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 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 pv. 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 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 + || and J_ to length of fibres ditto TRIDYMITE . >! )! + Feeble double re- fraction. Scales viewed _|_ (0001) isotropic ditto OPAL . . . Am. none Anomalous double refraction, due to strain, especially in hyalite ditto CORUNDUM . Hex. none ~ || and J_ c. Basal sections dark in all azimuths very strong 6 low, not above red of 1st ord. 2 EUDIALYTE . Hex. 2~~ (0001) + || and _L c. medium 3 very low, grey of 1st ord. PEROWSKITE Cub". ? p. cub. (100) + Anomalous double refraction extremely strong 7 very low, not above yellow of 1st ord. 40-44 SPHENE . . Mon. (110) + _L (102) or 500 43' with a in the obtuse angle /3. very strong 6 extremely high 4th ord. nearly as high as rutile 500 MONAZITE . Mon. (001) . (100) + very strong 6 high 23-50 APATITE Hex. (0001) imperfect medium 3 very low, not above yellow of 1st ord. 39 PHOSPHATES, ETC TABLE V. Colour in thin section. Sign in the direction of elongation. Notes. yellow, brownish-red, violet colourless, yellow, brown colourless, yellowish, reddish Crystals frequently more or less rounded. Twins common on (101) and on (301), the former geniculate, the latter kite- shaped. Also in small groups of needle-like crystals forming meshes (sagenite) in conformity with twinning. Interference figure anomalous at times, the cross breaking up into dark brushes. Zonal structure common. colourless Thin basal sections show a dark cross without rings. Inclusions of Ha O and C O 2 often present. The apparently optically negative character is due to a being the axis of elongation of the fibres, the elongation being at right angles to that of quartz. The fibres of chalcedony are biaxial. Tridymite generally occurs in aggregates of overlapping scales. Becomes strongly stained when moistened with solution of fuchsine (roseaniline). colourless, blue Crystals traversed by basal and rhombohedral solution planes, simulating cleavage. Surfaces of sections appear rough. colourless, reddish Occurs chiefly in the elseolite syenites. greyish-brown, violet, brownish-yellow colourless, yellowish, reddish- brown Descloizeaux regards the dodecahedron as built up of twelve hemipyramids (monoclinic), the lozenge-shaped base of each corresponding to a face of the rhombic dodecahedron. Optic axial plane parallel to the longer diagonal of each lozenge. Dispersion horizontal. In sphene the optic axial plane lies in the plane of symmetry. Pleochroism weak, in very thin sections barely perceptible. Dispersion very strong, inclined p> v. Twinning on (001). brownish, yellowish colourless, bluish Horizontal dispersion p < v. Twinning on (100). In the spectro- scope gives absorption bands between the green and orange, due to didyrnium. Crystals frequently show gas- and fluid-inclusions. Optical anomalies common. 40 E P I D O T E Cryst. Syst. Cleavage. Opt. Sign. Extinctions. Relief. Interf. Col. 2E. ZOISITE . . . Eh. (010) + straight marked 4 very low, bare- 10- 100 ly to yellow of 1st ord. THULITE . . . Eh'. (001) + >! ditto 00-40 EPIDOTE . . . (Pistacite) Mon. (001) - 2-30 w ith c strong 5 very high, 3rd and 4th ord. >18QO PIEDMONT i TE . (Withamite) Mon. )) + 4-5 with c very strong 6 ditto >180 ORTHITE . . . (Allanite) Mon. 1 - 30 with c ditto very high, but not so high as epidote n CHLORITE ErIPIDOLITE. . Mon. p. hex. (001) _[_ (001) weak gives no complete 2 extinction very low, v. small grey 1st ord. to PENNINE . . . Mon. n ditto ditto ditto ditto CLINOCHLORE . Mon. n + 12-15 with a ditto normal to (001) low, up to blue of 2nd ord. 0-75 DKLESSJTE . . V n - JL (001) medium 8 TALC .... Mon.? (001) - respectively || b and c weak 2 very high, 4th ord. 10-20 KAOLIN . . . Mon. p. hex. M 20 with a normal to (001) very weak large 41 GROUP. TABLE VI. Colour in thin section. Sign in the direction of elongation. colourless, green, greenish-grey ros-r*d colourless, yel- lowish-green red, yellow brown Notes. Crystals very seldom show terminal faces. No perceptible pleochroism. Pleochroism very strong, a reddish-white, fa rose-red, c yellow- ish. Dispersion p < v. Crystals often twinned on (100). Optic axial plane || (010). Pleochroism varies in strength : generally a colourless, fa yellowish-green, c siskin-green. Crystals elongated on the orthodiagonal b. Dispersion inclined p> v. Crystals elongated on the orthodiagonal. Pleochroism very- strong, a orange to lemon-yellow, fa amethyst to rose-red, c deep-red. Optic axial plane || (010). Crystals elongated on the orthodiagonal. Pleochroism, a clear yellowish-brown, fa chestnut-brown, c dark greyish-brown. Optic axial plane || (010). GROUP green colourless, green- ish, yellowish Pleochroism for vibrations _|_ (001) yellowish to reddish. || (001) green. Cleavage-laminae flexible but not elastic. He- suits from the decomposition of biotite, hornblende, etc. Pleochroism for vibrations J_ (001) brownish-red, brown, yellow. || (001) emerald-green, blue-green. Ditto ditto Twinning, as in the micas, common. Optic axial plane || (010). Dispersion inclined p < v. Occurs in spherulitic groups and rosettes, in the vesicles of basic lavas. colourless The crystallisation is generally regarded as rhombic. Dispersion weak. Optic axial plane inclined about 20 behind a normal to (001). Obtuse bisectrix J_ & Kaolin is an alteration product of felspars. 42 A L T E R A T I O N - P R O D U C T S OF FEL- SAUSSUKITE . KELYPHITE . . LEUCOXENE Cryst. Syst. Gran r . and Fibrous Fibrous Gran r . to Fibrous Cleavage. Opt. Sign. Extinction*. Relief. Interf. Col. 2E. NATROLITE . SCOLECITE STILBITE . . (Desmine) PHILLIPSITE THOMSONITE . ANALCIME Eh- 1 . Mon. Mon. Mon. Rh. Cub". (110) (010) (C01) . (010) (010) (100) straight 22 with c 8 with c + 3-15 with a straight Often shows weak double refraction very weak 1 ditto ditto ditto ditto ZEOLITE low, not 94-96 above blue of 2nd ord. low, 1st ord. n very high, to bluish -grey of 4th ord. ditto ; very low, not i above iron- grey 1st ord. 56 53 83 SERPENTINE CHRYSOTILE SERPENTINE Rh.? straight very weak 1 low, 1st ord. variable up to 50 43 SPAR, GARNET, AND ILMENITE. TABLE VII. Colour in thin section. Sign in the direction of elongation. Notes. Essentially a mixture of minerals of secondary origin, chiefly zoisite or epidote with a felspar, not unfrequently albite. It results from the alteration of plagioclastic felspars in gabbros and dolerites. Saussurite is grey, greenish-grey, sometimes with a bluish tinge, occasionally almost white. It is an aggregate of fibres and granules, usually of exceedingly fine texture. Garnet, tremolite, chlorite, etc., may be present. Kelyphite forms borders to garnets (pyrope) in peridotites and serpent- ines. According to Becke, the radially fibrous portion of kelyphite consists of picotite (isotropic) and hornblende (doubly refracting) with a clear outer border, formed of a mixture of hornblende, bron- zite, and diallage. He regards it as the result of a reaction between pyrope and olivine, i.e. Pyrope + Olivine = Spinel +Hornblende Mg 3 ALSi 3 O 12 + Mg 2 Si O 4 =Mg Al 2 O 4 +Mg 4 Si 4 O 12 . In the variety of spinel, picotite, Mg is partly replaced by Fe and A1 2 by Cr 2 . In reflected light kelyphite appears pale greyish-brown. Leucoxene is an alteration product of ilmenite, sometimes granular in structure, at others fibrous, the fibres lying at right angles to the surface of the ilmenite. In reflected light it is white, yellowish, greyish, or brownish. In transmitted light it is nearly opaque, except in very thin sections, when it shows strong double refraction. The brownish colour of some leucoxene is probably due to a sagenitic intergrowth of rutile. The alteration of ilmenite may sometimes result in anatase. GROUP. colourless, yellowish colourless + Optic axial plane H (010). Dispersion /> < i JL(OIO). * II (010). (010). . ,, II (001). GROUP colourless, greenish, yellowish ditto Ordinary serpentine and the fibrous variety, chrysotile, ai'e in most cases the result of the alteration of olivine or hornblende, and often of augite. The serpentines derived from olivine show an irregular mesh-structure; those from augite a rectangular grid-structure (90) and those from hornblende a lattice-structure (124). 44 SERPENTINE Cryst. Syst. Cleavage. Opt. Sign. Extinctions. Relief. Intcrf. Col. 2E BASTITE . . . ANTIGOEITK Eh. Eh c . (010) )i - straight weak 2 d itto low, not above yellow, 1st ord. not above blue of 2nd ord. 20-90 II C A R B o - CALCITE . . . Hex. 2 K (1011) - weak 2 very high, 4th ord. ARAGONITK . . Bh. (010) straight medium 3 ditto 80 DOLOMITE . . Hex. 2 K (1011) - ditto ditto CHALVBITE . . Hex. ~2~ ?) ditto ditto IRON NATIVE IHON . Cub". Colour in reflected light. Colour in transmitted light. iron-black opaque MAGNETITE . . Cub". (Ill) bluish-black Opaque in the thinnest sections, but in extremely thin crystals forming inclusions in mica, it is transparent and of pale-brown colour ILMENITE . . Hex. 2 K (1011) iron-black, with slight tinge of brown opaque HEMATITE . . Hex. 2 iron-black to greyish- black with tinge of red. Earthy condi- tion red In thick crystals, or when earthy opaque. In very thin crystals (eisenglimmer) red, orange, yel- low. Titaneisenglimmer pinkish madder-brown LIMONITE . . Am. none yellowish-brown, dull opaque PYRITES . . . Cub c . brass^yellow n GRAPHITE . . Hex. 2 /c(0001) greyish-black to grey. No metallic lustre opaque, even in the most minute grains and thinnest sections 45 GROUP (continued). TABLE VIII. Colour in thin section. Sign in the direction of elongation. Notes. greenish Optic axial plane | pyroxene. (100). Eesults from the alteration of rhombic yellowish Plecchroism feeble. Chiefly results from the alteration of augite. NATES colourless colourless, yellowish yellowish brown Lamellar twinning on K (0112) very common. No pleochroism, but strong absorption of light, O > E. Optic axial plane || (100). Dispersion />< v. Twinning || (110), but, in rocks, aragonite only occurs in rod-like aggregates or in compact General Note. All of these carbon- No pleochroism, but strong absorption, O > E. Often becomes altered to limonite. ates are soluble in acids with effervescence, the two former in the cold, the two latter in hot acids. ORES. Occurs in the basalts of Greenland and in the gabbros of the Western Isles of Scotland. The surface of a section containing native-iron when treated with a solution of copper-sulphate has a film of metallic copper deposited upon the iron. Crystals frequently give square or triangular sections. Parallel grouping common. Also occurs in irregular grains and patches, sometimes with intermixture of pyrites. Crystals of tabular habit, giving sections, which when || K (0001) are hexagonal or trigonal, when J_ K (0001), lath-shaped. Often altered to leucoxene. Tetartohedral faces met with on the larger crystals. Specular-iron mostly in six-sided tables with separation-planes || K (1011), due to lamellar twinning. The translucent crystals of micaceous hematite (eisenglimmer) show pleo- chroism O > E. O brownish-red, E yellowish- red. In earthy condition occurs as a finely disseminated pigment. Occurs as an alteration product, not only of other iron ores, but also forming pseudomorphs after biotite, hornblende, etc. Occurs in pentagonal dodecahedra or in cubes, also in irregular grains or patches, often inti- mately associated with magnetite or ilmenite. Frequently occurs in rocks as a finely granular pigment. The particles show no definite crystalline form. Often occurs in intimate admixture with magnetite, limonite, and pyrites. INDEX TO DETERMINATIVE TABLES. T1BLK TABM? TABLE Elaeolite . iii. Natrolite . vii. Actinolite i . Enstatite ii. Nepheline iii. Aegirine . i . Epidote . vi. Nosean . . . iii. Albite . Allanite . vi. Eudialyte V. Oligoclase i. Almandine . iv. Fayalite . Olivine . . . iv. Analcime . vii. Opal V. Andalusite Andesine iv. i. Garnet . Gedrite . iv. ii. Orthite . Orthoclase vi. i. Anorthite Anorthoclase . i. i. Glaucophane . Graphite . ii. . viii. Paragonite Pennine . . . iii. . vi. Anthophyllite ii. Grossular iv. Perowskite V. Antigorite . viii. Hatiyne . iii. Phillipsite . vii. Aragonite Arfvedsonite . . viii. ii. Hematite Hornblende . . viii. ii. Picotite . Piedmontite . Finite iv. vi. iv. Hyalite . V. Bastite . . viii. Hypersthene . ii. Pistacite . . Plagioclase vi. > Idocrase . Ilmenite . iii. . viii. Pleonaste Pyrites . IV. . viii. Bronzite . ii. Bytownite i. lolite , iv. Calcite . . viii. Iron, Native . . viii. Riebeckite ii. Cancrinite Cassiterite Chalcedony iii. V. V. Kaolin . Kelyphite vi. . vii. Ripidolite Eutile . vi. V. Chalybite Chlorite . . viii. vi. Labradorite . i. V. Sanidine . i. Chromite Chrysolite Chrysotile Clinochlore iv. iv. . vii. vi. Lepidolite Lepidomelane Leucite . Leucoxene iii. iii. iii. . vii. Saussurite Scapolite Schorl . vii. iv. Scolecite . . vii. Cordierite iv. Limonite . viii. Serpentine . vii. Corundum V. Siderite . . viii. Cossyrite OrossiijG ii. j-j Magnetite Malacolite . viii. ii. Sillimanite iv. Delessite . vi. Specular-iron . Sphene . . . viii. V. Melanite. iv. Desmine . . vii. Melilite . iv. Spinel ... iv. Diallage . ii. Microcline i. Stilbite . . vii. Dichroite iv. Monazite V. Dolomite . viii. Muscovite iii. Talc vi. GENERAL INDEX 47 TABI.K TABI.K TABLE Thomsonite . . vii. Tourmaline iii, Wernerite iv. Thulite . vi. Tremolite ii. Withamite . vi. Tinstone V. Tridymite V. Titanite . V. Topaz iii. Vesuvian iii. Zoisite vi. The microscope employed for petrological work should be provided with a rotating stage divided to degrees, Nicol's prisms and arrangements for convergent polarised light. As a rule, greater amplification than that afforded by a quarter-inch objective is not required, but one or two objectives of lower power are essential. A quartz-wedge, a quarter-undulation mica plate, and a bull's-eye condenser are necessary adjuncts. In the microscope devised by Mr. Allan Dick, manufactured by Messrs. Swift & Son, the stage is fixed, while the polariser and analyser, are capable of simultaneous rotation. By this arrangement the necessity for centring gear is abolished. The stage and tube-fittings of this instrument admit of a rapid change from parallel to convergent light. Should higher powers than a quarter-inch be required, it is desirable to use immersion objectives capable of focussing through moderately thick covering-glasses. GENERAL INDEX The Names of Structtires are Printed in Italics. PAGH l-AGK PAGE Allotriomorphic . 13 Eclogite . . 23 Holocrystalline . . 15 Alnoite . 22 Elseol ite-sye n ite . 18 Hornblendite . . 23 . 19 Elvan Group . . . 16 Hyalo-andesite . . 19 Andesite Group . 18 Epidiorite . 25 Hyalo-basalt . . 20 Aplite . 16 Eucrite . 22 Hyalo-dacite . . 18 Arkose . 24 Eudialyte-syenite . . 18 Hyalomelane . . 20 Augitite . . . . 21 . 23 Hyalo-phonolite . 18 Euphotide . 25 Hyalopilitic . 14 Basalt . 20 Eurite . 15 Hyalo-trachyte . 17 Basalt Group . . 20 Hypersthenite . . 23 Basanite . . 21 Felsite 15,24 Hypidiomorphic . 13 Borolanite . 18 Fehitic . 14 Hypocrystalline . 15 Bostonite . . 17 Felspar-porphyry . . 16 Bronzitite . 23 Felstone . 15,24 Idiomorphic . 13 Fluxion-structure . 14 Ijolite . 23 Camptonite . 19 Fourchite . . 22 Charnockite . 17 Foyaite . . 18 Keratophyre . . 15 China-stone . 24 Kersantite . 19 Columnar . . 14 Gabbro Group . . 22 Kimberlite . 25 Cortlandite . 23 Glassy-base . 13 Corsite . 19 Glomeroporpliyritic . . 14 Lamprophyre Group 17,19 Cryptocrystalline . 13 Granite Group . . 16 Leucite-basalt . . 21 Granophyre . 16 Leucitite . . 21 Dacite . 18 Granular . . 14 Eeucophyre . 22 Devitrification . . 13 Greisen . 16 Lherzolite . 23 Diabase Group . 24 Groundmass . 13 Limburgite . 21 Diallagite . 23 Litchfieldite . 20 Diorite Group . . 19 Halleflinta . 15 Ditroite . 18 Haplite . 16 Malchite . . 19 Dolerite Group . 21 Harzburgite . 23 Melaphyre Group . . 24 D unite . 23 Haiiyne-andesite . 20 Melilite-basalt . 21 48 GENERAL INDEX PAGE PARK PAOK Miarolitic . . 14 Peridotite Group . 23 Soda-felsite 15 Miascite . . 18 Per lit ic . 13 Soda-rhyolite . 16 Mica-peridotite . 23 Perlitic Obsidian . 15 Spheroidal 14 Mica-syenite . . . . 17 Picrite . 23 Spherulitic 14 Microcrystalline . 13 Picrite-porphyrite . . 25 Stockwerksporphyr . 16 Microfelsitic . 14 Phonolite . . 18 Syenite Group . 17 Microgranite . . 16 Pilotaxitic . 14 Syenite Group (Nepheline Microlitic . . 13 Pitchatone . 15 Series) . 18 JififJTCttioH totTUCtllTC . . 14 Plat>/ 15 Minette . 17 Porphyrite Group . . 24 Tachylyte 20 Monchiquite . 22 Porphyritic . 13 Tephrite . 21 Propylite . . 24 Teschenite 22 Nepheline-basalt . 21 Pumice , 15 Theralite . 22 Nephelinite . 21 Pyroxenite Group . . 23 Tinguaite 18 Norite . 22 Trachyte . 17 Obsidian . . 15 Quartz-andesite Quartz-felsite . . 18 . 16 Trachyte Group Troctolite . 17 22 Ophicalcite . 25 Quartz-norite . . 22 Ophite . 24 Quartz-porphyry . 16 Variolite . 19 Ophitic . 14 Rhyolite Group . 15 Vogesite . 17 Panidiomorphic Pantellerite . 13 . 16 Rhyolite . . 16 Websterite 23 Pegmatite . 16 Saxonite . . 23 Wehrlite . 23 Pele's Hair . 20 Scyelite . . 23 Peridotite Dykes . 25 Serpentine . 25 Zircon-syenite . 18 Butler & Tanner, The Selwood Printing Work, Frome, and London RETURN EARTH SCIENCES LIBRARY TO ^ 230 McCone Hall 642-2997 LOAN PERIOD 1 2 HOURS 2 3 4 5 6 2 HOUR RESERVE BOOKS CANNOT BE RENEWED BY TELEPHONE DUE AS STAMPED BELOW FORM NO. 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