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PROFESSIONAL PAPERS OF THE ENGINEER DEPARTMENT, U. S. ARMY. 

No. 18. 



OP THE 



GEOLOGICAL EXPLORATION OF THE FORTIETH PARALLEL 



MADE 



HY ORDER OF THE SECRETARY OF WAR ACCORDING TO ACTS OF 
CONGRESS OF MARCH 2, 1867, AND MARCH 3, 1869, 



UNDER THE DIRECTION OF 



BRIO. AND BVT. MAJOR GENERAL A. A. HUMPHREYS, 

CHIEF OK ENGINEERS, 



BY 



CLARENCE KING, 

C. . GEOLOGIST. 




i-n 



VOLUME VI. 



UNITED STATES GEOLOGICAL EXPLORATION OF THE FORTIETH PARALLEL 

CLARENCE KING, GEOLOGIST-IN-CHARGE. 



MICROSCOPICAL PETROGRAPHY 



BY 



FERDINAND ZIRKEL. 



SUBMITTED TO THE CHIEF OF ENGJNKKIiS AND PUBLISHED I!Y OUDEU OF THE SECRETARY OF 
WAB UNDER AUTHORITY OF CONGRESS. 



ILLUSTRATED BY TWELVE PLATES. 



WASHINGTON: 

OOVEBNMKNT PRINTING OFFICE. 
1876. 

V-VI 




TABLE OF CONTENTS. 



Pago. 

INTRODUCTORY LETTER xiii 

LETTER TO THE GEOLOGIST-IN-CHARGE xv 

CHAPTER I. INTRODUCTORY 1 

CHAPTER II. CRYSTALLINE SCHISTS AND BELATED ROCKS 14 

CHAPTER III. GRANITE AND GRANITE-PORPHYRY . 39 

SECTION I. GRANITE 39 

II. GRANITE-PORPHYRY CO 

CHAPTER IV. FELSITE-PORPHYRY AND SYENITE 71 

SECTION I. FELSITE-PORPHYRY 71 

II. SYENITE 81 

CHAPTER V. DIORITE, HORNBLENDE-PORPHYRY, DIABASE, MELAPHYRE, 

GABBRO 83 

SECTION I. DIORITE 83 

II. HORNBLENDE-PORPHYRY 94 

III. DIABASE 97 

IV. MELAPHYRE 103 

V. GABBRO 107 

CHAPTER VI. PROPYLITE, QUARTZ-PROPYLITE, HORNBLENDE-ANDESITE, 

DACITE 110 

SECTION I. PROPYLITE 110 

II. QUARTZ-PROPYLITE 117 

III. HORNBLENDE-ANDESITE 122 

IV. DACITE 134 

CHAPTER VII. TRACHYTE, EHYOLITE 143 

SECTION I. TRACHYTE 143 

II. KUYOLITE 103 

III. HYALINE- RHYOLITE 206 

vu 



Vlll TABLE OF CONTENTS. 

Page. 

CHAPTER VIII. BASALTS 216 

SECTION I. GENERAL REMARKS 216 

II. AUGITE- ANDESITE 221 

III. TRUE BASALTS 229 

IV. APPENDIX TO TEUE BASALTS 255 

CHAPTER IX. LEUCITE ROCKS 259 

CHAPTER X. CLASTIC ROCKS ' 2C2 

SECTION I. OLDEE CLASTIC ROCKS 262 

II. YOUNGEE CLASTIC ROCKS ; BEECCIAS, CON- 
GLOMERATES, AND TUFAS OP TEETIARY AGE. . 264 



LIST AND EXPLANATION OF PLATES. 



PLATE I. 

Fii,. 1. Double inclusion, consisting of an external (solid) zone (a), of liquid carbonic acid (b), and of 
a bubble (c) ; in tbe quartzes of many gneisses and granites. 

FIG. 2. Inclusion of a saturated solution of chloride of sodium, containing a sbarp, little salt cubo 
(and bubble); in tbe quartzes of many granites and gneisses. 

FIG. 3. Inclusion as in Fig. 2, containing also thin, pale-green microlitcs of hornblende, resting upon 
the walls of the including quartz, and projecting into tbe interior of the fluid. 

FIG. 4. Same inclusion in the quartz of granite ; black microlites accompanying the salt cube. 

FIG. 5. Same fluid-inclusion, containing grains and microlites of hornblende and plates of blood-red 
ozyd of iron, beside the salt cube ; in tbe quartz of granite-porphyries of Franklin Buttes, Nevada. 

FIG. 6. Apatite prisms, broken into many pieces and thrown into a curved bow ; in many gneisses 
and granites. 

FIG. 7. Apatite, with microscopic fluid-inclusions ; in gneiss and granite. 

FIG. 8. Apatite, filled with cylindrical pores, which lie parallel to the chief axis of tbe crystal, and 
confined to its middle ; longitudinal and transverse section ; from granites of Wacboe Mountains, Nevada. 

FlG. 9. Apatite, containing strange dust-like material, arranged in short lilies, which are combined 
in broom, and tuft-like forms; in andesite of Kamma Mountains, Nevada. 

FIG. 10. Section of apatite, with a black prismatic core through its whole length ; six thin lino-like 
prisms are affixed to the six vertical edges of this stouter individual ; in rock from Basalt Hill, near White 
1'lahiH, Nevada. 

FIG. 11. Section of a hornblende crystal, not a homogeneous individual, but built up of many accumu- 
lated tbin microlites, with distinct signs of aggregation from all sides ; in diorite from New Pass, Nevada. 

FIG. 12. Skeletou-like or cross-formed groups of magnetite crystals, arranged according to the axes 
of the regular system ; in the basalts from near Wadsworth. 

FIG. 13. Glass-inclusion in the oli vine of basalt, from the head of Clan Alpine Canon, Augusta Mount- 
ains ; the inclusion is pressed flat, stretching out many dendritical arms, which carry near their termina- 
tions small, dark bubbles. 

FlG. 14. Aggregations of microscopical tridymite in the reddish-gray (later) trachytes from Mount 
Rose and the frugar Loaf, Washoe, Nevada. 

FIG. 15. Hexagonal glass-inclusion, representing tbe form of qnartz in which it is imbedded ; a six- 
nuliuted, faint star over it, the rays of which apparently protrude beyond the border of the inclusion ; in 
quartz of rhyolite from Carico Valley, Nevada. 

FIG. lli. Glass-iuclnsion, the bubble of which is sac-like, curved, and twisted ; in quartz of rhyolite 
from New Pass, Desotoya Mountains, Nevada. 

FIG. 17. Hornblende-uiicrolite, partly surrounded by a bubble-bearing glass-drop, and itself con- 
taining two little glass-inclusions ; in qnartz of dacito from Shoshone Peak, Nevada. 

!':<;. 18. Glass-inclnsion, containing, beside the bubble, many black, short microlites, resting on the 
periphery, and stretching into the interior in parallels; in olivine of basalt from Buffalo Peak, North 
Park, Colorado. 

FlG. 19. Microlites of augite, on both sides of which are affixed many subtile prickles of augite at 
(lill'rrrnt angles, like tbe needles of a fir-tree ; in basalt from Snowstorm Canon, BlackRock Mountains, 

NYvilda. 

In.. -'!!. Colorless crystallites, the ends of wbich are terminated in two uciculur points, or iu regular 
Mair-like Bcrratinus, or irregularly lobed; iu globular prarlite of Montezuma Itangc, Nevada. 

IX 



X LIST AND EXPLANATION OF PLATES. 

Fui.21. Section of loucitc, with a concentric ring of glass-inclusions and augitc grains ; in the Icucite 
rock from tbo Leucito llills, northwest of Point of Rocks, Wyoming. 

FiO. 22. Section of leucito, with radially interposed, club-formed augito prisms; from last-named 
locality. 

FIG. 23. Section of leucite; green augite-microlites projecting into its substance; last-named locality. 

PLATE II. 

FIG. 1. Mica-schist, west slope of Humboldt Range, Nevada, containing quartz, brown mica, white 
mica, and singular knots, which consist of a dense web of microlites of fibrolite (distheue), imbedded 
in a quartz-mass. 

FIG. 2. Mica-schist, Spruce Mountain, Pcoquop Range, Nevada, consisting of quartz, much deep-green 
and little white mica; the lamina; of biotite contain innumerable extremely thin microlites, which 
sometimes show a hexagonal arrangement. 

FIG. 3. Staurolite in a silver-white mica-slate from Red Creek, Colorado, partly consisting of color- 
less, rounded quartz grains ; an arm of the mica-slate, with microscopical tourmaline and specular iron 
projecting into the staurolite-crystal. 

FIG. 4. Granite dike from north of Summit Springs, Havallah Range, Nevada ; quartz filled with 
black, hair-like microlites ; cleavable hornblende and lamellated biotite, showing a curious interlacing 
and interwreathing ; also brown titanite and dusty apatite. 

PLATE III. 

FIG. 1. Structure of feldspar, filled with microscopical interpositions (plates, laminae, microlites, 
grains) resembling those of true labradorite; from coarse-grained granite of Havallah Range. 

FIG. 2. Hornblende, of syenitic granite-porphyry, from the divide between Bingham Cafion, Oquirrh 
Mountains, and Tooelle Canon ; altered (with outlines conserved) into an aggregation of a leek-green 
substance (viridito), calcite, with rhombohedral cleavage, epidote, and black magnetite; all clearly 
observable secondary products. 

FIG. 3. Hornblende of diorite, from the southwest end of Wiunemucca Peak, Nevada, altered into 
green viridite, epidote and geode-like calcite, with residua of original hornblende. 

FIG. 4. Greenish-yellow nests of concentrically radiating epidote, showing, in their aggregation, the 
original form of the hornblende by the alteration of which they have been produced; in diorito from 
cast end of Winnemncca Peak, Nevada. 

PLATE IV. 

FIG. 1. Hornblende rock ; dike in granite of low hills northeast of Havallah Range ; consisting of 
colorless quartz, delicate green particles, and larger crystals of hornblende. 

FIG. 2. Mechanically altered crystals of hornblende, surrounded by black border; in hornblende- 
porphyry of Augusta Mountains, Nevada. 

FIG. 3. Melaphyre, from Berkshire Cafion, Virginia Range, Nevada, with transverse section of an 
amygdule, composed of regularly arranged quartz and green-earth. 

FIG. 4. Propylite, from Independence Valley, Tuscarora, Nevada, with characteristic greenish, pro- 
pylitic grouudmass, zonally built crystals of plagioclase, and two kinds of hornblende ; the predominating 
green one (somewhat fibrous and without a black border, altering into epidote), and a rarer browu form 
(strongly dichroitie, surrounded by black border, eminently cleavable, and entirely fresh) ; the latter 
playing the role of an accessory, almost a foreign element. 

PLATE V. 

FIG. 1. Brown biotite, in the quartzifcrous propylito of Wagon CaDou, Cortez Range, Nevada, con- 
taining interposed layers of pellucid calcite in the direction of the lamellation. 

FIG. 2. Very much broken and shivered hornblende-crystals, surrounded by dark border (probably 
a product of the chemical attack of the molten magma) ; from hornbleudo-andesite of Augusta Cafion. 

FIG. 3. Hornbleudc-andesite, from the Anuiu C'ruek, Cortez Range, with large, schistiform feldspars, 
cniitiiiiiiiig iiinumurablu glass-inclusions, arranged in regular bauds. 

FIG. 4. Leucite rock, from the Leucito Hills, Wyoming ; many rounded or eight-sided leucites, with 
grain-rings in their interior; prisms and microlites of pale-green augite; larger lamiuaj of peculiar, 
browuish-ydlow mica, dusty apatite, black microlites ; no feldspar. 



LIST AND EXPLANATION OF PLATES. XI 

PLATE VI. 

FIG. 1. Structure of rbyolite: pearlitic cracks run as a network through a light homogeneous glasp, 
associated on both sides with a narrow zone of mierofelsitic substance, with imbedded crystals of feld- 
spar; north of Wadsworth, Nevada. 

FIG. 2. Structure of rhyolite : fluidal bands of dark-brown grains form contorted undulations, which 
include axiolitic fibrous portions ; from foothills of Virginia Range, northwest of Wadsworth, Nevada. 

FIG. 3. Structure of rhyolite: tlnidal lines of dark-brown grains run in contorted undulations, and 
envelop homogeneous glass portions ; from last-named locality. 

FIG. 4. Ehyolito from the ridge at the head of Louis' Valley, Nevada, with lamellatcd brownish 
biotite plates, broken ami shivered into single leaves. 

PLATE VII. 

FIG. 1. Rhyolite of the Black Rock Mountains, Nevada; microfelsite (with some polarizing particles), 
containing single axially fibrous or axially cuneate bodies (axiolites) with distinct middle suture. 

FIG. 2. Rhyolite from Pahkeah Peak, Pah-tsou Mountains, Nevada, presenting a ramifying network 
of axially cuneate strings, in the meshes of which concentric, radially fibrous spbrerolitcs are placed; 
fractured aud contorted biotite; quartz and feldspar with glass-inclusions. 

FIG. 3. Rhyolite, northwest from Black Canon, Montezuma Range, consisting of a confused aggrega- 
tion of bunch-formed systems of parallel fibres, repeating the true structure of artificial porcelain; 
stripes of colorless, polarizing, angular grains run through this mass. 

FIG. 4. Rhyolito from the Mopung Hills, west of Humboldt Range ; yellowish and grayish-brown, 
axially fibrated, tail-formed strings of longer or shorter extension, and with distinct middle suture, ran 
through a light-gray, ferrite-bearing grouudmass, which is in the undeveloped crystalline state. 

PLATE VIII. 

FIG. 1. Rhyolito from Hot Spring Hills, Pah-Ute Range; fluidal stripes, composed of brown grains, 
and set with ciliated, thorn-like hairs, forming a network the meshes of which consist of axially or con- 
centrically fibrous portions. 

FIG. 2. Rbyolito from the pass north of Chatay.i Peak, Pah-Ute range; presents a delicate alternation 
of contorted, darker-brown, glassy layers (set with short, dark hairs), and of lighter microftlsitic or half- 
crystalline ones ; crystal of feldspar, with glass-inclusions. 

FIG. 3. Rhyolite, summit of ridge south of Squaw Valley, Nevada ; groundmass a dense aggregation 
of finely fibrous sphoirolites, which partly consist of thicker, reddish-brown ferritic needles in concen- 
tric grouping ; feldspars and quartzes with glass-inclusions. 

FlG. 4. Obsidian from Truckee Ferry, Truckee CaSou, Nevada, consisting of lamina) and layers of a 
nearly colorless aud of a pale-brownish glass, which are entangled and kneaded together in the most 
confused manner. 

PLATE IX. 

FIG. 1. Glassy rock from Truckee Range; the small, granular globulites in the glass are coagulated 
into little lumps, stars, needles, tendrils, spider-like forms, etc. 

FIG. 2. Pearlite from the Pah-tson Mountains, with globulites, aggregated into the form of crystal- 
lit i<; needles, and of cuneate and tendril-like bodies; large crystals of brown hornblende. 

FlG. 3. Pearlite from Grass Canon , west side Pah-tson Mountains, with colorless and black microlitea 
(belonites and tricbites), drawn straight ly or twisted, arranged in parallels by fluctuation. This micro- 
litic devitrification is quite independent from the pearlitic shell-texture, which produces concentric 
crack-lines in the section. 

FIG. 4. Pearlitic rock from Monteznma Range, west of Parker's Station, devitrified into subtile black 
trichites, usually sharply contorted, often curled into little indistinct flocks; the stronger ones sometimes 
powdered with pale, pellucid grains of extreme minuteness; distinct arrangement in parallel linear paths 
by fluctuation. 



X LIST AND EXPLANATION OF PLATES. 

Fiii.lil. .Section of leucito, with a concentric ring of glass-inclusions and augito grains ; in the leucite 
rock from the Leucito Hills, northwest of Point of Rocks, Wyoming. 

Fiu. 2. Section of leucite, with radially interposed, club-formed augito prisms; from last-named 
locality. 

FIG. 23. Section of leucite ; green augite-microlites projecting into its substance; last-named locality. 

PLATE II. 

FIG. 1. Mica-schist, west slope of Humboldt Range, Nevada, containing quartz, brown mica, white 
mica, and singular knots, which consist of a dense web of microlites of fibrolite (disthene), imbedded 
in a quartz-mass. 

FIG. 2. Mica-schist, Spruce Mountain, Pcoquop Range, Nevada, consisting of quartz, much deep-green 
and little white mica; the laminai of biotite contain innumerable extremely thin microlites, \vhich 
sometimes show a hexagonal arrangement. 

FIG. 3. Staurolite in a silver-white mica-slate from Red Creek, Colorado, partly consisting of color- 
less, rounded quartz grains ; an arm of the mica-slate, with microscopical tourmaline and specular iron 
projecting into the staurolite-crystal. 

FIG. 4. Granite dike from north of Summit Springs, Havallah Range, Nevada ; quartz filled with 
black, hair-like microlites ; cleavable hornblende and lameilated biotite, showing a curious interlacing 
and interwreathing ; also brown titanite and dusty apatite. 

PLATE III. 

FIG. 1. Structure of feldspar, filled with microscopical interpositions (plates, lamina?, microlites, 
grains) resembling those of true labradorito ; from coarse-grained granite of Havallah Range. 

FIG. 2. Hornblende, of syenitic granite-porphyry, from the divide between Bingham Cafion, Oqnirrh 
Mountains, and Tooelle Cafion ; altered (with outlines conserved) into an aggregation of a leek-green 
substance (viriditoj, calcite, with rhombohcdral cleavage, epidole, and black magnetite; all clearly 
observable secondary products. 

FIG. 3. Hornblende of diorite, from the southwest end of Winnemucca Peak, Nevada, altered into 
green viridite, epidoto and geode-liko calcite, with residua of original hornblende. 

FIG. 4. Greenish-yellow nests of concentrically radiating epidoto, showing, in their aggregation, the 
original form of the hornblende by the alteration of which they have been produced; in diorite from 
east end of Winnemncca Peak, Nevada. 

PLATE IV. 

FIG. 1. Hornblende rock ; dike in granite of low hills northeast of Havallah Range ; consisting of 
colorless quartz, delicate green particles, and larger crystals of hornblende. 

FIG. 2. Mechanically altered crystals of hornblende, surrounded by black border; in hornblende- 
porphyry of Augusta Mountains, Nevada. 

FIG. 3. Melaphyre, from Berkshire Canon, Virginia Range, Nevada, with transverse section of an 
amygdule, composed of regularly arranged quartz and green-earth. 

FIG. 4. Propylite, from Independence Valley, Tuscarora, Nevada, with characteristic greenish, pro- 
pylitic grouudmass, zonally built crystals of plagioclase, and two kinds of hornblende ; the predominating 
green one (somewhat fibrous and without a black border, altering into epidote), and a rarer brown form 
(strongly clichroitic, surrounded by black border, eminently cleavable, and entirely fresh) ; the latter 
playing the role of an accessory, almost a foreign element. 

PLATE V. 

FIG. 1. Brown biotite, in the quartziferous propylito of Wagon CaCou, Cortez Range, Nevada, con- 
taining interposed layers of pellucid calcite in the direction of the lamellation. 

FIG. 2. Very much broken and shivered hornblende-crystals, surrounded by dark border (probably 
a product of the chemical attack of the molten magma) ; from hornbleude-andesite of Augusta Cauon. 

FIG. 3. Hornblemle-andesito, from the Annie Creek, Cortez Range, with large, schistiform feldspars, 
containing innumerable glass-inclusions, arranged in regular bauds. 

FIG. 4. Lencite rock, from the Leucito Hills, Wyoming ; many rounded or eight-sided leucites, with 
grain-rings iu their interior; prisms and microlites of pale-green augite; larger lamioaj of peculiar, 
bron uish-yellow mica, dusty apatite, blaek microlites ; no feldspar. 



LIST AND EXPLANATION OF PLATES. xi 

PLATE VI. 

FIG. 1. Structure of ibyolite: pcarlilic cracks run as a network through a light homogeneous glasp, 
associated on both sides with :i narrow zone of ruicrofelsitic substance, with imbedded crystals of feld- 
spar; north of Wadsworth, Nevada. 

FIG. 2. Structure of rhyolite : fluidal bands of dark-brown grains form contorted undulations, which 
include axiolitic fibrous portions ; from foothills of Virginia Range, northwest of Wadsworth, Nevada. 

FIG. 3. Structure of rhyolito: fluidal lines of dark-browu grains run in contorted undulations, and 
envelop homogeneous glass portions ; from last-named locality. 

FIG. 4. Khyolite from the ridge at tbo head of Louis' Valley, Nevada, with lamellatcd brownish 
biotite plates, broken aud shivered into single leaves. 

PLATE VII. 

FIG. 1. Rhyolito of the Black Rock Mountains, Nevada; microfelsite (with some polarizing particles), 
containing single axially fibrous or axially cnneate bodies (axiolites) with distinct middle suture. 

FIG. 2. Rhyolite from Pahkeah Peak, Pah-tson Mountains, Nevada, presenting a ramifying network 
of axially cnneate strings, in the meshes of which concentric, radially fibrous spbterolites are placed; 
fractured aud contorted biotite; quartz and feldspar with glass-inclusions. 

FIG. 3. Rhyolito, northwest from Black Cafion, Montezuma Range, consisting of a confused aggrega- 
tion of bunch-formed systems of parallel fibres, repeating the true structure of artificial porcelain; 
stripes of colorless, polarizing, angular grains run through this mass. 

FIG. 4. Rhyolito from the Mopung Hills, west of Humboldt Range ; yellowish and grayish-brown, 
axially tibrated, tail-formed strings of longer or shorter extension, and with distinct middle suture, run 
through a light-gray, ferrite-beariug grouudmass, which is in the undeveloped crystalline state. 

PLATE VIII. 

FlO. 1. Rhyolite from Hot Spring Hills, Pah-Ute Range; fluidal stripes, composed of brown grains, 
and set with ciliated, thorn-like hairs, forming a network the meshes of which consist of axially or con- 
centrically fibrous portions. 

FIG. 2. Rhyolite from the pass north of Chataya Peak, Pah-Ute range ; presents a delicate alternation 
of contorted, darker-brown, glassy layers (set with short, dark hairs), and of lighter microfelsitic or half- 
crystalline ones ; crystal of feldspar, with glass-inclusions. 

FIG. 3. Rhyolite, summit of ridge south of Squaw Valley, Nevada; groundmass a dense aggregation 
of finely fibrous spbojrolites, which partly consist of thicker, reddish-brown ferritic needles in concen- 
tric grouping ; feldspars and quartzes with glass-inclusions. 

FIG. 4. Obsidian from Truukee Ferry, Truckee Cafion, Nevada, consisting of lamina) and layers of a 
nearly colorless aud of a pale-brownish glass, which are entangled and kneaded together in the most 
confused manner. 

PLATE IX. 

FIG. 1. Glassy rock from Truckee Range; the small, granular globulites in the glass are coagulated 
into little lamps, stars, needles, tendrils, spider-like forms, etc. 

FIG. 2. Pearlite from the Pah-tson Mountains, with globnlites, aggregated into the form of crystal- 
litic needles, and of cuneate and tendril-like bodies; large crystals of brown hornblende. 

FIG. 3. Pearlite from Grass Cafion, west side Pah-tson Mountains, with colorless and black microlites 
(belonites and trichites), drawn straightly or twisted, arranged in parallels by fluctuation. This micro- 
litic devitrification is quite independent from the pearlitie shell-texture, which produces concentric 
crack-lines in the section. 

FiG. 4. Pearlitie rock from Mouteznma Range, west of Parker's Station, devitrified into subtile black 
trichites, usually sharply contorted, often curled into little indistinct flocks; the stronger ones sometimes 
powdered with pale, pellucid grains of extreme minuteness; distinct arrangement in parallel linear paths 
by fluctuation. 



x ii LIST AND EXPLANATION OF PLATES. 

PLATE X. 

FIG. 1. Obsidian from the Ombc Huttc. Utah.consisting of yellowish-red and of nearly colorless glassy 
stripes, which are much contorted and kneaded through each other; long thin pores appear like black 
lines; quartzes with large inclusions of orange-colored glass. 

FIG. 2. Half-glassy rock from East Goose Creek Mountains, Nevada, containing quartz, sanidin, 
plagioclase, lamellated brown biotite, greenish augite, rare dark-brown hornblende, and magnetite in a 
mass, which consists of a throng of microlites, imbued with glass, and presents excellent phenomena of 
waving, damming, and encircling fluctuation. 

FIG. 3. Basalt, north of American Flat Creek, Washoe, Nevada, characteristic type, containing larger 
microscopical or even macroscopical crystals of feldspar and olivine, both contrasting with an extremely 
fine-grained crystalline mixture of rounded, drop-like or crippled augite grains of pale color and black 
sharp grains of magnetite. The aggregation of these two ingredients, which contains no perceptible 
glassy base, plays the role of a groundmass. Oliviue only appears as larger porpby ritical crystals ; angite, 
on the other hand, only as a subtile constituent of tho grouudmass. Borders of the feldspar ledges not 
vrry sharply denned. Extremely minute angite grains interposed in the larger feldspars, forming lines 
which correspond to the tricliuic striation. Olivine partly decomposed into greenish serpentineons 
matter. 

FIG. 4. Basalt from Lower Trnckee Valley, containing characteristic typical feldspars, a comparatively 
large number of which are sanidins, irregularly shaped augites, and larger, partly metamorphosed 
oliviues, imbedded in an uuindividualized amorphous base, which consists of a glass substance and of 
extremely small, dark globulitic grains (globulitically devitrified glass). This base is characteristically 
crowded in between the diverging crystalline elements in cuneiform points. 

PLATE XL 

FIG. 1. Altered basalt : the globulitic base is metamorphosed into amygdaloidal nests, consisting in 
the section of undulated and curled concentric rings, with an alternately lighter and darker grayish or 
brownish-yellow color. Mountain Wells Station, Overland Road, Augusta Mountains, Nevada. 

FIG. 2. Augite-audesite from Susan Creek Canon. The largo feldspar consists of spots which are almost 
wholly glass, the light-brownish inclusions being really woven together on the sides, so that the feld- 
spar substance scarcely appears between them ; yet the bubbles of the single particles which have been 
welded together are distinctly recognizable. 

FIG. 3. Basalt from the divide between North and Middle Parks, with large olivine, partly altered 
into serpeutineous matter, which forms strings and veins. 

FIG. 4. Basaltic rock of the Egyptian Canon, River Range, Nevada, with peculiar crystallitic ingre- 
dients. (See text.) 

PLATE XII. 

FIG. 1. Fragment of rhyolite, having resinous lustre, from rhyolitic breccia of Mullins Gap, Pyramid 
Lake, Nevada; pale-browuish-violet glass, with lighter spots; the glass containing delicate microlites, 
dark gas-pores (stretched out in the direction of fluctuation), and most remarkable fluid-inclusions, with 
moving bubble up to 0.012 mnl in diameter. The colorless prismatic crystal in tho middle (apatite) con- 
tains a glass-inclusion, which itself holds a liquid particle with moving bubble. 

FIG. 2. Chalcedony from Grass Canon, Pah-tsou Mountains, chiefly consisting of splendidly polar- 
izing siliceous sphrolitcs, which are made up of radiating fibres ; figure in polarized light. 

FlQ. 3. Palagonite-tufa, southeast from Hawes' Station. Nevada ; hyaline-breccia of differently col- 
ored glass fragments, containing crystals of plagioclase and dark-bordered gas-cavities, in many of which 
the inner walls and the immediately environing palagonite mass have been altered into a fibrous aggre- 
gation of short needles. 

FIG. 4. The same between crossed nicols; the isot rope, glassy DIMS becomes entirely dark, the altered 
walls of tho cavities presenting excellent aggregate polarization, showing even a colored cross, which 
changes its position and color by turning the object or the analyzer. The plagioclases show variously 
colored stripes. 



UNITED STATES GEOLOGICAL EXPLORATION 
OF THE FORTIETH PARALLEL, 

October, 1876. 

GENERAL: Herewith I have the honor to transmit Volume VI of the 
report of this Exploration. 

While American palaeontologists have materially aided field-geologists 
by their systematic assignment of fossil remains to proper horizons, the 
important study of petrography has suffered complete neglect, save by a 
few exceptional workers. 

Believing that the establishment of definite American rock-types could 
only be satisfactorily accomplished by minute comparisons with those of 
Europe, and that the refinements of microscopic investigation were essential 
to success, I naturally turned to Europe for aid. 

I am sure American men of science will welcome the present volume, 
from the distinguished pen of Prof. Ferdinand Zirkel, as one of the most 
important contributions ever made to our geology, and will give it the 
cordial intellectual greeting due so eminent a guest as its author. 
Very respectfully, your obedient servant, 

CLARENCE KING, 

Geologist-in- Charge. 
Brigadier-General A. A. HUMPHREYS, 

Chief of Engineers, U. S. Army. 



xiii 



LEIPSIC, 1876. 

SIB : Sending you herewith my report on the crystalline rocks along the 
Fortieth Parallel in the Western United States, I cannot fail to gratefully 
acknowledge how much invaluable assistance I owe to you and to your 
excellent fellow- workmen, Messrs. S. F. Emmons and Arnold Hague. You 
well remember that happy time in New York when for many weeks we 
made together the preliminary examination of that vast collection of rocks 
you had gathered under such difficulties, but with such eminent geological 
taste. 

You then enabled me to become acquainted with the geological distribu- 
tion, relative age, and reciprocal connections of the rocks ; and if I have 
been able to study their mineralogical and chemical constitution from a 
geological point of view, and to present more than a sterile and dry petro- 
graphical description, the merit is originally yours. Since the greater part 
of this investigation is directed to the microscopical composition and structure 
of rocks, it has appeared appropriate to offer in the beginning some brief 
general remarks upon that subject. 

You know that when we examined the collection macroscopically I 
entirely agreed with the determination and nomenclature you and your able 
colleagues had already arrived at in the field. There were only some 
doubtful occurrences, whose true nature could not at that time be decidedly 
cleared up. Now, after having carefully studied more than twenty-five 
hundred thin-sections under the microscope, I have only to testify again 
that your original designations should almost never be altered or corrected. 

May the results of this report as an American contribution to the gen- 
eral science of rocks, fulfil the expectation you cherished when you entrusted 
your classic collections to me. 

With sincerest respect, 

F. ZIRKEL. 

To CLARENCE KING, 

United States Geologist. 



XV 



CHAPTER I. 



INTRODUCTORY. 

Former examinations of the microscopical structure of rocks which 
are not clastic have established the existence of certain large and well-defined 
groups. These general divisions of micro structure are entirely independ- 
ent of the mineralogical composition of the rock and the special nature of 
its ingredients. They are the following : 

I. Purely crystalline type : rocks composed simply of macroscopical 1 
or microscopical crystalline individuals, which are in direct contact with 
one another, there being no amorphous substance between them. Granite 
is an excellent macroscopical example of this type of structure. Not only 
the apparently homogeneous mass of some so-called cryptocrystalline rocks, 
free from macroscopical crystals, but the groundmass of many porphyries, 
belong to this kind of microscopical structure. Moreover, although the 
entire absence of unindividualized substances is the strongest characteristic 
of this group, we must still class with it those members in which there is 
a minute quantity of a substance between the largely predominating crystal- 
line ingredients. 

II. Half-crystalline type : the crystalline individuals are either macro- 
scopical and microscopical, or microscopical alone, and constitute only a 
part of the rock. The rest is composed of an unindividualized amorphous 
substance of greatly varying character and quantity. This unindividual- 
ized mass exists microscopically in rocks of the second group, and in 
a certain sense plays the role of a foreign substance, is opposed to the 

"It is time this admirable word gained a fixed place in American petrography, and 
I have determined to use it throughout this geological series, with the accepted Euro- 
pean signification. 0. K. , 
IMP 



9 MICROSCOPICAL PETROGRAPHY. 

crystalline ingredients and varies much in its behavior, especially pos- 
sessing the following constitution : 

a. Purely glassy, consisting of a lighter or darker yellowish-brown, a 
grayish or a nearly colorless glass, which is simply refracting, yielding 
no colors in polarized light; some of the half-crystalline rocks being 
comparatively very rich in this pure glass, while in others it only imbues 
and impregnates the aggregation of predominating crystalline elements. 

b. Partly devitrified by the secretion of strange grains or needles, 
which do not belong to the crystalline constituents of the rock, but 
are of an entirely different nature. In this type, the unindividualized 
substance is not pure glass, but a glass in which sharply shaped, rounded, 
yellowish-brown or dark-brown grains, or black, hair-like needles, are im- 
bedded. The diameter of these somewhat translucent grains seldom exceeds 
0.005 mm , and they are often densely crowded together, almost totally replacing 
the glass. Since they do not polarize, they can only be considered as 
glass somewhat richer in iron. They are similar to the dark-green grains 
so often found in the colorless mass of the artificial, green slags of iron- 
furnaces. They belong to the so-called crystallites, and have been named 
by Vogelsang "globulites". 1 This granulated or globulitic devitrification 
of the amorphous mass is common in the half-crystalline basalts, melaphyrcs, 
and other basic rocks, but it seems to be rare in rocks rich in silica. The 
dark, almost wholly opaque needles or trichites, are usually aggregated within 
the glassy portions of the rock into branches, confused flocks, or singular 
skeleton-like nets. 

c. Such a dense aggregation of extremely small grains, needles, and 
hair-like bodies that very little glass, or none at all, appears between 
them. These little bodies, of which the very plentiful inclusion indicates 
an advanced stage of devitrification, are of quite an indistinct nature, not 
properly individualized, and certainly cannot be identified with the crystal- 
line ingredients of the rock, for they belong to the intermediate kingdom 
of undeveloped crystallites. This peculiar behavior of the amorphous mass 
may be termed the micro-crystallitic. It is, of course, connected by passage- 
members with the former. 

"H. Vogelsang, Die Krystalliten, 1875, 115. 



INTRODUCTORY. 3 

d. A peculiar amorphous substance, which, on the one hand, lacks 
the glassy appearance, possessing no transparency, and yet, on the other, 
cannot be resolved into single actually individualized particles. This 
microfeldsitic mass is usually composed of indistinct and imperfect little 
grains which blend into one another, or of well-nigh obliterated fibers. 
A typical development of this variety is generally perfectly dark between 
crossed nicols, but it sometimes sends out a very feeble, vague, general 
light. In places, the incomplete, undeterminable grains and fiber show 
a tendency to incomplete radial grouping. A light-grayish or yellowish mass 
of this sort specially enters into the composition of highly-silicated rocks, 
such as quartz-porphyries and rhyolites, but they are hardly ever developed in 
the basic rocks. The four principal types of the unindividualized substance 
in the half-glassy rocks are therefore the purely glassy state, the globulitic 
and tridiitic, the microcrystalline, and the microfeldsitic devitrification. 

III. Uncrystalline type: the rock consisting of an unindividualized 
amorphous substance, which is sometimes in the glassy and often in the 
microfeldsitic state, as obsidians free from crystals, tachylyte, ad some feld- 
site rocks. But just as those rocks in whose crystalline aggregation 
there is a trifle of glass may properly enough be named crystalline, so these 
which include a few small and rare secreted crystals in the largely 
predominating amorphous mass, are rightly called uncrystalline. Perhaps 
it is not superfluous to add that a rock which owes its specific place and 
name to the mineralogical nature of its ingredients, or to its macroscopical 
structure, does not of necessity always belong to one of these three types of 
microstructure. The basalts, which are characterized by the amount of 
plagioclase, augite, olivine and magnetite they contain, are, for instance, in 
one place developed as a purely crystalline, and in another as a half-crys- 
talline rock, the included amorphous mass varying in its condition. Even 
in the same coherent rock-mass, forming a geological whole, the behavior 
of the microscopical structure often entirely changes in very small distances. 
The well-defined diil'crences of these types cannot therefore be depended 
upon in the general or special classification of rocks, which, in the first place, 
must always be founded upon the mineral nature of the individualized con- 
stituents as the chariirtcristicof most constant importance; the development 



4 MICROSCJPICAL PETROGRAPHY. 

of the microscopical structure of the mass being wholly independent of the 
quality and combination of these. 

To prevent confusion, it seems best to employ the word "groundmass", 
in consonance with its present use, in the macroscopical sense only, signify- 
ing a mass which for the most part contains larger porphyritical crystals, 
appears.to the unaided eye homogeneous (dense) and insoluble, however it 
may behave under the microscope. That substance, however, which appears 
under the microscope as the proper unindividualized ground-paste, the bearer 
and holder, if one may say so, of both the microscopical and the macroscop- 
ical crystals, evidently merits a distinctive appellation, and has been named 
the "base", 1 by which, therefore, a purely microscopical conception is meant. 
In the groundmass, the base is very often accompanied by crystals. The 
base may be glassy, globulitic, microfeldsitic, etc., but never crystalline- 
granular. If the macroscopical groundmass is actually homogeneous 
throughout, as in many obsidians, of course both conceptions fail. 

In the glassy and half-glassy rocks, it is a widely-spread phenomenon 
for the colorless green and black, needle-formed, microscopical elements to 
be grouped together into strings, bands, and flocks. There are bodies 
among them which have the appearance of undulated and bent streams, 
damming up before a larger crystal, and flowing around it to unite on 
the other side, (giving the crystal something the appearance of an eye,) 
often also really scattered and dissipated by one of them. These appear- 
ances evidently indicate that the fluctuations happened in the stiffening 
glass magma, after the microlites or little needle-formed crystals had been 
solidified. Analogous phenomena of motion, fluctuation or fluidal struc- 
ture, invisible to the naked eye in the hand-specimens, are very often 
observed in the thin sections of partly or almost wholly crystalline massive 
rocks, such as basalts, trachytes, phonolites, melaphyres, and greenstones. 
The smallest ledge-formed sections of orthoclastic or plagioclastic feldspars, 
prisms of hornblende or augite, microlites of a variety of kinds ; in short, 
all the microscopical bodies possessing a longitudinal axis, are locally 
grouped parallel to one another, and form xindulating streams which diverge 
in the form of fans or ice-flowers. Where larger crystals lie in the paths of 
J F. Z., Die mikroskopisclie Beschaffenheit der Mineralien und Gesteiue, 1873, 208. 



INTRODUCTORY 5 

these crowded bands, the little needle-formed crystals encircle them on all 
sides with a tangential arrangement, are turned aside into different paths, or 
corne to an abrupt end before them, as if by a shock, the microlites being 
thrown asunder in all directions. Observations of these phenomena of 
fluidal microstructure are best made between crossed nicols, for the single 
crystals ai*e then colored and exhibit their characteristic direction much 
better than in ordinary light. A low magnifying power best enables one to 
overlook at once a larger portion of the thin section, and thereby to follow 
the lines of fluctuation. The shape of the little crystals is not with- 
out importance in the distinct observation of the form of the fluctuations. 
If they are needle-like or ledge-formed, even feeble movements of the mass 
will be unmistakably expressed ; if, on the contrary, they are of a roundish, 
granular form, it often happens that strong fluctuations which have taken 
place fail to leave a trace of their action. In some rocks, especially the 
rhyolites, this wavy structure is produced by small dark grains grouped 
into lines and bands. These lines of grains undulate in a most remark- 
able manner, so that the figures of their curvature resemble marbled paper. 
There are also curled and twisted stripes of felsitic material, differing in 
color and behavior, which render the waving motion evident. 

Three important points present themselves upon which light is thrown 
by this remarkable microstructure, connected with the fluctuations of 
the solidifying mass. It proves that the rock was at one time a magma, 
in a plastic state, and that, after larger crystals had- been secreted, 
a shifting and displacement of the small microlites happened. Soon after- 
ward, the mass seems to have been so suddenly solidified that the streams 
became fixed, and their fluctuation preserved for our observation. And, from 
these facts, the conclusion follows that the large and small crystals were not 
formed exactly where we perceive them, but that they have been thrown into 
their present place by the purely mechanical action of the surrounding plastic 
mass. It is worth mentioning that those rocks whose microfluidal structure 
is particularly distinct, are generally proportionately rich in broken crystals, 
shivered into detached, sharply angular fragments. And, lastly, this struc- 
ture proves that the smallest crystals of the rock have not altered their 
mutual grouping and form, which date back to their solidification ; and that, 



(? MICROSCOPICAL PETROGRAPHY. 

although secondary decompositions may have occurred in the lapse of time, 
these metamorphic influences have by no means been sufficient to obliterate 
the original characteristic structure. 

It is well known that the non-fragmentary, so-called crystalline rocks, 
are divided petrographically into the simple and the mixed. The latter are 
grouped, according to their general characteristics of structure, under the 
two names massive (not slaty, but for the most part granular) and slaty 
rocks. By far the greater number of the massive rocks contain feldspar, 
which is either orthoclase or plagioclase, or a representative of feldspar in 
the form of nepheline or leucite. But a very small part of them, like such 
comparatively very rare rocks as eclogite, tourmaline-rock, and cherzolite, is 
free from feldspar. It may be desirable to present, in this place the complete 
arrangement of the feldspar-bearing rocks according to the present mode 
of classification, and to add some considerations and remarks. The names 
which are not italicized are of rocks whose eruption antedates the Tertiary 
age, the ante-Tertiary and old massive rocks : those printed in italics have 
outflowed since the beginning of that age, and comprise the Tertiary and 
recent eruptive rocks. 

I. Orthoclase rocks. a. With quartz or excess of silica : granite, 
granite-porphyry, felsite-porphyry, rhyolite, glassy and half-glassy rocks rich 
in silica, obsidian, pearlite, pumice, and pitcJtstone. b. Without quartz, with 
plagioclase : syenite, augite-syenite, quartzless orthoclase-porphyry, trachyte, 
and augite-trachyte. c. Without quartz, with nepheline or leucite : foyaite 
and miascite, liebenerite, orthoclase-porphyry, phonolite, leucite, and sanidin 
rocks. 

II. Plagioclase rocks, a. With hornblende : quartz-diorite, diorite, 
porphyrite, hornblende-porphyrite, qmrtz-propylite, propylite, dacite, and 
(hornblende) andesite. b. With biotite : mica-diorite. c. With augite : 
diabase, augite-porphyry, melaphyre, augite-andesite, feldspar-basalt (with 
dokrite and aitamesite), and tachylyte. d. With diallage: gabbro. e. With 
hypcrsthene: hypersthenite. / With olivine : (serpentine) forellen stein. 

III. Nepheline rocks : ncphclinite and ncplidbic-liasalt. 

IV. Ltmcitu rocks: xuiittlw-li'itritt', rocks and lcii<-il<'-lt<ix<tlt. 

The rocks printed in italics are those of the true mineralogical and 



INTRODUCTORY. 7 

chemical, Tertiary and post-Tertiary, equivalents of the previously-mentioned 
ante-Tertiary rocks. It is curious that proper nephelino and leucite rocks 
arc not met with until the Tertiary age, no analogous types being known 
in older time. 

Some of the names of rocks require a more extended explanation. 
Felsite-porphyry has been preferred to the synonymous term of quartz- 
porphyry, because many of these rocks which are chemically identical 
with others do not contain macroscopical quartz, while the felsitic 
nature of the groundrnass is common to all. 

The name rhyolite was proposed early in 1860 by v. Richthofen 1 for 
certain rocks frequently occurring in Northern Hungary, distinguished 
mmeralogically from trachyte, which they otherwise resemble, by the pres- 
ence of quartz as an essential ingredient, and an almost infinite variety of 
texture, bearing clearer evidence than other rocks of having once flowed 
in a viscous state. He also united under this term the natural glasses, such 
as obsidian, pumice-stone and pearlite, which are geologically closely 
related to, and chemically identical with, the others. 

A long time before this (in 1820) Beudant had described certain non- 
glassy varieties of these rocks as trachytic porphyries. In 1861, the name 
liparite was given by J. Roth 2 to the same rock division, including the 
glassy modifications, of which well-characterized members occur in the 
Lipari Islands. Rocks of this kind, which certainly deserve to bo sep- 
arated from the trachytes, have been found in many parts of the globe, 
everywhere possessing one characteristic behavior. They are met with in 
Iceland, the Euganean Hills, Northern Italy ; the Siebengebirge in Rhenish 
Prussia ; the Aegseic Islands in New Zealand. Other geologists have sub- 
sequently named the non-glassy members quartz-trachytes. In a very 
valuable memoir presented to the California Academy of Sciences, 3 on the 
Natural System of Volcanic Rocks, v. Richthofen proposes the following as 

1 Studien aus den unRarisoh-siebenbiirgiselien Trachytgebir<*en, Jalirb. d. geolog. 
lt'idisanstalt, XT, 1860. 

'Die Gcstcinanalysen in tabdlariscber Ubersiobt mid mit kritisclien ErHlutcr- 
ungrii, IJrrlin, 1SC1. 

3 Vol. i, part ii, San l-'iaucisc... isii.s, tnmslaicd iu tlic ^citsdml't der d. guolog. 
l, XX and xxr. 



8 MICROSCOPICAL PETROGRAPHY 

subdivisions of the rhyolite group : a. nevadite, or granitic rhyolite, in which 
large macroscopical crystalline ingredients like quartz, sanidin, plagioclase, 
biotite, and hornblende predominate over the groundmass ; I. liparite, or 
felsitic and porphyritic rhyolite, in other words, the liparite proposed by 
J. Roth for the whole class, has been by v. Richthofen retained for those 
varieties which resemble quartz-porphyry or felsite-porphyry, possessing a 
porphyritic or felsitic structure ; c. rhyolite proper, or hyaline rhyolite, 
which includes the eminently glassy modifications, such as obsidian, pumice- 
stone, and pearlite. 

But this nomenclature does not seem to be suited to the natural man- 
ner of occurrence of the rocks, for, so to speak, the centre of gravity of the 
group appears rather to lie in the second series. The felsitic and porphy- 
ritic varieties seem more to merit the name of proper rhyolite than the 
glassy ones, which are always merely local, and are quantitatively inferior 
equivalents of the others. From this point of view, then, the division 
would be 'as follows : 

a. Nevadite, or granitic rhyolite. 

6. Proper rhyolite, the felsitic and porphyritic. 

c. Glassy rhyolite, obsidian, etc. 

Trachyte is a rock which repeats during the Tertiary age the mineral 
combination of the older syenite, being characterized by the predominance 
of sanidin over plagioclase, and by the absence of quartz as an essential 
ingredient. This being the general apprehension, it is strange that v. Richt- 
hofen should, in the memoir alluded to, admit an oligoclase (i. e., plagio- 
clase) trachyte. This variety does not belong petrographically to the tra- 
chytes, but to the hornblende-andesites, for it presents a groundmass in 
which the principal imbedded crystals are striated feldspar and hornblende. 
It is essential to limit the name trachyte to the sanidin-bearing rocks. 
The sanidin is indeed generally accompanied by hornblende ; but the 
examination of the trachytes of the Fortieth Parallel has proved that, in 
some cases, the black constituent, rich in lime, magnesia, and iron, and poor 
in alumina and the alkalies, is augite instead of hornblende. A new divis- 
ion, under the name augite-trachyte is therefore introduced into the petro- 



INTRODUCTORY. 9 

graphical system ; and this classification is sustained by the fact that Gr. von 
Rath has shown that among the old syenites of Tyrol and Norway, always 
considered as combinations of orthoclase and hornblende, there are real 
augite-syenites. This contemporaneous but independent enlargement of 
the older series and its newer equivalent is most interesting. 

In Hungary and Transylvania there occur singular rocks which in 
mineral composition closely resemble the old diorites, while they are inti- 
mately allied to the volcanic rocks geologically. They have, moreover, a 
commercial importance as the bearers of rich metallic veins; for instance, 
that of Kapnik, Nagybanya, etc. Having sufficient evidence of the Ter- 
tiary age of these rocks, in 1860 v. Richthofen designated them greenstone 
trachytes; for at that time rocks with prevailing plagioclase were still 
named trachytes. But subsequently he found rocks of the same distinctive 
petrographical behavior and the same geological position at Washoe and 
Silver Mountain; and since they have re-opened the eruptive activity of 
the Tertiaiy age, in all the localities where they have been met with, he 
called these precursors of all the Tertiary volcanic rocks propylites. 
Nevertheless, they always present the petrographical features of the old 
dioritic porphyries. 

In later periods of the Tertiary age, in Hungary and Transylvania, as 
well as in the western regions of the United States, has appeared another 
rock, which, like propylite, is chiefly composed of plagioclase and horn- 
blende hornblende -andesite. It was formerly called gray trachyte by 
v. Richthofen. The rock is also found in many other parts of the globe. 
It is the best proof of v. Richthofen's eminent geological perception that 
he should have separated these two rocks from each other, although he 
could not exactly explain the petrographical difference between them. 

"It escapes description. It may, at this present time, safely be founded 
on what the botanist would call 'habitus', a certain general character which 
is as easy to recognize by the eye as it is difficult to describe it in words 
and impossible to define its causes. It is probable that observations, such 
as Sorby has made in reference to those minute differences of texture which 
can only be detected with the aid of the microscope, and II. Rose in regard 
to the modifications of silica and their causes, aided by exact chemical 



10 MICROSCOPICAL 1'KTROGRAPHY. 

analyses and experiments made with the view of inquiring into the difl'er- 
(.m-.es of origin of such eruptive rocks as differ from each other in texture, 
will, if further prosecuted, reveal the true nature and cause of the proper- 
tics which distinguish these rocks." 1 

The vagueness of this diagnosis, founded upon geological properties 
alone and wanting well-defined, characteristic lithological distinctions, has 
prevented the propylite of v. liichthofen from receiving any considerable 
acknowledgment among European geologists, who doubted its specific 
independence or the necessity of separating it petrographically from horn- 
blende-andesite. But careful examination of the characteristic and typical 
propylites of the Western Territories of the United States has been the 
means of establishing a considerable number of constant microscopical 
peculiarities, sufficient to make it easy to discriminate between the two 
rocks, at least as they occur here. That petrographical differences exist 
between propylite and hornblende-andesite cannot be any longer doubted. 

Both these rocks have their quartz-bearing equivalents, agreeing with 
the old ante-Tertiary diorite and quartz-diorite. Gr. Stache was the first to 
discover 8 that rocks which were classified by v. Richthofen partly as green- 
stone trachytes (propylites) and partly as gray trachytes (homblende-ande- 
sites) contained quartz. He separated these rocks into greenstone-like 
quartz-trachytes and andesitic quartz-trachytes, and proposed dacite 3 as 
a name for both, and also for the granito-porphyric quartz-trachytes, 
although the last are really rhyolites or nevadites, with predominating sani- 
din. But because the assumed difference between greenstone-trachytes 
(propylites) and andesitic trachytes (hornblende-andesites) was not suffi- 
ciently obvious to geologists generally, the name of dacite is mostly 
confined to those quartz-bearing rocks which are marked by other peculiar- 

'Natnral System of Volcanic Rocks. P. von Richthofen, Mem. Cal. Acad., 1867. 

2 Fr. v. Hauer and G. Stache, Geologie Siebenbiirgens, Wien, 1863, 44, 102. 

3 Tho name of dacite was chosen because typical varieties of these rocks occnr in 
the ancient Roman province of Dacia. v. Richthofen is wrong when ho says Zeitschr. 
d. d. geol. Ges., xx, 1868, 692 that Stache's dacite is generally quartziferous propylite. 
Misunderstanding the observations of Austrian and other geologists, he assumes (Incite 
and <|iiurtz-propylite to lie identical, and fails to recognize in his groupings the quartz- 
bcariug incinlxT of tin- horiibk-nde-aiidesitrs his o\vu gray trachytes. 



INTRODUCTORY. 1 1 

ities as belonging to the proper hornblende-andesites. 1 Dacite is now 
always used in this sense where the specific existence of propylite has been 
established, and so it is not proper to apply it to the quartz-bearing members 
of both rocks. It may be proposed to limit the term dacite to the quartz- 
iferous homblende-andesites, and to call the equivalent member of propylite 
quartz-propylite. 

The differences between augite-andesite and feldspar-basalt will bo 
hereafter explained under the proper heads. 

v. Richthofen has shown that the succession of massive eruptions dur- 
ing the Tertiary and post-Tertiary ages in widely-separated parts of the 
earth has uniformly occurred in the following general order : 

a. Propylite, with quartz-propylite. 

b. Andesite, with dacite. 

c. Trachyte. 

d. Rhyolite. 

e. Basalt. 

mi i f i , , , P m 

1 liis order ot succession is also observable in the vast areas of Tertiary 
eruptive rocks along the Fortieth Parallel. 

T* C 1 ' J t A -U 11 u- 1 1 

It is, ot course, not designed to describe generally in this place the 
microscopical peculiarities of ordinary rock-composing minerals ; the 
diagnostic characters of single species being made a subject of occasional 
detailed statement in the following text. But this introductory chapter 
would seem to be the proper place for some remarks upon certain micro- 
scopical bodies which cannot always be identified with macroscopically- 
known minerals, and whose mineralogical nature is left more or less in doubt 
by the absence of distinctly characterizing features. It has been proposed 
(chiefly by H. Vogelsang) to designate the most frequent occurrences with 
special preliminary and subsidiary names. 

Microlites are thin, needle-formed, mostly cylindrical individuals. Many 
minerals are apt to occurinthis imperfectly crystallized form, such as feldspar, 
augite, hornblende, apatite, and mica. In many cases, it can be ascertained 
with perfect certainty towliat mini-nil a mirmlite I H'lonLi-s, when disqualifying 

'For instance, Uoelter, Tsehermak's Mini-raid^. JMil Uieilungou, 1873, 102. 



12 MICROSCOPICAL PETROGRAPHY. 

word is added, like feldspar-microlite, hornblende-microlite, etc. But, on the 
other hand, there occur a great many needle-formed products in the rocks 
which cannot be certainly referred to any macroscopically known mineral, 
either because they do not occur macroscopically or are not sufficiently well 
characterized. In such cases, where the closer signification is not to be deter- 
mined, the general group-name of microlite will be found very useful. The 
more minute the microlites are, the more the peculiarities of those belonging to 
different minerals are blended, until they are almost or entirely indistin- 
guishable. Sometimes the microlites have very curiously dichotomous, 
acicular, curled, and twisted dismembered forms, which will be described 
in detail hereafter. The regular crystals, like garnet and nosean, do not 
have a tendency to form microlites, on account of the isometric relation of 
their axes, and this is also true of the minerals which, like specular iron, 
occur macroscopically in lamellar plates. Microlites are generally the first 
product in the secretion of crystals from a molten mass. Sometimes one is 
inclined to think the microlites are the real embryos of crystals. It would, 
indeed, be possible, in the case of crystallographically and chemically 
closely related minerals, like hornblende and augite, or orthoclase and pla- 
gioclase, that the micz-olites should occasionally show a stage of primary 
development where the characteristic properties of neither have had time 
to assert themselves. For instance, a microlite may have been solidified 
so early that it failed to develop the characteristics of either hornblende 
or angite, say, but in a measure combines those of both; so that it 
really belongs to neither, having in a certain sense not decided which to 
become. 

Belonites are colorless, trichites, black and impellucid microlites. 
Both are of an uncertain mineralogical nature, and they very often occur 
in glassy rocks, like obsidian. 

Opacite : black, entirely opaque, amorphous grains and scales, which 
often appear as metamorphic products, resulting from the decomposition of 
other minerals. These little bodies may be of very different substances, 
formless magnetite, earthy silicates, amorphous metallic oxyds (especially 
oxyds and hydrous oxyds of titanium and manganese), graphite, etc. 

Ferrite: yellowish, reddish, or brownish amorphous earthy substances, 



INTRODUCTORY. 13 

which are not infrequently pseudomorphous after iron-bearing minerals. 
In most cases, this rust-colored, powder-like material doubtless consists 
of sesquioxyd of iron free from water or in the hydrous state, but it cannot 
usually be identified with any mineral. 

Viridite: green and transparent substances in the form of scaly or 
fibrous aggregations, which very often result from the decomposition of 
hornblende, augite, or olivine. Their composition is not always the same. 
They may belong chiefly to silicates of monoxyd of iron and of magnesia. 
The scales for the most part belong to a chloritic, the fibers to serpentinous 

1 1 , VI 1 

or delessite-Iike mineral. 

These names have been offered merely for the sake of convenience to 
obviate the necessity of repeated long descriptions, and w.ill serve only so 

long as our ignorance of the proper mineralogical nature of the substances 

,. , i, ,. 
in question shall continue. 









CHAPTER II. 

___ 

CRYSTALLINE SCHISTS AND RELATED ROCKS. 

Of the inetamorphic rocks of the Washoe district, through which 
diorites and afterward Tertiary volcanic rocks of the Virginia range have 
protruded, there may be only mentioned in this connection a slaty horn- 
blende rock [I], 1 which occurs on the hills above American City. It con- 
sists of a colorless quartz-ground, in which are distributed an innumerable 
quantity of small crippled prisms and irregular laminae of pale-green horn- 
blende, some lobes of brown mica, and only a very few striated plagioclases. 

The gneiss from the north end of the Lake Range, Nevada [2], repre- 
sents a remarkable variety, which not infrequently reappears among the 
crystalline schists of Nevada and Utah, but which is otherwise very rare and 
has not yet been particularly studied. The feldspar is almost entirely tri- 
clinic plagioclase, the colorless fresh sections of which bear the most rich, 
delicate, and variegated striation in polarized light. There is only a very 
little unstriated orthoclase present. Beside the brown mica (biotite) and 
quartz, this gneiss also contains beautiful green hornblende, so that it really 
becomes an equivalent of quartziferous mica-diorite, in the same manner 
as the common mica-gneiss corresponds to granite, and the usual horn- 
blende-gneiss to quartz-syenite. With reference to the nature of its feld- 
spars, it may indeed be named a diorite-gneiss. To this rock seem to be 
allied the gneisses rich in oligoclase, which Fischer has observed at Todtmoos 

'The actual baud-specimens from which these descriptions are written will receive, 
beside the collection-number, a special number corresponding to the bracketed numeral 
throughout this memoir. When, therefore, the collection finds its permanent resting- 
place in the National Museum, students will be able to identify, not only the species, 
but individual rocks, described by Professor Zirkel. 0. K. 

14 



CRYSTALLINE SCHISTS. 15 

and Gropbach in the Miinsterthal of the Black Forest, and v. Hochstetter 
at Adams' Peak in Ceylon; yet they differ by containing more orthoclase 
and less or no hornblende. The brown mica of this rock is, as always, 
easily to be distinguished by its excellent lamellation, by its strong dichroism, 
and by its powerful absorption when examined with one nicol. For the 
hornblende, which shows also important but somewhat less dichroism, the 
distinct cleavage according to the obtuse angle of the prisms (oo P) is highly 
characteristic. The transverse sections of the biotite plates become nearly 
black when the direction of their lamellation forms an angle of 90 with 
the short diagonal of the polarizer. The quartzes of this gneiss are strik- 
ingly poor in microscopical fluid-inclusions, those which are present being 
exceptionally large; on the other hand, both quartzes and plagioclases con- 
tain the most finished, sharply edged, brown laminae of mica, whose diameter 
often decreases to a few thousandths of a millimeter. The enormous quantity 
of microscopical apatite prisms found in this rock is remarkable, as in all 
gneisses which abound in hornblende. 

A series of curious crystalline slates occurs in Trinity Caflon, Monte- 
zuma Range, Nevada, and in some adjacent hills [3, 4, 5]. To the 
naked eye, they look almost wholly homogeneous or without any dis- 
tinct constituent; they have a dark, grayish-black color, a finely glitter- 
ing lustre, and are not easily fissile; so that at first sight they might be 
mistaken for certain fine-grained anamesites or basalts. The microscopical 
analysis discovered a wholly crystalline mixture of quartz, brown mica 
in large proportion, white mica (muscovite), black particles of magnetite, and 
a few grains and prisms of a pale-green mineral which seems to be a kind of 
hornblende. The brown and white mica laminae show a remarkable phenome- 
non, which has never been elsewhere observed, but which often reappears in 
tint rnica-bearing rocksof this region. 1 The same microlites are also imbedded 
in (he mica of the Kersanton from Brest in Brittany. They often contain 
very numerous, thin, delicate microlites, which have a tendency to cross each 
other after the manner of a net, and uniting in hurdles in the middle of the 
micas, or forming stars and groups whose members radiate from a centre. 
These microlites are often dichotome on the ends, and always spread out in 
'See representation of another rock, Plate II, fig. 2. 



16 MICROSCOPICAL PETROGRAPHY. 

the plane of lamination. Their proper color is not distinguishable, and their 
mineralogical nature must remain uncertain, but they may belong to the 
above-mentioned green mineral, for some varieties of these rocks exist 
[4] in which the latter is wanting, and hero the laminae of the brown mica 
are found to be entirely pure. 

The variety frt>m the ridge west of Pahkeah Peak, Pah-tson Mountains, 
Nevada, contains, beside the brown and white micas, an oil-green mica in 
thin laminae, larded with fine, limpid quartz grains. None of these rocks 
contain a trace of either monoclinic or triclinic feldspar. 

A similar metamorphic rock is found in the ridge east of Pahkeah Peak, 
Pah-tson Mountains [6]. Quite crypto-crystalline and almost homogeneous, 
light greenish-gray in color, it shows under the microscope a colorless 
quartz-ground, in which an immense quantity of very pale-green hornblende 
prisms have been associated and crowded together in single heaps, which 
are often elongated in one direction. As in the previously described rock, 
feldspar is here wanting. 

The Humboldt Range offers an excellent field for the study of the 
Archaean crystalline schists. On the west slope of these mountains, there 
occurs a brown mica-schist [7] with curious yellowish-gray knots which 
reach a diameter of 8 mm . The chief mass of the slate itself (Plate II, 
fig. 1) is a mixture of quartz, brown and colorless mica, the latter 
being remarkably laminated, so that its transverse sections, in ordinary 
light, might be mistaken for polysynthetic plagioclase; yet between the 
nicols, of course, in spite of the striation, no variegated lineation appears, 
only monochromatic polarization. A few very distinct crystals of plagio- 
clase are also present. Under the microscope, the knots present a very inti- 
mately entangled web of extremely thin, colorless, needle-formed microlites, 
heaped together primarily in bunches, which are again woven together. On 
the exterior of the knot-sections, the felt becomes looser, and, as may be 
clearly seen, is imbued with water-clear quartz. The outward margin con- 
sists of quartz, in which are isolated needle-tufts. As to the nature of these mic- 
rolites, it is highly probable that they belong to fibrolite or bucholzite, which, if 
not identical with sillimanite or disthene, is only a variety. The bucholzite 
from Bodenmais in Bavaria is a mineral so similar in every respect that it 



CRYSTALLINE SCHISTS. 17 

might be mistaken under the microscope for these Humboldt microlites. 1 
These fibrous bunches possess a further striking resemblance to those in the 
cordierites of the Saxon gneisses (Go'hren near Wecuselburg, environs of 
Rochsburg, Galgenberg near Mittweida), and especially the cordierite in 
that from Bodenmais, and which, according to all appearances, belongs like- 
wise to the fibrolite. 9 In the American specimen, the needle-bunches also 
lie abundantly in the independent quartzes and brown micas of the rock, 
just as both also occur in the gneiss from Bodenmais. Where the microlites 
are heaped together into sheaves, they are often curved, crooked, and dis- 
membered into single short pieces as in similar European rocks. Hydrous 
oxyd of iron has penetrated narrow fissures of these clots, and given them 
a yellowish tinge. At Spruce Mountain [8], Peoquop Range, Nevada, 
another mica-slate occurs which exactly agrees with this interesting variety. 

Clover Canon in the Humboldt Mountains yields an excellent and rich 
assemblage of varieties of Archaean crystalline schists. The gneisses are 
generally composed of orthoclase, quartz, brown mica, and some hornblende. 
Their feldspars are distinguished by the contained fluid-inclusions, which, 
as is well known, are elsewhere not often found in this rock-constituent. 
Here the feldspars in some instances contain more liquid-inclusions and 
empty cavities than the granitic quartzes, being actually surcharged with 
them. Here also the feldspars are decidedly richer in fluid-particles than 
the quartzes of these rocks: the inclusions have a rectangular or irregular 
shape, with numerous ramifications and branches, while those of the quartz 
are either rounded or more regular in form. This interesting peculiarity in 
the structure of feldspar, hitherto supposed to be very rare, is surprisingly 
common through the gneisses, granites, and younger eruptive rocks of the 
Fortieth Parallel ; but it can, of course, only be observed where the feldspar- 
substance remains clear and unaltered. Many fluid-inclusions which were 
once present in other feldspars may have been obliterated by decom- 
position, to which this mineral falls an easy prey. 

Most of the gneisses from Clover Cafion may also be distinguished by the 

1 See F. Z., Mikroskopiscbe Bescbaffenheit der Mineralien nncl Gesteine, 1872, 200. 
* Ibid., 209. 
2 M P 



18 MICROSCOPICAL PETROGRAPHY. 

surprising quantity of proportionally thick and strong individual prisms of 
microscopical apatite, which often have a length of 0.5 mm , are glaring and daz- 
zling, and are frequently traversed by basic cracks, broken into several pieces, 
dismembered into single joints which lie behind each other in a straight line 
or show the phenomenon of reciprocal dislocation. Some of the small members 
are moved out of a straight line, which sometimes breaks the apatites into a 
dozen pieces and throws them out of an even row into a curved bow behind 
each other (Plate I, fig. 6). Many of the larger of these apatites also show 
traces of the well-known fine, dust-like material rendering the interior impure, 
generally accumulated in the form of a thin strip along the chief axis ; but 
here, as elsewhere, this interposed substance is so fine that its nature cannot 
be determined. Long, cylindrical cavities are often visible in these apatites. 
All the prisms seem to be very sharply-featured, for the hexagonal sections 
of the transversely cut, vertically standing, or somewhat inclined columns 
(which let through a dazzling light) are very regularly edged, while in the 
other rocks these hexagons are nearly rounded. These apatites are found 
included in all rock-constituents, in the quartzes, feldspars, micas, and horn- 
blendes ; and they also show in this crystalline schist the familiar peculiarity 
of being gathered in numerous individuals upon a limited space, as if the 
phosphoric acid had not at the outset been equally spread through the rock- 
material. Moreover, it is remarkable that apatite should occur with such 
strikingly similar behavior in rocks which differ so widely in their genetic 
history. The apatites of the basalts and phonolites are, as such, not in 
general distinguishable from those in the crystalline slates, which, what- 
ever may be the opinion as to their origin, have certainly been formed 
differently from basalts or trachytes. 

In Clover Canon, there also occurs a gneiss [9] poor in mica, whose 
quartzes contain the very curious double inclusions, the interiors of which 
consist of liquid carbonic acid, (Plate I, fig. 1). Seen from above, they 
present three circular lines arranged concentrically. The outermost line is 
often somewhat angular, and marks the limit of the whole inclosure ; the 
middle one is the external limit of the carbonic acid ; the innermost circle is 
the bubble which lies in this fluid. There is not the slightest doubt that 
this liquid b is really carbonic acid, for the bubble c within it shows one 



CRYSTALLINE SCHISTS. 19 

of the most characteristic reactions. When subjected to a rising tempera- 
ture, a heat of only 31 C., it disappears by condensation, and reappears when, 
by cooling, that point of temperature is again reached. There is no other 
liquid, except nitrous oxyd, which possesses at so low a degree such an 
enormous expansive power. 

Vogelsang has shown, by a series of ingenious experiments, that quartzes 
containing a liquid having these peculiarities immediately gave, in a spectrum- 
apparatus, the excellent and characteristic lines of carbonic acid. The min- 
eral decrepitates upon being heated in an exhausted tube. Such quartz, 
powdered in lime-water, causes a precipitate of carbonate of lime, owing to 
the presence of the carbonic acid in little hollows. 1 Nearly at the same 
time that Vogelsang demonstrated the above, Sorby was engaged in show- 
ing, by measuring its expansive force, that the curious fluid in some sapphires 
is also carbonic acid. 8 As to the external zone a which surrounds the fluid 
and separates it from the quartz, Brewster has observed in Brazilian topazes 
inclusions analogous to these, and he believed that there were two distinct 
liquids, one outside the other. 3 But as against this explanation is the fact 
that by heating our inclosures even to and above the high temperature of 
120 C., the external boundary-line of the interior carbonic acid never 
shows either dilation or contraction, or, indeed, any alteration whatever, 
which probably ought not to be the case if the environing substance were 
likewise a liquid. Vogelsang, therefore, believed that the external zone 
was a solid, and he was inclined to regard it as a topaz substance of differ- 
ent density, which may have been produced by the'expansive nature of the 
interior fluid. If this interpretation is right, we should see in the external 
zones of our inclusions a quartz substance in a somewhat different state of 
pressure ; but it will be evident that upon this supposition the veiy sharp 
outermost boundary-line of the whole inclusion is somewhat striking. 
Moreover, all these inclusions are very similar to the elsewhere-occurring 
glassy inclusions which contain a fluid with a bubble ; but it may be 

1 PoggendoriFs Annalen, cxxxvn, 1869, 56, 265. 

* Proceedings of the Eoyal Society, xvir, 1869, 291 ; Monthly Microscopical Journal, 
1869, 222. 

3 Transactions of the Royal Society, Edinburgh, x, 1826, 407. 



20 MICROSCOPICAL PETROGRAPHY. 

declared, without further proof, that the inclusions of the gneiss-quartz 
cannot be identified with the latter. In the quartzes of our gneiss, the 
largest double inclusion of this kind measures 0.0072 mm in length 
and 0.0004 mm in breadth at the broadest part. The little gas-bubble 
in the carbonic acid is in constant spontaneous motion, rolling incessantly 
through the fluid, in a whirling dance, like a thing of life. Beside 
these, there occur other liquid inclusions, which do not possess the 
external zone a, but, when the quick absorption of the bubble is consid- 
ered, they too are seen to consist of the same remarkable acid. Liquid 
inclusions of carbonic acid are at present known to occur in the quartzes 
of the granitic gneiss of the St. Gotthard (very similar in every respect 
to those which we have been examining), in the quartzes of the gray 
gneiss from Freiberg in Saxony, and of the granite from Aughrushmore 
in Ireland ; in the topazes from Rio Belmonte in Brazil j 1 in some sap- 
phires; s in augites, olivines, and feldspars of basalts from Rhenish Prus- 
sia, Hesse, Wurternberg, Hungary; 3 in the greenish apatites from the 
Pfitsch Valley in Tyrol/ If the shortness of this list seems to warrant 
the conclusion that these highly interesting inclosures are very rarely 
found in the constituents of rocks, the following lines will prove that 
they occur in surprising frequency in the quartzes of the gneisses, mica- 
slates, and granites from the western part of the American continent, and 
that they are almost a common phenomenon in the rocks of the Fortieth 
Parallel. 

These double inclosures have been observed in the quartzes of many 
gneiss sections from different localities in Clover Canon, and in none are 
they more numerous, distinct or larger than in a variety poor in mica [10]. 
The quantity of quartz and mica in these gneisses varies considerably, 
some being poor in quartz, rich in mica [11, 12]; others showing an inverse 
proportion of these minerals. Apatite is by far more abundant in the 
gneisses which contain more or less hornblende than in those which bear 

1 Vogelsang, loc. cit. 2 Sorby, loc. cit. 

3 F. Z., Untersuchungen iiber die mikroskopische Structur uud Zusammensetzung 
der Basaltgesteine, 1870, 21. 33, 60. 

'Rosenbusch, Mikroskopische Physiographic der petrographisch wichtigsten 
Miueralieii, 1873, 220. 



CEYSTALLINE SCHISTS. 21 

only mica and are free from hornblende. Some of these crystalline schists 
contain an amount of carbonate of lime which causes the rock to effervesce 
when wetted with acids, and under the microscope it can sometimes be 
observed as small colorless particles of calc-spar traversed by oblique- 
angled fissures. 

Another rock from Clover Canon almost represents a diorite-gneiss 
[13]. It bears more plagioclase than orthoclase, the former being splendidly 
striated, considerable quartz, brown mica, much green, excellently cleavable 
hornblende, with included grains of black magnetite, and interesting phe- 
nomena of fracture, the ends of the thick prisms being often totally splin- 
tered, and the shivered pieces moved asunder and disjoined, but not so far 
separated that it is not plain to which individual hornblende they belong. 
Titanite appears here in pale grayish-yellow, sharp, oblique-angled sections 
having a rough surface, in all respects similar to those in the hornblende- 
bearing " Fundamental Gneisses" of the Loch Maree in Scotland, which 
there form the base upon which rest the Cambrian conglomerates and 
sandstones. This gneiss is rich in apatite, which is worth mention- 
ing because it contains, in an uncommonly distinct measure, large micro- 
scopical fluid-inclusions (see Plate I, fig. 7, representing a similar apatite 
from a granite). Sometimes they are rounded, sometimes heaped together 
in cylindrical form, sometimes more isolated and stretched out parallel 
to the chief axis of the crystal. The fluid here is not liquid carbonic 
acid, but is probably water, with perhaps a little carbonic acid; for by 
heating the section up to 110 C. no characteristic absorption of the 
bubble is discovered. Before now, such fluid-inclusions in apatites have 
only been observed in those of the hornblende-andesite from the Hem- 
merich near the Seven Mountains in Rhenish Prussia, 1 and in the 
up-grown macroscopical crystals from Schlaggenwald in Bohemia and 
the Pfitsch Valley in Tyrol, 8 where, however, they are not constituents 
of rocks. 

To these interesting rocks of Clover Canon also belongs a gneiss from 
the foot-hills north of Secret Pass in the same Humboldt Range [14]. This 

1 F. Z.. Mikroakopisehe Besclnift'enheit tier Miueralien, Gest., 223. 

2 Koscubnscb, Mikroskopiscbe Physiographic, 220. 



22 MICROSCOPICAL PETROGRAPHY. 

variety is grained more like granite. While not as slaty as those previously 
mentioned, it bears many apatites and numerous pale brownish-yellow, little, 
needle-formed, prismatic crystals, which are widely spread through the crys- 
talline schists of this region. These crystals have a very dazzling lustre 
and many faces, but their form is not sufficiently distinct to allow of recog- 
nizing them. In many cases, they appear to belong to the tetragonal system, 
and to present the combination of a pyramid, a prism of a different order, 
and of a ditetragonal pyramid (P. oo P co . n P n), a form which suggests 
the supposition that they may belong to zircon. They are certainly very 
similar to the microscopical zircon-crystals which occur in the eklogites of 
the Fichtelgebirge in Germany. The same somewhat obscure crystals are 
present in great numbers in the Saxon granulites, where it was also impos- 
sible to determine their nature with exactness. Zircon possesses the high 
index of refraction (M=1.95), and beside this the single faces are more 
difficult to recognize than in other crystallized minerals of the same size. 
Nevertheless, it is true that Professor Bunsen, of Heidelberg, who kindly 
consented to analyze one of these gneisses rich in the brown prisms, could 
find no trace of zirconium. With reference to this analysis, it should be 
remembered that the prisms are of microscopical fineness, that they con- 
stitute only a very small part of the bulk of the whole mass, and that the 
eminent analyst had at his disposal only a very small quantity of the rock. 
Beside this, as he says in a letter, the detection of minute quantities of 
zirconium is one of the most difficult processes in analytical chemistry. 1 
Associated with the gneisses and mica-schists in Clover Canon are some other 
interstratified rocks which are wont to accompany these crystalline slates. 

Among them is a pure hornblende-rock (amphibolite), remarkable for 
the absence of other microscopical constituents, there being neither quartz, 
apatite, nor titanite [15], a variety very rarely observed. The contrast is 
very nice between the long, parallel, fibrous, longitudinal sections and the 

1 Mr. R. W. Woodward, the Chemist of the Exploration of the Fortieth Parallel, 
afterward treated a much larger amount of this same gneiss, and succeeded, by a pro- 
cess of his own, in isolating a sufficient quantity of zirconium to subject to a rigid 
examination. The details of his process, together with the analysis of several other 
zirconiferous schists, will be found iu the chapter on Archaean Rocks, in Volume I of 
this series. C. K. 




CRYSTALLINE SCHISTS. 23 

transverse section of the dark-green hornblende, which shows prismatic 
cleavage in an unsurpassed manner. Hence it is evident that the individual 
hornblendes have crystallized into and through each other, and are mutually 
intertwined in the most complicated manner. Certain isolated fluid-particles, 
with spontaneously moving bubbles contained in the hornblende, are espe- 
cially worth mentioning, for they are the first liquid-inclusions which have 
ever been detected in this mineral. Hornblende must in general be very 
poor in them, even more so than augite, in which they have been known for 
a long time to occur occasionally. Perhaps it is allowable to presume that 
the usual fibrous texture of the hornblende is unfavorable to the incorpora- 
tion of fluid-particles. 

The quartzites from the western slope of the Humboldt Mountains, and 
from Clover Canon [16, 17, 18, 19, and 20], contain in the predominant quartz 
microscopical fluid-inclusions and empty cavities, and besides, laminae of 
colorless (never brown) mica, and some feldspar crystals, often light-brown 
hydrous oxyd of iron in fissures. Here and there long, thin microlites (per- 
haps belonging to hornblende) lie in the quartzes. These are so narrow that 
they appear, even under a high magnifying power, merely as fine, short, 
black lines. 

In a quartzite from Clover Canon [20], long, quite pale-green needles, 
which very probably are actinolite, are interspersed with crooked and curved 
ends, and present the most detailed phenomenon of fracture. One of these 
needles was found broken into not less than fifteen pieces. Rocks somewhat 
similar to these of the Humboldt Mountains are observed a little farther east, 
of which the following may be mentioned : a gneissic micaceous slate from 
Egan Cailon, Egan Range, Nevada, in which the zircon-like mineral, men- 
tioned above, occurs again very abundantly. Two thin, light-brown indi- 
vidual members of this are often grown together in the form of a right- 
angled joint ; nevertheless, it would seem to be improbable that a twin law 

exists here, for the angle is not always the same, being sometimes sharp 

, 
and at others obtuse. 

A mica-slate from Spruce Mountain, Peoquop Range [21], consists of 
quartz, much deep-green and a little white mica, and the zircon-like mineral. 
The laminae of biotite here are likewise filled with fine needles, which some- 



24 MICROSCOPICAL PETROGRAPHY. 

times show a hexagonal arrangement (Plate II, fig. 2). It is not impossible 
that they belong to the zircon-like mineral. They are confined to the mag- 
nesian mica, curiously enough, never having been found interposed in the 
white mica or quartz. 

Another mica-slate is observed interstratified in white quartz at Pilot 
Peak in the Ombe Mountains, the brown mica of which is entirely free from 
microlites, and the quartz is noticeable by reason of containing almost no 
fluid-inclusions. 

Ogden and Farmington Canons in the Wahsatch Range, Utah, present 
an excellent field for the examination of crystalline schists of the Archaean 
series. Hornblende-gneisses generally predominate in these canons. 
There is one specimen from the mouth of Ogden Canon [22] which 
is made up of orthoclase, considerable plagioclase, quartz, brown mica, 
and plenty of hornblende and apatite. It is seen that in the transverse 
sections of the biotites, sharp and limited layers of a colorless substance 
are imbedded parallel to the lamination of the individual biotites, or to 
the chief plane of cleavage. It seems most probable that they belong 
to white mica, although such a coalescence of the two species, as far 
as is known, has never been observed either macroscopically or micro- 
scopically. 

Another hornblende-gneiss from the same canon [23] is made up 
of like constituents, but it contains besides, short brown prisms of the 
zircon-like mineral svith a pretty distinct quadratic transverse section. In 
hand-specimens, the direction of the black prisms of hornblende shows very 
plainly a linear parallelism; elsewhere it is the mica that presents this phe- 
nomenon of macroscopic structure. The quartzes of this variety are very 
poor in fluid-inclusions. 

A characteristic hornblende-gneiss comes from Offden Point [241, 
is very coarse-grained, and contains orthoclase, a comparatively large 
amount of plagioclase, quartz, a little brown mica, and much horn- 
blende, to which nearly all the dark ingredients belong. Apatite is not 

wanting. 
a 

Other gneisses from this point [25, 26] are poor in hornblende, but 
they possess the zircon-like mineral very well developed and in consider- 



CRYSTALLINE SCHISTS. 25 

able frequency. Prof. Robert Bunsen, of Heidelberg, has very kindly 
analyzed this gneiss; he gives the following as the result: 

Silica 74. 95 

Alumina - 9. 42 

Sesquioxyd of iron 7. .47 

Lime 1. 65 

Magnesia 0. 13 

Potash 2. 02 

Soda 4. 05 

Water . 1. 02 



100.71 

Farmington Gallon in the Wahsatch has a gneiss [27] with grains of 
garnet more than l mm thick, which, under the microscope, give intensely 
red sections and appear perfectly dark between crossed nicols. They are 
traversed by a net-work of fissures having multitudinous ramifications in 
which some green matter has settled, running in narrow veins through the 
ganiet in all directions. Without doubt, this epigenetic substance is chlo- 
rite, which has been formed at the expense of the garnet bounding the cracks. 
This is the first step in the development of the true pseudomorphs of chlorite 
after garnet, which may be seen in the best quality and on the most mag- 
nificent scale in the iron-mine of Spurr Mountain in the Lake Superior 
region. 1 This example again demonstrates molecular alteration to be by the 
bursting and shivering of a crystal, as has been observed in a corresponding 
manner in olivine, 8 where it is decomposed into serpentine, and in cordierite, 3 
with its numerous pseudomorphous descendants. The substance of the 
garnets, as such, is quite free from impurities, as is usually the case with the 
garnets of crystalline schists and granulites. Very small rounded garnets, 
in form like dew-drops, the largest measuring 0.01 mm in diameter, are also 
interspersed in the feldspars and quartzes of the rock ; but there exists no 
transition in size between these and the macroscopical ones, which are a 
hundred times larger. In Farmington Canon, however, the garnets are 
not confined to hornblende-gneisses. 

' Kaphael Pnmpell;, Am. Jour. Sci., July, 1875, 17. 

'Tscbenuak, Sitzungsber. d. Wiener Akad., LVI, 18C7, I. Abtb.,Juli Heft, 1. 

3 F. Z., Mikroskopische Beschaffenheit der Mineralieu mid Gesteine, 211. 



26 MICROSCOPICAL PETEOGEAPHY. 

A mica-gneiss bearing garnet [28] also occurs, composed of alternating 
light and dark layers, in which there is no hornblende. The light layers 
are rich in feldspar and quartz, poor in brown mica and deep red garnet (or 
altogether without them) : the dark layers are abundant in brown mica and 
deep red garnet. The larger garnets lying along fissures are altered into 
green chlorite. Besides these macroscopical individuals, the rock only 
contains microscopical garnet grains, which are often found gathered into 
little heaps. Other beautiful hornblende-gneisses of this cafion are destitute 
of garnet. One of these rocks [29] which has little mica an*d feldspar, pre- 
sents again the yellowish-brown, zircon-like crystals. The quartz of this 
variety, as is commonly the case with these hornblende-bearing gneisses, is, 
in a striking measure, poor in fluid-inclusions, if not entirely wanting in 
them. 

This quantitative proportion of the ingredients often re-appears in Far- 
mington Canon, where gneiss [30], enormously rich in apatite, contains brown 
prisms and quartzes which are remarkably deficient in the liquid-inclusions 
that are elsewhere common to them. The peak north of the head of Farming- 
ton Canon is made up of a coarse gneiss, with not very much mica, no horn- 
blende, and no apatite : the feldspars are isabel-colored and decomposed into 
a rather impellucid substance ; the quartzes have numerous fluid-inclusions, 
and the brown zircon-prisms are wanting. 

The hornblende-schists [31] which occur in Farmington Canon appear 
under the microscope as splendid mixtures of dark-green, entirely pure and 
fresh hornblende and colorless quartz, which, like that in the hornblende- 
gneisses, is generally very poor in fluid-inclusions. 

Another metamorphic hornblende-schist [32] from the small cafion on 
the west slope of Twin Peaks, north of Little Cottonwood Canon, in the 
Wahsatch, consists of quartz in the form of a groundmass, penetrated by 
innumerable small, tender, nearly bluish-green-colored hornblende prisms, 
which unite inside into pretty bunches and stars : there are also thicker 
independent hornblende prisms and some brown mica. Sometimes 
quartzites are found in the neighborhood of the gneisses of Farmington 
Canon. 

tinder the microscope, one of these, having a grayish flesh-color 



CRYSTALLINE SCHISTS. 27 

[33], is seen to be composed only of rounded and evidently worn grains 
of quartz. The exact age is unknown, its relations to the neighboring 
gneisses obscure, and is probably referable to the Cambrian and not the 
Archaean series. The single fragments of quartz show evidence of an 
amassing by flowing waters, and possess the most diverse structure and 
behavior. Some of them are rich and others very poor in fluid-inclusions ; 
some are abundant in thin black microlitic hairs, which others entirely 
lack; in other quartz-grains, a dirty yellowish-gray, dust-like, and very 
fine grained matter, never before observed, is interposed in parallel bands, 
which are often curved or twisted. Its mineralogical nature cannot be dis- 
tinguished. The grains of quartz are also cemented by penetrating silica, 
which belongs not to opal, but to quartz, as was proved by examining it in 
polarized light ; but there is only a little of this luting material. 

At Twin Peaks Ridge, in the Wahsatch, an unmistakably Archaean 
quartzite occurs [34] which is filled up with microscopical laminae of brown 
mica, and shows numerous fluid-inclusions in the quartzes. 

A remarkable garnet rock is found as a member of the Archaean series 
at the head of Big Cottonwood Canon in the Wahsatch [35]. It is a quartz- 
iferous, rather coarse-grained mass, which contains macroscopically in its 
little hollows a fine, delicate spinning of green epidote. In the thin sections, 
the garnets have a feeble brownish-yellow color, and present a most excellent 
layer-structure, owing to the visible enveloping of individual schists during 
the growth of the crystals. The single zones which surround each other in 
the sections like concentric shells are only 0.0015 mm thick, so that 666 schists 
coine upon l mm . Pretty contrasts of color are often seen in the different 
layers and combinations of layers, mostly in darker and lighter tones of the 
yellowish-brown. Traces of lamellar polarization are in harmony with this 
structure, the sections showing between crossed nicols, single, colored lines 
in the dark principal mass. There can be no doubt that this polarization of 
single layers is produced by pressure. The schistiform structure of garnets 
has often been maroscopically observed in a rough development (as in 
the crystals from Cziklowa and Orawicza in Hungary); but as far as is 
known, it nowhere else appears in such distinct, detailed, and multifarious 
microscopical development as here. The crystals possessing this struc- 



28 MICROSCOPICAL PETROGRAPHY. 

turo have grown to their present size by repeated superposition of material, 
the process being marked by intervals of inactivity, each wrapper-like zone 
representing the result of a period of formation, and the planes between two 
of them indicating intermissions in the time of growth. The quartz accom- 
panying these perfectly pure garnets is rich in large fluid-inclusions, which 
are splendidly formed, sharp, hexagonal pyramids, so that one may look 
through the pellucid quartz substance and distinctly see even the corners 
and edges on the farther side of one of these negative crystals containing a 
liquid. It is but rarely that fluid-inclusions are found which so well repre- 
sent the exact form of the including crystal. This is, however, as is well 
known, a phenomenon that often occurs with imbedded glassy particles. 
Calcite is present in this rock in addition to quartz and garnet. 

Other remarkable crystalline schists, belonging also to the old Archasan 
series, form wide regions of country in Colorado. Here also hornblende- 
bearing rocks seem in general to predominate, in striking contrast to Ger- 
many, where very extensive areas of schists of the same geological position 
occur in which no hornblende-gneisses are developed. 

The fine silvery-white, scaly mica-slate from Eed Creek, Uinta Mount- 
ains [36], bears such a striking resemblance to the well-known beautiful 
paragonite slate from Monte Campione, near Faido, at the St. Gotthard, 
Switzerland, that it is difficult to distinguish one from the other the more 
so since it contains excellent large crystals of pale-blue disthene (cyanite). 
The rock also bears staurolite crystals in the same way that they accompany 
the disthene in the neighboring darker mica-slate from Airola on the southern 
declivity of St. Gotthard. Under the microscope, the American occurrence 
presents merely a mixture of almost wholly colorless, irregularly hexagonal 
laminae of paragonito, with ledge-formed transverse sections, which perfectly 
accords with the appearance of the chief ingredient of the Monte Campione 
rock. Here, however, the likeness ends, for neither disthene nor stau- 
rolite is found in microscopical crystals ; these constituents in this case 
are confined to the large macroscopical individuals. But very few dark 
oil-green and brownish-green laminae of mica are scattered through the 
aggregation of paragonite, and a number of bluish-gray sections of highly 
dichroitic tourmaline are present, of which the prismatic individuals are 



CRYSTALLINE SCHISTS. 29 

often broken into many pieces. The disthene here is like the Swiss, 1 and, 
like that in the Saxon granulite, totally free from any interposition: it cer- 
tainly seems that this mineral is one of the purest we know of. It was 
highly interesting to examine microscopically this new American occurrence 
of staurolite to prove whether it also belongs to those which are mixed so 
abundantly with quartz. Chemical analyses of staurolites from various 
localities, and even of those from one and the same locality, differ very 
considerably from each other, as has long been known. In general, the 
amount of silica varies from 27. to 51., that of alumina from 35. to 55., that of 
oxyd of iron from 13. to 23. per cent., so that it has been impossible to con- 
struct a formula suitable to all. The staurolite of St. Gotthard contained the 
lowest amount of silica of any. Lechartier 3 then pointed out that when 
the other staurolite occurrences, especially those of Brittany and from 
Bolivia, are powdered ; under the microscope, beside the usual red grains 
of the staurolite which correspond with the powder of those from St. 
Gotthard, a great number of unknown colorless grains become visible^ 
which can be extracted by hydro-fluoric acid, leaving the proper staurolite 
substance unattacked. The chemical composition of the Breton stauro- 
lite, originally containing up to 54. per cent, silica, after the removal of 
those pellucid, water-clear, interposed grains, entirely agrees with that 
from St. Gotthard in its natural state (with 28.5 per cent, silica). 
Curiously enough, Lechartier did not express the sufficiently- warranted 
supposition that the grains belong to quartz. Ascertaining the observations 
of Lechartier, H. Fischer 3 then demonstrated that the intermixed substance 
which adds to the natural quantity of silica was really quartz, v. Lasaulx, 4 
to whom we owe a careful examination of the different occurrences of 
staurolite, subsequently corroborated this, and showed that they here and 

1 Von Lasaulx has examined these analogous Swiss rocks microscopically Neues 
Jalirbuch f. Mineralogie, etc., 1872, 835. 

2 Bulletin de la Soc. chimique (2), III, 1865, 378. 

1 Kritische uaikroskopisch mineralogische Studien, erste Fortsetzung, Freiberg i. 
Br., 1871, 55. 

4 Mineralogische Mittheilungen gesammelt v. G. Tschermak, 1872, in. Heft, 173. 
See also, on the structure of staurolites, K. Peters, Sitzungsber. d. Wiener Akad., LVII, 
1. Abtheil., 646. 



30 MICROSCOPICAL PETROGRAPHY. 

there include still other microscopical minerals, such as dark mica, magnetite, 
garnet, and brookite. The yellowish or reddish brown sections of the 
new American staurolite (Plate II, fig. 3) microscopically agree in every 
respect with those of Brittany, the really stereotyped accompaniment of 
colorless rounded quartz grains being visible even in common light. The 
quantity of the inclosed quartz is so great that the staurolite would surely, 
upon a chemical analysis, show silica to an amount between 40. and 50. 
per cent. 

There also exists on Red Creek a beautiful hornblende- rock [37], the 
spaces between whose broad diverging prisms are filled with colorless quartz 
and a very few feldspar particles free from apatite. 

The granite-gneiss from Rawling's Peak, Wyoming [38], consists of 
quartz, feldspar (mostly plagioclase), and hornblende. In structure, it is 
between a granite and a gneiss, not sufficiently grained and without enough 
direction for the former and not sufficiently slaty for the latter. This horn- 
blende does not form any distinctly featured individuals : it presents mem- 
branes instead, which appear, under the microscope, to be composed of 
innumerable, little, regularly shaped microlites. One may distinctly see how 
these needles are associated with green grains in such a way as to make 
what appears macroscopically as an individual of hornblende. In the 
other constituents of the rock, pale-green, fine prickles, or needles, and 
grains of hornblende are abundantly scattered, especially in the plagio- 
clase; the quartz has less of them. The latter is, in striking contrast to 
the former, enormously rich in fluid-inclusions, which contain, with unusual 
constancy, little cubes of chloride of natrium. These remarkable inclu- 
sions have been considered till now as rather rare in the quartzes of some 
granitic, gneissic, and porphyritic rocks, and in some minerals, but they 
appear here in considerable frequency, and the following pages will show 
that they are found widely spread through the analogous rocks of the 
Fortieth Parallel. These microscopical inclusions (Plate 1, fig. 2) consist 
of saturated solutions of chloride of sodium, and are characterized by little, 
included salt cubes in addition to the bubble. The spontaneous motion 
of the bubbles here visible, as is often the case, prevents any doubt of 
the liquid nature of the surrounding medium. The small cubical crystals 



CRYSTALLINE SCHISTS. 31 

in the liquid look as if made from glass : they are so pellucid that the 
sharp corners and edges on the farther side appear distinctly through the 
mass, and all the dimensions of the hexahedric body can be viewed. 
Sometimes they are extended somewhat rectangularly or are rounded off 
at the corners. A very fine striation occurs here and there on their 
quadratic faces, running parallel to the edges, giving the faces something 
of the drawing of a chess-board, and recalling the same familiar phenom- 
enon which marks cubes of kitchen-salt. Other quartzes inclosing liquid 
particles, of the same kind, have been found, upon a careful examina- 
tion, actually to contain chloride of sodium. 1 The water in which such 
quartz as this has been finely powdered, produces an unexpectedly strong 
reaction for chlorine when a few drops of nitrate of silver are added. By 
spectrum-analysis, the presence of sodium in the same quartz may be 
again demonstrated. If it be held in a flame, at every decrepitation 
splendid repeated flashings characteristic of the sodium-line are seen, 
which indicate the moments when the small hollows, one after another, 
crack, and their contents enter the spectrum-flame of the apparatus. 
Such inclusions of dissolved chloride of sodium with included salt 
cubes have thus far been found in the quartzes of the zircon-syenite 
from Laurvig in Norway, the diorite from Quenast in Belgium, the granite 
from Johann-Georgenstadt in Saxony, Trevalgan near St. Ives, the 
Ding-Dong Mine near Penzanse in Cornwall, and the Groatfell in the 
island of Arran, Scotland ; in the felsitic-porphyry (elvan) from Withiel, 
Cornwall, and from the western coast of Arran ; in the post-Liassic syenite- 
porphyry near the Glamig in the Isle of Skye (Hebrides) ; in the propylitic 
rock from Borsa-Banya in Transylvania; in the gneissic crystalline slates of 
the Pass Trosachs near Loch Katrine, Scotland. They have also been observed 
in the calc-spar and nepheline of blocks ejected from Vesuvius, in some 
smaragde crystals, and in the unaltered cordierite substance of the praseo- 
lite from Brakke, near Brevig, in Norway. As mentioned above, the list of 
these occurrences has been so much amplified by the study of the rocks of 
the Fortieth Parallel that these remarkable microscopical inclusions must lose 
a good deal of their rareness and be considered as rather common pheno- 
1 F. Z., Neues Jahrbnch f. Mineralogie, etc., 1870, 802. 



32 MICROSCOPICAL PETROGRAPHY. 

mena. The inclusions in the quartz of the rock from Rawling's Peak, Wyo- 
ming, show besides a property which till now has never been observed 
except in a very indistinct and doubtful occurrence. In addition to 
the bubble and little cube in the salt solution, there are small, thin, 
pale-green microlites of hornblende, which are fixed to the wall of the sur- 
rounding quartz and project into the interior of the fluid (Plate I, fig. 3). 
These microlites are so abundantly interposed in the minerals which 
constitute rocks that there can be no doubt of this having been mechanically 
taken up by the liquid-particles. The dimensions of the largest inclusion 
in the quartzes of this rock are 0.08 mm in length and 0.065 mm in breadth. 
Another rock from Rawling's Peak [39] is quite similar to that last 
described. 

At Bruin Peak, Colorado, an excellent mica-slate occurs [40], which con- 
sists of quartz containing almost no fluid-inclusions, with dark and whijte mica. 
The dark mica, in some places more of a greenish and in others more of a 
brownish color, includes an abundance of narrow needles, whose parallel 
border-lines are so near together that it is almost impossible, even with a high 
magnifying power, to discover whether their substance is or is not colored. 
These short linear microlites cross each other with curious regularity at an 
angle under 60. For one to say to which mineral they belong is difficult ; 
surely not to hornblende, but perhaps to the colorless or white mica, because 
there are in this rock, beside the laminae of muscovite, wholly colorless prisms 
(not transverse sections through laminae), which can only belong to the latter. 
It is not surprising that mica appears here in the elsewhere uncommon form 
of long prisms, for we know by the examinations of Gustave Rose 1 that the 
very long prismatic crystals which are regularly interposed in the biaxial 
mica (phlogopite) of South Burgess in Canada and produce the famous 
asterism of that variety, belong to monaxial mica. In this gneiss, we possibly 
would have the opposite phenomenon, a regular interposition of biaxial 
white mica prisms in laminae of a monaxial, brown one. The same rock 
contains an exceptionally large amount of magnetite grains whose metallic 
lustre is shown off to splendid advantage in reflected light. In the larger 
octahedral crystals are included prisms of colorless mica, which look like 
'Monatsberichte der Berliner Akad. der Wissenschaften, 1869, 339. 



CRYSTALLINE SCHISTS. 33 

sharp incisions in the black, opaque substance. The mica-slate from 
Bruin Peak is accompanied by mica-gneiss containing some hornblende 
[41], and by a hornblende rock [42] strongly resembling that from Red 
Creek [37], except in possessing less of the quartz-ground and more 
feldspars, 'among which also are a few plagioclases. The region of 
Grand Encampment Creek in the Park Range consists of a similar suite 
of rocks. 

On Davis' Mountain, a very typical hornblende-gneiss occurs [43] with 
thick, alternating, white layers rich in feldspar and dark ones rich in horn- 
blende. Besides the quartz, this rock abounds in plagioclase and beautiful 
hornblende and apatite ; but it bears little orthoclase and almost no mica, 
and therefore approaches the diorite-gneisses. The quartz displays in great 
distinctness and beauty double inclusions with liquid carbonic acid in the 
interior ; also very numerous common fluid-inclusions, consisting only of 
water, with probably a little carbonic acid in solution. Curiously enough, 
the quartz, in another not less beautiful hornblende-gneiss from Grand 
Encampment Creek [44], contains hardly any microscopical liquid- 
inclusions. The Grand Encampment Peak offers a hornblende rock [45] 
which is, in a rare measure, free from other ingredients. 

Quartzite as white as snow is interstratified in the hornblende-gneisses 
at the north end of the Park Range [46]. Its chief mass appears in the thin 
section glass-like and pellucid, but is traversed in all directions by little 
dull, milky lines, bands, and spots, which, under a higher magnifying power, 
are seen to be a multitude of aggregated fluid-incluisons. They belong to 
two different varieties, some being composed of water with a movingbubble, 
which does not disappear in a temperature above 100 C , and others being 
the double inclusions, with carbonic acid in the interior, whose bubble may 
be driven off by the smoke of a cigar. It is remarkable that these 
inclusions of different chemical nature are associated also in the quartzes of 
the accompanying gneisses, a fact which may perhaps prove that the nearest 
geological connection exists between the two rocks, and that in origin they 
are the same. 

The metamorphic Archaean territory of Colorado presents also the follow- 

3 M P 



34 MICROSCOPICAL PETROGRAPHY. 

ing rocks worthy of mention: at Mount Zirkel, 1 Park Range, a gneiss [47] 
with some hornblende; among the feldspars a good many plagioclases ; 
zones with alternating rich bands of feldspar and hornblende, producing 
distinct layer-structure. 

On the south side of Clark's Peak, North Park, is an excellent horn- 
blende-bearing mica-gneiss [48] with an astonishing amount of apatite in 
quartzes as large as pin-heads ; 20 or 30 apatites lying horizontally, pointing 
in all directions, and showing transverse sections, being often included 
together; much beautiful plagioclase, also quartz almost free from fluid- 
inclusions. 

In the North Park, one of the best hornblende-gneisses is a decided 
diorite-gneiss [49], in which the naked eye detects many well-striated 
plagioclases, while the microscope shows that all the feldspars are tri clinic 
and richly lineated in polarized light; there is also hornblende, quite a little 
brown biotite, which borders the dark-green hornblende in places, some 
quartz ; in short, a slaty diorite, the hornblende being here filled with 
a great number of narrow cylindrical hollows drawn out parallel to the 
fibration. 

At French Creek occurs a mica-gneiss rather poor in mica [50], 
feeling somewhat sandy to the touch, and containing much plagioclase 
for a rock of this composition. The plagioclases of this latter, show in an 
eminent degree a combination of the two laws of triclinic twin formation, 
which was first described by Stelzner in the labradorites. 2 There is one twin 
composition parallel to the brachypinakoide M ( oo P oo ), and another par- 
allel to the base P (OP); so that there the poly synthetic twin striation 
appears as well upon the face P as upon M, in both cases going parallel to 
the edge P | Jf, and the two lamellar systems crossing each other under the 
angle P: M= 86 40'. So these crystals consist in a certain sense of staff- 
like individuals in nearly rectangular transverse section, and they present 

1 This interesting summit has been named by myself and corps in honor of Pro- 
fessor Zirkel, the author of this memoir ; and, without his knowledge, I have introduced 
the name in these pages. C. K. 

2 Berg. u. hiittenmiinnische Zeitung. xxix, 150; also Schrauf, Situngsber. d. 
Wiener Akad., LX, 1. Abth., Dec. 1869, 19, and F. Z., Mikroskopische Beschaffenh. d. 
Miu. u. Gest,, 133. 



CRYSTALLINE SCHISTS. 35 

in the sections a grate-work or lattice, here those of the one and there those 
of the other twin system prevailing as stronger ledges. 

Farther north, at Cherokee Butte [51], there occurs a hornblende- 
bearing mica-gneiss comparatively rich in titanite, whose brownish-red 
sections might at first sight, by reason of their unusual color, be mistaken 
for garnet; they distinctly polarize light, however, and possess besides the 
characteristic cuneated or sphenoidal figure and one unchanging form of 
cleavage. 

On the slope of Cedar Mountain, south of Cherokee Butte, there is a 
hornblende-rock [52], in which a colorless groundmass of quartz predomi- 
nates, filled in every part with large and very fine prismatic or irregularly 
shaped individual green hornblendes, thousands of which are scattered 
through the quartz of one small section. 

At Deer Mountain, a plagioclase-bearing gneiss [53] was collected, con- 
taining hornblende, another green but feebly dichroitic chlorite-like mineral, 
plenty of titanite in the characteristic sections, some apatite, and the zircon- 
like crystals. 

At Mill Peak Summit, a quartzite was found [54] appearing to the naked 
eye almost wholly homogeneous, but becoming under the microscope an aggre- 
gation of little colorless-edged grains averaging 0.015 mm in thickness; they 
are mixed with exceedingly small granules of ealeite, which show very dis- 
tinctly the well-known characteristic lamellar twin striation after the face 
( -Z?), which so often occurs in the crystalline individuals of granular lime- 
stones, and which shows in polarized light a variegated parallel lineature: 1 it is 
rare for calcite to figure in this manner as a constituent of crystalline quartzites. 

Long's Peak [55] and the entrance to Big Thompson Canon, Colorado 
Range [56], yield two beautiful, dark mica-slates, in which muscovite, 
unrecognizable to the naked eye and almost colorless, is contained: the first 
of these is enormously rich in apatite, and its quartz includes innumerable 
straight and extremely fine colorless prismatic microlites, which, however, 
belong rather to the muscovite than to apatite. 

The differences in mineralogical constitution between the mica-gneisses 

1 First detected one of the oldest microscopical observations on the structure of 
rocks by Oscbatz, Zeitsclir. d. d. geolog. Gesellsch., vii, 1855, 5. 



36 MICROSCOPICAL PETROGRAPHY. 

and the hornblende-gneisses in the above-described region may bo gener- 
ally summed up as follows : 

MICA-GNEISSES. 

Orthoclase largely predominating, but very little plagioclase. 
Fluid-inclusions in the quartz more abundant. 
Apatite rarer or wanting. 
Titanite entirely wanting. 
Zircon rare or wanting. 

HORNBLENDE-GNEISSES. 

Plagioclase frequent, sometimes predominating. 

Fluid-inclusions in the quartz more rare. 

Apatite generally very abundant, but sometimes wanting. 

Titanite sometimes present. 

Zircon abundant or wanting. 

The apatite is only to be found in those rocks rich in horn- 
blende which also contain feldspar. In the pure hornblende rock, as 
well as in the masses composed only of quartz and hornblende, it is ex- 
tremely rare, even when they are geological equivalents of or regularly 
interstratified in hornblende-gneisses, and when the hornblende in both has 
an exactly similar structure. There are two other rocks worthy of mention 
from the Archaean region of Colorado. 

Underlying Medicine Peak, Medicine Bow Range, a black, somewhat 
lustrous clay-slate occurs [57], which resembles roofing-slate, and is very 
easily fissile. Under the microscope, it is seen to consist of a colorless 
ground, which seems to be homogeneous, but polarizes between nicols in 
indistinctly outlined, colored spots. But this groundmass disappears by 
reason of innumerable black, opaque grains of the very smallest size, which 
are scattered through it, here being aggregated in heaps, there woven into 
flocks. Where the colorless mass is more or altogether free from these spots 
and stripes of dark points, a considerable quantity of small, pale brownish- 
yellow laminae of mica lies in it, and those standing obliquely prove that 
they are as well lamellated as those in fine-grained gneisses or mica-slates. 
The general composition here, therefore, does not agree very well with that 



CRYSTALLINE SCHISTS. 37 

of the externally and macroscopically similar Silurian and Devonian roofing- 
slates of the Rhine provinces, Westphalia, Thuringia, Saxony, and Cornwall, 1 
especially since the brown microlites, so characteristic of the latter, are here 
entirely wanting. In the same locality, and in near connection with that last 
described, is another confused and entangled slate [57], looking more crystal- 
line than the former, and having a dirty green and brown color. It is seen 
under the microscope to consist for the most part of broad rays of green 
hornblende, which present transverse sections of very well-formed and beau- 
tifully cleavable prismatic crystals. Between the individual hornblendes is 
a colorless mass, which in this rock must belong to brilliant polarizing quartz. 
Small heaps of fine black grains, like those seen in the former rock, are inter- 
posed in the quartz. Single feldspar crystals, which are wholly wanting in 
the former, are visible, beside the hornblende, in the latter. The brown spots 
seen in hand-specimens are produced by a superficial oxydation of the 
Fe amount of the hornblende, whose prismatic rays in the thin section are 
often still green on one end and brown along the fissures of the other. Both 
rocks bear the closest geological relation to each other, but petrographically 
it cannot be demonstrated that the second one has ever been like the first, 
or that it really is a more crystalline development of it. As a supplement 
to these old crystalline schists may be lastly mentioned a remarkable Juras- 
sic slate from north of Sahwave Mountains. 2 It is very similar to the old 
Silurian or Devonian roofing-slates, and even possesses a bright lustre upon 
its fissure-planes. It appears under the microscope to be wholly crystalline, 
notwithstanding its geological position, and presents nothing which indicates 
the presence of clastic material. It looks most like a mica-slate whose com- 
ponent parts are extraordinarily fine. Quartz and colorless or light-yellowish 
Render laminaj of mica, only a few hundredths of a millimeter in size, are its 
chief components. Through this mixture little black, prickly crystals and 
minute short needles of an unknown nature, together with some very fine 

1 F. Z., PoggendorfFs Aunalen, CXilv, 1871, 319. 

* While from its character and geographical position this slate is judged to bo 
Jurassic, its actual age is nnprovcn, and it may possibly represent a fragment of 
earlier rocks entangled in the later formations : its remarkable crystalline character 
may possibly point to a Cambrian or even Iluronian origin. The decided probabilities 
arc, however, that it is correctly referred to the .Jura. C. Iv. 



38 MICKOSGOriCAL PETEOGRAPIIY. 

black grains, are scattered ; the latter appearing with a low magnifying 
power like dust, being generally accumulated ha small heaps. Without 
doubt, this Jurassic slate is much more crystalline than the roofing-slates of 
the Silurian and Devonian formations, in which pronouncedly crystalline 
elements appear only as an occurrence in the generally prevailing clastic 
material. This is very unexpected, and the fact is the more striking 
because the Tertiary roofing-slates (as the excellent and famous ones from 
Glarus, Switzerland Eocene), which macroscopically cannot be separated, 
are made up entirely of microclastic ingredients. 

The beautiful crystalline-granular marble from Kinsley District, Nevada, 
should be spoken of before closing this chapter [58]. The grains of calcite 
in it show the most distinct twin striation (after J jR), and are remarkable 
for numerous fluid-inclusions, which, as is known, do not occur at all fre- 
quently elsewhere. They attain the size of 0.004 mm , and possess a rather 
mobile bubble, which does not disappear in a temperature of 100 C., and 
which is doubtless enveloped in water containing some carbonic acid. Then 
there is, too, a considerably crystalline altered Triassic limestone [59], south 
of Buffalo Canon, West Humboldt, which has good fluid-inclusions. 






CHAPTER III. 

GRANITE AND GRANITE-PORPHYRY. 

- 

SECTION I. GRANITE. 

SECTION II. GKANITE-POEPHYKY. 

tomini io1 



SECTION I. 
GRANITE. 

The granites of the Fortieth Parallel are doubtless partly eruptive rocks, 
which have evidently broken through sedimentary strata of a different 
geological age, and, in part, dependencies of the old crystalline schists, 
alternating with gneisses, etc., and showing no sign of eruptive character. 
Whatever may be the origin of the crystalline schists, that of the accom- 
panying granites must be the same. According to the most favorite theory, 
these granites that are not eruptive, and are at the same time generally 
stratified, should be called metamorphic granites. The decidedly eruptive 
granites may be divided into two classes: one embraces those older rocks 
that are of ante-Jurassic age; the other, those which have obtruded 
themselves through the Jurassic strata. For the enormous mass of 
eruptive granite of the Sierra Nevada, Professor Whitney has demonstrated 
a Jurassic origin; and, although not definitely proven, a similar age is 
iis8iiined for a considerable class of granites along the Fortieth Parallel, whose 
pctrographic constitution and habitus are identical with those of the Sierra 
Nevada. The full details of the reasons of this assignment will be found in 
the chapter upon granites in Vol. I of this series. When, therefore, in this 

39 



40 MICROSCOPICAL PETKOGKAPHY. 

memoir, Jurassic granite is spoken of, the intention is to designate that family 
of which the Sierra Nevada occurrence is the type in age and constitution. 
The purpose of this chapter is not only to describe the several occurrences 
which seem, from a general petrographical point of view, worth mentioning, 
but also subsequently to develop those macroscopical or microscopical 
peculiarities which will help to distinguish the single geological granite 
varieties above classified. The specimens as described are taken from 
localities ranging regularly from west to east. 

An older eruptive granite from Granite Canon, southeast of Winnemucca 
Lake, Nevada [60], consists of orthoclase, plagioclase, quartz, biotite, and 
comparatively little apatite, with no hornblende or titanite. The mica has 
brownish-green, richly lamellated, pure, homogeneous sections. The quartz 
is rich in liquid-inclusions ; this phenomenon can also be detected in the 
feldspars, which have a much fresher and clearer substance. The quartz 
also contains straight or curved needle-like microlites, which are so thin 
that, even with the highest magnifying power, they look merely like thin, 
black dashes (see Plate II, fig. 4, representing another variety of granite). 
They are generally found gathered in large numbers upon a small space 
or dispersed in all directions through the mass ; and it is always easy to dis- 
tinguish them from cracks or fissures. It has not been determined to which 
mineral these microlites so widely spread in the granitic quartzes belong. 

Going eastward, a series of eruptive granites referred to younger or 
probable Jurassic series is found. At the north end of the Truckee Range 
occurs a typical specimen of this rock [61]. It is made up of comparatively 
little orthoclase, much plagioclase, dark mica, hornblende (easily recognizable 
even in the hand-specimens, and showing deep-green sections in the slides), 
much quartz, relatively abundant apatite and magnetite, and the character- 
istic titanite. 1 The orthoclase shows here and there an excellent zoned 
structure, which is almost equal to that of the trachytic and rhyolitic sani- 
dins : it is also strikingly clear. The plagioclase, likewise, is not nearly as 
much decomposed as elsewhere in the granites, but is much more fresh and 
unaltered. The abundant quartz is conspicuously poor in fluid-inclusions, 

'Clarence King has long since shown that the eruptive Jurassic granites, anil 
only these, are characterized by the presence of macroscopical titanite. 



GRANITE. 41 

in this respect presenting a remarkable contrast with that of the first-men- 
tioned granite. Apatite is found everywhere in the mass, especially in and 
about the mica, but also in the feldspar, quartz, and hornblende. A gen- 
eral mutual inclusion and enveloping of the single constituents has taken 
place on an extended scale. The feldspars and quartzes contain delicate tables 
of brown mica, needles and lamellae of hornblende, thin yellowish-red plates 
of specular iron, hematite, and grains of magnetite, all of which are 
phenomena seldom found elsewhere in granites. The oxyd of iron, an 
almost invariable accompaniment of these post-Jurassic eruptive granites, is, 
moreover, of a twofold nature, appearing sometimes as a very sharp-edged, 
regular, little, six-sided lamina, ranking among the original constituents of 
the rock, having nothing to do with fissures, and often included in the com- 
pact mass of another mineral, for instance in the midst of quartz ; and at 
others showing a serrated and lobed dendritic formation, lying evidently in 
cracks and fissures, into which it has evidently been infiltrated in the course 
of time, being doubtless of secondary origin. The latter probably originates 
primarily from the decomposition of hornblende (less from that of brown 
mica), and is therefore wanting in all granites which are free from horn- 
blende, as are most of the older eruptive division. The titanite has brown- 
ish-yellow sections, with a rather rough surface, less pellucidity, and gen- 
erally sharp cuneiform outlines. The titanites are for the most part free 
from strange inclusions which may perhaps abound in the neighboring 
constituents of the same rock ; and, judging from this fact, a comparatively 
very early solidification might in all probability be rightly attributed 
to the titanite. In one instance, there was observed in this granite a 
crystal of titanite which had made a hole in an individual of mica at its 
border; it had squeezed and forced its way in, pushing asunder the 
lamella of the mica, and shivering them like fibrous wood. There is no 
trace of white mica in this granite, nor is there any in that of the 
same geological position. Then there is a comparatively large amount of 
thick, black grains of magnetite. No glass inclusions occur in any of 
the rock-constituents, nor is there any sign of an amorphous, imperfectly 
individualized base. The abundance of plagioclase, taken together 
with tlic ;mioiint of hornblende, removes this rock from the proper granites, 



42 MICROSCOPICAL TETEOGHAPUY. 

to which it is, however, allied by a large quantity of quartz and biotite. 
It really appears more to belong between granite and hornblende-bearing 
mica-diorite than between granite and syenite, possessing its true analogy 
in the series of gneisses. 

The hill west of Granite Creek Station, Granite Range, consists 
of granite [G2] similar to the latter, except in being a little poorer in 
mica and hornblende. The quartzes contain plenty of the thin black 
needles, which were mentioned as occurring in those of the first old 
eruptive granite ; so that the microscopical interpositions cannot help to 
determine the geological age. To these same granites also belong that 
from the Pah-supp Mountains, Nevada [63], whose quartz is somewhat 
richer in fluid-inclusions, and that from the summit of the neighboring 
Sahwave Mountains [64], which is comparatively poor in black mica and 
hornblende, and whose feldspars are evidently more altered. The larger 
orthoclases of this latter contain small included particles of beautifully stri- 
ated plagioclases, and the black needles in the quartz often reach a length 
of 0.35 mm , with a thickness of 0.0015 mm ; that is, 233 times longer than 
thick. All these granites belong to the class of possible Jurassic age. 

A totally different appearance and composition are seen in the granites 
next following from the Pah-tson Mountains. One from Granite Ridge 
[65] bears muscovite, gathered in groups, with concentric radiations like a 
rosette, which is a very uncommon phenomenon : no hornblende, no dark 
mica, no titanite, very little magnetite, and considerable apatite. The granite 
from the dike west of Pahkeah Peak, Pah-tson Mountains [66], has 
all these peculiarities. In Crusoe Canon, Pah-tson Mountains, occurs a 
granite dike [67] which is seen macroscopically to contain garnet and 
lepidolite (lithia-mica). Under the microscope, no smaller individuals of 
garnet can be found than those seen with the naked eye. There is also a 
colorless mica, which often forms, when seen through the microscope, star-like 
groups similar to the ice-crystals that form on windows. The mass contains 
neither biotite nor hornblende. When seen on a smaller scale, the lepidolite 
is found to be exactly like the lithia-bearing " mica-palniee " in the coarse- 
grained granite from Bagneres de Luchon in the Pyrenees, and that in the 
pegmatites from the neighborhood of Pressburg in Hungary. 



GKANITE. 43 

But there occur, in the Pah-tson Mountains, granites which belong 
geologically to the younger, Jurassic type, and which exactly agree 
petrographically with that from the Truckee Range, described above, 
strikingly contrasting with the just-mentioned older, muscovite granites 
of the same region. Of these, the following may be named : at Granite 
Ridge, Pah-supp Mountains, a massive hornblende-granite [68], in which 
quartzes and feldspars are again rendered highly impure by hornblende 
dust and little biotite plates, and which also contains primary specular 
iron. On the east side of Pahkeah Peak is a granite [69], having much 
biotite, but comparatively poor in hornblende. In those very rich in 
plagioclase, and also abounding in titanite, whose smaller grains are often 
densely associated in heaps, the titanite seems to search with peculiar per- 
sistence for the borders of the magnesian mica. At the head of Grass 
Canon, Pah-tson Mountains, is a hornblende-granite [70], with impure 
constituents, much biotite, doubtless more plagioclase than orthoclase, tita- 
nite, specular iron as in former and the following specimens, quartz some- 
what richer in fluid-inclusions ; hornblende and biotite being here grown 
together in a peculiar manner, so that as the sections of a green hornblende 
often show layers of intercalated brown mica, and inversely the brown mica 
lamels contain hornblende individuals, the limits between the two intimately- 
connected minerals being so sharp that it is scarcely allowable to explain 
this new phenomenon upon the supposition of alternating processes. 

Granite Peak, Pah-tson Mountains, yields a granite [71] likewise very 
rich in plagioclase, with somewhat less hornblende but with titanite in the 
same relative proportion; a large rounded grain of magnetite QA""" in 
diameter was seen bordered with fifteen apatite prisms standing out in 
every direction; and besides these, which had fastened on the outside, there 
were three other apatite needles perforating the magnetite, whose colorless 
sections looked in the black surrounding mass like so many sharp holes. 

Still another quite different constitution is represented by two granites 
from localities not very remote from tliis, namely, west of Rye Patch, Mon- 
tezuma Range, Nevada [7'2], and Montezuma Mine near Oreana [73]. They 
;int somewhat similar to each other, and are much more decomposed than the 
;ibove-described varieties. This latter is especially true of all the feldspars, 



44 MICROSCOPICAL PETROGRAPHY. 

which arc altered into a granular, mealy, or an indistinctly radiating 
substance with aggregate polarization. Hornblende and biotite display 
something of the behavior which is peculiar to these minerals in the 
gneisses, their flat prisms and laminae showing undulations and curvatures. 
They also contain microlites, included in the same way as in corresponding 
constituents of crystalline slates. Titanite is not present. In general, 
according to their whole microscopical aspect, neither of these granites 
have any characteristics in common with the typical titanite-bearing 
dioritic granites. They belong to the older eruptive family. 

The granite from Granite Peak, Pah-Ute Range, Nevada [74], is ex- 
tremely poor in mica, and a priori it is almost in vain to search for hornblende 
and titanite. The feldspars are somewhat decomposed. The quartzes contain 
splendid double inclosures of an unusually large size (see page 19). 
The innermost fluid is here also liquid carbonic acid, the bubble reappear- 
ing by a diminution of the raised temperature in all neighboring inclusions 
of this kind at the same time and at 31 C. Sometimes it can be distinctly 
seen that the outer boundary of the interior carbonic acid runs irregularly or 
presents a regularly angular form, but no well-rounded line. Judging from 
this, it would seem highly probable that the outermost zone of these curious 
double inclusions is not a liquid but rather a solid mass. If this peripheric 
substance were also a fluid, the included liquid carbonic acid would surely 
take the form of a globe ; but the outer form of the carbonic acid, of course, 
has the shape of the cavity in the peripheric mass. In addition to these 
double inclusions, the quartzes of this granite envelop also the more common 
single inclusions of liquid carbonic acid, and along with these are other 
cavities filled with a fluid whose bubble does not disappear even at the high 
temperature of 100. Without doubt, the latter is chiefly water, with per- 
haps a small amount of dissolved carbonic acid. It seldom happens that 
one has so good an opportunity to observe all together in one quartz grain, 
these three kinds of liquid-inclusions. This granite doubtless belongs to 
the metamorphic group. 

Those of the Augusta Mountains [75, 76] are rather poor in plagioclase, 
have hero less, and there more biotite, and are somewhat decomposed. Some of 
llu-sc granites contain quite a little hornblende, which is immediately accoiu- 



GRANITE. 45 

panied again by apatite, but there is not sufficient to make it at all charac- 
teristic, or to warrant separating the rock from the usual biotite-granites, to 
which the decidedly prevailing orthoclase also points. 

Highly interesting granite rocks occur in the Havallah Range, Nevada, 
north of Summit Springs. The predominating variety is a coarse-grained 
granite, with somewhat labradorizing feldspars, which are macroscopically 
similar to those from Frederiksvarn in South Norway ; very rich in 
brown mica and hornblende, with a comparatively enormous amount 
of apatite, which is often finely porous; but no titanite. Here also the 
quartz abounds in the three different sorts of fluid-inclusions : a. aqueous 
inclusions, with a bubble which is not condensable within the limits of obser- 
vation ; b. simple inclusions of liquid carbonic acid ; c. double inclusions, 
whose interior is liquid carbonic acid. The feldspars merit particular atten- 
tion ; a part of them containing many orthoclases show that tender and 
fine fibration which IB common to the feldspars of the zircon-syenites of 
South Norway and to those of the Saxon granulites, a fibration which has 
nothing to do with twin formation. These orthoclases are almost free from 
strange interpositions. In other feldspars, the orthoclases as well as plagio- 
clases contain in their very clear substance the most diverse microscopical 
bodies (Plate III, fig. 1); among them rhombic and six-sided, sharp, yellow, 
brown-black, and even gray-violet little plates, real microlitic needles of the 
same colors, also seeming needles, which the micrometrical screw proves 
to be laminae standing upon edge; pale-green hornblende-microlites, often 
with affixed magnetite grains ; in short, interpositions which produce a real 
picture of labradorite, in the making of which the external optical effect of 
the feldspars has a part. No certain lawful crystallographical grouping of 
these included individuals can be discovered. The needles and feldspars 
often very nicely present the well-known phenomenon of dissolution into 
single grains and ragged bunches. The thin, black, line-like needles some- 
times lie parallel and so close to each other, that with a low magnifying power 
they give a dark shading to the feldspars. Other parts of the feldspars 
have a very remarkable structure : their clear substance is thoroughly 
interwoven with a multitude of either colorless or very pale-greenish, long 
stripes and bands. These are often a little undulated and contorted, but 



46 MICROSCOPICAL PETROGRAPHY. 

they always run in regular parallelism. The transverse sections of 
the feldspars suggest graphic granite, but it is very improbable that the 
interpositions belong to quartz. It is more natural to suppose that they 
are muscovite. Moreover, there are no thin lamella; extended in two direc- 
tions, but only, as is seen in the transverse sections, plummet-shaped and 
vermiform objects having a prevailing direction. In these feldspars, the 
previously mentioned little tables and microlites are also imbedded, as in 
the other feldspars which are free from the vermicular interpositions, and 
they do not at all interfere with the direction of the stripes and bands. 
Yellowish-red oxyd of iron is plentifully infiltrated into microscopical cracks 
in the feldspars and quartzes as a secondary product ; primary plates of it 
as a genuine constituent of the rock being wanting. 

This coarse-grained granite, from the Havallah Range, is traversed by 
dikes which represent a wholly different variety [77], namely, one of the 
hornblende-titanite group, which everywhere are found to be the young- 
est. Hornblende and biotite often show the previously-mentioned intimate 
mutual interlacing and interwreathing (Plate II, fig. 4). The quartz is 
here filled with such a multitude of the black, hair-like microlites so 
often spoken of, as to surpass any other example of this phenomenon 
ever seen. They cross each other confusedly, often forming a web, or 
diverging from one point in all directions, like roots from a stump. 
The intertwining of the more curved and distorted ones is very beautiful 
when seen through the water-clear quartz-mass. The thicker hairs are feebly 
brownish, but transparent. In a quartz grain of one square millimeter lie 
over 120 of these hairs almost in one plane, so nearly in one, at any rate, 
that they may be seen without turning the screw ; hence, it may be calcu- 
lated that one cubic millimeter of quartz contains 10,000 of these microlites. 
Furthermore, the quartz abounds in liquid-inclusions containing the most 
beautiful cubic crystals of salt. Some of the inclusions contain short 
black hairs, which proves the simultaneousness of the general act of inclu- 
sion. Both salt cubes and minute black microlites can even be detected in 
one liquid-inclusion (Plate I, fig. 4). Of the fresh feldspars, many belong 
to the triclinic system. Primary plates of oxyd of iron are present. 

The granite from Ravenswood Hills, Shoshone Range [78], is in consti- 



GRANITE. 47 

tution totally different from the above. It represents a fine-grained variety 
containing white mica, but no trace of black mica, even under the 
microscope. Hornblende, therefore, is entirely wanting; and it would be 
superfluous to say there is no titanite, after having stated the prevalence 
of white mica. A little apatite appears here. Indeed, this ingredient is a 
component part of almost every rock, and its quantity rather than its mere pres- 
ence relates it to hornblende. The quartzes here are exceptionally poor in 
fluid-inclusions, resembling, in this respect, some of the younger granites. 

The granite from the western end of Winnemucca Peak, Nevada [79], 
is a dark, fine-grained variety, with feldspars that are much decomposed, and 
considerable magnesian mica. These and hornblende are the only con- 
stituents which are distinctly visible to the naked eye. This is one of the 
older eruptive granites. 

At Nannie's Peak, Seetoya Mountains, is another eruptive granite [80], 
rather fresh, containing biotite but no hornblende, splendidly striated un- 
decomposed plagioclase and little predominating orthoclase, which is more 
attacked, and shows here and there a distinct zone-structure, resembling the 
sanidins in trachytes and rhyolites. Some of the orthoclases are fibrated 
in the lately described manner. The quartzes bear numerous fluid-inclu- 
sions, some with beautiful included salt cubes and others with minute black 
hairs; while some grains are full of very small apatites. A microscopically 
fine-grained accumulation of pretty well crystallized quartzes, on an aver- 
age 0.1 mm , with less distinct little feldspar crystals, appears in some 
parts of the rock : a kind of aggregation like that which often forms the 
groundmass of felsitic porphyries, except that in the latter the component 
parts are still more minute. Other granites occur on this peak [81] having 
little particles of hornblende, as it seems, along the borders of the equally 
small brown mica. They contain quartzes in which every fluid-inclusion 
bears a salt cube, many of them even having two of these little crystals. 
Both varieties are entirely free from titanite. 

The old eruptive granite from Shoshone Knob, Shoshone Range [82], 
bears, beside magnesian mica, very much hornblende in prisms up to 5 mm 
long, neither white mica nor titanite, some apatite, and not much plagioclase. 
That from Woodranch Caflon, Shoshone Range, is similar to this [83], 



48 MICROSCOPICAL PETROGRAPHY. 

except that it has less mica and hornblende. Titanito is wanting. The 
clear orthoclases have very distinct fluid-inclusions. 

The rock of Agate Pass, Cortez Range, which possibly is a Jurassic 
eruption, even if the geological evidence is not entirely conclusive, forms 
an excellent example of the granites holding titanite. The plentiful horn- 
blende is splendidly cleavable : it is accompanied by almost no dark mica. 
The feldspars are highly altered, so that the comparative proportion of 
orthoclase and plagioclase is not recognizable. The product of this decom- 
position is curious, consisting of broader or narrower colorless prismatic 
rays, which are either massed confusedly together like felt, or are heaped 
together in the form of stars and bunches, presenting a beautiful aggregate 
polarization. That such phenomena cannot be well pictured is to be regret- 
ted. By this process of alteration, the entire mass of the feldspar crystals 
has been equally metamorphosed, so that former cracks and channel-ways 
are not at all preserved. The greater part of the feldspars in the granite 
of Granite Peak, Pah-Ute Range [84], is in a similar condition. The 
fresher portions of this constituent prove that it contains numerous liquid- 
inclusions and empty cavities, both as usual being long and rectangular in 
shape. The granite is poor in hornblende ; there is very little mica, and 
titanite is not present in the specimens which were examined. 

The granites back of Overland Ranch, Humboldt Range, are 
interesting, both geologically and petrographically [85, 86]. They are 
intercalated between the Archaean crystalline schists of that region. Much 
quartz is found in them, also black mica, but they are wanting in white 
mica. They differ petrographically from most of the eruptive granites 
previously described in this chapter, the points of difference being the 
possession by this rock of characteristics in general common to the granites 
which are members of the stratified crystalline territories and wanting proof 
of an eruptive nature. A long list of granites of this kind will be men- 
tioned hereafter. The distinguishing characteristics are: a. the mica shows 
here and there a tendency to form flat plates (Flasern), not all its lamina?, 
it is true, lying parallel, as is the case in crystalline schists, but there often 
appearing transverse sections of membranes woven together in more or less 
perfect parallelism, the single accumulations of this kind not being parallel; 



GRANITE. 49 

b. the small amount of plagioclase, even when compared with the granites, 
which are without hornblende; c. in the quartzes, there are hardly any 
fluid-inclusions; d. the linear apatite prisms are drawn out to an extreme 
length, are very thin, and often dismembered into numerous pieces, the 
apatites in massive granites being generally far more broad and short, 
and not so often or so minutely divided into pieces; e. the zircon-like 
mineral, so widely spread through the crystalline slates, immediately 
appears here in comparatively large individuals. 

The granite from Egan Canon, Egan Range, Nevada, bears much 
magnesian mica, considerable plagioclase, a little hornblende (fine green 
dust of which is plentifully scattered through the other constituents), and 
very -beautiful titanite in large cuneiform sections [87]. It is surprising to 
find that the zircon-like mineral also occurs here, for it has never before 
been observed in any granite. Yet the structure of this rock does not at 
all remind one of a gneiss, titanite being, besides, extremely rare in 
this species. There is a rather large number of liquid-inclusions in the 
quartz. 

The granites of Wachoe Mountains, Nevada [88, 89], possess a some- 
what porphyritic structure. Their orthoclases are highly metamorphosed. 
Curiously, the fresher and clearer feldspars all show distinct triclinic stria- 
tion. Large individuals of hornblende are comparatively numerous, titanite 
is abundant, and there are many apatites in large, thick, but short, prisms, 
with terminating pyramidal faces. These apatites are crowded with dash- 
like, cylindric, narrow pores extended parallel to the chief axis of the crystal, 
but confined to the middle of the prisms, the outer parts being of a pure 
substance. This phenomenon may be easily observed in the longitudinal 
and transverse apatite sections. Sometimes the longer pores are so thin 
that they look like solid black needles (Plate I, fig. 8). Other apatites 
are seen containing distinct fluid-inclusions, lying in clusters or bands which 
run rectangularly to the chief crystal axis (Plate I, fig. 7). In the quartz, 
whose liquid-inclusions bear little salt cubes, and in the feldspars, charac- 
teristic plates of specular iron are enveloped at intervals, and also black 
hair-like microlites. Biotite is frequent. 

In petrographical constitution, a great difference is shown by the 
4 M P 



50 MICROSCOPICAL PETROGRAPHY. 

granites from Granite Rock, Great Desert, Utah, where three varieties occur. 
The first [90] bears much dark magnesian mica, no white mica, a dirty- 
greenish mineral like chlorite, which is also highly dichroitic, but not so 
absorbing as biotite; hardly any proper hornblende, much titanite, and 
apatite with gigantic fluid-inclusions 0.03 mm in size, long black hairs in the 
quartz, which is rich in liquid-inclusions; the feldspars also having liquid- 
particles. The second [91] is a fine-grained granite, with only white mica, 
a very few microscopical scales of the greenish chloritic mineral, biotite 
totally wanting, titanite and apatite very rare, an abundance of quartz, and 
little plagioclase. Here it is pretty certain that the colorless prisms which 
are so widely spread through quartzes, especially those of granites, which 
lack the glaring and dazzling sections, and which are not apatite, belong to 
the white mica or muscovite. The only point which raises any doubt of 
this is that such microlites also occur in the quartzes of granite which 
do not have white mica among their proper independent constituents. 
The third variety [92] contains both black and white mica, the latter 
predominating. Here it appears that the black hairs in the quartz are 
exceedingly thin prisms of muscovite, for all the different stages of transition 
are seen between the most thin, straight, and curved black microlites and 
the very long colorless prisms, which without doubt belong to white mica. 
Little Cottonwood Canon, Wahsatch Range, presents eminently char- 
acteristic types of eruptive granites bearing hornblende and titanite [93, 
94]. All their ingredients, including hornblende and titanite, may be seen 
macroscopically ; rather fresh biotite, no muscovite, comparatively much 
plagioclase, abundant apatite, quartzes strikingly poor in fluid-inclusions 
as compared with those of the older eruptive granites. In the feldspars, 
quartzes, and micas, innumerable staff-formed, pale-greenish hornblende 
microlites are found, often divided by fracture into many single pieces and 
members, which lie one behind another, like chains of diatoms, associated 
with the most fine and regularly formed hornblende crystals of only 0.001 mra 
in length, and with roundish particles of hornblende like dew-drops. The 
little hornblende prisms in the feldspars and quartzes are often thickly cov- 
ered with a dust of fine magnetite grains. Largo individuals of hornblende 
are built out of single prisms which are not in immediate contact with each 



GRANITE. 51 

other, the spaces between being filled with colorless, beautifully polarizing 
quartz. In some varieties [94], the amount of titanite is extremely large. 
Red plates of oxyd of iron are found among the rock-constituents, more 
frequently in the feldspars, however. Many of the latter also include 
countless numbers of quartz-grains, the larger of which may be observed 
even by the naked eye in thin sections. 

The granite from Clayton's Peak, Wahsatch, is very similar to the 
above in the most characteristic points ; in the amount of titanite, and the 
association of hornblende and biotite. The colorless ingredients, such as 
feldspars (among them many plagioclases) and quartzes, are rendered very 
impure by foreign microscopical elements. The feldspars particularly 
contain unusually many violet-brown, red, and black laminae of specular 
iron, all the different colors of which depend upon the varying thickness of 
the plates. So many of these metallic interpositions are accompanied by 
black hair-like microlites that the sections of the feldspars labradorize more 
or less distinctly when observed in the sections with oblique reflected light. 
Here and there parts of the rock show a diverging radiation, like the petals 
of a flower. Of this it is difficult to say whether it belongs to matter that is 
not individualized, or whether a product of the alteration of feldspars occurs 
here. Since this substance is penetrated by the same plates of specular iron, 
and the same hair-like microlites, as the undecomposed feldspars, the latter 
supposition seems the most probable ; but it must then be acknowledged 
that the interposed bodies have escaped every alteration. These rocks 
are for the most part proportionally very rich in magnetite, well-formed 
crystals of which are also included in the titanite. The apatite crystals, 
curiously, are pressed flat, so that two opposite prism-faces are often four or 
five times larger than the four others; but there is no doubt that these sharp 
and glaring individuals are apatite. Accumulations of minute hornblende 
grains, biotite plates, and magnetite grains, without any individualized shape, 
occur among the rock-constituents. Titanite is a darker brown here than 
in other rocks. 

The granite at the mouth of Big Cottonwood Canon [95] is rich in dull- 
whitish feldspar, which appears macroscopically in the hand-specimen to 
form a real homogeneous groundmass; the microscope, however, shows 



52 MICROSCOPICAL PETROGRAPHY. 

that the crystalline individuals above mentioned are here associated. These 
feldspars also present an interesting manner of alteration, following most 
distinctly single, concentric inscribed zones. Within a slide, in the sections 
of the larger feldspars, three or four inner impellucid lines of a somewhat 
duller color appear, corresponding in their direction to the external outlines, 
and separated from each other by a clear unaltered substance. Numerous 
inclusions that are doubtless fluid can be detected with the microscope in 
the latter. The quartzes also are rich in liquid-inclusions. 

This type of granite extends into Little Cotton wood Canon [96, 
97, 98] with orthoclases which are sometimes a beautiful snowy white, 
especially in the coarser-grained varieties, excellently cleavable horn- 
blendes, and biotites which are very well lamellated and penetrated by 
numerous apatite needles. Very small light-brown laminae of mica are 
scattered through the quartzes and feldspars. The rocks of this canon con- 
tain the largest titanites that have thus far been observed in the granites of 
the Fortieth Parallel. When seen under the microscope, they are found also 
to have a straight and sharply denned contour, while the smaller titanites are 
often somewhat irregularly rounded. The rough surfaces of their sec- 
tions are just as characteristic as their dark-shaded border, which depends 
upon the high index of refraction, according to Des Cloizeaux, 1 in titanite 
ft = 1.905. 

Different geological characteristics are displayed by the granites next to 
be mentioned, which are, for the most part, members of the Archaean territory 
of the crystalline schists, and which at the same time represent other petro- 
graphical varieties. That from Bruin Peak, Park Range, Colorado [99], is 
uncharacteristic, being very poor in quartz and mica and rich in dull 
impellucid feldspars, traversed by light or dirty green veins of epidote. The 
granites of Davis' Peak, Park Range, are rather fine-grained [100, 101], and 
bear macroscopical garnet, black mica, no distinct hornblende, many ortho- 
clases which are fibrous like those in the Saxon metamorphic granulites, 
and quartzes strikingly poor in fluid-incluisons. The laminae of mica pos- 
sess, for the most part, a tendency to parallel grouping; and the pieces of 
rock knocked off in this direction give slides which present almost nothing 

1 Manuel de min^ralogie, part I, 1862, 149. 



GRANITE. 53 

hut the hasic sections of the mica plates, of course without the lamellation 
characteristic of orderless structure, and without dichroism. 

Typical Archaean granites are found at Grand Encampment Creek 
[102, 103, 104]. Their feldspars often have a reddish color, which is 
produced by infiltrated oxyd of iron deposited in the form of delicate 
orange-red, and manifoldly dentrically-lobed laminae upon the faces of 
microscopical fissures. These granites are partly very poor in mica [102] 
and partly they contain no real mica at all [104], while in the eruptive granites 
it is never wanting. They have instead a leek-green or dirty-green, rather 
strongly dichroitic chlorite, or perhaps mica-like mineral, which is much 
less cleavable and much more imperfectly lamellated than the proper com- 
mon biotite, and which also forms only short squames that are confusedly 
striated. This constituent can be distinguished at first sight from horn- 
blende, which is often similarly colored, by the entirely different cleavage. 
In some varieties of these granites, it is accompanied by groups of white 
mica, and in others the brown mica predominates. The qiiartzes of all 
specimens from this locality are conspicuously poor in liquid-inclusions. 

Very interesting granites occur at North Park. One from Clark's Peak, 
Medicine Bow Range [105], is an extremely fine variety. It bears considera- 
ble fresh plagioclase, black mica of the usual kind, splendidly lamellated, and 
apatite, but neither hornblende nor titanite. The quartzes are not very 
poor in liquid-inclusions. A granite from the southern base of Clark's 
Peak in the North Park is in many respects a remarkable and curious 
rock [106]. It is very coarse-grained and exceedingly rich in quartz 
which looks in the thin section like window-glass, and contains the 
largest number of fluid-inclusions that any rock-constituent has ever been 
seen to hold. They consist of water: liquid carbonic acid or saturated 
salt solutions do not occur among them. The bubbles in the single inclu- 
sions here are of unusual mobility and restlessness. Beside these fluid 
particles, the quartz contains innumerable, long, black, hair-like microlites 
that are extraordinarily thin and scattered without order all through the 
mass. There is very little of the feldspar, which is powerfully decomposed, 
and frequently shows well-nigh obliterated vestiges of broad, twin stria- 
tion. Biotite is present in large laminee : hornblende and titanite are 



54 MICROSCOPICAL PETROGRAPHY. 

wanting. A striking occurrence is found in the black bodies, the size of a 
pea, which show themselves in the slides as perfectly impellucid, even on 
the thinnest edges, and have in reflected light a distinct metallic lustre, so 
that they probably belong to magnetite. This seems the more likely 
because they often present quadrangular outlines, and sometimes little, 
short, yellowish-red fringes and edgings of oxyd of iron have apparently 
sweated out of them. It is very remarkable that generally these thick 
magnetite grains are surrounded in the first place by a nearly continuous 
margin of a very fine-grained mixture of quartz, feldspar, and colorless 
mica, the single particles of this aggregation being below 0.1 mm thick. 
This peripheric zone evidently owes its existence and the striking fineness 
of its structure to the presence of magnetite, and is on an average O.S"" 11 
broad. It is interesting because, a, the proper constituents of this granite 
are so coarse-grained; 6, colorless white mica does not otherwise occur as 
a constituent of this rock, being limited to the zone which surrounds the 
magnetite, and perhaps the latter used up in its formation all the contiguous 
iron, so that in the near neighborhood there was no opportunity for pro- 
ducing dark ferriferous magnesian mica; c, this zonal aggregation does 
not occur anywhere in the rock independent of the magnetite; d, it is 
immediately surrounded on its exterior outline by the coarsest granite 
constituents, most often by thick individuals of quartz, so that no passage 
or transition in the dimensions of the structure-elements can be discovered. 
The granite of Glacier Canon, south side of Clark's Peak [107], 
has more of an eruptive appearance than those next following. It is 
made up of colorless feldspar, considerable quartz, greenish-brown mica 
beautifully lamellated and penetrated by many apatite needles: besides, 
there are large reddish grains more than l ram in size, appearing microscopi- 
cally, with, on an average, five of these individuals in a common thin section. 
Neither hornblende nor titanite is present. At the first glance, this ingredient 
might be supposed to be garnet, but under the microscope the vivid yellow- 
ish-red sections intensely polarize the light, and it is most probable that the 
mineral, which is also pierced by apatite prisms, belongs to zircon. Its 
peculiar color perfectly corresponds with that of the zircon sections in the 
syenites of South Norway, and outlines appear which may, without hesitation, 



GRANITE. 55 

be pronounced longitudinal sections of the combined tetragonal prism and 
pyramid. This mineral is not generally disseminated through the rock in 
small microscopical individuals, but is limited to the grains which can be 
observed with the naked eye; a phenomenon which is repeated by the 
zircon of the Norwegian syenites and by the garnet of the Saxon 
granulites. 

The granite from Cherokee Butte, Medicine Bow Range [108], is 
doubtless metamorphic. The feldspar is highly decomposed, quite dull 
and impellucid, the mica brownish and rare, and there is no hornblende or 
titanite. Curiously, the quartz does not contain liquid-inclusions, and the 
empty cavities in it are not scattered without order through its whole mass, 
or gathered into clusters, which send out rays, as is the case in the quartzes 
of the eruptive granites, but are aggregated into rows, which are often par- 
allel to one another, and, with a low magnifying power, are seen to traverse 
the quartz-mass in straight black lines, running without deviation to its 
verge, and there ending abruptly against the adjoining mineral. The beha- 
vior of the quartzes in graywackes and sandstones is very similar to this. 
The course of these lines gives the quartzes something of the appearance of 
fragments, or even of worn fragments. The quartz-grains are at the same 
time very much rounded : it is also remarkable that the single particles are 
very different in size, large and small being associated. This is a phe- 
nomenon common to the clastic graywackes ; but in the eruptive granites 
the individuals of quartz do not vary so much, and are nearer of a uniform 
thickness. 

The same characteristics are found in the granites next to be named, 
which are likewise not eruptive ; and this tends to support the theory that 
these rocks are altered clastic sedimentary depositions. 

Other metamorphic granites are found at Bellevue Peak, Medicine 
Bow Range, very poor in mica, and bearing decomposed feldspars, 
containing also rounded grains of garnet the size of a pea, rich in 
macroscopical cracks and fissures, but not in microscopical ones [109]. 

Further, at Elk Mountain is a very fine-grained variety [110], which 
bears hardly any true mica, but instead that greenish, chloritic ingredient, 
already described, in the granite from Grand Encampment, Park Range. 



56 MICROSCOPICAL PETROGRAPHY. 

That from Long's Peak, Colorado Range [111], differs somewhat from 
the most characteristic metamorphic ones. In the hand -specimens, indeed, 
the feldspars are arranged with a certain degree of parallelism ; but the 
rock contains beautiful brown mica richly lamellated, and quartzes with 
irregularly disseminated liquid-inclusions and black microlitic hairs. In 
the quartzes and feldspars, a long prismatic mineral that is colorless 
and glaring is interposed, whose innumerable individuals look at first sight 
like apatite ; but a closer examination shows that the dazzling transverse 
six-sided sections are neither regular nor compressed hexagons of 120, 
but belong to obtuse prisms, with the faces truncating the acute angle. 
Obtuse quadrangular prisms also occur, and here it becomes distinct that 
the angle of the prism is about 124. Such sections have a splendid trans- 
verse striation, the infallible sign of twin formation, visible in common 
light, and between crossed nicols it causes variegated color-lines. Without 
much doubt, these crystals belong to grammatite or tremolite ( oc P . oo ^? oo), 
a variety of hornblende free from iron and alumina, which may replace 
here the usual hornblende. The crystallographical dimensions of the 
occurrence point to this conclusion as much as does the twin formation 
so characteristic of hornblende and actinolite ; so also the observation 
that these colorless prisms (which reach a thickness of 0.01 mm ) exhibit on 
their surfaces numerous longitudinal ribs and furrows parallel to the 
chief axis ; a system of striation that often occurs in actinolite, but never in 
apatite; and the twin formation (parallel oo'i? GO) mentioned is, of course, 
also impossible to apatite. Notwithstanding this, apatite is doubtless 
present in the rock. 

A number of granites next follow, which are inseparable members of 
the stratified territory of the crystalline schists. They are wanting in 
all geological evidence of eruptive character. The stratified granites of 
Granite Canon, Laramie Hills [112 113], show transitions into crystalline 
schists. The reddish color of these rocks is secondary, having been pro- 
duced by the abundance of yellowish-red oxyd of iron, which has entered 
the numerous capillary fissures in the form of dendritic lobes. There is 
biotite, and also a macroscopical ingredient of a dark substance, a little 
transparent on the edges, only having a feeble brownish or dark-green 



GRANITE. 57 

color and irregular shapes, and which seems to be a kind of mica similar to 
lepidomelane. The quartz here contains an enormous quantity of pale- 
brown, grayish-yellow, sharp, little lamina? of mica, scarcely larger than 
0.005 mm , placed in all directions, horizontally and obliquely, and standing 
on their edges. In a quadratic surface of quartz 0.2 mm in length, there 
were on one plane GO laminae of mica to be seen, which would be 1,500 
mica plates to a surface of one square millimeter. In the large quartzes are 
also some liquid-inclusions, among them several consisting of a saturated 
salt solution with cubic crystals. The feldspars of this granite include a 
very large quantity of quartzes presenting rounded or rough hexagonal 
sections, and showing in polarized light splendidly variegated spots upon the 
equally colored feldspar-ground. Mica, too, is dispersed through the feld- 
spars, so that they often actually swarm with foreign interpositions. 

Virginia Dale, Laramie Hills, produces a very coarse-grained meta- 
morphic granite [114]. It contains no characteristic biotite, but only that 
dark substance just now mentioned, whose feebly green transparent borders 
show here strong dichroism, and whose lamellated structure occasionally 
becomes distinct. The quartz is very poor in fluid-inclusions, and there is 
no hornblende and hardly any plagioclase. 

In the metamorphic granite from Signal Peak, Laramie Hills, wo 
find again, very characteristically, the dark-green, chlorite-like mineral in 
place of real mica. Limited to the aggregations of its laminae, and 
mixed with them, are very numerous, rounded, yellowish-red, or reddish- 
yellow grains up to 0.08 mm in thickness, which distinctly polarize ; they 
are not garnet, but they may possibly be zircon. There is much plagioclase. 
The quartz is just as poor as the above in liquid-inclusions, and all tho 
constituents here are very fresh and unaltered. 

A red metamorphic granite [115] is found on the west side of Laramie 
Hills, northwest from Sherman. It is very rich in quartz (with very few 
liquid-inclusions but more empty cavities), and contains, besides, fine fibrous 
orthoclase, considerable plagioclase, no characteristic mica; and the green 
chloritic mineral is very rare. All the fissures and cracks in and between 
the rock-constituents are filled with blood-red, reddish-brown, and brown- 
ish-black oxyd, and hydrous oxyd of iron. 



58 MICROSCOPICAL PETROGRAPHY. 

The gray metamorphic granite from Iron Mountain, Laramie Hills- 
however, bears richly lamellated real brown biotite. In the metamorphic 
granites from the six last-mentioned localities, microscopical apatite could 
not be detected. 

By generalizing the foregoing observations of this chapter, we find that 
the several geological varieties of granite in the examined territory are 
generally characterized by the following petrographical features : 

I. METAMORPHIC GRANITES. 

Rocks often colored reddish by secondarily infiltrated oxyd of iron. 

Frequent replacement of the usual biotite by a dark-greenish chloritic 
mineral. 

Rareness of hornblende and apatite. 

Absence of titanite and of primary specular iron. 

Frequent poorness of the quartz in fluid-inclusions. 

Quartz grains often rounded with fluid-inclusions arranged in lines, 
which end in such a manner at the limits of the grains that it makes them 
appear like worn clastic ingredients. 

II. OLDER ERUPTIVE GRANITES. 

Orthoclase generally predominating, no titanite, no primary specular 
iron, less magnetite, constituents not rendered very impure by strange, solid, 
microscopical, crystalline interpositions. There are two chief divisions: 

a. With white mica. The rarer- variety, hardly ever containing dark 
mica, always free from hornblende, and free from, or very poor in, apatite. 

b. With dark magnesian mica. The more frequent variety: 

1. "Without hornblende. 

2. With a hornblende that is very coarse-grained, and generally bearing 
comparatively much apatite. 

III. YOUNGER ERUPTIVE GRANITES. 

Richer in constituent minerals, which are generally more fresh. 
Characterized b}^ titanite, dark mica, and hornblende. 



GRANITE. 59 

Orthoclase, accompanied by proportionally a great deal of plagioclase. 

Feldspar never of a reddish color, but always white. 

Quartzes and feldspars rendered highly impure by microscopical dust 
like hornblende and biotite material. 

Never white mica. 

Generally rich in apatite. 

Frequently microscopical primary plates of specular iron. 

Proportionally richer in magnetite. 

Quartzes more often relatively rich than relatively poor in fluid-inclu- 
sions. 

It should be particularly stated that the described contrasts are valid 
only for the examined rocks of the Fortieth Parallel, and that it is not allow- 
able to generalize from them for other countries. 



60 MICROSCOPICAL PETROGRAPHY. 

SECTION II. 
GRANITE-PORPHYRY. 

Under this name, porphyritic rocks are collected which present the 
usual contrast between a groundmass and larger imbedded crystals of ortho- 
clase alone, or orthoclase and quartz. This groundmass looks to the unaided 
eye too fine-grained to allow of placing the rock among the porphyritic 
granites, and, on the other hand, it seems not to be homogeneous enough to 
permit of assigning it to the proper felsite-porphyries. These rocks stand 
petrographically between porphyritic granites and felsite-porphyries. 
Those occurrences which are rich in macroscopically prominent hornblende 
may be named syenitic granite-porphyry. Typical examples of this division 
are found in the German rocks from Beucha and Wurzen, near Leipsic, from 
Frauenstein and Altenberg, in the Saxon Erzgebirge, lately examined 
microscopically and described by J. Baranowski j 1 also in the rocks from 
Aschaffenburg, in Bavaria. Along the Fortieth Parallel, these rocks, else- 
where not very frequent, are developed in many places and in an eminently 
well-characterized degree. Sometimes they manifestly stand in close geolog- 
ical connection with the other granites of a given locality, presenting 
the same rock-mass in merely a somewhat petrographically different relation. 

Beginning at the west, and going east, the first member of this series 
found, is not very characteristic. It occurs in Granite Hills, Nevada, west ot 
Spanish Spring Valley [116], and is a light yellowish-gray rock with whitish 
feldspars and black hornblende points in an almost homogeneous ground- 
mass. Under the microscope, the presence of quartz is detected, and almost all 
these quartzes are yet only roughly crystallized, their sections most frequently 
showing the rhombic outlines of the longitudinally-cut, hexagonal pyramid. 
The groundmass is an aggregation of such quartzes whose crystallized 
forms are notoriously strange to the granites, and of feldspar individuals; 
between them being a substance of a light grayish-yellow color that is not 
individualized, and which polarizes between the nicols but feebly and indis- 

1 Uber die miueralogische mid cbeuiische Zasammuusutzuug der Granitporpbyrc, 
Inaugurol-Diaeertation, Leipzig, 1873; see Zeitschrit't d. d. geolog. Gesellseh., 1874, 
xxvr, 522. 



GRANITE-PORPHYRY. 6 1 

tinctly, and is full of numerous aculeate, prickly microlites. The same mass, 
occurring here in only a few spots, sometimes fills an important part in the 
composition of the felsitic groundmass of the real porphyries. There is 
some biotite and apatite besides the hornblende. The larger quartzes are 
almost free from fluid-inclusions. Whether some very small inclusions with 
an extremely delicate outline and a dark-bordered bubble are glass particles 
cannot, on account of their minuteness, be determined. 

Rocks from Nannie's Peak, Seetoya Range [117], connected with 
the granites occurring there, strongly resemble some from Maggie's Peak 
[118, 119]. Both belong to that class of granites which are almost felsite- 
porphyries. In the very fine-grained, yellowish-gray groundmass, feldspars 
and sharp, six-sided plates of dark mica, up to 2 mm in size, are abundant. 
Quartzes are less frequent. The groundmass of the rocks of Maggie's 
Peak is, curiously, an extremely fine-grained mixture of quartz and feld- 
spar, in which the larger quartzes again show pretty well-developed crystal- 
lized forms. Polarized light proves that this aggregation, at one position of 
the nicols, mostly resolves into colored, double-refracting particles, and the 
little spots which had appeared dark, become colored by turning the ana- 
lyzer, or the specimen section around its vertical axis. There is nothing, 
or extremely little, of amorphous, unindividualized, micro-feldsitic matter 
in this aggregation. A noticeable formation of sphoerolites occurs in 
the rock from Nannie's Peak, the process of formation often being far 
advanced. They have an evident centre and well-developed concentric 
rays, which consist either of indistinct crystalline grains arranged in linear 
form (granosphaerites) or of bunch-like felsitic fibres. The effect is pretty 
where these felsitic fibres are conglobated into sphaerolitic balls, entirely 
encircling many of the little quartzes, and having the appearance of a 
radiating garland. This same phenomenon has been observed with the 
quartzes in the rhyolite from Mount Baula in Iceland. The larger fresh 
feldspars of these rocks are for the most part plagioclases, or sanidin- 
resembling orthoclases, with imbedded ledges of striated triclinic feldspar. 
The thicker and most nearly macroscopical quartzes do not contain any 
glass particles, but a few liquid-inclusions, among which are some with 
included cubes of salt. Biotite, though a very rare ingredient in the corre- 



62 MICROSCOPICAL PETROGRAPHY. 

spending German rocks, is here defined with surprising sharpness, and is as 
remarkably regular in lamellation : it is pierced by many colorless apatite 
prisms. Green hornblende occurs infrequently. One light-gray variety 
of these rocks from Maggie's Peak, Seetoya Range [118], appears alto- 
gether homogeneous, being almost deprived of all larger crystals, and 
is easily mistaken for rhyolite; its felsitic groundmass, however, has pre- 
cisely the same structure as the described varieties, and the quartzes are 
very full of fluid-inclusions. 

A rock from Clover Cafion, Humboldt Range, is exactly similar to 
those above mentioned, the groundmass being distinctly grained without 
amorphous portions [120]. Nevertheless, quartzes the size of a pea here 
contain double inclusions, with liquid carbonic acid, in such abundance as 
is seldom seen. The many hundred inclusions which one field of view 
presents in one plane, and which look under a low magnifying power like a 
dust arranged in stripes, heaps, and bands, are all of this remarkable nature. 
Mica is also present here. 

Characteristic occurrences of the granite-porphyries, or rather of the 
syenitic granite-porphyries, are found at the Franklin Buttes, Nevada [121, 
122, 123, 124, 125, 126, 127, 128]. They are generally greenish or flesh- 
colored rocks, first one and then the other tinge predominating, and consist 
of an extremely fine-grained groundmass, which often passes into the fel- 
sitic, seemingly homogeneous state, and through which a comparatively 
large number of whitish feldspars, dark-green hornblende, gray quartzes, 
and sharp, black plates of biotite are disseminated. Under the microscope, 
a long series of hand-specimens of these rocks show considerable resem- 
blance in their respective behavior. Their groundmass, though as yet 
only in some varieties macroscopically crystalline, is, nevertheless, a crys- 
talline-granular aggregation without an amorphous unindividualized base, 
or, at any rate, without one appearing. Microscopically, this mixture is 
comparatively coarse-grained (the thickness of the single grains averaging 
0.03 mm ) ; and it presents in extraordinary completeness, the contrast between 
dull-gray, and poorly translucent little feldspars, often presenting rectangu- 
lar shapes and water-clear quartzes that are sometimes roughly crj^stallized. 
The minute feldspars of this groundmass have a good many little spherical 



GRANITE-PORPHYRY. 63 

or oval hollows. Sphaerolitic formations, and even indications of a tendency 
to form them, are generally rare here. The larger porphyritical quartzes 
are always entirely free from glassy particles, but they contain in some 
places, very many, and in others, strikingly few fluid-inclusions. These 
latter here consist for the most part of a saturated solution of chloride 
of sodium with minute salt cubes [124, 122, 127]. Sometimes these fluid- 
inclusions contain two beautiful cubic crystals ; and, again, the larger of 
them are remarkable by reason of including a great quantity of foreign 
substances, which have no doubt been partly taken up mechanically; 
inclosures 0.024 mm in length and 0.008 mm in width, often containing 
colorless salt cubes, pale-green round grains, and thin needles (most prob- 
ably hornblende), and even plates of blood-red oxyd of iron (Plate I, 
fig. 5). Sometimes there are so many of these foreign objects in the fluid- 
particles that the bubble cannot have its usual round shape, but appears 
bent or drawn out and bag-like ; and in some of the inclosures there is a 
confusion of actually undeterminable objects. Moreover, the larger quartzes 
of these granite-porphyries contain distinct " stone cavities " (Sorby) ; i. e., 
amorphous inclosures of a microfelsitically devitrified substance, often 
having a good hexagonal shape, like those which so often occur in the 
quartzes of genuine felsite-porphyries. There exists also another similar- 
ity with felsite-porphyries in the phenomenon where arms of the ground- 
mass penetrate into the quartzes in the form of wedges and bags. In a few 
varieties of these rocks, the larger feldspars in the groundmass are altered 
into a substance which is fibrous or confusedly radiating [128]. The 
amount of hornblende in some specimens is proportionally large, and apa- 
tite occurs in most of them. The presence of well-characterized titanite ia 
remarkable ; the mineral is scattered in microscopical crystals through many 
of these rocks, and in some places it is even visible macroscopically 
[124]. It is curious that in this series, lying as close together as they do, 
one occurs [129] which every petrographer would, from its macroscopical 
aspect, prefer to place among the genuine felsite-porphyries, and that the 
quartzes of this rock are seen under the microscope to contain very good 
glass-inclusions, which are entirely wanting in those of the granite-porphy- 
ries of this region, and which prove that the macro-petrographical percep- 



64 MICROSCOPICAL PETROGRAPHY. 

tion that discerned this occurrence from the accompanying ones was not mis- 
taken. 

To this same group also belong those from Marble Hill, Kinsly Dis- 
trict, Nevada [130, 131, 132, 133, 134]. Macroscopically, they are middle 
members, being less between granites and felsite-porphyries than between 
granite-porphyries and felsite-porphyries. They are very rich in crystals, 
but the groundmass looks macroscopically rather homogeneous and com- 
pact. Its color is a feebly greenish gray. The microscopical structure is 
again distinctly crystalline. Among the secreted crystals, beside quartz and 
feldspars, hornblende and black mica are to be found. The quartzes con- 
tain inclusions of the groundmass of such size that the larger ones can be 
seen in the sections with the naked eye. They bear also many empty 
cavities and fluid-inclusions, among the latter some double ones with 
liquid carbonic acid. Here also there are no glassy particles. The thin 
sections show that a part of the feldspars, are in the interior, still quite fresh 
and pellucid, while alteration has only surrounded them with a thin, milky- 
whitish exterior; but in other individuals decomposition has progressed farther 
toward the centre, and only a small kernel of fresh primary feldspar-sub- 
stance is left ; at last the whole feldspar section is found to be metamor- 
phosed into the usual impellucid kaolinic mass. All these interesting 
stages of transition may be seen in one section. Some varieties of 
these rocks bear excellently striated plagioclase [133]. The larger horn- 
blendes in the groundmass often have a fibrous appearance, produced by 
their being built up of innumerable acicular microlites, whose bunch-like 
ends spread out like sheaves of wheat. Hornblende of this construction 
seems sooner to undergo molecular alteration than the common form. Those 
rocks from this locality (Marble Hill) which are rich in secreted crystals, 
contain comparatively very many large plates of deep-black mica, which, 
under the microscope, are richly lamellated and frequently pierced 
by sharp needles of apatite. The apatite needles, however, occur quite 
plentifully independent of the biotite. A substance wliich is doubtless 
titanite has been observed microscopically in some varieties [132, 133], 
whose groundmasses are proportionally coarser-grained. Magnetite is some- 
times found in extraordinarily numerous and well-shaped crystals. The 



GRANITE-PORPHYRY. 65 

brownish spots which appear in some, parts of the groundmass are 
hydrous oxyd of iron, produced by the decomposition of hornblende, which 
has entered as a liquid between the feldspar and quartz grains of the ground- 
mass. One interesting variety of these granite-porphyries from Marble Hill 
resembles the others in every respect, except that the entire groundmass is 
not a microscopical aggregation of individualized quartz and feldspar, part 
of these substances having contributed to the formation of fine felsitic 
sphaerolites. There are not only evident tendencies to this process and 
elementary stages of it, but splendid complete sections through well-rounded 
radiating sphaerolites which feebly polarize, and bunches of their fibres are 
found fastened rectangularly upon the outlines of quartzes. This variety 
contains biotite plates up to the size of 4 mm . It is simply a sphserolite- 
bearing modification of the former rocks. Beautiful titanites are dissemin- 
ated through it, and its green hornblende sections do not show a fibrous 
structure, but a compact mass having splendid cleavage-directions. The 
association of titanite with hornblende in these rocks has a particular sig- 
nificance, if we remember that the first mineral is so nearly an exclusive 
characteristic of the later eruptive granites. Perhaps the presence of 
titanite would be sufficient to warrant the belief that the granite-porphyries 
of Marble Hill, as a petrographical modification of granite, belong also to a 
relatively late geological epoch. 

In the Goose Creek Hills, Nevada, granite-porphyries occur [135, 136], 
with very little or else with considerable hornblende, white orthoclase, some 
plagioclase, and quartz in a genuine, excellently fine-grained groundmass, 
in which the microscope detects, beside grayish feldspar particles and 
quartzes, rough elementary stages in the formation of sphserolites, sometimes 
in the shape of rather distinctly fibrous bunches. The larger feldspars are 
altered into an entirely dull, impellucid mass, in some places with feeble 
vestiges of a former triclinic striation. Some of these decomposed feldspars 
contain a large quantity of colorless, acicular and prismatic crystals, three 
or four being usually joined together in the form of a star, which may 
possibly belong to muscovite. A remarkable microscopical structure is 
developed in the larger individuals of hornblende : the characteristic out- 
lines are perfectly preserved, but the original mass is totally altered into 
5 M P 



66 MICROSCOPICAL PETROGRAPHY. 

leek-green fibres, which form single independent systems and bunches, fol- 
lowing one upon another at sharp or obtuse angles. Often these fiber-systems 
are undulating and curved, and sometimes parallel bands and strings appear 
in place of the fibers. Inside the hornblende, these substances are accom- 
panied by opaque, black, angular grains, which are surely magnetite, 
and as surely a product of decomposition, because they do not occur 
in the fresh hornblende prisms of analogous rocks. Dathe has shown that 
the development of magnetite out of the altering augites of diabases can be 
followed with certainty under the microscope; 1 and Gustav Bischof long ago 
determined that this process was probable iipon chemical grounds. 8 But in 
addition to these two products of alteration, the hornblende sections include 
dark greenish-yellow grains, and little accumulations of such grains, most 
probably belonging to newly formed epidote. It is further remarkable that 
while hornblende which has undergone so active a process of metamorphism, 
has changed into not less than three easily distinguishable epigenetic min- 
erals, the old apatite prisms which pierce it in all directions have been con- 
served without the slightest touch of alteration; a fact which may always 
be observed where apatite is included in highly decomposed minerals, and 
which leads to the conclusion that the water holding carbonic acid, which is 
generally regarded as the chief chemical agency in the decomposition of 
silicates, has no effect upon phosphate of lime, notwithstanding it is so easily 
dissolved by hydrochloric acid. 

The syenitic granite-porphyry from the divide between Bingham and 
Tooelle Canons, Oquirrh Mountains, Utah [137], belongs to this same series. 
It resembles those above described in the most characteristic points. Here 
the alteration of the hornblende has gone still further (Plate III, fig. 2). 
Its outlines have been as well preserved as in the last described specimen, 
but much colorless calcite having distinctly rhombohedral cleavage-fissures 
has settled in the sections of the primary mineral. The leek-green matter 
mentioned above, is reduced to single undulating strings and bands sepa- 
rated from the calcite by a botryoidal or bud-like boundary-line, which 
appears with a high magnifying power to be very delicately crenated. It 

1 Zeitschrift d. d. geolog. Gesellscb., xxvi, 1874, 29. 

*Lehrbncb der^cbepiisch. u. pbysikal. Gcologie, 2. Aufl., u, 913, 944. 



GRANITE-PORPHYRY. 67 

is remarkable that the general direction of the green stripes and 
bands plainly corresponds with the cleavage of the hornblende. The outer- 
most zone of these sections usually consists of the green matter, while with 
the interior portions are again mixed black magnetite and dark-yellowish 
epidote grains, the latter often presenting regular crystalline forms which 
are not unlike those of epidote. Sharp prisms of apatite are also present. 
These altered crystals are the most complex in composition, and, with refer- 
ence to the history and destiny of hornblende, are the most instructive of 
any that have ever been studied, and may not hereafter be easily surpassed. 

At the foot-hills between Tooelle and Stockton, Oquirrh Mountains, a 
granite-porphyry occurs [138], with little felsitic groundmass, much quartz, 
orthoclase, very beautiful hornblende, and macroscopical titanite. A crystal 
of yellowish-brown titanite reaching a length of 1.5 mm was found included 
in an orthoclase. 

A curious granite-porphyry forms a dike in the northern ridge of Twin 
Peaks, Wahsatch Range [139]. It contains, in an extremely fine-grained 
groundmass, large biotite plates, but neither macroscopical quartz nor feld- 
spar. The minute elements of the groundniass, which is entirely crystal- 
line, are well-shaped, dull feldspars, grains of quartz filling up the 
spaces between these and the brown biotite lamina which lie in all direc- 
tions; the horizontal ones appearing darker, those transversely cut lighter 
brown, and the smallest ones sharply hexagonal. There is also a compara- 
tively enormous quantity of apatite, but there is no hornblende, and there- 
fore no titanite. 

A macroscopical appearance similar to the last described is seen in a 
dike in the limestones of Big Cotton wood Cafion, Wahsatch Range [140], 
but the groundmass is somewhat coarser-grained and poorer in mica, 
though it has some very fresh feldspars, among them fine plagioclases, 
visible to the naked eye. The rock is extraordinarily rich in iron pyrites, 
which forms in transmitted light fine black macroscopical grains, appearing 
even with a low magnifying power as thick lumps, entirely opaque of a 
yellowish color, and having a metallic lustre in reflected light. It is curious 
that this mineral forms real veins through the quartz grains, the phenomenon 
suggesting fissures in the grains that have been filled up. The larger 



68 MICROSCOPICAL PETROGRAPHY. 

quartzes, proved by polarized light to be one individual, are traversed by 
such a mineral-net, which divides the quartz into isolated, single grains. 
Ramifications of the iron pyrites in the form of thin, delicate lines, also 
traverse the feldspars, always giving proof of its later injection. There is 
also an indistinct, green constituent in the rock, probably belonging in part 
to half-weathered hornblende and in part to epidote. 

A beautiful granite-porphyry is found north of Clayton's Peak, Wah- 
satch Range [141]. It has a greenish-gray groundmass, rich in plagioclase 
and iron pyrites, and formerly in hornblende, which, however, as proved 
already by the macroscopical aspect of the hand-specimens, is now for the 
most part often altered into coarse, radiated epdiote, and occasionally into 
quartz. The pale-reddish, flesh-colored feldspars seem from the beginning 
to have been rendered impure by much dust of oxyd of iron. Dirty reddish- 
brown spots, with contours similar to the magnesian mica, as shown in the 
corresponding fresh rock, seem to be the product of the alteration of this 
constituent. 

At Good Pass, east of Parkview Peak, between North and Middle 
Parks, Colorado [142], and on Parkview Peak [143], occur rocks belonging 
petrographically to the granite-porphyries, and bearing considerable resem- 
blance to the Saxon rocks of that name from Beucha and Wurzen, and still 
more to that from Altenberg in the Erzgebirge. The groundmass is a 
dirty yellowish-gray, and to the naked eye its place seems to be between 
the extremely fine-grained and the nearly homogeneous state. The rock 
from Good Pass is remarkable on account of its large and most excellent, 
roundly formed crystals of orthoclase. They have the most characteristic 
shape, resembling the perfect individuals in the fine-grained porphyritic 
granites from Thuringia and from Neubau near Hof in the Fichtelgebirge, 
Germany. They present the faces: 

T and I = <x> P = I 

M co ^ oo i - I 

P OP 

z = <x>-3 = i-3 

y = 2 oo 2 - i 

n = 2^oo= 2 -i 



GRANITE-PORPHYRY. 69 

These feldspars, sometimes an inch long, are generally of an adularia- 
like clearness in the interior, and on the outside milky, dull, and impellucid. 
Plagioclases also occur, but they are wanting in that extraordinary morpho- 
logical individualization. Macroscopically, both rocks contain some 
hornblende and quartz grains, but they have no glass and only a few 
liquid-inclusions. Fluid-particles are also observable in the fresher por- 
tions of the feldspars, and in greater frequency and distinctness than usual. 
The rock from Good Pass also bears brownish mica. That from Parkview 
Peak possesses a lighter color, and has more green, strongly fibrous horn- 
blende, in which bunches of finely radiated, pale-brownish, crystalline 
needles, probably epidote, are often interposed. Both rocks are rich in 
apatite and in beautiful titanite, and they also contain magnetite and 
splendid sharp cubes of iron pyrites, which have the color of brass in 
reflected light. The groundinass is almost entirely crystalline here, as is gen- 
erally the case through all these granite-porphyries. It may be mentioned 
that, in a locality not very far from this, Steve's Ridge, Elkhead Mountains, 
an undoubted Tertiary sanidin-trachyte exists, in which orthoclase-feldspars 
are developed, presenting in rich profusion precisely the same list of crystal- 
lographical faces as- the above orthoclase. The geological evidences of the 
age of the Good Pass rock are insufficient to warrant any determination from 
them, though it might be supposed from the great superficial resemblance 
of the feldspars that it belongs also to the eruptive rocks of the Tertiary 
epoch. But a number of petrographical circumstances combine to contra- 
dict this theory, founded only on the analogous form of the feldspars, and 
tend to prove a probable older origin. These petrographical facts are : a, 
their feldspar crystals in the above-described porphyry have the antique 
behavior which is characteristic of the orthoclases of the old granites, the crys- 
tals in the trachyte referred to presenting that really modern type which 
has received the proper name of sanidin ; b, the well-developed form, rich in 
rare faces, is certainly far less remarkable in orthoclases of older granitic 
than in sanidins of younger trachytic rocks ; c, the microscopical structure 
of the sanidins in question differs in many points from that of our ortho- 
clases ; d, the hornblende in the rocks from Good Pass and Parkview 
Peak, give green-colored sections, characteristic of the older, granitic rocks, 



70 MICROSCOPICAL PETROGRAPHY. 

the hornblende sections in the Tertiary trachytes being always brown here ; 
e, the quartzes of these two rocks contain only fluid-inclusions, those of the 
trachyte only glass-particles ; f, iron pyrites would be extraordinary as a 
secondary constituent in younger rocks ; g, the presence of titanite is just 
as usual in granitic rocks as it is highly rare in trachytic ones. So the 
theory that these two granite-porphyries belong to the Jurassic eruptive 
rocks, i. e., to the youngest division of the older ones, is generally strength- 
ened. 



CHAPTER 1Y. 

FELSITE-PORPHY11Y AND SYENITE. 



SECTION I. FELSITE-POEPHYEY. 
SECTION II. SYENITE. 



SECTION I. 
FELSITE-POKPHYRY. 

There are few questions in petrography which are of such importance 
and which have been so differently answered as that concerning the 
microscopical behavior of the groundmass of felsite-porphyries, which 
appears to the naked eye to be compact and homogeneous, or at least min- 
eralogically indeterminable. Abstractions founded upon analogies and 
interpretations of chemical analyses, were early formulated with a view 
of elucidating the subject, but none of these has ever risen above the rank 
of a more or less satisfying hypothesis. It is evident that a question of this 
kind can only be accurately solved by close microscopical examinations, and 
this branch of research and investigation is the most difficult of any that has 
been undertaken with this instrument. 

For a long time, two contradictory opinions on the point have 
confronted each other. Most of the earlier petrographers believed with 
Leopold v. Buch 1 that the groundmass of felsite-porphyries is an intimate 
aggregation of extraordinarily minute crystalline particles, especially of 
feldspar and quartz in microgranitic structure. Delesse, on the other hand, 

'Keise dnrch Norwegen u. Lap|>)an<l, 1S08. 1, 139. 

71 



72 MICROSCOPICAL PETROGRAPHY. 

pleaded for the opinion 1 that the seeming homogeneous paste which includes 
the porphyritical crystals was to be compared with a mother-water, present- 
ing in a certain respect the residuum of crystallization, and was not composed 
of individualized mineral particles, but a half-crystalline mass; an indefinite 
silicate consisting of silica and all bases which occur in the porphyritical 
crystals. This important conflict was even mooted after the microscope 
began to take an active part in the investigation of the question by the exam- 
ination of thin sections. But it should be remembered that the first studies in 
this new field were made without first-class instruments, with insufficient 
material, on sections which were not reduced to the extreme thinness neces- 
sary, and without exhausting all the resources of polarized light. In 1862, 
F. Z. arrived at the conclusion that the groundmass of half a dozen felsite- 
porphyries which he had examined microscopically had an entirely crystal- 
line structure, composed of little particles of impellucid feldspar and clear 
quartz. 2 H. Laspeyres 3 and E. Weiss 4 corroborated this testimony after a 
microscopical study of the felsite-porphyry from Halle in Prussia. Vogel- 
sang's 5 microscopical interpretation of the groundmass, however, approaches 
very nearly to that of Delesse. The groundmass proper, independent of 
the secreted crystals, is not resolved under the microscope into single minute 
crystalline particles, but appears in an unindividualized, half-crystalline 
state. Some groundrnasses are even really amorphous and simple-refract- 
ing. Then Stelzner came to the front 6 as an opponent of the theory of his 
predecessor, and reasserted the conclusions of the earlier investigators, 
namely, that the groundrnass is, under the microscope, a fine crystalline 
aggregation, all of whose elements become colored in polarized light, and 
therefore cannot be amorphous. E. Cohen, 7 after examining the typical 
felsite-porphyries of Odenwald, Germany, was the first to suggest that 
their groundmass is not generally the same, but in the several occurrences 

'Bulletin de la Soc. ge"olog. (2 s<r.), VI, 629. 

2 Sitzungsber. d. Wiener Akadeuiie, 1863, XLVII, 239. 

3 Zeitschrift d. d. geolog. Gesellsch., xvi, 1864, 402. 

* Beitrage zur Keimtniss der Feldspathbildung, Haarlem, 1866, 146. 

5 Philosophic der Geologic u. niikroskop. Gesteinsstudien, 1866, 133. 

6 Petrograph. Bemerkungen iiber Gesteine des Altai, 1871, 22. 

7 Die zur Dyas gehorigen Gesteine des siidlichen Odenwaldes, 1871, 37. 



FELSITE-PORPHYKY. 73 

of a different microscopical constitution. Some of them, indeed, are crys- 
talline-grained, while others contain a predominating glassy, amorphous mass 
(base). 

The more all these theories as to the microscopical constitution of the 
felsitic groundmass differed, the more necessary became a thorough 
examination of larger numbers of specimens, in the light of the experience 
which had meanwhile been accumulating. F. Z. therefore undertook, in 
1873, a new series of investigations of the subject, 1 the general result of 
which is that the microstructure of the groundmass is not at all equally 
constituted in each case, as it was believed to be before Cohen, and that 
the masses which are most similar macroscopically are found when viewed 
microscopically to be entirely different, There are, indeed, groundmasses 
which possess wholly, or almost wholly, a granitic, crystalline structure, 
being resolved by polarized light into double-refracting particles ; but there 
are, on the other hand, some which consist in a largely predominating 
measure of an indistinctly or wholly unindividualized substance, entirely, 
or almost entirely, indifferent to polarized light. Between these two extreme 
formations appear middle members, having some of the characteristics of 
both. And thus it appears that the earlier theories, notwithstanding their 
difference, were not so very far out of the way, except that it was not per- 
missible to generalize the single result gained by so few examinations. 

Genuine felsite-porphyries are not very frequent along the Fortieth 
Parallel, as compared with other countries; for instance, Germany or 
France. It almost seems as if the granite -porphyries here, elsewhere rather 
rare in this behavior, play the geological and petrographical pail which 
in France and Germany is assumed by the felsite-porphyries. In the first 
place, it may be said that the porphyries of the West generally possess a 
felsitic groundmass of an extremely pronounced microgranitic structure, 
made up almost wholly of crystalline particles; and that the unindividual- 
ized, glassy, indistinctly devitrified, or microfelsitic substance is rare. 

The rock from Miner's Canon, Truckee Range, Nevada [144], is a fel- 
site-porphyry, intercalated in beds referred to the Triassic formation, pre- 
senting that variety which, on account of its splintery fracture, is known as 

1 Mikroskopiscbe Beschaft'enheit d. Mineralien u. Qesteine, 1873, 324. 



74 MICROSCOPICAL PETKOGKAPHY. 

hornstone-porphyry, although the real constitution of its groundmass is 
related neither chemically nor mineralogically to that of the compact micro- 
crystalline quartz. The rock looks like certain Mlleflintas from Sweden, 
and in color it is a light greenish gray. Little dirty-green spots and quartz 
grains, up to the size of 2 mm , are its only prominent secretions that can be 
distinguished. The predominating groundmass appears extremely com- 
pact, but under the microscope it is entirely crystalline, though, even for a 
felsite-porphyry, unusually fine-grained ; no particle or little point appear- 
ing dark in polarized light by turning the analyzer or thin section. It 
nevertheless seems as if its single constituent (probably quartz and feldspar) 
had, in the form of extremely minute particles, balled together into very 
small lumps ; for in polarized light roundish spots are visible, which are 
really glittering little points, but which present as a total impression the 
same tinge of color ; for instance, yellowish or bluish. The macroscopical 
quartzes show pretty well-defined crystal-outlines, and contain some fluid- 
inclusions and beautiful, imbedded, isolated particles of the groundmass, 
with evidently the same structure, but no glassy grains. There is green 
hornblende in perfect little crystals and partly obliterated spots ; no larger 
feldspars. 

To the south of Willow Spring, Montezuma Range, just north of 
French Canon, a rock occurs [145] which might be taken at first sight for 
a rhyolite, but which surely belongs to the geologically older felsite-por- 
phyries. The orthoclases in the thin sections are quite milky-white and 
impellucid ; they seem highly altered, but certainly a part at least of the 
dust, which is usually found in connection with kaolinization, appears in a 
very high magnifying power with sufficient distinctness as very minute fluid- 
inclusions. The quartzes, which appear macroscopically the size of a pepper- 
corn, are, besides the feldspars, remarkably rich in large liquid-inclusions, 
which are superior in dimensions even to those of the granitic quartzes, and 
are, for the most part well-shaped hexagonal pyramids, with rather sharp 
edges. Glassy inclusions are wholly wanting. The light groundmass, 
probably entirely crystalline-grained, is made up of nearly colorless con- 
stituents, the feldspar particles being only a little less pellucid than the 
clear quartz, and containing as coloring materials only some very light yel 



FELSITE-rORPHYRY. 75 

lowish-green little grains of an undeterminable nature, which are of so pale 
a tinge that they evidently do not belong to hornblende. An analogous case 
to this is found at Ravenswood Peak, Shoshone Range, Nevada. Here, in a 
rhyolite country, a rock occurs [146] which a superficial examination would 
lead one to think was united with the predominating occurrence; but its 
macroscopic habitus shows some petrographical differences from the rhyo- 
lites. Through a gray groundmass unusual to the latter, milky irnpellucid 
feldspars, which have no similarity with rhyolitic sanidins, and large 
quartzes are distributed. With this antique behavior of the feldspars (nearly 
all of which belong to orthoclase), the abundance of fluid-inclusions in the 
quartzes agrees. Glassy inclusions are to be found very rarely (see the 
remarks on these objects at the end of this chapter on felsite-porphyries). 
The groundmass is for the most part crystalline, consisting of feldspar and 
quartz grains. Upon first viewing it between crossed nicols, one is inclined 
to believe that a thin, amorphous, single-refracting substance, which appears 
black, predominates ; but the greater part of these dark little spots show 
chromatic polarization when the section is turned around its vertical 
axis. Some amorphous unindividualized base is present, however, in the 
form of a yellowish-gray, granular, globulitically devitrified mass, and the 
behavior of this hidden base is entirely unusual in rhyolites. Larger sec- 
tions of altered hornblende are to be observed niacroscopically ; and dispersed 
through the groundmass are many pale-green, little needles and grains, 
wliich most probably also belong to hornblende. The feldspars include 
distinct prisms of apatite. This is remarkable for two reasons; first, be- 
cause generally apatite does not occur in feldspars, even if other rock- 
constituents are plentifully pierced by it ; secondly, because apatite has 
never been noted among the macroscopical accessory ingredients of felsite- 
poryhyries, although it has often been subsequently detected by microsco- 
pical examinations. 

Of the same geological age as the described granite from Granite Peak, 
Pah-Ute Range, Nevada, is the excellent felsite-porphyry [147, 148]. It 
shows niacroscopically feldspars and quartzes, and also some little plates of 
white mica. The feldspars are altered into an ifflpeltuoid substance, and 
the quartzes are full of fluid-inclusions, but contain no glassy ones. Here, 



76 MICROSCOPICAL PETROGRAPHY. 

also, the groundmass is distinctly microgranitic, consisting of grayish-yellow) 
granular feldspar and colorless quartz, which produce a sharp contrast and 
give a spotted appearance in polarized light, the single individuals wanting 
in distinct outlines. Even in the smallest microscopical quartz grains of the 
groundmass, scarcely 0.01 mm in size, fluid-inclusions are found interposed. 
Among the granite-porphyries of the Franklin Buttes, Nevada, described 
on page 64, there is one which is built up of a genuine, interesting felsite- 
porphyry, with a yellowish-gray groundmass that appears homogeneous, 
little quartz, feldspar, and lamina? of biotite. The mica individuals can be 
seen with the naked eye, and the use of the microscope does not increase 
their number, this ingredient not being present in smaller plates. The sec- 
tions have an extremely fine lamellation, are highly dichroitic and trans- 
versely perforated by colorless apatite needles. The groundmass appears 
under the microscope to be an entirely crystalline-grained aggregation, 
which resembles more than any other felsite-porphyries that of the Cornish 
elvan; and yet in the smaller quartzes of the groundmass occur the most 
distinct little glass-inclusions, with the usual included, dark-bordered, fixed 
bubble. There are rounded quartz sections of only 0.04 mm in diameter, 
which contain in one plane six or eight glass-particles very similar to those 
which are held in the quartzes of the famous rhyolite from Mount Baula in 
Iceland. All the glass-inclusions in the quartzes of this rock are not of like 
importance. Bodies of this kind are usually entirely wanting in the con- 
stituents, where the rock has proved to possess a crystalline structure 
throughout, and they occur, on the other hand, where a part of the original 
rock magma has passed into the glassy or otherwise amorphous, unindividu- 
alized state. Here the very rare phenomenon is presented 1 of glass-inclu- 
sions in the quartzes of a wholly crystalline rock, proving that arguments for 
a once molten state and for a crystalline rock structure do not necessa- 
rily exclude each other, as till now there has been reason to believe. This 
fact is invested with additional interest if we consider that the rock in ques- 
tion, composed of feldspar, quartz, and mica, is really nothing else but an 

1 Baranowski has observed glass-inclusions in the feldspars of the likewise-crys- 
talline granite-porphyries from Beucha in Saxony. Inaugural-Dissertation, Leipsic, 
1873. 



FELSITE-PORPHTEY. 77 

extremely fine-grained granite, and that therefore the objections made to 
the theory of the igneous origin of granite, on the ground of its crystalline 
structure and of the absence of glass-inclusions in its constituents, are very 
weak. 

The limestone of the Wachoe Mountains is traversed by dikes of a typi- 
cal felsite-porphyry [149], a crystalline aggregation being again presented 
in its groundmass. The large feldspars show to the naked eye, in the thin 
sections better than in the hand-specimens, that they are composed of a 
dim and dull exterior zone of a milky, impellucid condition, with a rather 
adularia-like, clear, interior substance, the passage from one to the other at 
the boundary being soft and gradual. The larger secreted quartzes are dis- 
tinguished by their entirely rounded dihexahedral shape, and by their 
containing the most beautiful liquid-inclusions, with salt cubes. One of 
these inclusions of saturated salt solution was 0.015 mm long and 0.004 mm 
broad, the contained salt cube measuring 0.001 7 mm in length on one of its 
edges. Most of the cubes are very clear and sharply denned, but some 
are rounded at the edges and even pass into real grains, yet all are 
connected by easily observable transitions. In some quartzes, these inclu- 
sions are extremely abundant, and perhaps in their nearest neighbors they 
will be found to be very rare or altogether wanting. It is remarkable 
that microscopical objects so identical in nature as these are interposed 
in the quartzes of genetically different rocks, namely, gneisses, granites, felsite- 
porphyries, and diorites. Long arms and short, obtuse wedges of the ground- 
mass penetrate far into the water-clear quartzes, which, beside these, 
include a lot of isolated particles of the groundmass, whose behavior is 
entirely like that of the surrounding or main mass, and which in outline, 
copy the rough pyramidal form of the quartz itself. The presence of such 
characteristic inclusions is of importance in the discussion of a genetic 
question. Vogelsang was inclined to think that the present crystalline or 
half-crystalline constitution of the porphyritic groundmass was not original, 
but rather the result of a secondary, molecular devitrification, which, in the 
wet way, had happened to a primary, glassy substance of tliQ nature of the 
hyaline pitchstones. 1 This theory is, in truth, supported by the fact that 

' Pbilosophie der Geologic u. s. w., 144, 153, 194; see also Kalkowski, Miueralog. 
Mittheilungen, gesammelt von Tschermak, 1874, 52. 



78 MICROSCOPICAL PETROGRAPHY. 

the glass mass of pitchstones is traversed by fractures along which a really 
felsitic devitrification has taken place, producing a mass which cannot gen- 
erally be distinguished in either ordinary or polarized light from the ground- 
mass of felsite-porphyries. But it is doubtful whether this devitrification 
is indeed a secondary hydro-metamorphic one, developed in the lapse of 
time, or whether it was not originally connected with the solidification of 
the pitchstone mass, as is doubtless the case in felsitic rhyolites. As against 
this, however, is the fact of the presence of genuine isolated particles of 
groundmass in the midst of compact quartzes and the absence of any micro- 
scopical fissures leading to them, circumstances which prove that these 
mechanically included little bodies can by no means be a product of a sec- 
ondary devitrification of glass-inclusions; and as in constitution they entirely 
agree with the general groundmass of the rock, the burden of their testi- 
mony is that at the time when the quartz crystals became solid, a felsitic 
substance of the behavior now exhibited was, as such, already present 
around them. 

In the neighboring limestones of Spruce Mountain, Peoquop Range, 
occurs an interesting felsite-porphyry [150] which presents macroscopi- 
cally only small quartzes and no feldspars. Under the microscope, the 
groundmass appears to be principally an aggregation of extremely nice, 
concentrically radiating sphserolites up to the size of 0.3 mm in diameter, and 
feebly polarizing without showing the colored cross. The globules have 
neither a different interior structure nor a defined peripheric zone, and no 
foreign centre. Some feldspars are dispersed between them. This sphsero- 
litic structure cannot be distinguished in hand-specimens, nor is it visible 
in the thin sections, even to the naked eye. Perhaps this characteristic rock 
is in some way connected with the granite-porphyries from Marble Hill, 
Kinsley District, some of which also bear sphaerolites (see page 65), but the 
rock in question is entirely free from hornblende. In later times, much 
attention was paid to these felsite-porphyries in which portions of the 
groundmass have undergone a process of aggregation and radial arrangement 
into globular masses, on account of the striking and genetically important 
analogy between them and their later Tertiary successors, the rhyolites, in 
which the development of sphserolites is much greater, and which also have 



FELSITE-PORPHYRY. 79 

the natural, and artificial, glassy masses of which the secretion of such 
globules is so characteristic. Stelzner has microscopically examined and 
described the sphserolitic felsite-porphyries from the Korgon and the 
Tscharisch in the Altai; 1 E. Cohen those from Apfelskopf and the Edel- 
stein in Southern Odenwald, Germany; 2 and Samuel Allport that from 
Corriegills on the island of Arran, Scotland, 3 which forms a dike near 
the well-known pitchstone. Other sphserolitic porphyries, which have not 
yet been examined, occur in Thiiringer Wald, Germany, at the Schneekopf, 
Regenstein, Meisenstein, at the Hauskopf near Oppenau, and at Ganzenbach 
near Baden in the Schwarzwald. 

Another felsite-porphyry forms a dike in the granite of Long's Peak, 
Colorado Range [151]. It is one of those varieties which is as rich in 
hornblende as most others are very poor. The groundmass is probably 
entirely crystalline. Many liquid-inclusions appear in the quartzes, and 
it is remarkable that numerous large and distinct specimens of them with 
a spontaneously moving bubble are also found imbedded in the ortho- 
clases, where their substance is pellucid enough to admit of seeing through. 
The contrasts in the shape of the liquid-inclusions here are very striking, 
those of the feldspars having a very irregular form and those of the quartzes 
being oval or nearly globular. The farther our studies of the structure of 
feldspar proceed the more it becomes probable that the microscopical dust- 
like material which is so often interposed in the clear unaltered individuals 
consists, for the most part, of extremely minute fluid particles. Hornblende 
occurs in the form of little, fresh, green prisms, abundantly scattered through 
the groundmass ; and the thin and delicate, almost colorless, and often broken 
microlites, which are taken up by the quartz, belong with certainty to horn- 
blende. 

If we compare the American felsite-porphyries of the Fortieth Paral- 
lel with those from other regions which have been examined microscopi- 
cally, principally German occurrences, the chief contrast, all other rela- 
tions being strikingly similar, seems to exist in the fact that the quartzes of 

1 Petrographische Bemerkungen iiber Gesteiue des Altai, 1871, 31. 
* Die zur Dyas gehorigen Gesteine des siidlichen Odenwaldes, 1871, 89. 
'Geological Magazine, 1872, ix, No. 12. 



80 MICROSCOPICAL PETROGRAPHY. 

the former are generally remarkably poor in glass-inclusions, or wholly 
devoid of them, containing only liquid ones. The typical felsite-porphyries 
of Europe, for instance on the other side of Halle, from the Odenwald, 
certain localities in Westphalia, and from Northern Saxony (Rochlitzer 
Berg, etc.), for the most part show in their quartzes some excellent glass- 
grains, beside the predominating fluid-inclusions. The rocks of the two 
continents are strikingly similar in all other points. Perhaps this difference 
warrants the conclusion that the above-described porphyries are generally 
of an older geological age, equivalent to that of the older eruptive granites, 
while those of Germany are chiefly younger than most of the granites 
occurring there. In accord with this theory of older origin is the fact of the 
total absence of titanite in the felsite-porphyries of the Fortieth Parallel, the 
significance of which is that these rocks probably are not petrographical 
modifications of the younger granites, if indeed the latter are as recent as 
has been supposed. 



SYENITE. 81 

SECTION II. 

SYENITE. 

Among the examined rocks of the Fortieth Parallel, genuine character- 
istic syenites resembling the classic German ones from the Plauenscher 
Ground near Dresden and from Weinheim on the Bergstrasse are extremely 
rare. Properly, there is only one really old syenite in this region. It 
forms the main mass of Clure- Hills, Cortez Range, Nevada [152]. Ma- 
croscopically, it consists of prevailing flesh-red, monoclinic feldspar and 
greenish hornblende, which are not fresh in appearance. Under the micro- 
scope, the feldspar is seen to possess the same behavior as the orthoclase of 
granites ; but it is remarbable that some of its individuals are altered, their 
exterior outline only being preserved, into an accumulation of short, radiated, 
gray, fascicular and radially arranged needles, which give an excellent 
aggregate polarization. The hornblende sections prove not to be homogene- 
ous individuals, every one being made up of an aggregation of light-green, 
broad rays, or narrower prisms, which are in one place straight and paral- 
lel, and in another curved and diverging, like sheaves or ice-flowers. On 
the outside, these polysynthetical hornblende crystals are often colored 
brownish-yellow by oxidation ; but their present structure is probably orig- 
inal and not the result of alteration, because in the quartzes of the rock the 
same component bodies, rays and prismatic needles, are found to be included, 
in precisely the same manner as they build up the hornblende. Here and 
there some of the feldspars present half-obliterated remains of a twin-stri- 
ation. This fact is interesting as showing that even this remote syenite is 
not free from plagioclase. It was believed formerly that syenite was only 
a combination of orthoclase and hornblende ; but even the famous rock 
from the Plauenscher Ground near Dresden, always considered to be the 
most typical syenite, and the first to receive this name from Werner, has 
been found, in polarized light, to contain some triclinic feldspar. It is most 
probable that there is no syenite at all free from plagioclase, just as a 
trachyte is not likely to exist, which does not contain this associated feld- 
spar. The rock also bears microscopical but very distinct grains of quartz, 
with many fluid-inclusions; another ingredient which was not formerly sup- 

C M P 



82 MICROSCOPICAL PETROGRAPHY. 

posed to exist in the German syenites, but which can be detected in most of 
them by the microscope. 

Amorphous matter does not exist between the crystalline ingredients, a 
characteristic of all genuine syenites. A rock from the south of Palisade 
Canon, Cortez Range, Nevada, should be mentioned [153], which seems 
macroscopically to be a porphyritic modification of the foregoing syenite. 
This syenite-porphyry contains, in a seemingly homogeneous, greenish-gray, 
felsitic groundmass, flesh-colored feldspars, altered green hornblendes, and 
small quartzes. Under the microscope, the felsitic mass is seen to be com- 
posed entirely of crystalline grains of feldspar, quartz, and altered horn- 
blende. Many of the larger feldspars are triclinic. The hornblende is 
decomposed into a green, earthy substance, and has caused the production 
of the yellowish-brown ochre-masses which surround its metamorphosed 
sections, and are accumulated elsewhere through the rock; for example, on 
the fissures of quartz grains. All the feldspars are evidently fragments. 



CHAPTER V. 

DIORITE, HORNBLENDE-PORPHYRY, DIA 
BASE, MELAPHYRE, GABBRO. 



SECTION I. DIORITE. 

SECTION II. HORNBLENDE-PORPHYRY. 

SECTION III. DIABASE. 

SECTION IV. MELAPHYRE. 

SECTION V. GABBRO. 



SECTION I. 
DIORITE. 

The Virginia Range is traversed by the canons of the Walker, Carson, 
and Truckee Rivers, and full eight-tenths of its mass is made up of younger 
volcanic rocks. Only at rare intervals, where deep erosion in the canons 
has laid bare the original range, or where its hard summits have been lifted 
above the volcanic flows, is there any clue to the materials or position of 
the ancient chain. Mount Davidson (7827 ft.) is one of the few remaining 
vestiges, being composed of diorite and forming the central mass of a bold 
outburst of this rock rising above the city of Virginia. This dioritic body 
is bounded upon the north by propylite, which forms the northern slope 
of Ophir Ravine. Crown Point Ravine marks its southwestern limit. It 
is really an insular mass, one of the ancient original summits, which is 
completely surrounded by the subsequent propylite. The overflow of pro- 
pylite has also failed to cover two narrow insular ridges, which still outcrop 



84 MICROSCOPICAL PETROGRAPHY. 

on the slope of Cedar Hill ; and in the bottoms of ravines, eroded in the 
volcanic material, it is shown that these diorites have obtruded through con- 
siderable masses of metamorphic rocks, schists, limestones, graphitic shales, 
and slates, whose folds date from the period of Jurassic upheaval. 1 The 
diorites of Mount Davidson [154, 155] sometimes contain coarse-grained 
secretions and traversing light zones rich in feldspars. The plagioclases 
are still partly fresh, having fluid-inclusions, are beautifully striated, and 
partly metamorphosed into dull milky spots. There is considerable quartz: 
no orthoclase is visible. Distinct hornblende appears in very fibrous indi- 
viduals of a dark-green color, but none quite imaltered. Epidote often 
occurs macroscopically, and under the microscope a great quantity of the 
very thinnest veins of this mineral, colored an intense greenish-yellow, is 
seen traversing the rock in all directions. The same capillary fissure is 
sometimes filled in one part with epidote and in another with calcite. It is 
plainly visible how the epidote enters into the hornblende. Where micro- 
scopical cracks are found in the hornblende individuals, their walls are 
discovered to have been altered for a wide extent into epidote, and often 
the borders of the hornblende consist partly of the yellowish products of 
alteration. Magnetic and titanic iron occur together. This diorite, made 
up chiefly of plagioclase and hornblende, and possessing an entirely crys- 
talline structure, is as regards the absence of orthoclase a very typical 
one, and a member of the quartziferous division. In the plagioclase rocks, 
the presence or absence of quartz is not of so much importance as in the 
orthoclase series ; in the former, it often happens that the same deposition 
is in one place free from, in another poor, and again rich in quartz; a 
phenomenon which does not occur in the orthoclase rocks. 

A small obscure outcrop of diorite in Basalt Canon, Washoe [156], 
contains, beside the hornblende, dark reddish-gray feldspars, almost all of 
which are triclinic. On the south side of Ophir Ravine are portions of the 
diorite which have become microcrystalline, and look quite homogeneous. 
The coarser varieties of this locality [157] bear, beside plagioclase and 
a little pale hornblende, some orthoclase and quartz. The plagioclase 
includes many foreign particles, among them excellent fluid-inclusions, 

1 See vol. in, Geology of the Washoe Mining District, by Clarence King, 13, 21. 



DIOK1TE. 85 

some of which are 0.003 mm in diameter, with distinctly moving bubbles. In 
larger fluid-inclusions, whose bubbles are usually immovable, small, short, 
greenish hornblende-microlites are seen. 

Very similar to these Washoe diorites are some fresh occurrences from 
Bevel-hyma Ledge, Peavine Mountain, Nevada [158, 159]. They are rather 
poor in hornblende, with proportionally much orthoclase. The hornblende 
principally forms irregular aggregations of grains, but all the feldspars are 
at intervals actually overladen with fine, light-green hornblende dust, as is 
the case also with the German diorites. In the larger and more compact 
hornblende individuals, the development of intensely greenish-yellow 
epidote may be easily followed. The rock is very poor in quartz, or free 
from it. 

The diorites of the railroad-cut in Truckee Canon vary somewhat from 
each other. One of them [160] has a highly distinct crystalline texture, 
containing plagioclase with fluid-inclusions, almost no orthoclase, splendid 
sections of dark-green hornblende, showing that the crystals possess also the 
faces ( oo P co) ; biotite often partially encircling the hornblende or included in 
it, without being a product of alteration, some quartz, apatite, magnetic 
iron ; in short, a most typical diorite, having all the ingredients possible 
to this rock, like the classic ones from Ihnenau in the Thiiringer Wald. 
Another [161] is also crystalline; but it is poor in hornblende and biotite, 
rich in quartz and apatite, and contains, besides, the zircon-like mineral. 
Still another variety [162] is a diorite-porphyry, consisting of a ground- 
mass with imbedded crystals, which distinctly appear only in the thin 
sections; feldspars rather strongly altered, often having an aggregate 
polarization and a fibration which is visible in ordinary light. The charac- 
teristic sections of the almost wholly decomposed hornblende consist on the 
outside, of a black, impellucid border, in the interior, of a nearly aquamarine- 
colored mass, sprinkled throughout with black grains, and showing fibrous 
aggregate polarization between crossed nicols. The yellowish-gray ground- 
mass is half granular, devitrified, and indistinctly polarizing ; yet small feld- 
spar-microlites appear in this insufficiently individualized base. Diorite 
bearing the same relation to the ancient series of metamorphic rocks, is found 
in different localities throughout the whole basin. Wherever observed, its 



86 MICROSCOPICAL PETROGRAPHY. 

manner of occurrence is always the same: it invariably accompanies the 
mountain-fractures presumably of middle geological age, and is ' always 
assumed to be later than the granite and earlier than the propylite. 1 

In the Kawsoh Mountains, a diorite forms a part of the region through 
which the basalts protrude : it contains hornblende, which has been attacked 
by decomposition, and biotite, but almost no quartz. 

On Nache's Peak, Truckee Range, is found an old rock [163] similar 
to that from Quenast in Belgium: it is highly decomposed and indistinct 
in texture, yet there is no doubt that it is a plagioclase-hornblende rock, 
with some quartz. 

The hills south of Rabbit Hole Spring in the Kamma Mountains, 
Nevada, are composed of a dirty-green rock, which appears under the micro- 
scope as an excellent, fine-grained diorite [164]. Notwithstanding the 
cleavage of the green hornblende is well conserved, it contains small 
light-brownish grains, which are probably epidote. The plagioclases are 
rather fresh, and include much finely distributed, pale hornblende material, 
together with spots of a light brownish-gray dust, consisting of grains so 
minute as to be undeterminable even with the highest magnifying power. 
No quartz is visible. 

The diorites of the Pah-Ute Range are, for the most part, distinctly 
coarse-grained, and, like most of the above, crystalline throughout, without 
any amorphous substance. These rocks are made interesting by the wide 
difference in the amount of quartz and by the replacement of hornblende by 
biotite, which produces the group of mica-diorites. The beautifully crys- 
talline diorite from the Hot Spring Hills, Pah-Ute Range [165], is almost 
wholly composed of plagioclase, traversed by dull opaque veins and spots 
and splendid fresh hornblende and magnetite. It is almost totally free from 
quartz. Another coarse-grained diorite from this range [166] shows white, 
porcelain-like, decomposed feldspar, whose twin striation is entirely obliter- 
ated; pretty hornblende, which is arranged with some degree of parallelism, 
making the rock somewhat slaty; biotite, little quartz, and much apatite. 
The black and very much broken mineral seems to be titanic iron. Another 
variety [167] is a fine-grained, light greenish-gray rock, with highly altered 

1 See vol. in, Geology of the Wasboe Miuing District, by Clarcucc King, 21. 



DIOEITB. 87 

plagioclase, and rich in quartz. But, in this neighborhood, a dioritc .also 
occurs, which is midway between the coarse and fine grained varieties 
[1G8]. The largest part of the feldspar in this rock has remained pellucid, 
and presents the most brilliant variegated lineature, so that there is no 
doubt of the triclinic nature of the above-mentioned decomposed feldspars. 
The rock has no mica. It can be easily seen how yellowish-brown horn- 
blende has been metamorphosed on the borders into a parallel fibrous green 
mass, which sometimes spreads over the whole individual, in which appear 
dots and spots of brownish hornblende, showing a soft, pleasant blending of 
the colors. On the other hand, the most typical mica equivalent of the 
above-described hornblende-diorites occurs in the Pah-Ute Range. The 
feldspars in these mica-diorites [169, 170] are not in any respect different; 
but biotite in brown laminae, half an inch in length, by far outweighs the 
hornblende. Apatite is more abundant here and quartz is wanting. Similar 
mica-diorites are known in Europe, at the Muhlberg near Dreihacken in the 
Bohemian Forest, between Schonfeld and Schlaggenwald in Bohemia, at the 
Kyffhauser in Thuringia, at Clefcy near Fraize, Vosges, at Vaugneray, 
Diipartement du Rhone, at Plerneuf, and Pont-des-iles in Brittany. 

Very coarse-grained diorites are found at the west foot of Augusta 
Mountains, north of Shoshone Springs [171], containing well-striated 
plagioclase, very cleavable brown hornblende, apatite in proportionally 
large prisms, and titanite. There is, beside these, a mineral in dark-brown 
sections without any cleavage, and seeming to be nearly homogeneous and 
compact. By these features, and by its outlines differing at first sight from 
the hornblende, the individuals appear pretty strongly dichroitic, and it 
seems most probable that they belong to tourmaline, notwithstanding the 
fact that this mineral has never till now been observed as a macroscopical, 
accessory constituent of diorites. 

The Jurassic diorite which occurs as country-rock hi the New-Pass 
Mines is a remarkable product [172, 173]. It contains large milky 
feldspars 4 mm in length, and green, fibrous crystals of hornblende 6 mm 
in length. The greater part of the feldspar shows vestiges of a former twin 
*t nation, and is filled up with a multitude of the most fine and delicate 
prickles, awns, needles, and grains of light-green hornblende. The larger 



88 MICROSCOPICAL PETEOGKAPHY. 

hornblende sections, which look to the naked eye like homogeneous indi- 
viduals, are really built up of aggregated fine microlites in as interesting 
and instructive a manner as may be seen anywhere (Plate I, fig. 11). Thin 
needles and delicate prisms of greater or less length have a parallel arrange- 
ment and are in immediate contact; the effect of this accumulation and 
joining being to represent roughly or distinctly the contours of a crystal. 
The larger crystals have been welded together of thousands or tens of 
thousands of minute hornblende staffs. This structure is the more evident 
because the borders of the sections are not sharply defined like those of 
really homogeneous individuals. Here some of the larger needles or 
bunches project out beyond an even line; there the outline curves inward, 
where the needles are not long enough to reach and help to form an e\en 
external section-line to the crystal. Hence the margins of these aggrega- 
tions often look corroded or gnawed. Sometimes these accumulations of 
microlites, imitating crystals, with their surrounding mass, have been 
solidified before the single needles employed in their construction had taken 
their place, so that some appear near the external borders of the crystals as 
if they had been stopped in the act of approaching. The fine hornblende- 
microlites are also heaped together in the form of pretty stars and irregular 
fascicles. Traversing the feldspars, and also more or less the whole rock, 
are lines of small, greenish-yellow, closely-crowded grains, whose secondary 
origin is evident, and whose epidotic nature is certain. The same sub- 
stance is also found in the hornblende, forming, beside the vein-lines, 
patches and spots which can be easily distinguished from the fibrous 
hornblende by their granular composition and color. The rock also con- 
tains colorless prisms, which have a rhombic section, with an angle which 
is even more obtuse than 120. When considered with reference to other 
occurrences, there is scarcely any doubt that this, in some places very 
abundant mineral, is tremolite. Beside all these, magnetite is found in the 
rock, together with long, black, opaque staffs, which probably belong to it, 
because they often have smaller appendages, of the same nature as them- 
selves, fastened rectangularly to a sti-ong, thick shaft. 

In Dale Canon, Havallah Range, Nevada, a dike is formed by an old 
greenish-gray rock with thick quailz grains. It belongs to the diorites, but is 



D10EITB. 89 

a very peculiar variety [174]. Palo short needles and laminae of green 
fibrous hornblende, often aggregated into stars and bunches, are in contact 
with totally decomposed feldspars, whose features are not easily distinguish- 
able, and with quartz grains and apatite, in a yellowish-gray, granulated, 
amorphous, and indistinctly polarizing base, in which a tendency to con- 
fused fibrous structure is often evident. 

In the cafion south of Ravenswood Peak, Shoshone Range, another 
similar diorite is found, except that it is more crystalline and more altered 
and bears considerable biotite [175]. 

Characteristic diorites were collected at Winnemucca Peak, Nevada. 
The porphyritic variety from the southwest end of the peak [176] has a gray 
groundmass, with feldspars, more prominent hornblende, and some quartzes. 
The green hornblendes (some reaching a size of 2 mm in length) appear very 
distinctly, especially in the slides, possessing a peculiar structure. They 
are not fresh, but in a stage of alteration into epidote, whose greenish-gray 
radiated or fibrous substance forms parts of the border and wholly replaces 
the smaller hornblende individuals. The larger hornblendes (Plate III, 
fig. 3) contain, beside thick rounded nests of epidote, smaller grains of it 
united into long bands, or stripes, which are curved like a paragraph-mark. 
The fibration of the hornblende, which is evidently secondary, and probably 
causes the development of the epidote, often runs in twisted lines resembling 
the curves of the letter S. The hornblendes also include newly formed 
geode-like particles of calcite somewhat resembling eyes. Moreover, com- 
pact brown spots belonging to the original crystal substance of the horn- 
blende are visible in its interior. Occasionally in one section all these 
stages of alteration may be seen surrounding each other in right order : 
a. brown hornblende ; b. green fibrous hornblende (viridite) ; c. epidote 
with calcite. The rock from the eastern end of Winnemucca Peak [177] 
is very similar to that from the southwestern, except that dull feldspar 
is more prominent in the groundmass. The vividly greenish-yellow 
aggregation of concentric-radiating epidote having the outline of horn- 
blende is very beautiful (Plate III, fig. 4). One frequent feature of 
these pseudomorphs shows several centra of little irregular balls, to which 
the epidote needles have been united. Titanic iron is found here, which has 



90 MICROSCOPICAL PETROGRAPHY. 

undergone the same curious alteration as that usually met with in diabases, 
except that the secondary mass is here a more dirty yellowish-gray. Black 
strokes run through it, cutting each other under 60 and 120, which prob- 
ably belong to the more resisting lamellae. In the German diabases, this 
metamorphic product covers the individuals of titanic iron as a whitish 
opaque crust. Its mineralogical and chemical nature is wholly unknown. 1 
The diorite from the southern slope of Winnemucca Peak is more granular 
than porphyritic, having fresher plagioclases than both former rocks [178] ; 
very rich in quartz, which contains innumerable fluid-inclusions that are 
mostly dihexahedral, and a part of them envelop little salt cubes like those 
in the quartzes of the famous Belgian diorite from Quenast, the pavement- 
stone of Paris. The abundant hornblende shows all the phenomena of 
alteration just mentioned, more or less advanced stages of which are indeed 
found pretty generally spread through all diorites, but seldom with all the 
stages as distinct as they are here. 

Other diorites are found on the divide between Grass and Cortez Valleys, 
Nevada. One [179] is a fine-grained quartziferous rock, somewhat decom- 
posed and without biotite. 

In Trout Creek Canon, Shoshone Range, a beautiful porphyritic rock 
occurs [180-81], which has a greenish-gray groundmass, containing quartzes 
nearly the size of a pea, with numerous fluid-inclusions, attacked horn- 
blende, striated feldspars, and some orthoclases, among others a crystal 
an inch long. Under the microscope, the groundmass shows itself as 
entirely crystalline, consisting of decomposed feldspar, little quartz, much 
titanic iron metamorphosed in the same manner as in a recently described 
variety, small apatites, and some leek-green mica, in which are very thin 
brownish bunches of extremely delicate needles crossing each other under 
60. 

A fine-grained diorite occurs at Ravenswood Peak, Shoshone Range 
[182]. Wherever the feldspar is fresher, it proves to be evidently striated, 
which throws light upon the nature of the more decomposed feldspars 
found in previously mentioned varieties. 

1 See Seuftcr, Neues Jabrb. f. Miuenilogio, 1872, 673 ; F. Z., Mikroskop. Beschaf- 
feuh. d. Mineral, u. Gesteine, 40!); Datlie, Zeitsclir. d. d. geol. Gesdlscli., xxvi, L'>. 



D1OEITE. 91 

The diorite fiom Mill Creek Canon, Cortez Range, is very peculiar 
[183]. Large plagioclases and short black prisms of hornblende appear 
macroscopically in a seemingly very fine-grained groundmass. The thin 
sections prove that the larger part of the feldspars are triclinic ; they are 
accompanied by a little orthoclase and considerable microscopic brown 
biotite. The fine crystalline groundmass, curiously, is found under the 
microscope to be enormously rich in quartz, whose colorless grains give 
out brilliant variegated colors between crossed nicols ; moreover, the ground- 
mass is composed almost wholly of quartz and hornblende. There is, 
indeed, almost too much quartz in it for a diorite, for this quantitative pro- 
portion of constituents is extremely rare. The rock is a plagioclase-bearing 
quartz-hornblende variety. It is remarkable that in some places the plagio- 
clases have precisely the structure of the well-known labradorite from Paul's 
Island on the Labrador coast. 1 They contain the same black needles, grains, 
and brown laminae, in exactly the same arrangement. The presence of 
such an abundance of quartz becomes the more curious because this 
behavior of plagioclase has never before been observed save in the very basic 
gabbros and hypersthenites, which are absolutely free from quartz. 

A typical diorite without any trace of an amorphous mass, rich in 
quartz and in brown biotite, beside the hornblende, is found near the mouth 
of Agate Pass, Cortez Range [184]. The feldspars are not yet so far decom- 
posed as to make their striation very indistinct. 

In BinghamCafion, Oquirrh Mountains, is a diorite which contains quartz, 
biotite, and apatite [185]. The laminae of mica, as is shown by the micro- 
scope, are broken and shivered, and the larger ones may be seen glancing 
on the fracture-planes of the rock. 

A remarkable member of the metamorphic Archaean series is a rock 
which has the composition of diorite, found at the mouth of Ogden Cafion, 
Wahsatch Range. Pale-red feldspar, hornblende, and quartz, in a wholly 
crystalline mixture, may be seen with the naked eye. At first sight, it would 
seem that the feldspar is orthoclase, and that the rock belongs to the syenites, 

'Vogelsang, Sur lo labradorite colord, Archives NcSerlandaises, 18C8, tome III ; 
see also Scheerer, PoggeudorfFs Aimaleii, 18-15, LXIV, 102, aud Schrauff, Sitzungsber. 
d. Wiener Akad., LX, 1. Abtli., Dec. 1809, 1. 



92 MICROSCOPICAL PETROGRAPHY. 

but under the microscope it becomes plain that with but rare exceptions all 
the feldspars, which are unusually fresh, bear a splendid twin-striation. On 
the borders of these plagioclases are variously crenated laminae of blood- 
red specular iron, appearing as dendrites on capillary fissures, and causing 
the pink color of the feldspar. The substance of the plagioclases is not much 
less impellucid than that of the quartzes, a dust-like material being inter- 
posed in them, usually accumulated into stripes. A high magnifying power 
(immersion-objective No. 10 of Hartnack) shows that this matter is composed 
of very fine pale-green grains, which are probably hornblende ; very small, 
hollow, rounded, or funnel-like pores ; and, an extremely rare phenomenon 
in the plagioclases of diorites, fluid-inclusions with distinctly moving bub- 
bles. The fresh hornblende has a very detailed cleavage, which makes the 
single transverse sections seem to be composed of innumerable oblique- 
angled rhombs. Quartz is rather abundant ; and, beside this, there appear 
under the microscope a comparatively very great quantity of apatite prisms 
included in all three of the chief constituents, and single titanites, wliich are 
elsewhere by no means frequent in diorites. In another variety from this 
locality [186], short and thick brown prisms of the zircon-like mineral are 
found. These diorites pass geologically into hornblende-schists. 

A dike on the divide between American Fork and Little Cottonwood 
Cafion, Wahsatch Range, is made up of a greenish-gray and seemingly 
nearly homogeneous groundmass, with crystals of hornblende and plagio- 
clase. The smaller and fresher individuals of hornblende are plentifully 
mixed with opaque, black magnetite ; nevertheless, alteration has already 
progressed in them, and the larger ones especially have become totally 
fibrous. Indeed, there are no parallel fibres developed here, but in their 
stead, bunches having a cross and transverse direction, like the crystals of 
frost on a window. Plagioclase is rendered very impure by a fine dust of 
hornblende. The groundmass appears to contain some glassy base between 
the net-work of microlites. 

One of the most beautiful diorites in this region is that from the west 
side of the Medicine Bow Range, between French and Brush Creeks [187], 
consisting, in the section, of interwreathed, prevailing dark-green, and color- 
less patches, whose alternations give the effect of mosaic work. The dark- 



DIOEITE. 93 

green spots are hornblende, or rather an irregular but intimate aggregation 
of grains and prisms of strongly dichroitic hornblende. The patches have 
no proper limits, but are irregularly rounded or angular. The borders are 
armed with beautiful delicate and bristly hornblende crystals, stretching into 
the colorless places which fill up in a certain sense the holes between the 
green hornblende. It is also noticeable that the borders of the hornblende 
are a darker green than the interior parts. Under the microscope, it is seen 
to be impregnated with finely-distributed hornblende dust, and to consist 
very largely of splendidly striated plagioclase, accompanied by some 
few orthoclases in the form of Carlsbad twins. There is no vestige of an 
amorphous or microcrystalline groundmass, no quartz, and no apatite ; in 
short, a typical diorite made up of the characteristic ingredients, but 
arranged in a singular manner. 

At the close of these pages on diorites, a hornblende rock may be 
mentioned, which forms a dike in the granite of the low hills northeast of 
Havallah Range [188]. It consists of quartz and hornblende in needles 
and prisms (Plate IV, fig. 1). The quartz constitutes a kind of colorless 
groundmass in which the hornblende individuals are distributed. Here and 
there occur larger hornblende members, irregularly shaped but showing an 
evident tendency to the characteristic hornblende features : their borders 
distinctly prove that they are built up of single needles and rays, yet they 
have in the interior the uninterrupted, oblique-angled cleavage of horn- 
blende. Larger quartz members also exist, in which the rarer hornblende 
needles are aggregated into the most delicate, looser, or denser bunches. 
Some of these places microscopically resemble the prasem from Breitenbrunn 
in Saxony, the more because the quartz contains fluid-inclusions : there is 
also brown mica. 



94 MICROSCOPICAL PETROGRAPHY. 

SECTION II. 

HORNBLENDE-PORPHYRY. 

Some rocks of the Augusta Mountains [189, 190] bear a striking 
macroscopical and microscopical likeness to the well-known horn- 
blende-porphyry from Potschappel in Saxony, belonging to the porphyritic 
tract which runs southwest from Dresden to Potschappel. They are 
dark-gray rocks, with a tinge of green, showing to the naked eye as 
porphyritical constituents, only small black hornblende prisms. The 
porphyry from Potschappel, which has been until the present referred 
to the rocks characterized by the preponderance of plagioclase, curiously 
contains, when examined microscopically, decidedly more orthoclase in 
single individuals and Carlsbad twins than striated feldspar ; and it is 
a striking analogy that the rock from Augusta Mountains, so similar 
also in its external aspect, likewise bears at least as much orthoclase 
as plagioclase, although the smaller ciystals seem chiefly to be triclinic. 
Under the microscope, the groundmass is seen to consist of a yellowish- 
gray, amorphous, somewhat indistinctly globulitic, devitrified substance, 
including small feldspar prisms, which are sometimes aggregated into 
larger forms, and, beside these, numerous black, point-like grains, which 
are often gathered into heaps or spread out into lines or chains. Horn- 
blende does not seem to fill much of. a place in the composition of the 
groundmass. It forms chiefly large crystals, the number of which is 
not much augmented by observations through the microscope. The 
larger hornblende sections having a darker or lighter brownish-yellow 
color, in both the American and the Saxon porphyries, are in a surpris- 
ingly similar, peculiar condition (Plate IV, fig. 2). Provided with an 
excellent cleavage, they exhibit, according to the direction of the sections, 
either a longitudinal fibration or two systems of cracks cutting under an 
obtuse angle; but none of them have a regular crystallographical shape, and 
often even lack straight outlines, having instead rounded ones : moreover, 
they are mostly fragments. All are encircled by a border of black grains, 
which, really belonging to hornblende, limits them externally. This 
is the same granular zone that plays so important a role around the horn- 



HORNBLENDE-PORPHYRY. 95 

blende of andesites and trachytes. The breadth of this black border varies. 

/ 

Sometimes it is so broad that it preponderates, and only a small spot of the 
hornblende individual appears in the interior ; again it is narrower, and the 
individual is larger and better shaped. Hornblende presenting this behavior 
has long been known in other, peculiarly basaltic, and andesitic rocks. With 
reference to the presence and origin of this remarkable black border, it was 
formerly explained as a peripheric aggregation of attracted magnetite 
grains, which had been more or less forced by the crystallizing power of 
the hornblende to follow its own form, in exactly the same manner as in 
the so-called sandstone from Fontainebleau, grains of sand are mechanically 
forced into the rhombohedral form of calcite, some carbonate of lime crys- 
tallizing between them. A careful study of the phenomenon, however, 
leads to another and more satisfying supposition. There is no proof that 
the black grains composing the border are really magnetite, and they may 
therefore be called by the non-committal name of opacite (page 13). The 
porphyries seem to support the view that the hornblende crystals, upon 
first becoming solidified bodies, were superficially altered by the still mol- 
ten surrounding rock-mass, and that the border of opacite grains is the relic 
of this conflict With the chemical reaction and change, the mechanical 
tendency present went hand in hand, and from this results the decidedly 
fragmentary nature of so many of the individuals, as does also the phe- 
nomenon where at the ends of sections the black border is dilacerated or 
torn, and the groundmass has penetrated into the hornblende substance. 
Both actions confirm and explain each other. The deeper the process of 
caustic alteration advanced, the more the form of the attacked crystal 
became obliterated. This supposition also makes it easy to conceive how 
it happens that the black border of opacite often grows gradually looser on 
the surface, and becomes dismembered into single isolated grains ; and it 
may not be impossible that many of the dark grains which are scattered 
through such rocks, and are generally taken for magnetite, are really finely- 
distributed, powder-like particles of the pyrogenous alteration-product of 
hornblende. 

In one of these rocks from the canon south of Granite Point, Au- 
gusta Mountains [190], the black-bordered hornblende has undergone a 



96 MICROSCOPICAL PETROGRAPHY. 

later, secondary process of decomposition in the wet way, the result being a 
vivid green substance which appears macroscopically in many hornblende 
individuals of the rock. This viridite substance, which is probably green- 
earth, penetrates the fissures of the crystals in the most distinct manner ; 
and so it comes that the brown hornblende, with its dark outline, shows first, 
naiTOw, green alteration-lines along all the parallel or obliquely crossing 
cracks. Gradually they become broader, and form a green net- work, includ- 
ing small, brown kernels of the original substance. The black border does 
not seem to be much attacked by this kind of decomposition. When it 
does happen, however, calcite must be formed, for it may be found 
macroscopically deposited in the form of small veins in the larger fissures of 
the rock. The larger feldspars, often built up zonally in great distinctness, 
are somewhat fresh, and bear beautiful glass-inclusions, which are sometimes 
found arranged in regular concentric bands. The small rectangular ledges 
of orthoclase even contain a large, rectangular, compact kernel of light- 
brownish glass. There is some dirty apatite, but no augite. This would 
be expected on account of the close analogy between this rock and hom- 
blende-andesite, of which it represents a real ante-Tertiary precursor. 



DIABASE. 97 

SECTION III. 

DIABASE. 

Some macroscopically indistinctly characterized rocks occur in Miner's 
Canon, Truckee Eange. It is uncertain whether they belong to diorites, 
diabases, or syenites, neither the nature of the feldspars being recognizable, 
nor is discrimination between hornblende and augite possible. A micro- 
scopical examination, however, discovers them to be diabases [191, 192, 
193, 194, 195]. These rocks are made up of plagioclase, augite, with its 
products of alteration, little quartz, magnetite, and sometimes apatite. The 
feldspars are still pretty fresh and well striated, in some places richly 
and brilliantly so; and they contain numerous strange interpositions, which 
seem to vary in the different specimens. In some cases, these interpositions 
consist of fine grains and crippled microlites of augite, accompanied by 
amorphous particles of the groundmass; in others, of more or less dis- 
tinct fluid-inclusions, associated with empty cavities. In general, these 
plagioclases are much less decomposed than is common in the German 
diabases. The augite often forms sections of a yellowish-brown color, 
with the characteristic generally eight-sided (oo P. 00^*00. oo'sPoo) features, 
of feeble or wanting dichroism, 1 and the typical directions of easy 
cleavage, crossing each other nearly rectangularly. Beautiful variegated 
parallel lines sometimes appear in polarized light, which proves the 
existence of the well-known twin lamellation parallel to ( oc J2 oo ). This 
augite substance is altered along the borders and fissures into a fine, dark- 
green, parallel and fibrous mass, which might easily be taken for the 
uralite-like hornblende, so often occurring as a secondary product of augite, 
until it is examined with the under nicol ; but this process demonstrates that 
it is not at all dichroitic. It must be, therefore, another of the numerous 
but indistinctly characterized epigenetic substances of augite which have 

1 Tscherinak was, as is known, the first to point out that hornblende and augite 
can be easily distinguished by their optical behavior if the polarizer be put into the 
microscope : the sections of the very strongly dichroitic hornblende very plainly change 
their color upon turning around that nicol ; the sections of augite then keep their color 
entirely, or almost entirely, being almost totally undichroitic. Sitzungsber. d. Wien 
Akademie, LIX (I), May, 1869. 
7 M P 



98 MICROSCOPICAL PETROGRAPHY. 

been recently collected under the general name of viridite (page 14). 
Perhaps it belongs to chlorite. Some of the crystals have been entirely 
metamorphosed, and yet have preserved very exactly their original outlines 
as augites. But the same green fibrous mass also forms many irregularly- 
shaped individuals found in the rock which have doubtless been augites, 
but whose features have been partly or totally obliterated by the alteration. 
Considered by themselves, these green spots would hardly be taken for 
decomposed augites, but the presence of all the members of transition 
between them and fresh augite crystals, puts the question of origin beyond 
doubt Through this product of alteration, numerous fine black opaque" 
grains of magnetite are scattered, which must have been taken up during 
the time of alteration, for they are not found in the unattacked augite. 
This is a process the chemical possibility of which was stated a long time 
since by Gustav Bischof, 1 and subsequently corroborated by Dathe in 
his excellent memoir on the microscopical constitution and structure of 
(German) diabases 2 (see page 67). In certain specimens from Miner's 
Canon [195], the product of the augitic alteration is a light green in 
color, not fibrous, and evidently isotrope, showing no colors at all 
between crossed nicols. Although- many black magnetite grains have 
developed, it must belong to some other substance than the above- 
mentioned dark-green fibres, which present a vivid chromatic polarization. 
Fischer has shown that the seladonite of Fassa Valley, Tyrol, is unaffected 
by polarized light, 3 and it is not impossible that the light-green substance 
in question may stand in close relation to it. These diabases contain quartz 
in single colorless grains. This ingredient was never formerly supposed to 
exist hi these basic rocks with augites and plagioclases poor in silica. The 
first diabases in which quartzes were observed as original constituents are 
those trap-rocks which form layers and dikes in the Lower Carboniferous 
sandstones in the island of Arran, Scotland. 4 And Dathe has proved that 
quartz is also frequent in the German diabases, especially in those of the 
Voigtland and of the Lausitz in Saxony, which had never before been 

1 Lehrbucb d. chemisch. u. physikal. Geologic, 2, edit. II, 913. 

2 Zeitschrift d. d. geolog. Gesellsch., xxvi, 1874, 30. 
'Kritiscbe mikroskopisch mineralogische Studien, 1869, 24. 
4 F. Z., Zeitscbrift d. d. geol. Gesellschaft, xxm, 1871, 28. 



DIABASE. 99 

examined microscopically. Quartz also occurs in a diabase from the neigh- 
borhood of Torquay, England, and in those splendid fresh diabase rocks 
(traps) which form intercalated contemporaneous layers and eruptive dikes 
in the Triassic sandstones near New Haven, Conn. 1 Titanic iron, which so 
frequently appears in the German diabases from Saxony and the Fichtelge- 
birge, Nassau, as the accompaniment of magnetite or replacing it, cannot 
be detected in these rocks from Miner's Canon. An amorphous yellowish- 
gray base fills up the spaces between the crystalline ingredients. It is 
devitrified in a globulitic manner (page 2), and is penetrated by thin col- 
orless feldspar-microlites. 

At Diabase Hills, Truckee Range, is another region of old diabases 
[196, 197, 198], where they have been overflowed by the younger Tertiary 
basalts. The more beautiful ones present under the microscope fresh 
striated plagioclase, but no orthoclase, brownish-green augite, often in a 
comparatively rather small quantity; olivine, which is mostly altered in the 
interior into a brownish-yellow, and on the borders into a darker yellowish- 
brown serpentineous matter; and a little black ore, which, according to its form, 
belongs rather to magnetite than to titanic iron. Beside these constituents, 
there are numerous, long, colorless, prismatic microlites, a part of which belong 
to apatite, while another part are probably of a feldspathic nature. Search 
for quartz here is in vain. The structure is entirely crystalline, without any 
trace of an amorphous, unindividualized base. This feature causes a con- 
siderable difference between the basalts and the diabases of the same region, 
although in the nature of their crystalline constituents there is the closest 
analogy between them, the basalt also containing olivine. The latter are 
here rich in a well-developed, half-glassy base. The decomposed olivines 
can be seen in the sections as small brown spots. These rocks strongly 
resemble some of the above-mentioned Scotch Sub-carboniferous traps, in 
the abundance of olivine they contain and the absence of titanic iron; but 
they are nearly as unlike the German diabases, as they are similar to the 
Scotch; for the German generally contain titanic iron, although they have 
no olivine. The general petrographical rule that olivine-bearing diabases are 

*E. S. Dana has made a very valuable microscopical study of these rocks, in 
which, however, he does not mention quartz. Ainer. Jour, of Sci. and Arts (3), VII, 390. 



100 MICROSCOPICAL PETROGRAPHY. 

free from quartz, and that quartziferous ones are free from olivine, has been 
proved as well by the Scotch as by the German varieties, and it is further 
corroborated by the study of the American occurrences. Some diabases 
present a singular macrocrystalline structure, which reappears in the younger 
basalts, but not in those of this country. Between the usually not very 
widely diverging rays and ledges of colorless, well-striated plagioclase 
are crowded dark-yellow, angular and rounded augite grains, up to the size 
of 0.01 mm , and black particles of magnetite. In this so-constituted aggrega- 
tion, which figures as a microscopical groundmass, larger brownish-red, 
altered olivines are distributed in a porphyritical manner. The olivines very 
often form the most characteristic rhombic sections, being very sharp-feat- 
ured, like those in the basalts. Larger augites do not occur, but the feld- 
spars sometimes attain dimensions equal to those of the olivine. This struc- 
ture becomes especially characteristic when the ledges of feldspar are crowded 
close together and the narrow spaces between them are filled up with grains 
of augite and magnetite, ranged behind one another lineally. Quartz is 
wanting in this rock also, but it contains some sanidin-like orthoclase. A 
few sharp rectangular sections are seen, which would at first sight be taken 
for nepheline, but in polarized light they become neither monochromatic 
nor polysynthetically striated, but are divided by a longitudinal middle 
suture into differently-colored halves, and belong to orthoclase twins, after 
the manner of the Carlsbad law. 

To this series of diabases also belongs one from the high peak at the 
south end of the Truckee Range [199]. It is rich in olivine, but richer 
in augite. There is both orthoclase and plagioclase, the latter of which 
shows a most excellent schistiform composition of very numerous water-clear 
and conformable layers surrounding each other. In a strict sense, it is 
rather difficult to understand how this structure, distributed regularly 
through the whole crystal, can exist together with so pronounced a 
polysynthetical lamellation. The rock also contains a little globulitic, halt- 
glassy base. Secondary oxyd of iron has infiltrated into small fissures, 
and formed delicate, yellowish-red, dendritic lobes. 

In the quartzites of Humboldt Caflon, West Humboldt Mountains, a 
dike of a dark, very fresh, and distinctly-grained rock occurs [200], which 



DIABASE. 101 

so closely resembles macroscopically, as well in the hand-specimens as in 
the prepared slides, a Tertiary dolerite, that one is inclined to consider it as 
a dependence or a variety of the neighboring basalts, although its geological 
relations do not favor this conclusion, but instead betoken a greater age of 
eruption. It is remarkable that the rock numbers among its constituents, 
beside quite_clear and splendidly linear plagioclase and entirely unaltered 
augite, beautiful characteristic grains of quartz, with moving fluid-inclusions, 
an ingredient which has never been observed in any dolerite or basalt in the 
world. So the microscopical quartz pronounces the true nature of the dike- 
rock to be akin to that of the older diabases, which agrees with its geolog- 
ical circumstances but differs from the suggestions of its macroscopical 
aspect. Between the individualized minerals, a little of an amorphous mass 
of felsite is distributed, but it does not at all partake of a globulitic, glassy 
nature, as is the case in the neighboring basalts. It is locally transformed 
into a dirty brownish-yellow substance. The black ore may be partly 
titanic iron. The large plagioclases contain glass-inclusions, with dark, 
fixed bubbles, which is rarely a distinguishable phenomenon in either dia- 
bases or dolerites. 

Another old diabase dike breaks through at Granite Peak, Pah-Ute 
Range, Nevada [201]. This is a distinctly grained rock, consisting of 
plagioclase in a somewhat more advanced state of alteration than in the 
diabases of Diabase Hills, Truckee Range ; considerable augite, some 
apatite sections, and a black mineral, which in this case is magnetite. Both 
olivine and quartz are wanting, and no amorphous base is visible. The 
augites contain glass-inclusions, and in some places a multitude of long and 
narrow, fine, cylindrical, empty cavities, placed in parallel arrangement near 
each other. 

The rhyolite from Owyhee Bluffs, Rock Creek, includes foreign frag- 
ments [202], which belong to an excellent fresh diabase, closely resembling 
that from the Truckee Range. It bears plagioclase, much pale-brown augite 
nicely crystallized, half-metamorphosed olivine, and small quantities of a 
globulitic base. The black mineral here seems to be titanic iron, which, 
however, is not connected with that curious and unexplained product ot 
alteration which so often appears in the German diabases. 



102 M1CEOSCOPJCAL PETKOGKAPHY. 

The limestone at Seetoya Peak, Nevada, is traversed by a dike of rock 
which probably must also be placed among the diabases. It is entirely 
decomposed, and of a dirty grayish-green color, and it contains calcite, mica, 
and a green product of alteration, which, judging from its features, should 
be referred to augite rather than to hornblende. 

In general, these diabases of the Fortieth Parallel are characterized by 
the relatively fresh condition of their augites, by the feeble development of 
a chloritic or viriditic secondary substance, and by the frequent occurrence 
of olivine ; and they therefore much more closely resemble the Scotch than 
the German diabases. 



MELAPYIIBE. 103 

SECTION IV. 

MELAPHYKE. 

Some other rocks from the Fortieth Parallel, which the German geol- 
ogists would place among the melaphyres, could, by reason of their close 
affinity to the diabases, be classified as such. Most of these rocks are char- 
acterized by the absence of easily distinguishable macroscopical ingredients, 
by their seemingly homogeneous mass of a dirty greenish-gray or brownish 
color, and by the presence of amygdaloidal calcite or green-earth (delessite). 
Self-existing amygdules of quartz or of other silicious matter seem not to be 
frequent. Such secretions, which here often reach the dimensions of a hazel- 
nut, are, as is well known, derived primarily from the decomposition of the 
augitic constituent. These macro-petrographical points give the rock a cer- 
tain degree of difference from common diabases, and stamp it with some 
peculiarity, which is the only apology for using the unfortunate name, 
melaphyre to classify them. Careful microscopical examination of a large 
number of the so-called rocks from Germany and Transylvania 1 shows 
that the single occurrences differ widely from each other, and that they are not 
much alike in general composition. This is not very astonishing when it is 
remembered that this class of rocks has been established merely upon their 
exterior aspect, without any certain knowledge of their real constitution, 
and without their exhibiting a normal occurrence which could be made a 
basis of comparison. So it has happened that for half a century all rocks 
whose constituents it has been found impracticable to determine macroscopi- 
cally, have been named melaphyre. 

A series of specimens from Berkshire Canon, Virginia Range, belongs to 
these melaphyres [203, 204, 205, 206, 207, 208]. For the most part, the feld- 
spars of these rocks are in a somewhat decomposed state, but here and there 
the tri clinic lamellation is doubtless still visible. In one variety [205], the 
larger plagioclases, having a somewhat parallel arrangement in the hand-speci- 
mens, appear under the microscope to be partly altered into calcite: the mass 

'G. I I.iann;mii, Zeitsclir. d. d. geol. Gesellscu., XXV, 1873; F. Z., Basultgt'Stviue, 
ly, 108 ; G. Dolter, Jahrbucb d. geolog. lieicbsuustalt, xxiv, 1874, 1. 



104 MICROSCOPICAL PETROGRAPHY. 

of the crystals is traversed by broad veins, all of which present in polarized 
light the characteristic picture, with delicately changing and playing colors, 
which is characteristic of calcite in very fine-grained aggregations. Ortho- 
clastic feldspars cannot with certainty be said to be present. These Western 
Nevada varieties bear a striking likeness to the melaphyres from Germany 
and Transylvania, in that they contain under the microscope poorly distinct 
augite. Augites are sometimes totally wanting here, and generally rather 
rare, but the rocks are rich in a greenish ingredient which seems to 
be a product of the decomposition of that mineral. The presence of 
olivine in these melaphyres is an interesting fact, for it has been by 
degrees discovered in a large number of macroscopically more or less 
similar European rocks, in which it was formerly supposed never to exist. 
Tschermak 1 found the first olivines in melaphyres from Breitenbrunn, 
between Kuchel and Smolenitz in the small Carpathians, from Falgendorf 
in the Lower Bohemian Dyas formation. Haarmann observed microscopical 
olivines in the melaphyres from Oberstein and Weiler at the Nahe (Rhine), 
from Ilmenau, Thuringia, from Wiirschnitz near Stollberg, from Wildenfels, 
Kainsdorf near Zwickau, Saxony, from the Mummel near Landshut, Sile- 
sia; and Doelter recognized this mineral in some melaphyres from Western 
Transylvania. The olivines are generally somewhat decomposed into ser- 
pentineous products, at least on the borders and along fissures. The pre- 
vailing color of this product of alteration is a deep reddish brown, and it 
is very slightly pellucid. The nearly colorless original substance of the 
olivines remaining unattacked, appears as kernels in the dark net-work of 
metamorphosed crack-walls. Some of the olivines have a dirty-green color, 
and it would seem that this hue is the symbol of an earlier stage of altera- 
tion. In the basaltic olivines, it is often observable that the reddish-brown 
colors follow upon the green. Thus brown veins here traverse the greenish, 
decomposed olivines, marking the ways by which the oxydation and hydra- 
tion of the protoxyd of iron have penetrated. In some of these old mela- 
phyres, the crystals of olivine can even be detected macroscopically, either 
in the hand-specimens or *in the slides [205]. Another, but uncommon, 
ingredient is apatite, which here shows in the middle of its colorless sub- 
1 Sitzuugsber. d. Wiener Akad., LII, 18G5, 1. Abth., 205. 



MELAPHYRE. 105 

stance the thin, black, nail-like, longitudinal prism so often occurring in 
those of the basalts, but which is entirely wanting in the thousands of 
apatites of crystalline slates that have been examined. Here and there 
between the diverging ledges of plagioclase some of a yellowish-gray, amor- 
phous, half-devitrified base is squeezed in, in the form of little spots. The 
amygdaloids in these melaphyres, some of a size of 5 mm (Plate IV, fig. 
3), but the general mass of them being smaller, consist in the transverse 
section of a prevailing grass-green, and of a colorless substance. In view 
of its compact mass, its pellucidity, and its vivid chromatic polarization, the 
latter of these is surely quartz. The green material is probably green- 
earth; in one place entirely structureless and unaffected by polarized 
light, and in another a fine fibrous mass producing a feeble aggregate 
polarization. The outermost zone of the amygdaloids usually consists of a 
narrow schist of pure quartz: in the interior, the green matter predominates, 
forming radially fibrous globules, heaped together into botryoidal and 
lenticular aggregations. The section running through these little balls 
presents concentric rings varying from a lighter to a darker color, so that 
one of the thin fibres possesses several tones of green, at different distances 
from the centre. The green material also forms peculiar, feebly fibrated, 
horseshoe-like semi-circles and three-quarter rings, like those which have 
been observed in the analogous amygdaloids of English toadstones occur- 
ring as contemporaneous layers in the Carboniferous limestones of Derby- 
shire. The spaces and gaps between the single spots and aggregations of 
the green-earth are filled with a little pellucid quartz. 

There is a melaphyre in Berkshire Canon [208], seemingly an almost 
homogeneous, dirty, yellowish-gray mass, which belongs geologically to the 
above-described rocks, but differs from them in its structure and composition. 
It appears in ordinary light under the microscope to be a colorless mass con- 
taining innumerable, angular, dark-yellow grains, some of which are 0.02 mm 
in length, most probably belonging to augite. In polarized light, the col- 
orless substance is not homogeneous, but an aggregation of doubly refracting 
particles: these are possibly feldspar, bvit they are without distinguishable 
twin-striation. The yellowish grains for the most part lie irregularly and 
without order, though very equally distributed through the mass; but they 



106 MICROSCOPICAL PETEOGEAPHY. 

are found in numerous places densely accumulated, and grouped into exact 
rings, which of course correspond to sections of balls. The interiors of these 
circles are filled with the structureless chief mass of the rock, which seems 
here, however, to be poorer in the yellow grains, or to contain smaller ones ; 
as if a part of those in the middle had been used for the construction of the 
outer rings. Sometimes smaller concentric rings are placed within the main 
ones: these are formed of very densely accumulated yellow grains, and in 
some places they are disturbed and pressed out of shape, or else two or more 
of them run into each other at the peripheries. All these phenomena surely 
prove a peculiar mode of solidification, but it is remarkable that no centre 
can be found from which the spherical arrangement of the grains could 
have been governed. The circles appear in the slides with a lens as little 
rings, from the size of l mm downward; but in the hand-specimens hardly any- 
thing can be seen of them. 

There is another entirely decomposed melaphyre, with secretions of 
calcite and green-earth [209]. The microscope shows that the calcite has 
also settled in the interior of the rock in small portions. Larger crystals 
which have been wholly altered into a pale-green, secondary matter, are 
surrounded on the outside by a very distinct series of blackish-brown grains, 
and these features seem to point toward augite. Pulverulent, brownish- 
black grains, probably a product of decomposition, are scattered through 
the whole rock, and are often found aggregated into loose heaps. 



GABBltt). 107 

SECTION V. 

GABBRO. 

A remarkable gabbro [210] forms a hilly dome in the gray metamor- 
phic granite east of Iron Mountain, Laramie Hills. It is composed almost 
entirely of bluish-gray plagioclases, which have a somewhat feeble play 
of colors, and whose broad faces M are, for the most part, nearly parallel in 
direction, so that the excellent polysynthetic twin-striation only appears in 
the transverse fracture. Thin sections show that another grayish or yellow- 
ish-green mineral takes part in the composition of the rock : this is far less 
distinct in the hand-specimens. The plagioclase shows in polarized light, 
where it lies obliquely, a splendid variegated lineature ; and, under the 
microscope, it is seen to have exactly the structure of the other gabbro- 
plagioclase, which is altogether similar to that of the genuine labradorite 
from Paul's Island, on the coast of Labrador. It contains a multitude of char- 
acteristic interpositions of little, sharp, dark, linear needles, which cause the 
grayish color of the crystals. These microlites are in part entirely black 
and opaque and partly brownish and transparent : their maximum length 
is 0.06 mm , but they decrease in size to the most minute proportions. In one 
section of a plagioclase crystal, they have for by far the most part a strictly 
parallel arrangement ; but there are some which tra verse the same system 
without any visible regularity. Small grains of the same nature accompany 
these needles ; but the well-known, little, flat lamina} included in the labra- 
dorite are here, as is often the case in gabbro-plagioclases, comparatively 
rare. Some sections seem, with a low magnifying power, to be filled with 
a dark dust, which, with a higher power, turns out to be needles and line 
grains. All these impregnations are, as is usually the case in such feldspars, 
abundantly gathered together into straight, dark, parallel lines or bands 
which correspond to the lamellation of the triclinic feldspars, or else thicker 
grains and stronger needles are interposed along such lines. No liquid- 
inclusions were found. In short, these plagioclases have the same character- 



108 MICROSCOPICAL PETROGRAPHY. 

istic microscopical structure that has been observed in the European gab- 
bros from Volpersdorf, Buchau, Ebersdorf, Schlegeler Mountains, in Silesia; 
from Harzburg, in the Harz; from La Prese, in the Veltlin, Northern Italy; 
from Valeberg, near Krageroe, Norway, and from the islands of Mull 
and Skye, off the western coast of Scotland. 1 It is very remarkable that 
wherever the plagioclase possesses this peculiar structure, it is accompanied 
by diallage or hypersthene; the triclinic feldspars associated with common 
augite or with hornblende never being filled with such interpositions as far 
as known. The American gabbro strikingly corroborates the conclusions 
reached by study of the European occurrences. Sometimes these pla- 
gioclases also show the lamellar structure, resembling a grate or lattice- work, 
which is mentioned on page 34. The rather obscure grayish or yellowish- 
green ingredient of these rocks is diallage, with one prevailing cleavage, 
not the double prismatic one of augite, and having here and there small 
lamellar interpositions that are not dichroitic, but are sometimes altered on 
the borders into fibrous, green hornblende ; a phenomenon comparable with 
the uralite originating from augite, well known in all European gabbros. 
No olivine can be detected in the American gabbro, although it has been 
recently discovered in many European specimens where it was formerly never 
suspected. Titanic iron appears in long, irregularly shaped members, 
wrapped in the whitish-gray crust produced by decomposition, which is so 
often seen covering it in diabases. The general structure of the rock is 
purely granular, without any trace of an amorphous, unindividualized mass. 
This also is a peculiarity common to all diallage-bearing gabbros without 
exception, and it is in contrast with the greater part of those rocks which 
are characterized by augite. This rock, with its strongly predominating 
amount of plagioclase (labradorite) seems not very far removed from the 
Norwegian norites, as described by Scheerer and Kjerulf. 2 A chemical 

'R. Hagge, Mikroskopische Untersuchungen iiber Gabbro u. verwandte Gesteine, 
Kiel, 1871 ; F. Z., Zeitschr. d. d. geol. Gesellscb., 1871, xxm, 59, 94 ; Karl Urba, Gabbro 
from tbe entrance of Lichtenan Fjord, Greenland, showing all the above-mentioned 
peculiarities, Sitzungsber. d. Wiener Akademie, LXIX, 26th February, 1874. 

'Scheercr, Gaea. Norvegica, n, 313, Neues Jahrb. f. Miueralogie, 1843, 668; 
Kjeiulf, Bulletin de la Soc. g6olog., xxix, 1862, 413. 



GABBRO. 109 

analysis of this rock made in the laboratory of Wiedemann in Leipsic gave 
the following result : 

Silica 52. 14 Oxygen ratio 27. 80 

Alumina 29.17 Do 13.60 

Protoxyd of iron 3.26 Do 0.72 

Lime 10. 81 Do 3. 09 

Magnesia 0. 76 Do 0. 32 

Potassa 0.98 Do 0.16 

Soda 3.02 Do 0.80 

Loss by ignition 0. 58 

100. 72 

The composition of the rock so much resembles that of the so-called 
labradorite that only a very small amount of diallage can be present. The 
oxygen ratio of Si O 8 : Al 8 O 3 : RO = 27.8 : 13.6 : 5.09 = 6.13 : 3 : 1.12, is nearly 
the same as that of a pure labradorite. There is also the closest resem- 
blance in chemical respects between this rock and the norite from Tronfield, 
Oesterthal, and the labradorite rock from Zaerdals Oeren in Bergenstift 
(both in Norway), which were once analyzed by Kjerulf. 



CHAP TEE VI. 

PROPYLITE, QUARTZ-PROPYLITE, HORN 
BLENDE-ANDESITE, DAOITE. 



SECTION I. PBOPYLITE. 
SECTION II. QUAETZ-PROPYLITE. 
SECTION III. HOENBLENDE-ANDESITB. 
SECTION IV. DACITE. 



SECTION I. 
PEOPYLITE. 

With .the exception of the diorite summit of Mount Davidson [211], 
the entire Virginia range in the Washoe district was formerly covered by 
an outflow of propylite, the first eruptive rock of the Tertiary age. On 
the north and sotith, propylite occupies the summit up to the boundary of 
the diorite mass, and descends to the plain on either side. In Ophir and 
Crown Point Ravines, propylite is found superimposed upon the older 
diorite, and penetrating it in well-defined dikes. The rock continues in a 
southeasterly direction to Carson Plain. In its course thither, it is uninter- 
rupted, except by occasional dikes of andesite, which cut it in north and 
south lines and overflow limited areas. Over all the upper portion, the 
propylite was a sub-aerial ejection, but as it approached the lowlands it out- 
poured below the level of the great fresh- water lake which formerly skirted the 
range. Evidence of this is afforded by the tufacious form of the propylite, 
which shows all the phenomena of aqueous arrangement and stratification. 

Leaves of Tertiary plants are found in the tufa at a height of about seven 
no 



PROPYLITE. Ill 

hundred feet above the present bed of Carson River. Following the 
propylite, but after a lapse of time which permitted a considerable erosion, 
three parallel fissures were broken through the propylite, and large volumes 
of andesite were thrown out. 1 The propylite in these regions has also an 
important connection with silver-veins, as in the Carpathian Mountains and 
in some parts of Mexico, where prominent silver districts occur either upon 
it or associated with it. It forms one of the walls of the famous Comstock 
Lode along some of its most productive portions. Moreover, it is connected 
with several of the veins in the Aurora District, with some of those in Silver 
Mountain, and with the Moss Lode of Arizona. Propylite is everywhere 
the oldest eruptive rock of the Tertiary formation, and it consists of the 
same ingredients as the always-younger hornblende-andesite, but has the 
characteristic older external habitus of the ante-Tertiary dioritic porphyries. 
Propylites are either free from quartz (proper propylites), or they con- 
tain it (quartz-propylites) : both rocks present the same contrasts as (quartz- 
less) andesite and quartz-andesite or dacite. The quartziferous modification 
of propylite is in reality its dacite, or, in an inverse sense, the propy- 
lite of the dacite. Some geologically well-defined occurrences of quartzless 
propylite will be first described. 

In Crown Point Ravine, Washoe, excellent propylites are found [212, 213, 
214, 215, 216]. They have a seemingly almost homogeneous, light greenish- 
gray groundmass, in which larger plagioclastic feldspars, measuring some 
millimetres, and often of a pale-greenish color, are imbedded. In the inte- 
rior, these feldspars are still rather fresh; but, on the outside, they show a dull 
product of alteration, and a net of the same decomposed material, indicating 
former capillary fissures traversing them inwards. All these crystals, as 
well as the microscopical ones of the rock, are closely impregnated with 
fine particles and a quite pale-greenish dust of hornblende, as is often 
the case with the feldspars of older diorites and porphyries, but hardly 
ever happening in those of the younger andesites. This causes the .char- 
acteristic color here. Where the green hornblende is still in some degree 
fresh, it presents a pretty good cleavage; but it is for the most part already 
decomposed, and the vivid yellowish-green product of alteration, which is 

Clarence King's Geology of the Washoe Mining District, vol. Ill, p. 17-25. 



112 MICROSCOPICAL PETROGRAPHY. 

evidently developed out of it, and which already fills narrow cracks, and 
has settled in the groundmass as grains arranged one after another, like 
beads on a string, seems surely to be epidote. The brown hornblende 
of andesites never produces secondary epidote. The formation of epi- 
dote has even taken place in the interior of the feldspars, where it origi- 
nates from the decomposition of the imbedded hornblende material; and 
it seems very probable that the so-called pseudomorphs of epidote after 
feldspars, which have been described by G. Bischof, 1 Blum, 2 and I. Lem- 
berg, 3 and which are chemically so very difficult to explain, do not depend 
upon an alteration of feldspar into epidote, but upon a development of 
epidote out of hornblende particles originally inclosed in the feldspars. 
This supposition is strengthened by the fact that all such pseudomorphs 
are porphyritical crystals in hornblende-bearing rocks. The crystals of 
hornblende in this propylite reach the size of O.l mm . Its groundmass 
also is very rich in hornblende in the form of pale-green grains and fine 
needles, like that of porphyritic diorites; for instance, those from the vicinity 
of Quenast, Belgium. Such an abundant presence of hornblende in the 
groundmass never occurs in andesites. Some excellently fissile, light- 
yellowish augite sections and apatites (pure and dirty brown ones curiously 
intermingled) are present in single varieties, all containing proportionally 
thick crystals of magnetite. 

The propylite from Gold Hill Peak, Washoe, is an interesting one [217]. 
It has a greenish-gray groundmass, which contains plagioclaseslhe size of 
a pea, externally decomposed but pellucid in the middle. The ground- 
mass includes some worn and washed fragments of dark-bordered, brown 
hornblende, the substance of which is pretty well preserved and fresh; some 
pale augites; and a larger quantity of a darker or lighter green, somewhat 
fibrous substance, showing a splendid aggregate polarization, whose outlines 
prove beyond a doubt that it is the product of the alteration of hornblende. 
This seems to be the proper hornblende of the rock which has never 
possessed a black border. The fresh hornblende, with the dark, crumbled 

1 Lehrb. d. cheni. u. physik. Geologic, II, 549. 

2 Die Pseudomorphosen Mineralreichs, dritter Nachtrag, 1863, 118. 

3 Die Gebirgsarten der Insel Hochland. Dorpat, 1867. 



PKOPYLITB. 113 

margin, has, at the first sight, nothing to do with the other, possessing the 
appearance of a strange, erratic body, which is much less easily decomposed 
than the other. In the groundmass, rich in feldspar-microlites, the product 
of altered hornblende (which does not here seem to belong to real epidote) 
takes an important part. Large, dirty black apatites are present. 

In the typical propylites from Ophir Ravine, Washoe [218, 219], the 
feldspars also are completely filled with hornblende material, and the larger 
hornblende crystals are entirely altered into vivid-yellow epidote, slightly 
tinged with pale-green. The forms of the hornblende, nevertheless, have 
been conserved in the most exact manner. Section-lines corresponding 
to the combination ( oo P . oo GO) bound a space which is an aggregation of 
broad, radiated, and confusedly fascicular epidote, often showing colors of 
varying intensity. Beside these long forms, rounded grains of epidote have 
been developed in the interior of the hornblende: these are of a somewhat 
deeper color, and are often strung out into curved lines which traverse the 
radiated aggregations. These epidote grains were formerly the external 
outlines of the hornblende sections, and the contours become evident, in 
especial distinctness, when two concentric series of darker grains form the 
border. Surely nobody would, from petrographical reasons, refer such 
rocks to the andesites, but would, without hesitation, place them among the 
old dioritic porphyries. Geologically, however, they are decidedly Tertiary 
eruptive rocks. Quartz grains are scattered here and there, but they are too 
small and rare to allow of considering the rock to be a quartziferotis propylite. 
Apatite forms curious, short and thick, rounded prisms, which sometimes 
have a pyramidal termination at one end. It is rendered impure by an 
intensely brown, dust-like material, which is confined to an inscribed hex- 
agonal prism surrounded by a water-clear zone of pure and pellucid apatite, 
so that the transverse sections bear a close resemblance to nosean. These 
rounded hexagons might the more easily be mistaken for nosean because 
the dust is sometimes arranged in black cross-lines; but, at the same time, 
the entirely unaltered condition of these seeming noseans would be inexpli- 
cable in a rock in which even the new formation of parasitic epidote is so 
far advanced, and the prisms lying horizontally give abundant proof that 
the mineral is double-refracting. 
8 M P 



114 MICROSCOPICAL PETROGRAPHY. 

A totally altered propylite of a deep yellowish-brown color occurs in 
the hill east of Steamboat Valley, Virginia Range [220]. 

In Sheep Corral Canon, Virginia Range, a gray propylite occurs [221], 
which is geologically of a decidedly older age than the neighboring ande- 
sites and trachytes. The feldspars of the rock are rather fresh ; but the 
hornblende is metamorphosed, its form being preserved, into an excellent 
radial aggregation of dirty-green fibres. Judging from its color, this 
secondary product is not epidote. Thick magnetite grains, slightly dusty 
apatites, and many hornblende particles are in the groundmass. 

The Truckee and Montezuma Ranges are connected by low hills of 
typical yellowish-green-gray propylite, consisting of triclinic feldspar and the 
product of the alteration of hornblende, which is here for the most part 
epidote, although the alteration did not happen in a proper pseudomorphosing 
manner, since the contours of the hornblende are almost entirely obliterated 
and no longer recognizable. The substance of such changed and irregularly 
shaped spots would scarcely be supposed to have any genetic relation to 
hornblende, if different sections did not present the various stages of pas- 
sage between this mineral and epidote, showing how the secondary mate- 
rial gradually comes entirely to occupy the place of the old hornblende. 

Some hills north of Storm Canon, Fish Creek Mountains, Nevada, are 
made up of a splendid propylite [222], which bears large feldspars. The 
hornblende is here and there rather fresh, and it has been built up out of 
thin, staff-like, green microlites, the accumulation being plentifully impreg- 
nated with little, black grains. These hornblende individuals are decom- 
posed, their outlines being preserved, into an aggregation of, a, calcite, with 
a rhombohedral cleavage; &, the usual epidote; c, a dirty-green mineral like 
viridite, which is not epidote. The rock also bears augite in very distinct, 
pale-yellow sections, but in far less abundance than hornblende; brown 
mica, often with somewhat curved, tender laminae (all propylites previously 
described were entirely free from biotite); and pure (not dusty) apatites. 
There are many hornblende particles of extreme minuteness in the micro- 
litic groundmass. 

The pale-yellowish or reddish-gray propylite from Storm Cafion, Fish 
Creek Mountains [223], also shows green hornblende crystals, which are 



PROPYLITB. 115 

here beautifully constructed of prismatic staffs joined in long, parallel 
grouping. Longer and shorter ones enter into the composition, and so the 
ends of the polysynthetic crystals appear fringed and notched, and the side- 
lines do not run in one direction, but show prominent curves outward and 
inward. Hornblende of such a structure as this never occurs in andesite ; 
but it is, on the other hand, a striking repetition of that in the older dioritic 
porphyries, in which the mineral is so abundantly scattered. Apatites 
often lie rectangularly against the microlitic fibration. Hornblende is also 
plentifully distributed through the groundrnass. Moreover, the secondary 
formation of epidote has begun here. There is neither augite nor biotite 
in this variety. 

A good greenish-gray propylite, with characteristic groundmass [224], 
occurs at Tuscarora (Plate IV, fig. 4), Cortez Range. It is worth men- 
tioning that it contains two kinds of hornblende ; first, a predominating 
green variety, characteristic of propylite, which is somewhat fibrous, not 
very distinctly fissile in the transverse sections, always without a black 
border, and sometimes in an early stage of alteration into epidote ; and it is 
this kind of hornblende which is found in smaller particles in the ground- 
mass and thicker microlites and grains of which are imbedded in the 
feldspars; secondly, a dark-brown, much more strongly dichroitic, horn- 
blende, having a black border; it is prismatically cleavable, never meta- 
morphosed, and always pretty fresh ; in short, a hornblende very similar 
to that in andesites. The individuals of the latter are much the rarer of 
the two, and are present only as an accessory : they may be a foreign 
ingredient. There is no augite or biotite. 

At Wagon Cafion, Cortez Range, a propylite occurs [225], with distinct 
macroscopical but dirty-greenish, and somewhat earthy, decomposed horn- 
blende, and large dull plagioclases, measuring 5 mm in length, in a reddish- 
gray groundmass. Larger apatites and some biotites are discernible with 
the microscope. The groundmass is no longer fresh, and the hornblende 
particles in it especially seem to be secondary and brownish. All of the 
described propylites also contain small quantities of monoclinic feldspar. 

The typical quartzless propylite from Sheep Corral Canon mentioned 
above was subjected to a quantitative chemical analysis in the laboratory 



MICEOSCOP1CAL PETEOGEArHY. 

of Professor Wiedemann in Leipsic, and was found to have the following 
composition: 

Silica 64. 62 

Alumina 11. 70 

Protoxyd of iron 8.39 

Lime 8. 96 

Magnesia 1. 18 

Potassa 1. 95 

Soda 3. 13 

Phosphoric acid trace. 

Loss by ignition 1. 02 

100. 95 

A relatively high amount of silica is here unquestionable; for another 
test for this constituent gave 64.60 per cent., and a third one gave even 
65.05 per cent. The quantity of silica in this propylite, therefore, is much 
higher than in the diorites and diorite-porphyries (50. to 60. per cent.), to 
which in other respects the rock bears the closest rnineralogical resemblance. 
It seems also to have more silica than the European hornblende-andesites, 
which on an average possess 58. to 62. per cent. The newly analysed 
andesite of the Fortieth Parallel likewise gave only 61.12 per cent, of silica ; 
and the Hungarian greenstone-trachytes (propylites), besides, are somewhat 
more basic. An analysis of the propylite from the Comstock Lode, made by 
W. Gr. Mixter, 1 gives of silica 58.68 per cent. ; but the enormous amount of 
water (6.53 per cent.) in the rock proves that it is no longer in its original 
state. 

i Vol. Ill, page 90. 



QU AKTZ-PEOP YLITE. 117 

SECTION II. 
QUARTZ-PROPYLITE. 

To facilitate the description, and better to explain the significant pecu- 
liarities of these rocks, they will not be arranged geographically ; but the 
most typical will be mentioned first. One of the best examples of this 
group forms the hills next east of Golconda [226]. It is a dark yellowish- 
gray rock, in which the unaided eye can only distinguish single, clear 
quartzes the size of a pepper-corn, and dull, whitish feldspars. The ground- 
mass has been attacked by decomposition, and is therefore not so easy of 
microscopical analysis. For the most part, it is an intimate mixture of 
isabel-colored, more or less impellucid, feldspar, the small individuals of 
which are sometimes distinctly defined, and of pale-green, granular and 
acicular hornblende-particles. Some amorphous base may perhaps be 
interspersed in the mass ; but, even in polarized light, it can scarcely be 
distinguished from the decomposing feldspar. Here and there, an abortive 
tendency to sphaerolitic formations appears. Decomposition has also begun 
upon the larger feldspars, which appear, in polarized light, to be covered with 
a glittering dust belonging to the products of alteration, which is surely, 
in this first stage, carbonate of lime (see the chemical analyses later). In 
the beginning, like those in the quartzless propylites, they were rendered 
exceedingly impure by hornblende particles, and in most of them traces of 
the former triclinic striation are still distinctly visible. The quartzes in this 
Tertiary eruptive rock are very remarkable, in that they do not contain 
glass, but fluid-inclusions, in great profusion, the best specimens of which 
contain briskly moving bubbles ; and in some double liquid-inclusions were 
observed, the interior consisting of liquid carbonic acid. As respects this 
point, indeed, the quartz of this Tertiary propylite, and, without an excep- 
tion, that of all which follow, behaves exactly like that of the ante-Tertiary 
dioritic porphyries, and differently from that of all other Tertiary quartzifer- 
ous rocks, dacites and rhyolites, which only contain glass-inclusions. 
Microscopical quartz does not seem to be disseminated through the ground- 
mass. Hornblende crystals, which are indistinguishable in the hand-speci- 



118 MICROSCOPICAL PETROGRAPHY. 

mens, become evident in tlie sections. They are decomposed, half fibrous, 
inclined to develop epidote, and most resemble hornblende of the old syenitic 
granite-porphyries. Black magnetite grains are present and apatites are 
comparatively very abundant. A chemical analysis of this rock has been 
made by Dr. Walter Kormann in Leipsic, with the following result: 

Silica 66. 336 

Alumina 14 803 

Sesquioxyd of iron 4. 068 

Lime I 2. 991 

Magnesia 0. 920 

Potassa 3. 190 

Soda _ 5. 160 

Loss by ignition 3. 341 



100. 809 

In the heavy loss by ignition, showing the far advanced stage of alter- 
ation of the rock, 1.034 per cent, of carbonic acid is included. The iron 
was determined as sesquioxyd because, at the ignition, the browning which 
happens if the protoxyd be present was not observed. 

The quartziferous propylite from West Gate, Augusta Mountains, is 
very similar to the above [227]. It contains hornblende in larger individ- 
uals, which are in part highly, and in part altogether, decomposed and altered 
into a greenish substance resembling epidote. The feldspars, plentifully 
filled with the dust of fresh or altered hornblende, look in every respect 
like those in the old diorites. The quartz has fluid but no glass inclusions, 
and many apatites are present. 

Other excellent quartziferous propylites occur at and in the vicinity of 
Cortez Peak, Cortez Range [228, 229]. These rocks have a light-greenish 
or brownish-gray groundmass, in which are macroscopical, dull feldspars and 
hornblendes that look as if decomposed. The groundmass presents an 
entirely crystalline aggregation of, a, dark, Isabel-colored, very slightly pel- 
lucid feldspar, resembling that of granites; b, half-altered hornblende par- 
ticles; c, quartz which cannot be distinguished inacroscopically, the maximum 



QUARTZ-PROP YLITE. 119 

length of its grains being only O.l mm . The presence of such an abundance 
of minute quartz grains has the effect of giving to the thin sections in trans- 
mitted light, when viewed with the naked eye, the appearance of being per- 
forated with innumerable pin-holes. The quartzes have no glass-grains, but 
fluid-inclusions (among which some with salt cubes) could be detected, 
making stronger its resemblance to the old dioritic quartzes. The large 
feldspar crystals are dull; but they still show that they have once been 
triclinic, the dimming lines, densely crowded together, crossing each other 
like lattice-work or the bars of a grate, and leaving small, somewhat clearer, 
fields between them. The best-preserved porphyritical hornblendes are 
splendidly built up of long prismatic staffs, therein repeating the peculiarity 
which is shown as well in the quartzless propylites as in the older diorites. 
No dacitic hornblende has ever grown in such a manner. There is apatite 
and also some characteristic titanite. An incomplete analysis of the quartz- 
propylite from Cortez Peak, executed in the laboratory of Professor Wiede- 
mann, Leipsic, showed the following: 

Silica 67. 79 

Alumina 16. 13 

Protoxyd of iron 3. 64 

Lime 2. 30 

Magnesia 0. 53 

Loss by ignition 1. 70 

Both quartz-propylites present a larger amount of silica than the 
quartzless variety. In composition, they very closely resemble the dacites 
(see the analyses on page 136), and there does not seem to be a sustained 
chemical difference between them. 

Another quartziferous propylite makes its appearance in Wagon Canon, 
Cortez Range [230]. It contains somewhat clearer feldspars, green horn- 
blende, and large, macroscopical, fresh laminae of brown biotite, which 
contain, curiously, in the direction of then- cleavage, interposed layers of 
pellucid calcite (Plate V, fig. 1), in one place being lamellated with twin- 
formation, and in another riven with distinct rliombohedral cracks. Very 



120 MICROSCOPICAL PETROGRAPHY. 

minute fluid-inclusions are found in the quartzes. The groundtnass seems 
to contain some globulitic glass of a brownish-yellow color. There are 
quite a good many apatites in the groundmass and biotites. 

The propylite [231] from Cross Spur, below Virginia City, shows the 
beginning of the introduction of carbonate of lime in those feldspars 
which are as large as peas, yet they are fresh enough to permit of deter- 
mining that they are chiefly plagioclase accompanied by a little sanidin. 
Hornblende of a beautiful green color, and partly composed of aggregated 
microlites, occurs. The only microscopical quartzes bear extremely small 
interpositions, which, vinder a high magnifying power, prove to be fluid- 
inclusions. Apatites are present. 

One of the finest members of this group of propylites occurs at Berk- 
shire Canon, Virginia Range [232]. The gray groundmass, rich in macro- 
scopical, dark-green hornblende, is seen under the microscope to be com- 
posed of colorless plagioclase and light green hornblende. An extraordinary 
quantity of the hornblende in the form of small, dust-like grains, short 
needles, thin laminae, and especially beautiful, long, acicular microlites, is 
disseminated through the larger feldspars. These foreign corpuscles are 
so thickly distributed in the feldspars, appearing under crossed nicols and 
in a low magnifying power like a glittering powder, that even the fresh 
feldspars sometimes show only faint rudimentary twin-striation. The way 
in which the hornblende needles are attracted to different centres and group 
themselves into crystals is not often as clearly seen as here. Hornblende 
needles growing together in crystals, the attracted microlites pushing and 
bending together from all sides, is rarely so distinctly observed ; and those 
stages are very interesting where only half a dozen needles are tending to 
form an individual, which, as yet, presents merely a bunch-like figure. 
The cleavage of the hornblende can be distinctly seen in the interior of 
the larger products, while their ends and borders still consist of disunited 
fibres. Single prisms are not yet, on their own part, cleavable, and it almost 
seems as if the cleavage was solely produced by aggregation. Some quite 
small grains of quartz are present. The quartz-propylites cf Washoe over- 
flow the propylites and the metamorphic rocks of American Canon with 



QUAKTZ-PEOPYLITB. 121 

their accompanying granite, and are in turn capped by basalt. That they 
succeed the propylite is evident, since they are found cutting it in dikes 
above the American Flat Road. Generally, they are of a very great variety 
of nature and constitution, and the quantity of quartz they hold differs 
widely, some specimens containing only here and there a grain and others 
being thickly studded with the mineral. 






122 MICROSCOPICAL PETKOGKAPHY. 

SECTION III. 

HORNBLENDE-ANDES1TE. 

In the "Washoe district were formed three zones of fissures, through which 
peneti-ated to the surface and outpoured thin, table-like masses of hornblende- 
andesitic 1 rocks. 2 Some of these fissures cut through the diorites of Mount 
Davidson, as well as the propylites, on both the north and the south of the 
peak. On the heights above Ophir Grade considerable fields of andesite 
cover the summit, and pour downward over the propylite to the north of the 
road. In ascending Crown Point Ravine, two of these dikes are passed 
whose outcrops show a thickness of one hundred feet. Another zone of 
andesites traverses American Flat and the plateau in front of Virginia, but 
its outcrops disappear to the northward under the later outpourings of 
sanidin-trachyte. A third, and by far the most important, lies about two 
miles to the eastward, where an almost continuous overflow of andesite covers 
the^ountry from near Devil's Gate in Gold Canon, and a large part of Silver 
Terrace Spur, reappearing in the Roman Catholic burying-ground. 3 The 
andesite from the first hill north of Gold Hill Peak, Washoe, is in 
many respects a thoroughly characteristic occurrence. Nearly all the feld- 
spars are fresh plagioclases: there are only very rare individuals and Carlsbad 
twins of sanidin, in which this variety is like the Siebengebirge andesites. 
Nevertheless, the included hornblende is decomposed into a light-green 
substance, surrounded by a black border, which preserves the old contours. 
The composition of this rock is remarkably simple ; neither augite, biotite, 
quartz, tridymite, nor olivine being found. Apatite is present, but it is very 
rare. The groundmass is of a dark-gray color, and is simply an aggregation 
of small ledge-formed feldspars and feldspathic microlites, presenting distinct 
fluidal lines, and of dark, opaque grains, which are mostly magnetite. 
Between these constituents, however, there is more or less of an isabel- 
colored, half-glassy base, although (as in the andesites of the Siebengebirge) 

1 The members of this group are in the following lines named simply audesites. 

2 See King's Geology of the Wasboe Mining District, vol. Ill, page 28. 

3 Ibid., page 30. 



ANDESITE. 123 

smaller hornblende grains and microlites are almost entirely wanting; a 
remarkable contrast with the groundmass of the propylites. 

The rock from the second hill-top north of Gold Hill Peak [233], with 
its splendid brown hornblende, is not a propylite, as might be supposed, but 
an andesite. 

Other hornblende-andesites having a dark-gray groundmass, and con- 
taining beautiful brown hornblende, occur west of Quartz-porphyry (dacite) 
Peak [234]. 

East of Gold Hill Cemetery are varieties [235] bearing large triclinic 
feldspars ; and at the Gould and Curry Road some quartzless members are 
found in the company of quartz-bearing dacites. Still another member, 
which was discovered at Silver Terrace, contains dirty-greenish, fibrous 
hornblende, and some quite pale-greenish, fresh augite, but no biotite. It is 
evident that the green color of the hornblende here is not primary (as in 
the propyh'tes), but a result of the alteration which has begun to take place. 
At the Cross Spur near the burying-ground, an andesite is found [236] 
which has a brcwnish-gray groundmass, and contains excellently striated 
plagioclases the size of a millimetre ; and indistinct hornblende, which seems 
to have been largely replaced by an abundant, dirty-yellowish, somewhat 
globulitic, base. 

A specimen of the andesite from the first hill north of Gold Hill Peak 
has undergone a quantitative chemical analysis at the hands of Dr. Walter 
Konnann, of Leipsic, who announces the following results : 

Silica 61. 12 

Alumina 11. 61 

Sesquioxyd of iron 11. 64 

Lime 4. 33 

Magnesia 0. 61 

Potassa 3. 52 

Soda 3. 85 

T , 

Loss by ignition 4. 35 



101.03 
The loss by ignition included 1.51 per cent, of carbonic acid. A part 



124 



MICROSCOPICAL PETROGRAPHY. 



of the iron discovered must be in the form of protoxyd, as shown by the 
change of color during calcination. In composition, this rock generally 
agrees with that of European andesites, as may be seen by the appended 
table of analyses, except that the amount of alumina present is compara- 
tively small. No. 1 is an andesite from the Walkenburg, Siebengebirge, 
Rhine, examined by Gr. Bischof j 1 No. 2 is from Macska, Hungary, anal- 
yzed by K. v. Hauer ; 2 No. 3 is from Schernnitz Kremnitzer Stock, Czifiar, 
analyzed by K. v. Hauer ; 3 No. 4 is from the railway-station at Tokaj, 
Hungary, analyzed by K. v. Hauer ; 4 No. 5 is from Monte Sieva, Euganean 
Hills, Italy, examined by Gr. von Rath; 5 No. 6 is from Gunung Patna, 
Java. 





1 


2 


3 


4 


5 


6 


Silica 


62.78 


61. 70 


60 10 


62 67 


62 21 


eg 84 




16.88 


14. OO 


17 62 


14 Q4 


12 4Q 


17 OO 




7. W 


6 is 










Protoxyd of iron ...... .... .... 






7 O1 


6 QC 


972 


10 61 




7. 40 


6 47 


2 24 


C o7 


5 O2 


7 O7 




O.82 


2 6:; 


I 8* 


O 71 


I 7O 






2. 04 


I 4H 


7 82 


3 80 


2 17 


o 83 


Soda 


4. 42 


6 10 


4 OI 


e !$ 


7^1 


2 12 




O,87 


2 OQ 


211 


2 OO 


2 7Q 




















99- *3 


100. 61 


98.78 


ioi. 32 


IOI. 21 


IOO.42 



A large and interesting group of varieties of andesite occurs on the 
south flank of the entrance to Truckee Canon [237, 238, 239, 240, 241, 242, 
243, 244, 245, 246]. They generally possess a gray groundmass, which passes 
into the pale-yellowish or reddish sort. Being rather porous and somewhat 
loose, they look rough and trachytic; but, in polarized light, all the distinctly 
observable feldspars are beautifully and thickly striated. Viewed macro- 
scopically, these large feldspars are seen to have a dull, opaque border. A 

1 Lelirb. d. chern. Geol., 1854, II. 

2 Verb. geol. Reicbsanst., 1870, 338. 

3 Ibid., 1869, 81, 59. 
4 1 bid, 1SGI), 140. 

5 Zeitscbr. d. d. geol. Ges., 1864, 502. 
8 Prolss, K Jabrb. f. Mineral., 18C4, 432. 



ANDESITE. 125 

high magnifying power shows that this is not, as is often the case, produced 
by molecular alteration, but by the presence of an enormous number of 
imbedded glassy and half-glassy particles and grains. Smaller feldspars 
of the same rock sometimes contain, inversely, a kernel which is rendered 
impure by the same inclusions, and with a clear border. The splendid zonal 
structure of most of the feldspars is not at all disturbed by these foreign 
interpositions. The relation between the hornblende and augite in these 
geologically inseparable rocks is curious. There are specimens which are 
rich in excellent, dark-brown hornblende sections, with a broad, black border, 
but free from discernible augite ; and in some of the same specimens, where 
a very small quantity of the pale-green augite accompanies the largely 
prevailing hornblende, the augites are always without a black border, but 
rich in glass-inclusions, which, on the contrary, are invariably wanting in 
the brown hornblende. The contrast here in color between hornblende 
and augite, and also in general behavior, is precisely the same as it is in the 
andesites of the Siebengebirge and of Hungary. Specimens with rather many 
augite and rather few hornblende individuals are in the suite; and at last 
some are found presenting only a very small quantity of augites and no large 
hornblendes at all. It should be especially mentioned that there occur, in 
the groundmass of these rocks, feebly transparent, dark-brown, undichroitic, 
needle-formed microlites, generally about 0.015 mm long, which cannot be 
unhesitatingly referred to hornblende, but which seem to be hornblende 
when their relation to the larger hornblende individuals of the rocks is 
noted. They are very rare or wanting when the latter are numerous, 
and are found in great abundance in the groundmass when the others are 
scarce ; one seeming, as it were, to fill the place of the other. The augite 
along the fissures of some specimens of these andesites is curiously accom- 
panied by a fibrous product of alteration, small denticles or teeth of which 
project into the augite, a point of behavior similar to that of the serpentine 
in olivine. This substance polarizes with different colors, and its mode of 
growth is easily observed between nicols. There are other specimens in 
which this alteration of augite, and the clouding or dimming which results 
therefrom, are further advanced. Some sections in places strongly remind 
one of fibrous diallage or enstatite, although the cleavage is still parallel to 



120 MICROSCOPICAL PETROGRAPHY. 

the prismatic faces. Ultimately, the augites become quite dull, retaining 
but slight traces of pellucidity. The groundmass of all these andesites 
contains, beside the above-mentioned brown needles (supposed to be 
hornblende), colorless, triclinic feldspar microlites, often producing splendid 
fluidal lines, black grains of opacite, and often yellowish-gray or pale- 
brownish, pure, or somewhat .glbbulitic, glass. 

Some reddish-brown rocks occur in this same part of Truckee Cafion 
whose color is not caused by secondary alteration, but by the presence of 
a primary, brown, half-glassy, amorphous material in the groundmass, 
whose long but irregularly formed fragments streak the feldspars, often being 
found in an enormous quantity. These varieties are for the most part 
finely porous, and they are comparatively rich in green augites. No 
biotite, tridymite, or olivine could be detected in any of them. 

A perhaps more typical andesite occurs in Berkshire Canon [247]. 
The plagioclase predominates and also forms the largest crystals; but there 
is beside not a little sanidin, beautiful brown hornblende without a black 
border, and some biotite. There is, however, no augite. A neighboring 
andesite [248] cannot be separated geologically from this, but the sanidin 
in it either really or apparently predominates. 

A brownish, half-glassy rock from the west shore of Pyramid Lake, 
Nevada [249], also belongs to the andesites. Feldspar of both kinds is 
present in individuals up to the size of 3 mm ; yet plagioclase decidedly 
predominates: all the larger individuals bear striatton. The rock is 
exceedingly impure, but the strange glassy inclusions are arranged in 
several zones, which are sharply separated by pure feldspar substance. 
It contains also strongly dichroitic, brown hornblende, with a broad, black 
border, which externally decomposes into opacite grains and tail-formed 
aggregations of the same grains without any trace of hornblende in the 
interior ; but having the form of those bodies which consist for the most 
part of a central mass of hornblende. Some good augite crystals are also 
found. The groundmass is made up of colorless (feldspar) and light- 
brownish (augite?) microlites, with much brown glass between them, and 
considerable magnetite. 



ANDESITE. 127 

Another sanidin-bearing andesite is found near Lander Spring, Kamma 
Mountains, Nevada [250]. 

In the Kamma Mountains other andesites occur. Those found at the 
Honey Lake Road Pass [251, 252], and to the southward [253], somewhat 
resemble each other. The feldspars, nearly all plagioclases, and most of 
the larger hornblendes, are somewhat decomposed. No augites could be 
detected. The groundmass is almost free from hornblende. It is chiefly 
feldspathic, but contains a great quantity of brownish-black grains and 
irregularly-formed particles, which are mostly opacite, but a small part of 
which belong to magnetite, and these are accompanied by some dark 
yellowish-brown grains of ferrite, the mass becoming slightly pellucid 
under a high magnifying power. Apatite is present in all these specimens 
in the form of dusty, dismembered prisms, with hydrous oxyd of iron 
between the horizontal cracks. Sometimes the strange, dust-like material in 
the apatites is arranged in short lines, which are gathered into broom-like 
and tuft-like forms (Plate I, fig. 9). 

A typical andesite occurs in the first canon north of Wright's Canon, 
West Humboldt. Macroscopically, it is almost a homogeneous, gray mass, 
without any crystalline secretions. The feldspdr is, with very few excep- 
tions, triclinic. Beautiful, brown, strongly dichroitic and absorbing horn- 
blende, surrounded by a narrow, black border, with a splendid cleavage, is 
present. Brownish microlites of hornblende are more frequent in the 
groundmass here than in that of any former andesite. 

The Augusta Mountains, Nevada, yield other characteristic andesites. 
In the dark specimen from the south head of Augusta Canon [254], the 
hornblende is for the most part altered into an extremely fine, fibrous sub- 
stance, almost the color of green malachite, while its outlines and even its 
black and occasionally brownish border are preserved; or in other words, 
the place of the hornblende, as exactly marked out by the border, is occupied 
by a texture of pale-green fibres and a compact colorless substance seeming 
to be homogeneous, which, judging from its general behavior and its vivid 
chromatic polarization, is nothing else than quartz (not calcite). The effect 
is very pretty where the thin and delicate greenish needles penetrate into the 
colorless substance. The groundmass is very rich in the most minute, brown 



128 MICROSCOPICAL PETROGRAPHY. 

ferrite grains, by no means to be mistaken for the dark, similar globulites, 
which are products of a simple devitrification of a glassy mass. The feld- 
spar-microlites of the groundmass show excellent fluidal lines. The aggre- 
gation is colored more or less brownish by microscopical spots. Here also, 
curiously, the apatite sections are not colorless, but a light brownish-yellow. 
There is no augite. 

One of the most interesting andesites of Augusta Canon [255] macro- 
scopically resembles that from the "Wolkenburg in the Siebengebirge. Its 
dark-gray groundmass contains some hornblende crystals a millimetre in 
length which present the most wonderful phenomenon of rupture, being 
partly visible even with a loupe in the sections (Plate V, fig. 2). The crystals, 
generally brown, but here and there found with greenish or reddish tinges, and 
environed by the well-known dark border, are divided by countless fractures. 
The lines of rupture of a single individual can be followed among the frag- 
ments of a neighborhood, and the broken crystals may be ideally recon- 
structed in their exact original form. Sometimes a real breccia of hornblende 
fragments and splinters, cemented by groundmass, appears. Many splinters 
are again broken, and the diverging particles hang together only at one end. 
Often one of the larger individuals is fractured into thirty or forty pieces. 
It is very remarkable that around the broken crystals, and winding among 
their fragments, bodies of the groundmass, plainly visible in polarized light, 
are found curving ; showing how the stream of the half-plastic magma pro- 
duced the fractures. Since these traces of movement remain, it follows that 
the rock-mass was solidified in a moment. Many angular, cuneiform splinters 
are disseminated through the rock which cannot be traced to a broken crystal, 
and which originated in some other place than where they occur, being 
in a certain sense erratic. They are, almost without exception, entirely 
surrounded by the black border, which proves that they had nothing to do 
with the original formation of the crystals, but have been added to the horn- 
blende since the fracture. So this phenomenon strongly supports the theory 
that the dark border is the product of a chemical reaction between the 
already solidified hornblende and the still half-molten, environing magma 
(see page 95). It is in no sense a contradiction of this theory where splinters 
are found which have the dark zone only on those contours which correspond 



ANDESITE. 129 

to the original faces of the crystal, and not on the lines of rupture as well ; 
for such are fragments of already peripherically metamorphosed crystals 
which were not afterward altered superficially. Bodies of the groundmass 
are frequently found which have broken through the black border and indented 
the crystals in the form of deep and broad inlets and bays; and the most 
distinct fluidal streams penetrate even into the middle of the hornblendes. 
It is very significant that the black outline is in such cases found on the 
limit between the hornblende substance and the intrusive groundmass. The 
penetrating groundmass is an aggregation of largely predominating, minute, 
colorless feldspar ledges and a few brown hornblende- microlites and black 
grains. 

A very similar rock occurs at the head of Clan Alpine Canon, Augusta 
Mountains [256]. 

The andesite from the head of Crescent Cation, Augusta Mountains 
[257], is rich in feldspar, poor in hornblende, and contains not a little quite 
pale-greenish augite. An amorphous base is present in the groundmass. 

An andesite from above Tuscarora, Cortez Range [258], is a peculiar 
one. It has a brownish-gray felsitic groundmass, seldom found in these 
rocks, and contains many large, macroscopical feldspars and some dark 
hornblende prisms. Under the microscope, feldspars are found to be, for 
the most part, well-striated plagioclases. In the sections, the hornblende 
has a vivid, dark grass-green color, and is fibrous and exceedingly dichroitic, 
changing from a light bluish-green into a deep, dark blackish-green. Tho 
black border of the hornblende is here lacking, but numerous angular grains 
which are doubtless magnetite are disseminated through its substance. 
Seen under the microscope, the groundmass between these crystalline ingre- 
dients is a light yellowish-brown in color, with a multitude of the finest and 
most delicate black hairs and grains strung together in lines, really trichite- 
like, short bodies. These minute bodies (measuring only 0.003 mm in length) 
are densely aggregated, and accompanied by thicker black grains, which are 
probably of the same nature, and they produce the most splendid, waving, 
fluidal phenomena. The direction of these trichite swarms makes it evident 
that the groundmass lias really flowed among the large feldspar and horn- 
blende crystals. The trichitic hairs are at intervals also grouped radially, 

9 M P 



130 MICROSCOPICAL PETROGRAPHY. 

and form dark, radiated sphaerolites. The mass in which these hairs are 
imbedded, and which is thickly filled with them where it occurs in the pure 
state, has a simple refraction, and is isotrope glass. There is neither augite, 
biotite, nor tridymite. Some apatite prisms perforate the groundmass. 
The unusual green color which the hornblende has here, certainly seems to 
be original; green products of its alteration being far less dichroitic. 
Perhaps it was the amount of iron in the glass base which prevented the 
hornblende from taking that deep-brown color which is most common to 
the andesites. Occasionally, a thicker, black, club-formed body is found 
lying in the groundmass, looking like a stout trichite, and surrounded by a 
remarkably pale glass zone. 

A neighboring andesite from the foot-hills at Tuscarora [259] does not 
possess the same structure. It is somewhat decomposed, and its hornblende, 
surrounded by a narrow black border of grains, has been in a measure 
altered into epidote, which appears in the form of grains arranged in lines 
and gathered into heaps. This is an extremely rare phenomenon in ande- 
sites. Feldspars are also attacked, and have produced carbonate of lime. 
There is also much somewhat dusty apatite. 

Brown andesite occurs in the Cortez Range south of Palisade Canon 
[260]. It does not, however, contain any good -hornblende. 

A peculiar variety is found in the rock from Wachoe Mountains [261]. 
It is a dark-gray rock, in which a great quantity of laminae of brown biotite 
can be seen macroscopically. Seen in section under the microscope, the 
number of these laminae is largely increased. The feldspars, arranged in 
splendid zones, are partly sanidins, but plagioclases predominate. They 
are without doubt very fresh; for they are entirely water-clear and com- 
paratively very pure, containing only extremely few remarkably small 
foreign inclusions of an undeterminable nature. Beside the predominating 
biotite, there is only a little hornblende, mostly in the form of aggregations 
of small, brown grains. There is no augite. This rock is therefore the 
mica equivalent of the hornblende-andesites; the former standing in the 
same relation to the latter as mica-gneiss to hornblende-gneiss. A few micro- 
scopical quartzes are present in the form of compact grains, having hexagonal 
and rhombic sections, and which show a strong 'chromatic polarization. 



ANDESITE. 131 

Another remarkable andesite occurs at the head of Annie Creek, Cortez 
Range [2G2], whose dark-gray groundmass has that microstructure which 
will be subsequently mentioned as characteristic of the augite-andesites, a 
dense felt composed of the smallest microlites and having more or less glass 
distributed through it. There are some light yellowish-brown, sharp augites, 
beside so many deep, dark-brown hornblendes, that these latter, no doubt, 
predominate. Both minerals are easily distinguishable by the cleavage, 
which is never lacking here. The schistiform feldspars are largely mono- 
clinic, but plagioclases surely prevail among the larger individuals. This 
point also places the rock in a certain relation with the augite-andesites. 
The feldspars (Plate V, fig. 3) contain remarkably good glass-inclusions 
(indeed, the best which have ever been observed) ; being of a yellow color, 
having a thick bubble, and sometimes including short, black needles which 
are very distinctly imbedded conformably with the directions of the zones 
of growth. There are even frame-like layers in the feldspar sections, con- 
sisting of a dust of glass grains less than 0.001 mm in diameter, densely crowded 
together into blackish lines. Signs of thicker and more yellowish glass- 
particles appear in the feldspar between these lines or bands. Some apatite 
is found. 

Emigrant Road, north of Palisade Canon, Cortez Range [263], gives a 
beautiful andesite, which in general resembles that last described. Here 
again the plagioclases contain an enormous quantity of a foreign substance: 
rounded glass-inclusions are less numerous; long strips and shreds of light- 
brownish glass, growing alternately wider and narrower, predominating; 
a perfect net-work of glass, the holes in which are filled with feldspar. 
Sanidin is also present. Hornblende is plentiful, and largely predominates, 
seeming to consist chiefly of larger or smaller macroscopical crystals, although 
there is a good deal of microlitic felt in the dark-gray, somewhat glass- 
bearing, groundmass. Its deep-brown individuals have a very broad, black 
border: some of them are almost all border, having only an extremely 
small and faint spot of the original hornblende substance in the interior. 
Ultimately, the individual is totally transformed into the same material 
as that which first appears merely as a border, and is simply a product of 
alteration, possessing the same rounded and often tail-formed outlines as in 



132 MICROSCOPICAL PETROGRAPHY. 

the original body. Some palo brownish-yellow augites, with sharp, well- 
defined crystal outlines, and lacking the black external zone, are also found. 
This and the last-mentioned rock present a good example of the phenome- 
non of a waving, fluidal groundmass. 

To this same variety (which curiously does not occur among the 
western andesites of Washoe and Nevada), the andesite from Traverse 
Mountains, Utah, also belongs [264]. It bears plagioclase, a little sanidin, 
abundant dark-brown hornblende in crystals reaching 2 mm in size, which are 
without, or possess only the rudiments of, a black border, and a small quan- 
tity of excellent augite. The groundmass is a microlitic felt rich in gray glass. 
By comparing these andesites from the Fortieth Parallel with the European 
specimens from Siebengebirge, Rhenish Prussia, Nassau, and Hungary, 
we find that in general structure there is the closest analogy between them, 
and it is therefore curious that the American rocks have proved, in nearly 
all cases, to be free from biotite and invariably to be without tridymite; 
two minerals which are seldom lacking as accessory ingredients in the 
corresponding European occurrences. Some sanidin and some augite, 
beside the prevailing plagioclase and hornblende, seem to be common to 
all the andesites of both hemispheres. 

The general question now presents itself, By what points may propylites 
and andesites be distinguished and separated petrographically? Their chief 
constituents are the same, and they belong to different epochs of the 
same Tertiary age. But this slight difference in geological time is of suffi- 
cient value to express itself in some characteristic petrographical peculiari- 
ties, which, in their combination, always indicate the distinction between 
propylites and andesites. Perhaps it may not be superfluous to insist that 
all the rocks described in the foregoing pages as propylites and andesites 
were first referred to one or the other group by geological observations in 
the field, and that the petrographical diagnosis and the classification of them 
have not been influenced by any artificial point of view or preconceived 
opinion. The examinations have proved that in every rock the geological 
and petrographical differences perfectly accord. The diagnostic differences 
between the two rocks may be summed up as follows: 

a. The general color of the propylitic groundmass has more of a 



ANDESITE. 133 

f 
greenish-gray, while the andesitic groundmass has more of a pure gray or 

brown tinge. 

b. In structure and in the behavior of its constituents, the propylite 
still resembles the older ante-Tertiary diorite-porphyries. 

c. The groundmass of the propylites is rich in minute particles of horn- 
blende, while in that of the andesites this mineral appears only in the larger 
individuals, fine hornblende dust being wanting. 

d. The propylitic feldspars are usually filled with a considerable 
quantity of hornblende dust, while the andesitic feldspars are entirely 
without it : the latter not infrequently containing glass-inclusions, which do 
do not seem to occur in the propylitic plagioclases. 

e. The color of the proper hornblende sections in propylito is always 
green (never brown), while the color of those in andesites is almost without 
exception brown ; and the propylitic hornblende never shows the curious 
black border which is so common to that of andesites ; and again, propylite 
in some cases contains, beside the largely predominating green hornblende, 
a few sections of the brown mineral, presenting, in many points, a strikingly 
peculiar aspect, while in andesites two kinds of hornblende never occur to- 
gether. 

At- 

f. I he propylitic hornblende is often distinctly built up of thin needles 

or staff-like microlites, and therefore is not regularly cleavable ; which has 
never been found to be the case in andesites. 

g. The production of microscopical epidote (mainly by the alteration 
of hornblende), so very common in propylites, has, with one exception, 
never been observed in these andesites, and it is also unknown in the 
European occurrences. 

h. Augite often occurs as an accessory constituent in andesites, but it 
is comparatively very rare in propylites. 

i. The andesitic groundmass here and there seems to possess a half- 
glassy development : a glass-bearing propylitic groundmass has never been 
found ; and herein is another point of resemblance to the old diorite- 
porpliyries. 

All these differences between propylitic and andesitic hornblende also 
extend to both of the quartziferous members, quartz-propylite and dacite. 



134 MICltOSOOPCAL PETKOGKAPHY. 

SECTION IV. 

DACITE. 

All the rocks described under this title in the following lines are from a 
few illustrative localities inferred to be younger than propylites and older 
than trachytes. They stand in close connection with the quartzless member, 
andesite, being generally of later eruption. 

At the West, the first excellent dacites occur in the hills above American 
City, Washoe [265, 266], where two varieties are presented. One has a 
brownish-gray and the other a greenish-gray groundmass. Both contain 
macroscopical feldspars, all, or nearly all, of which are more or less distinctly 
striated. They also bear quartz grains, which range in size from the dimen- 
sions of a pea downward. The first variety [265] is made especially inter- 
esting by the fact that its groundmass has a perfectly rhyolitic structure. Its 
crystalline secretions, plagioclase and hornblende, separate it from the 
rhyolites, and refer it to the andesitic group, and the quantity of quartz it 
contains pronounces it a dacite. The groundmass presents for the most 
part a sphserolitic microstructure of a perfection seldom observed. The 
balls (of which the average diameter is O.l mm ) are composed of fine, pale 
yellowish-gray fibres of various lengths, having a splendid radial convergence 
to a common centre. Beside the more perfect normal sphserolites, rudi- 
mentary and malformed ones are seen constituting only a sector,with a diverg- 
ing structure like an ice-flower. The sphserolitic material either polarizes 
feebly, with a very indistinct transmission of light, or not at all; and it must 
therefore be of a microfelsitic nature. The feldspars are no longer fresh in 
their sections. With a low magnifying power, they show only an indistinct 
and rudimentary striation; but it is evident that they have once been beauti- 
fully lamellated. A high magnifying power ascertains that carbonate of 
lime already constitutes a considerable part of their mass. This limCj 
present in the form of calcite, replaces the feldspar in some spots, and forms 
irregular masses, which have a very distinct rhombohedral cleavage. Who- 
ever is acquainted with the peculiar polarization of finely distributed particles 
of calcite, which appear like glittering polychromatic scales, will immediately 



DACITE. 135 

observe that calcite powder is also largely interposed in the plagioclases, a fact 
with which the obliteration of the twin-striation is connected. The whole feld- 
spar section appears finely stippled and speckled with the most minute calcite 
points. It may be allowable to account for this by the comparatively 
basic nature of the plagioclase and the relatively high amount of lime 
present in it. And C. Doelter has shown that the plagioclase of the typical 
dacites around Vorospatak in Transylvania belongs to labradorite and contains 
10. to 11.5 per cent, of lime. 1 The feldspars of the Washoe dacite also have 
small but very distinct fluid-inclusions: this is remarkable, for it is the 
first tune liquid-inclusions have been discovered in the feldspar of a trachytic 
or andesitic rock. Its quartzes, which are very fine ones, are wanting in 
fluid and also in proper glass-inclusions ; but they contain rounded, oval, 
and variously-shaped particles of the sphaerolitic fibrous groundmass, which 
exactly agrees with the main body of the mass in substance and structure. 
Groundmass enters the quartz crystals in the form of rounded bays and 
inlets, precisely as in the older felsite-porphyries and rhyolites. The horn- 
blende has a feebly black border, and sometimes, under the protection of this 
zone, has slightly altered, developing these substances : o, a pale greenish- 
yellow or yellowish-green substance (viridite), which seems to be homogene- 
ous, but polarizes, often found in abundance, traversing in veins; b, excel- 
lently rhombohedral, fissile calcite; c, red and brownish-red grains of oxyd 
of iron; d, dark yellowish-green, rounded grains, often arranged in lines or 
heaps, which is probably epidote. Aggregations of these four substances 
often entirely fill the space formerly occupied by the hornblende, and 
splendid polysynthetic pseudomorphs are exhibited. There is no augite 
present. Prisms of apatite are occasionally found. The other variety 
[266], having a greenish-gray groundmass, for the most part repeats the 
IK ruliarities of its above-described neighbor: there is the interposition of 
lime in the plagioclases, tbe remarkable phenomenon of hornblende altera- 
tion, etc. Its groundmass, however, is not at all sphaerolitic, but instead 

1 Mineralogische Mittheilungen, gesauiinelt vou Tscliermak, 1874, page 13; for 
instance, the plagioclane of the dacite from the Suligata has 9.95 per cent, of lime, and 
the plagioclase of the dacite from the Zuckerhut near Nagyag has 10.10 per cent, of 
lime ; and, again, the plagioclase of the dacite from tin; Haito, 11.42 per cent, of lime. 



136 MICROSCOPICAL PETKOGRAPUY. 

microfelsitic, possessing fine, indistinctly polarizing grains, of which egg- 
shaped inclusions (some even with a bubble) are interposed in the quartzes. 
A quantitative analysis of this dacite from the hills above American City, 
Washoe, was made by Mr. C. Councler, of Leipsic, with the following 
result : 

Sfflca 69. 3 

Alumina 17. 9 

Protoxyd of iron 4. 1 

Lime 1.6 

Magnesia 1.3 

Potassa 3. 6 

Soda 2. 

. Loss by ignition 2. 1 

101.9 

It will be seen by this analysis that the amount of silica present is 
much higher than in andesite (see page 124), and approaches in quantity that 
in rhyolite, and that the general composition of the rock resembles that of 
quartz-propylite. A parallel to the striking fact that, in this rock (the 
larger feldspars of which are triclinic) potash slightly predominates over 
soda, is seen in the analysis of Transylvania dacites. With reference 
to this, we shall not quote the analyses made by Sommaruga, who 
found, in some of these dacites, ten or fifteen times more potash than soda ; 
for it has been asserted 1 that all his conclusions concerning the alkalies are 
incorrect. Careful chemists like K. v. Hauer, however, have sometimes 
noted an amount of potash more or less in excess of the soda, or nearly 
equal to it ; for instance, potash : soda = 4.91 : 3.12 ; 5.40 : 3.86 ; 3.58 : 3.G4 
(Streit Tschermak) ; 3.33 : 3.59 (v. Andrian). This seems to prove that 
sanidin is often present in the groundmass of dacites in a not inconsider- 
able quantity. J. Roth is even inclined to believe that dacite and rhyolite 
are to be distinguished by the circumstance that the former contains 
plagioclase and quartz in a groundmass relatively rich in potash ; the latter, 
sanidin and quartz in a groundmass relatively rich in soda. 2 For the sake 

'C. Doelter, Minnalo-. Mittheiluugeu, geeainnielt von Tschcrnuik, 1873, 92. 
" BiMtriige znr Petrographic der plutonisclic'ii Gt-steinc, Merlin, 1809, 187. 



DAC1TE. 



137 



of illustration and comparison, we quote some newly-made analyses of 
European dacites. 

No. 1 is from Illova Valley near Rodna, Transylvania; 1 No. 2, from 
between Szekelyo and Rogosel, Transylvania ; 2 No. 3, from Sebesvdr, 
Transylvania ; 3 No. 4, from Nagy-Sebes, Transylvania ; 4 No. 5, from Kis 
Sebes, right bank of Koros, Transylvania; 5 No. 6, from Monte Alto, 
Euganean Hills, Italy ; 6 No. 7, from New Prevali, Karinthia. 7 





1 


2 


3 


4 


5 


6 


7 


Silica 


66.41 


66.10 


66.91 


67. 17 


66.32 


68.18 


67.44 




17.41 


15. 6-1 


H. 11 


16.06 


14. 11 


n. 6c 


10. 11 




A 12 


A 1O 


Soo 


3 At 


1 Cl 




1.61 










I. 2O 


O. 2S 


6.60 


i. 08 




1,O6 


2.76 


2. 1? 


4. 46 


4. 64 


2.21 


?. 07 




1.82 


I. 11 


O. Q<C 


I. <O 


2.4? 


O. 42 


I.Od 


Potassa 


1.65 


4.QI 


5. 4O 


I. "^ 


1.61 


I. 71 


?. ss 


Soda 


5.8-i 


1. 12 


1.86 


1. 7O 


1. QO 


6. oo 


r 64 




o. 81 


1.76 


1.42 


o 80 


I. 11 


o. cc 


2.06 




















100.01 


100.40 


IOO. O2 


100.88 


loo. 16 


99-45 


100.63 



Washoe dacites have a comparatively high amount of silica, but are 
not the richest in it; Marx having analyzed a dacite from Leon de Nicaragua 
el Cerrito, Central America, with even 71.27 per cent, of silica, and 18.46 
of alumina. 8 The low percentage of lime in our rocks is probably due to 
alteration, which 'is to be counted in with the high loss by ignition; a quan- 
tity of this substance having been carried off as a soluble carbonate. 

Dacites from the hills west of Devil's Gate, Washoe [267, 268, 269], 
generally bear a more or less close external resemblance to those above 
described; the behavior of the feldspar and hornblende being the same. The 
groundmass is a pale yellowish-gray, microfelsitic substance, in which a 



'Tschermak (Slechta), Wiener Akacl., Ber.,18G7, 295. 
*K. v. Hauer, Verhandl. d. geol. Eeichsanst., 18G7, 119. 
K. v. Hauer, ibid., 1867, 118. 

4 C. Doelter, Mineralog. Mittheil.,von Tschermak, 1873, 92. 
6 C. Doelter, ibid., 1873, 93. 

6 G. von Eath, Zeitschr. <1. d. geolog. Gesellscli., 1804, 500. 

7 Tscliermak (Streit), Wiener Akad., 13er.,18(57, 302. 
1 ZcitscL., d. d. geol. Ges., 1808, 524. 






138 MICROSCOPICAL PETKOGKAPIIY. 

feeble iudividualization has begun to show itself here and there, presenting 
between crossed nicols a quite dark isotrope base, out of which small points 
occasionally shine out indistinctly. Along undulating lines, a somewhat 
more decided individualization has happened, producing waved bands, 
which polarize better, and which present in ordinary light a somewhat micro- 
crystalline composition made up of colorless grains. By the curvature of 
these granular bands and stripes, which at both extremities soon pass into 
the microfelsitic base, fluidal wave-phenomena are well shown. In this 
groundmass also splendid sphserolites in different stages of development 
have been discerned. Beside entirely round, perfect balls, there are rudi- 
mentary tendencies to horseshoe-like forms, and many poor, little bunches, 
which scarcely constitute the tenth part of a circle. A ring of fine and 
delicate sphserolitic fibres (the single ones in radial position) is sometimes 
found wreathing a rounded kernel of a non-fibrous, prevailingly micro- 
felsitic, mass; and surrounding this is another ring exactly like the first one; 
the effect being that quite a gradual passage between the fibres and the micro- 
felsite takes place. The quartzes have rare but perfectly characteristic 
glass-inclusions but none of fluid. 

Another dacite [270] is found in Basalt Canon, Washoe. It also bears 
a really rhyolitic base, with excellent fluidal lines, produced by the zonally 
different, more amorphous, or more indistinctly crystalline-granular behavior 
of the groundmass. Here and there, black trichites, aggregated fascicularly, 
are interposed in isolated patches, being more or less dismembered into 
grains. Quartzes contain the most beautiful hexagonal and rhombic 
inclusions of brownish glass, and of groundmass pressed into the shape of 
quartz. The rock bears splendid brown mica instead of hornblende, and 
is therefore a mica equivalent of the common (hornblende) dacite. More- 
over, this dacite, as well as the former one, has a very little sanidin accom- 
panying the prevailing plagioclase. All these rocks are filled with fine 
pores; and it appears under the microscope that little plagioclase crystals, 
which in all probability were previously wholly altered into carbonate of 
lime, had suffered complete removal. 

Among the andesites of the Gould and Curry Quarry, Washoe, occur 
quartz-bearing members, some even rich in quartz [271]. The hornblende 



DACITE. 139 

is brownish, with a tinge of green. The quartzes contain magnificent glass- 
inclusions. 

Dacitic rocks also appear in Berkshire Canon [272, 273, 274, 275], 
They are yellowish-gray or bluish-gray felsitic masses, rich in dark, macro- 
scopical quartzes, which are for the most part pretty well crystallized. They 
are poor in larger feldspars. Where the latter are visible, they show distinct 
traces of striation, notwithstanding the development of carbonate of lime 
has made considerable progress. This circumstance may make it justifiable 
to classify these rocks as dacites, although no hornblende can be recognized, 
and dark plates of biotite can be seen only here and there. The ground- 
mass is in some places more truly crystalline than in the foregoing rocks, and 
is entirely rhyolitic, sometimes showing a tendency to form sphserolites and 
those curious, axially fibrous, longitudinal bodies which will be described in 
the chapter on rhyolites. These rocks are also geologically older than the 
sanidin-bearing rhyolites from Berkshire Cafion, and are, moreover, no 
poorer in quartz. 

One of the most typical dacites is that from Mullen's Gap, west side of 
Pyramid Lake [276]. In this grayish rock, it is easy to detect, with the 
unaided eye, many striated feldspars and beautiful quartzes. Its ground- 
mass, again of rhyolitic nature, is a simply refracting, homogeneous base, in 
which a great quantity of polarizing particles had begun to be secreted. 
Almost all the feldspars are triclinic and fresher than in the above-mentioned 
dacites: they are extremely rich in inclusions of glass and of the groundmass, 
and also in sometimes irregularly cylindrical, and sometimes flat-pressed, 
empty pores, in which, however, carbonate of lime is occasionally found 
parasitically deposited. The hornblende is no longer fresh, but it is very 
distinctly visible. Where its original condition has been in some measure 
preserved, it becomes evident that its color was brown. 

Another good dacite occurs on the east slope of the hills, south of 
Rabbit Hole Spring, Kamma Mountains [277]. The structure of its ground- 
mass is the same, but its color is a somewhat darker yellowish-gray. Many 
quartz grains appear under the microscope, and so also do distinctly pre- 
dominating, very well striated, but small plagioclases and some hornblende 



140 MICROSCOPICAL PETROGRAPHY. 

attacked by decomposition. Macroscopically, the felsitic rock does not 
present any crystals. 

The dacite from Shoshone Peak, Shoshone Range [278], is a dark 
greenish-gray rock, with quartzes nearly the size of a pea, filled with superb, 
almost colorless, glass-inclusions having a thick, dark bubble. The quartz 
also contains hornblende-microlites, which is a rare phenomenon in these 
rocks. In one instance, one of them was seen in the quartz partly 
surrounded by a bubble-bearing glass-drop, and itself possessing two little 
glass-inclusions (Plate I, fig. 17). The hornblende is rather distinct. Feld- 
spars have many gas-cavities, and are already in the early stages of de- 
composition. 

That from south of Palisade Canon, Cortez Range, a light grayish- 
brown variety, appears generally andesitic, but it contains beautiful 
quartzes [279]. Pellucid feldspars up to the size of 4 mm , nearly all of which 
are ti-iclinic, may be seen in the section with the unaided eye. The 
external part is a quite dull, milky layer, forming a plain frame. Under the 
microscope, one discovers that innumerable bubble-bearing inclusions, or 
fragments of a fine, porous, light grayish-yellow glass, are imbedded in the 
feldspar, almost wholly replacing the feldspar substance. The hornblende, 
under preservation of its outlines, is often altered into a radially fibrous, 
pale-green substance, and sometimes into a mixture of this and calcite. 
Less decomposed individuals show that their original color was brown. 
In strong contrast with this hornblende, the biotite Iamina3 present have 
remained entirely fresh. A large part of the groundmass has a microfelsitic 
base, in which a considerable quantity of delicate, brownish-black, trichitic 
needles are scattered without order. 

A peculiar dacitic rock [280, 281] occurs in Wagon Canon, Cortez 
Range. Its prevailing mass is a dark blackish-brown, in which, beside 
quartzes the thickness of a pepper-corn and some plagioclases, strange 
angular particles of a dull, milky-looking substance, which are seldom larger 
than a pea, are richly disseminated. The sections make it instantly apparent 
that the rock is a miniature breccia ; and it is surely an eruptive breccia, 
not of sedimentary clastic, material. Without doubt, the quartzes, and 
lrolal>ly the plagioclases, belong to the prevailing dark mass; for its 



DAOITE. 141 

secreted ingredients are not of a foreign, fragmentary nature. Included 
milky splinters, often densely crowded, have a felsitic microstructure, 
and contain plagioclases and quartzes. The rock is, in short, a dacite, 
which envelopes so many strange fragments of another variety of dacite as 
to form a real breccia. 

In conclusion, some comparative observations upon the relation of 
dacites, as well to quartz-propylites as to andesites, will not be amiss. Of 
course, these remarks refer only to the rocks examined in the foregoing 
pages, and a summary generalization as to those of foreign regions is not 
attempted. It should be understood, nevertheless, that these rocks arc 
thoroughly classic, and represent the best forms of their respective kinds. 
The difference between dacite and andesite does not consist only in the 
presence of quartz in one, while it is lacking in the other ; the microscopical 
structure of the groundmass of the two rocks being entirely different : that 
of the dacites has a rhyolitic structure, presenting a microfelsitic, here and 
there more or less granular-crystalline, mass, with a frequent tendency to 
form sphaerolites; while the andesitic groundmass is a simple aggregation of 
microlites. In its original state, the color of hornblende in both rocks 
is brown. Augite often accompanies the prevailing hornblende, aa an 
accessory element, in andesites ; but it is entirely wanting in the dacites. 
Herein is another point of close likeness between dacites and rhyolites. As 
to quartz-propylites and dacites, it has been shown (page 133) that all the 
differences between propylitic and andesitic hornblende also exist between 
the quartziferous equivalents of the two rocks. The microscopical structure 
of quartz-propylites being the same as that of quartzless members, it 
becomes apparent that a considerable difference must exist between it and 
that of dacites, and it also equally differs from the andesites. But there is 
another and more remarkable point of contrast : the quartzes of the quartz- 
propylites abound in fluid-inclusions (as in the older diorite-porphyries), 
but do not contain any glassy ones ; while the quartz of the dacites (as in 
the rhyolites) do not bear any liquid, but possess excellent glass-particles. 
Upon this point, it is interesting to know that the only hornblende- 
plagioclase rock of Transylvania, that from Borsa-banya, in the quartzes 
of which inclusions that are doubtless fluid have been observed, does not 



142 MICROSCOPICAL PETROGRAPHY. 

belong to the dacites (as has been suggested), but is, according to Richt- 
hofen, 1 a quartziferous propylite. J. C. Doelter once mentioned liqxiid- 
inclusions in real dacites of Transylvania, but subsequently withdrew the 
statement. 2 It was formerly thought that the limit between the fluid and 
glass-bearing quartzes was at the beginning of the Tertiary age; but now, 
after study of the quartz-propylites, it must be advanced and placed within 
the Tertiary, between propylites and dacites (andesites). While the quartz- 
propylites are rich, the dacites (again like rhyolites) are extremely poor 
in apatite. In general, therefore, there is even a stronger contrast between 
quartz-propylite and dacite than between propylite and andesite : the 
difference has a stronger expression in the quartziferous members. The 
contrast between quartz-propylite and propylite is confined merely to the 
presence in one of quartz, and its absence in the other ; but, between dacite 
and andesite, the discrepancy is far wider. 

1 Zeitschrift d. d. geolog. Gesellscb., XX, 1868, 687. 
Mineral. Mittheil., v. Tschermak, 1874, 14. 



Jtosii 








of T>H< 
s ',& 



CHAPTER VII. 

TRACHYTE, RIIYOLITE. 

1 fit 

~~ ~ stump 

SECTION I. TRACHYTE. 
SECTION II. RHYOLITE. 
SECTION III. HYALINE EHYOLITES: GLASSY 
AND HALF-GLASSY ROCKS. 



SECTION I. 

TRACHYTE. 

About the level of Virginia City, Washoe, and not far to the east 
of it, a line of trachyte outcrops. The most northerly is on Graveyard 
Spur, at what is known as The Quarry. Here a comparatively thin sheet 
of sanidin-trachyte is found capping an abrupt hill ; the greater mass 
being formed of inclined, rudely hexagonal columns, varying in size from a 
foot to four feet across. A second outcrop occupies a bench just above 
the Geiger grade, near the Sierra Nevada works. The main ejection of 
trachytic rocks occupies a broad zone extending from the Washoe foot- 
hills to Pyramid Lake, a distance of forty miles. It has flowed out, in a 
nearly meridional direction, along the heights of the range, and almost 
all the prominent eroded summits (Sugar Loaf, Mounts Rose, Kate, Emma) 
and elevated table-lands are formed of it. In the pass north of the 
Gould and Curry mill, a narrow dike of trachyte, less than one hundred feet 
wide, breaks through propylite, and up out of this small opening the whole 
mass of the formation has come. Wherever the country is eroded to any 

143 



144 MICROSCOPICAL PETROGRAPHY. 

great depth, for instance, at Six Mile Canon, at the base of Mount Rose, 
traces of an earlier variety of trachyte may be seen; fragments being 
imbedded in the breccia, which immediately succeeded it. These breccias 
form the greater part of the whole outflow, containing masses varying in 
size from a mere pebble to' blocks twenty feet in diameter. Capping the 
breccias is a broad thick overflow of normal sanidin-trachyte, varying in 
thickness from one hundred to one thousand feet. 1 In other localities along 
the Fortieth Parallel, two kinds of trachyte occur, which are distinguished by 
age. The younger eruptions break through the masses of the older trachyte 
formation, flow over their rounded forms, and generally occupy the summits 
and higher parts of mountains. Petrographically, they differ in essential 
points. The older ones still recall the andesites, which are just antecedent in 
geological age. They are rich in plagioclase (so that both feldspars often 
maintain a balance, and the sanidin but slightly predominates), and pro- 
portionally rich in hornblende, which is of a brown color, like that in ande- 
sites. The younger trachytes are poorer in plagioclase, contain more sanidin , 
much less hornblende, and in general possess many macroscopical lamina; of 
biotite. Moreover, they are somewhat rougher and more porous than the 
older ones. 

The older trachyte from Mount Rose and Sugar Loaf [282] also so 
far microscopically resembles the andesites that proper hornblende does not 
enter into the composition of the groundmass, which is composed of feld- 
spar microlites, dark brownish or blackish grains of ferrite, and opacite, 
with here and there some colorless glass. The reddish-gray, younger 
trachyte from these mountains [283] contains excellent microscopical aggre- 
gations of tridymite, precisely resembling those in the trachytes from the 
Siebengebirge in Hungary, the Euganean Hills in Northern Italy, and 
Montdore and Puy-de-D6me in Central France. The tridymite forms 
little thin and delicate colorless lamina? (which are seldom of more than 
0.02" in diameter), having a six-sided or irregularly rounded outline. 
These lamina; are arranged, generally in great numbers, one above the 
other, presenting something the appearance of roofing-slate or shingles 
(Plate I, fig. 14). The younger trachyte from Cross Spur Quarry, Washoe 

1 Clarence King, Geology of the Wasboe Mining District, vol. JII, 33. 



TEACH YTE. 145 

[284], is separated into large columns, and shows in the smaller sanidins 
beside minute inclusions, real kernels of yellowish-brown glass, which are 
rectangular in shape, like the crystal section. Their size is sometimes larger 
than the narrow outer zone of pure feldspar. The hornblende sections have 
a curious intermediate color between green and brown. Splendid, richly - 
lamellated biotite is perforated by numerous apatite needles ; and again 
the pores contain dense accumulations of tridymite. 

Among the trachytes of the Fortieth Parallel there are some peculiar 
rocks, which present a combination of prevailing ingredients that has never 
been observed elsewhere in these Tertiary eruptive rocks, namely, sanidin 

j r J 

and augite. 

The first of this new group of augite-trachytes forms the low hills 
between Sheep Corral Cafion and Wadsworth [285]. It is a dark brownish- 
black, half-glassy-looking rock, and is younger than propylite. All the 
larger crystals are doubtless sanidin, which places the rock among the 
trachytes: there is pale-green augite and some plagioclase and brown 
hornblende ; the latter being decidedly inferior to the augite. The very 
abundant groundmass consists of little colorless crystals and microlites, 
which are probably for the most part sanidin, and very pale-greenish 
microlites, belonging by analogy more to augite than to hornblende, 
imbedded together with magnetite in a nearly colorless glass-base; a micro- 
structure which in every respect resembles that characteristic of the 
groundmass of augite-andesites, to be described hereafter. At the time the 
examination of this and other American augite-trachytes was made, Gr. 
von Rath showed that the same peculiar combination also occurs among 
the older rocks. 1 He described as " augite-syenite " the rock which forms 
the largest part of the Monzoni Mountain in South Tyrol; the most charac- 
teristic variety occurring at the Toal dei Rizzoni. These older . rocks 
contain, besides the principal constituents, orthoclase and augite, some 
plagioclase, and, rather as accessory ingredients, titanites, hornblende, iron- 
pyrites, magnetite, and apatite. At the same time, he discovered that some 
varieties of the famous syenites from the southern coast of Norway, 

1 Sitznng der niederrheinischen Gesellschaft f. Natur- und Heilkunde, 8. Marz 
1875. 

10 M P 



146 MICROSCOPICAL PETROGRAPHY. 

especially the occurrence at Laurvig, belong to the angite-syenite group. 
These results are the more remarkable because it was formerly considered 
a petrographical law that augite is, without exception, confined to the basic 
rocks, and that it never occurs in conjunction with a member of the feldspar 
group as rich in silica as the sanidin or orthoclase is. A quantitative 
analysis of this augite-trachyte, made by Dr. Anger in Leipsic, gave the 
following results : 

Silica 68.81 

Alumina 13. 62 

Protoxyd of iron 3. 91 

Lime.' 4.30 

TUT r, 

Magnesia 2. 74 

,\ ;i;i ._, 

Potassa 2. 56 

n -l ^ nn 

Soda 2. 68 

8891'' _ , . _ .. 

Loss by ignition 2. 30 



100. 92 

The preponderance of sanidin in this highly silicated rock is indicated 
by the comparatively large amount of potash, as compared with the soda. 
Its high amount of silica, which even surpasses the sanidin, in spite of the 
presence of basic augite, is, as in the augite-andesites, produced by the glass. 

A genuine trachyte from the summit at the head of Sheep Corral Cailon, 
Virginia Range [286], is a peculiar reddish-gray variety, bearing, along 
fissures, excellent hyalite. Thin sections show an alteration-product of 
small gray and pale reddish-yellow spots; the first being the freshest 
portion of the rock. The groundmass consists of water-clear feldspar- 
microlites, opaque black grains, and a light-gray cementing and imbuing 
glass-base. In the reddish places, the color of which is doubtless secon- 
dary, all the microlites are colored brownish-yellow; and the hornblende, 
in other parts of the mass a good brown and entirely fresh, is here, wholly 
or in veins, altered into a dull, dirty yellow substance, inclining one at first 
sight to mistake it for serpentinized olivine. Beside the prevailing sanidin, 
there are some plagioclases and biotites present. There is no tridymite, 
however; probably on account of the abundant acid glass-base. 



TRACHYTE. 147 

Other trachytes occur in the Truckee Canon between Glendale and 
Clark's Station. One variety presents even macroscopically the two kinds 
of feldspar. 

A characteristic trachyte locality is met with at Truckee Ferry, 
Truckee Canon. On the side of the ferry, a beautiful variety appears 
[287], which is very rich, both in sharp crystals and glass. The 
included macroscopical and microscopical crystals, among which horn- 
blende and biotite are found in individuals 3 mm long, are a, sanidin, in 
some places macroscopical, charged with a remarkable quantity of glass- 
inclusions; 6, quite an unimportant amount of plagioclases; c, abundant, 
excellently fissile, deep-brown, fresh hornblende; d, biotite, in about the 
same quantity as hornblende; e, much rarer, light greenish-yellow augite, 
pierced by apatite, which does not occur as an independent constituent; f, 
magnetite. The copious groundmass, of resinous lustre, is very rich in 
almost colorless glass, and it also contains a large quantity of colorless 
microlites of the same material, and rectangular crystallites, each of the four 
corners of which are drawn out into a long acicular spire or thorn. No 
colored microlites which could belong to hornblende or augite are present. 
Macroscopical, pumicestone-like protuberances cover hollows in the rock. 

Purple Hill, at Truckee Feny [288], consists of rather dark and more 
common trachyte, with narrow and unstriated ledges of sanidin, 7 mm long, 
in an unusual degree charged with long microscopical fragments of ground- 
mass. In the groundmass, which chiefly consists of feldspar ledges, among 
them many that are triclinic, and microlites of feldspar, neither hornblende 
nor augite can be detected: a quantity of small black magnetites and 
yellowish-brown or brownish-yellow grains of ferrite accompany the feld- 
spar. 

A light-gray rock, which belongs to the augite-trachytes, but repre- 
sents a different variety from that above described, occurs on the left bank 
at the Truckee Ferry [289]. It is for the most part a nearly crystalline 
mixture of feldspar impregnated with augitic dust and pale brownish-yellow 
augite. Many of the feldspars are striated, but there is no doubt that the 
predominating quantity is monoclinic. Augite prisms are often found 
grouped in oval, imperfectly radial accumulations, measuring as high as 



148 MICROSCOPICAL PETROGRAPHY. 

0.3 mm in diameter. Long tails, composed of black hornblende grains, which 
appear in the light, thin sections as delicate, short, black lines, Avere among 
the phenomena noted. Here and there are feeble spots of a half-glassy, 
globulitic, brownish-gray substance ; but the abundance of imbuing glass, 
characteristic of the former augite-trachyte, is in this variety totally wanting. 
There is no olivine, which expresses the absolute separation of the rock 
from basalts. 

A peculiar trachyte forms the foot-hills north of Nevada Station, 
Truckee Range [290]. It is a brown and somewhat porous rock, appear- 
ing almost half-glassy, and contains many macroscopical biotite plates. 
Under the microscope, all the feldspars, which, although they approach 
the length of l mm , are not visible in the hand-specimens, are discovered to 
belong, with very rare exceptions, to sanidin. In two thin sections, only 
one very small, dark-bordered hornblende section could be detected; but light 
yellowish-green augite crystals were rather numerous. The biotite forms, 
as is often the case, only macroscopical, and no microscopical, individuals, 
so that the amount seen with the unaided eye is not increased under the 
microscope. These individuals are partly altered into a striped aggre- 
gation of small, dark grains, in which they resemble the rubellans in tho 
lavas around the Laacher See. The groundmass is an accumulation of 
colorless feldspar-microlites, pale-green microlites, which are most probably 
not hornblende but augite, black magnetites, and vestiges of glass. There 
is no apatite. 

At the north end of the Kawsoh Mountains, a somewhat rough, brown- 
ish-gray trachyte occurs [291, 292]. All the macroscopical feldspars and 
nearly all the larger microscopical ones are sanidin. Large hornblendes 
are present, but rare, and also a few augites: biotite is wanting. Larger 
feldspars are full of roundish, oval, and cylindrical pores, and unex- 
pectedly numerous, dusty-brownish apatite prisms. The groundmass con- 
sists of feldspar-microlites and very minute, indefinable, dark needles and 
grains. It also bears blood-red laminae of specular iron, which is also set- 
tled in fissures of the rock and on the borders of feldspars, and rather 
numerous aggregations of tridymite. 

The summit of the island in Pyramid Lake is formed of trachyte [293]. 



TBAOHYTE. 149 

It has a fine, reddish-gray groundmass, in which are larger sanidins and 
some plagioclases. The groundmass is a very intimate mixture whose parts 
can only be resolved microscopically with great difficulty. It seems to be 
composed mainly of feldspars, together with which are little black grains, 
black, granulated prisms, and irregular spots, the latter appearing macro- 

scopically in the thin section, and being, without doubt, totally altered brown 

i j 

hornblende. 

A gray trachyte appears in the range south of the Kamma Mountains, 
Nevada [294]. It contains feldspars measuring as high as 3 mm in diameter, 
and hornblende that is entirely decomposed (its border of dark grains being 
preserved) into a seemingly homogeneous, but aggregately polarizing, mass 
of an aquamarine or pale-green color, which might, at the first glance, lead 
one to mistake these sections for serpentinized olivine. This light-green 
product of alteration bears in its fissures dendritic laminse of specular iron, 
which are, therefore, of still more secondary nature. Apatite is present, 
but augite and biotite are wanting. There are no proper hornblende micro- 
lites in the groundmass. The mass contains, beside the feldspar, reddish- 
brown ferrite and black opacite grains, the latter of which, as in many of 
the above-described trachytes, after feldspar, are the chief components. 

At a point twenty -five miles north of Rabbit Hole Spring, Nevada, is 
a somewhat rough but quite fine-grained trachyte [295], in which all the 
feldspars are strikingly pure, and are all, with rare exceptions, sanidins. 
No macroscopical or microscopical crystallized ingredient is imbedded in 
the groundmass except the feldspars neither hornblende, nor mica, nor 
augite; but it contains numerous reddish-brown and brownish-yellow grains 
of ferrite, which surely do not belong to either hornblende or augite, but 
are a kind of substitute for them, in part presenting a chemical equivalent 
to fill their place. The black opacites here are so excellently quadrangular 
that they doubtless must be taken for magnetite. 

At Chataya Pass, Pah-Ute Eange, western Nevada, occur yellowish- 
gray trachytes, with splendid sanidins, sometimes as large as peas [296, 
297, 298], rich in gas-cavities. Quartz is wanting, and there is no proper 
hornblende present. These rocks somewhat resemble the rhyolites in the 

micr( (structure of their groundmass. One variety contains a groundmass 

j & 



150 MICEOSCOPJCAL PETROGRAPHY. 

which consists of undulating-, twisted, and entangled axially fibrous strings 
and bands, between which is a little felsitic substance that is nearly struct- 
ureless but rich in heaps of ferrite and opacite. This type of groundmass 
is as common in rhyolites as it is rare in trachytes. In another specimen, 
this structure is wanting; the groundmass being here, as in most other 
trachytes, an aggregation of feldspar, opacite, and ferrite, which are, in the 
usual manner, accumulated in little heaps with rounded outlines. Horn- 
blende is wanting. 

There is one variety among these trachytes which has been altered by 
solfataric action [296]. In one of its attacked feldspars, the microscope 
discovered even some grains of calcite with rhombohedral cleavage and 
twin-striation parallel to \ R. Hornblende is present, but it is entirely 
altered. It is remarkable that the brown and black grains whose intimate 
accumulation borders the former hornblende, and which usually encircle a 
central, pale-green or aquamarine-colored substance, exactly agree with the 
ferrite and opacite of the groundmass, and are even plainly seen to pass into 
them, the external margin gradually dissolving and becoming looser. If 
we consider that the groundmass is often very fresh and unaltered, and 
also make note of the regular and equal distribution of the ferrite and opacite 
grains through it, it can scarcely be believed that the latter had anything 
to do genetically with decomposed hornblende, of which only alteration 
products were left. There is some titanite. 

Trachytes are found on Coal Creek, Seetoya Range, Nevada [299]. 
A very fresh, light-gray trachyte possesses, beside sanidin, much plagio- 
clase; but it has no other secretions, except very rare, minute laminae of 
biotite. The feldspars are beautifully built up zonally, with excellent zonal 
inclosure-lines of half-glassy grains. The groundmass is nearly colorless 
or very light gray, finely microlitic and granular-feldspathic, containing 
but h'ttle ferrite and opacite. Some titanite is also met with. Another 
variety, from the Eiver Range, near Susan Creek, Nevada [300], is a 
somewhat earthy, pale-reddish, and domite-like trachyte, and presents 
macroscopically some feldspar and biotite. This rather remarkable rock 
possesses a light, globulitic, glassy base, in which numerous feldspar ledges 
almost wholly devoid of striation, and subtil, half-transparent grains or 



TKACHYTE. 151 

needles of a brown and brownish-red color, are disseminated. There is no 
microscopical biotite, hornblende, augite, or apatite. Nevertheless, there 
occur, appearing even macroscopically hi the sections, granular aggrega- 
tions of pale, rose-red, isotrope garnet, in seemingly broken grains, free from 
any interposition, and resembling in all respects the garnets in the Saxon 
granulites, which, as members of the old crystalline schist series, are doubt- 
less of a different geological origin from these Tertiary eruptive trachytes. 
Only in one other case has garnet been observed as an accessory ingredient 
of trachytes, namely, in the Castle Rock from the island of Ischia in Italy. 1 
Rarely but evenly disseminated through the groundmass, are some sharp 
grains which in color are an azure or Prussian blue, measure only 0.0025"", 
sometimes possess a distinct hexagonal shape, and in all probability belong 
to haiiyne: they perfectly resemble those microscopical blue crystals which 
are found well preserved in the sanidins and highly altered in the ground- 
mass of the trachyte from the Pferdekopf in the German Rhon Mountains. 3 
The rock also bears, besides aggregations of tridymite, water-clear portions 
of a wholly isotrope substance, rather strongly refracting, traversed by 
quite irregular cracks, and forming singular spires and denticles: the individ- 
uals of this substance seem to be thick, angular particles of glass, and 
precisely identical with an occurrence which has been observed in the 
rhyolite from the Hohenburg, near Berkum, Rhenish Prussia. 3 

The Wah-we-ah Range is an interesting trachytic region, and a large 
number of specimens from there were examined. One variety [301] is a 
very rough gray rock, with feldspars nearly as large as a pea, biotites, and, 
in spite of its roughness and richness in biotite, two characteristics common 
to the younger group of trachytes, very many plagioclases and predom- 
inating sanidin. The feldspars have the most distinct and well-developed 
glass-inclusions, which are not very common in trachytes. The micas are 
often cleft and broken, as well transversely and longitudinally as parallel 
to the lamellation, often showing bent stripes. They are no longer in 
their original condition, but present externally a loose aggregation of 

dark grains; which extend with a varying width somewhat into the inte- 

_ - _ 

1 J. Itotli, Der Vesuv u. die Umgebungeu von Neapel, 1857, 201. 

2 F. Z., Die mikroskopische BeschaflfeuL. d. M. u. G^st., 386. 
3 lbid., 343. 



152 MICROSCOPICAL PETROGRAPHY. 

rior. The many biotites present look like mere shadows of the mineral. 
Hornblende is altered from its original brown color. These two last-named 
ingredients do not occur in very small microscopical individuals. The 
groundmass has a microfelsitic base, showing here and there a tendency to 
form sphaBrolites, and containing feldspars, ferrite, opacite, and, very rarely, 
the blue haiiyne grains mentioned in a preceding occurrence. These haiiyne 
grains are also included in the feldspars near their borders. There is some 
anatite 

In other greenish-gray varieties from the same locality [302], with 
sanidins 2 mm long and many excellently lamellated and strongly absorbing 
biotites : the rare hornblende is altered into an impellucid gray substance, 
which is seen in reflected light to have an earthy surface. This is a very- 
strange product of decomposition, and it could scarcely be referred to horn- 
blende if the outlines and the directions of cleavage were less distinctly 
preserved. Rocks from the same region which are in other respects similar 
to this, do not contain any hornblende and only a little biotite, therein illus- 
trating the freedom of petrographical modification possible to a single rock- 
mass. Sometimes the brown ferrite corpuscles of the groundmass are 
found to have aggregated into needles and irregularly rectangular forms. 
Many of the groundmasses seem to contain not a few plagioclases in narrow, 
striated ledges. A beautiful trachyte [303] bears very fresh feldspars (sanidin 
predominating), splendid biotite with a black border, but no distinct horn- 
blende, quartz in good hexagonal sections, entirely surrounded by a narrow 
zone of a fibrous sphserolitic nature, only 0.01 mm in width, the limits of the 
zone both on the side of the groundmass and of the sections being sharply 
defined. The substance of these zones was, singularly, confined to the 
peripheries of the quartzes. 

The augite-trachytes are also represented in the Wah-we-ah Mountains. 
A very dark gray rock [304, 305]. with a groundmass that seems to be 
homogeneous, and macroscopically secreted feldspars, for the most part 
plagioclases, shows, in the sections, to the unaided eye, a large quantity of 
greenish-yellow grains, excellent augites rich in glass-inclusions, and biotite, 
but no hornblende. The groundmass is an aggregation of colorless micro- 
lites, imbued by considerable recognizable glass. By this structure, and by 



TKAOHYTE. 153 

the predominance of augite and the comparative richness in plagioclase, the 
rock shows a certain approach to augite-andesites ; but geologically these 
rocks belong to the trachytes. The feldspars contain the most beautiful 
inclusions of colorless or pale brownish glass. There is some apatite. 

The cliffs along Palisade Canon, Cortez Range, Nevada, are formed of 
a thoroughly typical trachyte [306]. The feldspars are nearly all sanidin, 
appearing as dull plates measuring as high as 4 mm in the brownish-gray 
groundmass, which also contains a large quantity of brown biotite lamina?, 
the individuals of which reach the extreme minuteness of only a few thou- 
sandths of a millimetre. There are some pretty thoroughly decomposed 
remnants of hornblende and some apatite, but no augite. In this somewhat 
decomposed rock, the magnetite grains have projecting from them very neat 
dendritical tongues of sesquioxyd of iron. 

A remarkable trachyte occurs in Wagon Canon, Cortez Range [307]. 
It is a yellowish-gray mass, with quite dim and dull, small feldspars and 
laminfB of biotite. Between crossed nicols, the feldspars seem to be covered 
with a glittering dust, partly a product of alteration (probably calcite) and 
partly an accumulation of strange, minute, greenish particles (probably 
hornblende). The feldspars have lost their pellucidity, but it is distinctly 
visible that most of them are simple monoclinic crystals or Carlsbad twins: 
polysynthetic twin-striation can be detected but rarely. It is sure, how- 
ever, that the latter structure is now visible wherever it has existed. So in 
this rock orthoclastic feldspar decidedly predominates, perhaps in a higher 
degree than in most other trachytic occurrences. Nevertheless, hornblende 
in small particles enters largely into the composition of the groundmass, 
which chiefly consists of dull feldspar, and the few tolerably well-pre- 
served crystals show that their original color was green. In nearly all 
these points, the rock exhibits a considerable measure of similarity to 
propylite, and it would be so classed if orthoclase did not unquestionably 
predominate. The biotite has this peculiarity, that quite colorless layers, 
which are probably muscovite, are intercalated between the brown laminse 
of its transverse sections, and that in the darker brown substance of the basal 
sections, poorly defined, colorless spots appear. Some apatite is found. (r /d 

A rock from north of Cave Creek, Humboldt Range, should be men- 



154 MICROSCOPICAL PETROGRAPHY. 

tioned among the trachytes, although it differs from them considerably in 
some points [308]. It is a somewhat rough, gray mass, containing macro- 
scopically very numerous biotite plates,' some feldspars, and here and there 
a grain of quartz : the latter, however, appear almost like strange inclusions. 
The structure of the groundmass is for the most part unmistakably crys- 
talline-granular. The large, dark mica plates are perforated by an enor- 
mous quantity of colorless microlites, part of them showing the most sharp 
hexagonal transverse sections ; and although there may be apatite among 
them, the needles are present in almost too large numbers (one biotite plate, 
0.5 mm in diameter, often containing as many as 40) for referring all of them 
to apatite ; and this theory is strengthened by the fact that independent 
prisms of apatite are abundantly disseminated through the groundmass. 
The feldspars are tolerably fresh, amongst them much plagioclase ; and 
there is some badly crystallized green hornblende. Fine particles of the 
hornblende also enter into the composition of the groundmass : this is a 
peculiarity of propylites, but rare in trachytes. The rock does not contain 
any quartz except that in macroscopical crystals. 

""?,( A more distinctly characterized trachyte occurs on the ridge crossing 
Peoquop Creek, Peoquop Range [309], containing dull feldspars 8 mm long. 
In its prevailing yellowish-gray groundmass, which has a somewhat globu- 
litic, pale, brownish base, are feldspar-microlites and opacite and ferrite 
grains. There is no trace of biotite ; but, there is a dirty, yellowish-green 
product of alteration, the connection of which with hornblende cannot 
be with certainty determined. Feldspars are largely plagioclases, and 
the rock is probably a trachyte of the older division. The small hollows 
of the rock are filled with silicious deposits, which appear macroscopically, 
in the section, with refracted light, as small, dim, white spots. External 
layers of these secretions are of a very fine-grained hornstone ; next comes 
a verrucose zone of coarser fibrous quartz, of which the pike-formed ends of 
the individuals project inwards, and the interior or kernel is composed of 
very finely fibrous quartz, which presents splendid aggregate polarization 
between the nicols. 

The rock from Emigrant Road, north of Palisade Canon, Cortez Range 
[310], has sanidin, which predominates, and a fine microlitic groundmass, 



TRACHYTE. 155 

probably bearing 1 a considerable quantity of glass base, but neither biotite 
nor augite. The hornblende is granulated, and has a brown or black color, 
and is encircled by a dark border, which at its outer margin is disin- 
tegrated into single grains. There are quite a good many aggregations of 
tridymite. 

The trachyte from the southern wall of Palisade Canon [311] resem- 
bles that last described. Some of its transverse sections of hornblende 
are entirely hollow in the middle, being only an empty frame of brown, 
granulated, altered border-material. In one place, the continuity of this 
border is broken; and through this gap the groundmass has entered and 
filled up the whole of the interior (once occupied by hornblende), with its 
microlitic mass, which is here of the same structure and state as the general 
surrounding material; a piece of testimony to the strength of the mechanical 
force which destroyed the hornblende. There are also some sections of 
quite fresh, pale, greenish-yellow augite. In the groundrnass, which con- 
tains some coarser elements, there is, beside the ledges and microlites of 
feldspar, a large quantity of yellowish-brown, indistinctly crystallized, 
crippled, and somewhat fibrous prisms. They measure as much as 0.05 mm 
in length and 0.01 5 mm in thickness. Although undichroitic, they may 
very probably belong to hornblende; surely not to biotite or augite, or to 
any other known mineral. They are chiefly found surrounding in great 
numbers, and often in tangential position, the hornblende lumps, so that 
the latter sometimes seem to be dissolving into the encircling periphery. 
Perhaps they are later hornblende, which crystallized out of the unsolidified 
rock-mass after the larger individuals of hornblende previously formed had 
been attacked and destroyed by the molten magma. 

On the east base of the Aqui Mountains, Utah, occurs a trachyte [312], 
which has a fine, porous, grayish-white groundmass containing black 
biotites and hornblende in quite small prisms. No macroscopical feldspar 
can be detected. Under the microscope, the rock is enormously rich in 
biotite, which forms delicate, lighter or darker, brownish laminae, the most 
regular hexagons, whose single sides are of different lengths, and even 
rhombs: sometimes two thin plates of diverse form are found one upon the 



150 MICROSCOPICAL PETROGRAPHY. 

other. But hornblendes smaller than the macroscopical ones are very 
rare. Much feldspar is scattered through the groundniass in ledge- 
formed and broader sections; so also are comparatively many plagioclases. 
In the groundmass are also some little brownish spots looking like cavi- 
ties filled with dust, rounded or irregular in shape, and always simply 
refracting light. They are exceedingly fine, globulitic, glass-stains, but 
they might very easily be mistaken for nosean, the more because they are 
usually encircled by a delicate colorless zone, which is indifferent to 
polarized light. The greatest diameter of these glass spots is 0.06 mm . The 
combination of so widely isolated glass-particles with purely crystalline 
ingredients (feldspar and mica) is rather uncommon. There is some 
apatite present. 

A very fine trachyte comes from the east end of Traverse Mountains 
[313]. It bears fresh, beautiful feldspars, built up in regular zones, and 
filled with splendid glass-inclusions having thick bubbles; abundant dark- 
brown, entirely unaltered hornblende, having a very distinct cleavage, and 
lacking the dark border; some biotite plates; not a little pale greenish- 
yellow augite, between which and the hornblende there is an excellent 
contrast of color; very much apatite, but no quartz. Most of the feldspars 
are sanidin, but not a few are plagioclases. The aggregation of feldspar 
microlites and magnetite grains constituting the groundmass is impregnated 
throughout with glass base. 

The very characteristic and typical trachytes of the Wahsatch Range, 
which are generally rich in well-developed macroscopical crystals, are similar 
to the last-mentioned specimen. That from City Creek [314] has a rough, 
dirty-gray groundmass, in which hornblende and biotite are included. The 
macroscopical feldspars, among them many plagioclases, are not very distinct. 
There is an abundance of deep-brown hornblende, with a narrow, black 
border which is partly well shaped and partly somewhat rounded. One 
hornblende section was quite pale in the interior, and became colorless 
by gradual passage ; nevertheless, cleavage was evident in it ; and in the 
centre of another a number of gas-cavities were found, also (an exceedingly 
rare phenomenon in hornblende) some subtil fluid-inclusions, with moving 
bubble. Intensely brownish-yellow mica, having a fine black border, is 



TRACHYTE. 157 

present, but it is rarer than hornblende. It often occurs in fragments and 
shivered pieces. The rock also bears excellent pale-greenish augite 
with glass grains, and is remarkable for containing tridymite in an abun- 
dance and distinctness of aggregation only surpassed by the trachyte from 
Cerro de San Cristoval near Pachuca, Mexico, where it was first found. It 
does not properly enter into the composition of the groundmass, being merely 
attached as an incrustation to the walls of microscopical hollows. There is 
apatite, but no quartz. 

Tridymite also appears in the similar trachyte from East Canon Creek, 
Wahsatch [315]. 

A trachyte from the Upper- Provo Cafion, Utah [316], is also very 
rich in crystals of feldspar and hornblende, but is wanting in biotite, augite 
and tridymite. The transverse sections of the larger hornblende prisms 
show that they are beautiful twins ; a line parallel to the truncation of the 
obtuse angle dividing them into two parts, which polarize at the same 
time with different colors. The apatite contains a yellowish-brown dust. 
Around the hornblende crystals, the groundmass shows splendid fluidal 
structure, and contains a large number of sharply hexagonal, blood-red 
plates of specular iron, some of which are as small as 0.003 mm in diameter. 

The brownish-gray trachytes from the divide between Provo and Sil- 
ver Creeks [317, 318] contain sanidin in predominating quantity. A 
plagioclase crystal, which contained, as do the other feldspars, very distinct 
glass-inclusions, had, in the interior of one of its glass grains (0.03 mm in 
diameter), an excellent fluid-inclusion with a moving bubble. This remark- 
able combination of glass and liquid is not unknown in some other rocks ; 
analogous phenomena occurring, for instance, in the leucites of the lava 
from Capo di Bove and from the Solfatara, Italy. The bubble in the fluid- 
inclusion was not absorbed when the thin section was heated up to 120 C. 
Hornblende is mostly blackish and decomposed ; but another greenish- 
gray variety of trachyte from the same locality contained entirely fresh 
and unaltered crystals. This ingredient here shows the most extraor- 
dinary phenomenon of rupture. On the walls of the pores of this rock, 
numerous lighter or darker, Isabel-colored, stalactitic or mammillated, finely 
stratified, secondary, silicious deposits appear. The tops and warts are usu- 



158 MICROSCOPICAL PETROGRAPHY. 

ally still covered with extremely delicate fibres. The same kind of material 
also penetrates through the rock in the form of small veins traversing the 
groundrnass, the feldspars, and even the hornblende. No tridymite exists 
here, perhaps because the substance just described as filling the hollows and 
traversing the fissures plays its usual part. The groundmass contains some 
dark-gray, very fine, globulitic or microfelsitic base. 

Trachytes from the mouth of Silver Creek [319] and from near Kim- 
ball's in Parley's Park [320] also very well represent this general type 
of the Wahsatch trachytes, being rich in crystals of sanidin, plagioclase, 
rather fresh brown hornblende, biotite, often augite, and occasionally tridy- 
mite. In the former specimen, the biotites are perforated by numerous 
apatite prisms, and the feldspar sections are richly set with glass-inclusions. 

Another characteristic trachyte of a brownish-gray color occurs on 
the divide between the North and Middle Parks, Colorado [321]. The 
beautiful, lucid feldspars, formed in regular zones, are mainly sanidins in 
simple individuals and Carlsbad twins. Sharp, yellowish-green augite and 
brown hornblende with a black border, occur together. The augite, rather 
than the hornblende, predominates. There is some biotite and dusty-brown 
apatite. The groundmass is an aggregation of microlites, with grains and 
needles of opacite and ferrite. This rock is in an almost entirely unaltered 
state, and presents excellent fluctuation phenomena. 

A second trachyte from this same region [322] is not unlike that 
last described. Like the above, it bears predominating sanidin, but also 
has a great deal of green augite. The hornblende present is in a much 
smaller quantity, and is generally found in the form of loose aggregations 
of black grains and dark-brown needles, which show more or less of the 
original hornblende contours. Small, but thick, yellowish-brown, indis- 
tinctly crippled prisms, which are entirely undichroitic, are disseminated 
through the groundmass in considerable abundance : they may be related 
to hornblende, but this is uncertain. Biotite is present. The groundmass 
contains colorless, dazzling, and cracked angular glass-grains, like those 
found in the trachyte from the Aqui Mountains, Utah (see page 156). 

Before concluding this section, the singular quartz-bearing trachytic 



TRACHYTE. 150 

rocks of the Elkhead Mountains, a part of whoso composition is very 
remarkable, must receive attention. 

The rough, gray trachyte from Skellig's Ridge, Elkhead Mountains 
[323], bears sanidin, hornblende, and biotite, accompanied by some grains 
which are doubtless quartz, distinctly visible to the naked eye. Nevertheless, 
the whole habitus of the rock is trachytic, rather than rhyolitic. And all 
those quartzes which are highly cracked, much fissured, split apart, and 
burst asunder, possess more of a dull greasy than a bright glassy lustre : 
they are rounded grains which easily drop out of their places, leaving little 
hollows. The highest magnifying power does not discover any more quartz 
than that visible to the unaided eye ; and while what is present may not 
properly be designated as a strange erratic body, it is in every case unim- 
portant and purely accessory, and does not at all influence the aspect of the 
rock : it is, in short, of no more significance than the presence or absence 
of tridymite in a trachyte. Under the microscope, this rock is extremely 
rich in small crystals of hornblende, and even richer in brown mica. It 
also bears not an inconsiderable quantity of pale, yellowish-green augite. 
The apparently homogeneous groundmass is composed of feldspar micro- 
lites, very small prisms of augite and hornblende, and minute biotite plates, 
all imbedded in a pale brownish, somewhat globulitic, amorphous base. A 
remarkable fact is that the quartzes are immediately surrounded by a zone 
of the most delicate and tender, pale-green spikes or needles, probably 
augite, gathered in a very intimate but confused aggregation, and appearing 
in the sections like a green ring of a prickly felt. That the substance of 
the quartzes is not a secondary infiltration into preexisting cavities, is proved 
by their sometimes containing splendid glass-inclusions. The same kind of 
microlitic ring also encircles the quartzes of the trachytes from the summit 
of Crescent Peak, Elkhead Mountains [324]. When, in preparing the 
section, the quartzes fall out, this ring keeps its place as a sort of frame in 
the cavities, and shows where the quartzes have been. 

A rough, reddish-gray trachyte from the summit of Whitehead Peak, 
Elkhead Mountains [325], is still more remarkable. It presents, beside san- 
idin, very many cracked quartzes as large as a pea, hornblende and augite, 
and, what is remarkable, not very numerous but doubtless characteristic 



1GO MICROSCOPICAL PETROGRAPHY. 

half-serpentinized olivines, the sections of which, measuring as high as 
0.75 min , are visible even to the naked eye in the slides. The peculiar quartz 
occurring here is, therefore, accompanied by a mineral which has never 
before been observed in a sanidin rock. Perhaps the explanation of the 
formation of this uncommon quartz out of a rock-magma of trachytic 
constitution may also account for the presence of the olivine in the same 
mass. It almost seems as if the uncalled-for secretion of free silica had 
been counterbalanced and neutralized by the contemporaneous production 
of as basic a mineral as the olivine. 

Another highly interesting quartz-bearing trachyte occurs at Steves' 
Eidge [326]. It bears remarkably good sanidins, measuring more than 
an inch in length. In other respects than the size of its sanidins, also, this 
rock strongly resembles the famous trachyte from the Drachenfels, on the 
Rhine; especially when one observes, with astonishment, a quantity of 
quartz grains the size of peas, which are riven by multitudinous cracks, and 
look glassy. The large, imbedded individuals of feldspar are especially 
remarkable, because, in spite of the proper and natural physical behavior 
of the sanidin, they possess crystal faces which we have been accustomed 
to observe only in the old, compact and dull orthoclases of the porphyritic 
granites, or in those of some felsite-porphyries. The crystals where the 
trachyte is somewhat decomposed, can easily be loosened and removed 
from the rock-mass, presenting the faces T (oo P) z (GO 3) ; M (oo -i? oo ) : 
P(OP); y(2-<x>); even n (2 i? GO). Sanidins of like richness of crystalli- 
zation have never been found in trachytes. Beside the sanidin and quartz, 
macroscopical black biotite is also present in the whitish-gray groundmass. 
Some plagioclase is dicovered with the microscope. The mica often has a 
black border. Hornblende is rare, and augite is entirely wanting. As usual, 
microscopical quartzes could not be detected. The larger quartz grains bear 
beautiful glass-inclusions. There are some thick titanites, and also apatite 
prisms. The groundmass is chiefly composed of feldspathic particles. A 
thin section was made of one of the large sanidins ; which thus prepared, 
showed an almost perfectly water-clear mass which looks at first sight 
somewhat homogeneous, but the microscope and polarized light prove the 
contrary. It contains other smaller, differently situated feldspars, simple 



TRACHYTE. 161 

sanidins and Carlsbad twins, and some (much rarer) striated plagioclases. 
None of these bodies appear distinctly before polarized light is used. 
The sanidin also exhibits excellent hexagonal and rhombic sections of 
pure and homogeneous quartz, measuring as high as 0.5 mm in diameter, 
and polarizing with intensely brilliant colors. Sometimes the quartzes are 
broken and the pieces separated by the sanidin substance ; but the frag- 
ments lie so close together that it seems as if they might easily be put 
together again, and each individual made complete. Lastly, the sanidin 
contains some groups of pale-green, sometimes dismembered microlites, and 
a quantity of empty cavities, but neither glass, nor fluid-inclusions. 

The trachytic rock from Camel Peak, Elkhead Mountains [327], 
also bears quartz with a green ring around it and olivine; but it has no 
large sanidins. Seen under the microscope, augite predominates ; but 
there is only a little hornblende, some biotite and much magnetite. This 
rock, surely of a more basic composition than the others, therefore resembles 
the basalts. Perhaps it is pretty closely related to the quartziferous basic 
rock [328] from the benches along the Upper Little Snake River (see Basalts). 

A trachyte from the Little Snake River, Colorado [329], has a dark, 
bluish-gray, seemingly homogeneous groundmass, in which are a great 
number of cracked quartzes of the unusual size of a hazel-nut, very glassy 
sanidins, and large, light brownish biotite plates. Under the microscope, 
considerable augite appears; but there is no distinct hornblende, and olivine 
is wanting. To this rather poorly characteristic group belong the rocks 
from the South Shoulder of Crescent Peak [330], and from Hantz Peak, 
Elkhead Mountains [331]. The latter, of a brownish-gray color, is some- 
what decomposed. One of these eastern trachytic rocks, which forms 
the mouth of Slater's Fork, Elkhead Mountains [332], contains 'what is 
most probably nepheline. It is a yellowish-gray mass, of which the only 
macroscopical ingredient is long stripes and rays of yellowish-brown mica 
similar to that in the interesting leucite rocks to be described hereafter. 
Under the microscope, very little striated feldspar can be detected, but 
considerable monochromatically polarizing sanidin is discovered; and the 
instrument also reveals a colorless mineral having short, sharply rectangular 

sections, which have sometimes become somewhat fibrous on the borders; a 
11 MP 



162 MICROSCOPICAL PETROGRAPHY. 

phenomenon strongly characteristic of decomposing nepheline. But hexa- 
gons of this mineral could not be found in sufficient distinctness to make the 
identification of it sure. When powdered and treated with hydrochloric 
acid, the rock very soon secretes flocculent silica; and the inference of the 
presence of nepheline based upon the microscopical examination is thus 
coiToborated. Neither hornblende nor quartz enters into the composition 
of the rock; but comparatively numerous, beautiful augites, and occasional 
olivines, are found. It is a remarkable fact that this sanidin rock, in all 
probability containing nepheline, and the nepheline-bearing basalts from 
Fortification Peak, Upper Little Snake River, Yampah River, etc., occur in 
general not far from leucite rocks. 









RHYOLITE. 163 



SECTION II. 

EHYOLITE. 

The scope of this section will be confined to a description of the 
proper felsitic or porphyritic rhyolites; for the almost granitic rhyolites 
(nevadites) are wanting in the examined territories, and the chemically and 
geologically identical glassy rocks (hyaline-rhyolites), like pearlite and 
obsidian, will be treated in a section devoted especially to themselves. Of 
all rocks, these rhyolites most excel in variety and diversity of microscopical 
structure ; and since better facilities for investigation than had ever before 
been enjoyed, were furnished in this case by the extraordinary number of 
occurrences at hand, it is highly probable that the following pages will be 
found to explain all, or nearly all, the most characteristic types of which 
the rhyolitic structure is capable. Particular attention has been paid to these 
interesting varieties, examples of which will doubtless be found in studying 
the comparatively unknown rhyolites of other countries. 

Proceeding from west to east, the first occurrence is found on the west 
side of American Flat Canon, Washoe [333]. It is an excellent specimen, 
and represents one of the most widely spread and characteristic types. Its 
color is a pale yellowish-gray, and it is apparently an almost perfectly 
homogeneous, felsitic mass. Beside its microscopical crystals, the rock 
under the microscope is found to be composed, first, of a light sphserolitic 
material, and, secondly, of lines and strings of dark grains. These latter 
appear in the slides as short, fathom-like, bent, and undulating stripes, made 
up of fine, dark-brown, closely aggregated grains. As usual, the undulations 
of these lines here produce a very distinct microfluidal texture, and wind 
around larger and smaller microscopical crystals, imparting to them the 
appearance of eyes. Generally, these stripes have, along their length, short, 
ciliated, or prickle-like hairs, which, for the sake of illustration, may be 
compared to thorns upon a stem, consisting of very minute, lineally grouped 
grains, growing finer towards the projecting end, so that they taper to a 
point. The spaces between these curved grain-stripes are now occupied by 
the sphaerolitic substance (see Plate VIII, fig. 1, which refers to another 



1G4 MICROSCOPICAL PETROGRAPHY. 

rock of precisely the same structural type). The sphscrolites are colorless 
and more or less distinctly fibrous. Often a granular composition of the 
single^ radiated fibres may be observed, particularly in the larger ones. 
The sphserolites are but feebly affected by polarized light, giving between 
crossed nicols merely an indistinct shimmer, which seems to be some- 
what more intense in the granulated fibrous members. But this is cer- 
tainly not such chromatic polarization as is proper to really crystalline 
bodies, even of the most extreme minuteness ; so we surely have here to do 
with an indistinctly crystallitic and imperfectly individualized substance. 
The dark strings do not pass through the sphserolites, but run between them, 
marking the outlines of the individuals: because of their fluidal structure, 
the brown grain-lines have in general, excepting their undulations and cur- 
vatures, a parallel direction. The sphserolites do not have the usual rounded 
shape, but are also for the most part drawn out lengthwise ; and very many, 
if not by far the greater part, of the sections must be derived more largely 
from longitudinal, cylindrical clubs, fibrated axially or concentrically, than 
from globular balls. Viewing the whole mass, the conclusion is that the 
sphserolites do not generally appear isolated, and several of them, or some of 
the rudimentary stages thereof, are often found developed in close connection 
between the dark strings ; so that the section of one sphserolitic spot some- 
times presents a number of centra, or axes of attraction, towards which 
the fibres tend. Fine hair-like or prickle-like appendages, attached to the 
surface of the grain-lines, protrude into adjacent sphserolites ; and the ap- 
pearance of the whole mass gives an impression that the intricate system 
of line-strings was formed first, and that subsequently included sphsero- 
lites were developed. As in individualized elements, so in this rock occur 
colorless, fresh feldspars not exceeding O.G mm in length, most of which 
belong to sanidin ; and, in one of them, unmistakable fluid-inclusions, with 
moving bubbles, was detected. In the light of all previously known upon 
the subject, this phenomenon of a liquid-inclusion in a rhyolitic feldspar at 
first seemed to us very strange, but it was frequently observed in other 
feldspars of these American rhyolites; and it was doubtless merely chance 
that liquid-inclusions were never before found in rhyolitic feldspars, for 
they have long been known to occur in the plagioclases of nearly contem- 



KHYOL1TE. 165 

poraneous basalts. The rock contains but very little quartz, and biotite is 
present only in the form of microscopical laminae. Some indistinct sec- 
tions which were noticed appeared to be decomposed hornblende. There 
were a very few opaque, black magnetite grains, but no augite. A charac- 
teristic feature of the rhyolites observed in this variety is the total absence 
of microlites. The crystalline ingredients do not appear in very great micro- 
scopical minuteness, an enlargement of 60 showing all that are present. 

West of Spanish Spring Valley, a rhyolite is found, forming a dike in 
granite-porphyry [334] , externally resembling the former, but having an 
entirely different microscopical structure. The strings of dark grains so 
abundant in the other, are wholly wanting here, and the main body of the 
mass is a true microfelsite (see page 3); an unindividualized substance 
which is neither homogeneous glass, nor an aggregation of single crystal- 
line particles discernible in ordinary or polarized light. It becomes quite 
characteristically dark between crossed nicols; indistinct shimmers, aa 
from pin-points, rarely appearing. Very minute, dark grains which are 
probably opacite, are sparingly disseminated through this feebly gray, 
almost colorless, typical microfelsite. Here and there a tendency to form 
fibres is seen ; the rudiments observed being always rough, and more fre- 
quently arranged along longitudinal axes than around a centre. There are 
no crystalline secretions at all, except a very few small feldspars : this is a 
general rule with microfelsitic masses. Microlites also are wanting. 

The region of Truckee Canon, Virginia Range, is very rich in rhyo- 
lites, which differ somewhat in macroscopical but still more in microscopi- 
cal structure. On the foot-hills at Sheep Corral Cafion is a reddish-brown 
rock [335], in which dull, milky feldspars and small quartzes can be detected 
with the unaided eye. The thin section appears, under the lens, like finely 
mottled marble, having reddish and colorless spots. In general, this variety 
is the same as the Washoe rhyolite first described; but, between the 
more pronounced and axially fibrous groupings, those thin, line-like strings 
common to the other are wanting, their place being taken by broader, 
brownish-red stripes and bands, which also consist of little grains. The 
aggregation forming these stripes is probably imbued with some glass. 

These stripes are sometimes shorter than the strings of the other, and are 

<nj 



166 MICROSCOPICAL PETROGRAPHY. 

often extraordinarily bent, winding like manifoldly twisted veins through 
the surrounding mass, or woven together in the form of a net. The 
nearly colorless fibres are sometimes distinctly seen to be composed of 
granular particles, more particularly on their thicker, outer ends. The 
number of quartzes visible macroscopically is not at all increased under 
the microscope. They are well shaped ; and contain even macroscop- 
ically the most excellent sphaerolitic particles, having a divergent fibration 
like the fibres of a quill: the quartzes also bear, beside these, splendid 
glassy but no liquid-inclusions. All the feldspars are sanidins (which fact 
pronounces the rock not a dacite), and they present a very remarkable 
microstructure. While naturally inclining to think that their dull aspect is 
produced by molecular alteration, one observes with astonishment that their 
quite fresh and clear mass is thickly filled with empty, round and cylin- 
drical cavities, and, surprisingly, with countless small, but for the most 
part rather distinctly recognizable, fluid-inclusions, containing moving 
bubbles. These interpositions give the rock its milky, opaque appearance. 
It is certainly very curious that the feldspars of this volcanic rhyolite 
should be as rich in liquid-inclusions as the orthoclases in granites or crys- 
talline schists ; and that, in striking contrast, its quartz bears only glassy and 
its feldspars only fluid-inclusions. Biotite, hornblende, and microlites are 
wanting in this rock. 

A collection of rhyolites obtained from the railroad-cut, Truckee Canon 
[336], well exhibits the great diversity of characteristics often found in differ- 
ent rhyolitic groundmasses. There are some in which the above-described 
brown grain-strings appear; the included roundish or angular portions 
being very well fibrated. The individual members of these roundish or 
angular portions consist of several systems of fibres arranged close against 
one another. Where the dark, granular lines run approximatively parallel 
for some distance, there the intervening mass becomes, not concentrically 
and radially, but longitudinally and axially, fibrous; and the effect is very 
beautiful where the parallel fibres, extending from the strings, meet in the 
middle and form a real linear suture. Aggregations of fibres evenly 
arranged along a linear axis like the calcareous fibres in a belenmite or in a 
stalactite, and which are more or less cylindrical in form, in general fill an 



BHYOLITE. 167 

important place in the rhy elites; and it is the more necessary to lay stress 
upon this phenomenon, because it has never before been described, and 
because it is coordinate with the long-known fact of sphserolites possessing a 
concentric radial structure. In the latter, the attraction acted from a centre, 
apparently with equal force, in all directions; while, in the longitudinal 
or axial bodies, it acted along a line; the substance, however, being the 
same as in the first instance was employed in the formation of sphaero- 
lites. This newly discovered manner of arrangement (see, for example, 
Plate VII, figs. 1, 4) might be named axiolite. It is remarkable that in 
the older felsite-porphyries, in which thoroughly typical spha3rolites are 
often found, axiolitic formations do not seem to play the same part as in 
rhyolites. 

Other rhyolites from the same railroad-cut present only indistinctly 
visible grain-strings; these strings or lines being often replaced by 
irregularly disseminated grains of opacite and ferrite. In these cases, 
the fibrous material, which develops from a microfelsitic substance, is 
distributed without any order, or else the groundmass is an indistinctly 
granular material, which becomes feebly fibrous in some places. But this 
groundmass is not at all crystalline-granular. It does not show any such state 
of development, as, for instance, the groundmass of most felsite-porphyries, 
which is in fact granitic and of the opposite type. There are no distinctly 
outlined grains here, all blending together in ordinary and polarized light, 
with the exception of some better-individualized particles which shine forth 
from the mass between crossed nicols. Rocks of this description contain 
only a very few of the larger crystalline secretions. In one variety, 
manifoldly undulating and curving bands of the undeveloped granu- 
lar material, varying in width, alternate with others which are rather 
distinctly crystalline-grained, the particles being very fine. The course of 
the latter, and the contrast of the two, however, are not easily visible 
without the use of polarized light. Between crossed nicols, the last- 
described mass is found to be an aggregation of vividly polarizing grains. 
All the feldspars in this specimen [337], measuring as high as 3 mm in 
length, are sanidins, and contain inclusions which (although one could not 
positively pronounce as to the mobility of the bubble), judging from their 



108 MICROSCOPICAL PETROGRAPHY. 

whole aspect, are of a fluid nature. The quartzes are rare, and they con- 
tain the most beautiful hexagonal inclusions of glass but none of liquid. 
There are a few thick grains of magnetite, but no biotite. 

In the ravine north of Truckee Road, between Glendale and Clark's 
Station, a light-gray rhyolite occurs [338]. Viewed macroscopically, it is 
a quite homogeneous rock, devoid of crystalline ingredients. The proper 
base is a microfelsitic substance, in which, nevertheless, numerous polarizing 
grains are disseminated, in some spots so abundantly that they produce a 
nearly angular aggregation. The rock contains many accumulations of 
tridymite, which are mostly oval in shape, together with isolated, splendidly 
fibrous and well-rounded sphasrolites that have a strong action in polarized 
light. 

Tridymites and sphaerolites were also found in another specimen from 
the same locality [339], of which the groundmass has the same structure 
as the other. The mass consists of a striped and spotted mixture of a pale 
brownish and a colorless substance in which very short, light and dark, 
prickle-like microlites are imbedded. In polarized light, between crossed 
nicols, the brownish mass appears dark over nearly its whole extent, being 
a real microfelsite ; while the colorless material, appearing in ordinary light 
to be homogeneous, is found to be an aggregation of polarizing granular 
particles. 

An entirely different type of structure is found in a rhyolite from 
above Clark's, Truckee Canon [340]. It is a light brown rock with a 
somewhat resinous lustre, and contains sanidins. Under the microscope, 
it is seen to be a glass-bearing mass, rich in crystals. The feldspars and 
microscopical quartzes are extremely rich in light brownish-yellow glass- 
inclusions, which have often become confusedly fibrous, but nevertheless 
contain a dark bubble. The groundmass is a felt-like aggregation of indis- 
tinct microlites, charged through and through with brownish-yellow glass. 

The rhyolite from Purple Hills, Truckee Ferry, possesses the micro- 
structure which is rather common in that region [341]. In color, it varies 
from a brownish-red to a brick-red. Its groundmass is a typical light-gray 
microfelsite, containing some indistinctly polarizing spots, and it has a very 
great quantity of fine grains of reddish or brownish-black ferrite and opacite 



KHYOLITE. 169 

disseminated through it, which are often crowded so close together as to 
produce rather distinct, curving, fluidal lines. Feldspars l mm in length show 
plainly, under the microscope, a partial alteration into carbonate of lime; the 
newly-formed calcite within the feldspar presenting not only the rhombo- 
hedral cleavage of the latter, but even the twin-lamellation after R 
Similar phenomena were to be observed in the feldspar crystals. There is 
some rather milky and dull biotite, but no quartz. 

Berkshire Cafion, Virginia Range, is a most excellent region for 
rhyolites. The varieties [342, 343, 344, 345, 346, 347, 348, 349] are 
chiefly gray, but in part somewhat reddish, and are usually veiy rich 
in quartz, and often extremely porous, resembling the so-called millstone- 
trachytes from the environs of Schemnitz, Hungary. They abound, like 
the latter, in little veins of a brown, jasper-like substance, and in other 
silicious concretions. All the rhyolites from this vicinity agree pretty well 
with each other, but they differ somewhat from those heretofore described. 
In ordinary light, their groundmass is apparently composed, for the most part, 
of little colorless grains, and polarized light proves that it is indeed chiefly 
constituted of angular, feebly double-refracting particles ; a structure which 
is not very common in rhyolites. When arranged in single bands, as they 
sometimes are, the grains become a little larger; and these coarser crystalline 
stripes present here and there signs of fluidal structure. Nevertheless, there 
run through the groundmass linear arcs, half rings, and perfect circles, 
composed of the very finest black grains densely strung together, compara- 
ble in their direction with the roundish cracks which the pearlite sections 
offer, by reason of their shaly, globular composition. Faint little fissures 
were sometimes observed running in the middle of these curved, granular 
lines. Usually, the dark grains are confined to the lines, and are not gen- 
erally scattered through the rock. These bent lines are often accompanied 
on either side by a narrower or broader zone, which has a peculiar modifi- 
cation of the prevailing groundmass, a variation of grain-dimension (for 
instance, a somewhat coarser-grained substance), or a development of fibres 
the most of which are arranged axially along the lines. Small sphaerolites 
are occasionally found in these rhyolites. Rings of sphjjerolitic fibres are 
often seen partly or wholly surrounding smaller quartzes and feldspars, 



170 MICROSCOPICAL PKTJIOGliAPllY. 

giving them the appearance of being set in frames. It is possible that some 
glassy or niicrofelsitic base is present in the granular aggregation of the 
groundmass ; but this cannot be decided with anything like certainty, on 
account of the fineness of the composition of the mass. Arms of the ground- 
mass intrude into the quartzes, which are pretty well formed, and their 
outlines sharply defined ; and they contain a few excellent glass-inclusions. 
The feldspar is partly plagioclase. The macroscopical sanidins are for the 
most part dull ; which, as the microscope shows, is produced by a great 
quantity of interpositions so extremely minute that their nature cannot be 
discovered even with Hartnack's immersion No. 10. They would seem to 
be fluid-inclusions and gas-cavities. Comparatively large fluid-inclusions 
were observed in the feldspars of one of these Berkshire Canon rhyolites. 
Here they measure O.OOG mm , but they are decidedly rare. The plagioclase has 
given rise to the formation of carbonate of lime, which in some cases has 
been transported from its native place, and deposited in cracks of the ground- 
mass. It may not be a mistake to suppose that the porosity of these rocks 
is produced by the far-advanced decomposition of a pail of the feldspars. 
Altered hornblende is present, but very rare. Tridymite and biotite are 
wanting. 

Southeast from Wadsworth, three brownish rhyolites, appearing to be 
half-glassy, were collected. They possess interesting individual peculiarities, 
but there is an unquestionable likeness between the three. All contain 
pearlite flaws, or narrow, dark, granular lines, which are at times rounded, 
semicircular, oval, and irregularly formed, traversing a glassy mass, giving 
the appearance of a net- work. In one variety [350] (Plate VI, fig. 1), the 
glass-mass is nearly colorless, and the cracks are bordered on either side 
by a narrow zone of true microfelsitic substance, varying from light to dark 
brown. Thus a manifoldly entangled vein traverses the colorless glass, and 
gives it the appearance of running hurdle-work. The cracks and their micro- 
felsitic walls are often cut in obliquely, and so the substance of the latter 
seems to be broader than it really is ; the thickness of one lateral zone not 
exceeding 0.008 mm . The microfelsite is quite isotrope, like the glass into 
which it gradually passes. By the use of the right focal distance, the chasms 
of the cracks may be observed on the surface of the thin sections. Often, 



KHYOL1TE. 171 

however, the cracks seemed to be cicatrized. There are no crystalline 
ingredients except the feldspars, which are clear and pure, and have many 
cleavage-fissures. So, in its microstructure, this rock in general resembles 
the well-known pitchstones from Meissen, Saxony; in the latter, however, 
the substance of the zones bounding the cracks on either side has a better 
crystalline development. It seems that the formation of the microfelsitic 
mass happened during the solidification of the rock along the previously 
opened cracks, instead of being produced in the lapse of time by a molecular 
alteration of the glass. In another of these three varieties [351] (Plate VI, 
fig. 3), the same figures are produced by faint, dark, granular lines, which, by 
their fluidal running, form a net with a multitude of meshes of a long-oval 
shape. It is impossible to determine with accuracy whether or not, in the 
midst of these lines, an extremely narrow crack is present. Sometimes it 
would seem that there is such an one. The lines surround pure glass spots, 
forming manifold undulations and curves ; the interior being brownish- 
yellow, gradually growing paler and paler towards the outlines, where they 
come in contact with the dark lines. Here, also, there are no crystalline 
secretions but feldspars, which latter are often broken into pieces; the single 
fragments lying close together. The third rock from the same locality [352] 
(Plate VI, fig. 2) has the same net of dark, granular, fluidal lines. But the 
included glass, pale yellowish-brown portions of which represent the meshes 
of the net, is not a homogeneous substance, but distinctly fibrous. The 
fibres of the single oval or roundish portions are at times concentrically and 
again axially grouped. Although the fibration is quite distinct, polarized 
light has but a very feeble effect Here also the brown color becomes decidedly 
pale towards the dark lines. Some of the parts, and among them the smallest 
ones, have not become fibrous, but remain pure, homogeneous glass. There 
are very few crystalline secretions : sanidins, rare quartzes, and in one slide 
two unmistakable augite sections were observed. 

The specimen from Haws' Station on the Carson River and the outcrop 
in the valley southeast of the Station [353, 354], are two very similar 
gray rhyolites, rarely containing feldspars. The groundmass is a mixture 
of fine, polarizing particles, and colorless glass, the latter appearing, under a 
high magnifying power, in considerable abundance and imich distinctness 



172 MICROSCOPICAL PETRCGKAPIIY. 

between the particles. Somewhat coarser-grained aggregations appear in 
some spots, where the individual,doubly refracting grains have a diameter of 
about 0.008""". Brown hornblende prisms occur rarely, and so also do 
richly laniellated microscopical biotite plates. Almost all the macroscopical 
feldspars are sanidins in excellent Carlsbad twins. 

A fine series of rhyolites outcrop in the vicinity of Pyramid Lake. 
The most remarkable occur in Astor Pass, between Honey and Pyramid 
Lakes [355]. This beautiful rock resembles the rhyolite from the Esterel 
Mountains in the south of France. It is unusually rich in large crystals ; 
bearing quartzes the size of a pea, feldspars half an inch long, small biotite 
plates and hornblende prisms not as large, with a small proportion of a light- 
gray groundmass. Under the microscope, the latter is almost wholly crys- 
talline-granular ; and it is comparatively so coarse-grained that feldspar 
and quartz particles seem to be distinguishable. This entirely crystalline 
microstructure of the groundmass, which bears some relation to the macro- 
scopical one of nevadite, is very rare in rhyolites. The microscope adds 
another to the crystalline rock-constituents, namely, apatite. 

The yellowish-gray, somewhat rough, and porous rhyolite, from the 
west shore of Pyramid Lake, Virginia Range [356], is a totally different 
variety from that last described. It looks like a quite homogeneous rock, 
containing macroscopically only extraordinarily small feldspars; and it has 
no microscopical quartz, biotite, or hornblende. The groundmass is a 
microfelsite, with imbedded, feebly polarizing grains and very unevenly 
outlined feldspar ledges; the latter chiefly Carlsbad twins, but partly 
triclinic. The microfelsite has become indistinctly fibrous in stripes, which 
undulate through the groundmass, and produce fluidal phenomena visible 
to the naked eye in the hand-specimens. 

A brownish-gray rhyolite from the ridge at the head of Louis Valley, 
Nevada [357], is similar to the last-described rock, except in color. Its 
microfelsitic base contains colorless, feebly polarizing grains, together with 
dark granules of opacite and ferrite. In the groundmass, and doubtless 
developed out of it, narrow, longitudinal, brownish-yellow bodies appear, 
most of them curving like a paragraph-mark, and having a more or less 



EHYOLITB. 173 

distinct axial fibration. Sometimes a series of thicker ferrite grains runs 
in the midst of these tails, more distinctly marking their axially fibrous 
structure. The microscopical aspect of the rock very much resembles 
that of one to be described hereafter, (represented in Plate VII, fig. 4). 
Neither quartz nor hornblende is present, but there are many microscopical, 
splendidly lamellated, brownish biotite plates, which are split and shivered 
into single leaves in the most extraordinary manner, the folia being often 
bent (Plate VI, fig. 4). 

In strong contrast with this, the brownish rhyolite from Mullen's Gap, 
Pyramid Lake [358], contains nearly as large a quantity of macroscopical 
quartzes as of the splintery groundmass. The quartzes bear a few unusually 
beautiful, light-brown, dihexahedral glass-inclusions, in some of which a little 
green microlite is present. In some cases, the microlite passes entirely 
through the inclosure, as if it were the axis of the little body. The ground- 
mass is principally a microfelsitic base, within which a large quantity of 
brown ferrite and dark opacite grains have been grouped into rounded 
lumps, and the base has become, at intervals, somewhat fibrous. Inclusions 
of brown glass in the quartzes probably date from the time when the separa- 
tion of the groundmass into light microfelsitic and dark granular matter had 
not taken place, and when the rock-magma, solidifying homogeneously, 
still produced a light-brown glass. 

Well-developed axiolites (linear aggregations of axially grouped fibres), 
in color nearly a reddish-yellow, are found in a brownish-Bed rhyolite from 
the immediate neighborhood of that last mentioned [359]. Here they are 
generally much curved and bent, presenting S-formed, semi-circular, and 
even horseshoe-like figures. 

In the hills east of Winnemucca Lake, Truckee Range, is a beautiful 
yellowish-gray rhyolite [360], containing quartz and feldspar which are 
almost devoid of foreign interpositions. Here also the groundmass is a 
microfelsite, having little polarizing grains imbedded in it. An alteration 
of lighter and darker, or of more homogeneous and more confusedly fibrous 
bands, has produced macroscopical fluidal drawings. 

The brownish rhyolite from the southern end of the Forman Mount- 
ains, Nevada [361], is more crystalline, and contains many grains and 



174 MICROSCOPICAL PETROGRAPHY. 

little, short clubs of ferrlte, some black-bordered biotite, and what seems 
to be altered hornblende. 

Rhyolites occur in the Forman Mountains, west of Cold Springs [362, 
363], with secreted feldspars and quartzes, in a single thin section, of which 
the groundmass is in one place of a finer crystalline-granular structure, and 
in others rich in quite light-brownish glassy particles. 

The light-colored rhyolites from the Black Rock Mountains are in 
strong contrast with (for instance) those of Berkshire Canon, Virginia 
Range, very poor in macroscopical and microscopical secretions, few of 
them containing any proper crystalline element at all. This is the com- 
position, too, of the rhyolites from Utah Hill [364], Ruby Canon[ 365], 
Hardin Mountain [366], and Star Canon [367]. The groundmass (Plate 
VII, fig. 1) is of a medium character, between that which is so 
extremely fine-grained as to be indistinct, and the microfelsitic; and it 
contains straight, curved, and almost circular stripes, composed of thicker, 
colorless, cuneiform grains placed axially along a central line, a section of 
which therefore shows two series of roughly wedge-formed grains, with a 
distinct suture running between them. Bodies of these granular axiolites, 
which are closely related to the fibrous ones, are very common in the 
groundmasses of rhyolites. They are more strongly affected by polarized 
light than fibrous axiolites or fibrous sphserolites. 

A rhyolite from Snow Storm Canon, Black Rock Mountains, con- 
tains many spots of colorless glass in a net-work of entwined strings, 
composed of axial fibres or cuneiform grains [368]. Some quartz crystals, 
which are partly broken, are each traversed by one of the little strings. 
Where the strings, or stripes, or cylindrical forms of axial structure, are cut 
transversely, there the section, of course, looks like that of a little, concen- 
trically radial sphaerolite. 

Another rhyolite from Snow Storm Ledge [369] possesses the same 
general microstracture as its neighbor, but it includes very small fragments 
of strange rhyolitic particles. The latter, with their sharp outlines, very 
distinctly contrast with the main mass, and are of a very fine-granular or 
microfelsitic structure; and they, together with angular, broken pieces of 
quartz, sanidin and plagioclase, and numerous black grains, seem more 



EHYOLITE. 175 

like foreign inclusions than original crystalline secretions of the rock. The 
dirty, light-gray specimens do not show macroscopically anything of this 
partially microplastic composition. It is evident, however, that the rock is 
not a real tufa, but a massive rhyolite, which has taken up vagrant splinters, 
grains, and scraps of other varieties with which it has come in contact. 
These strange fragments are also wanting in other specimens of the same 
locality [370, 371]. 

The Pah-tson and Kamma Mountains are rich in rhyolites, character- 
ized by a greater quantity of dark biotite than any described in the 
foregoing pages. The hills north of Rabbit Hole Spring consist of a very 
distinctly lamellated rhyolite [372]. The mass contains hardly any secre- 
tions, and is neither microfelsitic nor properly crystalline, even differing 
from that of most felsite-porphyries. It is constituted for the most part 
of very small and indistinct, uncertain grains and confused, short fibres, 
which have a rather feeble optical action. In this almost colorless mass, 
larger and better-polarizing grains (about 0.02 mm in diameter) are gath- 
ered in little heaps, contrasting with the rest in ordinary light by their 
yellowish-gray color, and between crossed nicols they appear very 
distinctly. The faint fluidal lines which give the rock the appearance of 
being lamellated, are bands, stripes, and layers of these densely crowded 
heaps, with their better crystalline development. 

A rock from the north end of the Kamma Mountains is veiy similar, 
possessing the crystalline grains in better development, polarizing so dis- 
tinctly that they must be considered as true crystalline individuals of feld- 
spar and quartz [373]. 

The rhyolite from the saddle in the main ridge north of Aloha Peak, 
Pah-tson Mountains, Nevada, bears macroscopical biotite [374] ; but the 
microscope does not reveal any more than the naked eye can detect. It 
has no other secretions. 

Rhyolite forms the ridge southwest of Pahkeah Peak, Pah-tson Mount- 
ains [375], being very rich in crystals and bearing much glass. It has 
a colorless glass-ground, with imbedded microlites and larger crystals. 
The latter are microscopical feldspars (some plagioclases), quartzes, and 
biotites. Feldspars have an enormous number of glass-inclusions; the 



176 MICROSCOPICAL PETROGRAPHY. 

inner portions of the crystals being in some cases a close aggregation of 
angular, bubble-bearing glass-grains, in such immediate contact with one 
another that no feldspar appears between them. The thicker, horizontally 
lying mica-plates remain almost wholly dark in thin section : the transverse 
sections, however, become greenish-brown. The rock includes an extra- 
ordinary quantity of microscopical biotites in the form of sharply outlined 
hexagonal laminae, the diameter of which is only 0.008 mm . They vary from 
dark to light brown, according to their thickness ; but their color does not 
depend entirely upon that alone, for pretty plates were observed, consisting 
of three or four concentric zones, of different shades of brown, arranged 
one around another, like frames. In the glass-base, many feldspar-micro- 
lites are scattered, and are accompanied by some quite pale-greenish needles, 
which appear to be related to thicker, indistinctly formed, green individuals 
probably belonging to augite rather than to hornblende. The base contains 
numerous oval and rounded, dark-bordered gas-cavities in the glass, whose 
pellucidity is chemically supplemented by black magnetites. 

The peak north of Pahkeah Peak, Pah-tson Mountains, is formed of 
a beautiful and interesting yellowish-gray rhyolite [376], containing small 
macroscopical feldspars, quartzes, and biotites (Plate VII, fig. 2). The 
groundmass is a ramifying net-work of pale-yellowish strings, having an 
axially arranged composition, of roughly cuneiform grains, or short, thick 
fibres, with a distinct suture running down the middle. In the most con- 
spicuous contrast, the intervals or meshes of this net-work have become 
concentrically and radially fibrous, like sphaerolites, with a well-expressed 
centre. These fibrous aggregations of a grayish-yellow color sometimes 
show, in the sections, alternating lighter and darker concentric rings. 
Quartzes and feldspars containing glass-inclusions are present. Some 
large, irregular masses of groundmass, with all its structural character- 
istics, imbedded in the larger quartzes, prove that this structure is primary. 
The biotite plates are often shivered and broken into pieces. 

A very similar type of rhyolite occurs on the rock from the east side of 
the Pah-tson Mountains [377]. The rhyolite from the second summit north 
of Pahkeah Peak possesses a different structure [378]. This rock is rather 
rich in crystals of quartz, feldspar, and biotite, and does not show any trace 



RHYOLITE. 177 

of axial fibration; its groundmass being composed of, long, rough, rudiment- 
ary, and perfect sphserolites with more or less distinct centres. The rhyolite 
from the south of Pahkeah Peak [379] does not show any fibration at all, 
but has an undeveloped, granular groundmass. Biotite is about the only 
secretion. 

A rock from hills north of Indian Spring [380] is in some places 
even devoid of biotite; other specimens [381] showing many inacroscopical 
feldspars and microscopical quartzes. 

The rhyolite from Aloha Peak, Pah-tson Mountains, contains Carlsbad 
twins of sanidin, very little plagioclase, and considerable quartz and biotite 
[382]. All but the biotite contain good glass-inclusions. The mica is. 
partly pretty fresh and partly altered into a dirty brown, granular substance. 
The groundmass is microfelsitic, and contains many brownish, partly 
pellucid, ill-shaped microlites of an uncertain nature. 

Rhyolites of Grass Canon, Pah-tson,, Mountains [383, 384, 385], are 
related to very characteristic glass-rocks or hyaline-rhyolites, and have 
the most varied behavior ; in one place containing good sphserolites ; in 
another, almost free from crystalline secretions ; again, devoid of sphaerolites, 
but bearing fine quartzes (with glass-inclusions), feldspars, and biotites. 
The largest part of the groundmass is very finely and indistinctly crystal- 
line, and is traversed by microfelsitic zones, which contain clumsy ferrite 
needles produced by aggregations of single grains. 

At Karnak Ridge, Montezuma Range, columnar rhyolites occur ; single 
columns attaining the thickness of a man's body. In light-gray rocks of 
this locality [386], large macroscopical biotite plates are secreted. The 
groundmass contains many fine crystalline particles, quite pale-green, ill- 
shaped grains, and dark grains of opacite. In many places, it has, by 
perceptible passage, become radially central-sphserolitic. Axial fibration is 
wanting here. The sphserolites, well formed and measuring O.l mm in diam- 
eter, consist of fine fibres, which are nearly colorless about the centre ; but, 
near the periphery, they have something of an isabel color, so that in low 
magnifying power many pale-yellowish rings are seen surrounding colorless 
centres. There is also much microscopical, dark -bordered biotite, of which 

the granular, outer zone often protrudes far into the interior. The rock also 
12 MP 






178 MICROSCOPICAL PETROGRAPHY. 

contains microscopical hornblende in splendid brown sections, clear feldspars, 
bearing good, thick glass-inclusions, apatite in prisms that are transversely 
cracked, and pale-green microlites (perhaps augite); the latter being very 
rare in rhyolitic groundmasses. Quartz is not present. It is a noteworthy 
fact that apatite only occurs in those rhyolites which are rich in crystals, 
or else possess a rather crystalline groundmass ; and it is wanting in the 
microfelsitic or indistinctly crystalline rhyolites, although the circumstances 
necessary to its thorough individualization are as complete here as in other 
varieties. Thin sections of the outer and inner parts of a little rhyolite 
column which were examined under the microscope did not at all differ, 
either in relation to general structure, or to the nature, quantity, and behavior 
of the included crystalline ingredients. 

The rhyolites from Lovelock's Knob, Montezuma Range [387, 388], 
are made interesting by their microstructure. They consist of, a, very fine, 
brownish-yellow, granular stripes; 6, light-yellow or reddish-yellow axially 
fibrated strings (both being very much bent and curved, and often present- 
ing section-forms like the letter G) ; and of roundish, longitudinal bodies, 
composed either of, c, a distinctly polarizing aggregation of small colorless 
grains, or of, d, concentric sphaerolitic bodies. The strings 6, often show in 
the middle a darker axis, or suture, and a high magnifying power discovers 
that this is produced by extremely minute black grains, arranged lineally 
between the ends of the short fibres or wedge-formed particles. There is 
no well-individualized, microscopical secretion. 

Back of Oreana, in the Montezuma Range, a reddish-gray rhyolite, with 
faint, finely undulating, bluish-gray stripes, occurs [389]. In the sections, 
short, dark reddish-brown h'nes are visible, which, under the microscope, prove 
to be highly altered biotite; the lenses showing them to be an aggregation of 
dirty reddish-brown grains, in the distribution of which the former lamellation 
can sometimes be recognized. The prevailing reddish-gray parts of the rock 
are rich in microscopical biotite plates of the same nature. They are, how- 
ever, inseparable from accompanying needles, evidently composed of brown 
ferrite grains; so that there exists here a curious relation between ferrite and 
decomposed biotite. Both contribute to form the somewhat darker color of 
these parts of the rock. The bluish-gray stripes show the indistinctly crys- 



RHYOLITE. 1 79 

talline groundmass to be free from both these elements. There are no 

secretions beside some very rare, small feldspars. 

f 

Rhyolites, northwest from Black Canon, Montezuma Range, are 
lithoidal and quite like hornstone, resembling isabel-colored or pale-reddish 
porcelain, with excellent, fine, undulating fluidal stripes [390, 391, 392]. 
The groundrnass has a very interesting microstructure (Plate VII, fig. 3). 
It has become very largely fibrous, but the fibres are not grouped into 
sphaerolites or axial, longitudinal bodies, but are arranged in bunches, 
with parallel aggregation and a slight divergence at the ends, so that 
they suggest tiny sheaves of grain. These fibrous bunches measure 
about 0.02 mm in length, are heaped together confusedly, and but very 
slightly affect polarized light. This is a really microscopical structure 
which strongly reminds the observer of that of artificial porcelain as 
described by H. Behrens. 1 Stripes about 0.05 mm broad, the borders of 
which are not sharply defined, composed of colorless angular (polarizing) 
grains, traverse the groundmass, which also contains minute grains and 
needles of ferrite. There are a few quartzes and feldspars, but no 
biotite. 

The groundmass of the highly lamellated grayish rhyolite from the 
ridge near White Plains, Montezuma Range, also possesses an interesting 
structure [393, 394]. With a strong lens there can be seen in the thin sec- 
tions, a large number of alternating, nearly colorless and duller isabel-colored, 
narrow, finely undulating stripes. The clearer ones consist chiefly of a 
water-clear substance, which might, in ordinary light, be taken for a homo- 
geneous glassy base, but between the nicols are discovered to be an aggrega- 
tion of more or less polarizing grains. A large number of nearly colorless, 
thin microlites and dark grains and knotted needles of ferrite are imbedded in 
the clearer stripes, the general direction of the two linear elements agreeing 
with that of the stripes. The isabel-colored bands appear under the micro- 
scope as an aggregation of excellent, parallel-fibrous bunches, almost devoid 
of polarizing particles, and poor in those microlitic constituents which abound 
in the lighter-colored stripes. The outlines of the two kinds of stripes are 
not sharply defined, the passage between them being gradual. This ground- 

1 Poggeudorif s Annalen, CL. 386. 



180 MICROSCOPICAL PETROGRAPHY. 

mass contains feldspar, large biotite plates, and some hornblende individuals. 
The stripes wind among and around these crystals. 

The gray rhyolite from the Karnak, Montezuma Range [395], has a 
somewhat trachytic behavior, containing in its groundmass, which is also 
rather rhyolitic, numerous feldspar microlites (an unusual thing in rhy elites), 
macroscopical individuals of hornblende and biotite, apatite, and more 
plagioclase than is common in other rhyolites. There is no quartz. The 
presence of such a quantity of microlites, however, seems to be merely 
accidental; for specimens occur at the east base of the Kamak, some of which 
are rich in microlites, while others are entirely free from them. There is no 
evidence of a tendency to fibrous structure. 

Quartzes nearly as large as a pea are imbedded in the reddish rhyolites 
from Bayless Canon, Montezuma Range [396, 397]. They are strikingly 
free from all microscopical interpositions. Excellent longitudinal, axial 
fibre systems run through the groundmass, being pale-yellowish along the 
axis and reddish-yellow along the borders. 

The greater part of the rhyolites from the Mopung Hills, at the southern 
end of the West Humboldt Range, are very poor in crystalline secretions. 
The rock is usually somewhat porous. Dark and light reddish-gray varieties 
predominate. They take their peculiar color mostly from grains and short 
needles of dirty-brown ferrite or from hydrous oxyd of iron, evidently 
infiltrated into microscopical fissures after the formation of the crystalline 
ingredients. Often the rocks are highly lamellated, presenting the most 
characteristic string-structure. In some of the varieties [398, 399], the lam- 
ellation is surprisingly complete (see Plate VIII, fig. 2, the representation 
of another very similar rock). Under the microscope, the rock is seen to 
consist of alternating colorless and brownish-yellow layers, the line of sepa- 
ration between each being sharply defined, without any passage. The 
colorless layers are indistinctly granular, or else somewhat fibrous in con- 
stitution, acting very feebly upon polarized light, and often containing 
many short, dark microlites. The brownish-yellow layers of this color 
are of a globulitic, glassy nature, and do not polarize at all. The deli- 
cate, hair-like microlites sometimes consist of a linear grouping of extremely 
fine, dark-brown grains; and, being generally attached to the surface of 



iqooaoaoiM 

EHYOLITE. 181 

the brownish, glassy layers while stretching into the colorless zones, 
they seem to spring from the globulitic glass; but they are also found 
isolated in the clearer layers. These are often so extremely thin that 
a dozen of them together measure only 0.03 mm ; and, under the microscope, 
the most delicate drawings appear on them, resembh'ng the finest agate. 
These systems of layers also curve and undulate very nicely, swelling into 
beautiful contours around cavities. The single layers are often interrupted, 
only to be continued at a greater or less distance beyond. The globulitic, 
brownish-yellow members are generally somewhat narrower than those 
which are colorless. In such varieties, there are hardly any secretions. 
Others of the Mopung Hills rhyolites present types of structure already 
described. Rocks occur [such as 400] which show, in unusual distinct- 
ness, the yellowish-brown, axially fibrated, longer or shorter, tail -formed 
strings, running through a light-gray, ferrite-bearing groundmass, which is 
principally in an undeveloped crystalline state. The strings are alter- 
nately thicker and thinner, show short, wedge-formed ramifications, and. 
are often darker along the suture, and invariably so on the borders (Plate 
VII, fig. 4). Brownish -green biotite is sometimes met with in such varieties. 
In other rocks [such as 401], a net- work of brownish-yellow lines, com- 
posed of ferrite grains, includes roundish or oval bodies which are observed, 
in varying distinctness, to be concentrically and radially fibrous. Other 
varieties possess only rough sphserolitic groupings of fibres in a mainly 
microfelsitic base. 

Other types from the Mopung Hills [402, 403] are also poor in crys- 
talline ingredients, having no quartz, only a little feldspar, and here and 
there some very much altered biotite ; and they are generally made up of a 
combination of sphaerolitic and axially fibrated masses. 

In conspicuous contrast with all the more recently described rhyolites, 
those of the Pah-Ute Range, Nevada, are, for the most part, comparatively 
very rich, even in macroscopical crystals. The brownish-gray specimen 
from McKinney's Pass [404] contains quartzes measuring 3 mm , and even 
larger sanidins. The first have very accurately dihexahedral forms, and con- 
tain some macroscopical inclusions of groundmtiss, besides numerous smaller 
glass grains. The sanidin is entirely fresh, and bears many layers of cylin- 



182 MICKOSCOPICAL PETEOGKAPHY. 

drical or oval, empty gas-cavities. The groundmass, for the most part, is 
divergingly fibrous, being composed of long bunches, like ice-flowers. In 
these bunches, an enormous quantity of dark-brown, bristly, ferritic needles 
is imbedded, which in their position, curiously enough, follow uniform 
h'nes of direction, regardless of the trend of the including fibrous mass. 
They form strings and bands of parallel needles, and also diverging bunches, 
which pass transversely through the rough sphaerolites. The perfect inde- 
pendence of direction, each -from the other, in these chemically different 
elements, is indeed striking. There is no hornblende: biotite cannot be 
detected with certainty. 

Quartzes in the similar rhyolite from the hills southwest of Granite 
Mountain, Pah-Ute Range [405], are filled with the most excellent glass- 
inclusions, measuring as high as l mm , and are formed in faultless dihex- 
ahedrons. The rock contains some long hornblende prisms. There is no 
sign of axial fibration in either of the two rocks. 

The yellowish-gray and brown rhyolites from the Hot Spring Hills, 
Pah-Ute Range, are, on the contrary, partly very poor and partly inclined 
to be rich in crystalline secretions. In one place will be found a base 
belonging between the poorly developed crystalline and the microfelsitic 
state ; in another, axially fibrous strings ; in another, more or less distinct 
sphserolitic aggregations predominating in the groundmass. After careful 
comparative examinations, no general relation between the microscopical 
structure of the groundmass and its individualized crystals could be detected. 
There is one variety [406] which contains large quartzes, measuring 4 mm , 
some sanidius, and small biotite plates, and is composed mainly of very 
excellent concentric-radial sphserolites in an unusually good stage of devel- 
opment, and they act more vividly upon polarized light here, therefore, than 
elsewhere. The sphairolites are pretty regularly rounded; and where 
several of the larger ones, say three, touch each other, the immediate 
mass around the point of contact has become fibrous ; sometimes radially, 
but always axially where the interval between them had a longitudinal 
direction. Sesquioxyd of iron has penetrated as a secondary infiltration 
into the rock, being attached in the form of dendritic lobes to the walls of 
the cracks in the quartz; and in planes where there has been stronger 



EHYOLITE. 

absorption, they color tl.e sphserolites, in part or wholly, reddish-brown. 
There also occur here light-brown varieties [for instance, 407], which per- 
fectly agree in microstructure with those from the west side of American 
Flat Canon, Washoe (see page 163: Plate VIII, fig. 1). 

The pass below Chataya Peak, Pah-Ute Mountains, is another place 
where typical varieties of rhyolite are met. Their structure, however, is such 
as has been already fully described. Most of them are comparatively very 
rich in moderately large crystals of quartz, sanidin, and biotite. In some of 
these rocks, the groundmass has this extraordinary composition : alternations 
of brown, wavy, glassy layers, often containing short dark hairs and lighter 
microfelsitic or half-crystalline layers (Plate VIII, fig. 2). In others, rough 
sphserolitic globules are traversed by axially fibrous strings, or by brownish- 
yellow, microfelsitic bands. The axial strings generally show a darker 
color along the borders, and are lighter near the axis. Still other ground- 
masses are chiefly made up of an indistinctly developed, crystalline- 
granular substance. Through these several varieties of groundmass, ferritic 
grains or needles are disseminated in more or less profusion. Rhyolites 
occur here, also, in which crystals constitute fully one-third of the whole 
mass. The quartzes generally bear good glass-inclusions, and the sanidin 
often forms distinct Carlsbad twins, and has a beautiful zonal structure. 
In some varieties of these rocks [such as 408], the sanidins are remark- 
able for exhibiting the most superb blue color in refracted light, and reach 
a size of B mm . This splendid color-phenomenon appears as well in the 
hand-specimens as in the thin sections. . In transmitted light, the thin sec- 
tions are quite colorless. The blue color is much more intense even than 
that of the famous " labradorizing " feldspar from Frederiksvarn in South 
Norway. But, while the luminous shimmer of the latter is connected with the 
numerous brown and violet-black laminae and needles which are microscop- 
ically interposed in its mass, the same strange bodies being also present in 
the proper labradorite from the coast of Labrador and from Kiew, Russia, 
in the feldspars of these rhyolites, no strange particles can be detected, 
neither needles, nor plates, nor grains, nor a dust-like powder, nor glass or 
fluid-inclusions. Moreover, these extremely fresh and unaltered feldspars 
have a strikingly compact mass, being sometimes in transmitted light, there- 



184 MICROSCJPICAL PETKOGKAPHY. 

fore, not easily distinguishable from the quartz. The latter, however, is 
always characterized by excellent dihexahedral glass-inclusions. The cause 
of this strange blue color must, for the present, therefore, remain uncertain. 
It reappears in the sanidins of some more eastern rhyolites. If the ele- 
ment which shows the color were plagioclase instead of sanidin, the phe- 
nomenon could easily be explained as a freak of polarization, produced 
by the passage of broken rays from one lamel into another, whose planes of 
vibration do not correspond. 1 The rhyolites of this locality which are rich 
in biotite, generally possess a peculiar fine porosity. There are varieties 
which only exhibit biotite macroscopically, quartz and sanidin being 
secreted in the form of very small individuals in the groundmass. Apatite 
is comparatively plentiful in these rhyolites, which have many crystals 

The greenish-gray rhyolite from north of Shoshone Spring, Augusta 
Mountains, Nevada, appears macroscopically to be decomposed ; but the 
microscope shows it is unaltered [409, 410]. It is rather rich in small feld- 
spars, and occasionally contains quartz. In some places, the groundmass is 
radially fibrous, and traversed by fine, undulating axial strings. 

An interesting half-glassy rhyolite occurs at Shoshone Springs, Augusta 

fit . 

Mountains [411], consisting of an intimate running-hurdle-work of color- 
less glass, and somewhat less pellucid bands and stripes of typical light 
yellowish-gray microfelsite. The contrast of the two ingredients here is 
very instructive as to the nature of the latter. It is evident that the 
microfelsite is neither proper glass nor a granular-crystalline aggregation, 
and it is not at all affected by polarized light. The glass, often finely 
porous, seems to be a little in excess of the microfelsite. Some feldspars 
and broken quartzes are imbedded in the rock; and here also bluish, opal- 
izing feldspars occur, as in those of the rock from the pass near Chataya 
Peak; and here, too, they all present perfectly pure substances. 

Excellent rhyolite forms the head of Antimony Canon, Augusta 

Mountains [412]. Its grayish-yellow groundmass consists almost wholly 

of longitudinal strings, with good axial fibration, 0.05 mm broad. Near the 

borders and axis, the strings are darker-colored. Feldspars are rarely met. 

1 Vogelsang, Sur le labradorite colore". Archives N^erlandaises, 1868, tome III. 



RHYOLITE. ] 85 

At the forks of Granite Point Cafion, Augusta Mountains, a greenish- 
gray rhyolite occurs [413], which is rather rich in quartz. 

In the rhyolite from the mouth of Granite Point Canon [414], the 
feldspars seem to bear inclusions of a liquid nature along their borders, but 
the mobility of the bubbles is uncertain. In the interior of the feldspars, 
unmistakable glass-inclusions are imbedded. The rock does not contain 
quartz. 

Other rhyolites from the Augusta Mountains are much richer in 
crystals. From the variety found north of Shoshone Springs [415], which 
contains sanidin, quartz (bearing many glass-inclusions), and biotite, the 
imperfectly crystalline groundmass develops in some places the most beau- 
tiful radially fibrated globules, their centres being very distinct. The single 
fibres constituting these sphrerolites are of varying lengths, and their sur- 
faces, therefore, have the appearance of being finely fringed. In other 
places, long axial fibrations traverse the groundmass. 

The bluish-green rhyolite, somewhat like hornstone, from the head of 
the ravine south of Shoshone Pass, Augusta Mountains [416], has the same 
porphyritical crystals, and strongly resembles the last-described rock in^ 
structure. But the sphaBrolites are more nicely fibrous; and these, as well 
as the axially fibrous strings present, are immediately surrounded by a 
zone of groundmass having a better crystalline development than the 
other; and between the nicols it has a spotted appearance something like 
a mosaic. 

A grayish-black rhyolite occurs in the ravine south of Shoshone Pass 

1 

[417], which has the porphyritical characteristics, together with brown 
glass, in considerable abundance, and sanidin. The groundmass is like 
that represented in Plate VI, fig. 3. The most of these rocks have very 
small angular fragments of strange varieties of rhyolite imbedded in their 
mass, of which the prevailing groundmass differs from that of the others in 
color and in microscopical structure. 

In the rhyolite from the ravine north of Shoshone Pass [418], the bluish, 
labradorizing sanidins again appear. 

Reddish-yellow varieties from this same locality [for instance, 419] 
bear, besides sanidin and quartz, an abundance of biotite, which is pene- 



186 MICROSCOPICAL PETROGRAPI1Y. 

tratod by numerous dazzling prisms of apatite. Light-brownish axial 
strings of great beauty traverse the groundrnass. 

Profuse secretions of quariz, sanidin, and biotite are also found in the 
rhyolites from Clan Alpine Canon, Augusta Mountains. That from Clan 
Alpine Mine [420] also contains plagioclase, and its sanidin is very rich in 
cylindrical and rounded, empty cavities. Long arms of the groundmass are 
often found protruding into the quartz, and, in its hexagonal sections, hex- 
agonal zones of the groundmass are imbedded; and the quartz is full of 
isolated rounded inclusions of pure glass, with bubbles, and of the half- 
fibrous groundmass. 

The variety from the head of Clan Alpine Canon [421] is rich in the 
same crystals. The brownish-yellow groundmass has become finely but 
more confusedly fibrous ; in some places, the fibres being arranged in par- 
allel, bunch-like systems. It does not act upon polarized light. Faint, 
black hairs are scattered without order through this mass; but they are 
more numerous in the immediate vicinity of the quartz crystals. There 
is no sign of axial fibration. 

A rock from the mouth of Clan Alpine Canon [422] bears, beside large 
quartzes and sanidins, colorless bodies which seem to be perfectly homoge- 
neous in ordinary light, but prove, in polarized light, to be aggregations of 
powerfully double-refracting grains, reaching a size of 0.05 mm (probably 
quartz and feldspar), and excellent sphaerolitic bodies having a comparatively 
strong action in polarized light, sometimes almost producing the black cross. 
In some cases, the better-developed sphaerolites of three concentric zones 
vary in color from dark to light. The colorless portions of the ground- 
mass contain an unknown mineral of a vivid yellow color, in the form of 
sharp, irregularly shaped, compact grains, and also sharp, rhombic, colorless, 
little plates 0.02 mm long, lying one above another, like tridymites, which are 
also indeterminable. 

The quartz in the rhyolite from the canon south of Clan Alpine Canon 
[423] bears as beautiful glass-inclusions as may be seen anywhere. 

Rhyolites from the Desatoya Mountains are also very rich in crys- 
tals. On the east side, a black variety, appearing to be very glassy, occurs 
[424], containing microscopically a, deep, dark-brown glass, which even in 



KHYOLITE. 187 

very thin sections is but slightly transparent, and, remarkably, is almost 
wholly free from microscopical products of devitrification. The larger 
quartzes, sanidins, and plagioclases of the rock are filled with inclusions 
of the dark glass, whose shapes are more like long stripes and rays than 
roundish grains. 

A rhyolite from the mouth of New Pass, near the stage-road [425], is 
enormously rich in quartz, which constitutes perhaps one-third of the whole 
mass. Its groundmass is in a peculiar state. The pale-yellowish sub- 
stance composing it does not polarize at all, except in the case of very rare 
and very minute, indistinctly individualized particles ; yet it is neither a 
real glass nor the common microfelsite. When carefully examined with the 
higher magnifying power, it is evident that the groundmass is composed of 
extremely small globules, heaped together like clusters of grapes or bunches 
of blackberries ; their forms being such as have been named by Vogelsang 
cumulites. 1 The single globules are distinctly concentric, at least, not evi- 
dently fibrous. Toward the centre,- they are a little darker, and merge 
into one another at the borders. Short, axially fibrous strings traverse this 
aggregation of isotrope cumulites. Vogelsang has observed similar cumu- 
litic groundmasses in Hungarian rhyolites. In the rhyolites of the Fortieth 
Parallel, this development seems to be very rare. 

Labradorizing sanidin is again found in another rhyolite "from the New 
Pass, Desatoya Mountains [426], and, as usual, it is free from interpositions. 
The rock contains quartzes, with fine glass-inclusions. 

A brownish rhyolite from Gilbert Creek [427] bears many, but very 
small, quartzes and feldspars, and microscopical biotites, and is traversed 
by fine axially fibrous strings. 

The reddish-brown rock from the head of South Canon, Desatoya: 
Mountains [428], is very rich in larger crystals, and bears comparatively 
very thick, dark, grayish-yellow to reddish-yellow sphaerolites, polarizing 
in flame-like stripes. The sections also show concentric, differently-shaded 
rings, often suggesting the appearance of an old tree-trunk. 

Large sphserolites, with distinct centres, are also developed in the rhy- 
'Die Krystalliteu, 1875, 134. 



188 MICROSCOPICAL PETROGRAPHY. 

. 

elites from the New Pass Mines [429]. In some places, axially fibrous and 
often dichotome bands run through the sphscrolitic aggregations ; in others, 
the substance of the latter, gradually becoming indistinctly fibrous, passes 
into a cumulitic matter, the shaly globules of which are unaffected by polar- 
ized light. The yellowish-gray groundmass of this variety, resembling 
hornstone, contains numerous crystals, among which are many biotites 
and microscopical apatites. 

Quartzes of the rhyolite from the Eastern foothills, New Pass Mount- 
ains [430], bear unusually large and clear glass-inclusions, measuring 
0.045 mm in diameter, the bubbles of which are often remarkably sac-like, 
curved and twisted (Plate I, fig. 16). Included particles of the ground- 
mass, also bearing bubbles, which have been pressed into sharply hexag- 
onal forms, accompany the dihexahedrons of pure glass in the same crystals. 

The saddle northeast of the New Pass Mines also consists of rhyolite. 

Rhyolites from Mount Airy, Shoshone Range [431, 432], show an 
excellent fluidal structure, in the form of groundmass-stripes differing some- 
what in color, several of them being axially fibrous. There is no sign of 
a tendency to form sphaerolites. The numerous quartzes and sanidins are 
entirely free from interpositions, excepting a few gas-cavities. 

The rhyolite from Jacob's Promontory, Shoshone Range [423], is of a 
quite different" type, being largely a half-glassy rock. The groundmass 
becomes dark gray in the section, and is a felty aggregation of small micro- 
lites, charged through and through with glass, so that it very much resembles 
the characteristic groundmass of augite-andesites. Nevertheless, the secreted 
crystalline ingredients are those of a genuine rhyolite. The rock contains 
quartz, largely predominating sanidin, accompanied by a little plagioclase, 
an abundance of excellent brown hornblende with a dark border, biotite, 
and some lighter augite crystals, with exceedingly abundant glass-inclusions. 

A rhyolite from the Hot Springs, Reese River Valley [434], is not 
glassy, but is rather rich in crystals, resembling those from the Desatoya 
Mountains. 

Rhyolites from the south of Ravenswood Peak, Shoshone Range 
[435], are much poorer in secreted crystals than the latter, and possess a 
brownish groundmass, which is a fine combination of very distinctly fibrous, 



EHYOLITB. 

sphgerolitic and axiolitic bodies. The latter often have a lighter-colored 
middle suture, because the axially arranged fibres do not here join each 
other closely. Delicate, brownish stripes and lines of ferrite-grains wind 
among the individuals, marking their limits distinctly. 

Rocks of Reese River Canon, Shoshone Range [436], are perhaps 
rhyolitic tufas. They consist of roundish, dirty-gray bodies of groundmass, 
rich in ferrite, especially so along the borders. Between these isolated 
rhyolitic particles, run bands and veins of a colorless substance. In ordinary 
light, they seem to be homogeneous; but, in polarized light, they are proved 
to be composed of single, wedge-formed grains, exhibiting a very vivid 
chromatic polarization. There is no doubt that they belong to a fibrous, 
granulated hornstone ; and it is highly probable that this quartzy material 
filled up the intervals between the rhyolitic fragments secondarily. 

At the north spur of Ravenswood Peak, Shoshone Range, a rhyolite 
occurs [437], which has been colored a brick-red by dusty ferrite grains, 
aggregated in lumps, heaps, and long stripes. Those parts of the ground- 
mass which are poor in ferrite, or free from it, have a delicate, sphserolitic 
fibration. In this mass, also, are fragments of other rhyolites, as large as 
1.5 mm , and macroscopically visible in thin sections, being more distinct here 
than in the hand-specimens. The groundmass is opaque, and of a dark, 
dirty greenish-gray color. 

The rhyolites of the Fish Creek Mountains are for the most part 
extremely rich in crystals. The quartzes are often evidently broken, and 
sometimes have a very dark color, like that of the so-called smoky topaz or 
cairngorm-stone. This color is produced in the quartzes by deposits of 
hydrous oxyd of iron, or of oxyd of iron, in the numerous fissures of the 
crystals. These rhyolites also occur on the summit of Mount Moses, Fish 
Creek Mountains [438]. Their sanidins are often accompanied by a little 
plagioclase, and both are rendered remarkable by containing an enormous 
quantity of glass-inclusions and empty cavities; which latter are rare 
in quartz. In one place, the groundmass is microfelsitic, with small 
and rare polarizing points ; in another, it is in an imperfectly-developed, and 
in still another a better-developed, crystalline-granular state, and ferritic 
powder is scattered at intervals through it. Sphserolitic or axial fibration is 



190 MICROSCOPICAL PETROGRAPHY. 

generally wanting. In one of the examined specimens, however [439], 
axially fibrated strings of a rather yellowish color were visible. These 
strings do not, as usual, run singly ; but two or three are generally found 
intimately associated. 

A gray rhyolite of Storm Canon, Fish Creek Mountains [440], has 
sanidins which are extremely rich in pores, and its groundmass shows a 
pretty good crystalline development. Occasionally, feeble rudiments of 
axial bands are seen. The thick magnetite grains of this rock, which are 
doubtless quadrangular, are covered with a thin, whitish crust, like that 
which is sometimes seen to veil titanic iron. 

The isolated ridge between Winnemucca and Fairbank Point [441] 
consists of a curious rock, seeming, in the hand-specimens, to be slightly 
roTigh, like a trachyte ; but it contains quartzes as thick as a pea, and very 
large sanidins, together with microscopical biotite and apatite. The ground- 
mass does not contain any trace of either sphserolitic or axiolitic fibration, 
but is entirely crystalline, polarized light showing it to be constituted of 
double-refracting particles, which are most probably quartz and feldspar. 
Nevertheless, this aggregation of colorless grains contains a great abundance 
of rounded, microscopical pores, a phenomenon extremely rare in crystalline 
groundmasses. 

At the west end of the Havallah Range, Nevada, is a brownish rhyolite 
[442], which is very rich in quartz, and bears highly porous sanidins. The 
larger part of the groundmass is remarkably sphserolitic, and this is the 
more plain because the centre and periphery of the fibrous globes are gener- 
ally of a somewhat duller and darker gray color. 

Golconda Pass, Havallah Range, yields a brownish-red rhyolite [443], 
which bears fewer crystals. There is some biotite, and here and there pretty, 
axially fibrous strings. Sphserolites are wanting. The color of the rock 
seems in part to be of secondary origin, resulting from infiltrated combina- 
tions of iron, but it also contains ferrite grains as a primary ingredient. 

The rhyolites from the base of the cliffs of Shoshone Mesa, Nevada 
[444, 445], do not have any signs of fibration, either radial or axial, except 
around the larger crystals of the rocks, which are encircled by feeble and 
confused fibres. Almost the whole of the groundmass has a pretty good 



RHYOLITE. 191 

microcrystalline development, being chiefly composed of colorless particles, 
with grains of black opacite and brownish ferrite, beside needles of the latter. 
No microfelsitic matter is visible Of the larger colorless ingredients, many 
have a strikingly accurate rectangular outline; but, although some forms 
occur which might be taken for irregular hexagons, it would not be warrant- 
ble to ascribe them to nepheline. 

One variety from this locality contains quartz, bearing especially good 
glass-inclusions, very little plagioclase, many sanidins, and proportionally 
considerable apatite. Another variety is devoid of quartz, both as an 
ingredient of the groundmass and in the form of larger crystals; but it 
contains much plagioclase. 

Rhyolites from the top of Shoshone Mesa, east side [446, 447], are 
less distinctly crystalline than the last described, and they are remarkable 
for containing tridymite. Long, prismatic bodies are seen in the sections 
of both specimens : in transmitted light, they appear black, opaque, and 
somewhat granular ; but, in reflected light, they have a dirty, brownish-red 
color. These bodies are most probably altered biotite. 

A light-gray rhyolite from the spurs of the River Range, in the 
region of Susan Creek [448], is free from macroscopical crystalline secre- 
tions, with the exception of a very few, little quartzes; and it is so homoge- 
neous that a likeness between it and porcelain is suggested. Under the 
microscope, the groundmass is seen to be chiefly an aggregation of small, 
polarizing grains, no thicker than 0.05 mm , which are probably quartz and 
feldspar. The mass is intricately striped with lines of a rough, axial 
structure in arabesque drawings, and is almost devoid of any optical action. 
These stripes sometimes form complete rings around small particles of the 
crystalline mass. 

The rhyolite from Sunset Gap, Rock Creek [449], Nevada, is extremely 
rich in biotite, with which is mingled sanidin and quartz; and the ground- 
mass bears excellent axially fibrous stripes. 

Another variety from Rock Creek [450] is a glassy, brown rock, having 
a groundmass like that represented in Plate VI, fig. 3; but it bears only 
sanidin, with a very little plagioclase, and here and there some vividly green 
augite. Quartz, hornblende, and biotite are not present. 



192 MICROSCOPICAL PETROGRAPHY. 

A most remarkable rhyolite is taken from the walls of Upper Canon, 
Rock Creek [451]. In the hand-specimens, reddish and grayish stripes, 
which are more or less indistinct throughout their whole length, may be 
observed, the first-named of these showing macroscopically a sphserolitic 
structure. The microscope shows the groundmass (Plate VIII, fig. 3) to 
be constituted almost wholly of 'an aggregation of more or less perfectly- 
formed sphaerolites, with a very fine and delicate but moderately distinct 
radial fibration, and quite an obvious centre. The usual, light-isabel-colored 
sphserolitic balls contain transparent, prismatic, ferrite needles of a dark- 
yellow, reddish-yellow, or brownish-red color, their maximum length being 
0.045 mm , arranged loosely but regularly around given centres. This 
appearance of ferritic stars in the section is very pretty ; the needles com- 
posing them often varying in length and thickness. Reddish stripes of the 
hand-specimens are composed of sphasrolites richer than usual in these 
ferritic microlites, grayish stripes being produced by the small number of 
them here interposed in the sphserolites. Quartz bearing the most perfect 
glass-inclusions is present. 

Rhyolites from Independence Valley, north of Tuscarora [452, 453], 
appear somewhat trachytic. The groundmass is not very distinctly granu- 
lar, and lacks all signs of any tendency to fibration or to waving fluidal 
structure. Quartz is comparatively abundant. Partly decomposed feldspar, 
altered hornblende, and much biotite are also present. 

Specimens from the west slope of Nannie's Peak [454, 455], Seetoya 
Range, are poor in crystals, containing only quartz with beautiful 
dihexahedrons of glass, sanidin, and biotite. The groundmass is in some 
places indistinctly sphaerolitic; but there is no sign of axial fibration. 

The brown variety from the east of North Fork, Humboldt [456], is 
somewhat decomposed, containing an enormous quantity of roundish, 
brownish-yellow ferrite grains; and there are no larger crystalline inclusions. 
A more typical rhyolite is found in the yellowish-gray specimen from 
Station 39, Toyabe Mountains [457, 458]. Quartz, sanidin, and biotite are 
present. The imperfectly granular groundmass, inclining to the micro- 
felsitic state, presents pretty good sphserolitic fibrations and axially fibrous 
strings. 



RHYOLITE. 193 

In a variety of hand-specimens from different localities, very small 
angular fragments of a blackish-gray, largely half glassy rhyolite occur, 
containing, under the microscope, colorless feldspar-microlites. The larger 
of these sharply-limited fragments, which strongly contrast with the includ- 
ing rock, appear macroscopically in the thin .sections as dark points or dots. 
It is strange that these imbedded particles should be so small in size, several 
having been found which were only 0.3 mm in diameter. 

Th,e dark-gray rhyolite from the divide between Susan Creek and the 
North Fork [459] is rather rich in crystals, with which is mingled some 
biotite and hornblende, the latter of a rust-red color. The whole of the 
groundmass is confusedly fibrous, the fibres being short. Brownish and 
reddish-yellow ferrite needles are scattered without order through the 
mass. In short, this groundmass has the same composition as that of the 
rock from the summit of Upper Canon, Rock Creek. There has been, 
however, no tendency to central attraction here: if there had been, spbsero- 
lites and radial groupings of ferrite needles would have been developed. 

At the top of the hill above Camp Canon is a confusedly fibrous 
rhyolite [460], lacking larger secretions, but bearing a great abundance of 
biotite. 

In Reese River Cafion, Shoshone Range, occurs a good rhyolite [461], 
which has but few secretions of quartz and sanidin, and in rare instances 
biotite. The pale brownish-yellow groundmass is a good combination of 
fibrous heaps, having a central radial structure and axially fibrous strings. 
The thick fibres show proportionally strong optical action. 

West of Carico Lake, in the foot-hills of the Shoshone Range, a grayish- 
white, very quartzose rhyolite occurs [462], Its feldspars are rather porous, 
and are decomposed along the outlines, producing a dull, milky border 
around the clear, transparent kernel; a phenomenon common to the old 
felsite-porphyries. The rock also bears many beautiful fresh biotites of a 
comparatively light color ; but hornblende is wanting. The groundmass is 
confusedly fibrous, its individuals being short. 

From a neighboring locality in the Shoshone Range, a splendid rhyolite 
was collected [463], bearing numerous quartzes as large as a pea, in which 
are excellent, pale-brownish, hexagonal glass-inclusions. These are often 
13 MP 



194 MICROSCOPICAL PETROGRAPHY. 

covered with a star of six rays, the rays apparently protruding over the bor- 
ders (Plate I, fig. 15). The rock also contains biotite, but no hornblende 
In the southern end of the Wah-we-ah Mountains are some rhyolites 
[464, 465], which are very rich in quartz, feldspar, and large, fresh plates of 
biotite measuring 4 mm in diameter. Of the quartzes, many have the dark- 
brown color of smoky topaz; but they do not possess any microscopical 
peculiarity of structure. Of the feldspars, a comparatively large number 
are plagioclases. The biotites have a narrow black border, and include a 
great quantity of black grains and long colorless prisms, a part of which are 
doubtless apatite. The groundmass of this rock is better crystalline-granular 
than are those of most rhyolites of this type. It consists of colorless particles 
which are probably quartz, somewhat duller bodies which have been taken 
for feldspar, and microscopical brown mica plates, all mingled into an intimate 
aggregation. In spite of its granular composition, a great number of dark- 
bordered pores are disseminated among its elements. 

The Roberts' Peak group offers a rhyolite [466], which has but a very 
little quartz ; its felsitic groundmass looking homogeneous, and being of a 
violet color. This mass is imperfectly crystalline, wanting in fibration, and 
bears blackish-brown and reddish-brown prickly and crippled ferrite needles, 
scattered without order, together with small, microlitic, colorless feldspar 
ledges, which are, for the most part, striated. This latter phenomenon is 
very rare in rhyolites. The larger feldspars contain many black opacite 
grains, and are devoid of proper glass-inclusions ; but devitrified, slaggy 
inclusions of feeble pellucidity are present in them. There is some biotite 
and apatite in the groundmass. This variety, therefore, resembles trachyte. 
Rhyolites of Wagon Canon and Rhyolite Canon, Cortez Range [467, 
468], are distinguished from most other rhyolites in these respects : a, their 
feldspars are, for the most part, altered into a dull, half-kaolinic substance ; 
&, they lack evidence of any tendency to develop fibrous, sphaerolitic or 
axiolitic aggregations ; c, their groundmasses, which are in a veiy imper- 
fectly crystalline state, and are rich in ferrite, contain colorless feldspar- 
microlites ; and, d, they are absolutely free from biotite. Most of the rocks 
from . this locality are rich in quartz, which is very pure, including only 
narrow lines of empty pores and beautiful, isolated glass-inclusions. The 



EHYOLITE. 195 

rhyolite from the Roberts' Peak group belongs, in certain respects, to this 
type. 

A specimen from north of Pifion Pass, Pifion Range [469], is a normal 
rock, exceedingly rich in crystals, particularly quartz and biotite, the latter 
very much shivered. The groundmass is in an undeveloped crystalline con- 
dition, and is rich in felrite, with here and there the rudiments of a con- 
fusedly fibrous state. 

At Pleasant Valley, south of Pine Nut Pass, Pifion Range, a remark- 
able rhyolite occurs [470], composed of large quartzes and sanidins, which 
are full of dark pores, but wanting in proper glass-inclusions. Under the 
microscope, the groundmass, which is not very compact, shows a remark- 
ably well-developed crystalline-granular structure, so perfect, indeed, as to 
surpass that of any rhyolite ever before seen, and to strongly resemble that 
of granite-porphyries. ' This microscopically coarse-grained aggregation 
consists of roundish, water-clear grains of quartz, more numerous, roughly 
quadrangular sections of feldspar, which are somewhat dull, and often 
somewhat fibrous, rare plates of brown biotite, and grains of ferrite. This 
decidedly Tertiary rock contains, moreover, some pale-reddish grains of 
perfectly isotrope garnet, measuring up to 0.2 mm in diameter. This, by 
the way, is not the first time garnets have been observed in rhyolites. 
Macroscopical individuals of garnet were found by v. Richthofen in the 
rhyolite from Mount Hradek, Hungary, and others were discovered by v. 
Hochstetter in the felsitic rhyolite from Mount Misery, Malvern Hills, 
South Island of New Zealand. 1 

In Clover Cafton, East Humboldt Range, a dark glassy rhyolite, hav- 
ing a resinous lustre, occurs [471]. It is remarkable for containing a large 
number of quartz-grains, which are traversed by a multitude of cracks, the 
cracks being filled with dark yellow ochre. The sanidin and plagioclase 
contain an enormous quantity of half-glassy inclusions, which are so thick 
as to form a sort of net-work. The quartzes of this rock, contrastingly, do 
not have any inclusions at all. Fresh crystals, which are quite undichroitic 
and of a grass-green color, (doubtless augite), and thick magnetites, are also 
present; but biotite and hornblende are wanting. The groundmass consists 
1 v. Hochstetter, Geologie von Nenseelaml, 1864, 203. 



196 MICROSCOPICAL PETROGRAPHY. 

of a brownish glass, densely crowded with colorless feldspar-microlites and 
pale-green microlites, which are in all probability augite rather than horn- 
blende. Here and there in the mass, however, may be seen larger micro- 
scopical spots of pure glass-base. 

A rhyolite from the Antelope Hills, south of Leech Spring, Nevada, 
presents macroscopically faint, undulating stripes or waving fluidal 
phenomena, which the microscope discovers to be an alternation of fine, 
sphaerolitic fibrous strings, with strings of an imperfectly granular nature, 
without any tendency to fibration. The rock contains the mos* character- 
istic concretions of tridymite, a few quartzes, some biotite, apatite, and highly 
altered hornblende. 

The specimen from the Antelope Hills, southeast of Leech Spring [472], 
does not contain tridymite. The sanidins bear extremely large glass-inclu- 
sions with indented, pronged outlines. This peculiarity of the periphery is 
quite common to the glass-inclusions of sanidins; but it never happens in 
quartzes, the glassy particles of which always have a linear border. 

Rhyolites of the Wachoe Mountains embrace many varieties. One, 
a very compact rock resembles hornstone, is pretty rich in quartz and 
feldspar, both with beautiful glass-inclusions [473]. Under the microscope, 
the grouridmass is made up of a combination of axial and central fibrations, 
the former varying from yellowish-gray to greenish-gray. There is no 
biotite. 

Another rock from the Wachoe Mountains [474] contains, in a dirty, 
rhyolitic groundmass rather rich in crystals, black, seemingly half-glassy 
stripes, possessing a greasy lustre. These stripes consist of colorless or 
brownish glass, in which an enormous number of extremely fine black 
grains are aggregated. They run parallel to one another; but are veiy 
much distorted and curved. The limits between these stripes and the rhy- 
olitic groundmass are very sharp; there being no sign of passage from one 
to the other, so that the stripes seem very much like included fragments. 
Quartzes of the rock contain the most beautiful inclusions of pale-brownish 
glass, often with several bubbles. 

At the north end of the Wachoe Mountains, a pale-reddish rhyolite 
occurs [475], in which a number of dark red, jasper-like stripes and spots 



KHYOUTE. 197 

are visible to the naked eye. Many quartzes and sanidins are also present. 
The dark-red parts of the mass are found, by the use of the microscope, to 
consist of a small quantity of a homogeneous, yellowish-red, glassy sub- 
stance. They are, for the most part, excellently sphserolitic, being made up 
of bristling globes densely heaped together. The centres of these balls are 
a more intense red ; the looser ends of the bristling fibres being more yellow. 
At their outer extremities, all of these globes and half-globes have their 
limits towards the lighter portions of the groundmass, and their bristling 
points project into it. This effect is particularly pretty where one globe 
intrudes into a quartz, and its tender, yellow fibres pierce the water-clear 
mass. The prevailing light groundmass has for a base a genuine pale- 
yellowish-gray microfelsite, in which, beside small, colorless, polarizing 
particles, are dispersed the most perfect, small, isolated, fibrous globules ; 
intimate aggregations of these forming the dark red stripes mentioned 
above. A lighter color is here merely the result of the presence of a 
smaller quantity of sphserolites; so it is evident that the red portions are only 
massive concretions of a primary rock-element, not strange fragments. 
Larger quartzes and feldspars are very poor in microscopical inclusions, 
with the exception of a few glassy and half-glassy grains in the quartzes. 
A remarkable phenomenon discovered in this genuine rhyolitic rock, was 
a quartz which contained the most characteristic fluid-inclusions, with mov- 
ing bubbles, in as great profusion as they are found in the quartz of granites 
and gneisses. This rhyolitic quartz individual is the only one of the thou- 
sands and thousands that have been examined with the microscope which 
has been found to bear fluid-inclusions. In this connection, the fact is 
significant that to this curious quartz was joined a quite dull and entirely 
decomposed feldspar, like those in granites ; all the rest of the observed 
feldspars in these rhyolites being extremely fresh and perfectly pellucid. 
Since so many of these rhyolites contain sharply angular, microscopical 
fragments of other rhyolitic varieties, the observer is permitted to conclude 
that this singular quartz and the adjoining altered feldspar are also foreign 
inclusions. 

The seemingly half-glassy rhyolite [476J from Spring Canon, Wachoe 
Mountains, is rich in crystals containing sanidin, a comparatively large 



198 MICROSCOPICAL PETEOGEAPHY. 

amount of plagioclase, beautiful dark-brown hornblende, less pale-yellowish 
augite (with penetrating apatite prisms), but no quartz, therein approach- 
ing trachyte. Hornblende sometimes has a black border of more or less 
intensity; and some black, impellucid, granular, tail-formed bodies, which 
are present, appear to be altered hornblende, although they lack the 
slightest trace of the original brown color. The groundmass is a dense 
aggregation of almost colorless microlites intimately imbued with glass. 
Along traversing cracks, the groundmass has become quite dull and almost 
impellucid. The glass-inclusions of all the crystalline ingredients except 
hornblende are enormously large: in the hornblende, they are small and 
rare. 

Desert Buttes, Utah, furnishes a normal rhyolite [477] of quite a light- 
grayish color, bearing quartz, which is rich in glass-inclusions and feldspars. 
The groundmass is imperfectly granular, showing, at intervals, a tendency 
to form rough spha3rolites. Biotite and hornblende are wanting, but the 
tridymite, occasionally found in hollows, seems to be of a secondary nature; 
for it is often found overlying iron-ochre or earthy ferrite. 

The interesting rhyolite from Passage Creek, Desert Gap, Nevada 
[478], has macroscopical stripes 2 mm broad, much curved and undulated, 
which wind around and among the sanidins and quartzes and also encircle 
some cavities with lithophyses, which are present. Under the microscope, 
these stripes are found to be individually composed of three different zones: 
a, grayish-yellow middle zone, quite sphaerolitic, with small fibres, and rich 
in ferrite needles, which give the zone a somewhat darker color, and are 
sometimes grouped radially in the sphserolites; I, bordering this, on both 
sides, dull-gray zones, not fibrous, and seemingly rather homogeneous, 
which have fewer ferrite needles but a considerable quantity of ferrite 
grains; c, boundary zones, extending along the two sides, of almost crystal- 
line aggregations of colorless grains, destitute of ferrite, developing out of 
zone 6 . the third zone, peculiarly, surrounds the cavities. 

The rhyolite from Forellen Butte, Nevada [479], seems to be a brecciated 
variety, composed of, a, broken crystals of sanidin and quartz, the latter 
shivered into a great multitude of pieces, one individual sometimes being 
separated into fifty parts, all lying close together; 6, pieces of dark-gray 



RHYOLITE. 199 

rhyolite, resembling hornstoue, rich in glass; c, stripes and bands of a dull 
whitish rhyolite, including an enormous number of sharply angular quartz- 
splinters, and winding 1 like a stream among the other ingredients. This is 

. 
surely not a clastic rock deposited in water, but an eruptive breccia. 

Upon the northeast slopes of the River Range, a light-gray rhyolite is 
found [480], which has excellent axially fibrous strings, splitting two or three 
times into diverging ramifications, and very much curved. The rock is 
extremely poor in opacite and ferrite. There are very fine microscopical 
crystals ; some feldspar and quartz being occasionally detected. Under the 
microscope, nearly all the feldspars are sanidins. Finely lamellated biotite 
is present. 

On the east slope of the Cortez Range, north of Palisade Station, a 
yellowish-gray' rhyolite, very similar to that last described, occurs [481]. 
It contains some plagioclase but no quartz, with sanidin as the only micro- 
scopical constituent. The groundmass is a finely sphserolitic body, with very 
thin and delicate black hairs grouped between the radial fibres. There are 
numerous cavities which are covered with a verrucose or papillary, light-gray 
crust, consisting of a number of fine layers, which are often of different colors. 
This substance, which is not fibrous and is not affected by polarized light, 
seems to be a hyalitic opal-matter. 

. The rhyolites next to be described are from the Mallard Hills, Nevada. 
In strong contrast with the last-described rocks, these varieties are all 
characterized by large quartzes, often exceeding the size of a pea. In 
the rock from North Point, Mallard Hills [482, 483], the light-brownish 
groundmass looks somewhat trachytic, macroscopically ; but, under the 
microscope, it is very beautifully sphserolitic. The large, fibrous sphserolites 
consist of loosely grouped, single bunches, the ends of which diverge, like 
the straws of a broom, from a usually darker centre. 

Light-colored rhyolite from Deer Canon, Mallard Hills [484], shows 
macroscopically feldspars and larger quartzes. The groundmass is chiefly 
a distinct aggregation of colorless ledges and grains, and black grains. 
Larger crystals are surrounded by a narrow ring or zone, visible even to the 
naked eye in the thin sections, which is an extremely fine, granular modifi- 
cation of the groundmass. Occasionally, this predominating aggregation 



200 MICROSCOPICAL PETROGRAPHY. 

gradually passes into dull, distinctly fibrous spots, whose outlines have 
become indistinct. 

In the Goose Creek Hills, a rhyolite is found [485] which looks very 
much like porcelain. The small macroscopical secretions of quartz and 
feldspar are distinct only in the thin sections. The groundmass is a mix- 
ture of colorless, polarizing particles and somewhat duller, pale, yellowish- 
gray bodies, which are either radially fibrous, or, as in the rhyolite of 
Trinity Mountains, back of Montezuma (see page 178), consist of confusedly- 
grouped bunches and systems of parallel fibres, showing hardly any action 
in polarized light. 

East of Goose Creek Hills, there occurs a rhyolite having something of 
a violet color [486]. It is rich in crystals, which are unusually free from 
interpositions. The groundmass contains beautifully fibrous globes; in 
some cases the spheres being perfect, and in others only single segments 
of diverging bunches appearing. Fully developed sphserolites reach a 
diameter of 0.5 mm , and are often found to have for a centre or kernel a small 
feldspar-individual. Ferrite needles are found scattered without order or 
grouped radially. This rock, too, bears tridymite. 

A very peculiar rhyolite is seen at White Rock, Cedar Mountains, 
Utah [487]. Its gray groundmass does not appear as compact as is usual 
in m6st rhyolites, but is very rough and trachytic, containing, macro-, 
scopically, many riven sanidins closely resembling those of trachytes, 
numerous brown biotite plates, and large quartzes. Beautiful augites, which 
cannot be seen with the naked eye, are discovered by using the microscope. 
The groundmass is an almost wholly crystalline aggregation of compara- 
tively large grains and individuals of feldspar, quartz, and augite. There 
are no microscopical biotite plates. The quartzes, and more especially the 
smaller ones, are remarkable for containing an unusually large quantity of 
little glass-inclusions, each with a dark bubble. In sections of quartz 
crystals which measured only 0.075 mm in diameter, as many as fifty glass- 
inclusions were observed in one plane; the most extraordinary surcharging 
of them ever seen in this mineral. And, in very curious contrast with this, 
the sanidin associated with the quartz is almost free from glassy interposi- 
tions. The biotite is noticeable for containing a great quantity of black 
* 



EHYOLITE. 201 

grains and brownish, acicular needles of the same material; the latter being 
isolated or grouped into thin bunches precisely as in the biotites of granites, 
gneisses, and mica-slates. It is impossible to distinguish this rhyolitic 
mica from that of the old crystalline slates; for each in turn repeats the 
characteristic phenomena of the other. Since augite is so abundant in this 
highly quartziferous rock, it is remarkable that hornblende is wanting. The 
crystalline groundmass contains locally some roundish, microfelsitic spots of 
a dull gray color, which cannot be resolved into individual crystals. 

Another very interesting rock is found on the summit of Hantz Peak, 
Elkhead Mountains [488]. It is a grayish-white mass, and, as the thin 
sections show better than the hand-specimens, rich in quartz and sanidins. 
No plagioclase was observed. Some of the quartzes have the most perfect 
dihexahedral glass-inclusions, with dark bubbles ; others, undoubtedly 
fluid-inclusions, grouped into heaps and strung out into lines, having 
rapidly moving bubbles : these two types of inclusion do not, how- 
however, occur in one quartz. Polarized light discovers that all the 
quartzes, but more especially those having fluid-inclusions, are broken 
into fragments. A greater quantity of fluid-inclusions than the quartzes 
contain is enveloped in the feldspar. They are generally irregularly 
shaped, and are far more distinct than is usually the case in this mineral. 
No inclusions were seen which could be taken for glass. The somewhat 
porous groundmass contains many minute polarizing grains, among which 
there seems to be some microfelsite that occasionally passes into indistinct 
fibrations. The rock is destitute of hornblende and biotite A little ferrite 
and opacite is present Particular attention should be called to the fact 
that this rock, the only one of this division whose doubtless primary quartz 
bears fluid-inclusions, can be pronounced a rhyolite by its other petro- 
graphical characteristics. It properly belongs to the trachytes, and there- 
fore dates further back than any other of the described rhyolites. 

In Good Pass, North Park, another rhyolite is found which contains 
single, large quartzes. 

The foregoing descriptions show in what abundance those fibrous 
bodies in which the fibres are not grouped radially around a centre, as in 
spliaerolites, but arranged nxially along a longitudinal line, are dissem- 



202 MICROSCOPICAL .PETROGRAPHY. 

inated through these rhyolites. The presence in these rocks of such fibra- 
tions was formerly unknown, notwithstanding they are also common in 
the often-examined Hungarian and Euganean rhyolites. These axiolites 
usually consist of distinct, uniformly thin fibres, or of wedge-like particles. 

Another phenomenon to which attention was never before directed is 
that found in some of these Fortieth Parallel rhyolites, where fibres of the 
same kind as above decribed are arranged in parallel form, producing 
bunches. So we see in the arrangement of the fibres in these rhyolites four 
different types: a,, centrally radial; ft, longitudinally axial; c, parallel; 
d, confused and orderless. The development of fibres is, indeed, a phe- 
nomenon very characteristic of rhyolites : though the tendency to fibration 
may in many cases be feeble and imperfect, it is seldom entirely wanting. 
Trachytes, on the other hand, are remarkable for lacking all signs of fibres. 

The rhyolites of the Fortieth Parallel are generally poor in tridyinite ; 
occurrences of the mineral being comparatively rare. This may in some 
way be consequent upon the great quantity of quartz present in most, of 
these varieties; for it has been stated, as a result of observations of European 
rocks, that an abundant secretion of quartz is unfavorable to the formation 
of tridymite in the same rock. 1 

Augite is more frequent in these rhyolites than was formerly supposed, 
being associated with hornblende. There are even occurrences where 
hornblende is wanting, and augite is associated with quartz and sanidin. 
In earlier times, as is well known, it was a petrographical law based upon 
macroscopical observations, that quartz-bearing rocks never contained augite 
as an ingredient. 

Neither macroscopical nor microscopical white potash-mica (muscovite) 
could be detected in any of the examined rhyolites of the Fortieth Parallel. 
The only mica found was a dark magnesian biotite. The muscovite seems 
to have died out and totally disappeared since the beginning of the Tertiary 
formation. 

In some rhyolites here, the feldspar contains fluid-inclusions, each with 
a moving bubble, in unexpected profusion ; being often as rich in them as 
the feldspars of old granites. 

1 F. Z., Poggendorfl's Aiinaleu, CXL, 492. 



KHYOLITE. 203 

But out of the many thousands of quartzes which have been carefully 
examined, only two were discovered with liquid-inclusions. (All the others 
were, characteristically, filled with the most perfect glass-inclusions.) And 
of these two quartzes bearing liquid, one occurred under such circumstances 
as to make it appear highly probable that it was nothing else than a foreign 
fragment in the including rock; while that bearing the other quartz was 
only a rhyolite petrographically, belonging geologically to the older 
trachytes. 

Our study of the rhyolites of the Fortieth Parallel, therefore, corrobo- 
rates the result obtained by the examination of similar European rocks, that 
the quartzes in genuine members of this group have no inclusions except 
of glass. 

One of the convictions which this section most strongly enforces is, 
that the microscopical structure of rhyolite develops in a far greater varia- 
bility and variety of types than that of any other rock. 

In conclusion, we shall try briefly to sum up the most characteristic of 
the many types of structure in which the rhyolitic groundmass appears in 
these extremely complex rocks: 

a. Crystalline throughout, entirely composed of individualized, polar- 
izing grains, and generally rich in large secreted crystals; the groundmass 
sometimes possessing an unquestionably microgranitic structure, being made 
up of easily determinable grains of quartz and feldspar: this is a rare type. 

b. Microfelsitic, becoming in spots and passing by the different steps 
of transition into an imperfectly granular structure, often containing more 
or less perfectly developed sphserolites, and generally bearing ferrite and 
opacite: a frequent type. 

c. Aggregation of colorless, polarizing particles and colorless glass: 
very rare. 

d. Alternating bands of light-colored genuine glass and microfelsite: 
rare. 

e. Predominating microfelsite, showing some polarizing particles, and 
bearing single, dark, tail-formed axiolites, or short, longitudinal, axially- 
fibrous bodies. 



204 MICROSCOPICAL PETROGRAPHY. 

/ Microfelsite, which is traversed by a net-work of axially fibrous or 
cuneate strings, having a distinct middle suture. 

g. Net-work of axially fibrous or cuneate strings, with concentric, 
radially-fibrous.sphserolites in the meshes 

h. Net- work of axially fibrous or cuneate strings, with more or less 
distinct crystalline-granular aggregations in the meshes: rare. 

i. Plain aggregation of sphserolites. 

j. Confused aggregation of bunch-formed systems of accurately parallel 
fibres. 

Jc. Confused, felt-like aggregation of short fibres. 

I. Aggregation of cumulites, occasionally mingled with sphseroh'tes. 

m. Half-glassy mass, made up of an aggregation of thin, little microlites, 
fully imbued with glass, passing into obsidian; rocks, generally rich in 
larger crystals of quartz, sanidin, and biotite. 

n. Fluidal bands of dark-brown grains, undulating and contorted, which 
include homogenous glass. 

0. Bands identical with those last described, which include fibrous 
sphserolitic or axiolitic bodies, instead of homogeneous glass. 

p. Light-colored homogeneous glass, traversed by pearlitic cracks, 
which are associated on both sides with narrow zones of microfelsite. 

It is doubtful if these sixteen different types represent all the varieties 
of the rhyolitic groundmass; but that they comprehend the most character.- 
istic and common ones is proved by the fact that the comparative study of 
more than one hundred and fifty thin sections of rhyolites from Hungary, 
Transylvania, Rhenish Prussia, the Euganean Hills, Iceland, and New 
Zealand, did not discover a single variety which is not represented and 
described among those of the Fortieth Parallel. 

Our examinations prove that the waving fluidal phenomena of rhyolites 
are produced: 

a. By the different amount of coloring particles (needles and grains 
of ferrite and opacite), alternating in layers, while the nature and structure 
of the main mass remains the same throughout the rock. 

1. By the band-like alternation of different varieties of structure (gen- 
erally with a gradual passage between one another) ; for instance, by the 



EHYOLITB. 205 

c. Alternation of more or less distinct crystalline-granular layers with 
sphserolitic ones; 

d. Of microfelsitic and more or less perfectly crystalline layers; 

e. Of imperfectly and distinctly granular layers; 

/ Lastly, of layers of colorless and fine brownish-yellow globulitic 
glass, of which the former have, very characteristically, either an indistinctly 
crystalline or a fibrous structure, and usually contain dark, hair-like micro- 
lites, which generally have their root in the darker glassy bands. 






206 MICROSCOPICAL PETROGRAPHY. 



SECTION III 

GLASSY AND HALF-GLASSY (HYALINE) RHYOL1TES. 

The rocks described under this head are distinguished by consisting, 
entirely or in very large part,, of glass. Obsidians, pitchstone-like rocks, 
pumicestones, and pearlites belong geologically to the rhyolites, of which 
they are only a petrographical modification. 

On the left river-bank at Truckee Ferry, in Truckee Canon, such 
rocks outcrop through and over trachytes. There is one curious occurrence 
[489], which shows in the hand-specimens a brown, obsidian-like glass, that 
has a vivid orange color in the thin sections. It is remarkable that this pure 
glass has no microlitic secretions whatever, except occasional rudiments of 
microscopical feldspars. 

Another beautiful variety of obsidian from the same locality (Plate VIII, 
fig. 4) has a greasy lustre, and consists of Iamina3 and layers of two kinds 
of glass, twisted and entangled together in the most confused manner. One 
is a quite colorless, pellucid glass, the other a somewhat pale-brownish 
glass, with extremely fine, brownish grains, measuring only 0.0005 mm , 
imbedded in it. Indeed, the intermingling is as if the thin layers of these 
two kinds of glass were kneaded together artificially ; and their entangle- 
ment or confusion, the sections appearing as narrower or thicker waving 
fluidal lines, is often comparable to a gnarled and knotty tree-trunk. 
Some of the layers are twisted into the shape of the blades of a screw- 
propeller. 

The largely semi-hyaline rock from the Rhyolite Peak north of Desert 
Station, Truckee Range [490], is a dark-gray glass-mass, containing many 
feldspars and biotite plates, with some hornblende. It is destitute of quartz. 
The thin, sharp, oil-green laminae of mica in the glass, often measuring 
only 0.01 mm in diameter, are very pretty. This rock is very instructive 
as to the character of aggregations of small globulites in glass (Plate IX, 
fig. 1). The simple elements are very pale-greenish, rounded or angular, 
isotrope grains, from 0.002 to O.OOS" 1 " 1 in size. Sometimes these grains are 
isolated ; at others, two or three, or more, are conjoined into a little lump or 



HYALIFE-RHYOLITE. 207 

star; and, at others, they are strung out in lines, like pearly strings, an 
interval between the grains being usually distinctly visible, but in rare cases 
they touch one another. Towards one end, the globulites gradually grow 
smaller, so that the needles seem to be pointed. Two of these strings are 
often seen diverging from a thicker globulite, or from a heap of them, like 
the arms of a pair of compasses. This is occasionally repeated three or four 
times, producing manifoldly knee-formed objects. In other cases, the aggre- 
gation of globulites takes the form of the most perfect, curved and twining 
tendrils. Sometimes a number of these radiate from a centre, suggesting a 
spider with many legs. The curvature is in some cases very great, and the 
torsion occurs in widely different planes, as in a cork-screw. In short, all 
the interesting phenomena are repeated here in natural glass, which Vogel- 
sang has described in the artificial slags from the Friedrich-Wilhelmshutte 
near Siegburg. 1 

South of Warm Spring, near old Fort Churchill, Nevada, there occurs 
a light pumicestone, or (as proved by the microscope) more properly a 
pumicestone-breccia [491], the rock being made up of angular pieces welded 
together. This is made evident by the variety and independence of direc- 
tion of the glass-lines in the single fragments. They consist of densely- 
woven, parallel strings, ropes, and lines of colorless glass, occasionally with 
long, cylindrical hollows between them. The few feldspars of the rock, 
which are mostly striated, are remarkable for the wide difference in size of 
the bubbles in their glass-inclusions : large lumps of glass are seen which 
have the most minute bubbles ; and thick, dark bubbles are found, 
surrounded with but a thin zone of glass. 

A dirty, yellowish-gray pumicestone occurs at Mullen's Gap, west 
side of Pyramid Lake [492], in conjunction with rhyolites and breccias. 
It is a more homogeneous rock than that last described, and bears feldspars 
with glass-inclusions, which are made remarkable by the number of their 
bubbles. Some of these glass-grains contain five or six small bubbles, 
and some are quite finely porous. The pumicestone-glass contains brown 
biotite plates, and, as foreign fragments, rounded, bluish-gray particles, l mm 
thick, of a felsitic rhyolite. 

1 Die Krystalliten, 1875, page 25, Plate II. 



208 MICROSCOPICAL PETROGRAPHY. 

A splendidly devitrified pumicestone comes from the same locality. 
It contains, in its light-colored glass (rich in large and small cavities), a great 
number of small, almost colorless, belonites, averaging 0.02 mm in length, and 
0.0015 mm in thickness, which are arranged parallel, and form excellent bands 
of waving lines that wind around the oval cavities ; also sharply hexagonal 
and triangular plates of specular iron, of which the thinner ones are orange- 
colored and pellucid, and the thicker brown-black and impellucid; and, 
lastly, some brownish-yellow microlites and quadrangular magnetites. The 
rock contains, porphyritically, large feldspars, measuring 3 mm . 

But the two best regions for glassy rocks are the Pah-tson Mountains 
and Montezuma Range, where they occur in connection with rhyolites. The 
beautiful pearlites found here, which contain grains and balls of obsidian, 
that are often as thick as a hazel-nut or walnut, merit particular attention. 
A bluish-gray pearlite from Basalt Ridge, Pah-tson Mountains [493], is 
shown by the microscope to be a nearly colorless glass-mass, with concen- 
trically curved cracks, not unlike the layers of an onion. They are never 
complete rings, but only segments of circles. This mass is devitrified by 
very small and rare, colorless, or feebly-greenish microlites; by larger dark- 
green prisms, very much crippled and indistinctly crystallized, which, though 
undichroitic, cannot be pronounced augite, because neither the cleavage nor 
the prismatic angle is to be observed ; by larger feldspars (of which a part 
are triclinic) ; and, lastly, by a considerable number of biotite plates, varying 
from light brown to black-brown in color, which are sometimes only rudi- 
mentary and sometime sharply outlined hexagons. 

A rather dull-looking, yellowish, or bluish-gray pearlite from Aloha 
Peak, Pah-tson Mountains [494], owes its appearance to an abundance of 
microscopical products of devitrification, which here also are rounded glob- 
ulites, isolated and in peculiar aggregations (Plate IX, fig. 2). The small 
grains are arranged in long needles, not unlike crystals and tendril-like 
forms; the latter often so much curved and twisted as to resemble knots. 
These twisted tendrils usually have undulated borders, produced by the 
lateral disappearance of the lineally arranged globulites. It would seem 
that these distorted and twisted, line-like cilia are characteristic of pearlites ; 
for they have also been observed everywhere in the classic pearlites from 



HYALINE-EHYOLITE. 209 

Hungary (for instance, in that from Glashiitte near Schemnitz, Telki-banya, 
Bereghszasz), Cattajo, Euganean Mountains, Italy, and Mount Sommers in 
the southern island of New Zealand; a development, on the other hand, 
strikingly rare in obsidians, pumicestones, and pitchstones, otherwise so 
closely related to them. All these crystallites are not straight, linear bodies, 
but present distinct, waving fluidal phenomena, being grouped together in 
undulating bands. The rock contains splendid brown hornblende sections, 
measuring 0.8 mm along the orthodiagonal. Some of these crystals are excel- 
lent twins (parallel to oo^oo); the field on one side of the orthodiagonal 
axis in the horizontal sections becoming, in one position of the nicols, 
entirely black, and at the same time on the other side dark yellowish- 
brown. Some individuals of feldspar are present. 

At Grass Canon, Pah-tson Mountains, a pearlite occurs [495] which has 
the most excellent onion-like or layer structure. The glass-bulbs vary from 
the size of a pea to that of hazel-nuts. Products of devitrification in the 
nearly colorless glass (Plate IX, fig. 3) are chiefly cylindrical, needle- 
formed microlites, not exceeding 0.01 mm in length, and either of a pale- 
greenish hue or almost colorless; shorter, prickly bodies, of the same kind, 
grouped into very minute, bristly lumps or loose stars; black, opaque 
grains; straight and undulating, short, black, line-shaped microlites (tri- 
chites). Small, pellucid grains often cleave to the surface of the black 
microlites, giving them the appearance of being covered in part with powder 
or a granular dust. In consequence of fluctuations, all these microlites are 
arranged in parallel bands ; a phenomenon well known from examinations 
of pearlites of other regions. 1 The grouping is without any reference to 
the concentric structure of the pearlitic glass globes; the bands traversing 
independently the shells of the glass-grains, and passing directly through 
many of them. This evidently proves that the microscopic devitrification 
and the concentric layers of the pearlitic shells are entirely independent of 
each other. The latter seems to be merely a phenomenon of contraction. 
Considering this intimate analogy, it is very remarkable that, in these 
typical American pearlites, sphaerolitic formations, which are so well and 
widely developed in the Hungarian varieties, are rarely found. 

'F. Z., Zeitscbrift d. d. geolog. Gesellschnft, XIX, 1807, 7G8. 
14 UP 



210 MICROSCOPICAL PETROGRAPHY. 

The obsidian which is found in the form of kernels and balls in the 
pearlite of Grass Canon [496], as in the Siberian marekanite from Ochotsk, 
is of an intensely dark, blackish color. It gives, under the microscope, curi- 
ously, a light-gray section, which is hardly at all devitrified. This is one of 
the purest obsidians known, being only surpassed by the green bouteillen- 
stein, or pseudochrysolite, from Moldauthein, Bohemia, and it contains only 
extremely rare and thin black microlitic lines, which are often knotted 
(trichites). Its compactness and freedom from gas-cavities is remarkable, 
because the other very pure obsidians, for instance, the marekanite, the 
pseudochrysolite, and the famous obsidian from the Hrafntinnuhryggr in 
Iceland, are extremely rich in microscopical pores. 

Some pearlite lumps which occur in the pumicestone-tufas of Grass 
Canon [497] are also hardly at all devitrified, with the exception of a sparse 
sprinkling 'of delicate, water-clear microlites; but they contain a great number 
of long and narrow, pointed-oval pores that are often twisted like a para- 
graph-mark. 

In the Montezuma Range, west of Parker's Station, a pearlitic rock 
occurs, bearing black, dull, conchoidal glass which has a resinous lustre, and 
rounded clods of obsidian [498]. The latter (Plate IX, fig. 4) is prettily 
devitrified by subtle black trichites, which are usually sharply twisted, and 
often entwined in indistinct little flocks. The surface of the straighter and 
stronger ones is often powdered with extremely minute, pale, pellucid grains 
and prickles. Here also the microlites show, by waving fluidal lines, a dis- 
tinct parallel arrangement, presenting excellent phenomena of distortion. 
In a single thin section, the lines of trichites here project into the plane of 
the section, and there form an angle with it. In the latter case, the ends 
of the black microlites appear like mere dark points or small grains. 

The intensely devitrified pearlite from above White Plains, Montezuma 
Range [499], is imperfectly conchoidal, and contains large' feldspars, 1.2 mm 
in size, which are mostly triclinic, together with thin, brown biotite plates, 
which, if cut transversely, form nothing more than short, black lines. 

A rock of a gray color, and possessing a dull lustre, which is found 
forming fine columns in the Montezuma Range, back of Lovelock's Knob 
[500], is an intermediate member between pearlite and rhyolite. It is rich 



HTALINE-RO YOLITE. 211 

in glass, has an imperfect globular structure, shows the interesting phenom- 
enon of globulites aggregated into needles and tendrils, and includes, in 
its almost colorless glass, beautiful quartz crystals, with glass-inclusions, 
some biotite plates, dark-brown hornblende in sharply developed crystals, 
possessing a splendid cleavage, and some lighter sections of undichroitic 
augite, characterized moreover by normal contours and the nearly rect- 
angular cleavage. This latter ingredient, which some time ago was not 
supposed to exist in such highly silicated rocks, will also be found men- 
tioned in some of the half-glassy masses hereafter to be described, where it 
is present in such quantity that it sometimes predominates over the horn- 
blende. It is very likely that augite will also be discovered in analogous 
European occurrences, most of which were studied microscopically before the 
distinctions between augite and hornblende were definitely known. Many 
of the dark-green' crystals occurring in obsidians, pitchstones, etc., which 
have been described as hornblendes, may, upon more careful examination, 
prove to be augites. 

Yet there occur, in Montezuma Range, typical, concentrically globular 
pearlites [501], which bear, beside the other elements producing devitrifi- 
cation that are mentioned in the rocks heretofore described, those colorless 
crystals or crystallites which probably belong to feldspar. Their ends 
either terminate in two acicular points of varying- length, are regularly 
serrated like stairs, or are irregularly lobate and deeply riven, so that they 
look like the shattered ruins of crystals (Plate I, fig. 20). Nevertheless, 
they are not broken at the ends, but are simply in a rudimentary state, 
imperfectly developed. Such a pearlite is rich in biotite plates, partly 
in the form of sharply outlined crystals, and partly as irregular, ragged 
patches. 

Some singular rocks were found on the foot-hills of the Montezuma 
Range, near White Plains [502, 503, 504]. They exhibit at once a structure 
very much like that of pearlite, and a linear lamellation produced by the 
varying composition of their zones. The microscope discovers a great 
quantity of products of devitrification, nearly all the bodies found in the 
above-described pearlites being present : pellucid globulites, dark grains, 
globulitic needles, spikes, tendrils, and cilia ; the screw-like and spider-like 



212 MICROSCOPICAL PETROGRAPHY. 

formations, and black trichites. The crystallites terminate in pinnacles. 
Feldspar crystals, some quartzes, rare apatite prisms, rather many biotite 
plates, and extremely beautiful brown hornblende, of which the transverse 
sections measure as high as Q.15 mm in length, are also met. The linear lamel- 
lation, forming in the sections dull, delicate, and often rather fine, undulated 
lines, is partly the result of the exceedingly dense aggregation into bands 
of some quite small, pellucid, round grains, through which the glass is 
hardly visible, partly of the zonal grouping of small, yellowish-gray, sphaar- 
olitic globules composed of exceedingly fine fibres. The sphserolitic stripes 
are generally somewhat broader than the granular ones. "That the sphser- 
olites do not occur elsewhere in the glass, being limited to these zones, is 
a point worth mentioning. 

A member intermediate between pearlite and rhyolite occurs at the 
Shoshone Mesa [505]. An arrested tendency to form pearlitic globules is 
seen in the gray, half-glassy mass, which also contains sphserolites nearly as 
thick as a walnut, that develop, by decomposition, the concentric layer- 
structure. Some of the single shells or layers are less easily decomposed 
than others, and those which are least affected become isolated ; so that the 
altered sphaBrolites seem to consist of single, partly loose, convex layers, like 
the crystal of a watch. There occur all the stages of transition between 
sphserolites in the natural state and cavities in which there are five or six 
shells with their isolated borders. Such occurrences in Hungarian rhyolites 
have been named lithophyses by v. Richthofen, 1 who described them as 
bladder-like, vesicular swellings of the molten material. J. Roth has 
expressed the well-founded opinion, however, that these lithophyses are 
nothing but mechanically and chemically altered larger sphajrolites; 2 a view 
which has also been put forward by Szab6 with respect to those from Tokaj, 
and which is doubtless confirmed by the American occurrences. In micro- 
scopical structure, the rock resembles the pearlites from Grass Canon, 
although the mass of the predominating small, pellucid microlites has a very 
pale, but distinct, greenish color. The rock also contains larger quartzes, 
with fine glass-inclusions, feldspars measuring 4 mm in length, brown horn- 

'Studieu aus den ungarisch-siebenbiirgischen Trachytgebirgen, 1861, 180. 
' Beitrage zur Petrograpbie der plutonischeu Gesteiue, 1873, 168. 



HYALINE-KHYOLITE. 213 

blendes, and some dark-green, undichroitic, larger grains and prisms doubt- 
less belonging to augite, notwithstanding their form and cleavage are not 
easily discernible. If the question were asked, whether the small colored 
microlites which are not feldspars belong to hornblende or to augite, it could 
only be said that they seem most like the latter. 

In the western foot-hills of the Owyhee Bluffs, a black rock, appearing to 
be very glassy, is found [506]. Under the microscope, it is seen to consist of 
colorless and brown stripes of glass, which are very much bent and of varying 
thickness. The colorless ones contain a great many straight and twisted 
pellucid microlites, while the brown are much poorer in them. There are 
not many larger crystalline secretions, but among those present are distinct 
green augites and sanidins with irregular, ragged inclusions of the brown 
glass variety. 

The summit of Mount Neva, Cortez Range, is formed of a rock rich in 
both crystals and glass [507] The base is a gray glass containing numerous 
microlitic prickles, needles, stars, loops, and tendrils, with quartz, sanidin, 
plagioclase, and an unusual amount of beautiful biotite in larger crystals. 
Neither hornblende nor augite is visible. 

Some very interesting obsidians occur at the Ombe Bluffs, Utah, just 
south of the Union Pacific Railway. One of them [508] is a red-brown 
glass rock, with imbedded feldspars and quartzes measuring 8 mm (Plate 
X, fig. 1). The quartzes bear, in unsurpassed perfection, macroscopical 
inclusions of the glass, retaining the natural color, and almost as thick as a 
pepper-corn. Such splendid macroscopical inclusions as these are very rare, 
though minute microscopical ones are not infrequent. There is only one 
other occurrence of the kind known. This is in the famous pitchstone from 
the island of Arran, Scotland, the feldspars of which include thick grains of 
the prevailing deep-green glass easily visible to the unaided eye. The micro- 
scope discovers that the chief mass of the rock is made up of two differently 
colored kinds of glass. The sections consist of prevailing yellowish-red and 
nearly colorless glass, the dividing lines between which are sharply drawn. 
They are in the form of much contorted, alternating bands and stripes. The 
thin sections look as if thin layers of red and colorless glass had been artifi- 
cially laid upon one another, thoroughly kneaded together, and then drawn 



214 MICROSCOPICAL PETltOGliAPIIY. 

out longitudinally. The red glass contains a great number of long and 
quite narrow cavities arranged parallel to the direction of the stripes, but is 
almost destitute of products of devitrification. The colorless glass is more 
compact, and bears quite an abundance of black grains and irregular trichites. 
Orange-colored, microscopical glass-grains with large bubbles are imbedded 
in great quantities in the quartzes. If the section passed through a bubble, 
the latter of course only appears as a delicate, rounded ring, being incapable 
of producing the optical effect of a cavity. 

In the same locality, a rock occurs [509] which is composed macro- 
scopically of black, red, and yellow bands, streaks, and dashes of glass, 
intricately woven together. The microscope reveals the proper ground- 
substance to be a water-clear glass, colored macroscopically, as above 
described, by a variety of mechanically included bodies. The black stripes 
are made up of little angular and rounded, opaque, black grains and thin, 
short trichites. The colorless glass also bears narrow, linear fathoms and 
broader, curved microscopical dashes of a reddish-yellow color, which are 
generally somewhat darker along the borders than in the middle. Where 
these streaks are abundant, the glass has a red color; where they are less 
frequent, it is yellow. Microscopical stripes and bands of the same nature 
are much contorted, and sometimes form fantastical undulations. Quartz 
and sanidin are the crystalline secretions. 

On the east slope of the Goose Creek Hills, a largely half-glassy rock 
is found [510], which shows a distinct tendency to form pearlitic globes, and 
contains quartzes nearly as thick as a pea. The microscopical crystalline 
ingredients (Plate X, fig. 2) are, a, quartz in imperfectly formed dihexahe- 
drons; ft, sanidin; c, plagioclase; d, highly lamellated brown biotite ; e, very 
beautiful, feebly yellowish-green augite (sometimes crystallized in form co P. 
ccj?co . oo ^B 00 . P) ; f, quite rare, dark-brown hornblende ; and, g, mag- 
netite. The quartz, sanidin, and plagioclase contain excellent inclusions of 
colorless glass. The mass between these ingredients is a throng of micro- 
lites, with an abundance of glass, and it presents the most perfect, waving, 
damming and encircling fluidal phenomena. The inicrolites are naturally 
a very pale green, and, judging by the secreted crystals, there can be little 
doubt that they belong to augite ; more especially since the pronounced 



HYALINE-RUYOL1TE. 215 

transition between augite crystals and thinner, lighter, and more irregu- 
larly shaped (augite) prisms can be easily traced. 

In the same locality, on the east slope of the Goose Creek Hills, there 
is another rather curious rock [511]. Macroscopically, it is composed of a 
pale grayish glass, in which many rounded and angular bodies, of a deep 
isabel-colored substance, having a dull or waxy lustre, are dispersed. These 
bodies reach the size of a pea, and are encircled by a somewhat darker, 
dirty, yellowish-gray border. In the sections, the central mass and its bor- 
der strongly contrast ; the dull portions being but slightly transparent. The 
glass which forms the main mass bears quartzes (including glass-inclusions 
with delicate, green crystals) and feldspars, together with the same microlitic 
products of devitrification found in the other hyaline rocks. The microlites 
conspicuously exhibit the phenomena of fluctuation, and they are dissem- 
inated so loosely that the pure, compact glass appears distinctly among them. 
It is surprising to see that these microlitic lines traverse the dull yellowish- 
gray portions without altering their direction at the entrance or exit, so that 
the spots present, with respect to the nature, quantity, and arrangement of 
microlites, precisely the same behavior as the prevailing glass-mass. And 
it is also very remarkable that in the isabel-colored portions no pure, homo- 
geneous glass lies between the microlites, its place being taken by a fine 
fibrous glass or sphaerolitic substance (if the expression is allowable), the 
direction of the extremely delicate fibres being, upon the boundary, iisually 
rectangular, even in the irregularly shaped spots. The fibrous substance 
produces the dullness and feeble pellucidity above mentioned, and between 
crossed nicols it in some places sends out an indistinct play of light. A 
development of sphaerolitic fibres has therefore happened locally in this rock. 
It did not, however, advance so far as the production of real sphserolites, and 
did not occur until after the microlites were all solidified and dispersed 
through the rock by fluidal waving, so that it did not at all alter their nature 
or direction. Moreover, these portions appear to be somewhat richer in iron 
than the main glassy mass of the rock. 



CHAPTER VIII. 

BASALTS. 



SECTION I. GENERAL EEMAEKS. 
SECTION II. AUGITE-ANDESITE. 
SECTION III. TRUE BASALT. 
SECTION IV. APPENDIX TO TRUE BASALT. 



SECTION I. 
GENEEAL EEMAKKS. 

Of the younger Tertiary eruptive masses, the basaltic, in a general 
sense, are the opposites of the trachytic rocks, namely, rhyolite, trachyte, 
phonolite, and hornblende-andesite. They are characterized by the presence 
of augite, the total absence, or very small amount, of quartz and sanidin, the 
frequent occurrence of olivine, an abundance of magnetite, a more basic 
constitution, a higher specific gravity, and a darker color. It is only in com- 
partively rare cases that they present their mineralogical constituents in 
forms visible to the unaided eye. 

A great multitude of dark, heavy, basic rocks have been collected for 
basalts. Their chief mass seemed macroscopically to be homogeneous, 
and their mineralogical composition gave risa to conjectures and interpreta- 
tions as untrustworthy as they were numerous, until microscopical study 
achieved a solution of this much-vexed question. 1 The examinations show 

1 F. Z., Untersuehungen iiber die mikroskopiscbe Structur und Zusammensetzung 
der Basaltgesteine, Bonn, 1869. 



BASALTS. 217 

that these rocks, very similar in their exterior behavior and in their 
chemical constitution, are not, as was accordingly inferred, made up of the 
same principal mineral ingredients ; but they are clearly divisible into 
three large groups, possessing different mineral combinations. The micro- 
scopical association of ingredients discovered here are not, as such, however, 
at all new or strange, but are merely a phanerocrystalline repetition of 
types which have long been known. As stated above, the basalts may be 
arranged into three divisions, the behavior of each being quite different 
from that of the rest; and, according to the principles valid in macroscop- 
ical petrography, they are three separate and well-characterized types of 
rock. With respect to the chief silicate, which is free from iron and rich in 
alumina, that always accompanies the never-wanting augite, which is rich 
in iron and poor in alumina, there exist the following: a, feldspar- 
basalts, characterized by the presence of plagioclase, usually wanting in 
leucite, occasionally with some nepheline, which correspond to the more 
distinctly grained dolerites and anamesites; ft, nepheline-basalts, occasionally 
containing some leucite, and, when rich in nepheline, usually free from feld- 
spar, corresponding to the nephelinite, for instance, from the Lobauer Mount, 
Saxony ; c, leucite-basalts, which are almost always free from feldspar, but 
generally contain nepheline in comparative abundance, although less than 
the leucite. Contrary to the previously entertained opinion, therefore, feld- 
spar is not a principal ingredient of all so called basalts. The members of 
all the three groups always bear magnetite, almost always olivine, and 
sometimes titanic iron. Mellilite and haiiyne only occur separately, and are 
limited to the nepheline and leucite-basalts. 

The microscope also proves that the separation into three groups not 
only refers to the massive, proper basalts, but to all the basaltic lavas. These 
latter are divided into : 

Feldspar-basalt-lavas. 

Nepheline-basalt-lavas. 

Leucite-basalt-lavas. 

And not only all the varieties of constituent combinations which are 
met in the proper basalts, but all the most special relations presented in 
their microscopical structure, are exactly repeated in the basaltic lavas. 



218 MICKO8COP1CAL PETKOGHAPHY. 

Whether a basalt is a feldspar, a nepheline, or a leucite rock must always 
be decided in each case with the microscope ; for the simple black ensemble, 
common to them all, completely hides the difference of their interior miner- 
alogical composition, and even the most careful chemical analyses do not 
afford material for a rigid determination. 

Yet it becomes evident, by a comparative review of the examinations 
thus far made, that, taken in general, the basaltic occurrences assembled 
together in one region differ but little in their composition The stronger 
contrasts are obtained when rocks from different regions are compared For 
example, the German basalts of the Siebengebirge, and the enormous basaltic 
and anamesitic depositions of Scotland, the Western Islands, the Faeroer, and 
Iceland, are all feldspar-basalts, and not a particle of leucite has yet been 
discovered in them. The proper basalts of the Erzgebirge, between Saxony 
and Bohemia, on the other hand, bear only leucite and nepheline, and are 
free from feldspars. No lava from the environs of the lake of Laach, 
Rhenish Prussia, has been examined in which there was not to be observed 
an abundance of leucite. For aught that is now known to the contrary, 
leucite is totally wanting in the numerous basalts and lavas of Central 
France (Auvergne, Velais, Vivarais, Cantal), which bear feldspars, and are 
free from, or poor in, nepheline. 

The nepheline-basalts sometimes contain leucite, and the leucite-basalts 
usually bear nepheline ; so that these two groups appear to be much closer 
connected with each other than either of them with the feldspar-basalts. 
Moreover, the nepheline and leucite groups often occur together in one 
region; for instance, at Erzgebirge, Rhb'n, in Germany, and Northern 
Bohemia. And where feldspar-basalts are abundantly developed, there is 
little probability of finding with them members which are rich in leucite 
and nepheline, the latter almost always occurring separately. 

These rules, deduced from a comparative study of European basaltic re- 
gions, are found to hold good of the occurrences along the Fortieth Parallel 
in Western America. Notwithstanding the enormous number and extent of 
development of the basaltic eruptions here, the rocks are, with very few 
exceptions, and those confined to the eastern limit of the examined territory, 



BASALTS. 219 

feldspar-basalts; which, in general, are no doubt the most frequent type in 
all parts of the globe. If on this account the petrographer finds himself 
confined to the monotony of one general type of composition, and searches 
in vain for those interesting mineral combinations exhibited by the leucite 
and nepheline-basalts, he is amply compensated by the great number of 
remarkable and characteristic varieties of microscopical structure offered by 
the numerous feldspar-basalts. 

Beside the proper and genuine feldspar (i. e., plagioclase) basalts, 
which always contain olivine, often in abundance, there are some closely 
allied rocks which show sufficient persistent differences to make it proper 
to arrange them into a subdivision. They are distinguishable from the 
proper feldspar-basalts, although the two are almost identical macroscopi- 
cally, in these respects: a, besides the plagioclases, there are always distinct 
sanidins, which are often in Carlsbad twins, but never predominant over the 
plagioclases; 6, olivine is generally wanting, which characteristic feature hap- 
pens where the sanidins are present in considerable quantity; c, the behavior 
of the microscopical structure is quite unusual in genuine feldspar-basalts, the 
chief mass of the rocks being a felty accumulation of small microlites evenly 
impregnated with glass, and accordingly the larger feldspars are much more 
highly charged with glass-inclusions than are those of the proper basalts, 
beside wluch the glass gives the rocks a rather distinct resinous lustre; d, 
they sometimes contain hornblende as an accessory ingredient, which is 
generally unknown to the true basalts; e, the chemical analyses show that 
they all have a higher amount of silica than the proper basalts, rich in 
olivine, free from sanidin, and poor in glass; /, lastly, it may be mentioned 
that the apatites of these rocks are generally dusty, which is more common 
in andesites and trachytes than in basalts. The points, therefore, in which 
the rocks in question differ from the genuine basalts are evidently of only 
secondary importance, the chief constituents of both being augite and 
plagioclase; and they will always, upon merely a macroscopical examina- 
tion, be classed as basalts, and geologically the two are closely connected, 
possessing the same forms of occurrence and being likewise of younger 
eruption than even the rhyolites. They differ fjpui the common trachytes 
in the predominance of plagioclase and the abundant presence of augite, 



220 MICROSCOPICAL PETROGRAPHY. 

although they are pretty closely related to that rare variety, the augite- 
trachytes, which contain, beside invariably though often but slightly predom- 
inating sanidin, a good deal of plagioclase, augite instead of hornblende, and 
are likewise free from olivine ; but geologically and geographically they are 
distinct from the trachytes. These basaltic rocks, of course, differ from the 
hornblende-andesites by the amount of augite they contain. If a name is 
to be conferred upon them, that of augite-andesite seems the most suitable. 
When the special chemical composition of the plagioclases was a leading 
feature by which rocks were determined, and when oligoclase and labrado- 
rite were considered as fixed species, this name was applied by J. Roth to 
the Tertiary combination of oligoclase and augite. But now, when only a 
contrast of the triclinic nature of the feldspar-constituent with that of the 
orthoclase or sanidin is valuable in petrographical nomenclature, such an 
application of this name becomes superfluous ; for the so-called rocks come 
within the general class of plagioclase-augite mixtures, becoming members 
of the basalts, the more as it has appeared highly probable of most of the 
latter that their plagioclase is richer in silica than the so-called labradorite. 
It therefore seems that the name augite-andesite may unhesitatingly be trans- 
ferred, without causing any confusion or changing its original meaning, to 
those feldspar-basaltic rocks in which (beside the predominating plagioclase) 
sanidin has a certain part, and which are either free from or extremely poor 
in olivine, whose presence is more characteristic of the wider-spread genuine 
basalts. At any rate, these rocks are the augite-bearing equivalent of the 
hornblende-andesites, but on that account are also petrographically more 
nearly related to the basalts. In a certain sense, however, they occupy an 
intermediate position, presenting a kind of passage between the two ; and, 
therefore, the description of the basaltic rocks may properly be begun with 
them. 



AUGITE-ANDESITE. 221 

. 

SECTION II. 
ADGITE-ANDESITES. 

To this sub-division of basaltic rocks belong those from Java, Gam- 
biran, Rogodjampi, Grad Takan, Widodarin, Sungi Pait, which Rosenbusch 
has examined microscopically. 1 These occurrences, which have also been 
named by him augite-andesites, would seem, according to his description, 
to agree perfectly with the similar American rocks which have been analyzed. 
The rocks from Tunguragua, Cotopaxi, and Antisana, in the Andes, which 
were called quartz-augite-andesites, on account of the large amount of silica 
they contain (63. to 67. per cent), have been proved to be totally wanting in 
quartz, but rich in brown glass. 2 But if these rocks agree petrographically 
with the other augite-andesites, the large quantity of silica they contain 
removes them from the basalts. The lavas of Santorin, poured forth in 1865, 
are also rightly classed among the augite-andesites by such writers as Roth, 
Stache, Leonhard, and Urba. The black rock with a resinous lustre, from 
Bagonya, Hungary, which was formerly taken for a trachyte, also belongs 
under this head. An excellent augite-andesite from the Palau Islands, 
Australia, was quite recently described by Wichmann. 3 Dr. Ulrich sent 
from Melbourne to the Mineralogical Museum of Leipsic some rock-speci- 
mens from Kyneton, Victoria, Australia, which represent the most typical 
augite-andesites. It is very astonishing that these rocks, from such remote 
parts of the earth, should offer in the sections so detailed a resemblance to 
one another that it is impossible to distinguish them except by reference 
to the labels. 

Among the augite-andesites of the Fortieth Parallel, one of the most 
typical varieties is that first found in passing from west to east on the knoll 
west of Basalt Creek, Washoe [512]. It is a brownish-black mass, with a 
somewhat resinous lustre, containing white, ledge-formed feldspar crystals, 
which are seldom tabular; and it is both macroscopically and microscopi- 

'tJber einige vulkanische Gesteine von Java, Ber. d. naturf. Gesellsch. zu Frei- 
burg, i. Br., 1872. 

*F. Z., Die mikroskopische Bescbaffenbeit u. s. w., 418. 
3 Journal des Museum Godeffroy, 1875, Heft VIII. 



222 MICROSCOPICAL PETROGRAPHY. 

cally very similar to the well-known Hungarian rock from Bagonya, which 
is found in most mineral collections, by reason of bearing excellent hyalite 
in its hollows and fissures. Under the microscope, the Washoe rock con- 
sists of a brownish-yellow glass-ground; larger spots and insular bodies of 
it appearing quite pure and unaltered in many places. An enormous quan- 
tity of yellowish -green augite, colorless feldspar-microlites, and black mag- 
netite grains, together with larger crystals of feldspar and augite, are imbedded 
in the rock. The feldspars are largely sanidins in simple individuals and 
Carlsbad twins ; but plagioclase, with its rich strise, evidently predominates. 
Both feldspars contain the most beautiful and numerous inclusions of pale- 
yellowish, glass-bearing bubbles. Of these inclusions, nearly all, even the 
smallest ones, are rectangular in shape, and they are often arranged in lines 
which run surprisingly near to a parallel with the outlines of the crystal 
section. There are also larger inclusions of glass of a darker color; 
their shape being irregular and disfigured. The thicker augites are sharply 
outlined, having the usual eight-sided section-forms (ooP. cc-P cc . oo-f GO), 
are traversed by nearly rectangular cracks, show only the slightest trace 
of dichroism, are entirely fresh and filled with oval, glassy inclusions. 
There is no vestige of hornblende in this rock, nor yet of olivine. Some 
dusty apatite prisms are present. The quantitative determination of silica 
gave 58.015 per cent., an amount which entirely agrees with that found in 
other characteristic augite-andesites. For example: 

Top of Pico Teyde, Teneriffe 59.68 (Bolton.) 

Widodarin, Java 58.35 (Rosenbusch.) 

Chimborazo (17,916 feet elevation).. 59.12 (Rammelsberg.) 

Masaja Nindiri, Nicaragua 56.58 (Marx.) 

Klausenthal, Hungary 57.79 (Doelter.) 

Tuhrina, Hungary 58.76 (Doelter.) 

Palau Islands, Australia 57.54 (Wichmann.) 

No genuine basalts possess so large an amount of silica as these typical 
augite-andesites contain. The newly discharged lavas from Santorin, which 
also belong to this group, are even still more acid. 

Another excellent augite-andesite occurs on the hill west of Steamboat 



AUGITE-ANDESITE. 223 

Valley, Nevada [513]. It is very similar to ihe last described rock, except 
that the glass-base impregnating the felt-like aggregation of microlites in the 
groundmass, is not brownish, but light-gray. Many larger, colorless feld- 
spars measuring up to 1.5""", among which are some sanidins, are present. 
Most of them are no doubt triclinic, and all are evidently built up by sur- 
rounding zones, and surcharged with glass-inclusions. Augites in sharply- 
defined crystals are present, and magnetite and apatite are found. Horn- 
blende and olivine are wanting. 

There are some rocks which are connected with the genuine basalts of 
the Truckee River, a part of which only seem to belong while another part 
without any doubt do belong to the proper augite-andesites. The occurrence 
in the ravine north of the Truckee Road, a few miles west of Clark's Station 
[514], macroscopically very much resembles the foregoing, and is likewise 
rich in brown glass, which is here, however, of a somewhat darker color. 
The feldspars are mostly sanidin. There is at least as much orthoclastic as 
plagioclastic feldspar present; and one might therefore be inclined to 
petrographically classify this rock, which also contains exceedingly well- 
crystallized, green augites and is devoid of hornblende, as an augite-trachyte, 
if it did not bear fresh, and, characteristically, half-decomposed olivine, 
which establishes its relation to the basaltic family. The association of an 
abundance of sanidin and of olivine in a single rock is, however, extremely 
rare. So that the characteristics of both augite-andesites and genuine 
basalts are here united. This olivine, curiously, is immediately surrounded 
by a circle of tangentially placed, little, yellowish-green augite prisms. 

In the near vicinity, there occurs another augite-andesite [515], which 
also bears olivine. But the glass in this variety is a little lighter brown 
than that of the other, and is filled with small, glassy granules of a darker 
color. The globulitic development of glass-mass is much more distinct 
and beautiful here than even in the so-called melaphyre-pitchstone from 
the Weisselberg, near St. Wendel, Rhenish Prussia, which is famous on that 
account. The inclusions of the feldspars also consist of this hyaline modifi- 
cation. These rocks from the ravine north of the Truckee Road, are, 
indeed, identical in composition, both as regards the nature and the quan- 



224 MICROSCOPICAL PETROGRAPHY. 

titative proportion of their crystalline constituents; but they are different 
in structure. 

Nevertheless, there occur in this region, near the Truckee River, south 
of Wadsworth, in the foot-hills, on the southern side, very well charac- 
terized augite-andesites [516, 517]. Among the larger feldspars, there are 
as many sanidins as plagioclases; but the smaller individuals seem chiefly 
to be triclinic. In the aggregation of very small feldspar and augite- 
microlites, and magnetite, which forms the groundmass, glass seldom, 
appears, except indistinctly as a cement. Occasionally, however, it may 
be seen in pure, homogeneous, little, brown spots. The feldspars are 
highly charged with foreign substances ; the longer sections showing even 
macroscopical kernels of groundmass set in narrow frames of colorless 
feldspar substance. These rocks contain some hornblende beside the 
prevailing augite, and are instructive as to the difference between the 
two minerals, presenting the peculiarities of each in striking contrast: 
the augite sections are always very sharply outlined in the usual form, 
have nearly rectangular cracks, are of rather a pale-yellowish color, and 
are almost undichroitic : the hornblende is of a light-brown color, sur- 
rounded with a narrow, black border (never found in the augite), having 
obtuse angles of cleavage, very high, dichroism, and changing under the 
polarizer from a light brown to a deep, dark brown, with a very strong 
absorption. The hornblende never assumes its regular shape, but always 
appears in larger, broken crystals and fragments: all the smaller individuals 
and microlites belong, without exception, to augite. In short, here, as in 
so many other basaltic rocks, the hornblende has the appearance of an 
erratic, secondary ingredient, originally foreign to the rock. Olivine is 
here wanting ; a fact, as has been seen, characteristic of augite-andesites 

Rocks of this class next appear in Antimony Canon, Augusta Mountains 
[518]. The specimens from this locality can hardly be distinguished, either 
macroscopically or microscopically, from those heretofore described. The 
base is a brownish glass without any globulitic secretions, penetrating 
everywhere. It cements microlites of plagioclase and augite and grains of 
magnetite, and in some places it forms more noticeable spots. The larger 
feldspars (a few of which are sanidins) and the yellowish-green, sharply 



AUGITE-ANDESITE. 225 

outlined augites imbedded in this groundmass, are filled with an enormous 
quantity of egg-like and irregular bodies, and often large stripes of the brown 
glass. Oliviue is wanting. 

The rock from the north slopes of Jacob's Promontory, Reese River 
Valley, Nevada, is also quite similar [519]. It contains very little sanidin; 
most of the feldspars being larger plagioclases, bearing a great quantity 
of excellent inclusions of brown glass with thick bubbles, accurately rect- 
angular in shape. The glassy grains in the augites are so large that they 
sometimes include augite-microlites. No olivine can be detected, nor any 
hornblende. As usual, it is superfluous to mention the groundmass, which 
is the common glass-cemented aggregation of feldspar and augite-microlites 
and magnetite grains. 

At Susan Creek Canon, Nevada, a real augite-andesite occurs, which 
looks half-glassy, and has a somewhat resinous lustre [520.] The larger, 
zonally built feldspars (some of which are orthoclastic) bear the most beautiful 
inclusions of the yellowish-brown glass which constitutes the cement of the 
rock, together with microlites and eggs of glass, containing one or more 
dark bubbles, being often finely porous, sometimes possessing a crenate 
border. Some feldspars have an insular position in almost entirely pure 
glass; the light-brownish inclusions being woven together on the sides so 
that the feldspar scarcely appears between them ; but the bubbles of the 
single particles are easily seen (Plate XI, fig. 2). A still greater quantity of 
glass-inclusions are found in the yellowish-green augite; one crystal section, 
0.3 mm long and 0.12 mm broad, having in one plane not less that 95 oval 
particles, each with a bubble. In this ratio, there would be 2,650 glass- 
grains on the surface of one square millimetre of augite; and assuming 
that these inclusions are evenly disseminated through the entire crystal, 
a cubic millimetre of augite would have more than seven millions (7,022,500) 
of glass-inclusions. Microlites are often set tangentially around the thick 
magnetite grains, encompassing them on all sides and presenting distinct 
traces of former fluidal phenomena. Here also olivine is absent. 

In the foot-hills of the Cortez Range, Independence Valley, Nevada, 
an augite-andesite occurs, which closely agrees with that from the hill west 
of Steamboat Valley [521.] Plagioclases up to the thickness of a hazel-nut 
]5 M P 



226 MICROSCOPICAL PETROGRAPHY. 

predominate, exhibiting macroscopically rag-like, grayish-yellow particles 
of glass measuring O.l mni , whose edges curiously ramify like digits, as if 
they had been squeezed flat by the crystallization of the feldspar. At the 
periphery of the largest feldspars, which appear quite impellucid in the 
sections, the minute glass-grains decidedly predominate over the crystal sub- 
stance. There are many microlites in the sharply crystallized augites, im- 
bedded parallel to the zonal structure. Olivine is wanting. The glass base 
cementing the microlites of the groundmass is of a gray color. 

A blackish augite-andesite, having a resinous lustre, occurs in Wagon 
Caiion, Cortez Mountains [522]. Beautifully zonal feldspars are visible 
macroscopically (all of which are plagioclases), and so also are some dark 
yellowish-brown crystals of augite, both being rich in glass-inclusions. 
There is no olivine. Glass cementing the microlitic aggregation constitut- 
ing the groundmass is not brownish, but of a light-gray color. It is evident 
that all these rocks which possess a grayish glass-base, are much richer in 
larger and microlitic augites than those with brownish glass ; and it there- 
fore seems highly probable that the iron of the original magma in the first 
case mostly entered into the augites, and in the second has remained in the 
glass. Considering this, it is interesting to remark that the feldspars of the 
rocks with a gray glass-base often contain inclusions of brown glass, show- 
ing that at the time they were taken up, the differentiation of the magma and 
the secretion of augite had not happened. Occasionally, the glass-base 
appears in somewhat purer little spots, which are sometimes darkened by 
straight and curved black trichites interspersed with small, light-colored 
grains, similar to the products of devitrification in obsidians. 

One specimen of augite-andesite from the same locality contained 
some hornblende in dark-bordered and apparently erratic fragments [523]. 

At South Point, Wachoe Mountains, is an augite-andesite [524] so 
similar in the most subtle details to that from the North Pass, Cortez Range, 
that the sections cannot be told apart without reference to the labels. It 
bears augite crystals 3 mm long. 

The rock from the foot-hills of Spring Canon, Wachoe Mountains 
[525], contains considerable sanidin; but plagioclase decidedly predomi- 
nates. Olivine is wanting. There are also rather dark-yellowish, well- 



AUGITE-ANDESITE. 227 

crystallized, fissile, and faintly dichroitic augite-sections, and some very 
strongly dichroitic, yellowish-brown hornblende-fragments, with dark bor- 
ders and the characteristic cleavage. There is no olivine. The glassy base 
is of a light-gray color. 

An augite-andesite which seems to be half-glassy occurs at the west of 
White Rock, Cedar Mountains [526]. Almost all the larger feldspars are 
plagioclases. The augites are built up zonally. There are very few dark 
hornblende sections here. Some brown, sharply and straightly lamellated 
biotite is present this is a rare ingredient in these rocks, and, in connection 
with the hornblende, produces a resemblance to hornblende-andesites. The 
groundmass contains a very light glass-base, and exhibits good fluidal 
phenomena. There is no olivine. 

It is perhaps worth mentioning that the augite-andesites possessing a 
pale-gray glass-base, herein examined, are largely limited to the eastern, and 
those with a brownish glass-base to the western, regions of the examined 
territory. 

The foregoing descriptions suffice to show the extraordinary similarity 
between the augite-andesites, both macroscopically and microscopically, 
notwithstanding the individual occurrences are widely separated. 

A remarkable rock from the north bank of Palisade Canon, Cortez 
Range [527], which occupies a quite isolated position petrographically, 
should be mentioned before closing this section. Upon examination with 
the unaided eye, it would appear to be a hornblende-andesite ; but the 
real composition of the rock cannot be discovered macroscopically, for it is 
very cryptomerous. Under the microscope, it is found to consist of predom- 
inating plagioclase, which looks something like that in the German basalts, 
a little sanidin, not very abundant brownish-yellow augite, a considerable 
brown biotite, and some curiously angular quartz grains, with very minute 
glass-inclusions. Hornblende and olivine are absent. The presence of 
the quartzes is surprising; but they are easily identified by their compact 
substance and their very vivid chromatic polarization. It is remarkable 
that in the larger individuals, but more especially in the feldspars, numer- 
ous extremely fine crystals, scarcely larger than 0.003 mm , are imbedded, 
which exhibit the hexagonal pyramid very accurately formed, and for the 



228 MICROSCOPICAL PETROGRAPHY. 

most port belong to quartz, though in some measure to apatite. The rock 
has a wholly granular-crystalline structure, and it shows no vestige of a 
glass-base or other amorphous substance. This occurrence seems best to 
merit the name of quartziferous augite-andesite. In a certain sense, it is 
the dacite of augite-andesite, but with a different structure. The common 
augite-andesites are in every case entirely free from quartz, being always 
essentially half- glassy rocks; and the amount of biotite present gives the 
rock a certain resemblance to the hornblende-andesites rather than to dacites. 
The quantitative analysis of this rock made by Mr. Reinhard, at Leipsic, 
resulted as follows : 

Silica 62. 71 

Alumina 12. 10 

Sesquioxyd of iron 14. 79 

Lime 8. 34 

Magnesia 1.31 

Potassa 1. 15 

Soda 0. 73 

101. 13 



BASALTS. 229 

SECTION III. 

TRUE BASALTS. 

The proper basalts are, as is well known, subdivided into feldspar- 
basalts, anamesites, and dolerites. These three varieties differ only in point 
of structure : the same principal constituents are common to all. Dolerite 
comprises the middle and coarse-grained members. The granular structure 
of anamesite is visible macroscopically, but the individual elements cannot 
be detected with the naked eye. The basalts include those widely spread 
rocks whose mass would appear homogeneous but for the macro-porphyritical 
crystals that occasionally occur in it. 

Beginning at the west, the first basalt found has a very characteristic 
type of microscopical structure. It occurs north of American Flat Creek, 
Washoe [528]. In every point, it bears the closest resemblance to the rocks 
from the Kieshiibel near Dilln, and not far from Schemnitz, Hungary, from 
the Tungfernberg in the Seven Mountains, Rhenish Prussia, from Tunchal 
and the Puico Rivo, Madeira, and also to those from many other localities. 
The most important characteristics of this interesting structure-type (Plate 
X, fig. 3), which so often reappears along the Fortieth Parallel, are the 
absence of any amorphous, glassy or half-glassy base, and the way in which 
its macroscopical and larger microscopical porphyritical crystals of feldspar 
(which are often accompanied by similar olivines) contrast with its very 
fine-grained mixture of rounded, drop-like, or crippled augite grains of a pale 
color, and sharp, black grains of magnetite. The aggregation here described, 
whose individuals are seldom larger than 0.01 mm , take the part of ground- 
mass. Feldspar crystals, some of which are sanidins, are imbedded in it; and 
sometimes colorless particles are visible which would at first be considered 
as colorless glass, but which prove in polarized light to be feldspar. Olivine 
never occurs in this ground-mixture, but only in larger porphyritical crystals, 
which are generally well-developed, like the feldspars. The augite of the 
rock, however, is confined to the groundmass. Other characteristic points are, 
that the two long, parallel borders of the ledge-formed feldspar sections are not 
usually drawn very sharply ; that extremely minute augite grains are often 



230 MICKOSCOP1CAL PETKOGIiAPUY. 

interposed in the large feldspars, being arranged in lines which correspond 
with the triclinic striation ; and that two feldspar individuals lying close 
together have a line of augite grains running between them. 

The basalts from the second ridge of American Flat Canon, Washoe, 
are rather peculiar [529]. They are not characteristic types, but present 
exceptions to the general behavior consisting of white plagioclase and a 
blackish-green constituent, which would seem, by its outlines, to be augite, 
but it possesses a singular fibration, which at first suggests uralite, the well- 
known pseudomorph of hornblende after augite. The thin sections prove, 
however, that the mineral (which is here dark green) is not at all dichroitic, 
and therefore cannot belong to hornblende ; so that the only alternative is 
to consider it some other fibrous product of augitic alteration. The plagio- 
clases are beautifully lamellated, and are, besides, extremely well built up 
in single wrappings : crystal sections, 0.5 mm in length, consist of more than 
a dozen striae of involution. There is neither olivine nor apatite nor any 
amorphous substance ; but there are some magnetites. The rocks are in one 
place coarser-grained, or doleritic, in another fine-grained, or anamesitic. 

The next excellent basalt occurs in the Virginia Range, east of Spanish 
Spring Station [530]. The plagioclase is well striated, and is often built 
up zonally. With it is greenish augite, and olivine also occurs, its larger 
crystals altered along the borders and cracks and its smaller filled with 
a brownish-red, somewhat fibrous substance, which is, without doubt, of 
a serpentineous character. It may here be mentioned, in advance, that 
this phenomenon of decomposition is evident in all the olivines of our 
American basalts, in which respect they correspond with those from other 
parts of the globe which have been examined (see Plate X, figs. 3 and 4, 
showing the half-metamorphosed crystal of olivine belonging to another 
rock). Olivine is the substance in all rocks first falling a victim to 
alteration, which process begins on the exterior of the grains and crystals 
and pushes inward along the microscopical fissures which traverse the indi- 
viduals in all directions; and since the walls of these irregularly ramifying 
cracks are first altered, the larger crystals in one stage of metamorphism 
appear to be checkered with veins of a strange material, which divide the 
original substance into little insular spots or grains, which remain quite fresh. 



BASALTS. 231 

The product of this alteration (as appears in the wet way) is in most cases 
serpentine, possessing in one place a darker or lighter and in another a 
reddish-brown, brownish-red, or even a yellowish-red color, and a delicate, 
fibrous structure. There is scarcely any olivine to be found which does 
not show at least a tendency to decomposition. A quite common phenom- 
enon is to find smaller olivines which have been totally metamorphosed by 
the action that is manifest in the larger ones only along the cracks and 
borders. The gradual transformation of these insular spots and grains of 
the normal olivine substance marked out in the larger individuals by the 
serpentineous lines of alteration, which represent the middle stage of change, 
can be easily traced, even in different parts of a single thin section. In this 
rock, olivine sections measuring l mm , and encircled by a red border, the 
interior being somewhat yellowish, may be seen macroscopically in both 
the tliin sections and the hand-specimens, the latter of which are sometimes 
a little porous. Magnetite is the fourth constituent Apatite and sanidin are 
wanting. The rock is not altogether crystalline; for there exists between 
the individualized elements a small quantity of an extremely fine, brown, 
globulitic, amorphous mass, which is sometimes pierced by delicate, color- 
less rays. This is better observed between the diverging sections of feld- 
spars than between the augites. It forms quite thin, interposed layers, 
and can be examined to the best advantage when cut obliquely. Irregular 
bodies and stripes of this mass are inclosed in the larger feldspar crystals 
parallel with their twin-striation. 

Between Peavine Mountain and Virginia Range, a somewhat decom- 
posed and bleached basalt occurs [531], which bears macroscopical olivines. 

The hill behind Steamboat Spring, at the foot of Geiger Grade, is 
composed of basalt [532]. 

Truckee Canon is rich in basaltic varieties. The augite-andesites of 
tin's canon have been already mentioned (page 124). Common feldspar- 
basalts occur here [533], which are rather coarse-grained microscopically, 
yet contain very little brown, globulitic, half-glassy substance between the 
crystalline elements. It is exceptional to come upon so small an amount of 
this base; for, where it occurs at all, it is generally much more abundant. 
There are macroscopical feldspars in the sections, but the augites and 



232 MICROSCOPICAL PETROGRAPHY. 

olivines are confined to microscopical individuals. A great many very 
small, always brownish, metamorphosed olivine grains are disseminated 
through the rock. 

A basalt from the south flank of the entrance to Truckee Canon [534] 
is similar to that last described; but it is still more coarse-grained. Indeed, 
it is nearly anamesitic, resembling the famous typical anamesite from Steiii- 
lieim near Hanau, Germany, with which it in other points corresponds; 
namely, by being almost or wholly destitute of olivine, and by possessing 
an abundance of a brownish-gray, partly globulitic and partly fibrous, 
amorphous mass, which is flattened out between the crystals, and is altering 
into a dirty-green, fibrous substance, precisely as described and represented 
in the anamesite from Steinheim. 1 There is a very gradual and distinct 
passage between the original substance and the alteration-product. There 
is some apatite, also, as in the Steinheim rock. 

Another variety of the Truckee Canon basalts, occurring in the Truckee 
Valley, ten or twelve miles below Wadsworth [535], is a very porous arid 
even somewhat vesicular rock. It is rich in comparatively large feldspars, 
whose prevalence can be detected with the unaided eye in the thin sections. 
A small part of these feldspars are sanidins. The remainder of the rock- 
mass is, as it were, only crowded in between the long and broad ledge- 
formed feldspar sections. The beautiful plagioclases include very neat, 
roundly crystallized, but somewhat altered, olivines, measuring up to 0.012 mm 
in length. This phenomenon was never before observed. The rock is 
poor in augite, rich in small olivines and aggregations of magnetite grains, 
and contains considerable amorphous matter. The latter is a pale-yellow- 
ish glass-mass, bearing the usual small, brown, roundish globulites, and 
pierced by numerous long, colorless, acicular rays. These prisms or rays 
probably belong to some undeterminable product of devitrification, for 
they cannot be identified with any of the rock-constituents; and they stand 
parallel with one another, like the teeth of a comb, or are grouped in the 
shape of fascicles and bundles. Their surfaces, strangely, are often thickly 
covered with very minute, black, opaque grains, which are probably mag- 
netite. 

'R Z., Basakgesteine, 98. 



BASALTS. 233 

On the left bank of the river, at Truckee Ferry, Truckee Canon, a 
black, vesicular basalt [536] makes its appearance. It is distinguished from 
the others by containing many sanidins among the larger feldspars, most of 
which, however, are triclinic. Its dark groundmass consists of feldspar 
crystals and a smaller number of greenish-yellow augites distributed in a 
globulitic, glassy mass, exactly as in the basalts below Wadsworth. Olivine 
is entirely wanting here, so that the rock in one point resembles the augite- 
andesites of the region. 

Some basalts occur in Berkshire Cafion, Virginia Range, which are not 
well characterized, and are so highly altered that their composition is some- 
times undeterminable [537, 538, 539]. 

One of the most representative basalt regions is found on both sides of 
the Truckee Valley, in the foot-hills of the Virginia and Truckee Ranges, 
in the vicinity of Wadsworth, and the surrounding hills, where a rich 
variety of the most splendid and highly characteristic types are found 
overflowing older diabase rocks. The predominating black variety appears, 
for the most part, to be rather homogeneous, is seldom distinctly grained, 
and has a somewhat resinous lustre, which indicates the presence of a 
half-glassy substance. Little hollows of the rock are covered with a thin, 
milky-blue, cacholong-like deposit of silica. These rocks resemble, in themost 
minute details, the nature and the relative proportion of the constituents, 
and the microscopical structure of that peculiar basaltic type which has 
been described in specimens from Strathblane, near Dunglass, Scotland; 
from Mount Smolnik, near Kremnitz, Hungary; from Soleyjarhofdi, Ice- 
land; from Beauh'eu, Auvergne; and from Mount Hecla [540]. And it is 
worth the while to pause, here and remark that in these widely remote 
quarters of the globe, the product of the solidification of a molten mass, 
although exposed to many casualties, has, nevertheless, maintained a sur- 
prisingly close identity of microscopical composition. These basalts (see 
Plate X, fig. 4) are made up of crystalline elements in an unindividualized, amor- 
phous base, which is sometimes present in considerable quantity. This latter 
is generally a nearly colorless or very pale yellowish-gray glass substance; 
some little globules, a product of devitrification, being imbedded in it. 
These globules have been mentioned in the foregoing pages as occurring in 



234 MICROSCOPICAL PETEOGEAPIIY. 

some of the rocks, but they have not been found characteristically developed 
until now. They measure on an average 0.0015 Inm in diameter, have a dark 
brown color, and show with a high magnifying power a lighter central spot. 
They do not polarize, and are no doubt of a glassy nature, corresponding 
to the globulites in artificial slags. The glass often contains, beside these 
globules, long, narrow, acicular needles and rays, which are sometimes 
dichotome, and either lie parallel, or cross each other confusedly, like the 
fibres of a felt. The longer and stronger needles not infrequently pierce 
the adjoining feldspar crystals. These globulites are always much better 
characterized than the undeterminable microlitic secretions in the glass. 
The base is crowded in between the diverging crystalline elements of the rock, 
forming wedge-shaped bodies, whose microscopical structure is best studied 
where they are cut obliquely, and in an extremely thin layer overlap the 
border of a colorless feldspar crystal. In thicker layers, this glass-base, rich 
in dark globulites, appears grayish or brownish-black, and is almost per- 
fectly impellucid. Cases are rare where it does not occur in narrow, cunei- 
form or arrow-head-shaped masses, but in larger, rounded, insular spots. It 
sometimes shows a tendency to molecular alteration. The peculiar wedge- 
shaped places between the feldspar crystals, which it usually occupies, are 
instead occasionally filled with amygdaloidal formations, consisting in the 
sections of undulated and curled concentric rings, having an alternately 
lighter and darker grayish, or brownish-yellow color (see Plate XI, fig. 1), 
where it is easy to see how the glassy mass is gradually altering, the process 
beginning on the surface and working toward the middle. While the centre 
is still fresh, a gradual passage between it and the surrounding rings of 
alteration is plainly visible. In some cases, even in the same rock, this 
globulitic base is replaced by a pure glass-mass destitute of grains; and it is 
interesting to observe that this mass has a dark brown color, showing that 
the equivalent of the iron which formed the globules in the pale glass-mass 
is here evenly distributed, and produces the uniform color. A considerable 
proportion of the feldspars of this characteristic basalt variety are monoclinic; 
but plagioclases, which are often Carlsbad twins, largely predominate. These 
plagioclases are sometimes built up zonally, in one case more than one 
hundred zones being found in the thickness of 0.3; and they generally 



BASALTS. 235 

contain inclusions of the globulitic glass in tlie form of long, irregular 
bodies, parallel with the twin-striation, a phenomenon which is common to 
all like rocks throughout the globe. Here again, curiously, the plagioclases 
include minute crystals of olivine and drop-formed, greenish grains of augite. 
Larger crystals of augite are comparatively very rare; the mineral here 
generally forming only small irregular and crippled individuals, destitute of 
that morphological beauty which is a feature of those seen in the augite- 
andesites. Moreover, some occurrences are rather poor in augite; and it is 
interesting to observe that the rocks which are rich in the brown amor-, 
phous base, are the same that prove to be comparatively poor in augite, 
and vice versa; so that it seems highly probable that one is the equiv- 
alent of the other. Olivine is hardly ever lacking, being usually present 
in numerous individuals that sometimes attain macroscopical size. The 
olivines exhibit the well-known phenomenon of successive alteration into a 
serpentineous mass, which is mostly of a reddish-brown color. They are 
often well crystallized, and contain very distinct but minute, sharply octa- 
hedral crystals having a greenish-brown color, translucent on the edges, 
which belong to picotite, a variety of chromiferous spinel, a mineral found 
occurring in precisely the same manner in innumerable basaltic olivines 
of Germany, Bohemia, Hungary, Italy, and Scotland. They resist altera- 
tion, and are consequently found entirely fresh when the whole surround- 
ing crystal mass has become decomposed into serpentine. They never 
occur as independent constituents of the rock, but are confined to the interior 
of olivines. Here more than in any other known basalt variety the mag- 
netite is strongly inclined to form skeleton-like or cross-formed groups of 
crystals, which are very pretty; three longer or shorter lines of grains 
standing perpendicularly one above the other, according to the axes of the 
regular system (Plate I, fig. 12). Apatite is occasionally present in small 
quantities, and is sometimes altogether wanting: in general, it is propor- 
tionally rare. Hornblende and nepheline never occur here. Experiments 
have proved that the globulitic glass base is not at all affected by hydro- 
chloric acid. A powder of these basalts boiled five hours, and treated four 
days with this acid, showed under the microscope that the small fragments 
of the base were still fresh and unaltered. The glass, therefore, cannot be 



23G MICROSCOPICAL PETROGRAPHY. 

of a tachylytic nature, but must possess a more acid character. The 
feldspars also remained unattacked after the long treatment with acid. 
Olivine and magnetite alone disappeared tinder the test; and so it is compre- 
hensible why these rocks gelatinize so little with hydrochloric acid. The 
quantity of silica in the whole rock (i. e., in one of the most typical speci- 
mens of this division) was determined by Mr. Councler, of Leipsic, to bo 
56.53 per cent. This is considerably more than the average amount in 
basalts. In the quantity of silica it contains, and in the frequent presence 
of sanidin, this variety resembles the augite-andesites; but its abundance of 
olivine precludes the observer from classifying it with these rocks. 

Nearly all of the basalts of this region are constituted as above described, 
but with them are some exceptional rocks [541, 542, 543] which are entirely 
crystalline, the amorphous glass base being wholly wanting. Nevertheless, 
these varieties are also rich in olivine and comparatively poor in augite, or 
at least in well-developed individuals of this mineral. There are larger 
feldspars 3 mm long, and large metamorphosed reddish olivines ; and between 
these porphyritical crystals is a kind of groundmass which is a wholly 
crystalline, small-grained aggregation of feldspar prisms and microlites, 
exhibiting a good fluidal structure, and of little, crippled, greenish-yellow 
augites and magnetite. 

The low hills in the region of Diabase Hills, Truckee Range, present a 
brownish, somewhat porous basalt rock [544], although its structure is the 
same as that of the predominating variety. But its feldspars are smaller 
and narrower, its augites and olivines extremely minute, and the globulitic 
grains in the glass-base are not blackish-brown but brownish-red, producing 
the peculiar color of the rock-mass. 

The above-mentioned type of basalts, which is porphyritically crystalline 
throughout, is also found scattered here and there in the remoter neighbor- 
hood of Wadsworth. The hill three miles north of Diabase Hills has some 
excellent, specimens of this variety [545.] 

At the same place (Diabase Hills), a basalt was discovered [546] repre- 
senting, in striking contrast with the rest, the other variety which abounds 
in a globulitic glass-base. 

This interesting occurrence of the union of the two types of structure ;it 



BASALTS. 237 

one point is repeated on the road from Clark's Station to the entrance of the 
Truckee Canon, where a crystalline, micro-porphyritical basalt [547] is 
accompanied by a globulitic half-glassy one [548]. In some parts of that 
first named, fine, predominating magnetite grains have been grouped with 
augite grains into dark roundish heaps, whose outlines gradually pass into 
the general groundmass. A curious and rare phenomenon is that where a 
countless number of the most minute, brown or black grains are irregularly 
scattered through the mass of the somewhat larger augites, giving it the 
dusty appearance so common to apatites and noseans. 

In one case, the micro-porphyritical type, which is elsewhere entirely 
crystalline, was observed bearing also an amorphous base. A basalt discov- 
ered near King's Station [549] contains in the groundmass, in which the 
larger feldspars are imbedded, an excellent light-brown glass, occurring in 
comparatively large insular spots and patches. This glass is either quite 
pure or is penetrated by delicate, prickly microlites, and its color is the same 
as that in the lavas of Vesuvius. All the other peculiarities of the genuine 
micro-porphyritical type are faithfully detailed here: feldspars with indis- 
tinct outlines; the minuteness and paleness of the crippled, grain-like augites; 
augitic interpositions in the feldspars; the crystals of black magnetite 
unusually sharp; a few small olivines altered into a dirty -green serpentineous 
substance. 

The basalts from the high peak in the Truckee Range, northeast of 
Wadsworth, are in part very similar to those from the region of the Diabase 
Hills. There is one [550] in which the globulites of the interwedged, half- 
glassy base have a light yellowish-brown color and are comparatively large. 
The larger feldspars contain kernels composed of loosely aggregated, irregu- 
lar bodies of this mass, through which the feldspar substance can hardly be 


seen. 

Other varieties [for instance, 551] represent the evenly granular type; 
a regular mixture of crystalline constituents, without any tendency to micro- 
porphyritical or macro-porphyritical structure, and destitute of any con- 
spicuous amorphous substance. Apatite is more often found in these basalts 
than in any other modification; but this mineral seems to be rarer in the 
American basalts than in the corresponding German rocks. They abound 



238 MICROSCOPICAL PETROGRAPHY. 

in larger or smaller olivines, whose altered brownish mass sometimes imparts 
to the rock a dark yellowish-brown color [550.] One specimen [552] con- 
tains a great quantity of quite small reddish-yellow and yellowish-red 
spangles, lamina?, and lobes of specular iron disseminated throughout the 
rock, and sometimes pierced with numerous holes. It seems to be of secon- 
dary origin, which is connected with the fact that there is not much fresh 
and unaltered augite in the rock. 

Basalt from the low hill west of Carson River [553] contains cuneiform 
masses of globulitic base, and has the structure and composition of the pre- 
vailing type of the Truckee Valley. 

The basalts from the Kawsoh Mountains and their environs are often 
somewhat porous ; thin, pearl-gray deposits covering the small hollows. 
That from the east end of these mountains [554] is rich in a globulitic, 
half-glassy mass, which, although it has the behavior so often described, 
does not occur in interwedged cuneiform bodies, but penetrates the rock like 
a ground-paste. This amorphous mass, together with its included brown 
globules, and the delicate, colorless prickles, which also occur here, is plainly 
seen decomposing into a seemingly quite homogeneous, yellowish-brown 
substance, which in polarized light is resolved into a number of parts like 
mosaic, each showing a chromatically different or an aggregate polarization. 
Considerable sanidin accompanies the prevailing plagioclase. 

The principal rock from the central peak of the Kawsoh Mountains 
[555] is a basalt. It contains a globulitic base; beautiful, zonally built 
feldspars; larger, red-spotted olivines; quite small augites, and a thick grain 
of magnetite inclosing a particle of olivine, a new phenomenon. The feld- 
spars appear in the slides in colorless sections 2.5 mm long. 

At West Spur, Kawsoh Mountains, an almost crystalline, even-grained 
basalt occurs [556], in which, nevertheless, there is a small quantity of a 
thickly globulitic, amorphous base. Many laminae and serrated dendritic 
lobes of specular iron unite with altered, smaller and larger, brownish-red 
olivines, to give the rock a dirty -brown color. Even in this variety, augites 
are not very frequent, and are poorly shaped. The feldspars have many 
gas-cavities. Numerous colorless microlites, which are often dichotomous, 



BASALTS. 239 

arc scattered through the rock, and they would appear to belong to those 
so often found secreted in the globulitic base, but surely not to apatite. 

In the south end of the Kawsoh Mountains, there is a basalt [557] 
whose structure is intermediate between the micro-porphyritical and the 
even-grained. Curiously enough, it contains microscopical aggregations of 
tridymite in the form of subtile, often regular hexagonal laminae, which partly 
cover one another like tiles and are grouped into rounded heaps, exactly as 
in the rhyolites and trachytes; but they do not look as if they were produced 
by a secondary infiltration of silica. This is the first time tridymite has 
ever been observed in basaltic rocks. Since this basalt comes to the surface 
either through or near strata of infusorial silica, it is not impossible, as Mr. 
Clarence King has sug^sted, that an included fragment of silica is the true 
origin of the tridymite, in which case it would of course be a substance 
originally foreign to basalt. 

There also occur in the same locality [558], in the southern end of the 
range, nearly due west from Wadsworth, and at Fossil Hill, at the extreme 
northern end of the Kawsoh Mountains [559], some excellent basalts, 
having the cuneiform masses of amorphous base, which is here a beautiful 
brownish glass with only a few pale, globulitic secretions. Some of the 
feldspars are colored a quite dirty yellow by hydrous oxyd of iron, which 
is deposited in innumerable confused cracks. 

All other basalts from the environs of the Kawsoh Mountains which 
were examined, exhibited a globulitic base. That from the central peak 
[560] is a dark reddish-brown rock, containing feldspars 3 mm long, the 
capillary fissures of which are filled with numerous lobes of oxyd of iron, 
evidently secondary. The abundant olivine present is altered into a reddish- 
brown substance, and the interwedged base has here and there received the 
color of iron-rust, and its structure is gradually becoming obliterated. 

The amorphous mass in the rather porous rocks from the Basalt Hills, 
at the south end of the Kawsoh Mountains [561, 562, 563, 564, 565], has 
some peculiarities. This light-brown, glassy substance sometimes contains, 
beside the globulites, short, black, impellucid, little needles, which appear 
to be embryo trichites. Augites of these specimens are remarkably pale. 
In other rocks, the globulitic mass only forms extremely thin septa between 



240 MICROSCOPICAL PETEOGKAPHY. 

the polysynthetic individuals of plagioclase. If the section is oblique, this 
almost immeasurably narrow wall strongly contrasts with the colorless mass 
bounding it on each side, and its structure can easily be studied. The mass 
consists of colorless glass, with a multitude of very dark globulites, so that 
there is little of the colorless substance to be seen, and the thicker layers 
are impellucid. And as the thin sections show narrow feldspar ledges in 
this dark mass, it looks hachured. Here and there the amorphous base has 
begun to alter into a dirty -green substance, with which, however, there was 
only previously joined a feeble and indistinct fibrous formation. All these 
rocks are rather poor in olivines. 

The basalt from the western foot-hills of the Truckee Range, four miles 
northeast of Wadsworth [566], is totally different, jpjng an entirely crystal- 
line, even-grained rock, composed of plagioclases; comparatively thick, 
roundish augite grains; olivine and magnetite. Its macroscopical olivines 
are very accurately formed and bear excellent glass-inclusions, with bubbles. 

In the Lake Range, on the east shore of Pyramid Lake, are some 
basalts with large pores, which are wholly crystalline, and destitute of 
amorphous base [567, 568, 569]. Plagioclases 4 mm long and 1.5 mm broad 
are sometimes far more highly charged with devitrified half-glassy inclusions 
and grains of augite and magnetite than is common in basalts; and in this 
they are like the plagioclases of andesites and trachytes. There are many 
crippled individuals of augite and olivine. Much lamellar oxyd of iron has 
settled on the rock, and the surfaces of the augites seem browned. One 
specimen from this locality is remarkable for the unusual length of its 
augitic microlites and its poorness in olivine. 

The basalts with a somewhat resinous lustre, from the Lake Range on 
the northwest shore of Winnemucca Lake, have, on the contrary, an abun- 
dance of a quite pure or feebly globulitic glass-base [570, 571]. The 
augites are pale and stunted, and they very rarely show crystal faces. The 
olivines are rare but comparatively large and fresh. In one specimen from 
this locality, feldspar crystals occur which measure three-quarters of an inch 
in length, and appear very impure even in the hand-specimens. The 
strange particles imbedded in them are nearly as thick as a poppy-seed, 
and are simply fragments of the basalt-mass with the structure and elements 



BASALTS. 241 

complete. There are also innumerable, smaller, irregular particles of 
brownish, globulitic glass, nearly colorless, flat glass-inclusions, with bubbles 
which are only a few thousandths of a millimetre large, together with many 
;is-cavities, imbedded in the feldspars. And yet, curiously, these enor- 
mous feldspar crystals do not show any striation, and prove in polarized 
light to contain smaller individuals of sanidin, which are oriented in a dif- 
ferent position. All the smaller feldspars are doubtless triclinic. 

Basalts from the Lake Range, at the east shore of Pyramid Lake, are 
not very different [572]. The glass-base contains very thick and dark 
globulites ; the augites are better crystallized, some being well formed ; the 
feldspars, again measuring up to 4 mm in length, are very rich in glassy frag- 
ments, but all are true plagioclases, of which the fine striation is not at all 
disturbed by innumerable foreign inclusions: the glassy lobes often form a 
kind of net. 

The most characteristic dolerites occur at Black Rock Mountains, 
Nevada, and in their immediate neighborhood, among them being a variety 
new to the herein-examined regions. That from Black Rock Hill, at the 
southernmost point of the group, consists of plagioclase, intensely colored 
augite, olivine, magnetite, and apatite. It shows, both in hand-specimen 
and thin section, a mass that appears macroscopically to be rather coarsely- 
crystalline throughout ; but the microscope discovers that a little amorphous 
mass is present. The plagioclases are beautifully clear and fresh, and are 
splendidly lineated. There is no sign of sanidin. The augite is very fresh, 
of a light brownish-yellow color, and perfectly pure, with the exception of 
some glass-inclusions. Feldspar is better crystallized than augite, masses 
of which devoid of individual forms fill spaces between the plagioclases. 
Olivine is slightly altered. Magnetite is in fresh, thick grains. There is no 
titanic iron. Quartz is wanting here ; indeed, it seldom occurs in the 
Tertiary dolerites, although it is very often present in the otherwise similar 
ante-Tertiary diabases. The augite differs from that in the freshest older 
diabases in that it shows no trace of chloritic alteration. The globulitic 
amorphous base, seen in thin section to be interwedged between the crys- 
talline constituents in cuneiform masses, is traversed by colorless and very 
pale, yellowish rays, which sometimes protrude from it and pierce the 
16 MP 



242 MICROSCOPICAL PETROGRAPHY. 

feldspars, and by short, black microlites. Its mass is here and there slightly 
browned. 

The dolerite from the south end of the Black Rock Mountains [573] 
agrees in every point with that just described. 

A remarkably beautiful amorphous mass is found in a quite similar 
coarse dolerite which forms a butte in Black Rock Desert [574]. It is 
dotted with fine, black globulites and broad, acicular rays, composed of 
small, black grains arranged like beads on a string. The olivine is altered 
into a brownish substance. 

To this same class of rocks also belongs the dolerite from Round Hill, 
Black Rock Mountains [575]. 

These dolerites, which are rich in olivines and darker augites, are a 
very characteristic type, which does not seem to be at all represented in other 
basaltic regions ; for instance, they are not found among the rocks of the 
Lower Truckee Valley. 

Neighboring basalts from Snowstorm Ledge and Canon, Black Rock 
Mountains, are entirely different from those of this locality thus far described 
[576, 577, 578] They are homogeneous, destitute of macroscopical con- 
stituents, and the presence of much glassy mass is proved by their resinous 
lustre. In some varieties, the globulitic base occurs in extremely minute 
particles, and the crystalline elements are also strikingly small. In all 
the rocks of these localities, the infrequent augite is very pale, and it is 
often found in small, thin prisms or thicker microlites, on both sides of which 
subtile prickles of augite have fastened, pointing at different angles like 
the needles of a fir-tree, and some of them are covered with real bristly 
cilia (Plate I, fig. 19). A striking contrast between these rocks is produced 
by the fact that they do not all contain distinct olivine. In some of them, 
the magnetite is altered into brownish-yellow, hydrous oxyd of iron, which 
is somewhat translucent at the edges. This decomposition is a rare phe- 
nomenon, even in the oldest diabases and diorites. The passage from the 
original substance to the product of alteration can be nicely observed where 
one of those crossed or dendritic aggregations so common in this globulitic 
variety has suffered metamorphism in all its particles. A most frequently 
observed phase of transition consists of a black, opaque kernel with light- 



BASALTS. 243 

brown border ; usually, however, only a feeble obliteration of the outlines 
happens, the quadratic form being generally well preserved. One specimen, 
which appears somewhat altered and bears green-earth in small pores, con- 
tains titanic iron. In its fresh state, it can hardly be distinguished from 
magnetite ; but it is sufficiently characterized when covered with the dull, 
porcelain-like crust of alteration, which makes it very conspicuous in 
reflected light. 

A low hill west of the Kamma Mountains [579] yields a good dolerite, 
resembling those from the Black Rock Range, except by being less coarse- 
grained. It is more beautiful than the others, however, on account of the 
distinctness of the cuneiform bodies of dark, globulitic base between the 
feldspars and augites. This base is easily studied by reason of its distinct- 
ness. Thicker, almost black, globulites are rather rare, and lie isolated in 
the colorless or pale-grayish glass-mass, which also bears the acicular rays 
so often mentioned. When the contrast between these rays and the light 
glass is observed, it is discovered that they are not also colorless, but possess 
a pale yellowish-green tint. It is, therefore, probable that they are augitic 
microlites. They are often grouped in the form of stars, and arranged 
parallel, like the teeth of a comb. In some spots, this intermediate mass has 
begun to alter, and the black grains disappear, producing a seemingly 
homogeneous, and often rather intensely yellow material, in which the long, 
difficultly altered microlitic needles lie unchanged. The last stage of change 
is to really amygdaloidal products, built up of differently colored, concentric 
layers. The yellow substance polarizes indistinctly, yet evidently reacting 
under polarized light, the fresh glassy base being, of course, quite isotrope. 
Beside the fine magnetite grains, the rock bears rather many sharply hexa- 
gonal laminae of specular iron, measuring 0.02 mm in diameter. These laminae 
are violet-brown when very thin, and dark-brown when thicker. There are 
large augites and a very little olivine. 

The dolerite from the desert between the Kamma and Pah-supp Mount- 
ains [580] is in every respect similar to the above. 

Basalts from the Pah-tson Mountains closely resemble those from 
the Lower Truckee Valley, but they are richer than the former in micro- 
scopical olivines. The rocks from the west ridge of Blue Peak [581], Basalt 



244 MICROSCOPICAL PETROGRAPHY. 

Peak [582], the northwest ridge of Black Peak [583], and the Grass Canon 
Camp [584, 585], possess larger or smaller cuneiform bodies of amorphous 
globulitic base, which is more or less translucent, and often appears with a 
low magnifying power in the thin sections as a black, opaque mass, with 
light, short, linear incisions produced by the imbedded feldspars. The 
microscope also discovers that the feldspars contain numerous, long, brown 
glass-inclusions. There is more olivine (also with glassy grains) found here 
than the other basalts of the same type generally contain. 

A rock capping the north peak of the Pah-tson Mountains [586] is 
somewhat different. In places, it is almost wholly crystalline, and it is 
composed of plagioclase, greenish augite, much olivine, and magnetite, 
exhibiting in parts of a single thin section light-brown and in other parts 
deep, dark-brown insular spots of homogeneous or slightly globulitic glass. 

The same type, characterized by a globulitic base, occurs still further 
on, and is found in the basalts from Montezuma Range, their external 
aspects agreeing with those from the Truckee Valley. 

In the basalts back of Oreana, Montezuma Range [587], the glassy 
mass is less frequently found in interposed cuneiform bodies, but serves more 
as a microscopical groundmass. As usual, the augite seldom occurs in the 
exactly formed, thick, dark crystals common to the more crystalline 
varieties, but in pale, crippled, and distorted, long prisms. The olivines 
are, without doubt, much better crystallized than the augit.es. 

The glass-base of a rock from Basalt Hill, near White Plains, Mon- 
tezuma Range, has an interesting structure [588]. It is a dark, grayish, 
globulitic glass, and contains, beside the globulites, opaque, quadrangular 
individuals of magnetite, whose well-known forms are powdered with 
grains, small augite crystals, and prisms of apatite, the sections of which 
often shine like little six-sided holes in the thicker, dark parts of the base. 
The apatites are very thin and delicate, and occasionally have a black, 
longitudinal centre or axis running from end to end. Sometimes extremely 
thin, line-like prisms are affixed to the six vertical edges of a larger indi- 
vidual (Plate I, fig. 10). It will, therefore, be seen that the base is unusually 
differentiated. There are very fresh, large olivines, with many included 
crystals of picotite. As a proper constituent, equivalent to plagioclase and 



BASALTS. 245 

olivine, augite is almost wanting; being confined to the extremely small 
grains and crystals in the glassy base. There are splendidly built crosses 
of magnetite grains. The globulites of the base are often not isolated, but 
are sometimes joined and form half-circles and horseshoe-shapes. 

Another specimen from the same locality [589] contained only globu- 
lites in its base, being even destitute of the acicular rays; so the rock is, of 
course, much richer in independent individuals. Pale augites, of course, 
occur. 

As a contrast to these, the basalts of the West Humboldt Mountains 
generally have an almost entirely crystalline structure. That near Buffalo 
Cafion [590] shows only a feeble trace of a globulitic, amorphous base, the 
presence of which would hardly be recognized if the observer had not 
become familiar with its behavior by recent previous examinations. The 
larger feldspars are highly charged with half-glassy inclusions. Here also 
the augites are much crippled. There are larger oli vines, which are some- 
times very nicely crystallized. It is here easily observable how the brownish 
yellow product of alteration of the olivines works in long, delicate fibres 
from the fissures toward the interior, through the still fresh mass. 

Better crystallized augites occur in the almost wholly crystalline basalt 
east of Oreana, in the West Humboldt Range [591, 592]. Under the micro- 
scope, this rock very closely resembles the German basalts from the Seven 
Mountains; but macroscopically it appears somewhat altered, and has 
developed calcite in its mass. 

A rather rare composition is that of the basalt from Eldorado Canon, 
West Humboldt Mountains [593]. It is an entirely crystalline rock, and 
its only macroscopically visible constituent is olivine, which occurs in the 
thin sections in numerous almost colorless grains. There are no larger 
feldspars, but only microscopical ones, and these are generally in the 
undeveloped state of microlites. Augites are very abundant, without doubt 
predominating over the feldspars. The augites are not especially large, but 
most of them are well crystallized, the sections in the different directions 
presenting very sharp and straight linear outlines; the horizontal sections, 
shaped by ( <x P, oo^Poo, oo^oo), showing the most beautiful and fault- 
less zonal structure. They are accompanied by many rounded or irregular, 



246 MICROSCOPICAL PETROGRAPHY. 

brownish-yellow augite grains; and these grains and small prisms are often 
included in the larger, sharply outlined augite crystals. The olivines con- 
tain finished crystals of picotite, which are often gathered into heaps or 
groups, and are metamorphosed along the borders and cracks. There is a 
faint trace of colorless glass between the crystalline constituents. 

The porous basalts from the hills north of Sou's Springs, Pah-Ute 
Range, resemble those from Buffalo Canon [594, 595]. 

Specimens of all these rocks in which there is no glassy base, or 
only minute bodies of it, are quite dull and destitute of the resinous lustre. 
The striation of the feldspars is uncommonly rich, and the larger ones 
contain many inclusions. All the feldspars are plagioclases. An abun- 
dance of sanidin, in basalts, in fact seems to depend upon the presence of a 
globulitic glass-mass. There are olivines measuring 2 mm , which have been 
metamorphosed into a vivid red substance. The augites are poorly crys- 
tallized. 

A basalt from Mountain Wells Station, south end of the Pah-Ute Range 
[596], and another from the divide at the head of Clan Alpine Canon, Augusta 
Mountains [597], belong to the type which has a globulitic glass-base. 
That first named bears feldspars measuring up to 3 mm , which one can discern 
with the naked eye to be sanidin. All the lesser feldspars belong to the 
largely predominating plagioclase. The glass-base has the usual form, inter- 
wedged between crystals ; and it is thoroughly altered into amygdaloidal 
nests (Plate XI, fig. 1). That the feldspars of the latter locality are built 
up of numerous schists, although very highly charged with foreign, half- 
glassy particles, can be seen in unusual distinctness. There are some 
olivines which bear glass-inclusions that are pressed flat and stretch out 
many dendritical arms, carrying near the end a small, dark bubble (Plate I, 
fig. 13). It may perhaps be remembered that a typical augite-andesite 
occurs in Antimony Canon, Augusta Mountains. 

Along the western foot-hills of the Fish Creek Mountains is a fine and 
large stream of porous basalt [598], containing feldspar crystals an inch 
long, part of which do not show any striation in the hand-specimens or any 
lineature in polarized light; they, therefore, belong to sanidin. But the 
microscope proves that all the smaller feldspars are plagioclases. The chief 



BASALTS. 247 

mass of the rock is in a veiy micromerous condition. There are no larger 
augites, and smaller individuals of this substance are not abundant. The 
rock is destitute of olivines. Its prevailing dark mass, which contains only 
macroscopical and microscopical feldspars, is very feebly transparent, and 
generally appears to be in the globulitic, half-glassy state ; which condition 
agrees with the fact that delicate, irregular bodies of a globulitic base lie 
in the pellucid mass of the larger feldspars. Leucite and nepheline are 
surely not contained in this basaltic lava. Incrusting all the protuberances 
of the interior of the pores is an isabel-colored material, which is evidently 
a product of alteration ; but its nature cannot be ascertained either macro- 
scopically or microscopically. 

A reddish-brown rock, from east of Winnemucca, near the mouth of 
Little Hurnboldt River [599], is seen, both macroscopically and microscop- 
ically, in the thin section, to be an excellent, genuine dolerite, bearing 
plagioclase, augite, many small olivines, and magnetite. The rock would 
appeal- at first sight to be entirely crystalline, but a closer examination 
proves that there are a very few globulitic, amorphous particles present. It 
is an interesting fact that these bodies, notwithstanding their rareness and 
smallness, have the common modification of solidification. 

The low hills northeast of the Havallah Range are composed of a true, 
medium-grained basalt [600]. It would be exactly like the most common 
German variety, if it were not for the occasional finding of a very thin wall 
of globulitic glass-base between two plagioclases lying close together. The 
glass-mass is as rare in the German, Bohemian, and Scotch feldspar-basalts, 
as it is almost universal in those of the Fortieth Parallel. 

At the top of the plateau at Stony Point, Shoshone Mesa, is a very 
small-grained basalt [601], which is rather poor in augite. Included 
between the constituents, more frequently, however, between the plagio- 
clases, is an amorphous mass of the usual structure. But the glass is a 
light-grayish yellow, and the globulites are, therefore, comparatively pale. 
The constituents of the rock appear somewhat blended and confused on 
account of their minuteness. The small, reddish-brown, somewhat trans- 
parent grains scattered through the rock are olivines. Close observation is 



248 MICROSCOPICAL PETROGRAPHY. 

necessary to distinguish them from the magnetite grains, which here often 
possess an ochreous, peripheric zone, the centre or kernel being opaque. 

The cliffs of the Shoshone Mesa exhibit a thoroughly characterized 
dolerite [602], similar in composition to those from Black Rock and the 
Kamma Mountains. Its well-formed plagioclases include rounded, 
greenish-yellow, drop-like augite-grains and little olivines, which are often 
crystallized, and even here are partly altered into a brownish substance. The 
rock also contains well-individualized augites, rather much olivine, magnetite, 
some apatite, and a little amorphous mass which bears more rays than 
globulites. There is no titanic iron. The rock is porous, the pores being 
sometimes as large as peas: this is not common in coarser-grained dolerites. 
A similar dolerite occurs on the summit of the Shoshone Mesa, in com- 
pany with the above-mentioned basalt [603]. 

The basaltic rocks of Egyptian Canon, Mallard Hills, Nevada, afford a 
great variety of rare types. One of the less curious occurrences [604] has 
for its chief mass an extremely fine mixture of almost indistinguishable 
microlites and grains of plagioclase and augite, the largest elements being 
scarcely longer than 0.003 mm . This groundmass contains, without gradations 
of dimension from the smaller members up, macroscopical porphyritical, 
colorless feldspars, which are largely sanidin, and yellowish -brown augites. 
There are no olivines. 

A very strange blackish-gray homogeneous rock [605] occurs in this 
canon, bearing constituents which are in a certain sense porphyritical, 
although not exactly macroscopical, namely, impure sanidins and predomi- 
nating plagioclases and augites. Olivines are wanting. These elements are 
imbedded in a groundmass which a high magnifying power proves to be in 
the globulitic, half-glassy state, relieved here and there with a secondary 
brown color. In this rather opaque groundmass, very peculiar products 
appear (Plate XI, fig. 4). They are colorless, line-like bodies looking like 
incisions or notches in the mass, most probably a feldspathic crystalline 
product of devitrification which are usually only 0.005 mm wide, and are 
straight, crooked, or curved into two-thirds of a circle. These parts of 
circles are sometimes arranged concentrically, sometimes with a tail like a 
paragraph-sign. The bent ones are sometimes placed radially, and sometimes 



BASALTS. 249 

they represent little trees, irregular branches projecting from a stronger stem. 
Often thinner, curved bodies are joined to a larger, like ribs to a spine, and again 
thinner, straight individuals are arranged like the teeth of a rake, not being 
on either side very sharply separated from the globulitic glass-mass. These 
curiously shaped and arranged lines pass into better-individualized bodies, 
which are recognized as identical with some that often occur in obsidians, 
pitch-stones, and artificial slags. 1 They have rectangular crystalline forms, 
the four corners tapering out into long teeth or prongs (see Plate I, fig. 20). 
The structure developed by these crystalline products of secretion is else- 
where entirely unknown to basalts; and when a thin section is considered 
only superficially, it might easily be supposed to belong to the rhyolites; 
but a globulitic base never occurs in the latter, and besides, the combina- 
tion of plagioclase and augite is sufficient to prove the rock a member of 
the basalt family, amongst which it constitutes an exception: apatite, for 
instance, is far more plentiful than in other basalts. 

At Whirlwind Peak, Shoshone Range, is a usual type of fine doleritic 
basalt [606]. It contains plagioclase, augite, olivine, magnetite which is 
rather coarse-grained under the microscope; and between these are small 
bodies of blackish-gray, globulitic base, its rays being powdered with minute 
grains. 

In Agate Pass, Cortez Range, the same type reappears: it is finer- 
grained, however, and contains more base, paler augite, very little olivine, 
and unusually thick grains of magnetite. Fine specimens of chalcedony 
are found in this basalt; large stalactitic pieces, which in the interior often 
pass into a dull, milky, cacholong-like substance. 

A common type of somewhat lustrous, porous basalt appears at 
Shepherd's Ranch, on the south fork of the Humboldt River [607]. The 
grayish-black globulitic base, however, serves more as a pervading ground- 
mass than merely as an interwedged body. In some places, the base is 
altered into the familiar isabel-colored fibres; and it now becomes evident 
that the interposed rays, which are powdered with minute grains, belong to 
augite, for the thicker and stronger ones are decidedly green. The rock is 
extremely poor in olivine. 

1 Vogelsang, Die Krvstalliten, Plate VII, figs. 10, 11, 12. 



250 MICEOSCOPICAL PETROGRAPHY. 

In the Ruby Valley Range, the basalts bear a curious, finely porous, 
blackish-brown mass, which has a pitch-like lustre, and appears largely glassy 
[608], It is destitute of macroscopical secretions. Under the microscope, 
it proves to be a brown glass free from any trace of a devitrification-product 
excepting some feeble traces of fine grains. Along some irregularly 
running, narrow lines, the glass has become pale and quite colorless; but 
these places do not show any polarizing action, the whole thin section 
appearing entirely dark at every point between crossed nicols. This basaltic 
glass-mass would be mistaken at first sight for a very pure tachylyte; but 
it is easily ascertained that it will not gelatinize, even when treated with 
boiling hydrochloric acid, the only effect of which is to give it a slight 
yellowish tinge. It is therefore only a tachylytic-looking substance; for it is 
destitute of the most characteristic feature of real tachylyte, namely, a 
readiness to form a thick gelatine of silica. It belongs rather to that divis- 
ion of basaltic glasses (the obsidians of basalt) which has been named hya- 
lomelane (pseudotachylytes), since the rock is unaffected by acids. In 
Germany, these basaltic glasses, which were referred to the tachylytes on 
account of their external appearance, but have since been proved not to 
gelatinize, occur at Ostheim, in the Wetterau, and at the Sababurg, in the 
Reinhardswald. 1 

At the northern end of the Ombe Mountains is a common middle- 
grained basalt [609], containing but a very little amorphous, globulitic base. 
It is richer in augite than most of the other basalts, but, curiously, there is 
no olivine to be seen. 

At Watch Hill, Elkhead Mountains, a rather coarse-grained typical 
dolerite occurs [610]. It bears large feldspars, augite, olivine, and magne- 
tite. Much dark globulitic substance is interwedged between the crystals. 

The top of Anita Peak, Elkhead Mountains, is composed of an excel- 
lent basalt [611] representing that well-characterized variety in which the 
amorphous base is free from globulites, being a pure glass of a chocolate 
or coffee-brown color, with occasional acicular microlites. Olivine alone 
appears macroscopically in the rock and in the thin sections as rather large 
grains, containing sharply outlined crystrals of picotite and glass-inclusions. 

1 Uoscnliiisch, Mikroskopisoho Physiographic, 134. 



BASALTS. 251 

The augites are of a darker-brown color, and are generally very well 
crystallized. All the feldspars are beautifully striated. Its augites include 
many quadrangular magnetite grains. An abundant brown glass-base is 
not interwedged in cuneiform shapes between the crystals, but serves as a 
pervading groundmass. In short, the rock combines all the peculiarities of 
the analogous German rocks from the Stillberg, in the Habichtswald, from 
Elfershausen, and from Weissholz, near Liitgeneder. 

A rather remarkable rock is found in that from the Benches of the 
Upper Little Snake River [612]. It is a grayish-black mass containing 
small, greenish-black grains and large grains of quartz. Narrow, ledge- 
formed, microscopical plagioclases exhibiting a distinct fluidal texture, little 
crystals, and grains of pale greenish-yellow augite and black magnetite 
grains unite to form a mixture which appears macroscopically in the thin 
sections as a dark-grayish groundmass. Very dark grains, measuring l mm , 
may be seen macroscopically in this mass. These are the greenish-black 
grains of the hand-specimens, and a high magnifying power discovers that 
they are either a very intimate aggregation of opaque, black grains of 
magnetite, with augite grains, which sometimes possess crystal outlines, or 
a micaceous mineral larded with black grains. There is no olivine present. 
The quartzes, which appear in a certain sense as a foreign substance, are 
each surrounded by a narrow, coroniform zone of small augite grains. 
Possibly this rock is in some way connected with the curious quartziferous 
trachytes of the neighboring Elk Mountain. 

Near the fork of the Yampah River is a common, medium-grained 
basalt, which has only faint signs of a globulitic base [613]. The plagio- 
clases are fresh and richly linear ; the augites pale green and poorly shaped ; 
the olivines altered into an intensely brownish-red substance; and the 
colorless feldspars include a great quantity of angular, Prussian-blue 
grains, which are somewhat transparent, hardly 0.003 mm long, and of an 
unknown nature (possibly haiiyne). 

A very good basalt is found in the dikes west of Buffalo Peak in the 
ridge between North and Middle Parks, Colorado [614]. It is a plagio- 
clase rock, rich in augite, bearing olivine, generally of a crystalline 
structure. The augites are mostly noteworthy for the excellence of their 



252 jMiciioscorjcAL PETROGRAPHY. 

crystallization, the well-defined green kernels in their prevailing brownish- 
yellow mass, and the completeness of their zonal structure. One horizontal 
angite section O.OG"" 11 square was composed of no less than 42 layers. The 
oh'viucs (Plate XI, fig. 3) are also very evenly shaped, the smaller ones 
being entirely metamorphosed into a brownish-yellow substance, which 
at first sight would not be easy to distinguish from that of augite. In the 
larger individuals, alteration has not proceeded beyond the borders and the 
walls of cracks, so that serpentineous veins wind in all directions through 
their sections. Each of these alteration-bands is made up of several layers 
of varying colors, slightly undulated ; some of them showing brown, yellow- 
ish, and green tinges, and the effect is very pretty where these variegated 
veins traverse the otherwise fresh and almost colorless olivine-substance. 
In many places between the crystalline elements of the rock are the most 
perfect bunches of trichites, aggregations of straight and curved, black, 
opaque, hair-like microlites of varying thickness, and with exactly the same 
behavior as they exhibit when scattered through obsidians. Basalts bearing 
groups of such trichites are not rare among the German occurrences, which 
are in general similar to this. Rectangular, dendritic, or skeleton-formed 
groups have not been observed here. The long microlites surely do not 
belong to magnetite, for there is no sign of passage between them and the 
common angular magnetite grains. It is probable that they are not proper 
ingredients of the rock, but are rather confined to small patches of colorless 
glass, which are not distinct. The rock contains a few quite small laminae, 
of strongly dichroitic and absorbing brown biotite. And it has one strange 
phenomenon : some augite crystals standing close together form a compact 
group measuring about 3 mm in diameter. These crystals have sharp out- 
lines, are traversed by the characteristic fissures of a yellowish-brown 
color common to the rest, and their substance here is perfectly pure. 
Inside they are much paler, becoming quite light aquamarine and pale 
greenish-blue, and a large quantity of foreign bodies is here interposed; 
rounded, opaque, black grains; straight and black, longer or shorter 
needles of varying thickness arranged parallel into two systems pointing 
different ways, so that they cross each other obliquely in a similar manner 
to those which have been observed in diallages and hornblendes ; and small 



BASALTS. 253 

brown biotite plates, especially in those augites which contain the black 
grains, for those which bear the needles are generally destitute of other inter- 
positions. Inclusions of biotite plates have never before been observed in 
basaltic augites, and are even wanting in the smaller and more isolated augite 
individuals in the same rock. Between the augites of this group are large 
sections of apatite, a mineral which does not appear elsewhere in the rock. 
Perhaps the black needles (0.02 mro long and generally O.OOS" 1 " 1 thick) are in 
some way connected with the above-mentioned trichites. 

Buffalo Peak, North Park, presents a light basalt [615] which is entirely 
crystalline, rich in plagioclase and well-shaped light olivines, and rather 
poor in augite, mostly in the form of pale microlites Many sharp, six 
or three-sided, thin, brownish-violet plates of specular iron are scattered 
singly and in little groups through the rock. There is no brown mica. 
The glass-inclusions abounding in the olivines often contain, beside the 
bubble, many short, black microlites, which project from near the border 
toward the interior (Plate I, fig. 18). 

''f O'i'Jj tfoi/B ^0 3(JfrOT2 

Upon trying to separate into groups the different varieties of micro- 
scopical structure in these feldspar-bearing basaltic rocks of the Fortieth 
Parallel, we find the following distinctive groups : 

a. Rocks of an evenly granular-crystalline composition, without any 
disposition to porphyritical microstructure, and poor in amorphous, glassy 
or half-glassy base. But a very slight trace of globulitic glass is often 
interwedged between the crystalline ingredients. This type of structure is 
presented by the true coarse or medium-grained dolerites, and by some of 
the seemingly homogeneous, genuine basalts. Nevertheless, the rocks pos- 
sessing it are rather rare along the Fortieth Parallel, as compared with Ger- 
many or Northwestern Europe (Ireland, Scotland, Faeroer, Iceland), where 
this type is the most common one. 

b. Rocks possessing a microscopically very fine-grained, totally crys- 
talline aggregation of crippled microlites, largely feldspar and augite, which 
serves as a groundmass in which micro-porphyritical and macro-porphyriti- 
oal, larger crystals of feldspar and olivine, with occasional augites, are 
distinctly and sharply imbedded. This type is rather restricted. 



254 MICROSCOPICAL PETROGRAPHY. 

c. Rocks in which a homogeneous, pure glass-base, usually of a yel- 
lowish-brown color, is largely developed, but hardly in such abundance as 
to exceed the crystalline ingredients. This type is very rare. 

d. Rocks composed of larger and smaller crystals, with a globulitic 
glassy base interwedged in cuneiform bodies between them. Without doubt, 
this variety has the closest affinities to augite-andesite. This well-charac- 
terized type, which occurs in other basaltic regions only as a well-known 
exception, is the most common one along the Fortieth Parallel ; a fact that 
tends to sustain a previously mentioned result, namely, that the mode of 
structure which most frequently occurs in the European basaltic regions, is 
rare in the herein-examined territory of North America. 



,as odr > 






BASALTS. 255 



SECTION IV. 

APPENDIX TO TKUE BASALTS. 

There are two rocks which differ somewhat in mineralogical composition 
from the true feldspar-basalts described in the preceding pages, and yet 
are most intimately connected with them geologically. These basaltic 
occurrences are the subject of the following notes. 

The principal rock of the hills between Haws's and Reed's Stations, near 
the Carson River, south end of Kawsoh Mountains [616], is of a blackish- 
gray, but not so dark or dull as most of the basalts, possessing a peculiar 
shimmering lustre. It looks something like a phonolite, and is fissile in rather 
thin and flat plates. Macroscopically, it is perfectly homogeneous, without 
any distinct porphyritical secretion. Under the microscope, a confused mass 
of badly formed crystals and needles appear, their substance ranging in 
color from a pale brownish-yellow to totally colorless. By comparison with 
other occurrences, these crystals and needles prove to belong to augite. 
Between the thin, delicate prisms, which are arranged without order, and are 
seldom longer than 0.045 mm or thicker than 0.015 ram , lie colorless sections of 
feldspar, mostly quadrangular in shape, and for the greater part polarizing 
monochromatically. But this is not proof that they are all sanidins, for the 
sections might belong to plagioclases cut parallel to oo P oo. Distinctive 
striated plagioclase is, however, quite rare. There is no hornblende or 
biotite, leucite or nosean, and, curiously, no olivine, in this rock. Another 
irregularly shaped and even externally unindividualized, colorless, polarizing 
ingredient is found between the augite prisms. Judging from its behavior, 
this substance can only be considered as nepheline ; for it occurs here in 
the manner so often found in the genuine nepheline-basalts. A great many 
small magnetite grains are disseminated through the mass. Upon being 
treated with hydrochloric acid, the rock immediately produces a gelatine, 
which is not very abundant or very stiff. This gelatine is occasioned by 
the presence of nepheline ; for olivine is entirely wanting, and no other 
ingredient of the rock is thus decomposed by the acid. 

Some conical hills at the north end of Kawsoh Mountains produce a 



256 MICROSCOPICAL PETROGRAPHY. 

variety similar to this in both macroscopical and microscopical respects 
[617, 618.] Larger fresh augite crystals, possessing the characteristic shape 
and cleavage, occur here, and they throw light upon the numerous, acicular, 
light brownish-yellow prisms belonging to the same mineral. There are also 
larger feldspars, which form small, white, macroscopical spots in the light- 
gray rock-mass; but here the larger and microscopical feldspars are, for 
the most part, plagioclases, accompanied by some sanidin. These specimens 
are also destitute of olivine. Apatite is present in many dusty, brown 
prisms, occurring precisely as so often found in phonolites, trachytes, and 
andesites, but rarely in genuine feldspar-basalts. Sometimes one apatite 
prism is partly imbedded in a larger feldspar and partly in the groundmass, 
another proof of the early solidification of this mineral. There is also con- 
siderable magnetite and some of the colorless, unindividualized nepheline- 
substance. 

A comparatively very light rock from the eastern point of the Kawsoh 
Mountains is precisely the same as the above-described. The small prisms 
and ledges of feldspar show a distinct fluidal structure. 

At the north end of the Kawsoh Mountains, the same variety is found 
in an altered state [619.] In the thin sections, numerous, little, pale, macro- 
scopical veins can be seen traversing the dark-gray mass in reticular forms. 
These veins indicate avenues of alteration along which the rock is decom- 
posed into a dirty, greenish-gray substance, whose nature cannot be deter- 
mined. It is remarkable that the parallel layers of which this product of 
alteration consists, in many places contain, as the innermost portions of these 
veins of decomposition, small, rounded aggregations of tridymite laminae, 
which, more than probably, are secondary depositions, although tfiey have 
all the peculiarities of the mineral where it is supposed to be a primary, 
original constituent; such as the scaly accumulation, the tenderness, the 
regular, but more often irregular, six-sided shape, as in those within the 
pores of andesites, trachytes, and rhyolites. 

The rock from Fortification Peak, Colorado [620], is a microscopically 
rather coarse-grained basalt rich in well-shaped, zonally built augites. At 
first sight, the rock appears closely to resemble the famous dolerite from 
the Lowenburg, in the Seven Mountains, on the Rhine. There are larger 



BASALTS. 257 

crystals of olivine. The colorless ingredient between the augites, olivines, 
and magnetites, however, in part only belongs to triclinic feldspar; other 
parts, polarizing monochromatically in colors of low orders, are unindividu- 
alized nepheline. Neither rectangular nor sexangular forms of this mineral 
could with certainty be detected. The sharply pointed ends of the augite 
crystals pierce this colorless nepheline mass in the usual manner, and it 
is also streaked with long, thin augite microlites. This ingredient is some- 
times slightly altered into a. less pellucid, dull substance, exactly like the 
first product of the decomposition of nepheline in the typical nepheline- 
basalts from the environs of Urach, in Wiirtemberg, from the Wartenburg, 
near Donaueschingen, Germany, and from the Fiji Islands, near Aus- 
tralia. Sharp, six-sided laminae of brown biotite are not rare here, being 
more abundant than in any true and pure feldspar-basalt. There is much 
magnetite. Olivines are rich in beautiful crystals of picotite and in glass- 
inclusions. The augites also have glass particles imbedded in them, which 
are remarkable for their thickness and the included bubbles. This rock, 
like those next to be described, much more strongly gelatinizes when treated 
with hydrochloric acid than could olivine alone; and the presence of 
nepheline is thus chemically substantiated. 

The rock from the summit of Navesink Peak, Elkhead Mountains, is 
exactly the same [621], except that the nepheline, and with it the biotite, are 
a little less abundant. Glass-inclusions of the olivines measure as high as 
0.008 mm in length. 

A dike on the Yampah River, south of Fortification Peak, also presents 
precisely the same composition. The less abundant augites are well crys- 
tallized, which is especially observable where they project into the colorless 
nepheline. 

The mixed, coarsely and finely porous rock from the summit of Bastion 
Mountain, Elkhead Mountains, would appear, also, to belong to this variety 
[622]. Nevertheless, it is rich in narrow plagioclases, which in some places 
possess an excellent fluidal structure. Some colorless spots are probably 
nepheline. There occur, beside the dark augites, some unknown needles, 
which are apparently flat and of a citron-yellow color when very thin, and 
varying from an orange to brownish-red when thicker. They are totally 
17 MP 



258 MICROSCOPICAL PETROGRAPHY. 

undichroitic, with rudimentary and crippled ends. In some places, these 
prisms are very numerous, and measure O.OG mm . They are neither horn- 
blende, augite, olivine, nor biotite; but perhaps they belong to gothite; yet 
the rock contains but little less magnetite than the others. A very little 
globulitic glass-base is occasionally seen. 

The rock from the ridge running east from Hantz Peak, Elkhead 
Mountains [623], surely belongs to this series. It bears plagioclase, unin- 
dividualized nepheline, augite, olivine, some biotite and apatite. 

A very curious rock was found at Fortification Rampart, Elkhead Mount- 
ains [624]. It belongs geologically to the basalts, although it differs from 
them at first sight by the lighter-gray color of its specimens and by the 
presence of a not inconsiderable quantity of macroscopical brownish-black 
biotite plates, which appear as foreign elements even macroscopically. The 
thin sections do not show in polarized light a single triclinic feldspar. A 
great quantity of freely cleavable and partly well-crystallized yellowish- 
green augites, finely lamella ted biotite, laminae measuring l mm , with sharp 
borders and a darker or lighter brown color to their sections, according to 
their thickness, and a colorless ingredient, which is equal in amount to both 
the others, and polarizes monochromatically without any striation, are 
present. Notwithstanding there is no olivine, the rock gelatinizes, which, 
by all analogy, proves nepheline. But only a part of the colorless spots 
above mentioned can belong to nepheline; for after the powdered rock had 
suffered a long treatment with boiling hydrochloric acid, a large quantity of 
the colorless grains and fragments remained unaltered. It is unquestionable 
that these latter belong to sanidin. Sometimes the colorless bodies are bor- 
dered on each side by parallel lines, perpendicularly upon which the sub- 
stance has become somewhat fibrous, as so often happens when alteration 
has just begun in nepheline. Apatite is more abundant than in any genuine 
plagioclase-basalt. Hornblende is wanting. 



CHAPTER IX, 



LEUCITE ROOKS. 

More than twenty years ago, Alexander von Humboldt published his 
conclusion that leucite was a mineral only found in Europe ; and it is 
rather curious that this casual remark has not been disproved until very 
recently. This mineral, up to the year 1868, was only known as a con- 
stituent of several lavas of Italy, of the Laacher See, and of the Kaiser- 
stuhl, in Baden. Since that year, it has been discovered to be a microscop- 
ical ingredient of many basalts of Saxony, Bohemia, the Thiiringer Wald, 
and the Rhon Mountains, occurring in unexpected frequency l But all 
these localities were European; so the remark still held good; and the other 
extra-European basic rocks, examined in large quantity, were never found 
to contain leucite. 

In 1874, Vogelsang discovered an Asiatic leucite. It occurred in a 
basaltic rock from the Gunung Bantal Soesoem, upon the small island of 
Bawean, north of Java. 2 

And now the microscopical study of the rocks of the Fortieth Parallel 
establishes the existence in America of the most classic leucite rocks. 
Moreover, these rocks are richer in the mineral than any occurrence in the 
Old World, besides which their general composition is very peculiar. 
Leucite was always considered, as is well known, one of the most perfect 
members of the regular system, until, in 1872, G. von Eath stated 3 that it 
belonged to the tetragonal or quadratic system, the apparent icositetrahedron 

1 F. Z., Die mikroskop. Structur u. Znsammensetzung der Basaltgesteiue. 

*P. Z., Neuea Jahrb. f. Mineralogie, 1875, 175: which was a communication 
after the lamented death of Vogelsang. 

'Monatsber. d. k. Akad. d. Wissensch. zu Berlin, 1. Aug. 1872. 

259 



260 MICROSCOPICAL PETROGRAPHY. 

being a combination of (P . 4 P 2). The colorless crystals, which generally 
show in the section a more or less regular or rounded octagon, have the 
peculiarity of containing a great quantity of strange, little crystals and grains 
grouped into a small, central heap or (which is more often the case) con- 
centric zones, of which the sections are also octagonal or roundish. These 
corpuscula, which are supposed to be intruded into the leucite, are, instead, 
situated on the surface of the leucite forms or globular figures. 

The first occurrence of these American leucite rocks was found in the 
Leucite Hills, northwest of Point of Rocks, Wyoming Territory [625, 626]. 
They have a light yellowish-gray, felsitic-looking, and very finely porous 
mass, in which the only macroscopical inclusion is some brownish-yellow 
and reddish-brown mica. This mica is not in six-sided or rounded plates, 
but in the form of remarkably long stripes and dashes, such as have seldom 
been observed. No other ingredients are visible to the naked eye, and the 
specimens do not disclose their rich secretions of leucite. At the first glimpse 
of the rock under the microscope (Plate V, fig. 4), the leucite appears, with 
its innumerable, very sharply outlined, colorless, octagonal sections 035 mra 
in diameter. None of the known European rocks are as rich in leucite as 
these, and there is scarcely one in which the forms of the sections are so 
regular and so similar. As is the rule with all such small bodies, the sections 
are entirely dark between crossed nicols, and do not show the curious 
systems of alternating dark and polarizing lines caused by polysynthetic 
twin-formation. The most minute leucites, measuring but 0.003 mm , with their 
fine, clear octagons, seem to have perfected their crystalline form. All 
of these leucite sections (Plate I, figs. 21, 22, 23) include quite pale-green 
augite-grains, which themselves bear very minute glass-grains, with included 
bubbles, arranged into pretty wreaths or rings, being, in fact, the American 
counterpart of the leucites in the famous lava stream flowing from the Alban 
Mountains to the Tomb of Caecilia Metella, near the Capo di Bove, near 
Rome. From five to eight rounded grains are grouped in these rings. A very 
nice phenomenon is that where light-green augite-microlites, radially arranged 
and very evenly distributed, occasionally protrude from the surrounding 
rock-mass into the larger leucites. In rare cases, leucites are found which 
entirely include club-formed augite-needles. These needles are not arranged 



LEDCITE-KOCKS. 261 



zonally or tangentially, but radially; the long axis of each pointing to a 
common centre, precisely as the phenomenon has been observed in some 
leucites of Vesuvian lava streams. 1 There are mixed with the leucites in 
this rock, as independent ingredients, pale-green prisms, acicular needles, 
and microlites, which surely belong to augite; although their shape is indis- 
tinct, and larger, better-crystallized individuals do not occur. In this fine 
aggregation and intermixture of leucite and augite, the large biotite stripes 
are imbedded, and none of them of microscopical size was observed. This 
curiously colored mica, which resembles ormolu, and whose long, thin streaks 
appear in surprising distinctness in the light rock-mass, is remarkable for its 
comparatively very feeble absorption. When examined with one nicol, its 
transverse sections never appear deep-brown or black, but only reddish- 
brown. Sometimes several delicate zones, each of a different color, compose 
the mica plates. These plates seem for the most part to be scattered through 
the rock with some measure of parallelism ; and hence the sections prepared 
parallel to the rock-cleavage show no transverse sections of mica, but only 
basal ones. There is no trace of monoclinic or striated feldspar, and horn- 
blende, olivine, mellilite, haiiyne, and nosean are wanting. A small quantity 
of magnetite is present, and also a considerable number of comparatively 
thick apatites, possessing a basal cleavage, and often a longitudinal dust- 
line in the interior. Occasionally, indistinct, colorless, rectangular or oblong 
bodies appear, which possibly belong to nepheline. No corresponding hexa- 
gons are visible; but, in any case, this mineral must be relatively veiy 
rare. Some brownish-black, opaque microlites occur at intervals, and a few 
of them are included in the mica. 

The external aspect and the mineralogical composition of these rocks 
differ not a little from the other leucite-bearing masses. Their unusual light- 
gray color is produced by the extraordinary abundance of leucite, and their 
comparative poorness in augite. Moreover, the augite occurs only micro- 
scopically. The European leucite rocks commonly bear thicker individuals 
of augite, and much more of it, and also more magnetite, so that their color 
is a great deal darker. The entire absence of feldspar is as remarkable as 
the abundance of large macroscopical biotites. 

1 F. Z., Neues Jahrbuch f. Mineralogie, 1870, 810. 



CHAPTER X. 

CLASTIC EOCKS. 



SECTION I. OLDER CLASTIC BOCKS. 

SECTION II. YOUNGER CLASTIC KOCKS BRECCIAS, CONGLOMERATES AND 
TUFAS OF TERTIARY AGE. 



SECTION I. 
OLDER CLASTIC ROCKS. 

Only a small number of occurrences belonging to this group has been 
examined, for the series does not possess great petrographical interest. The 
following rocks, however, seem to deserve mention. 

An old Carboniferous conglomerate from Penn Canon in the River 
Range, Nevada [627], is mostly made up of angular grains of black and 
brown lydite, quartz, and hornstone. The quartz-grains bear a great multi- 
tude of fluid-inclusions and frequently also delicate lamellae of mica; both of 
which inclusions prove that the grains are derived from shattered and disin- 
tegrated old granites or crystalline slates. . 

Another old Carboniferous rock from Penn Canon [628], which occupies 
a middle place between sandstone and graywacke, consists of rounded grains 
of quartz and decomposed feldspar and lydite. Here, also, the quartz 
grains include very neat, sharp, six-sided mica lamellae, measuring only 
0.003 mm .' 

A reddish conglomerate occurs in the same locality, which bears grains 
of quartz and kaolinized feldspar in a cement that is sprinkled with unknown 
black grains and is itself entirely indifferent to polarized light. This cement 
is therefore an opal-like substance or an amorphous silicate. 



OLDEK CLASTIC ROCKS. 263 

In another old clastic rock found in the Fountain Head Hills, Nevada 
[629], which is a rather coarse-grained graywacke similar to arkose, the 
microscope discovers a phenomenon that was previously known only macro- 
scopically, namely, the impressions made by egg-shaped fragments of 
neighboring rocks, as they occur, for instance, in the pebbles of the nagelflues 
in Switzerland. Here rounded, oval, worn grains of quartz have caused very 
distinct and often rather deep impressions in the surface of other clastic con- 
stituents with which they come in contact, especially in the pebble-like 
pieces of decomposed feldspars of a mass similar to felsite, and also in lydite. 



264 MICROSCOPICAL PETROGRAPHY. 

SECTION II. 

YOUNGER CLASTIC ROCKS BRECCIAS, CONGLOMERATES AND TUFAS 

OP TERTIARY AGE. 

The character of a clastic rock is easily recognized under the microscope 
if the thin section is made up of several fragments and not of a single one. 
Rhyolitic and trachytic rocks of this series are still rather fresh, while 
the basaltic and andesitic ones are much altered or decomposed, which 
renders them very difficult to study. If little bits of rock form the clastic 
material, the characteristic structure of certain mineral constituents can be 
seen very distinctly; for instance, the quartzes in the rhyolitic fragmentary 
rocks are seldom free from glass-inclusions, and the olivines in the basaltic 
group show the traversing veins of serpentineous matter. 

It should be remembered that these clastic rocks are to be divided 
into two genetically different classes; on the one hand those in which the 
cement binding the larger and smaller fragments, is only a very finely ground 
detritus of the same petrographical material, and on the other hand an 
eruptive crystalline rock-mass, which, for the most part, presents a variety 
of cemented fragments. In the latter case, the imbedded clastic pieces 
generally have a remarkably sharp-edged form. The Tertiary clastic rocks 
to be examined in the following pages may be separated into two divisions: 
one made up of rhyolitic or acid, and the other of basaltic or basic material. 
Trachytic and andesitic clastic rocks are very rare about the Fortieth Parallel. 

RHYOLITIC CLASTIC ROCKS. The rhyolites of "Western Nevada are 
often accompanied by numerous tufas and breccias. They are abundant, 
for instance, on the ridge at the head of Winnemucca Valley and about the 
Warm Springs and the Hot Springs. 

West of the Warm Springs, near old Fort Churchill, Nevada, a tufa 
[630] forms a lake deposit, consisting of predominating, light-colored and 
brown rhyolitic detritus. 

In the neighborhood of the Hot Springs, west base of Kawsoh Mount- 
ains, a rhyolitic breccia is spread out [631]. It is very finely clastic and 
highly porous. The microscope discovers the walls of the pores to be 
incrusted to the depth of Q.2 mm with a homogeneous substance of a pale- 



YOUNGER CLASTIC ROCKS. 265 

yellowish color, entirely isotrope in polarized light, and without doubt opal 
as a product of decomposition. 

The Tertiary hills between the Kawsoh Moimtains and Montezuma 
Range are rhyolitic tufa deposits [632, 633], in former fresh- water lakes, 
made up of nearly colorless, predominating, little, microscopical splinters, 
chips, and shards of very porous glass, accompanied by some fragments of 
diatomes (Melosira, Navicula). 

One of the most remarkable clastic rocks of those herein described is 
an obscure rhyolitic breccia from Mullens' Gap, west side of Pyramid Lake 
[634]. It is composed of sharp-edged fragments of a light-gray and a dark- 
gray rhyolitic rock, as large as hazel nuts, imbedded in a predominating 
dirty-gray material. The structure of the clastic particles of the light-gray 
and dirty-gray rhyolitic substance are not especially noteworthy ; but the 
fragments of the dark -gray rhyolite, which -have a somewhat pitch-like 
lustre, merit particular attention. Under the microscope, they are found to 
consist (Plate XII, fig. 1), in the main, of a pellucid glass, which has a very 
strange tone of color, best described as a pale-brownish-violet. Some feld- 
spar crystals, which are mostly sanidins, a considerable quantity of little 
feldspar microlites, and some black-edged grains, probably belonging to 
magnetite, are scattered through this glass-mass. Distinct and beautiful, 
waving, fluidal phenomena are caused by the direction of those secreted 
bodies which have a longitudinal axis. The glass also bears dark-bordered 
gas-cavities, roundish in form, egg-shaped, or drawn out parallel to the direc- 
tion of the fluidal lines. But the most remarkable feature of all is, that the 
glass contains the most perfect fluid-inclusions, with a movable bubble in each. 
For a moment the bubble will roll slowly around in the liquid, presently it 
shows a trembling motion, and again rests immovable ; but it can easily 
be set in motion by a slight elevation of temperature, for instance, by the 
heat of a lighted cigar held under the thin section. These liquid-inclusions 
attain the comparatively large size of 0.01 2 mm , being seen at the first glance, 
and there is a great quantity of them, for the most part of an oval form, but 
many are stretched out parallel to the lines of fluctuation. Since heating the 
thin section to a high temperature will not cause the bubble to be absorbed, the 
liquid cannot be pure carbonic acid. It is probably water holding in solution 



266 M1CKOSGJP1CAL PETltOGKAPHY. 

a slight amount of the acid. The presence of fluid-inclusions in a real glass- 
mass is indeed striking. To be sure, such inclusions were known in the con- 
stituents of rocks that had doubtless been solidified from a molten material, 
as in the leucites and olivines of basaltic lavas. But these latter masses have, 
by cooling, become a wholly, or almost wholly, crystalline aggregation which 
does not contain any, or hardly any, glassy base as a solidified residuum of 
the molten magma. On the other hand, fluid-inclusions in the larger crys- 
tals of glassy and half-glassy rocks were, as far as is known, totally wanting. 
In the feldspars, quartzes, hornblendes, etc., of the pitchstones, pearlites, and 
obsidians, and of the trachytes and rhyolites, rich in glass, all included amor- 
phous particles are, conformably, o'f a hyaline nature. But in the rhyolitic 
fragments of this American rock, a part of a liquid, or of a gas condensed to 
a liquid, has been arrested by the molten mass and preserved in the glass 
produced therefrom by cooling. The conclusions which are to be drawn 
from these observations as to the physical state of the former molten matter, 
are so near at hand that they should be explained in this connection. The 
presence of liquids does not necessarily exclude the idea of a former molten 
state, and vice versa. The fact that fluid-inclusions occur, for instance, in the 
quartzes of granites cannot, by any means, be used as an argument against 
the igneous origin of this rock. This interesting breccia presents yet another 
rare phenomenon. A colorless crystal, 0.1 4 mm long and 0.024 mm thick, one 
end terminating in a point, lies in the violet glass. On account of its daz- 
zling clearness, the roughness of its surface, its transverse basal cracks, and 
its association with hexagonal sections of the same substance, it would appear 
most probable that it belongs to the almost omnipresent apatite. This crystal 
contains an inclusion of light brownish glass, 0.02 mm long and 0.009 mni thick, 
imbedded parallel to the chief axis of the crystal, as is usual in apatites ; 
and there is a moving bubble within this fluid-inclusion. This isolated 
hyaline particle, entirely separated from the crystal mass, therefore possesses 
precisely the same peculiarity as the chief glassy mass of the rock. This 
phenomenon invests with new importance an older and perhaps half-forgotten 
observation made in 1868, where the leucites of the lava from Capo di Bove, 
near Rome, of that from the Solfatara, near Naples, and from the Burgberg, 
near Rieden, Lake of Laach, were found to contain analogous glass-inclu- 



CLASTIC KOCKS. 267 

sions, 1 which bore, instead of the usual interior empty cavity, a liquid in 
which rolled a moving bubble. Beside these combined interpositions, there 
occur in the same leucites, single glass and fluid inclusions. Since then no 
such curious associations have been seen. They merit attention now, because 
here, in one microscopical object, proofs of the presence of both factors in the 
rock-formation of molten material and liquid (or gas) are evidently united. 

At Cold Spring, Forman Mountains, Nevada, a characteristic rhyolitic 
breccia occurs [635]. It is a predominating brownish-red rhyolite, in 
which there are so many small splinters and chip-like, sharp-edged frag- 
ments imbedded, that in a thin section the size of one's finger-nail more 
than thirty can be seen with the naked eye. It seems to be a real friction- 
breccia, not a product of aqueous accumulation, nor yet a solidified, ejected 
tufa ; for under the microscope the brown-red rhyolite mass is seen to fill up 
the very smallest spaces between the strange fragments. There is great 
similarity between it and the massive quartziferous rhyolite found in the 
Forman Mountains [362]. The gray fragments of this mass very closely 
resemble the material of which a rhyolitic breccia of Snow Storm Canon, 
east slope of the Black Rock Mountains, is mostly composed [636], and 
which again appears in the desert, near Utah Hills, Black Rock Mountains 
[637]. It may be remembered in this connection that among the described 
massive rhyolites there are many that contain a few very small fragments 
of strange varieties without exhibiting either to the unaided eye or under 
the microscope the true characteristics of a breccia. 

In Snow Storm Cafion, Black Rock Mountains, there is a pumice- 
breccia [638] which the microscope discovers to be composed of numerous 
fragments welded or cemented closely together. The single fragments are 
mostly skeins of a gray glass, with a few crystals and ledges of feldspar. 
Individual glass-lines follow different directions in the adjoining pumice 
fragments. Under the microscope, the character of the mass as a breccia is 
very distinct Broken pieces of a brown rhyolite mass are also interposed 
between the pumice particles. 

Another series of rhyolitic tufas and breccias develop in the Kamma 
and Pah-tson Mountains. Three varieties were examined from the Kamma 



1 F. Z., Zeitschrift d. d. geolog. Gesellsch., XX, 1868, 117, 132. 



268 MICROSCOPICAL PETROGRAPHY. 

Mountains [639, 640, 641], one from the ridge north of the Kamma Mount- 
ains [642], and four from the vicinity of Grass Canon, in the Pah-tson 
[643, 644, 645, 646]. The rocks from these places are made up of very 
small fragments of rhyolite heaped together, sometimes in actual contact 
with each other, and sometimes included in a cement, which is not of a 
clastic nature, and often itself predominates. The most diverse varieties 
are represented here; strongly half-glassy ones, little sphaerolitic rhy elites, 
including glass, and possessing microscopically an excellent fluidal structure; 
some whose texture is entirely orderless, and brownish-gray ones with a 
reddish tinge. They chiefly belong, however, to varieties which are very 
poor in larger crystals or entirely free from them. One of the nicest rocks 
of this neighborhood is a pumice-like breccia from Ball Rocks, Grass Canon, 
Pah-tson Mountains [644]. It is for the most part composed of pieces of 
variously colored glass, which are very rich in pores. There are no crys- 
talline secretions, but the single glass-fragments contain shards and bits of 
differently colored glass. 

Tufas and breccias are also associated with rhyolites at Lovelock's 
Knob, foot-hills of the Montezuma Range It is difficult to determine 
whether the principal pail of a dirty-gray product from this place [647] is a 
solid, massive rock, or a very intimate accumulation of fine, clastic fragments. 
To be sure upon this point would be rather interesting, for the cavities of 
the rock contain an enormous quantity of the most beautiful and charac- 
teristic aggregations of comparatively large tridymite crystals. This occur- 
rence of tridymite, supposing the rock to be composed of fine clastic frag- 
ments, can only be explained as a secondary formation, which would be 
very remarkable genetically; there being no unquestionable proof of the 
epigenetic nature of this mineral. 

Another specimen from Lovelock's Knob is an accumulation of entirely 
isotrope shards and splinters of glass, varying in color from yellowish-gray 
to almost colorless, and very rich in pores. On account of its porosity, the 
rck shows no polarizing ingredient, and for the same reason it passes into 
the pumice-like variety. 

In the Mopung Hills, West Humboldt Mountains, a compact crystal- 
line quartz and some which is distinctly fibrous, forming the nicest sphsero- 



YOUNGER CLASTIC ROCKS. 269 

lites, with an admirable aggregate polarization, serve as the cementing 
material of the rhyolitic fragments [648]. 

The rhyolites of the Mopung Hills are associated with fine breccias, in 
which massive, not clastic, rhyolite material is the chief binding mass of the 
small fragments [649, 650]. Very coarse, 1'ght-colored bits of a sphserolitic 
rhyolite, in a quite dark, brownish-red cement, appear in the breccias of the 
Sou's Springs, Pah-Ute Range [651], bearing in its hollows small, botryoidal 
and well-stratified deposits of siliceous matter. The sphasrolites are rather 
poorly developed, consisting merely of the first rudiments or of segments of 
a circle: the formation of a somewhat regularly outlined, globular secretion 
has nowhere taken place. 

Grayish-green rhyolite-breccias are found in the Desatoya Mountains 
[652, 653]. They are chiefly made up of a pale-greenish, twisted, conchoidal 
glass, which is traversed by curious stripes of colorless glass, sometimes 
straight and sometimes curved like a sabre, a linear aggregation of the 
most fine, green, microlitic prickles running like a longitudinal axis from 
end to end of each. This aggregation is bounded on both sides by the 
colorless glass. The latter bears quartzes which include apatites containing 
glassy particles and monoclinic and triclinic feldspars. In the prevailing 
glassy rock-mass, there is a great number of sharp-edged little fragments of 
glass of the most diverse texture, but usually possessing a very distinct 
fluidal structure, the direction of which, of course, varies a great deal in 
the single shards. So many kinds of glass are here fused together that 
with a magnifying power of 300, as many as six glass splinters of a different 
texture may often be seen at once included in the chief mass. 

Real pumice-tufa occurs in the Fish Creek Mountains. It is, for the 
most part, an accumulation of very porous and undulated streaks of yellowish- 
gray glass. 

A breccia from Mount Airy, southern end of Shoshone Range [654], 
consists of sharp, angular fragments of dirty-grayish and greenish rhyolitic 
varieties, isolated crystals of quartz, which are often broken, with perfect 
glass-inclusions, feldspar, and biotite. The pores of the rock are covered 
over with curious microscopical crystals, the nature of which is unknown. 
These crystals measure O.l mm in length, are entirely colorless, sharply 



270 MICROSCOPICAL PETROGRAPHY. 

crystallized, and belong either to the tetragonal or the rhombic system, 
having a rectangular, and often apparently quadratic, prism, with two hori- 
zontal terminating faces. The prismatic faces bear a vertical striation. It 
is difficult to think of a mineral known macroscopically which could be 
supposed to occur in this manner, and capable of connection with these 
secondary crystals. May it not be the zeolite, comptonite, or thomsonite, 
the prismatic angle of which is 90 40', and whose prismatic faces are 
vertically striated? 

Penn Canon, River Range, contains very fine clastic rhyolitic tufas 
[655, 656, 657], which contrast with many of the above-mentioned rocks, 
that contain strange fragments in a predominating crystalline mass. Here 
are striped and striated, gray, brownish, and yellowish varieties which are 
perfect likenesses of the felsitic tufas or claystones of the Lower Permian 
(Dyas, Rothliegendes) in Germany, originating in the old felsite-porphyries; 
in every respect, the true precursors- of the. Tertiary rhy elites. 

Dirty-gray rhyolitic tufa from Carico Canon, Shoshone Range, whose 
individual particles on an average measure only 0.05 mm in diameter, is 
similar to the last described [658]. 

A coarser tufa from the slope toward Indian Creek [659] has, upon 
the surface of its rounded, clastic particles, crystals like those in the rock 
from Mount Airy, here also protruding into the chasms. 

The same locality has another variety which looks like a light, coarse- 
grained graywacke. Under the microscope, there are to be observed: a, 
several kinds of rhyolitic-felsitic groundmasses; 5, fragments of feldspars 
that are partly striated; c, rounded quartzes, in some of which glass- 
inclusions appear; d, broken pieces of biotite; e, brown hornblende. All 
these ingredients are closely massed together without any visible cement, 
but there are minute spaces between them. 

Rhyolitic tufas of Sacred Pass, Humboldt Mountains, are veiy finely 
clastic [660, 661, 662]. One of them was made up in pretty equal parts 
of fragments of rhyolitic rubbish, and bits of crystals measuring O.l mm . 

A clastic rock from Citadel Cliff, Holmes' Creek Valley, is a very inter- 
esting one. It is a nearly compact brown mass, which the loupe shows to 
be made up of little splinters of glass. The composition of the rock is a fine 



YOUNGER CLASTIC ROCKS. 271 

volcanic ash, consisting of very thin splinters of obsidian fused together into 
a cohering mass. Under the microscope, the thin sections have a very beau- 
tiful appearance. The thin bits of glass, destitute of secretions, are of a 
lighter or darker brownish-yellow color. They are orderless, pointing in 
different directions, and can often be seen to be welded together on the 
border crosswise. Between them, as independent clastic ingredients, are 
fragments of feldspar and quartz, the latter bearing comparatively large 
glass-inclusions. 

Before closing this section, some occurrences which are genetically or 
geologically connected with the rhyolitic, clastic rocks, should be mentioned. 

The tufas from Boone Creek, Shoshone Range, include opal, the color- 
less mass of which shows, in the thin sections, white, milky stripes, less 
pellucid than the surrounding substance [663, 664]. They are an intimate 
aggregation of little siliceous sphaerolitic globules (Vogelsang's cumulites 1 ), 
sharply separated at the borders from the predominating colorless material. 
The latter appears perfectly homogeneous in ordinary light, and it is aston- 
ishing to observe between crossed nicols that they form an aggregation of 
single-edged polarizing grains which are set together like mosaic. Where 
this aggregation is coarser, but particularly where it is finer, there appears in 
polarized light a very pretty speckled, spotted, and stippled surface, such as 
could not possibly be reproduced by artificial coloring. In some places, the 
larger grains have very fine, radiating fibres. Independent of the fact of 
the presence of water in the mass, it is not very probable that the grains 
belong to crystalline quartz. It seems more likely that they are particles 
of opal, which have been endowed with double refraction by mutual 
pressure. The polarizing qualities of the noble opal were detected by 
Reusch in 1865, who has been corroborated by Behrens. 8 

A chalcedony from Grass Cafion, Pah-tson Mountains, possesses an 
exquisite structure [665]. It does not originate from the tufas, however, but 
from the massive rhyolites. Under the microscope, the section shows little 
globules and botryoidal concretions in a seemingly homogeneous, colorless 

1 Die Krystalliten, Bonn, 1875, 134. 

2 In bis capital examinations of the microstructurc of opals Sitzungsbcr. d. 
Wiener Akad., LX1V, 1871, Dec. Heft. 



272 MICROSCOPICAL PETROGRAPHY. 

substance. The glolmles are very nicely concentric, and become entirely 
dark between crossed nicols. Viewed through the nicols, the colorless 
substance proves to be an aggregation of siliceous sphaBrolites, which 
richly polarize (Plate XII, fig. 2). They are fibrous, the fibres radiating, 
although the outlines are not rounded, but have become polygonal by 
reciprocal compression. 

A whitish, sinter-like, siliceous deposit resembling chalcedony, from 
the hills between Kawsoh Mountain and Montezuma Range [666], gives a 
thin section, in which small pellucid spots alternate with less pellucid ones. 
The microscope discovers that the whole mass is crystalline, and it presents 
a variegated, glittering picture. It is a pure, proper hornstone, made up of 
an aggregation of the finest quartz particles, closely resembling the horn- 
stone from the metalliferous veins of Schneeberg, Saxony, which is, how- 
ever, somewhat coarser crystalline. The dull spots are caused by an 
enormous quantity of angular and rounded, dark-bordered cavities, aver- 
aging 0.01 mm in diameter, which lie associated in this hornstone. They 
are always empty, never showing a bubble, the sign of a surrounding 
liquid. Otherwise this siliceous deposit is wholly free from anything like an 
inclusion, and no particle which could possibly belong to an opal substance 
can be detected in it. It is remarkable, and, as far as is known, hitherto 
unobserved, for a sinter produced by siliciferous springs to take the form 
and condition of cryptocrystalline quartz instead of that of amorphous, 
simply reflecting opal, called a siliceous sinter. 

BASALTIC CLASTIC ROCKS. The massive basalts do not seem to be as 
often accompanied by corresponding fragmentary rocks as the rhyolites. 
Only those which are in some respects especially interesting will be men- 
tioned in these closing pages. 

At the Black Rock (which will be mentioned hereafter as the location 
of a palagonite tufa), a blackish-gray rock [667, 668] is found which appears 
rather homogeneous and looks like a basalt; but under the microscope it 
proves to be a basaltic tufa. Angular, dark splinters and little bits of 
basalt are first distinguished, together with colorless ledges of plagioclase. 
They seem to belong to a very dark, globulitically devitrified variety of 
basalt, and are often browned. Other basalts are represented, but they are 



YOUNGER CLASTIC ROCKS. 273 

much decomposed, and cannot well be made out. There are also rounded 
spots, mostly of a green color, sometimes composed of undulated stripes and 
sprinkled with black grains. These are probably products of the alteration 
of olivine and augite. The cement is mostly of a calcareous nature. Even 
larger and purer portions of calcite bearing the characteristic twin-striation 
appear in some places. 

A remarkable basaltic tufa comes from Basalt Ridge, east of Grass 
Cafion, Pah-tson Mountains [C69]. It is principally composed of many- 
cornered splinters, of a brown, somewhat porous, and entirely isotrope 
glass, in which only the traces of crystalline secretions can be detected. 
There are also some polarizing fragments of dark-brown augite and color- 
less feldspar crystals. These clastic constituents are joined without cement, 
so that the rock is very loose and easily triturable. The brown glass will 
not gelatinize, even after a long boiling with hydrochloric acid : it therefore 
does not belong to tachylyte, but to hyalomelane (page 250). Indeed, it is 
allowable to call this rock a hyalomelane tufa. It was probably once in 
the state of volcanic sand ; and it may be remarked in this connection that, 
among the clastic, dust-like particles of ejected volcanic material, glass- 
masses far more largely predominate than in the massive lavas which have 
flowed out of the same crater. 1 Petrographically, if we disregard their 
insolubility in acids, such materials are not very different from palagonitic 
tufas. 

At Bastion Mountain, Elkhead Mountains, a curious basaltic tufa occurs 
[670] which the microscope shows is composed of, a, fragments of a basaltic, 
light greenish-yellow glass, as thick as peas, with augite crystals, color- 
less feldspar-microlites, arranged like stars, black magnetite grains, and 
numerous small, oval cavities; fc, rounded quartzes, traversed by many 
band-like lines of fluid-inclusions, and the long, often curved, very thin, 
blackish needles that are so often observed in the quartzes of granites or 
crystalline schists ; c, fragments of a decomposed, dull, untransparent min- 
eral, which appears to be altered orthoclase ; and, d, a cement of calcite. 
It is probable that predominating basaltic detritus has united with metamor- 
phosed granites to form this rock-mass. 

PALAGONITE TUFA. The mineral known as palagonite, which M as first 

1 Neues Jabrb. f. Mineralogie, u. s. w., 1872, 16. 
18 M r 



274 MICROSCOPICAL PETROGRAPHY. 

detected by Sartorius von Waltershausen in the basaltic tufas from Militello, 
Sicily, and from several places (but especially Seljadalr) in Iceland, has 
since been found in many other localities as a constituent (sometimes pre- 
dominating) of basic fragmentary rocks ; for instance, at the Beselicher 
Kopf, near Limburg, and at the Lahn, Nassau ; in the Eifel, at the Kaules- 
berg, in the Westerwald, Germany ; at Le Puy en Velay, France ; near 
Montferrier, north of Montpellier ; at the Szigliget Mountain, and at 
Leanyvar, near Battina, Hungary ; at James Island, Galapagos ; in the 
district of Dyampang-Kulon, Java ; in the Canary Islands ; at the foot of 
Mount Somers, New Zealand. This list may be lengthened with some 
excellent occurrences of palagonite in Nevada. They have been hitherto 
unknown in the United States. Localities where these occurrences are 
found are in the Tertiary strata at the south end of the Kawsoh Mount- 
ains, near the Overland Road ; west of the Kawsoh Mountains, near Hot 
Spring ; and at Black Rock, Nevada. They are tufas, composed for the 
most part of grains and little fragments of a lighter or darker, yellowish- 
brown, amorphous mass, totally indifferent to polarized light. This 
substance indeed looks glassy, and is enormously rich in very dark- 
bordered, larger and smaller, microscopical gas-cavities, usually of a rather 
regular, oval form. There is no trace of augite, olivine, magnetite, or 
nepheline. Colorless ledges of striated plagioclase are the only secretions. 
Here and there sharply outlined bodies of a glass-mass are seen to be 
imbedded, differing from the surrounding mass by a different tone of color ; 
so that this is most probably a hyaline breccia (Plate XII, fig. 3). In some 
places, the inner walls of the larger empty hollows have been remarkably 
altered, particularly in a stratum which outcrops southeast of Haws' Sta- 
tion. The alteration progresses in zones from the walls of the cavities 
inward through the surrounding mass, changing this originally homogene- 
ous substance into a fibrous aggregation of short needles, the color only 
being retained. A section running through such a cavity shows plainly 
the structure of the walls, although it is not easy at first sight to distinguish 
the radiating external circle, or altered outer wall, from the fibrous mass 
which forms the inner parts, and the bottom of the hemispherical cavity 
laid open by cutting through the hollow. Between the nicols, these objects 
aggregately polarize; and even a colored cross, changing its color and 



YOUNGER CLASTIC EOCKS. 275 

position by turning the thin section or the analyzer, may be distinctly seen 
running over them (Plate XII, fig. 4). The rocks from near White Plains 
and from Black Rock are finer-grained tufas. That from Fossil Hill, Kawsoh 
Mountains, is more of a breccia. In it the black palagonite grains are often 
distinctly arranged in the form of schists. According to Sartorius von 
Waltershausen, 1 whose opinion formerly received general approbation, the 
palagonitic substance is to be compared with a hydraulic mortar, being the 
secondary product of a submarine alteration of basaltic tufa rocks. It 
would therefore be a hydrous, iron-bearing silicate which belonged to the 
class porodine, amorphous bodies produced like opal by the solidifying of a 
mass resembling gelatine. 

As the result of his recent microscopical study of several palagonites, 
Rosenbusch has expressed the view 2 that these tufas are chiefly accumula- 
tions of ejected hyaline volcanic sands and ashes, consisting of basic glass. 
But he is inclined to think that the amount of water in the palagonites is 
not primary, but is derived from a molecular alteration of this glass, which 
is poor in silica. The substance which has been analyzed and named 
palagonite is indeed a mechanically inseparable mixture of the primitive 
anhydrous glassy palagonite (the sideromelane of Sartorius von Walters- 
hausen, with only 0.349 per cent, of water), and the products of its easy 
decomposition. Rosenbusch's conclusions would appear to be accurate, for 
the microscopical study of this new Nevada palagonite does not at all con- 
tradict him. But the differently colored, red, yellow, and brown, band-like 
zones which appear in the amorphous substance of, for instance, the Ice- 
landic palagonites, as well as others, and which probably represent separate 
stages in the progress of alteration, do not occur here. The only proof of 
decomposition in the Nevada occurrences is the fibrous walls of the cavities. 
This American palagonite being, therefore, a comparatively rather fresh 
one, approaching sidromelane, it may be allowable to conclude that a chem- 
ical analysis will demonstrate it to be poorer in water and richer in silica 
than are most of the more highly metamorphosed types. 

1 Die vulkan. Gesteine v. Sicilieii u. Island u. deren submarine Umbildung. 
* Nenes Jahrb. f. Mineralogie, 1872, 152 ; Mikrosk. Physiographic d. petrogr. wich- 
tigst. Mineral., 141. 



GENERAL INDEX. 



Page. 

Actinolite in quartzite of Clover Canon 23 

Adrian, von, cited 136 

Allport, Samuel, cited , 79 

Amorphous base. (See Base.) 
Ampli i In ilitc. Humboldt Range, moving bub- 
ble in fluid-inclusion in hornblende of 23 

Amygdules in melapbyre 103 

Analyses, andesite, by C. Conncler 135 

European andesites 124 

dacite 137 

Analysis, andesite, by Dr. Walter Eormann . . 123 

angite-andesite, by Reinbard 228 

trachyte, by Dr. Anger 146 

by W. O. Mixter 116 

gabbro, by Prof. Wiedemann 109 

gneiss, Ogden Point, by Prof. Robert 

Bunsen 25 

qnartz-propylite, by Dr. Walter 

Kormann 118 

qnartz-propylite, by Prof. Wiede- 

mann 119 

Anamesite, classified 6 

ofSteinheim 232 

Andosite, amorphous base in 129 

analyses of, by C. Conncler 136 

analysis of, by Dr. Kormann 123 

Annie's Canon, Cortez Range 131 

apatite in 127,130,131 

angitein 125,131 

(augite) 123,221 

analysis of, by Reinhard. 228 
angite-microlites in, 224, 225, 22C 

apatite in 222,223 

Angusta Mountains 224 

Bagonya, Hnngary 221 

Basalt Creek, Washoe .... 221 

biotiteiu 227 

Cedar Mountains 227 

classified C 

Cotopaxi 221 



Page. 

Andesite, (angite) feldspar-microlites in .. 222,224, 

225 

foreign, silica in 222 

glass-inclusions in 225 

glass-inclusions in feld- 
spar of 223,225 

globulitic glass in 223 

Independence Valley, Nev. 225, 
226 

Java 221 

Kyneton 221 

near Clarke's Station, 

TruckeeCaflou 223 

ofGambira"n 221 

of the Palau Islands, Aus- 
tralia 221 

ofRogodjampi 221 

ofSantorin 221 

ofSnngiPait 221 

ofTunguragua 221 

of Victoria 221 

ofWidodarin 221 

olivinein 223 

Palisade CaBon, Nevada. 227 

quartziferous 228 

qnartzin 227 

Reese River Valley, Nev. 225 

snnidin in 224 

south of Wadsworth, Nev. 224 
Steamboat Valley, Nev . . 222, 223 
subdivision of basalts. . . 219, 220 
Susan Creek Canon, Nev. 225 

trichites in 226 

Wachoe Mountains 226 

Wagon Canon, Cortez 

Range 226 

Augusta Canon 128 

Angusta Mountains, Nevada 127 

Berkshire Canon 126 

biotiteiu... 130 

277 



278 



GENERAL INDEX. 



Page. 

Andesite, black outline of hornblende in 129 

carbonate of lime in 130 

Clan Alpine Cafion 129 

classified 6 

compared with propylite 132 

Crescent Cafion, Augusta Mount- 
ains 129 

diagnostic comparison of with 

propylite 133 

enstatite in 125 

epidote iu 130 

(European), analyses of 124 

European, compared with Ameri- 
can 132 

flu iila I groundmass in 132 

glass in 130 

glass-inclusions in feldspar of 131 

plagioclase of . 131 

glassy inclusions in plsgioclase of. 126 

globulitic base in 123 

Gold Hill Cemetery 123 

(hornblende) 9,10,110,122 

classified ,10 

Hungarian 9 

Hungarian 125 

microlites in gronndmass of 132 

of feldspar in ground- 
mass of 126 

of hornblende in ground- 
mass of 125,127,129 

near Palisade Canon 131 

north of Gold Hill Peak 123 

opacite in 127 

order of eruption 11 

quartz in 130 

eanidin-bearing, Kamma Mount- 
ains 127 

sanidin in 126,130, 131 

spbterolites in 130 

Traverse Mountains, Utah 132 

trichites in 129 

Truckee Canon 124 

Tuscarora 129, 130 

Wachoe Mountains 130 

Washoe 122 

west shore Pyramid Lake 126 

Wright's Canon 127 

Andesitic groundmass. (See Groundmass.) 
Anger, Dr., analysis of augite-trachyte by. .. 146 
Apatite, abundance of in gneiss, Farming- 
ton Cafion 26 

containing fluid-inclusions, in gneiss, 

Clover Cafion 21 

in andesite 127, 130, 131 

augite 148 



Page. 

Apatite in augite-andesite 222, 223 

basalt 235, 241, 244, 248, 249, 253, 258 

basaltic rock 256 

dacite 135 

diorite 85,87,90 

felsite-porphyry 76 

glass of rhyolitic breccia 266 

gneisses, Clover Cafion 17, 18 

gneiss, Deer Mountain 35 

granite, Granite Canon 40 

granite .....51,53,54,56 

Granite Rock, fluid-inclu- 
sions in 50 

porphyry 62,64,67,69 

Truckee Range 41 

Wachoe Mountains 49 

hornblende-porphyry 96 

leucite rock 261 

melaphy re 104, 105 

mica-gneiss 34 

mica-slate, Long's Peak 35 

propylite 113,115 

quartz of rhyolitic breccia 269 

quartz-propylite 118, 119, 120 

rhyolite .. .172, 386, 190, 191, 194, 196, 198 

semi-hyaline rhy elite 212 

trachyte.. 145, 147, 149, 152, 1E3, 154, 156, 

157, 158 

inclusion of, in feldspar of felsite- 
porphyry 75 

superabundance in gneiss of Lake 

Range 15 

Archsean schists, Clover Canon 17, 18, 19, 20, 

21,22,23 

Humboldt Range 16 

Arkose 263 

\ngite-andesite. (See Andesite.) 

in andesite 125,131 

basalt, inclusions of biotite in 252, 253 

hyaline-rhyolite 211 

pearlitic-rhyolite 214 

propylite 112 

rhyolite 171, 188, 195, 198, 200, 202 

semi-hyaline-rhyolite 213 

trachyte .. .147, 148, 152, 155, 157, 158,159, 

161 

inclusion of, in leucite 260 

plagioclose of ba- 
salt 235,248 

Angite-porphyry. (See Porphyry.) 
Augite-syenite. (See Syenite.) 
Augite-trachyte. (See Trachyte.) 

Axial fibration in rhyolites 164, 165, 166 

trachyte 150 

Axially fibrous groundmass. (See Groundmass. ) 



GENERAL INDEX. 



279 



Page. 

Axiolites in ihyolite . . 173, 174, 170, 178, 180, 183, 188, 
189, 190, 193, 194, 196, 202, 204 

Baranowski, J., cited GO 

Basalt 216,229 

Basalt, Agate Pass, Cortez Range, Nevada .. 249 

American Flat Cation, Washoe 230 

Anita Peak, Elkhead Mountains 250 

appendix to 255 

angite-audesite subdivision 219,220 

angite-microlites in.. .240, 242, 243, 248, 253 

Augusta Mountains 246 

Au vergne 233 

Berkshire Canon 233 

biotitein 252,257,258 

Black Rock Desert 242 

Black Rock Mountains, Nevada 241 

Buffalo Peak, Colorado 251,252 

calcite in 245 

chalcedony in .. 249 

constitution of 217,218 

cuneiform development of base in.. 234, 243 

devitrification-phenomena in 232 

Diabase Hills 236 

dichotomous microlites in 238,239 

Elkhead Mountains 250 

(feldspar) classilied 6 

feldspar-microlites in 284 

FitOi Creek Mountains 246 

gas-cavities in feldspar of 238, 240 

glass-inclusions in feldspar of. .241, 244, 246 

olivine of.. .246,250,253 

globulites in . .232, 234, 236, 240, 243, 244, 245 

globulitic base iu 342 

gothitein 258 

half-glassy-inclusions in feldspar of . . 245 
plagioclaseof 240 

Hanau .. 232 

haiiynein 251 

Havallah Range, Nevada 247 

Hungarian 233 

hyalomelane of 250 

Icelandic 233 

inclusion of augite in plagioclase of .235, 248 

biotite in angite of 252, 253 

olivine in magnetite of.. 238 

plagioclase of. 232, 

235,248 

picotite in olivine of 250 

Kamma Mountains 243 

Kawsoh Mountains 238,239 

Kieshiibel 229 

Lake Range, Nevada 241 

Mallard Hills, Nevada 248 

Montezuuia Range 244 

near Carson River 238 



Page. 

Basalt near Clarke's Station 237 

King's Station, Nevada 237 

Winnemucca, Nevada 247 

(nepheline) 256,257 

classified. 6 

Fiji Islands 257 

Urach 257 

Wartenburg 257 

nepheline in _ 258 

Ombe Mountains 2. r >0 

order of eruption . 11 

Pah-tson Mountains 243, 244 

Pah-Ute Range 246 

Peaviue Mountain, Nevada 231 

peculiar products of secretion in 249 

picotite in 244, 246, 257 

oliviueof 235 

pseudotachylytes of 250 

PuicoRivo 229 

Pyramid Lake 240 

quartz in 251 

Ruby Valley Range 250 

sanidin in, 232, 233, 234, 238, 241, 246, 248,258 

Scotch 233 

Schemuilz 229 

serpentineous alteration of olivine in . 252 

olivine iu 237 

Seven Mountains, Germany 245 

Shoshonee Mesa , 247,248 

silica determination of, by C. Couu- 

cler 236 

siliceous deposit in 233 

similarity of occurrences in separate 

regions 218 

Snow Storm Ledge 242 

Spanish Spring Station, Nevada 230 

specular iron in 238 

Steamboat Spring, Washoe 231 

subdivision of Fortieth Parallel 253,254 

tachylyte in 236 

titanic iron in 243 

trichites in 239,252 

tridymite in 239 

TruckeeCafion 231,232 

Ferry 233 

Valley 232 

Tunchal 229 

Tungfernberg 229 

Upper Little Snake, Wyoming 251 

Washoe 229 

West Humboldt Mountains 245 

Whirlwind Peak, Shoshonee Range, 

Nevada 249 

Winuemucca Lake, Nevada 240 

Yauipah River 251 



280 



GENERAL INDEX. 



Page. 

Basaltic clastic rocks 272 

lavas characterized ....'. 217 

rock, Bastion Mountains, Colorado. 257 
Fortification Peak, Colorado.. 256 

Fortification Rampart 258 

Hautz Peak, Colorado 258 

Kawsoh Mountains 255,256 

nepheline in 255, 256, 257 

sanidiu in 256 

tridymite in 256 

Yampah River 267 

rocks characterized 216, 217 

tufa, Bastion Mountains, Colorado. 273 

Black Rock, Nevada 272 

caleite in 273 

fluid-inclusions in quartz of.. 273 

Pah-tson Mountains 273 

Basalts, European and Fortieth Parallel, com- 
pared . 218,219 

foreign, contrasted 218 

general remarks on 216 

(lencite) classified 6 

near Wadsworth, Nevada 233 

nepheline and leucite, relations of.. 218 
of Anita Peak compared with Gor- 
man 251 

picotitein 235 

serpentineous alteration of olivine 

in 230,231 

Base 4 

(amorphous) in andesite 129 

uielaphyre 105 

cuneiform development of, in basalt ..234,243 

globulitically devitrified in diabase 99 

felsite - por- 
phyry... 75 

(globnlitic) in andesite 123 

basalt 242 

diabase 100 

trachyte 150, 158, 159 

indistinctly polarizing, in diorite 89 

(microfelsitic) in rhyolite 173 

Behrens cited 271 

Belonites characterized 12 

in pnmicestone 208 

Beudant cited 7 

Biotite in andesite 130 

augite-andesite 227 

basalt 252,257,258 

dacite 138 

diorite 86,87 

diorite-gneiss, North Park 34 

lencite rock 261 

propylite 114,115 

inclusions of, in augito of basalt 252, 253 



Page. 

Bischoff, Oustav, cited 98, 124 

Breccia .. 2G4 

comptonitein.... 270 

(rhyolitic). (See Rhyolite.) 

zeolite in 270 

Brewster cited 19 

Brookite 30 

Bubbles, inclusions with. (See Inclusions.) 

Buch, Leopold von, cited 71 

Bucholzite in mica-schist of Humboldt Range 16 

Bunsen, Prof. Robert, analysis of gneiss by.. 25 

Caleite in basalt 245 

basaltic tufa 273 

dacite 134,135 

diabase 102 

diorite 89 

garnet rock, Big Cottonwood 

Canon, Wahsatch Range 28 

melaphyre 103,104 

propylite 114 

quartzite, Mill Creek 35 

quartz-propylite 119 

rbyolite 169,170 

trachyte 150, 153 

Carbonic-acid inclusion. (See Inclusion.) 
Carboniferous conglomerate, River Range, 

Nevada 262 

Cavities (empty) in angite of diabase 101 

feldspars of groundmass 

of granite-porphyry.. 62 

quartz of rhyolite 195 

sanidin of rhyolite 195 

(gas) in feldspar of basalt 238 

dacite 140 

rhyolite 189 

glass of rhyolite 176 

hornblende of trachyte 156 

quartz of rhyolite 182 

sanidin of rhyolite ]86 

(glass) in quartz of rhyolite 194 

in quartz of granite 55 

Chalcedony 271 

in basalt 249 

Cherzolito G 

Chlorite after garnet, gneiss, Farmington 

Canon 25 

Chlorite after garnet, mica-gneiss, Farming- 

ton Canon 26 

Chlorite in granite 53,57 

Chloritic mineral in gneiss, Deer Mountain.. 35 

granite 55 

Classification, general 1 

Clastic rocks 262 

(older) 2G2 



GENERAL INDEX. 



281 



Page. 

Clay-stones (Permean) of Germany 270 

Cohen, E., cited 72 

Comptonite in breccia 270 

Conglomerate (carboniferous), River Range, 

Nevada 2G2 

fluid-inclusion in quartz of 262 

mica-inclusions in qnartz of .. 262 

Conglomerates 264 

Councler, C., analyses of audesito by 13G 

silica determination of basalt 

by 236 

Crystalline groundmass. (See Gronndmass.) 
schists. (See Schists.) 
slate, Trinity Canon, Monteznma 

Range,Nevada 15 

type 1 

Crystallites, rectangular, in gronndiuass of 

trachyte 147 

Cumnlites in Hungarian rhyolites 187 

rhyolite 187,188,204 

tnfa 271 

Cnneate strings of rhyolite 204 

Cyanite in mica-schist, Red Creek, Uinta 

Mountains 23 

Dacite 11,110,134 

American Flat City, Washoe 134 

apatite in 135 

axially fibrous groimdmass in 139 

Basalt CaBon, Washoe 138 

Berkshire CaBon 139 

biotite in 138 

calcitein 134,135 

carbonate of lime in 139 

classified 6 

comparative observations npon 141, 142 

Devil's Gate, Washoe 137 

epidote in 135 

European, table of analyses of 137 

flnidal phenomena in 138 

fluid-inclusion in feldspar of 135 

Gould and Curry quarry, Washoe 139 

inclusions of glass and gronndmass in 

feldspar of 139 

inclusions of glass in qnartz of .138, 139, 140 
inclusions of glass, with bubble, in 

feldspar of 140 

Inclusions of raicrofelsitic gron ml nines 

in quartz of 136 

inclusions of sphaerolitic groundmass 

in quartz of 135 

Kainrna Mountains, Nevada 139 

Leon de Nicaragua 137 

microfelsitic gronndmass of 135, 137 

Mullen's Gap, Pyramid Lake 139 



Page. 

Dacite, order of eruption 11 

origin of name 10 

Palisade Canon, Cortez Range 140 

sanidin in 138 

Shoshonee Peak 140 

sphserolites in 133 

spharolitic gronndmass of 135, 139 

microstrncture in ground- 
mass of 134 

Transylvanian 135 

trichites in 138,140 

viridite in 135 

Wagon CaBon, Cortez Range 140 

Dacitic breccia 140,141 

Dathe cited 98 

Delesse cited 71 

Delessite in melaphyre 103 

Des Cloizeanx cited 52 

Devitrification 2 

phenomena in basalt 232 

Diabase 83,97 

apatite-microlites in 99 

augite-microlites in 97 

calcite in 103 

classified 6 

empty cavities in angite of 101 

felsitic groundinass in 101 

feldspar-microlites in 99 

fluid-inclusions in feldspar of 97 

qnartz of 101 

German 98 

glass-inclusions in angite of 101 

plagioclase of ... 101 

globulitic base in 100 

globnlitically devitrified base in 99 

mica in 102 

New Haven 99 

olivinein 99 

orthoclase in 100 

Owyhee Bluffs 101 

Pah-Ute Range 101 

Saxon 99 

Seetoya Peak 102 

serpeutiueons olivine in 99 

south end Truckee Range 100 

Torquay, England 99 

Truckee Range.... 97,99 

West Humboldt Mountains 100,101 

Diallage 108 

rocks classified 6 

Diatomes in rhyolitic tnfa 265 

Diorite 83 

Agate Pass, Cortcz Range. 91 

apatite in 85 

Augusta Mountains 87 



282 



GENERAL INDEX. 



Page. 

Diorite, Basalt Canon, Washoe 84 

biotite in 86,87 

calcito in 89 

Curtoz Valloy, Nevada 90 

classified 6 

Crown Point Ravine 83 

epidotein 84,85,86,89 

fibratcd feldspar in 85 

fluid-inclusions in plagioclase of 84, 85 

gneiss 14 

Clover Cailon, II um bold t Range 21 

Grass Valley 90 

green mica in 90 

Hot Spring Hills, Pah-Ute Range.... 86 
inclusions of .salt solution, with cubes, 

ill quartz of 90 

indistinctly polarizing base in 89 

K a m ma Mountains 86 

Kawsoh Mountains 86 

labradoritic structure of plagioclase 

in 91 

magnetite in 84 

Medicine Bow Range 92 

(metaniorphic), Ogden Canon, Wah- 

satch Range 91 

(mica), classified 6 

(micaceous), European 87 

microlite of horn blende in fluid-inclu- 
sion in plagioclase of 85 

microlitesin 92 

microlites of hornblende in 87 

Mill Creek Canon, Cortez Range 91 

Mount Davidson, Washoe 83,110 

Mount Davidson, Washoe, fluid-inclu- 
sion in plagioclase of 92 

moving bubble in fluid-inclusion in 

feldspar of 85 

Nache's Peak, Truckee Range 86 

Now Pass 87 

of Ilmenau 85 

of middle geological age 86 

Ophir Ravine 83 

Oquirrh Mountains 91 

Pah-Ute Range 86 

Peavine Mountain, Nevada 85 

polysy nthetic structure of hornblende 

in 88 

quartziferons groundmass in 91 

quartziferous, Havallah Range 88 

Quenast, Belgium 112 

Ravenswood Peak 90 

Shoshonee Range 89 

specular iron in feldspar of 92 

titauitoin 87,92 

titanic iron in 84,86,89 



Diorito, tourmaline in 87 

tremolite in 88 

Truckee Canon 85 

viridite in 89 

Winuemucca Peak 89 

zircon in 160 

Dioritic-gneiss, North Park, biotite in 34 

Disthene in mica-schist of Humboldt Range. 16 

Doelter, C., cited 11,104,135,136 

Dolerite classified C 

Lowenbnrg 256 

Dolerites 242 

Eklogite C 

Elvan, Cornish 76 

Empty cavities. (See Cavities.) 

Enstatite in andesite 125 

Epidote grains in granite-porphyry 67 

in andesite 130 

dacite 135 

diorite 84,85,86,89 

garnet rock (Archajan), Big Cot- 
tonwood Canon, Wahsatch 

Range 27 

granite-porphyry 66, 68, 69 

propylite 112,113,114 

quartz-propylite 118 

traversing feldspar in granite 52 

Eruptions, massive, succession of 11 

Feldspar, aggregation of 48 

altered in concentric, inscribed 

zones 52 

basalt. (See Basalt.) 

bearing rocks, classification of 6 

caolinized, in granite-porphyry . .. 64 

decomposed, in granite 43 

fibrated, in diorite 85 

fibrationof 45,47 

in foreign zircon-sye- 
nites 45 

in groundmass of granite-porphyry, 

empty cavities in 62 

labradoriziug, in granite, Havallah 

Range 45 

monoclinic, in propylite 115 

of basalt, gas-cavities in 238, 240 

diorite, moving bubble in fluid- 
inclusion of 85 

felsite-porphyry, inclusion of ap- 
atite in 75 

granite, epidote traversing 52 

rhyolitc, gas-cavities in 189 

opaliziug, in rhyolite 184 

radiated, in syenite 81 



GENEltAL INDEX. 



283 



Pace. 

Feldspar, reddish, iu granite 53 

rrin a rk a 1 ilc structure of 45, 46 

rocks, ante-Tertinry 6 

Tertiary and recent 9 

zone-structure of 47 

Felsite-porphyry. (See Porphyry.) 

Felsitic fibres in granite-porphyry 61 

groundniasg. (-SfeGronndmass.) 

Ferrite characterized 12, 13 

in :i mil-sit ic gronndmags 128 

rhyolite . 167, 168, 172, 174,177, 178, 179,189, 

190, 191, 192, 193, 194, 195, 198, 200, 204 

trachyte ..144, 147, 149, 150, 152, 154, 158 

Fibres, aggregation of, in rhyolite 204 

felgitic, in granite-porphyry 61 

leek-green, in granite-porphyry 66 

sphairolitic, in trachyte 152 

Fibrolite, mica-schist, Hnmboldt Range 1C 

Fibrous groundmass. (See Groundmass.) 

Fischer, H., cited 14,29 

Fluctuation 4 

phenomena in rhyolite 204,205 

Fluidal groundmass. (See Groundmasg. ) 

lines in andesitic grouudmass 128 

rhyolite 171, 172 

microstructure 5 

phenomena 4 

indacite 138 

Fluid-inclusion. (/See Inclusion.) 

Forellenstein, classified C 

Foyaite, classified - 6 

Fundamental gneiss, Loch Maree, Scotland . . 21 

Gabbro 83,107 

analysis of, by Prof. Wiedemann 109 

classified 6 

Iron Mountain, Laramie Hills 107 

microlites in plagioclase of 107 

Gabbros (European), peculiar microscopical 

structure of plagioclase in 108 

(European), uralite in 108 

Garnet, chlorite after, in gneiss of Farming- 
ton CaSon 25 

mica-gneiss, Fann- 

irjgton Canon 26 

iu gneiss, Farmington Canon 25 

granite 52 

Crusoe Cation 42 

metamorphio granite 55 

mica-gneiss, Farmington Cuuon . 26 

rhyolite 195 

Saxon granulite 55 

trachyte 151 

rock, Big Cottonwood Canon, Wah- 

satch Range,'calcite iu 28 



rge. 
Garnet rock, (Archaean), Big Cottonwood 

Canon, epidoto iu 27 

Big Cottonwood Canon, fluid- 
inclusions in quartz of 28 

Hungarian, schistiform structure of. 27 

sch isti form structure of 27 

Gas-bubble. (See Bubble.) 

Gas-cavities. (See Cavities.) 

German basalts compared with Anita Peak 

occurrences 251 

Glass-cavities. (See Cavities.) 

Glass (globulitic) in angite-andesite 223 

grouudmass of quartz- 

propylite 120 

rhyolite 180 

in andesite 130 

andesitic groundmass 126 

Glass-inclusions. (See Inclusions.) 

Glassy type 2 

Globulites 2 

in basalt 232,234,236,240,243, 

244,245 

half-glassy rhyolites 206,207 

byaline-rhyolite 211 

pearlite 208,211 

Globulitic base. (See Base.) 

Gneiss, Adam's Peak, Ceylon 15 

Clover Canon 17 

apatite in 17,18 

fluid-inclusions in apa- 
tite of 21 

fluid-inclusions in feld- 
spar of 17 

Deer Mountain, apatite in 35 

chloritic mineral in. 35 

(diorite) 14 

Clover Cafion, Humboldt 

Range 21 

titanite in 21 

Farmington Canon, Wahsatch Range, 

Utah 25 

Farmington Cafion, abundance of apa- 
tite in 26 

Farniington Cafion, chlorite after gar- 
net of 25 

Farmington Cafion, fluid-inclusions 

numerous in 26 

fundamental, Loch Maree, Scotland.. 21 

(granitic), Rawling's Peak, Wyoming. 30 

hornblende-bearing, Mount Zirkel... 34 

hornblende-microlites in 30 

(hornblende) 36 

Davis Mountain, Park 

Range, Colorado 33 

Farmiugton Canon 26 



284 



GENERAL INDEX. 



Page. 
Gneiss, (hornblende) Grand Encampment 

Creek, Park Range.. 33 
inclusions of water, and 
liquid carbonic acid 
and 'water, in quartz 

of 33 

OgdenCafion 24 

Ogden Point 24 

zirconin 26 

Lake Range, fluid-inclnsions in quartz 

of 15 

Lake Range, superabundance of apa- 
tite in 15 

(mica), apatite in 34 

chlorite after garnet in 26 

French Creek 34 

garnet in 26,35 

hornblende-bearing 35 

hornblende-bearing, Clarke's 

Peak 34 

plagioclase in 34 

titanite in 35 

FarmingtonCafion.garnetin. 26 
near Secret Pass, Huiuboldt Range.. 21,22 

north end Lake Range, Nevada 14 

Ogden Point, Prof. Robert Bunsen's 

analysis of 25 

(oligoclase), Todtmoos 14 

pseudomorph chlorite after garnet in. 25 
Rawling's Peak, Wyoming, sponta- 
neously moving bubbles in saline 
solution of inclusion in quartz of.. 30 
spontaneous motion of bubble in car- 
bonic-acid inclusion in quartz of. .. 20 

titanite in 35 

water and carbonic acid inclusions in 

quartz of 18,19 

zircon in 22,24 

Gneissic slate, Eagan CaBon, Eagan Range, 

Nevada 23 

Gothite in basalt 258 

Granite 39 

Agate Pass, Cortez Range 48 

apatite in 51,53,54,56 

Augusta Mountains 44 

black microlites in quartz of 42,46,53 

Bruin Peak, Park Range, Colorado. 52 

cavities in quartz of 55 

CherokeeBntte,MedicineBowRange 55 

chlorite in 53,57 

chloride mineral in 55 

Clarke's Peak 53 

classified 6 

Clayton's Peak, Wahsatch 51 

Crusoe Cafion, garnet in 42 



Page. 

Granite, Davis' Peak, Park Range, Colorado. 52 

decomposed feldspars in 43 

dioritic tendency of 42 

dike, Crusoe Canon 42 

Havallah Range 46 

Eagan Canon 49 

Elk Mountain 55 

epidote traversing feldspar in 52 

eruptive, Granite Cafiou 40 

titanite in 40 

fibrous orthoclase in 57 

garnet in 52 

general remarks on 39 

Glacier CaSon, North Park 54 

Grand Encampment Creek 53 

Granite Cation, apatite in 40 

Granite Creek Station 42 

Granite Peak, Pah-tson Mountains.. 43 

Granite Rock, Utah desert 49, 50 

fluid-inclusions in ap- 
atite of 50 

Havallah Range 45 

labradorizing feld- 
spar in 45 

(hornblendic), Granite Ridge 43 

GrassCafion 43 

inclusions (fluid) in feldspar of 52 

quartz of 43, 52, 53 

inclusion (liquid carbonic acid) in 

quartz of.... 44 

inclusions (liquid) in feldspar of ... 48 
in quartz and feld- 
spar of 40 

in quartz of 49,56 

with salt cube, in 

quartz of 46,49,57 

(mica) in feldspar of 57 

in quartz and feld- 
spars of......... 52 

inquartzof 57 

(miorolitic) in feldspar of. 45 

(quartz) in feldspar of 57 

(water and liquid carbonic 

acid) in quartz of 44,45 

(Jurassic). 40 

large magnetite in 54 

lepidomelane in 56,57 

LittleCottonwoodCafion,Wahsatch. 50, 52 

Long's Peak, Colorado Range 56 

(metamorphic), BellevuePeak, Med- 

iciiie Bow Range. 55 

characterized 58 

garnet in 55 

Granite Canon, Lar- 

amie Hills.. 56 



GENERAL INDEX. 



285 



Page. 

Granite, (metamorphic), Granite Peak 44 

Huniboldt Kango .. 48 
Iron Mountain, Lar- 

amie Hills 58 

Sherman, Laramie 

Hills 57 

Signal Peak, Lar- 
amie Hills 57 

Virginia Dale, Lar- 
amie Hills 57 

zircon in 49 

microlites in labradorizing feldspar 

of 51 

quartz of 40,56 

of feldspar in quartzes of. 50 
hornblende in feldspar 

of 50 

hornblende in mica of. 50 
mnscovite in quartz of. 50 

Monteznma Range 43 

mouth of Big Cot ton wood Cation .. . 51 

muscovite in 42,47 

Nannie's Peak 47 

of Pyrenees, lepidolite in 42 

older eruptive, characterized 58 

oxyd of iron, hydrous, infiltrating.. 57 

in feldspar of. 51 

infiltrating 41,46,56 

feldspar of. 63 

PahkeahPeak 42 

Pah-enpp Mountains 42 

Pah-tson Mountains 42 

porphyry. (See Porphyry.) 

Ravenswood Hills 46,47 

reddish feldsparin 53 

Sahwave Mountains 42 

Shosbonee Knob 47 

Sierra Nevada 39 

specular iron in 43 

feldspar of 49,51 

spontaneously moving babbles in 

water-inclusion in quartz of 53 

titanite in 41, 43, 48, 49, 50, 51 

unusually large in 52 

tremolite in 56 

Truckee Range 40 

apatite in 41 

mutual envelopment 
of constituents in. 41 

Wachoe Mountains 49 

apatite in 49 

Winnemucca Peak 47 

Woodranch Cafion, Shoshonee Range 47 

younger eruptive, characterized 58, 59 

zircon in 49,54,57 



Page. 
Granitic gneiss, Rawling's Peak, Wyoming .. 30 

Granospbasrolites in granite-porphyry 61 

Granular gronndmass. (See Groundmass.) 

Granulite (Saxon), garnet in 55 

Gray trachyte 9 

Graywacke, Fountain Head Hills 263 

Greenstone-trachytes 9 

Gropbach cited 15 

Gronndmass (andesitic), ferrite in 128 

fluidal lines in 128 

glass in 126 

(axially fibrous) of dacite 139 

constitution of, in felsite-por- 

phyry 77 

(crystalline) of felsite-porphyry 75 

defined 4 

(felsitic) of diabase 101 

granite-porphyry.. 62,67 

(fibrous) of rhyolite 167, 179 

radiating alteration of, 

in granite-porphyry 63 

fine-grained and felsitic.. 62 

(fluidal) in andesite 132 

inclusions. (See inclusions.) 

(microfelsitic) of dacite 135,137 

rbyolite....l68,172, 
177, 197 

(microlitic) of rhyolite 168 

(microlitic and granular) of 

trachyte 150 

of granite - porphyry, empty 

cavities in feldspars of. .. 62 
quartz-propylite, globulitio 

glass in 120 

trachyte, crystallites rect- 
angular in 147 

(quartziferous) of diorite 91 

(sphserolitic) of dacite 135, 139 

rhyolite 199 

of felsite-porphyry 73 

Hagge, R., cited 108 

Half-crystalline type 1 

Half-glassy rhyolite 204 

Haner, K. von, cited 124,136 

Haiiynein basalt 251 

trachyte 151,152 

Hochstetter, F. von, cited 15 

Hornbleude-andesite. (See Andesite.) 

bearing gneiss, Mount Z irkel ... 34 
mica - gneiss, Cherokee 

Butte 35 

mica-gneiss, Clarke's 

Peak 34 

black outline of, in andesite.. . 129 



286 



GENERAL INDEX. 



Pago. 
Hornblende gneiss. (See Gneiss.) 

microlitic fibration of, in propy- 

lite 115 

of amphibolite, Hiuuboldt 
Range, moving bubble in fluid- 
inclusion of 23 

of trachyte, gas-cavities in 156 

polysynthctic structure of, in 

diorite 88 

porphyry. (See Porphyry.) 

rock, Cedar Mountain 35 

Eed Creek, Unita Mts 30 

fluid-inclusions in quartz 

of 93 

schist, Farmington Cafion 26 

Twin Peaks, Wahsatch 

Range 26 

Hornstono, granulated, in rhyolite 189 

Schneeberg 272 

Hiuuboldt, Alexander von, cited 257 

Hyaline ash 271 

Hyaline-rhyolites 143 

glass-inclusion in quartz 

of 211,215 

characterized 8 

Hyalomelane of basalt 250 

Ostheim 250 

Sababurg 250 

tufa 273 

Hyalite in trachyte 146 

Hypersthene rocks, classified 6 

Hypersthenite, classified 6 

Inclusions (fluid) in 

apatite of gneiss, Clover Cation 21 

granite, Granite Hock 50 

calcite of marble, Kinsley District 38 

Triassic limestone, Buffalo 

Canon 38 

feldspar of dacite 135 

diabase 97 

diorite, moving bubble in 85 

gneiss, Clover CaSon 17 

granite 52 

granite, Granite Eock 50 

granite-porphyry 69 

rhyolite 167,168,170 

trachyte 157 

glass of rhyolite-breccia 265 

hornblende of amphibolite, Humboldt 

Eange, moving bubble in 23 

hornblende of trachyte 156 

microscopical quartz-grains of felsite-por- 

phyry 76 

plagioclase of diorite 84,85 



Page. 

Inclusions (fluid) in 
plagioclase of diorite, microlite of horn- 
blende in 85 

moving bubble in. 92 

quartz of basalt tufa 273 

conglomerate . 26*) 

diabase 101 

ffl.site-porphyry 75 

garnet-rock, Big Cottonwood 

Canon, Wahsatch Range 28 

gneiss, Lake Range 15 

granite 43, 52, 53 

granite -porphyry 62 

hornblende rock 93 

quartzite 23 

quartz-propy lite 1 18, 120 

moving bubble 

in 117 

rhyolite 197 

moving bubble in 201 

syenite 81 

sanidin of rbyolitc 164 

moving bubble in 166 

Inclusions (fluid) 

numerous in gneiss, Farmington Cation.. . 26 
Inclusions (fluid) with 
salt-cube, in quartz of gneiss, Eawling's 

Peak, Wyoming 30 

salt-cube, in quartz of granite, Nannie's 

Peak 47 

Inclusions (glass and groundmass) in feld- 
spar of dacite 139 

Inclusion (glass) bubbles in 207 

Inclusions (glass) in 

apatite of rhyolitic breccia 266 

augite-audesite 225 

angite of diabase 101 

leucitc rock 260 

rhyolite 188 

trachyte 152,157 

biotite of rhyolite 177 

feldspar of andesite 131 

augite-andesite 223,225 

basalt 241,244,246 

hornblende-porphyry 96 

pumicestone 207 

rh volite 175, 176, 178, 185, 

187, 189 

trachyte 157, 1 58 

olivine of basalt 246,250,253 

pitchstone of Arran, Scotland 213 

plagioclase of andesite 131 

diabase 101 

half-glassy rhyolite 214 

quartz of dacite 138,139,140 



GENERAL INDEX. 



287 



Page. 

Inclusions (glass) in 
quartz of groundmass of felsitc-porphyry. 70 

hyaline-rhyolite 211,215 

obsidian 213 

rhyolite . 166, 168, 170, 172, 176, 177, 181, 

182,183,184,186,187,188,191,192,193, 

200,201 

semi-byaliue rhyolite 212 

trachyte 159,160 

rbyolite 198 

sanidin of rbyolite 196 

Inclusion (glass) with bubble, in feldspar of 

4acit 140 

Inclusion (glassy) in andesite 126 

Inclusion (half-glassy) in feldsparof basalt... 245 

rbyolite . 195 
trachyte. 150 
plagioclase of basalt 240 
Inclusion (saline fluids!) in quartz of gneiss, 

bornblcnde-niicrolites in 32 

Inclusions (liquid) in 

feldspars of felsite-porpbyry 74 

granite 48 

lencite of European lavas 266 

quartz and feldsparof granite 40 

of felsite-porpbyry 74,79 

granite 49,56 

rhyolite-breccia, moving bubble in 265,266 

Inclusions (liquid) with 

salt cube, in quartz of granite 46,49,57 

granite-porphyry... 61 
quartz-propylite... 119 

Inclusions (microlitic) in feldspar of granite. 45 
Inclusion of 

apatite iu feldspar of felsite-porphyry 75 

angitein leucite 260 

plagioclase of basalt 235, 248 

biolitein augite of basalt 252,253 

carbonic acid in quartz of gneiss, spon- 
taneous motion of bubble in 20 

fluid carbonic acid and water 19 

fluid carbonic acid in quartzes of granitic 
gncissofSt Gotthard; Freiberg, Saxony; 
Align rush more, Ireland; in topazes of 
Rio Belnionte, Brazil ; in sapphires ; in 
angites, oli vin%s, and feldspars of basalts 
from Rhenish Prussia; and in apatites 

from the Pfitsoh Valley, Tyrol 20 

groundmass in quartz of felsite-porphyry.. 77 

granite-porphyry. 64 

liquid carbonic acid in quartz of granite.. 44 

mica in feldspar of granite 57 

quartzes and feldspars of granite . . 52 

quartz grains of conglomerate 262 

of granite 57 



Tage. 
Inclusion of 

microfelsitic groundmass in qnartz of 

dacite 136 

muscovite in decomposed feldsparof gran- 
ite-porphyry 65 

oliviue in magnetite of basalt 238 

plagioclase of basalt 232,235,248 

picotite in oli vine of basalt 250 

rhyolite 203 

salt solution, witb cubes, iu quartz of 

diorite. 90 

salt solution, with cubes, in quartzes of fel- 
site-porpbyry 77 

salt solution, with cubes, iu quartz of 

granite-porphyry 63 

spbajrolitic groundmass in quartz of 

dacite 135 

water and carbonic acid in quartz of gneiss. 18, 19 
water, and fluid carbonic acid and water, 

iu quartzite 33 

water, and liquid carbonic acid and water, 

in quartz of hornblende gneiss 33 

water and liquid carbonic acid in quartz 
of 

granite 44,45 

granite-porphyry 62,64 

quartz-propylite 117 

water in qnartz of granite, spontaneously 

moving bubble in 53 

Inclusions (qnartz) in feldspar of granite 57 

(saline) in quartz of gneiss, spon- 
taneously moving bubbles in.. 30 
(slaggy) in feldspar of rhyolite. .. 194 
with dark-bordered bubble in 

granite-porphyry 61 

Iron oxyd in feldspar of granite 51 

infiltration of, in granite 41,46,56 

specular, in feldspar of diorite 92 

Jurassic granite. (See Granite.) 

Kormann, Dr. Walker- 
analysis of andesito by 123 

quartz-propylite by 118 

Labrador! to, Kiew, Russia 183 

Paul's Island, Labrador 107 

rock of Zaerdals Oeren 109 

Labradorizing feldspar in granite, Havallah 

Range 45 

Laspeyres, H., cited 72 

Lavas (European) lencite in 259 

(Italian) leucites in 157 

Lemberg, I., cited 112 

Leonhard cited 221 



288 



GENERAL INDEX. 



Page. 

Lepidolite in granite dike, Crusoe Canon... 42 

ofPyrenecB ..... ..... 42 

Lepidomelaue in granite ................... 56, 57 

Leucite, Asiatic ........................... 259 

augite-inclusion in ................. 260 

angite-niicroliteg in ................ 261 

basalts, classified .................. 6 

Bawean Island ................... 259 

classified ......................... 6 

crystallography of ............... 259, 260 

inKuropean lavas ................. 259 

liquid-inclusions in 266 

Italian lavas ................... 157 

rock, augite-microlites in .......... 260 

biotite in .................... 261 

CapodiBove ................ 260 

glass-inclusion in augite of ... 260 

nepheline in ................. 261 

rocks ............................. 259 

classified .................... 6 

Leucite Hills, Wyoming ...... 260 

(sanidiu) rocks, classified .......... 6 

Liebenerite classified ......... . ............ 6 

Lime, carbonate of, in andesite ............. 130 

dacito ................ 139 

quart z-propylite ..... 117, 120 

Limestone, Triassic, Buffalo Cafioii, fluid- 

inclusions in calcite of ................... 38 

Liparite characterized .................. ... 7,8 

Euganean Hills, Northern Italy ____ 7 

Iceland ......................... 7 

Siebengebirge, Rhenish Prussia ___ 7 
Liquid-inclusions. (See Inclusions.) 

Lithophyses in rbyolite ............... . ____ 198 

semi-hyaline rbyolite ....... 212 

of Tokaj ..................... 212 

Lydite .................................... 263 

in conglomerate .................... 262 



] 

84 
214 
208 
165 
156 



Macroscopical, term defined 
Magnetite in diorite 

pearlitic rhyolite 

pumice stone 

rbyolite 

trachyte .................... 1 

large, in granite ... 
zonal periphery of 
Marble, crystalline-granular, Kinsley Dis- 

trict ....................... _____ 38 

Kinsley District, fluid-inclusion in 
calcite of ....................... 38 

Marekanite, Siberian ...................... 210 

Marx cited ................................ 222 

Massive crystalline rocks .................. 6 

eruptions, their succession ......... 11 



Page. 

Melaphyre 83,103 

amorphous base in 105 

amygdules in 103 

apatite in .104,105 

Berkshire Canon 103 

calcite in 103,104 

classified 

delessite in 10:i 

pitcbstone, Weisselberg 223 

Melaphyres, foreign 104 

German 103 

serpentiueous oli viues'in 104 

Metamorphic diorite, Ogden Canon, Wah- 

satch 91 

granite. (Sec Granite.) 

Miaseite, classified 6 

Mica, South Burgess, Canada 32 

diorite. (See Diorite.) 
gneiss. (See Gneiss.) 

green, in diorite 90 

inclusions. (See Inclusions.) 

in diabase 102 

schist. (See Schist.) 
slate. (See Slate.) 
Microfelsitic base. (See Base.) 

devitrification 3 

groundmass. (See Gronndinass.) 
Microlite knots in mica-schist of Hnmboldt 

Range 16 

of hornblende in fluid-inclusion in 

plagioclase of diorite 85 

Microlites 

(apatite) in diabase 99 

(augite) 126 

in angite-andesite 224, 225, 226 

basalt 240,242,243,248,253 

diabase 97 

gronmlmass of augite-trachyte.. 145 

half-glassy rhyolite 214 

leucite 261 

rock 260 

rhyolite 176,178,196 

black, in quartz of granite 42,46,53 

quartzite 27 

characterized 11, 12 

dichotomons, in basalt 238,239 

feldspar, in iingite-andesite 222, 2<!4, 225 

basalt 24r) 

diabase 99 

granite-porphyry 64 

groundmass of amli-site 126 

propylite 113 

tuichyte . ....146,148 

quartzes of granite 50 

rhyolite 170,196 



GENERAL INDEX. 



289 



Pnge. 
Microlites 

feldspar, in trachyte 154, 155 

(hornblende) in diorite 87 

feldspar of granite 50 

qnartz-propy- 

lite 120 

felsite-porphy ry 79 

gneiss 30 

groundmassof andesite. 125, 137, 

129 

groundmassof trachyte.. 148 

mica of granite 50 

propylite 114 

Quartz of quartzite 23 

saline fluid-inclusion in 
quartz of gneiss. ...... 32 

in andesitic gronndinass 131 

diorite 92 

granite-porphyry 61 

gronnduiassof andesite 132 

trachyte 147,158 

labradorizing feldspar of granite 51 

mica of Kersanton, Brittany 15 

schist 15 

mica of trachyte ' 154 

plagioclase of gabbro 107 

pumicestone 208 

quartz of granite 40, 56 

rhyolit-e 175,198 

sanidin of trachyte 161 

trachyte 159 

trachytic groumliuass 152 

(mica) in mica-slate 32 

(muscovite) in mica-slate 35 

quartz of granite 50 

(plagioclase) in rhyolite 194 

Microlitic gronndinass. (See Gronndmass.) 

Microstructnre, fluidal 5 

Mixter, W. G., analysis of, cited 116 

Moving bubble. (See Bubble.) 

Muscovite in crystalline schist 15 

granite 42,47 

mica-slate 23 

trachyte 153 

inclusions of, in decomposed feld- 
spar of granite-porphyry 65 

Nepheline-basalt. (See Basalt.) 

in basalt 258 

basaltic rock 255,256,257 

leucite rock 261 

trachyte 161,162 

rocks, classified 6 

Nepbelinite, classified 6 

Nevadite, characterized 8 

Norite, Tronfield 109 

19 M P 



Page. 
Norites, Norwegian 108 

Obsidian 206 

classified 6 

glass-inclusion in quartz of 213 

Grass Canon 210 

Hrafntinnuhryggr, Iceland 210 

Ombe Bluff, Utah 213 

trichites in 214 

Ochre in quartz of rhyolite 195 

Olivine in augite-andesite 283 

diabase 99 

trachyte 159,160 

of basalt, glass-inclusions in.. 246, 250, 253 
serpentineous alteration of, in ba- 
salt 230,231,252 

in basalt 237 

diabase 99 

melaphyres 104 

serpentinized, in trachyte 146 

Opacite characterized 12 

in andesite 127 

hornblende-porphyry 95 

rhyolite . . 165, 167, 168, 172, 177, 191, 204 

trachyte 144, 149, 150, 152, 154, 158 

Opal in rhyolite 199 

Orthoclase, fibrous ... 52 

in granite 57 

in diabase 100 

porphyry, classified 6 

remarkable crystals of, in gran- 
ite-porphyry 68 

rocks, classified 6 

with nepheline or loncite, 

classified 6 

quartz, classified 6 

without quartz, classified. 6 

zoned structure of 40 

Orthoclases of felsite-porphyry, spontane- 
ously moving bubbles in 79 

Oschatz cited 35 

Oxyd of iron, hydrous, infiltrating granite.. 57 

infiltrating feldspar in granite . 53 

Palagonite tufa 273,274,275 

Black Eock, Nevada 275 

Kawsoh Mountains 275 

mode of origin 274 

White Plains, Nevada 275 

tufas, foreign 274 

Paragonite-slnte, St. Gotthard 28 

Pearlite, devitrification-products of 208,209 

classified 6 

globulites in 208,211 

Parker's Station, Montezuma Range . 210 
trichites in 210 



290 



GENERAL INDEX. 



Page 

Pearlito, White Plains, Montezuma Range. . . 210 

Pearlites, foreign, devitrification-products of. 209 
Pearlitic rhyolite. (Sec Rhyolite.) 

Pegmatite, Hungarian 42 

Phlogopite, South Burgess, Canada 32 

Phonolite, classified 6 

Picotite in basalt 244,246,257 

European basalts 235 

olivtne of basalt 235 

Pitchstone, classified 6 

Arran, Scotland, glass-inclusion 

in 213 

Pitchstones of Meissen 171 

Plagioclase, abundance of in granite of 

Truckee Range 41 

Plagioclase-augite rocks, classified 6 

biotite rocks, classified 6 

gneiss, Deer Mountain 35 

hornblende rocks, classified 6 

in mica-gneiss 34 

in pearlitic rhyolite 214 

labradoriticstructureof, indiorite 91 

olivine rocks, classified 6 

peculiar microscopical structure 

of, in European gabbros 108 

rocks, classified 6 

Plastic magma 5 

mechanical action in 5 

Porodine 275 

Porphyrite, classified 6 

(hornblende), classified 6 

Porphyry 

(augite), classified 6 

(felsite) 7,71 

apatite in 76 

classified 6 

constitution of groundmass 77 

crystalline groundmass in 75 

dikeof,ingraniteofLong'sPeak.. 79 

Wachoe Mountains 77 

foreign 79 

from Korgon and the Tscharish, in 

the Altai 79 

globulitically devitrified base in.. 7ft 

Granite Peak, Pah-Ute Range 75 

groundmass of 73 

inclusion of apatite in feldspar of. 75 
salt solution, with 

cubes, in quartzes of 77 
inclusions (fluid) in microscopical 

quartz grains of. 76 

in quartz of 75 

(glass) in quartz in 

groundmass of. 76 

(liquid) in feldspars of. 74 



Page. 
Porphyry 

(felsite,) inclusions (liquid) in quartz of ... 74, 79 
of groundmass in quartz 

of 77 

microlites of hornblende in 79 

Miner's Canon, Truckee Range... 73 

Odeuwald, Germany 72 

of Fortieth Parallel compared with 

foreign 79,80 

Ravenswood Peak, Shoshonee 

Range 75 

sphserolites in - 78 

spontaneously moving bubbles in 

orthoclases of 79 

Spruce Mountain, Peoquop Range. 78 

various theories concerning 71,72 

Willow Springs,MontezumaRange 74 

(granite) 39,60 

apatite in 62,64,67,69 

Big Cottonwood Cation, Wah- 

satch Range 67 

caolinized feldspar in 64 

classified 6 

Clayton's Peak,Wahsatch Range. 68 
Clover Canon, Huruboldt Range. 62 
empty cavities in feldspars of 

groundmass of 62 

epidote in 66, 67, 68, 69 

European '. 68 

feldspar-niicrolites in 64 

felsitic fibres in 61 

groundmass in 62,67 

Good Pass, North Park 68 

Goose Creek Hills, Nevada 65 

granosphserolites in 61 

inclusions (fluid) in feldspar of. . 69 
quartz of... 62 
(liquid),with salt-cube, 

in quartz of 61 

of gronndmass in 

quartz of 64 

muscovite in decom- 
posed feldspar of . 65 
salt -solution, with 

cubes, in quartz of. 63 
water and Hqu id car- 
bonic acid in quartz 

of 62,64 

with dark - bordered 

bubble in 61 

Kinsley district 64 

leek-green fibres in 66 

microlites in 61 

Maggie's Peak - 61 

Nannie's Peak 61 



GENERAL INDEX. 



291 



Page. 
Porphyry Continued. 

(granite) Oqnirrh Mountains, Utah 66 

pyrites in - 67,69 

radiating alteration of fibrous 

groundmass in 63 

remarkable crystals of orthoclase 

in 68 

Spanish Spring Valley 60 

spbaerolites in 61,65 

syenitio 62 

titanite in 63,65 

Tooelle 67 

Twin Peaks, Wahsatch Range .. 67 

typical foreign examples ... 60 

hornblende) 83,94 

apatite in 96 

Augusta Mountains 94,95,96 

foreign 94 

glass-inclusions in feldspar of. 96 

opacite in > 95 

viridite in 96 

(orthoclase), classified 6 

Porphyries (granite), stone - cavities in 

quartzes of 63 

Propylite 9, 10, 110 

apatite in 113,115 

angitein 112 

Aurora district Ill 

Berkshire CaHon 120 

between Truckee aud Montezuina 

Ranges 114 

biotitein 114,115 

calcitein 114 

Carson Plain 110 

characterized lit 

classified 6 

connection with silver veins of 

Europe and America Ill 

Crown Point Ravine, Washoe...llO, 111 
diagnostic comparison of andesite 

with 133 

epidotein 112,113,114 

feldspar-microlites in gronndmass 

of 113 

Fish Creek Mountains, Nevada .. 114 
foot-hills of Virginia Range, near 

Steamboat Valley 114 

Gold Hill Peak, Washoe 112 

hornblende-microlites in 114 

microlitic fibration of hornblende 

in 11 

monoclinic feldspar in 11 

Ophir Ravine, Washoe 110, 113 

order of eruption 11 

Prof. Wiedemann's analysis of ... 115 



PSRO. 

Propylite, quartz-propylite later than 121 

(quartz) 110,117 

apatite in 119,120 

carbonate of lime in 117, 119, 

120 

classified 6 

Dr. Walter Eormann's 

analysis of 118 

epidotein 118 

globulitic glass in gronnd- 
mass of 120 

Golconda, Nevada 117 

hornblende-microlites in 

feldsparof 120 

inclusion (fluid) in quartz 

of 118,120 

inclusion (liquid), with 

salt cube, in quartz of. 119 
inclusion of water and 
liquid carbonic acid in 

quartz of 117 

later than propylite 121 

moving bubble in fluid- 
inclusion in quartz of. . 117 

order of eruption 11 

Prof. Wiedemann's analy- 

sisof 119 

sanidin in.... 120 

sphaerolites in 117 

Wagon CaOon, Cortez 

Range 119 

West Gate, Augusta Mts., 

Nevada 118 

Sheep Corral Cafion, Virginia 

Range 114 

Silver Mountain Ill 

Storm Caiion, Fish Creek Mts. .. 114 

Transylvanian 9 

Tuscarora, Cortez Range 115 

Wagon Canon, Cortez Range 115 

Washoe District 110 

Propylites, andesites compared with 132 

of Kapnik 

Nagyba'nya 9 

Propylitic tufa, Tertiary leaves in 110 

Psendochrysolite from Moldanthein, Bohemia 210 
Psendomorph chlorite after garnet in gneiss, 

Farmiogton, Canon.. 25 
after garnet in Lake 

Superior 25 

Psendotachylites of basalt 250 

Pumice classified 

Pumicestone, belonites in 208 

glass-inclusions in feldsparof. 207 

magnetite in 208 



292 



GENEKAL INDEX. 



l-nge. 

Pumicestono, microlites in 208 

Mullen's Gup 207,208 

near Fort Churchill, Nov 207 

specular iron iu 208 

Pumice-tufa, Fish Creek Mrs., Nev 269 

Pumpelly, Raphael, cited 25 

Pyrites in granite-porphyry 67,69 

Qnartz-diorite, classified 6 

inclusions. (See Inclusions.) 
propylite. (SeePropylite.) 

Quartziferons diorite, Havallah Bange 88 

groundmass. (See Grouudmass.) 
Quartzite (Archaean), Twin Peaks, Wahsatch 

Range 27 

black microlites in quartz of 27 

calcite in 35 

(Cambrian ? ), Farmington Canon. 27 

Clover Cafion, actinolite in 23 

inclusions (fluid) in quartz of 23 

of water and fluid car- 
bonic acid and water 

in 33 

mica-slate in 24 

microlites of hornblende in quartz 

of 23 

Mill Creek 35 

ParkEange 33 

west slope of Huinboldt Mount- 
ains 23 

Rath, G. von, cited 9,124,145,259 

Reinhard, analysis of augite-andesite by 228 

Reusch cited 271 

Rhyolite 7,143,163 

aggregation of fibres in .... 204 

Aloha Peak 175, 177 

Antelope Hills, Nevada 196 

Antimony Cafion, Augusta Mount- 
ains 184 

angite-microlites in 176, 178, 196 

Bay less Canon, Monteznma Range. . 180 
Black Cafion, Montezuma Range... 178 

Black Rock Mountains, Nevada 174 

calcite in 169,170 

Camp Cafion 193 

characterized 8 

ChatayaPass 183 

classified 6 

Clover Cafion, Humboldt Range 195 

Cortez Range, Nevada 194, 199 

crystalline-granular structure of 195 

cnmulites in 187,188,204 

cuneatc strings of 204 

Deer Cafion, Mallard Hills, Nevada. 199 



1'apo. 

Rhyolite, Desert Buttes, Utah 198 

eastern foot hills New Pass Mount- 
ains 1H8 

empty cavities iu quartz of 195 

sanidin of 195 

Esterel Mountains, France 172 

feldspar-niicrolites in 176, 196 

ferrite iii . 167, 168, 172, 174, 177, 178, 179, 189, 
190, 191, 192, 193, 194, 1S5, 198, 200, 204 

fibrous groundmass of 167, 179 

Fish Creek Mountains 189, 190 

fluctuation-phenomena of. 204, 205 

fluidal lines in 171,172 

fluid-inclusions in feldspar of.. 167, 168, 170 

quartz of 197 

sauidin of 1C4 

foot-hills, Shoshonee Range 193 

Forellen Butte, Nevada 193 

Foruiau Mountains, Nevada 173 

garnet in 195 

gas-cavities in feldspar of 189 

quartz of 182 

sanidiu of 186 

glass-cavities in quartz of 194 

inclusions in 198 

angiteof 183 

biotiteof 177 

feldspar of 175, 176, 

178, 185, 187, 189 

quartz of.. .166, 168, 170, 

172, 176, 177, 181, 182, 183, 184, 186, 

187, 188, 191, 192, 198, 200, 201 

inclusions in sanidin of 196 

globulitic glass in 180 

GolcondaPass 190 

Good Pass, North Park 201 

Goose Creek Hills 200 

Granite Mountain, Pah-Ute Range.. 182 
Granite Point, Augusta Mountains. . 185 

granulated hornstone in 189 

Grass Canon, Pah-tson Mountains.. 177 
half-glassy inclusions in feldspar of. 195 

Hantz Peak, Nevada 201 

Havallah Range, Nevada 190 

inclusions of 203 

Independence Valley 192 

Indian Springs 177 

Jacob's Promontory, Shoshouee 

Range 188 

Kamma Mountains 175 

Karnak, Montezuma Range 177 

labradorizing sauidin in 183 

lithophyses in 198 

Louis' Valley, Nevada 172 

Lovelock's Knob, Montezuma Range . 178 



GENERAL INDEX. 



293 



Page. 

Kbyolitc, magnetite in 165 

Mallard Hills, Nevada 199 

microfelsite of 203 

microfelsitic base in 173 

microfelsitic groundmass of .. 168, 172, 177, 

197,201 

microlites in 175,198 

microlitic gronndmass in 168 

Mopnng Hills, West Hnraboldt 

Range 180 

Mount Baula, Iceland 76 

moving bubble in fluid.-inclusion in 

quartz of 201 

moving bubble in fluid-inclusion in 

sanidin of 166 

Mullen's Gap, Nevada 172 

Nannie's Peak, Seetoya Range 192 

near Carico Lake 193 

Clarke's, Truckee Cation 168 

Susan Creek, Nevada 191 , 193 

Wadswortb, Nevada 170 

Winnemucca, Nevada 190 

New Pass Mines 188 

New Pass, Nevada 187 

North Fork, Hnmboldt 192 

north of Pahkeah Peak 176 

north of Ravenswood Peak, Shosho- 

nee Range . 189 

north of Shoshoneo Springs, Au- 
gusta Mountains 164 

ochre in quartz of 195 

opacite in .... 165, 167, 168, 172, 177, 191, 204 

opal in 199 

opalizing feldspar in 184 

order of eruption 11 

Pahkeah Peak 175 

Pab-tsou Mountains 175 

Passage Creek, Nevada 198 

Pifion Range 195 

plagioclase-microlites in 194 

Pleasant Valley, Nevada 195 

Pyramid Lake 172 

Rabbit Hole Spring 175 

Railroad Cut, Truckee Canon 166 

Ravenswood Peak, Shoshoneo Range. 188 

Reese River Valley, Nevada 188 

Canon 189,193 

River Range, Nevada 199 

Robert's Peak Group 194 

Rock Creek, Nevada 191,192 

sac-like bubbles in glass of 188 

Shoshonee mesa 190, 191 

Springs, Augusta Mount- 
ains 184 

slaggy inclusions in feldspar of 194 



Page. 
Rhyolite, Spanish Spring Valley, Nevada .. . 165 

spbrerolites in 164, 166, 168, 169, 174, 

176, 177, 178, 181, 182, 183, 185, 186, 187, 188, 
389, 190, 192, 193, 194, 196, 197, 198, 200, 204 

sphcerolitic groundmass of 199 

material in 163 

Storm Canon, Fish Creek Mountains. 190 

Sunset Gap, Nevada 191 

Toyabe Mountains 192 

tridymitein 168,196,198,200,202 

Truckee Road 168 

types of, summarized 203,204 

Wah-we-ah Mountains 194 

Washoe 163 

White Plains, Moutezuma Range. .. 179 
White Rock, Cedar Mountains, Utah. 200 

Winuemncca Lake 173 

(half-glassy) 204 

augite-microlites in... 214 
glass-inclusions in pla- 

gioclase of 214 

globulites in 206, 207 

(hyaline) 143,206 

characterized 8 

globulites in 211 

glass-inclusions in quartz 

of 211,215 

Mount Neva, Nevada 213 

Owyhee Bluffs, Nevada .. 213 

sphajrolites in. 212 

spbierolitic fibres in 215 

Truckee Ferry 206 

(pearlitic), Goose Creek Hills 214 

magnetite in 214 

near Lovelock's Knob, 

Nevada 210 

near White Plains, Ne- 
vada 211 

plagioclase in 214 

(semi-hyaline), glass inclusions in 

quartz of 212 

lythophyses in 212 

near Desert Station, 

Truckee Range.. 200 
Shoshonee Mesa, Ne- 
vada 212 

Rhyolites, Berkshire CaOon.Virginia Range. 169, 170 
ClanAlpiueCauon.AugnstaMonnt- 

ains - - 186 

Desatoya Mountains 186 

Euganean Hills, Northern Italy.. 202 

foreign, containing garnets 195 

glassy 206 

east slope, Goose Creek 
Hills.. 215 



294 



GENEEAL INDEX. 



Page. 

RLyolitcs, Hungarian 169 

riuniiliti's in 187 

McKinney's Pass 181 

Moutezuuia Range 178 

Mount Airy 188 

Moses, Fish Creek Mount- 
ains 189 

near Shoshonee Pass 185 

Sheep Corral Cafion, Virginia 

Range 165 

Snow Storm Ledge 174 

Spring Cafion, Wachoe Mountains 197 

Truckee Canon 165, 166, 167, 168 

Wachoe Mountains 196, 197 

Rhyolitic breccia, Black Rock Mountains. .. 267 

Desatoy a Mountains 269 

fluid-inclusions in glass of. 265 
Forman Mountains, Ne- 
vada 267 

Kamma Mountains 267,268 

Kawsoh Mountains 264 

Mopnng Hills 268,269 

Mount Airy, Nevada 269 

quartz in 269 

Sou's Springs 269 

thomsonite in 270 

clastic rock 264 

opal 265 

pumice-breccia, Pah-tson Mount- 
ains 268 

tufa, (li a tomes in 265 

FortChurchill 2f ; 4 

Indian Creek, Nevada 270 

Lovelock's Knob, Montezuma 

Range, Nevada 268 

Mouteznnia Range 265 

River Range, Nevada 270 

Sacred Pass, Humboldt 

Range 270 

Shoshonee Range, Nevada. . . 270 

tridymitein 268 

Winnemncca Valley 264 

Richthofen, F. von, cited 7, 8,9, 11, 195, 212 

Roofing-slate . 33 

Roofing-slates, foreign 37 

Rosenbnsch cited 221 

views of, concerning palagonite . 275 

Roth, J., cited 7,8,151,212,220,221 

Saline inclusions. (See Inclusions.) 

Salt-cubes. (See Inclusions.) 

Sandstone from Foutainblean 95 

Sanidin in andesito 126,130,131 

aiigite-andegite 224 

basalt 232,233,234,238,241, 

246,248,258 



Sanidin in basaltic rock 

dacite 

quartz-propylito 

labradorizing 

from Frederiksvarn, 

Norway 

in rbyolite 

lencite rock, classified . 

rocks, classified 

Scheerer cited 

Schist, microlitcs in mica of 

(Archrean), Clover Canon. . 17, 18, 19, 

Colorado 

Hnmboldt Range 

(crystalline), muscovito in 

(hornblende), Farmingtou Canon 

Twin Peaks, Wahsatch 

Range 

(mica), Humboldt Range, bucholzite 

in 

Humboldt Range, disthene in 
Humboldt Range, fibrolite in. 
Humboldt Range, microlite 

knots in 

Red Creek, Uinta Mountains, 

cyanite in 

sillimanite in 

stanrolite in 

tonrmaline in 

Schists (crystalline) and related rocks 

Humboldt Range 

Ogden Cafion, Wahsatch 

Range, Utah 

Pahkeah Peak, Pah-tson 

Mountains, Nevada .. 

(mica), Spruce Mountain, Peoquop 

Range, Nevada 

Seladonite 

Semi-hyaline rhyolite. (See Rhyolite.) 

Serpentine, classified 

Sideromelane 

Silica, stanrolite in 

Siliceous cement of conglomerate 

deposit, Monteznma Range 

Sillimanite in mica-schist 

Slate (clay), Medicine Peak 

(gneissic), Eagan caBou, Eagau Range, 
Nevada . . . 



zircon in 

(Jurassic), Sahwave Mountains 

Medicine Bow Range 

(mica), apatite in , 

Bruin Peak, Colorado... 

in qnartzite , 

inivrolites of mica in. . 



256 
138 
120 

187 

183 

ia3 

6 

6 

108 
15 

20, 21, 
22,23 
28 
16 
15 
26 

26 

26 
16 
16 

16 

28 
16 
28 
28 
14 
16 

24 
16 

17 

98 

6 

275 

29 

262 

272 

16 

36 

2?. 
23 
37 
37 
35 
32 
24 
32 



GENERAL INDEX. 



295 



Page. 

Slate (mica), microlites of muscovite in 35 

muscovite in 23 

Spruce Mountain, Peoquop 

Kange 23 

white, Bed Creek, Uinta 

Mountains 28 

zircon in 23,24 

(paragonite), St. Gotthard 28 

Slaty-crystalline rocks 6 

hornblende rock 14 

Sommaruga cited 136 

Sorby cited 9,19,63 

Specular iron in basalt 238 

feldspar of granite 49,51 

granite 43 

pumicestone 208 

trachyte 148,157 

Spbrerolitesinandesite 130 

dacite 138 

felbite-porpbyry 78 

granite-porphyry 61,65 

hyaline-rhyolite 212 

qnartz-propylite 117 

rhyolito ....164, 166, 168, 169, 174, 176, 

177, 178, 181, 182, 183, 185, 186, 187, 

188, 189, 190, 192, 193, 194, 196, 197, 

198, 200, 204 

rhyolitesof Mt.Baula, Iceland. 61 

rhyolitic breccia 268,269 

tufa 271 

Sphaerolitic fibres in hyaline-rhyolite 215 

trachyte 152 

groundmass. (See Groundmass.) 

material in rhyolite 163 

microstructure in gronudmaaa of 

dacite 134 

Stache, G., cited 10,221 

Stanrolite 30 

in mica-schist 2 

of Brittany, Bolivia, and St. Gott- 
hard 29 

silica in 29 

Stelzner cited 72,79 

Stone cavities, in quartzes of granite-por- 
phyries 63 

Syenite 81 

(angite), classified C 

of Laurvig 146 

South Tyrol 9,145 

classified 6 

Clnro Hills, Cortez Range, Nevada.. 81 

fluid-inclusions in quartz of. 81 

of Sontb Norway 54, 55 

( porphy ri tic), Cortez Range, Nevada . 82 
radiated feldspar in 81 



Page. 

Syenites, foreign 81 

old 9 

(zircon), foreign, fibration of feld- 
spar in 45 

Syenitic granite-porphyry 62 

Szabo cited 212 

Tachylyte, classified 6 

of basalt 236 

Tertiary eruptive clastic rock 264 

volcanic rocks 14 

Tbomsonite in rhyolitic breccia 

Titanite in eruptive granite 40 

diorite 87,92 

diorite-gneiss 21 

gneiss 35 

granite 41,43,48,49,50,51 

granite-porphyry 63,65 

mica-gneiss 35 

trachyte 150 

unusually large in gran ite 52 

Titanic iron, and whitish-gray crust of 108 

in basalt 243 

diorite 84,86,89 

Tourmaline iu diorite 87 

mica-schist. 28 

rock 6 

Trachyte 143 

altered by solfataric action 150 

Auaho Island, Pyramid Lake 148, 149 

apatite in 145, 147, 149 

augito in 147,148 

(augite), characterized : 8 

classified 6 

Dr. Anger's analysis of. . . 146 
near Wadsworth, Nevada. 145 

Truckee Ferry 147 

Wah- we-ah Range, Nev . . 152 

axial fibration in 150 

calcite in 150,153 

Cammel Peak, Colorado 161 

characterized 8 

City Creek, Wahsatch Range 156 

classified 6 

Coal Creek, Seetoya Range, Nevada 150 

Colorado Parks 158 

ferrite in.. .144, 147, 149, 150, 152, 154, 158 

fluid-inclusion in feldspar of 157 

hornblende of 156 

garnet in 151 

gas-cavities in hornblende of 156 

glass-inclusion in augite of 152, 157 

feldspar of..... 157, 158 

quartz of 159,160 

globulitic base in 150, 158, 159 



296 



GENEEAL INDEX. 



Page. 

Trachyte, gray 9 

HantzPeak 161 

haiiyne in 151,152 

Huuiboldt Range 153, 154 

hyalite in 14fi 

Kanima Mountains, Nevada 149 

Little Snake Eiver, Colorado 161 

magnetite in 153,156 

microlites in 159 

groundmass of 147,158 

micaof 154 

sail id in of 161 

of augite in groundmass 

of 145 

feldspar in 154,155 

feldspar in ground- 
mass of.' 146, 148 

hornblende in ground- 
mass of 148 

microlitic and granular groundmass 

in 150 

Mount Rose, Washoe 144 

muscovite in 153 

nepheline in 161, 162 

Nevada Station 148 

north of Rabbit Hole Springs 149 

ofDrachenfels 160 

Ischia 151 

the Pferdekopf 151 

olivinein 159,160 

opacite in 144, 149, 150, 152, 154, 158 

order of eruption 11 

Pah-Ute Range 150 

Palisade Caflon, Nevada 154, 155 

Peoquop Range 154 

Provo Canon 157 

rectangular crystallites in ground- 
mass of 147 

(sanidin), Virginia City. 143 

Seetoy a Range, Nevada 150 

serpentinized olivinein . 146 

Sheep Corral Canon, Virginia 

Range 146 

Slater's Fork, Colorado 161 

specular iron in 148,157 

sphserolitic fibres in 152 

Steves' Ridge, Colorado 160 

Susan Creek, Nevada 150 

titanitein 150 

Traverse Mountains, Utah 156 

tridymite in ... .144, 148, 151, 155, 157, 158 

Wah-we-ah Range 151 

Trachytes, Aqui Mountains, Utah 155 

Central France 144 

Chataya Pass, Pah-Ute Range. . . 149 



Page. 

Trachytes, earlier , 144 

(greenstone) 9 

Kawsoh Mountains 148 

Purple Hill, Nevada 14? 

qnartziferous,Elkhead Mountains, 

Colorado 159 

SU'bengebirge, Rhenisli Prussia.. 144 

Silver Creek 157,158 

Truckee Canon 147 

Ferry 147 

Wahsatch Range .. 156, 157, 1!>8 

Washoe 143 

Trachy tic groundmass, microlites in 1G2 

Trap-rocks of Scotland 98 

Trernolite in diorite 88 

granite 56 

Triassic limestone, Buffalo CaSon, fluid 

inclusions in calcite of. . . 38 

Trichites 2 

characterized 12 

in andesite 129 

augite-andesite 226 

basalt 239,252 

dacite 138,140 

obsidian 214 

pearlite 210 

Tridymite in basalt 239 

basaltic rock 256 

rhyolite 168,196,198,200,202 

rhyolitic tufa 268 

trachyte .... 144, 148, 151, 155, 157, 158 
Cerro de San Cristoval, Mexico.. 157 

Tsehermak, Streit, cited 97,104,136 

Tufa 264 

(basalt), fluid-inclusions in quartz of.. . 273 

Boone Creek, Shoshonee Range 271 

cumulites in 271 

(hyalomelane) 273 

(palagonite). (See Palagonite.) 

(pumice), Fish Creek Mountains 269 

(rhyolite). (See Rhyolite.) 

sphserolites in 271 

Ulrich, Dr., cited 221 

Uncry stalline type 

Uralite in European gabbros 108 

Urba cited 221 

Viridite, characterized 13 

in dacite 135 

diorito - 89 

hornblende-porphyry % 

Vogelsang, H., cited. -.2, 11, 19, 72, 77,184, 187,207, 

259,271 

Waltershausen, 8. von, cited 274,275 



GENERAL INDEX. 



297 



Page. 
Water-inclusion. (See Inclusion.) 

Weiss, E., cited , ?2 

Whitney, Prof. J. D., cited 39 

Wicbmaun cited 221 

Wiedeuiaun, Prof., analysis of gabbro by.. . 109 

propylite by. 115 
quartz-propy- 

liteby 119 

Woodward, R. W., determination of zirconi- 
um by 22 

Younger clastic rocks 264 



1'agf. 

Zeolite in breccia 270 

Zircon in diorite 5, 92 

gneiss 22,24 

gneissic slate 23 

granite 49,54,57 

hornblende-gneiss 20 

metamorphic granite 49 

mica-slate 23, 24 

Zirconium, determination of, by R. W. Wood- 
ward 22 

Zircon-syenite. (See Syenite.) 



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