Louis V. (Louis Valentine) Pirsson.
Rocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools online
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Online Library → Louis V. (Louis Valentine) Pirsson → Rocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools → online text (page 34 of 35)
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acquire it by shearing movement. The oolitic structure
indicates a sedimentary rock. In general, structure must
be considered in connection with texture and other prop-
erties.
Texture. Certain textures are of definite assistance in
determining the family to which a rock belongs. Thus
the glassy texture is definite proof that the rock is of
igneous origin; a porphyritic texture shows the same
thing, especially if the phenocrysts are well crystallized,
and of quartz, or feldspar, or both. Metamorphic rocks also
contain at times well crystallized minerals, which are
similar to phenocrysts, as, for instance, garnet and stauro-
lite, but in general they also possess at the same time a
well foliated structure, which helps to distinguish them.
Sedimentary rocks do not exhibit this texture. The mere
contrast in color of a few dark mineral grains among many
lighter ones must not be mistaken for a porphyritic
texture. In general, a hard, firm, highly crystalline tex-
ture, alike in appearances in all directions through the rock,
is indicative of an igneous origin; but there are many
exceptions to this rule, as shown in various marbles and
quartzites. If a rock has a highly crystalline texture,
and at the same time a foliated structure, it is probably
metamorphic.
Hardness. This character, which can readily be tested
in a rough way in the field, is very useful in distinguish-
ing between certain classes of rocks. Thus very fine-
grained compact sandstones (or quartzites), limestones, and
dense igneous rocks often look much alike in specimens.
A simple test of hardness with the knife-point will at once
THE DETERMINATION OF ROCKS 401
distinguish between the limestones (carbonate rocks, soft)
and the others mentioned (silicate, or silica rocks, hard).
If the rock is not very firm, care must be taken not to con-
fuse the mere breaking down or crushing of the rock fabric
with actual scratching of its component minerals. If the
rock itself is used to scratch with, care must also be taken
to test a number of corners, or edges, so that some single
grain, harder than the average, may not produce a false
impression of the average hardness.
Fracture. This is of less importance than the foregoing
characters, but yet in some ways is of value. Most rocks
which are firm and solid enough to have a distinct fracture
exhibit a more or less rough, hackly one. Those which
are fine-grained, or dense and compact, and which contain
a large amount of silica, or are wholly composed of it,
such as felsites, quartzites, flint, etc., possess a more or less
distinct conchoidal fracture, and the surface may be
splintery. Some dense limestones also have a splintery
fracture, and may even approach the conchoidal. Natural
glasses, such as compact obsidian, have a beautiful con-
choidal fracture.
Specific Gravity. This property is of much greater
value in the determination of minerals than of rocks. It
cannot of course be used in the field, as it requires definite
apparatus to determine it, as described in Chapter III,
under minerals; nevertheless, even in the field, a rough
distinction may be made between light and heavy rocks,
by weighing them in the hand. Rocks that are dark-
colored and very heavy, in general, are composed largely,
or chiefly, of iron-bearing minerals, and are apt to be of
igneous, or of metamorphic origin.
Treatment with Acid. This is particularly useful in
distinguishing the carbonate from the silicate rocks.
The method of treatment has been fully described in
Chapter V. and need not be repeated here. If necessary
almost any acid may be used, such as vinegar (acetic
acid), or lemon juice. For field use a few crystals of
402 ROCKS AND ROCK MINERALS
citric acid powdered up may be conveniently carried, and
dissolved, when needed, in a little water; the test for
effervescence can thus be readily made. The test for
gelatinization, as also described in Chapter V, is also very
useful in determining the nephelite syenites from other
syenites and from granites, and also its effusive represen-
tative, the phonolite variety from the other felsites. It
should be remembered that olivine, which, however, chiefly
occurs in the dark ferromagnesian rocks, gabbros, perido-
tites and basalts, also gives this gelatinization test.
Determination in the Field. The best method of deter-
mining the family to which a rock belongs, that is, whether
it is sedimentary, igneous, or metamorphic, is to study its
characters in the place in which it occurs, and its relation
to other rock masses. For these features, and the larger
ones of its structure, may be very apparent in the field,
while a simple hand-specimen may entirely fail to show
them. The structure of a granite-gneiss, for instance,
may be very clear on the surface of a field exposure, and
be quite inappreciable in a small specimen. It is not
necessary to give here the characters and relations by
which the class may be determined; this is geological
rather than petrographical, and has been sufficiently com-
mented upon in Chapters VI, VIII and X. If the family
has been determined in the field, and the rock is coarse-
textured, so that the mineral grains can be seen, and if
necessary handled, Table I (p. 124) of Chapter V may
be used for their identification, and by then referring to
the classification of the appropriate family, its place in
general can be readily determined.
Table for Rock Determination. Appended to this chap-
ter is a table which may be used for the determination of
the more important kinds of rocks. It is based essentially
on the one given by Geikie, in his Textbook of Geology,
which has, however, been considerably modified, and
extended to meet the needs of this work. As the tests
which it demands are very simple, consisting for the most
THE DETERMINATION OF ROCKS 403
part of those relating to hardness and effervescence with
acid, it may be readily used, even in the field. It must be
remembered, however, that a table of this nature can be
only quite general in character, and applicable to rocks of
well-defined types. Rock kinds grade into one another in
so many ways, as has been described in a number of places
in this work, that not only the student, but even the
experienced geologist, will sometimes be puzzled as to
the proper designation a particular type should receive.
But if this fact is borne in mind, it is believed the table
will prove useful in aiding one to classify the common
rocks.
404 ROCKS AND ROCK MINERALS
TABLE FOR DETERMINING THE COMMON ROCKS.
The newly fractured surface of the unaltered rock shows
one of the following cases:
a. It is wholly or partly glassy. See A beyond.
6. Not glassy; of a dull, even appearance or stony; with-
out particular texture, or so compact that the indi-
vidual grains cannot be seen or recognized. See B.
c. Distinctly grained and crystalline; the grains can be
seen and determined. See C.
d. Has a distinctly foliated or gneissoid structure.
SeeD.
e. Has a clearly fragmental composition. See E.
A. Wholly or partly glassy.
1. Wholly of glass ; solid ; strong vitreous luster. Obsidian, p. 262.
2. Wholly of glass; solid; resinous or dull pitchy luster. Pitch~
stone, p. 265 (Obsidian and pitchstone may contain spherulites.)
3. Wholly of glass, but cellular or froth-like. Pumice, p. 266.
4. Of glass, but enamel-like, and composed of small, concentric
spheroids. Perlite, p. 265.
5. Partly of glass and partly of distinct, embedded crystals.
Vitrophyre, p. 267.
(The above forms are generally associated with, or pass into,
felsite lavas.)
6. Glass associated with, or passing into, basalt; rare. Tachylite,
p. 268.
B. Compact close-grained, and dull or stony; not glassy.
a. Very soft; can be scratched with the finger-nail.
1. Has a strong earthy or clay odor when breathed upon; rubbed
strongly between the fingers has ultimately a smooth, greasy feeling;
does not effervesce with acids. Various colors. Clay, p. 327.
2. Friable; crumbling; soils the fingers; little or no clay odor;
lively effervescence with acids; color white or light yellowish, etc.
Chalk, p. 310 or perhaps marl, p. 313. (Marl may give a good clay
odor.)
THE DETERMINATION OF ROCKS 405
TABLE FOR DETERMINING THE COMMON ROCKS
Continued.
3. General characters as in 2, but does not effervesce with acids.
Diatomaceous earth, p. 298.
4. Harder, more compact than 1, 2, and 3. No clay odor; does
not effervesce; composed of a mineral with a good cleavage; some-
tunes fibrous; occurs in beds or veins. Gypsum, p. 293.
5. White to green, or gray; does not effervesce; no clay odor;
mass has a soft, greasy feel ; is often foliated or shows a micaceous
cleavage; folia inelastic; marks cloth. Talc-rock, p. 374.
6. Not scratched by the nail, but easily scratched or cut
with the knife.
1. Composed of excessively fine, almost imperceptible particles;
dull, even appearance; gives clay odor when breathed on; no effer-
vescence or but feeble ; has a laminated or stratified structure and
usually breaks easily into chippy flakes ; generally gray, but often red,
yellow, brown, bluish, or black. Shale, p. 327.
2. No clay odor, or but feeble ; brisk effervescence with acid; white
streak; commonly gray; sometimes white to brown or black. Lime-
stone, p. 304.
3. As in 2, but feeble effervescence in acid, which becomes brisk
when the acid is heated; generally white, yellowish or pale brown.
Dolomite, p. 307.
4. Pale to dark green or black, sometimes reddish; soapy or
greasy feel; translucent on thin edges; waxy or oily appearing;
subconchoidal or splintery fracture; no effervescence. Serpentine,
p. 392.
c. Not scratched or cut with the knife; scratches glass;
does not effervesce with acid.
1. Various colors, white to red or purple, brown to dark gray;
often gives a clay odor; frequently shows banded flow structure.
Felsite, p. 248.
2. Very hafd; any corner or angle scratches feldspar; no clay
odor; scratches steel readily; light colors to brown or black; pro-
nounced conchoidal fracture; glimmering horny appearance. A
siliceous rock; either flint, p. 297, or perhaps the rhyolite variety of
felsite, p. 249.
406 ROCKS AND ROCK MINERALS
TABLE FOR DETERMINING THE COMMON ROCKS
Continued.
3. Not so hard as 1 and 2. Does not scratch feldspar; color
black, very dark gray or green; is heavy; sometimes shows a cellular
or slaggy structure; sometimes contains amygdules. Basalt, p. 254.
C. Distinctly grained and crystalline; grains wholly or partly
determinate.
a. Is easily scratched with the knife.
1. Effervesces briskly with acid. Limestone p. 304, or more prob-
ably marble, p. 384.
2. Effervesces briskly only when the powdered rock is treated
with hot acid. Dolomite marble, p. 390.
3. Does not effervesce; probably granular crystalline. Gypsum,
p. 293, or anhydrite, p. 295.
4. Soluble, with distinct saline taste. Rock-salt, p. 295.
b. Hard; cannot be scratched with the knife, or scratches
with difficulty. Silicate rocks, two cases arise, x and y.
x. It is composed of grains of approximately equal size;
i.e., it is even-granular, like common granite. See X.
y. It is composed of larger, distinct crystals embedded
in a finer-grained groundmass; i.e., it is a porphyry.
SeeT.
X. An even-granular, massive, silicate rock. See p. 155.
1. Mainly or wholly composed of quartz and feldspar. Granite,
p. 205. See also aplite, p. 214.
2. Mainly or wholly composed of feldspar without quartz.
Syenite, p. 218. See also nephelite syenite, p. 221 and anorthosite,
p. 224.
3. Composed of feldspar and a dark ferromagnesian mineral ; the
latter equals or exceeds the feldspar; a, the dark mineral is mostly or
wholly hornblende. Diorite, p. 226; b, the dark mineral is mostly
or wholly pyroxene. Gabbro, p. 229 ; c, the dark mineral is indeter-
minable. Dolerite, p. 235.
4. Composed entirely, or almost entirely, of ferromagnesian min-
erals; generally heavy and dark green to black (sometimes yellowish,
dunite). Peridotite, pyroxenite, etc., see p. 238.
THE DETERMINATION OF ROCKS 407
TABLE FOR DETERMINING THE COMMON ROCKS
Continued.
5. Composed of grains of quartz ; scratches glass or feldspar readily
Sandstone, p. 323, or quartzite p. 366.
6. Much less commonly than the above, massive silicate rocks
produced by metamorphism may occur in this division. There are a
number of different varieties, depending on the particular mineral, or
minerals. Epidote-rock, garnet-rock, etc., would be examples. See
contact-metamorphism, pp. 180, 186, and carbonate-silicate rocks,
p. 384.
Y. A porphyry (see p. 156), composed of phenocrysts and
a groundmass.
1. Phenocrysts of quartz and feldspar and, perhaps, of a ferro-
magnesian mineral in a groundmass of the same. Granite-porphyry,
p. 243.
2. Phenocrysts of feldspar (and often of a ferromagnesian min-
eral) in groundmass of predominant feldspar. Syenite-porphyry,
p. 243.
3. Phenocrysts of ferromagnesian minerals, or feldspar, or both, in
a groundmass of feldspar and ferromagnesian minerals; feldspar
phenocrysts frequently striated. Diorite-porphyry, p. 244.
4. Phenocrysts of quartz, or feldspar, or both, and sometimes of
ferromagnesian minerals, in a predominant groundmass of light color
and dense feldspathic aspect. Felsite-porphyry, p. 251.
5. Phenocrysts of feldspar, or of a ferromagnesian mmeral, or
both, in a dense, dark to black, and heavy groundmass. BasaU-
porphyry, p. 254.
D. It has a distinctly foliated, gneissoid, or slaty structure.
1. It contains feldspar, and generally quartz, with mica (some-
times hornblende). Gneiss, p. 351.
2. It consists mainly or largely of mica; often considerable quartz
is present, but feldspar is absent, or indeterminable. Frequently
contains crystals of dark red to black garnet, more rarely staurolite,
cyanite, etc. Mica-schist, p. 361.
3. Medium green, dark green or black; consists mostly of a felted
or matted mass of small, to very fine or microscopic, bladed, or needle-
like crystals arranged mostly, in one general direction, which promotes
the schistose cleavage. Other minerals, such as garnet, may be
pKsen.t.Hornblende-8chist or amphibolite (p. 379).
408 ROCKS AND ROCK MINERALS
TABLE FOR DETERMINING THE COMMON ROCKS
Continued.
4. Very compact, or dense and fissile, splitting easily into thin,
more or less tough, ringing slabs ; usually dark gray, or green to black,
but sometimes showing other colors. Slate, p. 369. (Sometimes
contains large crystals of staurolite, andalusite, etc.)
5. Very fissile, but soft to the feel ; laminae not tough, but often
brittle or crumbling; pronounced silky luster on the cleavage face.
Phyllite, p. 372.
6. Soft, greasy feel; marks cloth; easily scratched with the finger-
nail ; usually whitish to light gray, or green. Talc-schist, p. 374.
7. Smooth feel; soft; green to dark green; glimmering luster.
Chlorite-schist, p. 376.
E. Has a clearly fragmental composition; is seen to be com-
posed of fragments or pebbles of other rocks, or of smaller
angular or -rounded mineral fragments; if the latter,
frequently shows evidences of stratification.
1. The pebbles range from the size of a pea up and are rounded;
quartz ones are common; they are embedded in more or less of a
cement. Conglomerate, p. 320.
2. The pebbles are angular in shape. Breccia, p. 321.
3. Composed of various-sized angular fragments of volcanic rocks,
such as felsite and felsite porphyry, of bits of pumice, or cellular lava,
or of rounded, vesicular, volcanic bombs, etc., mixed with fine com-
pacted material (volcanic ash). Volcanic tuff and breccia, p. 272.
4. Composed of more or less angular, but sometimes rounded
grains, in size from that of a pea down; the grains are mostly, or
wholly, composed of quartz, and scratch feldspar. Generally some
cement is present, which, if the rock is light colored, is apt to
effervesce with acid (lime carbonate); if red or brown does not.
Sandstone, p. 323.
5. As ,in 4 but more or less feldspar is also [present among the
quartz grains. Arkose, p. 326.
INDEX
Actinolite, 61, 63.
Adinole, 188.
Adobe, 331.
Aegirite, 55, 58.
Aeolian rocks, 275.
Albite, 34.
Alkalic feldspar, 34, 35.
Alumina, test for, 117.
Amphibole, 60.
" determination of, 66.
Amphibolite, 379, 391.
Amygdaloidal structure, 159
Amygdaloid, basalt, 256.
Analcite, 103.
Andalusite, 76.
Andesite, 250.
Anhydrite, mineral, 113.
Anhydrite, rock, 295.
Anorthosite, 224.
Anorthite, 34.
Anthracite, 319.
Apatite, 95.
Aplite, 214.
Arfvedsonite, 61, 63.
Argillite, 369.
Arkose, 326.
Aschistic rocks, 169.
Ashes, volcanic, 141.
Associations of minerals, 29.
Augite, 55, 57.
Auqitophyre, 257.
Average rock, composition, 18.
Basalt, basalt-porphyry, 254.
" amygdaloid, 256.
" quartz, 256.
Bathylith, 139.
Bauxite, 97.
Biotite, 50, 51.
Bituminous coal, 318.
Black-band ore, 302.
Bog iron ore, 301.
Bombs, volcanic, 141.
Border zones in igneous rocks,
165.
Border zones, origin of, 170.
Boss, 138.
Bostonite, 254.
Breccia, friction, 322.
" sedimentary, 321.
" volcanic, 140, 272.
" " origin, 269.
Brown coal, 317.
Brownstone, 326.
Breunerite, 110.
Buhrstone, 368.
Calcite, 105.
Calcium, test for, 119.
Calcareous tufa, 312.
Camptonite, 257.
Cancrinite, 48.
Carbonates, test for, 115.
Cementation, zone of, 338.
Chalcedony, 86.
Chalk, 310.
Chemical elements, 18.
Chert, 297.
Chlorine, test for, 121.
Chlorite, 98.
Chlorite-schist, 376.
Chloritoid, 54.
Chondrodite, 82.
Chrysotile, 101.
Cipolin, 390.
Classification, general, 6.
" fragmental volcanic
rocks, 271.
Classification, glassy rocks, 261.
" igneous rocks, 191,
194, 203.
Classification, igneous rocks, table,
195.
Classification, metamorphic rocks,
348.
Classification, stratified rocks, 290.
409
410
INDEX
Clay, 96, 327, 278, 280.
Clay ironstone, 301.
Cleavage of minerals, 26.
effect of, 28.
Coal, 315.
" hard, 319.
" soft, 318.
Color of minerals, 23.
" " rocks, 399.
" " sedimentary rocks, 286.
Columnar structure, 162.
Comagmatic regions, 174.
Complementary dikes, 167.
Conglomerate, sedimentary, 320.
" volcanic, 322.
Consanguinity of rocks, 173.
Contact metamorphism, 180.
" " effect on
limestone, 187.
Contact metamorphism, effect on
sandstone, 186.
Contact metamorphism, effect on
shale, slate, 188.
Contact metamorphism, endomor-
phic, 181.
Contact metamorphism, exomor-
phic, 183.
Contact metamorphism, modes of
occurrence, 185.
Contact metamorphism, pneumato-
lytic, 189.
Contact metamorphism, ore bodies,
190.
Coquina, 311.
Cortlandtite, 238.
Corundum, 86.
Corundum-syenite, 226.
Crystals, denned, 21.
" form in rocks, 22.
" twinning of, 36.
Cyanite, 78.
Docile, 250.
Decay of rocks, 276.
Determination of minerals, 114.
" " tables,
122.
Determination of rocks, 396.
" " table,
404.
Diabase, 235.
Diaschistic rocks, 169.
Diatomaceous earth, 298.
Differentiation, 164, 169.
Dikes, 134.
" complementary, 167.
Diopside, 55, 57.
Diorite, 226.
" rock relations, 229.
Diorite-porphyry , 244.
Dolerite, 235.
" use of word, 198.
" alteration of, 237.
Dolerite-porphyry , 244.
Dolomite, mineral, 108.
Dolomite, rock, 307.
" " origin, 308.
Dolomite-marble, 390.
Dunite, 238, 240.
Earth's crust, composition, 17.
" interior, state of, 14.
Eclogite, 382.
Elements, geologically important
18.
Emery, 397.
Epidosite, 389.
Epidote, 73.
Eutaxitic Structure, 164.
Exotic mineral colors, 24.
Extrusive igneous rocks, 139.
Feldspars, 34.
alteration of, 44.
" cleavage of, 40.
" color of, 41.
crystal form of, 35.
determination of, 46.
twinning of, 36.
Feldspathoid minerals, 47.
Felsite, felsite-porphyry, 248.
" sheared, 373.
" varieties of, 249.
Ferromagnesian minerals, 146.
Field classification, 6.
Flint, 86, 296, 297.
Fluorine, test for, 121, 81.
Fracture of minerals, 29.
" conchoidal, 29.
" of rocks, 401.
Fragmental volcanics, 269.
Freestone, 325.
INDEX
411
Gabbro, 229.
" alteration of, 232.
" iron ores in, 234.
Garnets, 70.
Garnet-rock, 389.
Gelatinization test, 115.
Geyserite, 296, 298.
Glassy rocks, 260.
" alteration of, 269.
" " classification, 261.
Glaucophane, 64.
Glaucophane-schist, 383.
Gneiss, 351.
" field study of, 358.
" inclusions in, 356.
" texture of, 353.
" varieties of, 355.
Granite, 205.
complementary dikes of, 214.
contact of, 215.
" graphic, 212.
" orbicular, 211.
" pegmatites in, 212.
" porphyritic, 207.
" weathering of, 216.
Granite-porphyry, 243.
Granulite, 360.
Gravel, 278, 279.
Graywacke, 326.
Greenstone, 229, 377.
Greenstone-schist, 377, 383.
Greensand-marl, 325.
Grit, 325.
Ground mass denned, 156.
Gruss, 216.
Gypsum, mineral, 111.
Gypsum, rock, 293.
Halite, 113, 295.
Hammer, geological, 11.
" trimming, 12.
Hardness of minerals, 30.
" " rocks, 400.
Hauynite, 48.
Hematite, 91, 302.
Heulandite, 104.
Hornblendes, 60.
Hornblende-schist, 379.
Hornblendite, 238.
Hornfels, 188.
Hornstone, 188, 297.
Hydromica-schist, 373.
Hypersthene, 55, 57.
Ice, 20.
Igneous rocks, 132, 205.
" consanguinity of,
173.
Igneous rocks, classification of,
191, 194, 203.
Igneous rocks, crystallization in,
146.
Igneous rocks, dense types, 247.
" " general characters,
132, 141.
Igneous rocks, inclusions in, 163.
jointing of, 161.
minerals of, 145.
occurrence of, 134.
origin of, 164.
petrology of, 132.
post intrusive work
of, 174.
Igneous rocks, structure of, 158.
" " textures of, 150,
154.
Igneous rocks, variation of minerals
in, 142.
Ilmenite, 90.
Inclusions in granite, 213.
" gneiss, 356.
" igneous rocks, 163.
Injection of schists, 345.
Intrusive sheets, 135.
Infusorial earth, 298.
Iron ores, 88, 299, 395.
Iron oxide rocks, 395.
Iron, test for, 118.
Itabirite, 395.
Jadeite, 389.
Jade, 389.
Jasper, 86, 297.
Jaspilite, 297, 396.
Jet, 319.
Jointing, 161.
" in granite, 210.
Kammererite, 99, 393.
Kaolin, 96.
" from feldspar, 44.
Kimberlite, 242.
412
INDEX
Labradorite, 35 (rock, 224).
Laccoliths, 136.
" zoned, 166.
Lamprophyre, defined, 168.
Lamprophyres, 257.
Lapilli, volcanic, 141.
Laterite, 332.
Lava flows, 139.
Lepidomelane, 52.
Lepidolite, 52.
Leucocratic rocks, 167.
Leucophyre, 251.
Leucite, 49.
" rocks, 259.
Lignite, 317.
Lime-carbonate-silicate rocks, 387.
Limestone, 303.
" oolitic, 309.
Lime, test for, 119.
Limonite, 93, 301.
Listw'dnite, 375, 391.
Lithographic stone, 306.
Lithology defined, 2.
Lithophysae, 264.
Loam, 332.
Loess, 330.
Lydianite, 297.
Magmas, 134.
" composition of, 141.
" variations in, 142.
Magnesia-silicate rocks, 390.
Magnesite, 110.
Magnesium, test for, 119.
Magnetite, mineral, 89.
Magnetite-rock, 396.
Marble, 384.
Marble, onyx, 313.
Marl, 313.
" greensand, 325.
Megascopic, defined, 7.
Melanocratic rocks, 168.
Melaphyre, 255.
Metadiorite, 229.
Metamorphic rocks, 333.
" age of, 346.
" " classification
of, 348.
Metamorphic rocks, composition
of, 344.
Metamorphic rocks, injection of,
345.
Metamorphic rocks, minerals of,
339.
Metamorphic rocks, occurrence of,
346.
Metamorphic rocks, older struc-
tures in, 343.
Metamorphic rocks, origin of, 333.
Metamorphic rocks, textures of,
340.
Metamorphism, 333.
" agents of, 335.
" constructive, 338.
" effect of depth, 337.
" " " heat, 336.
" " " liquids, 337.
Miarolitic structure, 159.
Micas, 50.
Mica-schist, 361.
Mica-trap, 257, 215.
Microcline, 36.
Microscopical petrography, 7.
Minerals, associations of, 29.
" cleavage of, 26.
" color of, 23.
" exotic color of, 24.
" defined, 21.
" determination of, 114.
" fracture of, 29.
" hardness of, 30.
" specific gravity of, 31.
" streak of, 25.
Mineralizers, 149.
Minette, 237, 257.
Muscovite, 50, 51.
Natrolite, 103.
Necks, volcanic, 138.
Nephelite, 47.
Nephelite-syenite, 221.
Nephrite, 390.
Norite, 229.
Noselite, 48.
Novaculite, 297.
Obsidian, 262.
Occurrence of igneous rocks, 134.
Ocher, 328.
Olivine. 67.
" nodules, 258.
" rock, 238, 240, 390.
Onyx marble, 313.
LNDEX
413
Oolite, iron, 303.
" limestone, 309.
" siliceous, 368.
Opal, 86.
Ophicalcite, 391.
Order of crystallization, 146.
Ore bodies, 190, 391.
" formation of, 170.
Origin of igneous rocks, 164.
" " metamorphic rocks, 333.
Orthoclase, 34.
Oxides, important, 20.
Paragonite, 51.
Peat, 316.
Pebbles, 279.
Pegmatite dikes, 175.
" " origin, 178.
Peridotite, 238.
ores in, 241.
relation to gabbro, 240.
Perlite, 265.
Petrographic provinces, 173.
Petrography defined, 2.
microscopical, 7.
indicates a sedimentary rock. In general, structure must
be considered in connection with texture and other prop-
erties.
Texture. Certain textures are of definite assistance in
determining the family to which a rock belongs. Thus
the glassy texture is definite proof that the rock is of
igneous origin; a porphyritic texture shows the same
thing, especially if the phenocrysts are well crystallized,
and of quartz, or feldspar, or both. Metamorphic rocks also
contain at times well crystallized minerals, which are
similar to phenocrysts, as, for instance, garnet and stauro-
lite, but in general they also possess at the same time a
well foliated structure, which helps to distinguish them.
Sedimentary rocks do not exhibit this texture. The mere
contrast in color of a few dark mineral grains among many
lighter ones must not be mistaken for a porphyritic
texture. In general, a hard, firm, highly crystalline tex-
ture, alike in appearances in all directions through the rock,
is indicative of an igneous origin; but there are many
exceptions to this rule, as shown in various marbles and
quartzites. If a rock has a highly crystalline texture,
and at the same time a foliated structure, it is probably
metamorphic.
Hardness. This character, which can readily be tested
in a rough way in the field, is very useful in distinguish-
ing between certain classes of rocks. Thus very fine-
grained compact sandstones (or quartzites), limestones, and
dense igneous rocks often look much alike in specimens.
A simple test of hardness with the knife-point will at once
THE DETERMINATION OF ROCKS 401
distinguish between the limestones (carbonate rocks, soft)
and the others mentioned (silicate, or silica rocks, hard).
If the rock is not very firm, care must be taken not to con-
fuse the mere breaking down or crushing of the rock fabric
with actual scratching of its component minerals. If the
rock itself is used to scratch with, care must also be taken
to test a number of corners, or edges, so that some single
grain, harder than the average, may not produce a false
impression of the average hardness.
Fracture. This is of less importance than the foregoing
characters, but yet in some ways is of value. Most rocks
which are firm and solid enough to have a distinct fracture
exhibit a more or less rough, hackly one. Those which
are fine-grained, or dense and compact, and which contain
a large amount of silica, or are wholly composed of it,
such as felsites, quartzites, flint, etc., possess a more or less
distinct conchoidal fracture, and the surface may be
splintery. Some dense limestones also have a splintery
fracture, and may even approach the conchoidal. Natural
glasses, such as compact obsidian, have a beautiful con-
choidal fracture.
Specific Gravity. This property is of much greater
value in the determination of minerals than of rocks. It
cannot of course be used in the field, as it requires definite
apparatus to determine it, as described in Chapter III,
under minerals; nevertheless, even in the field, a rough
distinction may be made between light and heavy rocks,
by weighing them in the hand. Rocks that are dark-
colored and very heavy, in general, are composed largely,
or chiefly, of iron-bearing minerals, and are apt to be of
igneous, or of metamorphic origin.
Treatment with Acid. This is particularly useful in
distinguishing the carbonate from the silicate rocks.
The method of treatment has been fully described in
Chapter V. and need not be repeated here. If necessary
almost any acid may be used, such as vinegar (acetic
acid), or lemon juice. For field use a few crystals of
402 ROCKS AND ROCK MINERALS
citric acid powdered up may be conveniently carried, and
dissolved, when needed, in a little water; the test for
effervescence can thus be readily made. The test for
gelatinization, as also described in Chapter V, is also very
useful in determining the nephelite syenites from other
syenites and from granites, and also its effusive represen-
tative, the phonolite variety from the other felsites. It
should be remembered that olivine, which, however, chiefly
occurs in the dark ferromagnesian rocks, gabbros, perido-
tites and basalts, also gives this gelatinization test.
Determination in the Field. The best method of deter-
mining the family to which a rock belongs, that is, whether
it is sedimentary, igneous, or metamorphic, is to study its
characters in the place in which it occurs, and its relation
to other rock masses. For these features, and the larger
ones of its structure, may be very apparent in the field,
while a simple hand-specimen may entirely fail to show
them. The structure of a granite-gneiss, for instance,
may be very clear on the surface of a field exposure, and
be quite inappreciable in a small specimen. It is not
necessary to give here the characters and relations by
which the class may be determined; this is geological
rather than petrographical, and has been sufficiently com-
mented upon in Chapters VI, VIII and X. If the family
has been determined in the field, and the rock is coarse-
textured, so that the mineral grains can be seen, and if
necessary handled, Table I (p. 124) of Chapter V may
be used for their identification, and by then referring to
the classification of the appropriate family, its place in
general can be readily determined.
Table for Rock Determination. Appended to this chap-
ter is a table which may be used for the determination of
the more important kinds of rocks. It is based essentially
on the one given by Geikie, in his Textbook of Geology,
which has, however, been considerably modified, and
extended to meet the needs of this work. As the tests
which it demands are very simple, consisting for the most
THE DETERMINATION OF ROCKS 403
part of those relating to hardness and effervescence with
acid, it may be readily used, even in the field. It must be
remembered, however, that a table of this nature can be
only quite general in character, and applicable to rocks of
well-defined types. Rock kinds grade into one another in
so many ways, as has been described in a number of places
in this work, that not only the student, but even the
experienced geologist, will sometimes be puzzled as to
the proper designation a particular type should receive.
But if this fact is borne in mind, it is believed the table
will prove useful in aiding one to classify the common
rocks.
404 ROCKS AND ROCK MINERALS
TABLE FOR DETERMINING THE COMMON ROCKS.
The newly fractured surface of the unaltered rock shows
one of the following cases:
a. It is wholly or partly glassy. See A beyond.
6. Not glassy; of a dull, even appearance or stony; with-
out particular texture, or so compact that the indi-
vidual grains cannot be seen or recognized. See B.
c. Distinctly grained and crystalline; the grains can be
seen and determined. See C.
d. Has a distinctly foliated or gneissoid structure.
SeeD.
e. Has a clearly fragmental composition. See E.
A. Wholly or partly glassy.
1. Wholly of glass ; solid ; strong vitreous luster. Obsidian, p. 262.
2. Wholly of glass; solid; resinous or dull pitchy luster. Pitch~
stone, p. 265 (Obsidian and pitchstone may contain spherulites.)
3. Wholly of glass, but cellular or froth-like. Pumice, p. 266.
4. Of glass, but enamel-like, and composed of small, concentric
spheroids. Perlite, p. 265.
5. Partly of glass and partly of distinct, embedded crystals.
Vitrophyre, p. 267.
(The above forms are generally associated with, or pass into,
felsite lavas.)
6. Glass associated with, or passing into, basalt; rare. Tachylite,
p. 268.
B. Compact close-grained, and dull or stony; not glassy.
a. Very soft; can be scratched with the finger-nail.
1. Has a strong earthy or clay odor when breathed upon; rubbed
strongly between the fingers has ultimately a smooth, greasy feeling;
does not effervesce with acids. Various colors. Clay, p. 327.
2. Friable; crumbling; soils the fingers; little or no clay odor;
lively effervescence with acids; color white or light yellowish, etc.
Chalk, p. 310 or perhaps marl, p. 313. (Marl may give a good clay
odor.)
THE DETERMINATION OF ROCKS 405
TABLE FOR DETERMINING THE COMMON ROCKS
Continued.
3. General characters as in 2, but does not effervesce with acids.
Diatomaceous earth, p. 298.
4. Harder, more compact than 1, 2, and 3. No clay odor; does
not effervesce; composed of a mineral with a good cleavage; some-
tunes fibrous; occurs in beds or veins. Gypsum, p. 293.
5. White to green, or gray; does not effervesce; no clay odor;
mass has a soft, greasy feel ; is often foliated or shows a micaceous
cleavage; folia inelastic; marks cloth. Talc-rock, p. 374.
6. Not scratched by the nail, but easily scratched or cut
with the knife.
1. Composed of excessively fine, almost imperceptible particles;
dull, even appearance; gives clay odor when breathed on; no effer-
vescence or but feeble ; has a laminated or stratified structure and
usually breaks easily into chippy flakes ; generally gray, but often red,
yellow, brown, bluish, or black. Shale, p. 327.
2. No clay odor, or but feeble ; brisk effervescence with acid; white
streak; commonly gray; sometimes white to brown or black. Lime-
stone, p. 304.
3. As in 2, but feeble effervescence in acid, which becomes brisk
when the acid is heated; generally white, yellowish or pale brown.
Dolomite, p. 307.
4. Pale to dark green or black, sometimes reddish; soapy or
greasy feel; translucent on thin edges; waxy or oily appearing;
subconchoidal or splintery fracture; no effervescence. Serpentine,
p. 392.
c. Not scratched or cut with the knife; scratches glass;
does not effervesce with acid.
1. Various colors, white to red or purple, brown to dark gray;
often gives a clay odor; frequently shows banded flow structure.
Felsite, p. 248.
2. Very hafd; any corner or angle scratches feldspar; no clay
odor; scratches steel readily; light colors to brown or black; pro-
nounced conchoidal fracture; glimmering horny appearance. A
siliceous rock; either flint, p. 297, or perhaps the rhyolite variety of
felsite, p. 249.
406 ROCKS AND ROCK MINERALS
TABLE FOR DETERMINING THE COMMON ROCKS
Continued.
3. Not so hard as 1 and 2. Does not scratch feldspar; color
black, very dark gray or green; is heavy; sometimes shows a cellular
or slaggy structure; sometimes contains amygdules. Basalt, p. 254.
C. Distinctly grained and crystalline; grains wholly or partly
determinate.
a. Is easily scratched with the knife.
1. Effervesces briskly with acid. Limestone p. 304, or more prob-
ably marble, p. 384.
2. Effervesces briskly only when the powdered rock is treated
with hot acid. Dolomite marble, p. 390.
3. Does not effervesce; probably granular crystalline. Gypsum,
p. 293, or anhydrite, p. 295.
4. Soluble, with distinct saline taste. Rock-salt, p. 295.
b. Hard; cannot be scratched with the knife, or scratches
with difficulty. Silicate rocks, two cases arise, x and y.
x. It is composed of grains of approximately equal size;
i.e., it is even-granular, like common granite. See X.
y. It is composed of larger, distinct crystals embedded
in a finer-grained groundmass; i.e., it is a porphyry.
SeeT.
X. An even-granular, massive, silicate rock. See p. 155.
1. Mainly or wholly composed of quartz and feldspar. Granite,
p. 205. See also aplite, p. 214.
2. Mainly or wholly composed of feldspar without quartz.
Syenite, p. 218. See also nephelite syenite, p. 221 and anorthosite,
p. 224.
3. Composed of feldspar and a dark ferromagnesian mineral ; the
latter equals or exceeds the feldspar; a, the dark mineral is mostly or
wholly hornblende. Diorite, p. 226; b, the dark mineral is mostly
or wholly pyroxene. Gabbro, p. 229 ; c, the dark mineral is indeter-
minable. Dolerite, p. 235.
4. Composed entirely, or almost entirely, of ferromagnesian min-
erals; generally heavy and dark green to black (sometimes yellowish,
dunite). Peridotite, pyroxenite, etc., see p. 238.
THE DETERMINATION OF ROCKS 407
TABLE FOR DETERMINING THE COMMON ROCKS
Continued.
5. Composed of grains of quartz ; scratches glass or feldspar readily
Sandstone, p. 323, or quartzite p. 366.
6. Much less commonly than the above, massive silicate rocks
produced by metamorphism may occur in this division. There are a
number of different varieties, depending on the particular mineral, or
minerals. Epidote-rock, garnet-rock, etc., would be examples. See
contact-metamorphism, pp. 180, 186, and carbonate-silicate rocks,
p. 384.
Y. A porphyry (see p. 156), composed of phenocrysts and
a groundmass.
1. Phenocrysts of quartz and feldspar and, perhaps, of a ferro-
magnesian mineral in a groundmass of the same. Granite-porphyry,
p. 243.
2. Phenocrysts of feldspar (and often of a ferromagnesian min-
eral) in groundmass of predominant feldspar. Syenite-porphyry,
p. 243.
3. Phenocrysts of ferromagnesian minerals, or feldspar, or both, in
a groundmass of feldspar and ferromagnesian minerals; feldspar
phenocrysts frequently striated. Diorite-porphyry, p. 244.
4. Phenocrysts of quartz, or feldspar, or both, and sometimes of
ferromagnesian minerals, in a predominant groundmass of light color
and dense feldspathic aspect. Felsite-porphyry, p. 251.
5. Phenocrysts of feldspar, or of a ferromagnesian mmeral, or
both, in a dense, dark to black, and heavy groundmass. BasaU-
porphyry, p. 254.
D. It has a distinctly foliated, gneissoid, or slaty structure.
1. It contains feldspar, and generally quartz, with mica (some-
times hornblende). Gneiss, p. 351.
2. It consists mainly or largely of mica; often considerable quartz
is present, but feldspar is absent, or indeterminable. Frequently
contains crystals of dark red to black garnet, more rarely staurolite,
cyanite, etc. Mica-schist, p. 361.
3. Medium green, dark green or black; consists mostly of a felted
or matted mass of small, to very fine or microscopic, bladed, or needle-
like crystals arranged mostly, in one general direction, which promotes
the schistose cleavage. Other minerals, such as garnet, may be
pKsen.t.Hornblende-8chist or amphibolite (p. 379).
408 ROCKS AND ROCK MINERALS
TABLE FOR DETERMINING THE COMMON ROCKS
Continued.
4. Very compact, or dense and fissile, splitting easily into thin,
more or less tough, ringing slabs ; usually dark gray, or green to black,
but sometimes showing other colors. Slate, p. 369. (Sometimes
contains large crystals of staurolite, andalusite, etc.)
5. Very fissile, but soft to the feel ; laminae not tough, but often
brittle or crumbling; pronounced silky luster on the cleavage face.
Phyllite, p. 372.
6. Soft, greasy feel; marks cloth; easily scratched with the finger-
nail ; usually whitish to light gray, or green. Talc-schist, p. 374.
7. Smooth feel; soft; green to dark green; glimmering luster.
Chlorite-schist, p. 376.
E. Has a clearly fragmental composition; is seen to be com-
posed of fragments or pebbles of other rocks, or of smaller
angular or -rounded mineral fragments; if the latter,
frequently shows evidences of stratification.
1. The pebbles range from the size of a pea up and are rounded;
quartz ones are common; they are embedded in more or less of a
cement. Conglomerate, p. 320.
2. The pebbles are angular in shape. Breccia, p. 321.
3. Composed of various-sized angular fragments of volcanic rocks,
such as felsite and felsite porphyry, of bits of pumice, or cellular lava,
or of rounded, vesicular, volcanic bombs, etc., mixed with fine com-
pacted material (volcanic ash). Volcanic tuff and breccia, p. 272.
4. Composed of more or less angular, but sometimes rounded
grains, in size from that of a pea down; the grains are mostly, or
wholly, composed of quartz, and scratch feldspar. Generally some
cement is present, which, if the rock is light colored, is apt to
effervesce with acid (lime carbonate); if red or brown does not.
Sandstone, p. 323.
5. As ,in 4 but more or less feldspar is also [present among the
quartz grains. Arkose, p. 326.
INDEX
Actinolite, 61, 63.
Adinole, 188.
Adobe, 331.
Aegirite, 55, 58.
Aeolian rocks, 275.
Albite, 34.
Alkalic feldspar, 34, 35.
Alumina, test for, 117.
Amphibole, 60.
" determination of, 66.
Amphibolite, 379, 391.
Amygdaloidal structure, 159
Amygdaloid, basalt, 256.
Analcite, 103.
Andalusite, 76.
Andesite, 250.
Anhydrite, mineral, 113.
Anhydrite, rock, 295.
Anorthosite, 224.
Anorthite, 34.
Anthracite, 319.
Apatite, 95.
Aplite, 214.
Arfvedsonite, 61, 63.
Argillite, 369.
Arkose, 326.
Aschistic rocks, 169.
Ashes, volcanic, 141.
Associations of minerals, 29.
Augite, 55, 57.
Auqitophyre, 257.
Average rock, composition, 18.
Basalt, basalt-porphyry, 254.
" amygdaloid, 256.
" quartz, 256.
Bathylith, 139.
Bauxite, 97.
Biotite, 50, 51.
Bituminous coal, 318.
Black-band ore, 302.
Bog iron ore, 301.
Bombs, volcanic, 141.
Border zones in igneous rocks,
165.
Border zones, origin of, 170.
Boss, 138.
Bostonite, 254.
Breccia, friction, 322.
" sedimentary, 321.
" volcanic, 140, 272.
" " origin, 269.
Brown coal, 317.
Brownstone, 326.
Breunerite, 110.
Buhrstone, 368.
Calcite, 105.
Calcium, test for, 119.
Calcareous tufa, 312.
Camptonite, 257.
Cancrinite, 48.
Carbonates, test for, 115.
Cementation, zone of, 338.
Chalcedony, 86.
Chalk, 310.
Chemical elements, 18.
Chert, 297.
Chlorine, test for, 121.
Chlorite, 98.
Chlorite-schist, 376.
Chloritoid, 54.
Chondrodite, 82.
Chrysotile, 101.
Cipolin, 390.
Classification, general, 6.
" fragmental volcanic
rocks, 271.
Classification, glassy rocks, 261.
" igneous rocks, 191,
194, 203.
Classification, igneous rocks, table,
195.
Classification, metamorphic rocks,
348.
Classification, stratified rocks, 290.
409
410
INDEX
Clay, 96, 327, 278, 280.
Clay ironstone, 301.
Cleavage of minerals, 26.
effect of, 28.
Coal, 315.
" hard, 319.
" soft, 318.
Color of minerals, 23.
" " rocks, 399.
" " sedimentary rocks, 286.
Columnar structure, 162.
Comagmatic regions, 174.
Complementary dikes, 167.
Conglomerate, sedimentary, 320.
" volcanic, 322.
Consanguinity of rocks, 173.
Contact metamorphism, 180.
" " effect on
limestone, 187.
Contact metamorphism, effect on
sandstone, 186.
Contact metamorphism, effect on
shale, slate, 188.
Contact metamorphism, endomor-
phic, 181.
Contact metamorphism, exomor-
phic, 183.
Contact metamorphism, modes of
occurrence, 185.
Contact metamorphism, pneumato-
lytic, 189.
Contact metamorphism, ore bodies,
190.
Coquina, 311.
Cortlandtite, 238.
Corundum, 86.
Corundum-syenite, 226.
Crystals, denned, 21.
" form in rocks, 22.
" twinning of, 36.
Cyanite, 78.
Docile, 250.
Decay of rocks, 276.
Determination of minerals, 114.
" " tables,
122.
Determination of rocks, 396.
" " table,
404.
Diabase, 235.
Diaschistic rocks, 169.
Diatomaceous earth, 298.
Differentiation, 164, 169.
Dikes, 134.
" complementary, 167.
Diopside, 55, 57.
Diorite, 226.
" rock relations, 229.
Diorite-porphyry , 244.
Dolerite, 235.
" use of word, 198.
" alteration of, 237.
Dolerite-porphyry , 244.
Dolomite, mineral, 108.
Dolomite, rock, 307.
" " origin, 308.
Dolomite-marble, 390.
Dunite, 238, 240.
Earth's crust, composition, 17.
" interior, state of, 14.
Eclogite, 382.
Elements, geologically important
18.
Emery, 397.
Epidosite, 389.
Epidote, 73.
Eutaxitic Structure, 164.
Exotic mineral colors, 24.
Extrusive igneous rocks, 139.
Feldspars, 34.
alteration of, 44.
" cleavage of, 40.
" color of, 41.
crystal form of, 35.
determination of, 46.
twinning of, 36.
Feldspathoid minerals, 47.
Felsite, felsite-porphyry, 248.
" sheared, 373.
" varieties of, 249.
Ferromagnesian minerals, 146.
Field classification, 6.
Flint, 86, 296, 297.
Fluorine, test for, 121, 81.
Fracture of minerals, 29.
" conchoidal, 29.
" of rocks, 401.
Fragmental volcanics, 269.
Freestone, 325.
INDEX
411
Gabbro, 229.
" alteration of, 232.
" iron ores in, 234.
Garnets, 70.
Garnet-rock, 389.
Gelatinization test, 115.
Geyserite, 296, 298.
Glassy rocks, 260.
" alteration of, 269.
" " classification, 261.
Glaucophane, 64.
Glaucophane-schist, 383.
Gneiss, 351.
" field study of, 358.
" inclusions in, 356.
" texture of, 353.
" varieties of, 355.
Granite, 205.
complementary dikes of, 214.
contact of, 215.
" graphic, 212.
" orbicular, 211.
" pegmatites in, 212.
" porphyritic, 207.
" weathering of, 216.
Granite-porphyry, 243.
Granulite, 360.
Gravel, 278, 279.
Graywacke, 326.
Greenstone, 229, 377.
Greenstone-schist, 377, 383.
Greensand-marl, 325.
Grit, 325.
Ground mass denned, 156.
Gruss, 216.
Gypsum, mineral, 111.
Gypsum, rock, 293.
Halite, 113, 295.
Hammer, geological, 11.
" trimming, 12.
Hardness of minerals, 30.
" " rocks, 400.
Hauynite, 48.
Hematite, 91, 302.
Heulandite, 104.
Hornblendes, 60.
Hornblende-schist, 379.
Hornblendite, 238.
Hornfels, 188.
Hornstone, 188, 297.
Hydromica-schist, 373.
Hypersthene, 55, 57.
Ice, 20.
Igneous rocks, 132, 205.
" consanguinity of,
173.
Igneous rocks, classification of,
191, 194, 203.
Igneous rocks, crystallization in,
146.
Igneous rocks, dense types, 247.
" " general characters,
132, 141.
Igneous rocks, inclusions in, 163.
jointing of, 161.
minerals of, 145.
occurrence of, 134.
origin of, 164.
petrology of, 132.
post intrusive work
of, 174.
Igneous rocks, structure of, 158.
" " textures of, 150,
154.
Igneous rocks, variation of minerals
in, 142.
Ilmenite, 90.
Inclusions in granite, 213.
" gneiss, 356.
" igneous rocks, 163.
Injection of schists, 345.
Intrusive sheets, 135.
Infusorial earth, 298.
Iron ores, 88, 299, 395.
Iron oxide rocks, 395.
Iron, test for, 118.
Itabirite, 395.
Jadeite, 389.
Jade, 389.
Jasper, 86, 297.
Jaspilite, 297, 396.
Jet, 319.
Jointing, 161.
" in granite, 210.
Kammererite, 99, 393.
Kaolin, 96.
" from feldspar, 44.
Kimberlite, 242.
412
INDEX
Labradorite, 35 (rock, 224).
Laccoliths, 136.
" zoned, 166.
Lamprophyre, defined, 168.
Lamprophyres, 257.
Lapilli, volcanic, 141.
Laterite, 332.
Lava flows, 139.
Lepidomelane, 52.
Lepidolite, 52.
Leucocratic rocks, 167.
Leucophyre, 251.
Leucite, 49.
" rocks, 259.
Lignite, 317.
Lime-carbonate-silicate rocks, 387.
Limestone, 303.
" oolitic, 309.
Lime, test for, 119.
Limonite, 93, 301.
Listw'dnite, 375, 391.
Lithographic stone, 306.
Lithology defined, 2.
Lithophysae, 264.
Loam, 332.
Loess, 330.
Lydianite, 297.
Magmas, 134.
" composition of, 141.
" variations in, 142.
Magnesia-silicate rocks, 390.
Magnesite, 110.
Magnesium, test for, 119.
Magnetite, mineral, 89.
Magnetite-rock, 396.
Marble, 384.
Marble, onyx, 313.
Marl, 313.
" greensand, 325.
Megascopic, defined, 7.
Melanocratic rocks, 168.
Melaphyre, 255.
Metadiorite, 229.
Metamorphic rocks, 333.
" age of, 346.
" " classification
of, 348.
Metamorphic rocks, composition
of, 344.
Metamorphic rocks, injection of,
345.
Metamorphic rocks, minerals of,
339.
Metamorphic rocks, occurrence of,
346.
Metamorphic rocks, older struc-
tures in, 343.
Metamorphic rocks, origin of, 333.
Metamorphic rocks, textures of,
340.
Metamorphism, 333.
" agents of, 335.
" constructive, 338.
" effect of depth, 337.
" " " heat, 336.
" " " liquids, 337.
Miarolitic structure, 159.
Micas, 50.
Mica-schist, 361.
Mica-trap, 257, 215.
Microcline, 36.
Microscopical petrography, 7.
Minerals, associations of, 29.
" cleavage of, 26.
" color of, 23.
" exotic color of, 24.
" defined, 21.
" determination of, 114.
" fracture of, 29.
" hardness of, 30.
" specific gravity of, 31.
" streak of, 25.
Mineralizers, 149.
Minette, 237, 257.
Muscovite, 50, 51.
Natrolite, 103.
Necks, volcanic, 138.
Nephelite, 47.
Nephelite-syenite, 221.
Nephrite, 390.
Norite, 229.
Noselite, 48.
Novaculite, 297.
Obsidian, 262.
Occurrence of igneous rocks, 134.
Ocher, 328.
Olivine. 67.
" nodules, 258.
" rock, 238, 240, 390.
Onyx marble, 313.
LNDEX
413
Oolite, iron, 303.
" limestone, 309.
" siliceous, 368.
Opal, 86.
Ophicalcite, 391.
Order of crystallization, 146.
Ore bodies, 190, 391.
" formation of, 170.
Origin of igneous rocks, 164.
" " metamorphic rocks, 333.
Orthoclase, 34.
Oxides, important, 20.
Paragonite, 51.
Peat, 316.
Pebbles, 279.
Pegmatite dikes, 175.
" " origin, 178.
Peridotite, 238.
ores in, 241.
relation to gabbro, 240.
Perlite, 265.
Petrographic provinces, 173.
Petrography defined, 2.
microscopical, 7.
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