyke-rocks of holocrystalline-porphyritic character, consisting of orthoclase, anorthoclase, and quartz in a groundmass composed of small lath- shaped crystals of orthoclase. These dykes occur chiefly in the Lake Champlaiu. district between New York and Vermont, U.S.A., and also near Montreal in Canada, where they are associated with elseolite syenites. 6. SYENITE GROUP (Holocrystalline-granular or porphyritic rocks). SYENITE (Hornblende-syenite) is essentially composed of orthoclase and horn- blende ; AUGITE-SYENITE of orthoclase and augite; and MICA-SYENITE of orthoclase and biotite. 18 INTERMEDIATE SERIES Ace. Con. Oligoclase, apatite, sphene, zircon, magnetite, ilmenite, rutile. Sec. Con. Chlorite, talc, epidote, calcite, kaolin, hematite. True syenites contain no quartz, or but a vei-y small amount. An increase in quartz causes these rocks to pass over into granites. 7. TRACHYTE GROUP (NEPHELINE SERIES). HYALO-PHONOLITES. Vitreous rocks of rare occurrence. They occasionally form crusts on phonolite lavas, as in the Canary Isles. PHONOLITES. Holocrystalline- or hypocrystalline-porphyritic rocks. The por- phyritic constituents are sanidine, nepheline, or leucite with pyroxene (either malacolite or aegirine). The groundmass is mostly holocrystalline and composed of the above minerals, but it is sometimes hypocrystalline or vitreous. Nepheline and leucite may both be present. Ace. Con. Hornblende, biotite, sodalite, haiiyne, nosean, melanite, spinel, olivine, wollastonite, tridymite. Sec. Con. Natrolite, analcime, diaspore, muscovite, chlorite, kaolin, quartz. 8. SYENITE GROUP (DYKE-ROCKS OF THE NEPHELINE SERIES). BOROLANITE. Ess. Con. Orthoclase, decomposition products after nepheline, melanite, augite and biotite. Occurs at Loch Borolan in Sutherlandshire. TINGUAITE. Ess. Con. Orthoclase, elaeolite, and aegirine in a crystalline- granular groundmass of the same minerals. Lavenite and rinkite usually present. Chiefly met with in Portugal and Brazil. 9. SYENITE GROUP (NEPHELINE SERIES). The rocks of this series are holocrystalline-granular or porphyritic. They consist of orthoclase and elaeolite, with one or more of the following minerals : augite, hornblende, arfvedsonite, biotite, zircon, sodalite, eudialyte, (cancrinite, which may replace elseolite). Ace. Con. Sphene, apatite, magnetite, ilmenite. Sec. Con. Natrolite, calcite, rutile, anatase, limonite. FOYAITE. Ess. Con. Orthoclase, elseolite and augite. ELJ;OLITE SYENITE. and hornblende. ZIRCON SYENITE. DlTROlTE. EUDIALYTE SYENITE. MlASCITE. hornblende and zircon, cancrinite, hornblende, and sodalite. elaeolite, ai'fvedsonite, sodalite, eudialyte. ,, biotite and quartz. 10. ANDESITE GROUP. HYALO-DACITES differ from the vitreous rocks of the rhyolite group in contain- ing oligoclase or aiidesine instead of sanidine. Small crystals of biotite, hornblende, or augite and quartz occur in a yellowish or brownish glass, often denitrified by globulites. Spherulitic growths not uncommon. DACITES (Quartz-andesites) consist of a groundmass which may be a holo- crystalline aggregate of felspar and quartz, or it may be microfelsitic, or, again, it may be composed of small crystals or microlites of felspar with a certain amount of vitreous matter, in which case the microlites lie with their longest axes in the INTERMEDIATE SERIES 19 direction of flow. In one or other of such groundmasses porphyritic crystals of plagioclase (generally andesine or labradorite), biotite, hornblende and quartz occur. Augite and enstatite sometimes present. HYALO-ANDESITES. The main difference between these rocks and the hyalo- trachytes is in the nature of the felspar and in the chemical composition of the glasls. There are andesite-pitchstones, andesite-obsidians, and andesite-pumice. Variolites are devitrified, coarsely-spherulitic hyalo-andesites. ANDESITES. Holocrystalline or hypocrystalline-porphyritic rocks. The ground- mass may be microlitic, microfelsitic or cryptocrystalline, sometimes partly vitreous, at others wholly so. A hyalopilitic groundmass is of common occurrence. As a rule the groundmass contains more or less vitreous matter. The porphyritic minerals are plagioclase and a ferro-magnesian silicate. Oligoclase, andesine, and labradorite generally occur in the hornblende-andesites and mica-andesites, while bytownite and anorthite are most common in the augite- and enstatite-andesites. The essen- tial constituents of the different varieties are shown on the Table, p. 10; Ace. Con. Apatite, zircon, sphene, garnet, dichroite, olivine, orthite, magne- tite, ilmenite, specular-iron, tridymite. Sec. Con. Chlorite, epidote, kaolin, muscovite, leucoxene, calcite, aragonite, chalybite, chalcedony, opal, alunite, limonite, pyrites. 11. LAMPROPHYRE GROUP (in part). KERSANTITES differ from minettes in the felspar being chiefly plagioclastic. In addition to biotite, they sometimes contain augite or hornblende. Ace. Con. Orthoclase, quartz, sphene, ilmenite, magnetite, apatite, olivine, pyrites. Sec. Con. Talc, chlorite, epidote, calcite, uralite. CAMPTONITES consist of plagioclase, hornblende, augite, and a little biotite, often with a small amount of vitreous matter. Ace. Con. Apatite, ilmenite, magnetite. Sec. Con. Chlorite, delessite, analcime, calcite. Camptonite dykes occur in New Hampshire, U.S.A., and near Montreal. MALCHITE occurs as dykes in the Odenwald, Germany. The groundmass con- sists of labradorite, hornblende, and quartz, forming a granular aggregate, in which porphyritic crystals of biotite, labradorite, and quartz occur rather sparsely. 12. DIORITE GROUP. Holocrystalline-granular rocks consisting essenti- ally of a plagioclastic felspar (of any species from oligoclase to anorthite) and a ferro-magnesian silicate which in NORMAL DIOEITES is hornblende ; in AUGITE DIORITES, pale-green augite ; in ENSTATITE DIORITES, enstatite ; and in MICA DIORITES, biotite. There are also quartz-diorites, quartz-augite diorites, and quartz-enstatite diorites. TONALITE is a quartz-mica diorite. The CORSITES are those diorites in which the felspar is anorthite. Ace. Con. Biotite, zircon, sphene, garnet, hypersthene, spinel, apatite, orthite, quartz, magnetite, ilmenite. Sec. Con. Chlorite, epidote, actinolite, uralite, calcite, quartz. 13. ANDESITE GROUP (NEPHELINE SERIES). The existence of a nepheline-bearing series of andesites is, as yet, scarcely established, but it is rendered probable by the occurrence of haiiyne-andesites in the Canary Islands, Nassau, and elsewhere. Sauer considered that a little nepheline was present in the Canary Islands andesites. 20 BASIC SERIES HAUYNE-ANDESITES consist of plagioclase, hornblende, augite, haiiyne, sphene, magnetite, apatite, and a little zircon in a hyalo-pilitic groundmass, consisting of plagioclase, augite, magnetite, and vitreous matter crowded with opaque crystallites and trichites. Rosenbusch regards these rocks as closely related to the nepheline- tephrites. LITCHFIELDITE, consisting essentially of nepheline and albite, may be regarded as a plutonic representative of the nepheline series of andesites. Rocks of similar composition, but containing in addition biotite, hornblende, sodalite, calcite, etc., occur in Ontario, Canada. 14. BASALT GROUP. HYALO- BASALTS. These basic vitreous rocks, known as tachylyte, hyalomelane, etc., are not of very common occurrence, nor do they ever constitute independent rock-masses, but occur either as crusts on the surfaces of lavas, or as selvages to the margins of dykes, and seldom, in the one case or the other, do they generally exceed one or two inches in thickness. The basalt-glass of the Sandwich Islands is often quite spongy in texture, owing to the numerous vesicles which it contains. These are due to the expansion of aqueous vapour during the solidification of the lava, and the thin glassy walls of such vesicles, becoming drawn out in cobweb-like threads, constitute the well-known " Pele's hair " of these volcanoes. Sometimes the basalt-glass is but slightly, if at all, vesicular, and forms a thin, slaggy crust on the lava. Small crystals of olivine and felspar are usually present, the former often in delicate skeleton crystals of the chiasmolitic type. The tachylytes forming the selvages of dykes are mostly of dull black or dark brown colour, and nearly opaque, owing to separation of magnetite. They are frequently spherulitic. They pass by decomposition into yellow, brown, or greenish palagonite. BASALTS. Holocrystalline-porphyritic to hypocrystalline-porphyritic basic rocks, frequently ophitic. Groundmass sometimes partly, occasionally wholly, vitreous. Ess. Con. A plagioclastic felspar, generally labradorite or anorthite, with (in NORMAL BASALTS, augite), (in OLIVINE BASALT, augite and olivine), (in HORNBLENDE BASALT, augite and hornblende), (in MICA-BASALT, augite and biotite), (in QUARTZ-BASALT, augite, olivine, and quartz). HYPERSTHENE-BASALTS consist essentially of plagioclase, augite, olivine, and hypersthene, with a large amount of dark-brown vitreous matter forming the groundmass, to which numerous microlites of plagioclase and augite impart an almost hyalo-pilitic character. The hypersthene-basalts form a connect- ing link between the basalts and andesites. They occur chiefly in Oregon, U.S., and Salvador, Central America. Ace. Con. Apatite, zircon, bronzite, magnetite, ilmenite, native-iron, specular- iron, pseudobrookite, rutile, perowskite, and occasionally tridymite and graphite. Sec. Con. Chlorite, delessite, chlorophaeite, epidote, calcite, aragonite, chaly- bite, leucoxene, serpentine, kaolin, muscovite, quartz, chalcedony, and zeolites. 15. BASALT GROUP (NEPHELINE SERIES). This series may be separated into two divisions. In the first, both felspar and nepheline are present. In the second division, felspar is absent, and nepheline is not present in all cases. Nevertheless, the rocks of this division may be regarded as potentially felspathic, leucitic or nepheline-bearing, since the chemical composition of the vitreous matter which often forms their groundmass is such as to warrant 21 the conclusion that, under suitable conditions, plagioclastic felspars, leucite and nepheline would have crystallised out. 1. Felspathic Division. (Headed " NEPHELINE SERIES " in Table, p. 9.) TEPHRITES consist essentially of a basic lime-soda plagioclase with nepheline or leucite and augite (basaltic augite), and in some cases glassy matter, which in certain Vesuvian tephrites is sufficient to impart a strongly vitreous aspect. BASANITES differ from tephrites in containing olivine in addition to the other constituents. In the tephrites and basanites the nepheline may be replaced by leucite, or both minerals may be present. Ace. Con. Aegirine, hornblende, biotite, haiiyne. These rocks are related, not only to the basalts and non-felspathic rocks of the basalt group, but also to the phonolites. 2. Non-felspathic Division. (Headed " NON-FELSPATHIC " in Table, p. 9.) NEPHELINITE. Essential constituents, nepheline and augite. Ace. Con. Apatite, sphene, haiiyne, melanite, biotite, hornblende, aegirine, magnetite, ilmenite. NEPHELINE-BASALT differs only from nephelinite in containing olivine. The nephelinites and nepheline-basalts are for the most part holocrystalline-porphyritic. LEUCITITE differs from nephelinite in containing leucite instead of nepheline. LEUCITE-BASALT differs from nepheline-basalt in containing leucite instead of nepheline. MELILITE-BASALT consists essentially of melilite and augite. A glassy basis has in some cases been noted. Ace. Con. Olivine, biotite, apatite, chromite, perowskite, picotite, magnetite. Sometimes haiiyne and nepheline. Melilite-basalt is closely related to alnoite. AUGITTTE consists essentially of augite and magnetite, the former occurring in crystals and microlites, the latter in a granular condition, or in the form of trichites embedded in a vitreous base. Ilmenite and apatite almost always present. Ace. Con. (occasionally in small quantity). Plagioclase, nepheline, leucite, haiiyne, sphene, perowskite and chromite. In some cases the augite may, to a limited extent, be represented by hornblende. LIMBURGITE merely differs from augitite in containing olivine. The augitites and limburgites were included by Boricky under the title " Magma Basalt." 16. DOLBRITE GROUP (Diabase of Continental writers). Holocrystalline- granular basic rocks, occurring not only as dykes and intrusive sheets or sills, but passing upward into the basalts, so that they must, at times, be in part regarded as truly volcanic rocks. Lower portions of basalt lava-streams often doleritic in character. Ophitic structure common, also frequently porphyritic and amygda- loidal. Vitreous matter generally absent, never plentiful. The essential constituents of dolerites are a plagioclastic felspar (which may be oligoclase, andesine, bytownite, but generally labradorite or anorthite) with augite (basaltic augite, sometimes approximating to diallage, less frequently mala- colite). NORMAL DOLERITE = plagioclase + augite. HORNBLENDE-DOLERITE OR PROTEROBASE = plagioclase + augite + hornblende. OLIVINE-DOLERITE = plagioclase + augite + olivine. 22 BASIC SKBIKS ENST ATiTE-DOLEiu'f E = plagioclase + augite + enstatite. MICA-DOLKRITE = plagioclase + augite + biotite. LEUCOPHYRE is a name given by Giimbel to a rock composed of plagioclase, generally passing into saussurite, a little augite, ilmenite, and some quartz. Ace. Con. of dolerites are apatite, ilmenite, magnetite, all so common that they may almost be regarded as essential ; glaucophane, sphene, zircon, pyrites. Sec. Con. Chlorite, serpentine, epidote, calcite, leucoxene, actinolite, uralite, quartz, chalcedony, opal, limonite. 17. BASIC NEPHELINE SERIES. DYKE ROCKS OF THE NON- FELSPATHIC DIVISION. FOURCHITE, from Arkansas, consists of augite, hornblende, and biotite in a vitreous or microfelsitic base. MONCHIQUITE, occurring in the Lake Champlain Valley, Vermont, U.S., is similar in composition to fourchite, but contains olivine. The chemical corn-position of the base in these rocks renders it probable that, under suitable conditions, nepheline and plagioclase would have crystallised out. These dykes are associated with nepheline-bearing rocks. The same may be said of ALNOITE, which consists of biotite in large plates, olivine, augite and magnetite in a groundmass composed of melilite and' biotite, with a small quantity of augite and magnetite. Ace. Con. Apatite, perowskite. Sec. Con. Garnet after melilite and calcite. Occurs as dykes in eleeolite syenite, Sweden. Fourchite, monchiquite, and alnoite maybe regarded as lamprophyres of the basic series. 18. GABBRO GROUP. Holocrystalline-granular basic rocks, often coarse in texture, consisting essentially of a plagioclastic felspar (commonly anorthite, sometimes labradorite or bytownite) and a ferro-magnesian silicate which, in NOR- MAL-GABBRO, is augite ; in HORNBLENDE-GABBRO, augite and hornblende ; in OLIVINE- GABBRO, augite, hornblende, and olivine. TESCHENITE consists of plagioclase, augite, hornblende and biotite. EUCRITE consists essentially of anorthite and augite. TROCTOLITE (Forellenstein) contains plagioclase and olivine. NORITE consists of plagioclase and enstatite, or plagioclase and hypersthene. When olivine is present, these rocks are termed OLIVINE-NORITES. QUARTZ NORITE occurs in the Baltimore gabbro area in Maryland, U.S., and consists chiefly of bytownite, quartz, hypersthene, and secondary hornblende. Ace. Con. Apatite, zircon, picotite, chromite, garnet, quartz, pyrrhotine, mag- netite, titaniferous magnetite, ilmenite, native-iron, biotite, rutile, pyrites, ortho- clase. Sec. Con. Chlorite, epidote, serpentine, saussurite, calcite, garnet, spinel, leu- coxene, magnetite. 19. GABBRO GROUP (NEPHELINE SERIES). THERALITE. A holocrystalline-granular rock, consisting of a lime-soda felspar, nepheline and augite. Biotite is also present, and apatite ; occasionally olivine. Magnetite and ilmenite are poorly represented. Theralites occur in Montana, U.S. They bear the same relation to the tephrites that the elasolite-syenites bear to the phonolites. ULTRA-BASIC SERIES 23 20. BASIC NEPHELINE SERIES. PLUTONIC ROCKS OF THE NON- FELSPATHIC DIVISION. IJOLITE (pronounced lolite, like the mineral of that name), from Iwaara in Finland, consists mairly of nepheline, with a little hornblende or mica. It is a holocrystalline rock with a granitoid structure. A nepheline syenite from Dun- gannon, Ontario, described by Dr. F. D. Adams, approximates in places to ijolite. 21. PERIDOTITE GROUP. Holocrystalline, hypidiomorphic-granular, ultra-basic rocks. Either olivine, or secondary products after olivine, constantly pi'esent. Felspar absent, except, occasionally, a very small amount in the picrites. In addition to olivine, minerals of the amphibole, pyroxene, garnet and spinel groups may occur in these rocks. PICRITE. Essential constituents, olivine and augite ; a very small amount of plagioclase is sometimes present as an accessory constituent. HORNBLENDE PICRITE consists of olivine and hornblende ; a little plagioclase or its decomposition products may be present, and occasionally a micaceous mineral allied rather to the chlorite than to the mica group. Ace. Con. Apatite, biotite, magnetite, ilmenite, and, in some cases, diallage and hypersthene. Sec. Con. Chiefly serpentine, talc, and chlorite. SCYELITE. An altered hornblende-picrite, occurring in Caithness. It consists of hornblende (actinolite), a peculiar micaceous mineral of secondary origin, ser- pentinised olivine, and a little magnetite and chromite. MICA-PERIDOTITE consists essentially of serpentine (after olivine), biotite and perowskite. Ace. Con. Apatite and magnetite. Sec. Con. Chlorite, calcite, etc. The biotite is an original constituent, and in this and in the absence of hornblende the rock differs from scyelite. WEHRLITE. Ess. Con. Olivine and diallage ; often, however, only represented by the decomposition products, serpentine and chlorite. EULYSITE may be regarded as a garnetiferous wehrlite. Often serpentinous and the garnets altered to kelyphite. LHERZOLITE. Ess. Con. Olivine, diallage, and a rhombic pyroxene (generally enstatite or bronzite). One or more minerals of the spinel group are usually present (picotite, chromite, pleonaste), together with some magnetite and ilmenite. SAXONITE or HARZBURGITE. Ess. Con. Olivine and a rhombic pyroxene (bronzite, often altered to bastite). Ace. Con. Magnetite, ilmenite, hornblende, biotite, chromite, picotite, and occasionally a very little plagioclase. CORTLANDITE. Ess. Con. Olivine, hornblende, and hypersthene. DUNITE consists essentially of olivine and chromite. Picotite, pyrope, mag- netite, ilmenite, apatite, and, in some cases, a very little enstatite are present. Through decomposition the rocks of the peridotite group pass into serpentines. 22. PYROXENITE GROUP. This group includes certain holocrystalline- granular plutonic rocks, which consist of one or more species, or varieties, of pyroxene or amphibole. HORNBLENDITE = hornblende + augite. ECLOGITE = smaragdite + omphacite + garnet. BRONZITITE = bronzite. HYPERSTHENITE = hypersthene. WEBSTERITE = enstatite + diallage. DIALLAGITE = diallage. 24 ALTERED ERUPTIVE ROCKS ALTERED ERUPTIVE ROCKS. The most important altered eruptive rocks constitute several groups, the names of which are given on the tables in italics. 23. Felsite Group. There are felsites which are essentially apophyses of plutonic rocks, such as granites and syenites, those of granitic origin being more or less closely allied to the haplites and microgranites, while those originating from syenitic masses would be naturally poorer in silica. Such rocks do not belong to the group under consideration, which includes only those felsites which have resulted from the devitrification of lavas of the rhyolite and occasionally of the trachyte groups. The felsites of the former differ from those of the latter group in the proportion of silica present and in the nature of the felspathic constituent. The general characters of such felsites have already been given under the " Rhyo- lite Group," p. 15. 24. Arkose. This is a more or less coherent rock, consisting of the debris of granite, and either without a cementing material, or cemented by kaolin, etc. China-stone results from the decomposition of granite, and consists of kaolin and quartz. Kaolin results from the decomposition of the felspars. 25. Porphyrite Group. Porphyrites are altered andesites, the felspars having been more or less converted into kaolin, the hornblende and augite into chlorite, serpentine, etc., the hypersthene into bastite, and the magnetite into hematite or limonite. Biotite also is often represented only by pseudomorphs of limonite. Each member of the andesite group may have its representative among the porphy- rites ; thus the hornblende andesites pass into hornblende porphyrites, the mica andesites into mica porphyrites, and so on. The porphyrites resulting from the alteration of rocks of the dacite sei'ies contain more or less quartz. The glassy matter present in the andesites is converted in the corresponding porphyrites into devitrification products. 26. Propylite Group. The rocks termed propylites represent an altered con- dition of hornblende- and augite-andesites and dacites. The alteration appears to have been effected by solfataric agency and by the action of H 2 S, and consists in the decomposition of the felspars, amphibole, pyroxene, mica, etc., and the develop- ment of calcite, chlorite, epidote, actinolite, and pyrites. The glassy matter in the original rocks has also been completely altered. Valuable ore deposits are -not unfrequently associated with propylites. 27. Melaphyre Group. The melaphyres are altered basalts, and bear the same relation to those rocks that porphyrites do to andesites. There may, therefore, be as many varieties of melaphyre as of basalt. The secondary minerals present in melaphyres are chlorite, epidote, serpentine, dolomite, calcite, magnetite, and limon- ite. The cavities in the amygdaloidal melaphyres are often filled with chlorite, delessite, calcite, and chalcedony, or with zeolites (scolecite, thomsonite, natrolite, mesolite, stilbite, heulandite, phillipsite, gismondine, chabazite, etc.). 28. Diabase Group. The rocks of this group are more or less altei'ed dolerites, and, like the melaphyres, are rich in secondary minerals. Among these, chlorite is one of the most prevalent. Epidote, serpentine, leucoxene, magnetite, and calcite are also of frequent occurrence, as well as zeolites (analcime, natrolite, etc.). Diabase constitutes much of the " greenstone " of the older authors. 29. The Ophites of the Pyrenees are closely related to diabase. They contain ALTERED ERUPTIVE ROCKS 25 no olivine and are essentially plagioclase-augite rocks, in which the augite is mostly converted into uralite. Ilmenite, generally altered to leucoxene, apatite, epidote, chlorite, etc., are frequently present. In these rocks the so-called ophitic structure is typically developed. 30. Epidiorites are altered dolerites in which the augite is represented by secondary hornblende (actinolite, smaragdite, uralite). The felspars are generally much altered. Chlorite, ilmenite, and magnetite are commonly present. 31. Euphoiides are altered gabbros, in which the felspar has been converted into saussurite arid the augite into hornblende. 32. Serpentines. Although serpentine occurs in not inconsiderable quantity in the basic rocks, owing to the alteration of olivine, pyroxene, and amphibole, yet the ultra-basic rocks, which are composed almost exclusively of such minerals, yield, upon decomposition, rocks consisting wholly of serpentine. These true ser- pentines are often veined by the fibrous variety, chrysotile, and by steatite. They sometimes retain vestiges of olivine, diallage, etc., in an unaltered condition. The structure of the original minerals, irregular cracks, as in olivine, or regular cleavage- planes, as in augite and hornblende, may frequently be detected under the micro- scope. Thus, serpentine after olivine shows an irregular mesh-structure ; after hornblende a lattice-structure, and after augite a rectangular grid-structure, the bars of the lattice- and grid-structures respectively intersecting in angles which correspond, in the first case with those formed by the prismatic cleavages of hornblende, in the second with those formed by the corresponding cleavages in augite. Any considerable admixture of calcite with serpentine constitutes the rock known as ophicalcite. 33. Peridotite Dykes. Dyke representatives of the peridotite group have been described by Diller as ti'aversing slates and sandstones of carboniferous age in the East of Kentucky. They have the general character of Dunite and consist of olivine and pyrope in a more or less altered condition with enstatite, ilmenite, magnetite, and about 14 per cent, of dolomite. The olivine is partly altered to serpentine, and the pyrope to biotite and picotite. 34. Kimberlite and picrite-porphyrite may be regarded as volcanic representa- tives of the peridotites, the former occurring at the Kimberley diamond mines, South Africa, and consisting of olivine, bronzite, and biotite, more or less altered, together with some ilmenite, pyrope, etc., in a serpentinous groundmass. This rock in a still more decomposed condition and containing fragments of carbonaceous slate constitutes the " blue-ground " of the miners, in which the diamonds are found. Picrite-porphyrites consist of olivine, augite, ilmenite, magnetite, apatite, etc., with a not inconsiderable amount of vitreous matter. Those hitherto described appear to be mostly of Jurassic and Cretaceous age. 35. Tuffs are essentially pyroclastic rocks, or such as consist of fragments of lavas (scoria, pumice, lapilli), of lavas reduced to mere dust, or of rock matter originally ejected in a molten condition, and ranging from masses a foot or more in diameter (bombs) to microscopic drops of vitreous matter. Among such heterogeneous material small crystals, entire or broken, and occasionally fragments of plutonic or sedimentary rocks may occur, the latter having been brought up the volcanic pipe from considerable depths. These ejectamenta may eventually become cemented and strongly coherent. From their mode of origin, tuffs naturally vary greatly in their composition, and, when it can be ascertained, it is custom- ary to prefix the name of the lava from which the constituents have been mainly derived, as melaphyre- tuff, andesite-tuff, etc. Strictly speaking, the term tuff is restricted to the finer grained or pulverulent volcanic ejectamenta ; but, in a broader sense, it may include not only accumulations of scoria and lapilli, but also the materials resulting from the decomposition and atmospheric degradation of lavas. / 26 GRANITES AND GREENSTONES EXPLANATORY NOTES ON THE DETERMINATIVE TABLES. 1. In the column headed Cryst. Syst. the following abbreviations are used: Am. = amorphous. Gran r . = granular. Fib 8 . = fibrous. Cub". = cubic. Hex.= Hex. Hex. hexagonal. ~ = rhombohedral. ~~T~ = tetartohedral. Tet. = tetragonal. Rh c .=rhombic. Mon. = monoclinic. Tri. = triclinic. A symbol such as n ' p. hex. indicates that the mineral is really monoclinic in crystallisation, although ap- parently hexagonal. 2. In the column headed Cleavage, only the most easy cleavages are, as a rule, given. The symbols of the forms to -which the cleavages are parallel are in the Weiss-Miller notation. The corresponding symbols in Naumann's notation are : Cubic System: O = (111). ooOoo=(100). ooO=(110). Hexagonal System: P= (1011). ocP=(10lO). ooP2=(1120). OP=(0001). (Rhombohedral): R = ic(10ll). R = ic(10lO). -|R=*c(OOl2). OR = K (0001). Tetragonal System: P = (111). ooP=(110). ooPoo =(100). OP=(001). Rhombic System: P=(lll). ooP=(110). ccPdb=(100). ooPob=(010). OP=(001). Monoclinic System : o>P=(110). o>Poo=(100). ccPdb =(010). OP=(001). Triclinic System: ooP/=(110). co' ; P=(110). ooPdb=(100). ooPob = (010). OP=(001). 3. In the column headed Opt.' Sign, the positive + or negative character of the double refraction of the mineral is shown. In + uniaxial crystals, i.e., those crystallising in the hexagonal and tetragonal systems, c c, while in uniaxial crystals a=c. (Determined by mica-plate or selenite-plate.) In optically biaxial crystals, those in which c is the acute bisectrix (determined by quartz-wedge, Klein's plate, selenite-plate or mica-plate) are +, while those in which a is the acute bisectrix are . 4. In the column headed Extinctions, the extinction in uniaxial crystals is parallel and at right angles to the principal crystallographic axis, while sections at right angles to that axis remain dark during a complete revolution between crossed nicols. The only exception to this rule is to be seen in thick basal sections of crystals possessing circular polarisation (e.g. quartz and cinnabar), but in thin sections circular polarisation gives no appreciable sign of its existence. In the rhombic system the directions of maximum extinction are parallel to crystallographic axes (straight extinction). In the monoclinic system the extinction angle is measured from the vertical crystallographic axis c, or from an edge parallel to that axis. In the triclinic system it may be measured from an edge. In the plagioclastic felspars the edge chosen is that formed by the basal and brachypinacoids, OP/ooP6b . These measurements are best made upon cleavage-plates GRANITES AND GREENSTONES 27 taken respectively parallel to (001) and to (010). Such extinctions are termed positive or negative according to the direction in which the crystal has to be turned in order to attain its maximum extinction. These direc- tions are indicated in the annexed figure, where E B re- presents the edge 01/010. The expressions positive and negative, as here applied, bear no relation to the + or sign of double refraction. (Determined between crossed nicols only, or in conjunction with Bertrand's stauroscope eye-piece or Calderon's, or Brezina's plate.) 5. In the column headed Relief, the following scale, that of MM. Michel Levy and Lacroix, is employed. Mean Refraction N= or 3 1. 2. 3. 4. 5. 6. 7. ^1'55 Little or no relief = 1-55 to 1-60 1-65 1-70 1-75 2- = 1-61 = 1-66 = 1-71 = 176 very weak. . weak, medium, marked. . strong. 5 very strong. 6 extremely strong. 7. The relief in which a mineral appears is pi'oportional to the difference between the mean refraction of the mineral and that of the medium which surrounds it. Thus albite with N"=1'535 is barely visible when mounted in Canada balsam in which N"=1'530, while rutile, with N=2'712, mounted in that medium, appears strongly outlined and with a very broad black border due to total reflection. In some cases, owing to the same cause, the surface of the section looks rough or shagreened. 6. In the column headed Inter/. Col., the colours seen in polarised light, in sections of average thickness, i.e., from about 0'04 to 0'06 of a millimetre, are given. The interfei-ence colour is due to the unequal retardation of the two polarised beams of light in their passage through a doubly refracting crystal. The colour is influenced (1) by the thickness of the section, (2) by the direction in which the crystal is cut, (3) by the colour of the light in which it is examined. The strength of the double refraction, y a or n g n p , attains its maximum, in uniaxial crystals, in sections parallel to the optic axis and in biaxial crystals in sections parallel to the optic axial plane. To compare, therefore, the relative strength of double refraction in any two minerals it is necessary that the sections should be cut in that direction in which they exhibit the strongest colours in polarised light. The expression very low indicates that part of Newton's colour-scale repre- sented by the greys and pale-yellow of the first order, as seen at the thin end of a quartz-wedge. Low is employed for colours which do not range much above the first order. High denotes colours of the second, and lower colours of the third order. Very high implies colours of the third order, and extremely high those of the fourth order. 7. In the column marked 2 E, the apparent angle of the optic axes is given. 28 GEANITES AND GREENSTONES This may be roughly estimated by means of an eye-piece micrometer, in which the value of the divisions in degrees has been previously ascertained by measuring the separation of the vertices of the dark brushes of the interference-figure, when seen in the diagonal position, in a biaxial mineral, of which the optic axial angle has already been determined on an axis-angle goniometer, or by means of a stage- goniometer attached to the microscope. 8. In the column headed Colour in thin section, the colours mentioned are those seen in ordinary transmitted light. In some minerals, such as cossyrite and chro- mite, it is necessary that the section should be excessively thin in order to transmit light. 9. In the column headed Sign in the direction of elongation, the positive or negative character of the double refraction, with reference to the zone of elongation or of flattening of the crystal, is given. In order that the signs in this column may not be mistaken for those given in the column relating to the double refraction of the mineral, they are not so heavily printed. 10. The final column includes notes on pleochroism, character of dispersion 1 in interference-figures, position of optic axial plane and other matters which could not conveniently be tabulated on so small a page. The absorption schemes indicate the relative absorption of light for vibrations respectively parallel to a, ft, and c. Thus : a>b>c indicates that the absorption of light for vibrations || a is greater than that for those || 6, while the absorption for vibrations || b is greater than that for those || c. Also c^6>a denotes that the absorption for vibrations j| c may, in some cases, be greater than, in others equal, to those || b, while the absorption for vibrations || ft is greater than that for vibrations II a- a, b, and c respectively denote the maximum, mean, and minimum axes of optical elasticity, respectively corresponding with the minimum, mean, and maximum re- fractive indices, a or n p , (3 or n m , and y or n g . In biaxial crystals a and c always lie in the optic axial plane and are the bisectrices, while b is the optic normal. a, 6, and c are crystallographic axes. Of these c is always the vertical axis. In the rhombic and triclinic systems a is the brachy- and b the macro- diagonal, while in the monoclinic system a is the clino- and b the ortho-diagonal. The pleochroism may be determined by means of the polariser only, the analyser being removed, or by using a double-image prism over the eye-piece, both polariser and analyser being removed and a diaphragm inserted in the focus of the eye- piece. The latter method is equivalent to an adaptation of Haidinger's dichroscope to the microscope. Haidinger's instrument can only be employed upon th'in sections when the crystal to be examined occupies a comparatively large area. . l To ascertain the character of dispersion with the Dick Microscope the lower tuhe lens should be used. The employment of an immersion objective of large angular aperture is also advan- tageous. 29 DETERMINATIVE TABLES. PAOB ;I. FELSPAR GROUP . . . . , . . . 30 II. AMPHIBOLE AND PYROXENE GROUPS . . . . .32 III. MICA GROUP, FELSPATUOID GROUP, TOURMALINE, IDOCRASE, AND TOPAZ . . .34 IV. SCAPOLITE, MEIONITE, MELILITE, ANDALUSITE, SILLIMANITE, OLIVINE, FAYALITE, DICHROITE, GARNZT, SPINEL, PLEONASTE, PICOTITE, GHROMITE .... 36 V. RUTILE, TINSTONE, ZIRCON, QUARTZ, CHALCEDONY, TRIDYMITE, OPAL, CORUNDUM, EUDIALYTE, PEROWSKITE, SPHENE, MONAZITE, APATITE . . . . .38 VI. EPIDOTE GROUP, CHLORITE GROUP, TALC, KAOLIN 40 VII. SAUSSURITE, KELYPHITE, LEUCOXENE, ZEOLITES, SERPENTINE, CHRYSOTILE . . 42 VIII. BASTITE, ANTIGORITE, CARBONATES, IRON ORES, GRAPHITE ...... 44 ABBREVIATIONS AT HEADS OF COLUMNS. Cryat. Syst. = Crystallograp7iic System. Opt. Sign = Optical Sign. Interf . Col. = Interference-colour. 2E = Apparent angle of the optic axes. 30 FELSPAR Cryst. Syst. Cleavage. Opt. Sign. Extinctions. Belief. Interf. Col. 2E ORTIIOCLASE . (Sanidine) Mon. (001) (010) 90 on (001)=0 on (010)= +5 very weak 1 low in 1st ord. 125 MICROCLINE . Tri. p. mon. n n 90 16' - on (001) = + 15 on (010) =+5 n n 130 ANORTHOCLASE Tri. p. mon. n n 902(y - on (001) =+lo to 6 on (010) =+6 n n 720-88 ALBITE . . . Ab Tri. n n 93 36' + on (001) =+5 on (010) =+20 a 1st ord. very large OLIGOCLASE . Ab 4 An 1 Tri. n 93 50' - on (001) =+2 to 5 on (010)= +4 to 7 n n Ab 3 Anj on (001)= GO on (010)= GO ANDESINE . . Ab 3 An 2 Tri. H " on (001) =-8 6' on (001)= -1 to 3 6 weak 2 n LABRADORITE AbjAn! Tri. n 93 20' + on (001)= -50 on (010)= -16 n n n Ab 3 An 4 on (001) =-7 on (010) =-21 BYTOWNITE AbjAn 3 Tri. n on (001)= -17 W on (010) = -29 28' n n ANORTHITE . An Tri. n n 940 10' - on (001)= -33 on (010) =-37 n n NOTE. In the orthoclastic felspars the optic axial plane lies as a rule at right angles to the plane of symmetry, making an angle of from 3 to 7, with the plane of the crystallographic axes a, b, in the obtuse angle /3. In soda-orthoclase this inclination may amount to 10 or 12. In certain cases (abnormal orthoclase) the optic axial plane lies in the plane of symmetry, i.e. || (010). GROUP TABLE I. ! Sign in the Colour in thin section, j direction of ! elongation. When free from in- clusions and de- composition pro- ducts, colourless. Ditto Ditto Ditto Ditto Ditto Ditto Ditto Ditto Notes. Twinning on Carlsbad type common, on Baveno and Manebach types less so. Dispersion horizontal p > u, inclined p<v. In Carlsbad type elongation || c, in Baveno type || a. Crystals of Sanidine have a tabular habit || (010). Polysynthetic twinning on Albite and Pericline types. Dispersion p>v. Dispersion p>v. Twinning on (010) polysynthetic (Albite type), also twin axis the macro-diagonal 6, and plane of composition the " rhombic section " (Pericline type). Dispersion horizontal p < v. General Note. Twinning on the Carlsbad type (plane of composition 010), on the Baveno type (plane of composition 021), and on the Manebach type (plane of composition 001), may also occur in the plagioclastic felspars. Repeated twin lamellation, although not restricted to these minerals, is very characteristic of the plagioclastic felspars. Being colourless in thin section none of the felspars exhibit pleochroism. Anorthite and Labradorite are soluble in hydrochloric acid. When doubt exists concerning the species of a felspar, Szabo's flame-test may be advantageously employed. (See Cole's " Aids in Practical Geology," pp. 74-81. Also " Bock-forming Minerals," Rutley, pp. 8-12. Dispersion inclined p< v. Broad twin lamellae and wide extinction angles are very characteristic of anorthite. Dispersion crossed p > v. Dispersion inclined p > v. Dispersion crossed p > v. NOTK. In microcline the optic axial plane is approximately perpendicular to the macrodiagonal axis b (82-83). In the plagioclastic felspars the position of the optic axial plane varies progressively from albite at one end to anorthite at the other end of the series. The extinction angles of the plagioclastic felspars, given in the above table, are the observed angles of Max Schuster, which differ but little from the calculated angles of M. Michel-Levy. 32 AMPHIBOLE Cryst. Syet. Cleavage. Opt. Sign. Extinctions. Relief. Interf. Col. 2K ANTHOPHYLTITE Eh. (110) 124 + straight medium 3 high, 2nd ord. 1770 GEDRITE . . . Eh". (110) - > > )j 88 ACTINOLITE . . Mon. (110) 124 30' - 15 with c i> high, 2nd & 3rd ord. 770 HORNBLENDE (common var.) Mon. H - 15-22 with c >i >180 HORNBLENDE (basaltic var.) Mon. >t - 0-10 with c > ARFVEDSONITE . Mon. (110) 123 55' + 14 with c marked 4 higher than horn- blende GLAUCOPHANE . Mon. (110) 124 30' - 4-6 with c medium 3 lower than arf ved- sonite CROSSITE . . . Mon. (110) 126 ? 13 with c EIEBECKITE . . Mon. (110) + 5 -7o with c marked 4 low large COSSYKITE . . Tri. (110) (110) 114 9' ? 39 with c, on (010) ? PYROXENE ENSTATITE . . Eh". (110) (010) straight marked 4 < yellow 1st ord. BRONZITE . . . Eh'. i 5> HYPERSTHENE . Eh. (HO) 92 - up to red 1st ord. DIALLAGE . . Mon. (100) (110) + 39 with c high, 2nd & 3rd ord. MALACOLITE Mon. (110) 87 + 38 with c little higher than diallage 112 AUGITE . . Mon. )i + up to 45 with c strong 5 higher than malacolite AEGERINE . . Mon. (110) (010) 87ir 3-5 with c > higher than augite NOTE. In all rhombic amphiboles and pyroxenes the optic axial plane lies || (010), except in the altered pyroxene, bastite, in which it lies || (100). 83 GROUP. Colour in thin section. TABLE II. colourless, yellowish, yellowish-green ditto grass-green, greyish- green, weak-green green, brown brown olive-green, deep green- ish-blue blue blue, yellowish-blue deep- blue or green deep - brown. Only translucent in very thin sections Sign in the direction of elongation. Notes. Pleochroism not perceptible in thin sections. Dispersion p< v. Crystals traversed by cracks || (001). Pleochroism not perceptible in thin sections. Dispersion p > v. Pleochroism weak, in green tints, c > fa > a. Dispersion p< v. (TuEMOLiTE. Colourless, not pleochroic, otherwise like actinolite.) Pleochroism strong, c and fa green, a yellowish, generally c^fa > a. Dispersion p<v. Pleochroism strong, in brown and yellow tints, r^b > a. The common var. shows like opt. characters after heating. Pleochroism strong, a greenish-blue, fa blue, c greenish-grey, a > fa > c. Pleochroism very strong, c blue, fa violet, a pale-yellow. Crystals traversed by cracks. Pleochroism very strong, a sky-blue to dark-blue, fa reddish to purplish-violet, c yellowish-brown to greenish-yellow, a = fa > c. Pleochroism very strong, c green, fa blue, a deep-blue, a > fa > c. Occurs in pantellerites. GROUP. almost colourless, yel- lowish ditto greenish, reddish, greenish-brown greenish, yellowish colourless, greenish green, brown, violet green (acmite, brown) not very translucent Pleochroism feeble, generally inappreciable. Dispersion p < v. Pleochroism feeble, a yellowish, fa yellowish, c greenish. Cleav- age-flakes || (010) give no interference-figure, but, in the altered form (bastite'), a negative bisectrix is seen. Pleochroism, a reddish-brown, fa reddish-yellow, c green. Ab- sorption weak. Dispersion p > v. Pleochroism variable, weak. Cleavage-flakes || (110) show one of the ring-systems in convergent polarised light. Dis- persion inclined. Ditto ditto Ditto ditto Pleochroism, a green, fa olive- to sap-green, c grass-green (ae- girine) ; a brown, fa clear-brown, c greenish-yellow (acmite). NOTE. In all monoclinic amphiboles and pyroxenes the optic axial plane lies in the plane of symmetry, i.e. || (010). C 34 MICA Cryst. Syst. Cleavage. Opt. Biga. Extinctions. Relief. Interf. Col. 2B. MUSCOVITE . . Mon. p. hex. (001) parallel to the traces of the basal cleavage weak 2 very high up to 3rd & 4th ord. 40-70o PAKAGONITE . Mon. n ditto ditto 70 LEPIDOLITE . . Mon. n __ ditto ditto 50-70 BIOTITE . . . Mon. n ditto medium 3 ditto 00-73 LEPIDOMELANE . Mon. n ditto ditto n n FELSPATHOID NEPHELINE . . (Elseolite) Hex. (0001) (1010) when altered || and J_c. Basal sections dark in all azimuths very weak 1. very low, not above yellow Ibt ord. LEOCITE . . . Cub c . p. rh c . Isotropic Anomalous double refraction ditto very low, grey of 1st ord. SODALITE . . Cub c . (110) difficult Isotropic ditto HAUYNE . . . Cub c . ditto i) ditto NOSEAN . . . Cub<=. ditto n ditto TOURMALINE . Hex. 2 no cleavage seen in thin sections - || and _l_ c - Basal sections dark in all azimuths medium 3 high, 2nd ord. IDOCRASE . . Tet. (110) (001) very difficult ditto strong 5 very low, grey of 1st ord. TOPAZ . . , Bh. (001) + straight medium 3 low up to red of 1st ord. 70-120 GROUP. 35 TABLE III. Colour in thin section. Sign in the direction of elongation. Notes. colourless, yellow- ish, greenish ditto ditto brown, green deep-brown Twinning plane usually J_ (001) and lying in the zone 001 : 110, the plane of composition being (001). In muscovite, paragonite, and in most lepidolites (also in anomite, a mica allied to biotite), the optic axial plane lies at right angles to the plane of symmetry, also at right angles to the guide-line of the percussion-figure. In biotite, phlogopite, lepidomelane, and zinnwaldite, the optic axial plane lies in the plane of symmetry, also parallel to the guide-line of the percussion-figure. GROUP colourless ditto colourless, blue, green, yellow ditto ditto Often altered to natrolite and analcime. Soluble in hot HC1, with separation of gelatinous silica. In elseolite, inclusions of amphibole, hematite, etc., common. Cancrinite is allied to nepheline, but is a distinct species, containing Na Co 3 and H 2 O. It occurs in ditroite. Lamellar twinning common in leucite, except in very small crystals. The latter are isotropic. The larger crystals show weak double refraction, but become isotropic at a temperature of 500 C. Readily soluble in H 01. with separation of gelatinous silica, which on evaporation yields crystals of common salt. Readily soluble in HC1, with separation of gelatinous silica, which on drying yields crystals of gypsum, if the acid used be sufficiently dilute. Ditto for nosean, except that, on drying, few or no crystals of gypsum are formed. brown, blue, vio- let, green, etc. Rarely colourless colourless, brownish colourless Pleochroism strong. O > E. Crystals often hemimorphic. Zonal structure at times. Radial aggregates common. Optical anomalies common, but not appreciable in thin sections. Double refraction occasionally +. Idocrase occurs mostly in altered limestones. Pleochroism not perceptible in thin sections. 36 SILICATES Cryst. Syst. Cleavage. Opt. Sign. Extinctions. Relief. Interf. Col. 2E. SCAPOLITK . . Tet. (110) sometimes (100; - || and J_c. Basal sections dark in all azimuths weak 2 higher than quartz, but very variable MEIONITE . . Tet. ditto - ditto ditto MELILITE . . Tet. (001) + ditto medium 3 very low, grey 1st ord. ANDALUSITE . Eh. (100) - straight medium up to blue 8 2nd ord. >180 SlLLIMANITE . Eh'. I) + n marked high, up to 4 greenish-blue 3rd ord. 40 OLIVINE . . . Eh". (100) (010) + straight marked 4 very high, up to 4th ord. >1800 FAYAUTE . . Eh". n i - n ditto higher than olivine DICHROITE . . Eh". (010) (001) straight very not above red weak of 1st ord. 1 64-150 GARNET . . . Cub c . Isotropic. Anoma- lous double re- fraction at times very strong 6 SPINEL . . . Cub c . Isotropic strong 5 PLEONASTE . . Cub". n ditto PICOTITE . . Cub c . n ditto CHROMITE . . Cub". n ditto 37 AND OXIDES. TABLE IV. Colour in tbin section. Sign in the direction of elongation. Notes. colourless Basal sections generally give a good imiaxial interference- figure in convergent light. This at once distinguishes scapolite from felspars or dichroite, while its negative double refraction distinguishes it from quartz, and its higher inter- ference-colours from apatite. colourless or yellowish Plug structure common. Gelatinises with H Cl. colourless Pleochroism sometimes strong, frequently absent. Optic axial plane || (010). Optic axial plane || (100). Dispersion p > v. Occurs chiefly in crystalline schists. colourless, green- ish, yellowish, less often red to reddish brown colourless, red- dish, yellowish Crystals often much rounded or corroded and traversed by irregular cracks. The cleavages are only seen in sections of decomposed crystals. Gelatinises with H Cl. Dispersion p<v. Optic axial plane || (001). Surfaces of sections appear rough or shagreened. Frequently altered to serpentine. Fayalite gelatinises with H Cl. Dispersion p < v. Occasionally met with in the lithophyses of rhyolites. colourless Insoluble in HC1. Often altered to pinite (=muscovite+ chlorite, etc.). Dispersion p<v. Pleochroism a blue-grey, b dark blue, c yellow. The pleochroism is barely perceptible in some thin sections. Optic axial plane || (100). colourless, green (grossular) red (almandine) brown (melanite) reddish (common garnet) Garnets occur in rhombic dodecahedra or in rounded and sometimes very irregularly-shaped grains. Zonal structure common. Slowly acted on by acids. Difficultly fusible with Na 2 Co 3 . colourless, rose- red green brown brown Not acted upon either by acids or by heating with Na 2 Co 3 . Ditto Ditto Only translucent in extremely thin sections. 38 OXIDES, TITANATES, Cryst. Syst. Cleavage. Opt. Sign. Extinctions. Relief. Interf. Col. 2E. HUTILE . . Tet. (110) 90 + || and J_ c. Basal sections dark in all azimuths extremely strong 7 extremely high 4th ord. TINSTONE . Tet. (100) 90 + ditto ditto ditto ZIUCON . . Tet. (110) 80 + ditto very strong 6 ditto QUAKTZ . . Hex. Inappreciable + ||and_|_c. Circular polarisation in- appreciable in microscope - sec- tions weak low, not above 2 red of 1st ord. 4 CHALCEDONY Hex. )> + || and J_ to length of fibres ditto TRIDYMITE . >! )! + Feeble double re- fraction. Scales viewed _|_ (0001) isotropic ditto OPAL . . . Am. none Anomalous double refraction, due to strain, especially in hyalite ditto CORUNDUM . Hex. none ~ || and J_ c. Basal sections dark in all azimuths very strong 6 low, not above red of 1st ord. 2 EUDIALYTE . Hex. 2~~ (0001) + || and _L c. medium 3 very low, grey of 1st ord. PEROWSKITE Cub". ? p. cub. (100) + Anomalous double refraction extremely strong 7 very low, not above yellow of 1st ord. 40-44 SPHENE . . Mon. (110) + _L (102) or 500 43' with a in the obtuse angle /3. very strong 6 extremely high 4th ord. nearly as high as rutile 500 MONAZITE . Mon. (001) . (100) + very strong 6 high 23-50 APATITE Hex. (0001) imperfect medium 3 very low, not above yellow of 1st ord. 39 PHOSPHATES, ETC TABLE V. Colour in thin section. Sign in the direction of elongation. Notes. yellow, brownish-red, violet colourless, yellow, brown colourless, yellowish, reddish Crystals frequently more or less rounded. Twins common on (101) and on (301), the former geniculate, the latter kite- shaped. Also in small groups of needle-like crystals forming meshes (sagenite) in conformity with twinning. Interference figure anomalous at times, the cross breaking up into dark brushes. Zonal structure common. colourless Thin basal sections show a dark cross without rings. Inclusions of Ha O and C O 2 often present. The apparently optically negative character is due to a being the axis of elongation of the fibres, the elongation being at right angles to that of quartz. The fibres of chalcedony are biaxial. Tridymite generally occurs in aggregates of overlapping scales. Becomes strongly stained when moistened with solution of fuchsine (roseaniline). colourless, blue Crystals traversed by basal and rhombohedral solution planes, simulating cleavage. Surfaces of sections appear rough. colourless, reddish Occurs chiefly in the elseolite syenites. greyish-brown, violet, brownish-yellow colourless, yellowish, reddish- brown Descloizeaux regards the dodecahedron as built up of twelve hemipyramids (monoclinic), the lozenge-shaped base of each corresponding to a face of the rhombic dodecahedron. Optic axial plane parallel to the longer diagonal of each lozenge. Dispersion horizontal. In sphene the optic axial plane lies in the plane of symmetry. Pleochroism weak, in very thin sections barely perceptible. Dispersion very strong, inclined p> v. Twinning on (001). brownish, yellowish colourless, bluish Horizontal dispersion p < v. Twinning on (100). In the spectro- scope gives absorption bands between the green and orange, due to didyrnium. Crystals frequently show gas- and fluid-inclusions. Optical anomalies common. 40 E P I D O T E Cryst. Syst. Cleavage. Opt. Sign. Extinctions. Relief. Interf. Col. 2E. ZOISITE . . . Eh. (010) + straight marked 4 very low, bare- 10- 100 ly to yellow of 1st ord. THULITE . . . Eh'. (001) + >! ditto 00-40 EPIDOTE . . . (Pistacite) Mon. (001) - 2-30 w ith c strong 5 very high, 3rd and 4th ord. >18QO PIEDMONT i TE . (Withamite) Mon. )) + 4-5 with c very strong 6 ditto >180 ORTHITE . . . (Allanite) Mon. 1 - 30 with c ditto very high, but not so high as epidote n CHLORITE ErIPIDOLITE. . Mon. p. hex. (001) _[_ (001) weak gives no complete 2 extinction very low, v. small grey 1st ord. to PENNINE . . . Mon. n ditto ditto ditto ditto CLINOCHLORE . Mon. n + 12-15 with a ditto normal to (001) low, up to blue of 2nd ord. 0-75 DKLESSJTE . . V n - JL (001) medium 8 TALC .... Mon.? (001) - respectively || b and c weak 2 very high, 4th ord. 10-20 KAOLIN . . . Mon. p. hex. M 20 with a normal to (001) very weak large 41 GROUP. TABLE VI. Colour in thin section. Sign in the direction of elongation. colourless, green, greenish-grey ros-r*d colourless, yel- lowish-green red, yellow brown Notes. Crystals very seldom show terminal faces. No perceptible pleochroism. Pleochroism very strong, a reddish-white, fa rose-red, c yellow- ish. Dispersion p < v. Crystals often twinned on (100). Optic axial plane || (010). Pleochroism varies in strength : generally a colourless, fa yellowish-green, c siskin-green. Crystals elongated on the orthodiagonal b. Dispersion inclined p> v. Crystals elongated on the orthodiagonal. Pleochroism very- strong, a orange to lemon-yellow, fa amethyst to rose-red, c deep-red. Optic axial plane || (010). Crystals elongated on the orthodiagonal. Pleochroism, a clear yellowish-brown, fa chestnut-brown, c dark greyish-brown. Optic axial plane || (010). GROUP green colourless, green- ish, yellowish Pleochroism for vibrations _|_ (001) yellowish to reddish. || (001) green. Cleavage-laminae flexible but not elastic. He- suits from the decomposition of biotite, hornblende, etc. Pleochroism for vibrations J_ (001) brownish-red, brown, yellow. || (001) emerald-green, blue-green. Ditto ditto Twinning, as in the micas, common. Optic axial plane || (010). Dispersion inclined p < v. Occurs in spherulitic groups and rosettes, in the vesicles of basic lavas. colourless The crystallisation is generally regarded as rhombic. Dispersion weak. Optic axial plane inclined about 20 behind a normal to (001). Obtuse bisectrix J_ & Kaolin is an alteration product of felspars. 42 A L T E R A T I O N - P R O D U C T S OF FEL- SAUSSUKITE . KELYPHITE . . LEUCOXENE Cryst. Syst. Gran r . and Fibrous Fibrous Gran r . to Fibrous Cleavage. Opt. Sign. Extinction*. Relief. Interf. Col. 2E. NATROLITE . SCOLECITE STILBITE . . (Desmine) PHILLIPSITE THOMSONITE . ANALCIME Eh- 1 . Mon. Mon. Mon. Rh. Cub". (110) (010) (C01) . (010) (010) (100) straight 22 with c 8 with c + 3-15 with a straight Often shows weak double refraction very weak 1 ditto ditto ditto ditto ZEOLITE low, not 94-96 above blue of 2nd ord. low, 1st ord. n very high, to bluish -grey of 4th ord. ditto ; very low, not i above iron- grey 1st ord. 56 53 83 SERPENTINE CHRYSOTILE SERPENTINE Rh.? straight very weak 1 low, 1st ord. variable up to 50 43 SPAR, GARNET, AND ILMENITE. TABLE VII. Colour in thin section. Sign in the direction of elongation. Notes. Essentially a mixture of minerals of secondary origin, chiefly zoisite or epidote with a felspar, not unfrequently albite. It results from the alteration of plagioclastic felspars in gabbros and dolerites. Saussurite is grey, greenish-grey, sometimes with a bluish tinge, occasionally almost white. It is an aggregate of fibres and granules, usually of exceedingly fine texture. Garnet, tremolite, chlorite, etc., may be present. Kelyphite forms borders to garnets (pyrope) in peridotites and serpent- ines. According to Becke, the radially fibrous portion of kelyphite consists of picotite (isotropic) and hornblende (doubly refracting) with a clear outer border, formed of a mixture of hornblende, bron- zite, and diallage. He regards it as the result of a reaction between pyrope and olivine, i.e. Pyrope + Olivine = Spinel +Hornblende Mg 3 ALSi 3 O 12 + Mg 2 Si O 4 =Mg Al 2 O 4 +Mg 4 Si 4 O 12 . In the variety of spinel, picotite, Mg is partly replaced by Fe and A1 2 by Cr 2 . In reflected light kelyphite appears pale greyish-brown. Leucoxene is an alteration product of ilmenite, sometimes granular in structure, at others fibrous, the fibres lying at right angles to the surface of the ilmenite. In reflected light it is white, yellowish, greyish, or brownish. In transmitted light it is nearly opaque, except in very thin sections, when it shows strong double refraction. The brownish colour of some leucoxene is probably due to a sagenitic intergrowth of rutile. The alteration of ilmenite may sometimes result in anatase. GROUP. colourless, yellowish colourless + Optic axial plane H (010). Dispersion /> < i JL(OIO). * II (010). (010). . ,, II (001). GROUP colourless, greenish, yellowish ditto Ordinary serpentine and the fibrous variety, chrysotile, ai'e in most cases the result of the alteration of olivine or hornblende, and often of augite. The serpentines derived from olivine show an irregular mesh-structure; those from augite a rectangular grid-structure (90) and those from hornblende a lattice-structure (124). 44 SERPENTINE Cryst. Syst. Cleavage. Opt. Sign. Extinctions. Relief. Intcrf. Col. 2E BASTITE . . . ANTIGOEITK Eh. Eh c . (010) )i - straight weak 2 d itto low, not above yellow, 1st ord. not above blue of 2nd ord. 20-90 II C A R B o - CALCITE . . . Hex. 2 K (1011) - weak 2 very high, 4th ord. ARAGONITK . . Bh. (010) straight medium 3 ditto 80 DOLOMITE . . Hex. 2 K (1011) - ditto ditto CHALVBITE . . Hex. ~2~ ?) ditto ditto IRON NATIVE IHON . Cub". Colour in reflected light. Colour in transmitted light. iron-black opaque MAGNETITE . . Cub". (Ill) bluish-black Opaque in the thinnest sections, but in extremely thin crystals forming inclusions in mica, it is transparent and of pale-brown colour ILMENITE . . Hex. 2 K (1011) iron-black, with slight tinge of brown opaque HEMATITE . . Hex. 2 iron-black to greyish- black with tinge of red. Earthy condi- tion red In thick crystals, or when earthy opaque. In very thin crystals (eisenglimmer) red, orange, yel- low. Titaneisenglimmer pinkish madder-brown LIMONITE . . Am. none yellowish-brown, dull opaque PYRITES . . . Cub c . brass^yellow n GRAPHITE . . Hex. 2 /c(0001) greyish-black to grey. No metallic lustre opaque, even in the most minute grains and thinnest sections 45 GROUP (continued). TABLE VIII. Colour in thin section. Sign in the direction of elongation. Notes. greenish Optic axial plane | pyroxene. (100). Eesults from the alteration of rhombic yellowish Plecchroism feeble. Chiefly results from the alteration of augite. NATES colourless colourless, yellowish yellowish brown Lamellar twinning on K (0112) very common. No pleochroism, but strong absorption of light, O > E. Optic axial plane || (100). Dispersion />< v. Twinning || (110), but, in rocks, aragonite only occurs in rod-like aggregates or in compact General Note. All of these carbon- No pleochroism, but strong absorption, O > E. Often becomes altered to limonite. ates are soluble in acids with effervescence, the two former in the cold, the two latter in hot acids. ORES. Occurs in the basalts of Greenland and in the gabbros of the Western Isles of Scotland. The surface of a section containing native-iron when treated with a solution of copper-sulphate has a film of metallic copper deposited upon the iron. Crystals frequently give square or triangular sections. Parallel grouping common. Also occurs in irregular grains and patches, sometimes with intermixture of pyrites. Crystals of tabular habit, giving sections, which when || K (0001) are hexagonal or trigonal, when J_ K (0001), lath-shaped. Often altered to leucoxene. Tetartohedral faces met with on the larger crystals. Specular-iron mostly in six-sided tables with separation-planes || K (1011), due to lamellar twinning. The translucent crystals of micaceous hematite (eisenglimmer) show pleo- chroism O > E. O brownish-red, E yellowish- red. In earthy condition occurs as a finely disseminated pigment. Occurs as an alteration product, not only of other iron ores, but also forming pseudomorphs after biotite, hornblende, etc. Occurs in pentagonal dodecahedra or in cubes, also in irregular grains or patches, often inti- mately associated with magnetite or ilmenite. Frequently occurs in rocks as a finely granular pigment. The particles show no definite crystalline form. Often occurs in intimate admixture with magnetite, limonite, and pyrites. INDEX TO DETERMINATIVE TABLES. T1BLK TABM? TABLE Elaeolite . iii. Natrolite . vii. Actinolite i . Enstatite ii. Nepheline iii. Aegirine . i . Epidote . vi. Nosean . . . iii. Albite . Allanite . vi. Eudialyte V. Oligoclase i. Almandine . iv. Fayalite . Olivine . . . iv. Analcime . vii. Opal V. Andalusite Andesine iv. i. Garnet . Gedrite . iv. ii. Orthite . Orthoclase vi. i. Anorthite Anorthoclase . i. i. Glaucophane . Graphite . ii. . viii. Paragonite Pennine . . . iii. . vi. Anthophyllite ii. Grossular iv. Perowskite V. Antigorite . viii. Hatiyne . iii. Phillipsite . vii. Aragonite Arfvedsonite . . viii. ii. Hematite Hornblende . . viii. ii. Picotite . Piedmontite . Finite iv. vi. iv. Hyalite . V. Bastite . . viii. Hypersthene . ii. Pistacite . . Plagioclase vi. > Idocrase . Ilmenite . iii. . viii. Pleonaste Pyrites . IV. . viii. Bronzite . ii. Bytownite i. lolite , iv. Calcite . . viii. Iron, Native . . viii. Riebeckite ii. Cancrinite Cassiterite Chalcedony iii. V. V. Kaolin . Kelyphite vi. . vii. Ripidolite Eutile . vi. V. Chalybite Chlorite . . viii. vi. Labradorite . i. V. Sanidine . i. Chromite Chrysolite Chrysotile Clinochlore iv. iv. . vii. vi. Lepidolite Lepidomelane Leucite . Leucoxene iii. iii. iii. . vii. Saussurite Scapolite Schorl . vii. iv. Scolecite . . vii. Cordierite iv. Limonite . viii. Serpentine . vii. Corundum V. Siderite . . viii. Cossyrite OrossiijG ii. j-j Magnetite Malacolite . viii. ii. Sillimanite iv. Delessite . vi. Specular-iron . Sphene . . . viii. V. Melanite. iv. Desmine . . vii. Melilite . iv. Spinel ... iv. Diallage . ii. Microcline i. Stilbite . . vii. Dichroite iv. Monazite V. Dolomite . viii. Muscovite iii. Talc vi. GENERAL INDEX 47 TABI.K TABI.K TABLE Thomsonite . . vii. Tourmaline iii, Wernerite iv. Thulite . vi. Tremolite ii. Withamite . vi. Tinstone V. Tridymite V. Titanite . V. Topaz iii. Vesuvian iii. Zoisite vi. The microscope employed for petrological work should be provided with a rotating stage divided to degrees, Nicol's prisms and arrangements for convergent polarised light. As a rule, greater amplification than that afforded by a quarter-inch objective is not required, but one or two objectives of lower power are essential. A quartz-wedge, a quarter-undulation mica plate, and a bull's-eye condenser are necessary adjuncts. In the microscope devised by Mr. Allan Dick, manufactured by Messrs. Swift & Son, the stage is fixed, while the polariser and analyser, are capable of simultaneous rotation. By this arrangement the necessity for centring gear is abolished. The stage and tube-fittings of this instrument admit of a rapid change from parallel to convergent light. Should higher powers than a quarter-inch be required, it is desirable to use immersion objectives capable of focussing through moderately thick covering-glasses. GENERAL INDEX The Names of Structtires are Printed in Italics. PAGH l-AGK PAGE Allotriomorphic . 13 Eclogite . . 23 Holocrystalline . . 15 Alnoite . 22 Elseol ite-sye n ite . 18 Hornblendite . . 23 . 19 Elvan Group . . . 16 Hyalo-andesite . . 19 Andesite Group . 18 Epidiorite . 25 Hyalo-basalt . . 20 Aplite . 16 Eucrite . 22 Hyalo-dacite . . 18 Arkose . 24 Eudialyte-syenite . . 18 Hyalomelane . . 20 Augitite . . . . 21 . 23 Hyalo-phonolite . 18 Euphotide . 25 Hyalopilitic . 14 Basalt . 20 Eurite . 15 Hyalo-trachyte . 17 Basalt Group . . 20 Hypersthenite . . 23 Basanite . . 21 Felsite 15,24 Hypidiomorphic . 13 Borolanite . 18 Fehitic . 14 Hypocrystalline . 15 Bostonite . . 17 Felspar-porphyry . . 16 Bronzitite . 23 Felstone . 15,24 Idiomorphic . 13 Fluxion-structure . 14 Ijolite . 23 Camptonite . 19 Fourchite . . 22 Charnockite . 17 Foyaite . . 18 Keratophyre . . 15 China-stone . 24 Kersantite . 19 Columnar . . 14 Gabbro Group . . 22 Kimberlite . 25 Cortlandite . 23 Glassy-base . 13 Corsite . 19 Glomeroporpliyritic . . 14 Lamprophyre Group 17,19 Cryptocrystalline . 13 Granite Group . . 16 Leucite-basalt . . 21 Granophyre . 16 Leucitite . . 21 Dacite . 18 Granular . . 14 Eeucophyre . 22 Devitrification . . 13 Greisen . 16 Lherzolite . 23 Diabase Group . 24 Groundmass . 13 Limburgite . 21 Diallagite . 23 Litchfieldite . 20 Diorite Group . . 19 Halleflinta . 15 Ditroite . 18 Haplite . 16 Malchite . . 19 Dolerite Group . 21 Harzburgite . 23 Melaphyre Group . . 24 D unite . 23 Haiiyne-andesite . 20 Melilite-basalt . 21 48 GENERAL INDEX PAGE PARK PAOK Miarolitic . . 14 Peridotite Group . 23 Soda-felsite 15 Miascite . . 18 Per lit ic . 13 Soda-rhyolite . 16 Mica-peridotite . 23 Perlitic Obsidian . 15 Spheroidal 14 Mica-syenite . . . . 17 Picrite . 23 Spherulitic 14 Microcrystalline . 13 Picrite-porphyrite . . 25 Stockwerksporphyr . 16 Microfelsitic . 14 Phonolite . . 18 Syenite Group . 17 Microgranite . . 16 Pilotaxitic . 14 Syenite Group (Nepheline Microlitic . . 13 Pitchatone . 15 Series) . 18 JififJTCttioH totTUCtllTC . . 14 Plat>/ 15 Minette . 17 Porphyrite Group . . 24 Tachylyte 20 Monchiquite . 22 Porphyritic . 13 Tephrite . 21 Propylite . . 24 Teschenite 22 Nepheline-basalt . 21 Pumice , 15 Theralite . 22 Nephelinite . 21 Pyroxenite Group . . 23 Tinguaite 18 Norite . 22 Trachyte . 17 Obsidian . . 15 Quartz-andesite Quartz-felsite . . 18 . 16 Trachyte Group Troctolite . 17 22 Ophicalcite . 25 Quartz-norite . . 22 Ophite . 24 Quartz-porphyry . 16 Variolite . 19 Ophitic . 14 Rhyolite Group . 15 Vogesite . 17 Panidiomorphic Pantellerite . 13 . 16 Rhyolite . . 16 Websterite 23 Pegmatite . 16 Saxonite . . 23 Wehrlite . 23 Pele's Hair . 20 Scyelite . . 23 Peridotite Dykes . 25 Serpentine . 25 Zircon-syenite . 18 Butler & Tanner, The Selwood Printing Work, Frome, and London RETURN EARTH SCIENCES LIBRARY TO ^ 230 McCone Hall 642-2997 LOAN PERIOD 1 2 HOURS 2 3 4 5 6 2 HOUR RESERVE BOOKS CANNOT BE RENEWED BY TELEPHONE DUE AS STAMPED BELOW FORM NO. 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