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

is quite analogous to what has already been described as
the effect of contact metamorphism of igneous rocks on
impure limestones in a previous part of this book, and the
chemical reactions which take place are the same as those
there mentioned. The resulting rocks are also quite simi-
lar, with, however, one difference. In contact metamor-
phism the chief agency is heat, while pressure and
shearing are either wanting, or are relatively of slight
importance, but in regional metamorphism these are
factors of great intensity. Thus the rocks of contact
metamorphism are massive and with little or no schistose
cleavage, while those produced by regional metamorphism
may strikingly exhibit it; that cleavage is not always
present is due to the reason given above under the des-
cription of marble.

Important minerals which thus occur in limestone are
pyroxenes (especially wollastonite, CaSiOs, and diopside,
CaMgSi 2 6 ) ; garnets (especially grossularite, Ca 3 Al 2 (Si04) 3 ) ;
hornblendes (especially tremolite, CaMg 3 Si 4 12 ) ; feldspar
(especially anorthite, CaAl 2 Si20g); vesuvianite; epidote;
fluorite, etc. A whole host of minerals occurs, but many
of them, such as graphite, magnetite, spinel, titanite,
tourmaline, apatite, phlogopite, etc., come chiefly from
the impurities in the original rock, which have been
recrystallized.

It is clear from this, that, depending on mineral com-
bination, a great variety of these lime carbonate-silicate
rocks exist, but only a few of the most important types
can be mentioned.

Wollastonite-rock. Marble not infrequently contains crystals of the
pyroxene-like mineral, wollastonite, CaSiO 3 , and this may increase
until the rock is practically composed of it. It is apt to be accom-



DESCRIPTION OF METAMORPHIC ROCKS 389

panied by diopside, hornblende, etc. The rock is white, generally
massive, and resembles marble, from which it is easily distinguished
by its superior hardness. It occurs in California, the Black Forest,
Brittany, etc.

Garnet-rock. This is a granular aggregate of grains of garnet,
generally accompanied by various other minerals in smaller, variable
amounts. If some calcite is yet present the garnets may show more
or less crystal form ; sometimes the calcite has been leached out and
the rock is porous. Apt to be yellowish, to reddish brown, in color.
Considerable magnetite is often present. New England, northern
New York, Montana, Germany, Alps, etc.

Epidote-rock, or Epidosite. Composed chiefly of epidote with
other minerals, quartz, garnet, etc. Sometimes massive granular,
sometimes schistose. Greenish in color, especially of a yellow-
green. Often very tough under the hammer. New England, Brazil,
Germany, etc. Sometimes the ferromagnesian igneous rocks, basalt
and dolerite, under proper metamorphic conditions, are converted
into a rock consisting chiefly of epidote, instead of hornblende or
chlorite as previously described, and of a yellowish green color.
They may resemble the above, but can usually be distinguished by
their mode of occurrence, geologic relations, greater uniformity, and
often by the remains of special structures, such as the amygdaloidal.
Instances occur in Pennsylvania, Virginia, etc.

Pyroxene-rock. In this case the rock consists chiefly of pyroxene,
of which the variety diopside is prominent. Other minerals, quartz
or calcite, etc., may occur. White, greenish, to dark green in color,
massive or schistose. Is found in Massachusetts, northern New York,
Germany, Bohemia, Sweden, etc. Under the head of metamorphic
pyroxene rocks there may be mentioned in this connection jade,
which, although extremely rare, is of great interest from its eth-
nological and artistic importance. Jade is a fine-grained, and
usually compact, aggregate of grains and fibers of the soda-pyroxene,
jadeite, NaAlSi 2 O 6 . It is sometimes snow-white, resembling marble,
but usually greenish (or with a violet shade) to dark green. The
greenish colors are also clouded, veined, or specked through the
white. When polished it has a soft, somewhat greasy luster. The
extraordinary toughness of the rock is one of its most marked
characters and on this account it was greatly prized in the early
history of mankind, before the discovery of metals, for the manu-
facture of weapons and implements, as shown by its distribution in
these forms, and in unworked pieces over the world. It has long
been greatly valued by the Chinese, who have devoted the most
laborious work to fashioning it into objects for personal adornment
and use. These objects, such as vases, bowls, etc., are often carved
with wonderful skill and taste and are greatly prized for their



390 ROCKS AND ROCK MINERALS

artistic value. The rock is only known in place in upper Burma
and in the Kuen-lun Mountains of Turkestan. Its origin is uncertain,
but its chemical composition suggests that it may be a metamor-
phosed igneous rock of high soda content, such as nephelite-syenite.
A green hornblende rock called nephrite, from Siberia and New
Zealand, has similar properties and uses and is frequently mistaken
for jade.

Cipolin is a marble full of -mica, which may show transitions to
calcareous mica-schist. Usually other minerals, sometimes in con-
siderable variety, are also present.

Dolomite Marble, Magnesia-Silicate Rocks. As men-
tioned under dolomite limestones, the rock name does not
necessarily mean that the substance composing it is pure
dolomite, in the mineralogical sense. There is generally an
excess of lime carbonate present, so that the composition
is a mixture of dolomite, MgCa(C0 3 )2, and calcite, CaCO 3 .
Just as marble is related to ordinary limestone, so is
dolomite marble to ordinary dolomite. In a practical
way no distinction can be drawn between the two vari-
eties of marble, except chemically. See dolomite under
the rock minerals. Like ordinary marble, dolomite is
one end of a series of metamorphic rocks, which,
beginning with a pure carbonate, becomes a mixture of
carbonates and silicates, and ends in pure silicate rocks.
The causes and processes are identical with those
described under marble, only in this case the presence of
magnesia causes the formation of silicate minerals, in
which this element is either the only metal, or a vsry
important one. Thus in distinction to the lime carbonate-
silicate series, this may be called the magnesian carbonate-
silicate series. The magnesian silicates thus produced
in the zone of constructive metamorphism may be anhy-
drous, or nearly so; on the rocks rising, by erosion or other-
wise, into the zone of hydration, they may be secondarily
converted in serpentine, H 4 Mg 3 Si 2 9 , or sometimes into
talc, H 2 Mg 3 (Si0 3 ) 4 . Thus these rocks are in many cases
closely connected with the talc-schists previously described,
while their relation to serpentine is mentioned under that



DESCRIPTION OF METAMORPHIC ROCKS 391

rock. The more important magnesia silicates which take
part in the series are olivine, enstatite, chrondrodite, diop-
side, tremolite, phlogopite, etc., and secondarily serpen-
tine and talc, as stated above. Of the varied rocks formed
by these mixtures, only a few of the most important can
be mentioned.

Crystalline dolomites, or dolomitic marbles filled with variable
mixtures of minerals, chrondrodite, phlogopite, pyroxenes, etc., with
others, such as magnetite, spinel, apatite, graphite, etc., coming from
original impurities, are found rather commonly in the metamorphic
areas in the eastern United States and Canada, but have received no
distinctive names, as rocks. They appear to have been formed some-
times by contact, sometimes by regional metamorphism, often by a
combination of both.

Ophicaldte is a mixture of white calcite and green serpentine, the
latter often in veins, spots, or clouded through the rock. A part of
the " verde antique " marble of the ancients, used for ornamental
purposes, appears to have been a variety of ophicalcite. It occurs
in Canada, northern New York, and various places in Europe.

Soapstone and talcschist. Part of the rocks included under these
names belong in this series: they have been already described
under a previous section. Listwanite, which occurs in the Ural
Mountains, and in Spain, is a mixture of magnesia carbonates (mag-
nesite, MgCO 3 , and dolomite), with talc, and with more or less quartz
Sagvandite, from Norway, is a granular mixture of varieties of mag-
nesite and enstatite (MgSiO 3 ) containing ferrous iron.

Amphibolites. Many of the hornblende-schists or
amphibolites, previously described in a separate section,
are the result of the transformation of impure limestones
and dolomites into metamorphic rocks. This has been
already discussed, but it should be again mentioned here,
because the amphibolites, made in this way, form one of
the most important members of the lime and magnesia
series of carbonate-silicate rocks described above.

Occurrence of Minerals and Ores. The crystalline marbles and
dolomites, in addition to the minerals mentioned above, not infre-
quently, owing to local causes, contain a great variety of others.
Thus at Franklin, New Jersey, owing to the presence of zinc and
manganese, a number of minerals containing these metals have been



392 ROCKS AND ROCK MINERALS

produced, forming useful ores. Ore-bodies are mostly developed in
these rocks, however, by contact metamorphism, but in some cases
the minerals developed by regional metamorphism are of such a
character, and in such quantity, that they may be usefully exploited.
Many of the famous mineral localities, specimens from which are
commonly seen in collections, are in these rocks. The minerals
thus found embedded as crystals in calcite and dolomite, are apt to
have the angles between the faces more or less rounded, and to be
veined with calcite in their cracks.

SERPENTINE.

General Properties. No close distinction between ser-
pentine, as it has been described as a mineral, and serpen-
tine as a rock can be made. As a mineral the chemically
pure substance was considered, but serpentine as a rock
is generally more or less impure from the presence of
other minerals which are mixed with it. Serpentine
rocks are generally compact, of a dull to waxy luster, and
of a smooth to splintery fracture. If tolerably pure they
are soft and can be cut by the knife, but they are some-
times saturated by deposited silica, which makes them
much harder. The general color is green, characteristically
a yellowish-green; but sometimes yellow, yellow-brown,
reddish-brown, and dark green to black. On smooth sur-
faces the rock has a somewhat greasy feel, recalling talc-
schists, from which it is, however, readily distinguished
by its superior hardness. Talc leaves its mark on cloth,
while serpentine does not. The yellow-green color re-
sembles also that of epidote rocks, but here again the
superior hardness of the epidote serves as a distinction.

Associated Minerals. Other minerals which may accom-
pany the serpentine, and which may at times be seen in it,
are remains of the magnesia silicates from which it has
been formed, olivine, pyroxene, and hornblende. Metallic-
looking specks or crystals of ores are common, magnetite,
chromite, etc. In some varieties garnet occurs, chiefly
pyrppe, and that which is used for gems comes in large
part from a serpentine in Bohemia. In the Ural Mountains



DESCRIPTION OF METAMORPHIC ROCKS



393



serpentine is the source of platinum, and in other places of
nickel ores. Serpentine is apt to be accompanied by
other secondary minerals, by chlorite (sometimes the pur-
ple-red variety kammererite containing chromium), by
talc, and by magnesium carbonates, magnesite, MgCOs, and
breunnerite, MgFeCOs, etc. Serpentine rocks are usually
massive but sometimes schistose, serpentine-schist. Not
infrequently they are seamed by veins of the finely
fibrous variety of the mineral called chrysotile, which
has the structure of asbestus and is often so called.

Chemical Composition. The chemical composition of
serpentine rocks approaches that of the pure mineral, but
generally differs somewhat on account of the other minerals
present. This is seen in the appended analyses:





SiO 2


AI 2 3


Cr 2 C>3


Fe 2 O 3 '


FeO


NiO


Mg


CaO


H 2


COa


XyO


Total.


I..


40.4


1.9


0.3


2.8


4.3


0.5


36.0


0.7


10.7


1.4


0.6


99.6


II.


36.6


1.0


0.3


7.3


0.4


0.3


40.3


0.1


13.4





0.4


100.1


Ill


38.6


1.3


0.5


5.6


2.2


0.1


39.1


0.9


11.3


0.5


0.2


100.3


IV


36.9


1.4





6.9


4.0





36.0


1.4


13.1








99.7


V.


42.3


0.8





_


2.6





40.3


1.3


12.5





0.5


100.3


VI


44.1

















43.0


~


12.9





~~


100.0



I, Serpentine, dark-green, Rowe, Massachusetts; II, Serpentine,
from pyroxenite dike, Mount Diablo, California; III, Serpentine,
Iron Mountain, Oregon; IV, Serpentine, from hornblende-schist,
Vosges Mountains, Germany; V, Serpentine, white, selected pure
mineral, Brewsters, New York; VI, Theoretical composition of pure
mineral, H 4 Mg 3 Si 2 0,.

The presence of small quantities of nickel and chrome
oxides is a very common feature.

Origin. Serpentine rocks are secondary in nature,
being formed when previously existent rocks, consisting
wholly or chiefly of magnesian silicates, are exposed to
the processes at work in the zone of hydration. Their
origin may thus be twofold: they may be formed from



394 ROCKS AND ROCK MINERALS

igneous rocks, such as peridotite, dunite, etc.; or when
amphibolites or hornblende-schists, which have been
made from sediments in the zone of constructive meta-
morphism, are brought by erosion into the zone of hydra-
tion, they may be converted into serpentines. Thus the
origin of the material may be igneous or sedimentary,
but, whereas the igneous rocks pass directly into serpen-
tine, the sedimentary ones first pass through an interme-
diate metamorphic stage (hornblende-schists, etc.), and
are then converted. In this connection what has been
said elsewhere concerning the alteration of the peridotites
and allied rocks should be read This also explains in
part at least the origin of the chromium and nickel. No
formula can be given for the recognition of which origin
a serpentine has had; the geologic mode of occurrence and
relation to other rock masses is often a help, while the
presence of nickel and chromium, substances to be
expected in igneous, but not in sedimentary rocks, if
it can be shown, is very significant.

Occurrence. Serpentine is a common rock, and, while
it rarely forms large masses or covers extensive areas, it is
widely distributed over the world. In the form of layers,
lenticular masses, etc., it is common in metamorphic
regions from the alteration of both igneous and meta-
morphic rocks, and it thus occurs in eastern Canada,
New England, New York, Pennsylvania, Maryland, Cali-
fornia, Oregon, and other states; in southern England,
Germany, the Alps and various other places. It also
occurs in non-metamorphic sedimentary areas due to the
conversion of igneous rocks which have penetrated the
strata, as in places in Quebec, New Brunswick, New York
state, etc.

Alteration* Uses. Serpentine shows great resistance to
the action of the weathering agencies at the surface, but
eventually breaks down into a mixture of carbonates and
silica, mixed with ferruginous matter. The soils thus
formed, on account of the lack of alkalies and lime, are



DESCRIPTION OF METAMORPHIC ROCKS 395

extremely barren, and often little or no vegetation grows
upon them.

On account of its beautiful coloring, serpentine has
been largely quarried for use as an ornamental stone, being
used for interior purposes much as highly colored marbles
are. It is sometimes employed for the same objects
for which soapstone is used; in many cases its softness is
an objection to its employment. In some places the
seams of fibrous chrysolite which it contains are mined for
use as asbestus. Its value as a source of ores of nickel,
chromium, etc., has been already commented upon, and is
further mentioned under peridotites and allied rocks.

IRON OXIDE ROCKS.

Itabirite. This rock is composed chiefly of micaceous
hematite and quartz. The micaceous hematite, or " spec-
ular iron ore " as it is often called, is in very thin tablets
or leaves of irregular outline, while the quartz is in aggre-
gates of grains. It much resembles mica-schist, and if one
were to imagine the mica of such a schist replaced by a
substance of mica-like thinness but with the metallic
luster of polished iron, he would have a good idea of the
appearance of this rock. Micaceous hematite is indeed
of not infrequent occurrence in genuine mica-schists,
and by its increase transition forms to itabirite are pro-
duced. Also, just as the relative quantities of quartz
and mica vary in different layers of mica-schist, so do the
micaceous hematite and quartz vary in itabirite; thus there
are layers poor in quartz, and others quite rich in it, of very
variable thickness. In addition to the mica, magnetite,
pyrite, talc, garnet, and others may occur as perceptible
accessory minerals. The rock is generally granular to
fine granular; very schistose; of a dark color on the cross-
fracture, and exhibits on the chief fracture the shining
steel-like luster of the specular iron ore. Sometimes the
amount of the iron mineral is so great as to practically



396 ROCKS AND ROCK MINERALS

conceal the quartz. Itabirite forms extensive areas in
Brazil and on the Gold Coast of Africa, and in these places
carries native gold. It also occurs in North and South
Carolina, in Canada, Norway, Germany, etc. It has
probably been formed by the metamorphism of sandstones
and shales rich in deposited ferruginous matter, limonite,
etc.

Jaspilite is a name given to somewhat similar rocks
which consist of layers of red chert and hematite. They
occur in the Lake Superior Region. See page 297.

Magnetite Bock. This is a compact to granular aggre-
gate of grains of magnetite; dark-colored to black, and
heavy. The properties are those described under the
mineral. Hematite is very commonly mixed with it, and a
variety of other minerals, such at ilmenite, pyrite, quartz,
calcite, garnet, etc., according to the mode of occurrence.
The origin of magnetite rock is various; thus it may occur
as masses included in, or associated with, igneous rocks,
and is then regarded as a differentiated phase of such rocks,
as mentioned under them, and in this case the associated
minerals vary with the kind of rock, as nephelite and augite,
when with nephelite-syenite (Arkansas, Brazil, Sweden);
olivine, pyroxene, lime-soda feldspar when with gabbros
(Adirondacks, Sweden, Canada, Colorado, etc.). In other
cases it occurs as a contact formation where igneous rocks
have metamorphosed beds of limonite, siderite, etc.
Finally it occurs in regional metamorphosed areas, in the
form of layers and lenses, in the midst of gneisses and
schists, and often associated with metamorphosed lime-
stones and dolomites. It then often contains carbonates
of lime and magnesia, as well as the more common of the
silicate minerals described as associates of marble, such
as garnet, pyroxene, hornblende, etc. It is probably due
to the metamorphism of beds of impure limonite, clay-
ironstone, etc. Deposits of magnetite rock occur in many
places in the United States and Canada, in Scandinavia,
Germany, the Ural Mountains, etc., and are of great



DESCRIPTION OF METAMORPHIC ROCKS 397

importance as sources of iron ore. Those which are
situated in genetic connection with igneous rocks are,
however, generally useless on account of the presence of
ilmenite, titanic iron ore, which prevents their being
profitably smelted.

Emery. This is a granular rock of a dark-gray to black
color, consisting mainly of grains of gray or bluish corun-
dum, often mixed with magnetite, and associated with
other minerals. It is sometimes quite schistose. It is
easily told by its weight and excessive hardness (corundum
= 9). It occurs, as layers of relatively small volume in
the crystalline schists, in Asia Minor, Island of Naxos,
Germany, Massachusetts, etc. Its use as an abrasive
material, on account of the corundum it contains, is well
known.



CHAPTER XII.
THE DETERMINATION OF ROCKS.

THE determination and classification of rocks presents
itself as a problem, whose difficulty depends on what is
sought to be done, and the means at command for carrying
it out. It is obvious that the fine distinctions made by
petrographers among rocks, especially the igneous ones,
cannot be carried into ordinary practice, unless the same
methods for the study of rocks the use of the micro-
scope on sections ground thin and chemical analyses are
employed which they use. This, of course, cannot be ordi-
narily done, and we are thus limited to the means of
observation which have been used in this work, and to
simple classifications and the limited number of kinds
which they afford. This has already been commented
upon, in discussing the classification of the igneous rocks,
and need not be repeated.

Bock Characters used in Determination. The characters
of rocks which may be used for their megascopic determi-
nation are of two kinds, mineral and general. By the
mineral characters it is meant, that if the rock is composed
wholly or in part of mineral grains, which are large enough
to be distinctly seen with the eye or lens, and which may be,
if necessary, handled and tested, then the determination
may proceed along the line of a study of the minerals, their
kinds, relative abundance, and relation to each other
(rock-texture) . In this case there is no essential difference
between the microscopic and megascopic study of rocks;
one can accomplish, in the main, on the fractured surface
of a coarse rock, what the microscope does on the thin
section of a compact one. The individual minerals may

398



THE DETERMINATION OF ROCKS 399

be studied and tested according to the methods given in
Chapter V; if in the field, the simple tests of Table No. I
may be used; if the conveniences of a laboratory are at
hand, the more complete one, Table No. II can be employed.
If it has been already determined, perhaps in the field,
whether the rock is igneous, sedimentary, or metamorphic,
its place can then be usually very quickly settled. Even
if all the different kinds of minerals cannot be told, the
determination of one or more will generally be of service.

The general characters are those which are resultant
from the combination of mineral grains; they might be
termed composite features of rocks. They include color,
structure,* texture, fracture, hardness, and specific gravity.
Of these the specific gravity is of the least general applica-
bility, because it requires a special apparatus to determine
it. The reaction of the rock with acids is also at times
extremely useful as a test, and may be added to the list.
These general characters are so useful that they deserve
some separate mention in regard to their employment in
rock determination.

Color. The rock color is the general resultant of those
of the combined mineral grains. Certain general conclu-
sions may be drawn from the color of a rock; thus if it is
pure white or nearly so, it is certain that compounds of
iron are either wanting in it, or are only present in traces,
and in general the rock is either a sandstone, quartzite,
limestone-marble, gypsum, or a nearly feldspathic igneous
rock, such as anorthosite, aplite, syenite, or felsite. Red,
brown, and green colors indicate the presence of iron com-
pounds ; black or stone gray may also, but in a sedimentary
rock, these colors may indicate carbonaceous material.

Structure. If the rock has a pronounced structure of
some kind, it may be of great assistance in determining
the general class to which it belongs, and this may be of
especial assistance if the geological relations of the rock

* The difference between the use of " structure " and " texture "
has been already explained, p. 158.



400 ROCKS AND ROCK MINERALS

mass cannot be determined. Thus if a rock mass possesses a
pronounced columnar, or a highly vesicular, or an amygda-
loidal structure, it is almost certainly of igneous origin;
if a laminated or banded structure, it is probably sedimen-
tary; but this cannot be definitely relied on, because
igneous rocks, especially lavas, may assume a banded
structure by flowage, while metamorphic rocks may