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 
 
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