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

they have resulted from several different sources, as seen
in the following analyses.



SiO a


A1 2 O 3


Fe 2 O 3


FeO


MgO


CaO


Na 2 O


K 2 O


H 2 O


XyO


Total


49.2
26.2


15.1
23.7


12.9
15.7


14.5


5.2
8.3


10.6
1.7


3.6
0.5


1.5
0.6


1.9
7.3


0.8


100.0
99.3



I, Chlo rite-schist, east of Roton, Sweden; II, Chlorite-schist
Benguet, Luzon Island, Philippines.



DESCRIPTION OF METAMORPHIC ROCKS 377

No. I has a composition very similar to that of the group
of igneous rocks known as gabbros, as may be seen by
reference to their analyses, and to which also the dolerites
and basalts belong, these being merely textural varieties of
magmas similar to gabbros. No. II, on the other hand,
is very different, and does not correspond to any igneous
rock; it suggests rather a very ferrugineous clay.

The chlorite-schists are of wide distribution, forming
subordinate layers or masses in the midst of gneisses,
mica-schists and other such rocks, characteristic of meta-
morphic areas. Thus they occur in Canada, New England,
New York, Pennsylvania, etc. They are also common
in Europe, in the Alps, Germany, Sweden and other places.

Greenstone. Transitions of chlorite-schist into mica-
schist, into slates, into schistose serpentine, and into
hornblende-schist occur in places. Under the descrip-
tion of gabbro and of dolerite it was mentioned that these
rocks by alteration, through processes of regional meta-
morphism, passed into hornblende-schists and into so-
called " greenstone " or " greenstone-schist." In such
cases the original ferromagnesian minerals, or the horn-
blende produced from them, have been largely changed
by alteration into chlorite which gives the rock its green
color. Such greenstones (if massive), or greenstone-
schists (if schistose), which thus represent altered dolerites,
basalts, and gabbros, form transition types to chlorite-
schist. The alteration of hornblende in diorites to
chlorite also produces greenstones. It is conceivable that
a dolerite might pass directly by alteration into a chlorite
rock, or greenstone, and thus be of massive character, or it
might be first changed into a hornblende-schist and this
secondarily alter into a chlorite-schist. But since horn-
blende-schists are produced, not only from igneous but
also from sedimentary beds, as described in the account
of these rocks, the mere fact that transitions from horn-
blende-schists into chlorite-schists occur does not alone
prove these latter have been derived from igneous rocks



378



ROCKS AND ROCK MINERALS



in any given case. Transitions from dolerite into
chlorite rocks, or greenstone-schists, have been observed in
many regions; in Michigan, Maryland, Connecticut, in the
south of England, in the Alps, Germany, etc. A green-
stone-schist from the Menominee River, Michigan, which is
known to be an altered dolerite, has the following com-
position.



Si0 2


A1 2 3


Fe 2 O 3


FeO


MgO


CaO


NazO


K 2


H 2 O


CO 2


Total


44.5


16.4


5.1


5.5


7.5


7.9


2.6


0.5


5.0


5.4


100.4



It is composed of chlorite, with some feldspar, quartz
and calcite. This should be compared with the analyses
of gabbro which represent the gabbro-dolerite group.

The greenstones vary in color from pale gray-green
through yellowish green to dark green. The color depends
on the proportion of chlorite to other minerals. They are
generally too compact for the megascopic determination of
the individual mineral particles. They are generally rather
soft rocks. Sometimes, when the original dolerite or
basalt was an amygdaloid, this amygdaloid structure is
retained and the rock is filled with little balls of calcite
or quartz. In other cases, where the rock has been strongly
sheared, these have disappeared, but are still represented
in the schist by ovoid spots of a different color and mineral
composition from the main mass. In some rare cases
they have been replaced by ores. The amygdaloidal
structure is a good proof of the original igneous character
of the rock.

Soapstonc, Steatite. As an appendix to talc and chlorite-schists
may be mentioned soapstone or steatite, a massive rock, usually of a
gray or green, but sometimes of a dark color; the lighter colors often
with a silvery or shimmering fracture surface. It is very soft,
easily cut or worked, without cleavage or grain, and resists well
heat and the action of acids. For these reasons it has been exten-



DESCRIPTION OF METAMORPHIC ROCKS 379

sively used in prehistoric times for the manufacture of pots and other
vessels, and is employed at present in table tops, sinks, and other inte-
rior fittings where its qualities render it valuable. It is usually a
variable mixture of interwoven scales of talc and chlorite mixed with
various minerals; in some cases carbonates are present. The better
qualities incline more nearly to pure talc. The minerals are in
general too fine for megascopic determination. It occurs in con-
nection with talc and chlorite rocks, and sometimes with serpentine,
in various parts of the world, in areas of metamorphic rocks.

AMPHIBOLITE OR HORNBLENDE-SCHIST.

The amphibolites are a large group of metamorphic
rocks whose distinguishing characters are, that they con-
sist partly or largely of hornblende, and that they possess
a more or less pronounced schistose structure. There are
a number of varieties in the group, depending on the kind
of hornblende present, and on the minerals associated
with it, so that it is difficult to give a general description
which will cover all cases. It is best therefore to describe
the most common kind first, and then give a brief mention
of some of the less common varieties.

Common hornblende-schists or amphibolites are rocks
which vary in color from green to black; the green is of
varying tones, clear light green, gray-green, yellowish
green to dark green, greenish black to black; the darker
colors are more common. The color is given by the
hornblende, though in a considerable degree, in some
cases, it is influenced by admixed chlorite. The grain of
the rocks varies from coarse to fine, the latter being more
common. When coarse, the hornblende, which is almost
always present in slender prisms or blades and rarely in
grains, is easily recognized by the eye from its form and
bright, good cleavage. In such cases the prisms may be
an inch or more in length and have the thickness of a
slender match stick; from this, in the finer-grained
types, they sink to tiny needle- or hair-like prisms which
can only be seen by careful observation with a good
lens. The prisms are usually arranged in the direction of



380 ROCKS AND ROCK MINERALS

schistosity and thus approach parallel positions; it is this
which chiefly gives the rock its cleavage. It also gives
the rock, especially in the finer-grained types with needle-
like prisms, a shimmering or silky luster on cleavage sur-
faces, which is rather characteristic. In some cases the
grain is so extremely fine that not even with the lens
can the individual minerals be seen; such rocks may
appear very much like slates, and are indeed difficult to
distinguish from them; they are however, not very com-
mon types.

The amphibolites are rather hard rocks, not easily
scratched by the knife. In the more schistose types
they are brittle, but as they become more massive in
character they are very tough and difficult to break.
They are heavy, the specific gravity ranging from 3.0-3.4.

In addition to the hornblende other minerals are present in
varying kinds and quantities; prominent ones are quartz, feldspar,
and mica. The quartz and feldspar in grains are best observed with
the lens on the cross fracture; often they are too fine and too much
masked by the hornblende to be seen; the quartz also at times
forms little lenses or masses, or fills fractures in the shape of veins, as
in other metamorphic rocks, and has then been secondarily deposited
from solutions. The mica can be generally seen on the surface of
chief fracture ; both biotite and muscovite occur and may increase
to such an extent as to produce formal transitions to mica-schist.

Other minerals which may be detected megascopically are iron
ore, pyrite, garnet in small dark red crystals, chlorite, calcite;the
latter sometimes in veins, etc., like quartz. Pyroxene, epidote, and
other minerals occur, but partly on account of the fineness of grain,
and partly on account of their resemblance to hornblende, it is
usually impossible to detect and identify them without microscopic
study.

The chemical composition of amphibolites has not yet
been as thoroughly investigated as it should be, but
what has been done shows, in agreement with facts
to be presently mentioned, that the origin of these
rocks is various. The following analyses will do for
examples.



DESCRIPTION OF METAMORPHIC ROCKS 381



I..

TT


SiOj

49.9
50.4


A1 2 O 3


Fe 2 O 3


FeO


MgO


CaO


NajO


K 2


H 2


XyO


Total


15.5
13.3


3.0
6.3


8.0

9 3


7.8
5 6


8.9
7.9


3.3
2.1


0.7
1 1


1.5
1 7


1.7

?,


100.3
99.7


III.
IV .
V


55.0
55.6
45.6


2.9
16.3
14.2


0.8
1.2
1.2


6.3

7.2
9 8


21.0
5.6
6 8


11.5
9.2
2.3


0.3
0.9
1.6


0.2
0.2
1 ?,


1.0
3.1
5 1


1.5
1.0
1? fi


100.5
100.3
100.4


VI.


52.4


13.6


2.7


9.8


5.5


10.0


2.3


0.4


1.7


1.6


100.0



I, Thin schistose amphibolite, Whitman's Ferry, Sunderland, Mass.;
II, Amphibolite, Crystal Falls district, Michigan; III, Grass-green
Amphibolite, Chiavenna. XyO = Cr 2 O 3 ; IV, Amphibolite, Goshen,
Massachusetts; V, Amphibolite, pyritiferous, Conrad Tunnel, Ophir
district, California. XyO = Pyrite 7.9, CO 2 3.0, TiO 2 l.l, plus traces;
VI, Olivine-basalt, main lava flow, Pine Hill, South Britain, Con-
necticut.

Of these analyses Nos. I and II have compositions very much
like that of the gabbro-basalt magmas, as may be seen by comparison
with No. VI ; No. Ill has the general composition of the peridotite-
pyroxenite group of igneous rocks and may be compared with
Analysis No. Ill of that group. The presence of Cr 2 O 3 in III is also
significant of an igneous origin. On the other hand No. IV is
thought on geological grounds to be derived from an impure lime-
stone, probably full of clay, and this supposition is rendered probable
by the fact that the high alumina is accompanied by an almost
entire lack of alkalies, a feature not seen in igneous rocks. In No. V,
while alkalies are present with the alumina, high magnesia and
ferrous iron are not accompanied by high lime and these make a
combination not seen in igneous rocks. Compare again No. VI.
This rock, No. V, is probably derived from an impure ferrugineous
arkose or silt.

Origin of Aonphibolites. As just shown, the composition
of amphibolites sometimes corresponds with that of
igneous rocks, and sometimes does not, and this agrees
with the results of geological investigation in the field.
For in some places we find them under conditions which
strongly suggest their derivation from igneous rocks, and
in other places such evidence is either wanting, or the
contrary is indicated. The use of the microscope on thin



382 ROCKS AND ROCK MINERALS

sections, by which the inward textures and associated
minerals may be seen, also leads to the same conclusions.

Under the description of gabbro and dolerite it was
mentioned under alteration, how these rocks by pressure
and shearing became converted into hornblende-schist or
amphibolite. Gradual transitions, without geological
break, from one kind into th.e other, are found. Thus as
the feldspathic igneous rocks give rise to gneisses, phyllites,
etc., so the ferromagnesian, especially the pyroxenic,
igneous rocks give rise especially to hornblende-schists,
and also to talc-schists, chlorite-schists, and to serpentine.

Sedimentary beds of impure mixed character, such as
limestones containing sand, clay, and more or less of the
hydroxides of iron, limonite, etc., or marls of a somewhat
similar nature, if subjected to metamorphism might, under
suitable conditions, be converted largely into hornblende,
mixed with other minerals. In this case the volatile con-
stituents the water, carbon-dioxide, etc., are mostly
driven out; the bases, lime, iron, magnesia and alumina,
combine with the acid silica, to form silicates, of which
hornblende is the chief and determinant one of the result-
ing rock. Thus hornblende-schists result from the meta-
morphism of sedimentary strata, and may be one form of
the alteration of limestone, as described later under
marble.

Varieties of Amphibolite. In the midst of gneisses and mica-
schists amphibolite sometimes assumes a very massive character.
The prisms and grains of hornblende, instead of being arranged in
parallel position, and thus producing a schistose cleavage, are inter-
woven without arrangement and cleavage is wanting. Especially
in such massive types is the hornblende liable to be accompanied by
feldspar. If th? feldspar should increase and dominate, transitions
to hornblende gneiss would be produced. There is a tendency on
the part of some to restrict the term amphibolite to such massive
varieties and to use hornblende-schist for those with distinct cleavage,
but this distinction has not yet come into general use.

Eclogite is a variety of hornblende-schist of a rather light green
color sprinkled full of red garnets. In the typical examples of this
rock the hornblende is accompanied, or more or less replaced, by



DESCRIPTION OF METAMORPHIC ROCKS 383

a green pyroxene. Other minerals also occur in subordinate amount.
It has been found in various places in 'Europe, and has recently
besn described as occurring in a Californian locality. A closely
related hornblende-schist full of garnets is found also in Hanover,
New Hampshire.

Glaucophane-schist is a variety in which ordinary hornblende
is replaced by the soda-bearing species glaucophane, and for this
reason the rock is colored blue instead of green. Various other
minerals may be present, depending on the occurrence, such as quartz,
epidote, pyroxene, chlorite, garnet, etc. Sometimes they are coarse
grained and these other minerals may be seen, sometimes dense and
appear as slaty blue or blue-gray rocks. Studies which have been
made of them show that sometimes they have been produced by the
metamorphism of sedimentary, sometimes of igneous, material.
While comparatively rare they have been found widely distributed,
in California, Brittany in France, the Alps, Island of Syra, Greece,
Japan, Australia, etc.

Greenstone-schist in its relation to amphibolites has been already
mentioned under chlorite-schist. As the use of the term " green-
stone " has been vague, applying rather to color than to a deter-
mined mineral composition, many rocks which are hornblende-
schists, rather than chlorite-schists, have been included under it.

Occurrence of Amphibolites. These form layers and
masses in other metamorphic rocks, especially in gneisses
and mica-schists, rather than extensive independent for-
mations. -They often occur in gneisses in long bands or
veins in such a manner as to suggest that they are meta-
morphosed dikes of doleritic rock. In size the masses may
vary within the widest bounds, from one foot to thousands
in diameter. In some places, what they lack in size, they
make up in frequency of occurrence. The manner in
which they are interlaminated in places with other meta-
morphic rocks suggests that they may have been some-
times formed from intrusive sheets of igneous rock, and
sometimes from interbedded. sediments, but in general
this can only be rendered certain by further chemical and
microscopical investigation. Their occurrence as mantles
around igneous masses has been already mentioned.

The amphibolites are extremely common rocks, in all
metamorphic regions. Thus they are found commonly



384 ROCKS AND ROCK MINERALS

distributed in New England and New York State, and
southward to Georgia; in Canada, the Lake Superior region,
the Sierras, in England, Scotland, the Alps, etc.

Alteration. It has been already pointed out under
chlorite-schists that the hornblende of these rocks may be
changed into chlorite. In another form of alteration it
may be turned into serpentine with other minerals, and
thus give rise to serpentine rocks, whose character is
described later. These changes take place in the upper
belt of metamorphism, that of hydration and cementa-
tion, and are secondary to the processes which have pro-
duced the amphibolite from something else. They
might thus be spoken of as tertiary changes.

By the ordinary process of weathering on the surface,
these rocks change to masses of limonite, clay, calcite, etc.,
which form ferrugineous soils.

MARBLE AND CARBONATE-SILICATE ROCKS.

Marble is the metamorphic condition of sedimentary
rocks which are composed of carbonate of lime, CaCOs,
and which in their ordinary stratified form are known as
limestone, chalk, etc. It is distinguished from them by
its crystallization, coarser grain, compactness and purer
colors. But just as we have ordinary limestones which
contain only carbonate of lime, and dolomitic limestones
which contain magnesian carbonate, MgC0 3 , in variable
quantity associated with the lime carbonate, so we have
lime marbles and dolomite marbles. As this distinction
is a purely chemical one which is rarely made, and indeed
rarely can be made in ordinary and commercial usage, the
rock is, therefore, called marble, without regard to whether
it contains magnesia or not. But geologically, especially
from the petrographical standpoint, there is an important
difference between the two rocks in respect to the asso-
ciated minerals they are apt to contain when impurities
were present in them originally, and therefore they are



DESCRIPTION OF METAMORPHIC ROCKS 385

treated separately in this work for reasons which will
presently appear.

General Properties. Marble is a crystalline granular
rock composed of grains of calcite; sometimes these are
cemented by a fine deposit of calcite between them.
The grain varies from very coarse to such fine compact
material that individual grains cannot be distinguished; in
the coarsest varieties, the cleavage surfaces of individuals
may attain a breadth of half an inch or more, but this
is unusual. The fracture surface of the finest-grained
kinds has a soft, shimmering luster, while the appearance
of coarser kinds is like that of loaf sugar. The normal
color is white, like that of the best statuary marble, but
the rock is usually more or less colored by various sub-
stances which act as a pigment, the principal ones being
carbonaceous matter and the oxides of iron. It thus
becomes gray, yellow, red or black, and while the color is
sometimes uniform, it is more generally spotted, blotched,
clouded or veined, producing that effect which is known
as " marbled." The hardness is that of calcite, 3; the rock
is thus readily scratched or cut by the knife, a ready means
of distinction from quartzite or sandstone, which may
resemble it. It is readily soluble in weak acids. Unless
the grain is too fine, the good rhombohedral cleavage of the
calcite grains can be easily seen with a lens. The chemi-
cal composition of a perfectly pure marble would be that
of calcite, CaO = 56, CO 2 = 44 per cent, but there are
usually small quantities of magnesia, alumina, iron and
silica present, coming from traces of sand, clay, dolomite,
etc., mixed with it; these may increase until the impure
marbles are produced, which are described in a later
paragraph.

Unlike most metamorphic rocks, marble, if pure, is very
massive and shows no sign of schistose cleavage, even
where its association with schists is such as to indicate
that it must have been subjected to enormous pressure
and shearing stresses. If impurities in the form of other



386 ROCKS AND ROCK MINERALS

minerals are present it may then assume cleavage, caused
by their presence. The reason for this want of cleavage
has caused much speculation; it is probably due to several
causes, to the purity of the rock, to a rolling of the grains
among themselves, but chiefly to a curious property which
calcite possesses of permitting movement among its mole-
cules, whereby new crystal forms are produced without
destruction of its substance; this results in a complicated
microscopic twinning, somewhat similar to that explained
under feldspar. As a result of this the stresses are
absorbed molecularly, instead of producing changes in the
outward structure, as in most rocks.

Varieties of Marble. The varieties of marble from the technical
point of view are chiefly those which are based on color. Statuary
marble is the purest and whitest kind. Architectural marbles are those
of the most uniform tones of color, while ornamental marbles are
those distinguished by striking effects of varied colors, as mentioned
above. In the trade, the term marble is used for any lime carbonate
or dolomite rock which can be procured in large, firm blocks, and is
susceptible of a high polish ; under this definition many limestones are
included. Shell marble is thus a hard, firm limestone in which a
certain pattern is given by the presence of certain fossils, shells of
brachiopods and remains of crinoid stems being the most common.
The different yellow, red, and black marbles, most of them veined
and clouded, of Italy, Greece, and the East have long been distin-
guished by a host of names.

Those varieties which depend on the presence of some mineral,
additional to the calcite, are treated in the following section on car-
bonate-silicate rocks.

Occurrence of Marble. The great deposits of marble,
from which the material used for structural purposes is
taken, are the result of regional metamorphism and it
is thus found in regions of metamorphic rocks associated
with gneisses, schists, etc., in the form of interbedded
masses, layers, or lenses. These vary in size within wide
bounds, from a few feet to many miles in length. It
forms immense interbedded layers, or masses, in the
Laurentian rocks of Canada; it occurs in quantities in



DESCRIPTION OF METAMORPHIC ROCKS 387

Vermont, Massachusetts, Georgia and Tennessee, where
it is extensively exploited, in Colorado and other places
in the west. The marbles of Greece and Italy have at-
tained celebrity from their use by the ancient Greeks and
Romans in statuary and buildings. It is found in the
Alps, Germany, and Scandinavia in Europe, and in
various other places in the world. Marble is also produced
from limestone (and chalk) by the contact action of in-
truded igneous rock. Although some very coarse-grained
material may be formed in this way, it is usually quite
limited in amount.

Lime Carbonate-Silicate Rocks. As described under
the general properties of sedimentary rocks, all transitions
occur between limestones and sandstones, between lime-
stones and shales, and between the three combined. This
means merely, that the original lime deposits may have
had sand, clays, silt, and ferrugineous material in variable
amounts, mixed with them. Chemically, it means that
the carbonate of lime has silica, the oxides of aluminum
and iron, and usually small amounts of other things, such
as magnesia, potash, and soda mixed with it. Under the
conditions of metamorphism the carbon dioxide, C0 2 ,
is driven out, to be replaced by an equivalent amount of
silica, SiO2, and thus silicates of lime, of lime and alumina,
of lime and iron, or mixtures of these, or combinations
containing other elements as well, are formed. Also
volatile substances, liquids and gases, such as water
vapor furnishing hydroxyl, fluorine, boron, etc., emana-
tions from magmas resting below or being intruded simul-
taneously with the crustal movements which give rise to
the metamorphosing conditions, may enter the rock mass
and thus, in adding new substances, produce additional
mineral combinations. The amount of silica present may
be sufficient to completely replace the carbon dioxide and
the resulting rock is then composed entirely of silicates,
or it may not be sufficient to accomplish this, and the
mass then consists of a mixture of lime carbonate, calcite,



388 ROCKS AND ROCK MINERALS

mixed with silicates. Thus all transitions may be found
from pure marble, through varieties containing bunches,
masses, and individual crystals of some mineral, or miner-
als, into rocks completely made up of sometimes one sili-
cate, but usually of a mixture of them. The whole affair