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

which they have afforded.

Nephelite syenites usually occur in rather small stocks
or large dikes; relatively large areas of them are known in
only a few places, Greenland, South Norway and Lapland.
They are of rare occurrence and geologically are of small



224 ROCKS AND ROCK MINERALS

importance compared with granites, gabbros and diorites.
In the United States they are found at Litchfield, Maine;
Red Hill, New Hampshire; Salem, Massachusetts; Beem-
erville, New Jersey; Magnet Cove, Arkansas; Cripple
Creek, Colorado; in western Texas and a few other local-
ities. In Canada, at Montreal; Dungannon, Ontario; Ice
River, British Columbia. Noted localities for these rocks
and their associated minerals in Europe are in South
Norway; Alno Island, Sweden, Kola Peninsula and Miask,
Ural Mountains, Russia; Foya, Portugal and Ditro, Tran-
sylvania. The rock is too uncommon to be of commercial
importance but makes an excellent building stone where
it occurs. That in the neighborhood of Magnet Cove,
Arkansas, has been thus used.

Anorthosite. This rock is composed wholly, or nearly
so, of a soda-lime feldspar, usually the variety described
as labradorite. Sometimes small quantities of a ferro-
magnesian mineral, pyroxene, is sprinkled through it in
grains and specks, or small masses of magnetite or some
other iron ore can be seen. This simple mineral com-
position makes it resemble in the hand specimen, especially
when the grain is rather fine and the colors light, both
marble and quartzite, also rocks consisting of a single
mineral. From the former it is easily told by its superior
hardness, since the feldspar cannot be scratched by the
knife, while marble is easily cut or scratched, and from the
latter by the cleavage of the crystal grains which can
usually be easily seen with a lens. While these characters
help to distinguish the rock, its identification can only be
made certain by the determination of the kind of feldspar
present ; otherwise it can merely be referred to the general
group of syenites. This can only be done in the field
when the cleavage surfaces of the feldspars are sufficiently
large to permit one to see on them the characteristic
twinning striations of plagioclase (see page 38). In
the laboratory the feldspar can be identified by blowpipe
and chemical tests.



DESCRIPTION OF IGNEOUS ROCKS 225

The color of the rock is normally white, and this is
sometimes seen, but generally it is colored yellowish to
brown, or, more commonly, some shade of gray, light
gray, blue or smoky to dark gray and almost black.
The very dark exotic color is due to an included pig-
ment, perhaps ilmenite dust, but it is notable that
where these rocks have been subjected to erogenic pres-
sure, and especially when they have been sheared and
granulated and have assumed gneissoid structure, the
dark color tends to disappear and they become lighter.
The chemical composition is practically that of a labra-
dorite feldspar (Anal. IV, page 43).

Mineralogically, the anorthosites are related to the
gabbros, for they contain the same kind of feldspar and
often, as stated above, there is more or less pyroxene; if
this latter increases in amount, passages into gabbro may
occur; and in gabbros, phases poor in pyroxene, and thus
transitional to anorthosite, are found. Geologically, how-
ever, they occur quite independently of gabbros. They
are not common rocks, so far as the number of occurrences
is concerned, but are of importance from the large and
sometimes vast masses which they form, notably in
Canada and Norway. They are found in Canada in
separate areas from the west coast of Newfoundland and
the east coast of Labrador down through Quebec into
eastern Ontario. One of these areas drained by the
Saguenay River covers nearly 6000 square miles while one
near Montreal comprises about 1000 square miles.

Another region is in the Adirondack Mountains in
northern New York state, which is in large part com-
posed of this rock. Small occurrences of a nearly related
type are found also in the White Mountains, New Hamp-
shire. It is found again in considerable masses in Minne-
sota on the Lake Superior coast.

In Europe, anorthosite occurs in Norway in large areas
on the west coast at Bergen, at Ekersund and on the
Sognfiord. It is also found in Volhynia in Russia.



226 ROCKS AND ROCK MINERALS

The labradorite of this rock sometimes shows a beautiful opal-
escent play of colors, especially a deep blue. Cleavage pieces from
the coarse and massive rock of the coast of Labrador have long been
known and cut as ornamental stones. Similar material comes from
near Zitomir in Volhynia.

Corundum Syenite. In all the different varieties of syenite
described above, instances have been found in which the rock con-
tains, in addition to the usual constituents, a notable amount of
corundum. The appearance of this mineral is due to the fact that
the magma contains more alumina than the alkalies and lime present
can turn into feldspars and feldspathoids, and this excess is forced
to crystallize out as corundum (A1 2 O 3 ), just as in granites the excess
of silica is compelled to form quartz (SiO 2 ). The mineral occurs in
crystals, either hexagonal prisms or barrel-shaped, or in grains and
lumps, and is usually of a gray color. It is easily identified by its
excessive hardness.

Such occurrences have been found in central Montana in common
syenite; in the counties of Renfrew, Hastings and others in Ontario,
Canada, in syenite and nephelite syenite; these rocks have been
traced in a belt a distance of over a hundred miles; it occurs in a
similar manner in the Ural Mountains and in Coimbatore district,
India. Anorthosites containing corundum are known from Clay
County, North Carolina; Lanark County, Ontario, and from the Ural
Mountains. Some of these occurrences, notably the ones in Canada,
are of economic value as a source of this valuable abrasive. Corun-
dum also occurs in other kinds of igneous rocks as mentioned under
dunite.

DIORITE.

Composition. The diorites are granular igneous rocks
composed of hornblende and feldspar of any kind, in
which the amount of hornblende equals or exceeds the
amount of feldspar. Usually more or less iron ore in
fine grains can be seen, and very often considerable biotite
is present in shining flakes, with sometimes a bronzy
luster. The hornblende is usually black, sometimes
dark green, and, while often in bladed or prismatic forms,
it is also often in short thick crystals or grains and some-
times in small masses of them and of biotite separated by
the light-colored feldspar. For its recognition and dis-
tinction from pyroxene see page 66. While any kind of
feldspar may be present, in the great majority of cases,



PLATE 18.




A. ORBICULAR DIORITE,
CORSICA.



B. DIORITE.




C. DIORITE, COMMON TYPE.



DESCRIPTION OF IGNEOUS ROCKS 227

as learned from microscopical studies, it is a soda-lime
variety, containing considerable lime. This latter point
however can rarely be determined on the hand specimens
because the rock is not often coarse grained enough to
permit the recognition of twinning striations on their
cleavage surfaces. It is not uncommon for some quartz
to be present and this can sometimes be identified with
the lens.

While the rocks determined as diorites by this megascopic classi-
fication will correspond in a general way with the greater part of the
diorites of the more strict classifications founded on microscopic and
chemical methods they also include some less common rocks which,
for one reason or another, have been given various names by
petrographers.

General Properties. The color of diorites is dark-gray
or greenish, running into almost black in some varieties.
It results from the color of the hornblende and the pro-
portion of this to feldspar. The different varieties are
due to the color, coarseness of grain, etc. The texture of
the rock is the granular one. The porphyritic texture,
while not unknown, is far less common than in granite.
Sometimes the black hornblende prisms are distinct
enough to produce an impression of porphyritic texture
which is dispelled as soon as one compares the average
size of the crystal grains. Orbicular structures are known
to occur. A rock from Corsica exhibiting it has been used
somewhat as an ornamental stone; it is illustrated on
Plate 18. Miarolitic cavities occur as in granite; they are
often masked by being filled with calcite. Pegmatite
dikes also occur and the minerals are somewhat different
from those in the granites. Fluidal or somewhat parallel
arrangements of the component minerals are not uncom-
monly seen, and these produce tendencies to gneissoid
structure. Diorites are also frequently cut by com-
plementary dikes, of much the same general appearance
as those in granites, or these are found in their immediate
neighborhood in dikes and sheets. Thus they may be



228



ROCKS AND ROCK MINERALS



traversed by light-colored aplites and felsites and by
dark, heavy, basaltic traps.

Their jointing is like that described for granites.

Chemical Composition. This varies considerably with
the relative amounts of feldspar and hornblende, with
the particular varieties of these two which are present, and
is also somewhat influenced by the accessory minerals
which may occur. The following table illustrates this
and it shows also how the increase of lime, iron and
magnesia over -the proportions of these oxides in granites
and syenites, causes the increase in the amount of horn-
blende.

ANALYSES OF DIORITES.





SiO 2


A1 2 3


Fe 2 3


FeO


MgO


CaO


Na 2 O


K 2


H 2 O


XyO


Total.


I


55.1


20.2


1.5


4.3


1.8


7.0


4.3


2.8


1.1


1.7


99.8


II


58.0


18.0


2.5


4.6


3.6


6.2


3.6


2.1


0.9


1.2


100.7


III


47.1


18.1


3.0


8.5


7.3


6.6


2.4


2.8


3.6


0.5


99.9


IV


43.9


16.2


4.0


10.1


5.1


9.6


2.9


1.5


1.6


4.9


99.8



I, Diorite, Little Belt Mountains, Montana; II, Electric Peak,
Yellowstone Park; III, Malvern Hills, England; IV, Belknap Moun-
tains, New Hampshire. XyO = Small quantities of various oxides,
TiO 2 , MnO, etc.

Occurrence, Uses, Etc. While diorites in many places
are found as independent intrusions, they are also very
apt to be, on the one hand, connected with granites, on
the other with gabbros, and usually these pass into each
other. They also do not form such vast batholiths or
stocks as the granites or the gabbros, and, especially as
independent masses, are more liable to be found as small
stocks, large dikes, etc. They have a very wide dis-
tribution and are found in all parts of the world where
the older deeply-seated igneous rocks are laid bare by
continued erosion. In the later formed mountain regions
they are also found as stocks and dikes. What has been
said of granites in this respect would be largely true of



DESCRIPTION OF IGNEOUS ROCKS 229

diorite. Owing to its dark color diorite is not so exten-
sively used for architectural purposes as granite, though,
so far as strength, durability and capacity for receiving a
high polish is concerned, it would furnish in many places
excellent material. It is somewhat heavier than granite,
its specific gravity ranging from 2.8-3.1; at 3.0 a cubic foot
of it would weigh about 187 pounds.

Relation to Other Rocks. As mentioned under its
description as a rock mineral, pyroxene through meta-
morphic processes changes into hornblende. Generally
this is accompanied in the rock by the production of
schistosity as described under metamorphism and horn-
blende schist. The description of uralite should also be
consulted. It sometimes happens that this change takes
place in gabbro without causing the rock to lose its
massive character or becoming schistose. In this case,
if of sufficiently coarse grain to permit the recognition of
the hornblende and feldspar, it would be classed as a
diorite. If it can be proved that a diorite has been
derived from gabbro, it may well be termed a metadiorite
to indicate its secondary origin. Usually, however, the
grain of such rocks is quite fine, too much so to permit the
individual hornblende prisms to be definitely determined,
and the rock would be classed under dolerite. They are
very apt to have a green color and for this reason have
been called greenstones. The green color is partly due
to hornblende, partly to chlorite. These rocks are
further mentioned under dolerite.

GABBRO.

Composition. The gabbros are granular igneous rocks
consisting chiefly of pyroxene and feldspar of any kind, in
which the amount of pyroxene equals or exceeds that of
the feldspar. Usually more or less iron ore in black
metallic-looking grains can be seen, and in some varieties
considerable olivine may occur. This latter can some-
times be detected with the lens as yellowish or green



230 ROCKS AND ROCK MINERALS

grains. Careful inspection will often show occasional
bronzy flakes of biotite. Of the two chief minerals the
pyroxene is usually dark greenish when examined with
the lens, often black to the eye alone, and sometimes it is
of the variety diallage with a pronounced apparent
cleavage in one direction, of a gray-green color and often
almost micaceous appearance, at times somewhat brassy
or semi-metallic in luster. A test with the knife point for
cleavage shows at once its non-micaceous character. The
feldspar in the great majority of cases is a soda-lime
variety, generally labradorite, as may often be seen by
the twinning striations on a cleavage surface. It is
usually in formless masses or grains like the other minerals,
but not unfrequently it has a tabular or book-like form
and the sections on the rock face have an elongated shape.
In this case the striations run parallel with the elongation.
Sometimes the feldspar is fresh and glassy; in this case the
two feldspar cleavages are good and the striations if
visible are distinct; sometimes the feldspar is waxy in
appearance, of a glimmering luster to dull, often with a
bluish tone; in this case the cleavage is poor or even
apparently wanting and striations cannot be seen. In
the latter case the feldspar is more or less affected by
alteration to other minerals as described on page 44.

A distinction is made by petrographers by which gabbros are
divided into two groups, depending on the variety of pyroxene
present. If this is the monoclinic, lime-bearing augite or diallage,
the rock is called gabbro, if it is the orthorhombic hypersthene which
is without lime the rock is called norite. This distinction cannot
be made in megascopic determinations unless some of the pyroxene
is extracted from the rock and tested chemically, hence the norites
are here included under gabbro. A rarer type consists of plagioclase
and olivine without pyroxene and is called Troctolite. Some rare
rocks with alkalic feldspar are also here included under gabbro which
are variously classified and named by petrographers.

General Properties. The color of gabbros is usually
dark, dark gray or greenish to black; very rarely reddish.
In some varieties in which diallage is the kind of pyroxene



DESCRIPTION OF IGNEOUS ROCKS



231



present and the grain is moderately coarse the rock is
much lighter in tone and of a medium gray or greenish-
gray. The same is true in many cases where the rock is
more or less altered; compare with what is said of the
feldspars above. The texture is granitoid or granular,
sometimes with a porphyritic tendency from the elonga-
tion of the feldspars, but true porphyritic texture is very
rare. Miarolitic cavities are much less frequent than in
granite and syenite. Orbicular gabbro has been found
in California. A fluidal or banded structure which is
produced by drawn-out layers of varying composition and
which simulates a gneissoid structure has been described
from several localities, from the Hebrides, California and
Minnesota. Pegmatites are also occasionally found in
gabbros; they consist of the usual minerals of the rock.
In South Norway the pneumatolytic processes attending
the intrusion of gabbros have formed much scapolite and
other minerals in the gabbro at its border and in dikes in
the contact zone; of these minerals apatite is the most
prominent and occurs sometimes in large masses. Com-
plementary dikes, etc., occur in gabbro masses but are not
perhaps so notable a feature as in the foregoing groups.
In this connection what is said concerning peridotites may
be consulted.

ANALYSES OF GABBRO.





SiO 2


A1 2 3


Fe 2 3


FeO


MgO


CaO


Na 2 O


KjO


H 2


XyO


Total.


I


47.9


18.9


1.4


10.5


7.1


8.4


2.7


0.8


0.6


1.7


100.0


II


48.9


8.8


1.0


9.5


15.2


14.7


0.6


0.1


0.6


0.7


100.1


III


49.9


18.5


2.1


8.4


5.8


9.7


2.6


0.7


1.0


1.4


100.1


TV


52.8


17.8


1.2


4.8


4.8


12.9


3.0


0.5


1.2


0.5


99.5


V


40.2


9.5


9.7


12.2


8.0


13.1


0.8


0.2


0.5


5.5


99.7



I, Adirondacks, New York State; II, Orange Grove, Maryland;
III, Pigeon Point, Minnesota; IV, Band rich in feldspar, poor in
pyroxene, Isle of Skye, Hebrides. V, Band rich in pyroxene, poor
in feldspar, Isle of Skye. XyO = small quantities of various oxides,
chiefly TiO 2 .



232 ROCKS AND ROCK MINERALS

Chemical Composition. The gabbros, as a rule, contain
larger amounts of lime, iron and magnesia, and less of
silica and alkalies than any of the previously described
rocks as may be seen from the table annexed.

Analyses IV and V show how the chemical composition
of the banded gabbros varies in the different streaks with
corresponding variation in mineral contents.

Occurrence. Gabbros are widely distributed and com-
mon rocks. They are found as large stocks and bath-
yliths and in dikes in the older rock complexes, similarly to
granite. They are also found as stocks and necks of old
volcanoes cutting the stratified beds of the younger moun-
tain regions. In these they may also be found as thick
intrusive sheets. Gabbros have been held to occur also
as forming the central portion of thick extrusive sheets,
as in the Hebrides, in Sweden and in the Lake Superior
region. If this is the case it is due to the low freezing
point of the magma, its liquidity and ready crystallization.

In the United States, gabbros are found in many places
in New England, as in the White Mountains. They are
found in the Adirondacks and at Cortlandt on the Hudson
River in New York State, in Maryland, etc.

They occur in the Lake Superior region and elsewhere
in Minnesota and in various places in the Rocky Moun-
tains and in California. They are extensively distributed
in Europe, in southern England, in northern Scotland,
especially on the islands of the Hebrides, in Norway and
Sweden and in Germany. They are in fact almost as
widely known as granites though they do not form, as a
rule, such large masses.

Alteration of Gabbro. It is common to find that where
gabbro massives occur in the older rock complexes and in
folded mountain ranges that they are surrounded by a
mantle of hornblende schist into which the gabbro grad-
ually passes, by transitional phases. The origin of this
is the pressure, shearing and other metamorphic agencies
brought about by the orogenic processes, as mentioned



DESCRIPTION OF IGNEOUS ROCKS 233

under metamorphism, hornblende, diorite, etc., which
have acted upon the pyroxene of the gabbro converting
it into hornblende and producing the schistose structure.
It may happen through pressure and shearing that a
schistose or, perhaps better, a gneissoid structure may be
induced in the gabbro without change of the pyroxene
to hornblende and we would have in this case a gabbro-
gneiss or gabbro-schist produced, but generally the change
to hornblende occurs. If olivine is present it also forms
amphibole. Very often garnet appears as a new mineral
resulting from the process. While the change to horn-
blende is usually accompanied by the assumption of a
more or less pronounced gneissoid or schistose structure,
this is not always the case; the rock sometimes retains a
massive granular character and, if its constituent feldspar
and hornblende can be recognized, it would be classed as a
diorite, as mentioned under that rock. In another mode
of alteration of gabbros the feldspar is changed into a
substance called saussurite, which was formerly thought
to be a distinct mineral, but which the microscope has
shown to be a mixture of albite, zoisite and other minerals.
The feldspar, or rather that which replaces it, has no
cleavage and is waxy looking. The pyroxene is changed
to hornblende, which tends to have a bright to grass-
green color and is the variety called smaragdite. Other
minerals are also formed, but megascopically the waxy-
looking saussurite and green hornblende predominate.
This may take place without formation of schistose
structure and it seems probable that in this case the
alteration is due more to the chemical and less to the
dynamic agencies of metamorphism. Such rocks have
been called saussurite-gabbro.

In the process of weathering through the agencies of
the atmosphere, gabbros give rise to clay soils deeply
colored by the oxides of iron and mingled with fragments
of still undecomposed minerals.

Iron and Other Ore Deposits. There are frequently



234 ROCKS AND ROCK MINERALS

found in large gabbro intrusions masses of iron ore,
sometimes consisting of magnetite, but generally of
ilmenite or mixtures of the two. Usually these are more
or less mingled with the minerals of the gabbro itself,
especially pyroxene and olivine. The character of the
occurrences, their lack of definite form and the manner in
which they gradually shade into the normal gabbro, show
that they are only a phase of the rock in which the iron
ore, usually scattered through it in small grains, is here
locally concentrated in great abundance. Such ore de-
posits are sometimes found at the border of the intrusion,
though often scattered in masses through it or at the
center. They are known in many places, in the Adiron-
dacks, in northern Minnesota, in Canada, Norway, Sweden
and elsewhere. If titaniferous iron ores could be success-
fully smelted, such deposits would undoubtedly be in
many cases of great value.

In other cases sulphide ores are developed in gaboro
rocks in a similar manner. This is especially true of the
sulphide of iron called pyrrhotite, which is often nickel-
bearing and hence of great value as a source of this useful
metal. In some places these deposits are accompanied
by valuable amounts of copper in the form of chalcopyrite,
copper-iron-pyrites, and it has been remarked that as the
percentage of copper rises that of nickel falls. Such
deposits in gabbros, or in rocks derived from them, are
known and have been worked in Norway and Sweden, in
Lancaster County, Penn., and at Sudbury, Ontario.

The origin of this kind of ore deposit in an igneous rock
has been described on page 170.

Use of Gabbro. The gabbros are well suited for con-
structional work and architecture, but as a rule have not
been extensively used, probably very largely on account
of their dark color. In Sweden they have received con-
siderable attention for monumental and other uses. In
the United States they have been used for building in the
Lake Superior region, as at Duluth, and quarries of them at



DESCRIPTION OF IGNEOUS ROCKS 235

Keeseville in the Adirondacks and in Vergennes, Vermont,
have been worked. They take a high polish, are suf-
ficiently durable and much easier to work than granite.

DOLERITE.

Definition and Minerals. The dolerites, as already
explained in the section on classification, comprise those
forms of diorite and gabbro in which, generally on ac-
count of increasing fineness of grain, the hornblende
and pyroxene cannot be safely determined or distin-
guished from one another, although the eye or lens
clearly sees that the rock is composed of feldspar min-
gled with an equal or greater amount of a ferromagnesian
mineral.

This term as here used comprises not only the finer-grained
diorites and gabbros but much also of what is termed " diabase "
by the petrographers, as well as occasional rare rocks which need no
mention here.

The feldspar, which is seen in larger or smaller grains
and sometimes in more or less extended lath-shaped
sections, is known from microscopic studies to be chiefly
a soda-lime variety, though alkalic ones are also present to
some extent and in some cases may replace the former;