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

these distinctions cannot be made megascopically, and the
plagioclase twinning can very rarely be seen on a cleavage
face with the lens. The ferromagnesian minerals are in
dark grains, perhaps short columnar; their cleavage sur-
faces can usually be seen but it cannot be said whether
they are hornblende or pyroxene or a mixture of both.
Sometimes olivine is also present and if its yellow-green
grains can be detected it is very probable that pyroxene
is the chief ferromagnesian mineral and not hornblende.
In addition the lens will often show bronzy-looking
flakes of biotite and metallic steel-like specks of iron
oxide (magnetite or ilmenite) or sometimes brass-like
crystals or grains of pyrite.



236 ROCKS AND ROCK MINERALS

Color. Owing to the equal or predominant amount of
ferromagnesian minerals, the color of these rocks is dark,
medium or dark gray or greenish to black. As in most
rocks the tone of color is best observed in viewing the
rock at a little distance, so that the individual grains
become indistinguishable and only their mass effect is
seen.

General Properties. The texture of these rocks is granu-
lar to fine granular, they are sometimes porphyritic but
these cases are described in the following section on the
porphyries. Their chemical composition, in the great
majority of cases, is similar to that of the diorites and
gabbros already given and need not be repeated. They
are heavy, the specific gravity being from 3.0-3.3. Their
jointing is usually small cuboidal, wedge shaped or platy,
often columnar and sometimes on a very large scale,
though generally this structure is not so perfect as in the
finer-grained basalts. It is most apt to occur in dikes,
very thick intrusive sheets and in massive extrusive flows.

Occurrence. The dolerites do not occur in large stocks
and bathyliths like the diorites and gabbros, though not
infrequently these latter rocks pass into an endomorphic
phase of dolerite at the margin of the intrusion. As in-
trusives they belong in the minor class, being found in
dikes, small laccoliths and intrusive sheets, the latter often
of great thickness, and in thick massive lava flows whose
cooling has been slow.

In the eastern United States the most conspicuous
examples are found in the intrusions and flows of " trap "
of the Triassic formations stretching from Nova Scotia to
Georgia. Through faulting and erosion they now give
rise to definite topographic features, such as the ridges in
Connecticut and the Palisades opposite New York City.
Similar masses of these rocks are found in the Lake Superior
region and in the great lava flows of the western United
States. In all these occurrences they are associated with,
and pass into, the denser forms of basalt. These larger



DESCRIPTION OF IGNEOUS ROCKS 237

occurrences of dolerite mostly contain pyroxene as the
dominant ferromagnesian mineral and are largely the
rock called " diabase " by petrographers, while cases where
hornblende is dominant are mostly confined to dikes and
smaller intrusions, especially in the older rocks.

Dolerites are also very common rocks in Great Britain
in various localities, in dikes and intrusive sheets, and
especially in the north of Scotland and Ireland where they
are often extrusive and associated with denser basalts.
They are in fact very common rocks in all parts of the
world.

Relation to Other Rocks Alteration. From what has
been said it is easy to see that the dolerites are a class
of rocks based largely on convenience. On the one hand
they form a transition group, based on texture, between the
diorites and gabbros and the dense basalts, and on the other
they cannot depend wholly on texture, because relatively
coarse-grained rocks may occur in which one cannot dis-
tinguish between hornblende and pyroxene and which
must therefore be placed in this class.

The case might occur in which, instead of hornblende or pyroxene,
biotite was the dominant mineral associated with the feldspar.
Such rocks are not very common but sometimes occur, especially in
dikes and sheets and with quite fine grain. They form the rocks
called mica trap or minette, mentioned later under basalt.

The pyroxenic members of this group, by regional
metamorphism, become converted into hornblende rocks,
generally into hornblende schists, and both varieties by
alteration may produce chlorite and pass into the so-called
" greenstones." These alterations are quite similar to
what has been described under gabbro. By weathering
they become brownish and discolored and ultimately
yield brown ferrugineous soils.

Uses. The rocks of this group are too dark and somber
for general use in fine architectural or interior work,
except for monumental purposes. The " trap " of the



238 ROCKS AND ROCK MINERALS

eastern states has been considerably employed in rough
masonry, and where good natural joint faces can be used
for wall surfaces, the brown weathering color gives pleas-
ing effects. The toughness of the material, which the
traps afford, has however caused it to be considerably
used for block paving and the crushed stone for road
making.

PERIDOTITE PYROXENITE.

Composition. Under this group are comprised all of
those granular igneous rocks composed of ferromagnesian
minerals alone, or in which the amount of detectible feld-
spar is so small as to be entirely negligible as a component,
and in which the mineral grains are sufficiently large to be
determined. The chief minerals which form these rocks
are olivine, pyroxene of both the augite and hypersthene
varieties, and hornblende. These may occur alone or in
various mixtures, and according to these the group has
been sub-divided into types, some of the more prominent
of which are as follows:

Pyroxenes and Olivine Peridotite.

Hornblende and Olivine Cortlandtite.

Olivine alone Dunite.

Pyroxenes alone Pyroxenite.

Hornblende alone Hornblendite.

The first three, which contain olivine, are comprised
under the general name of peridotites, from peridot, the
French word for olivine. But all the different types,
while they sometimes occur independently, also occur
together, with transition forms grading into one another,
and it is difficult, and sometimes impossible, to distinguish
them megascopically and therefore they are best treated
together as one general group and not as separate rocks.

Beside the minerals mentioned, a brown biotite some-
times occurs in these rocks, giving rise to the variety called
mica peridotite. Additional accessory minerals, some of
which are common and some confined to certain occur-



DESCRIPTION OF IGNEOUS ROCKS



239



rences, are titanic iron ore, spinels, of which chromite is of
importance, and garnet.

Texture. The texture is granitoid or granular; its
appearance depends somewhat on the minerals present
and their arrangement. When pyroxene or hornblende
is the dominant mineral the grain is often very coarse and
may exhibit large cleavage surfaces. Dunite is not apt
to be coarse grained; it commonly has a sugar-granular
texture like many aplites, sandstones, marbles, etc.
Porphyritic texture is rare or wanting. A common
texture is one in which the cleavage surfaces of the
pyroxenes or hornblendes are seen to be spotted with
grains of olivine included in the larger crystal. Such a
spotting of the shining cleavage surfaces of one mineral
by smaller included crystals of another, which have no
crystal orientation, either with respect to one another or to
their host, is called luster mottling and is known as the
poikilitic texture. It is sometimes well exhibited in these
rocks. The included crystals are of course older than
their host.

Chemical Composition. This varies according to the
minerals in the rocks but general characters are the very
low silica, the small amount or virtual absence of alkalies
and alumina, and the large quantities of iron and magnesia.

ANALYSES OF PERIDOTITES. ETC.





SiO,


AljO,


Fe 2 3


FeO


MgO


CaO


NajO


KjO


H,0


XyO


Total.


I
II
III

TV


40.1
43.9
53.2
46 4


7.8
1.6
1.9
10 8


7.3
8.9
1.4
5.9


8.6
2.6
7.9
5.6


23.7
27.3
20.8
22.2


6.5
6.3
13.1
3 7


1.2
0.5
0.1
0.3


0.5

b.i

1 fl


4.0

8.7
1.0
3.8


0.6
0.3
0.5


100.3
100.1
100.0
100.1


V


38.4


0.3


3.4


6.7


45.2


0.4


0.1




4.3


1.4


100.2



I, Peridotite, Devonshire, England; II, Peridotite, Baltimore
County, Maryland; III, Pyroxenite, Oakwood, Maryland; IV, Horn-
blendite, Valbonne, Pyrenees; V, Dunite, Tulameen River, British
Columbia. XyO = small quantities of other oxides.



240 ROCKS AND ROCK MINERALS

Color. The color of these rocks ordinarily varies from
dull green to black. The dunites, which are practically
composed of the one mineral olivine, are at times much
lighter. They may show various shades of light green,
medium yellow and light brown, passing into one another,
and from these through dull yellowish green into dark
green. They may thus be exceptions to the general rule
that ferromagnesian rocks are dark colored.

Occurrence. Relation to Gabbro. The peridotites
and allied rocks sometimes occur independently as dikes,
sheets, laccoliths or small intrusive stocks. In this way,
as small isolated occurrences they have been found cutting
the Paleozoic rocks, usually in a more or less altered con-
dition, at Syracuse and other localities in New York State,
in Kentucky, in Arkansas and elsewhere. But generally
speaking they are most liable to occur in connection with
greater intrusions of gabbros. Sometimes they form phases
of the gabbro mass, with transitions between the two; some-
times they cut the gabbros in dikes or are found in small
intrusions in their neighborhood. This dependence upon
the gabbros has led to their being held in such cases as
products of differentiation of the gabbro magma in which
they represent the lamprophyres of other rock groups.
In this way a great number of occurrences are known
in all parts of the world where gabbros are common
rocks.

Dunites occur in masses intrusive in the gneisses of
western North and South Carolina and Georgia. Asso-
ciated with them are smaller amounts of other peridotites
and pyroxenite. These occurrences are of importance on
account of the deposits of corundum of commercial value
associated with them. The mineral is thought to have
formed in them in the same manner as described under
syenite. Dunite also occurs in considerable masses in New
Zealand, especially in the Dun Mountains, from which came
the name. Pyroxenite and hornblendite are compara-
tively rare and of relatively small geologic importance.



DESCRIPTION OF IGNEOUS ROCKS 241

Alteration. Serpentine. The peridotites are extremely
liable to alteration, so much so that unchanged occurrences
are not at all usual. The most common form of alteration
is that in which the olivine and other magnesian silicates
are changed to serpentine.

This is illustrated by the following reactions:

Olivine Enstatite Water Serpentine

Mg 2 SiO 4 + MgSiO 3 + 2 H 2 O = H 4 M g3 Si 2 O 9

2 Mg 2 SiO 4 + CO 2 +2 H 2 O = H 4 M g3 Si.A + MgCO 3

3 MgSiO 3 + 2 H 2 O = H 4 M g3 Si 2 O 9 + SiO 2 .

Other magnesian minerals such as talc are also formed
by the alteration of these rocks, but that to serpentine
is the most important. All stages of transition to pure
serpentine occur, and studies which have been made in
recent years show that a large part, perhaps the greater
part, of the occurrences of this mineral are to be assigned
to the alteration of rocks of this group.

The peridotites ultimately weather down into brown
ferrugineous soils which, on account of their lack of potash,
do not favor vegetable growth and are therefore barren.

As a Source of Valuable Minerals. The magmas which
form the peridotites usually carry small amounts of
chromic oxide which often crystallizes with iron oxide to
form the mineral chromite, FeCr 2 O 4 , one of the spinel
group. It is often seen in dunite and usually forms small,
black, pitchy-looking grains. Sometimes this mineral is
concentrated in sufficient amount so that it becomes a
useful ore, supplying the chromium used in the arts.

The olivine of these rocks has been found by analysis to
contain a minute amount of nickel oxide; when they
change to serpentine it sometimes happens that this
nickel is concentrated in the form of nickel silicate, some-
times in amounts sufficient to form deposits of value as a
source of this metal, as in Douglas County, Oregon, and
in the Island of New Caledonia.

The peridotites, and to some extent their allies the



242 ROCKS AND ROCK MINERALS

gabbros, are also the source of platinum, which occurs in
them as the native metal or as sperrylite PtAs2j by the
decay of the rock it is washed down and, like gold, concen-
trated in alluvial deposits. The precious garnet, pyrope,
used as a gem, also comes from a decayed and serpentinized
peridotite from Bohemia, South Africa, etc. Lastly, the
diamonds of South Africa have their source in decayed
and greatly altered peridotite rocks. This altered rock,
which was originally a mica peridotite, is known as kim-
berlite, by the miners as " blue ground." Some have held
that the carbon forming the diamonds was derived from
the shales through which the magma passed, others hold
that it was original in the magma and that the diamond
is a true crystalline constituent of the igneous rock like
any other of its accessory minerals.

PORPHYRIES.

Definition. As explained in the former section treating
of the classification of porphyries, these rocks may be
divided into two main groups; one in which, on account of
its coarse texture, not only the phenocrysts but the grains
of the groundmass can be determined or the determinable
phenocrysts form so large a proportion of the rock that a
good idea of its mineral composition can be obtained and
the small amount of dense groundmass may be neglected,
and a second group in which the amount of dense ground-
mass is large and the phenocrysts are not abundant
enough to determine safely the mineral character of the
rock. It is the first of these two groups which is described
in this section, the one which we may call the group of
determinable porphyries; the second group will be con-
sidered later in connection with the dense igneous rocks
the felsites and basalts of which they form a por-
phyritic variety.

In this first group, porphyries are mainly confined to the
feldspathic division of the igneous rocks, apparently for
the reason that the magmas which furnish the ferro-



PLATE 19.




A. GRANITE-PORPHYRY, MONTANA.




B. SYENITE-PORPHYRY,
MONTANA.



C. SYENITE-PORPHYRY,
NORWAY.



DESCRIPTION OF IGNEOUS ROCKS 243

magnesian rocks have relatively so low a freezing point
and crystallize so readily that they are not apt to form
porphyries under conditions where the feldspathic rocks
often do so readily. Thus granite porphyry is very com-
mon, while gabbro and peridotite porphyries are so rare
as to be of no practical importance. In the group of
dense igneous rocks porphyries of both divisions are com-
mon. The rocks to be treated then are granite porphyry,
syenite porphyry and diorite or dolerite porphyry. There
are so many points in which they are similar that they
are best treated as a group.

Granite Porphyry. This consists of distinct pheno-
crysts of quartz and of feldspar in a granular groundmass
of the same minerals whose grains can be determined as
such, or one in which the abundance of the phenocrysts of
quartz and feldspar give a distinct granite-like character
to the rock and make the dense groundmass of less impor-
tance. Sometimes the rock consists of these minerals
alone, or very nearly so, and sometimes biotite and horn-
blende are present, perhaps in considerable amount. The
biotite and hornblende may be present separately or
together, though hornblende alone is rare. They may
occur as distinct phenocrysts, usually smaller than the
quartz and especially the feldspar, and also in the ground-
mass, in which case the tiny specks of biotite are most
easily detected.

When the groundmass is so coarse as to be equivalent to an
ordinary granite it is customary to speak of the rock as porphyritic
granite, as explained under granite.

Syenite Porphyry. This rock consists of distinct pheno-
crysts of feldspar in a groundmass, which, if determinable,
must be made up mainly of grains of feldspar and with
very little or no quartz. If the groundmass is not deter-
minable the amount of phenocrysts must be large enough
to give the rock a distinctly syenitic character. The
ferromagnesian minerals, biotite, hornblende and pyroxene,



244 ROCKS AND ROCK MINERALS

while they may be absent or practically so, are usually
present, either as phenocrysts, or in the groundmass, or
both. They may occur in considerable amount, but must
not equal or exceed the total amount of feldspar, or the
rock becomes a diorite porphyry. They occur separately
and together, but the combination of all three or of
biotite and pyroxene is not so common as biotite and
hornblende.

The rock defined above is that which corresponds to the common
one of the three varieties of syenite described on page 219 and
following, and represents it in porphyritic development. Anortho-
site porphyry is unknown. Nephelite syenite porphyry is known
but is a very rare rock.

In more' exact classification based on microscopic research a
distinction is made as to whether the feldspars are chiefly alkalic
or mainly soda-lime feldspars, both phenocrysts and groundmass
being considered together. In the latter case petrographers term
the rock diorite porphyry and only apply the term of syenite por-
phyry where they are mainly alkalic. So far as the groundmass is
concerned this distinction cannot be made by megascopic exam-
ination and but rarely, as described later, with the phenocrysts.
Hence, just as in the case of syenite, both kinds are classed here
together.

Diorite and Dolerite Porphyry. Diorite porphyry would
be composed of phenocrysts of hornblende and feldspar,
either separately or together, in a determinable ground-
mass of the same minerals, or if the groundmass is not
determinable the diorite character must be clearly shown
by the great abundance of the hornblende and feldspar
phenocrysts. Also the total amount of hornblende must
equal or exceed that of the feldspar. Some biotite may
also be present, as well as iron ore grains.

Such rocks occur and it may be possible at times to
determine them megascopically, but in the great majority
of instances it will be found that, while the hornblende
which is present in phenocrysts may be recognized, that
which is present in the groundmass cannot. It can often
be seen in these cases that the groundmass is composed of



DESCRIPTION OF IGNEOUS ROCKS 245

feldspar and a ferromagnesian mineral, either hornblende
or pyroxene, but megascopically it is impossible to say
which. In fact such groundmasses correspond to the
definition and description of dolerite previously given and
such rocks therefore are most conveniently called dolerite
porphyry. The phenocrysts are either feldspar, horn-
blende, or pyroxene, or mixtures of them. The feldspar in
these rocks is generally a variety of the soda-lime group,
usually labradorite. By increase in the amount and
density of the groundmass they pass insensibly into the
basalt porphyries, or melaphyres, described later.

Phenocrysts of Porphyries. As the phenocrysts of
porphyries have crystallized freely in the fluid magmas
they generally show distinct crystal shapes, such as are
described in the foregoing part devoted to the rock
minerals. A few words in regard to their crystal habits
may be added here. Quartz, as a phenocryst, tends to
take the form shown in Fig. 43, but is usually spherical;
the crystals may be a half inch in diameter but are usually
much smaller, the size of coarse shot or peas; it is usually
smoky in color. The feldspars tend to assume the forms
shown by Figs. 5-7; they are often twins, Fig. 8; they
are white, pink to red, or yellowish and gray; if feldspars
of two colors are present and one of them is a reddish
tone it is probably orthoclase, the other albite or a soda-
lime feldspar. They not infrequently form very large
phenocrysts, an inch or even more in length; the model-
like feldspars seen in mineral cabinets often are the pheno-
crysts obtained from porphyries. Hornblende occurs in
dark greenish or black prisms, usually elongated, and some-
times quite slender and with glittering cleavage surfaces
if fresh; the terminal faces are poor or wanting; some-
times it is weathered out and only a rusty mass left in
its place. Pyroxene is also dark green to black, in short,
stout prisms, and commonly its cleavage and crystal faces
lack the luster of hornblende. The method of distil
guishing them has been already explained. Rusty spots



246 ROCKS AND ROCK MINERALS

also show the former presence of pyroxene, but less com-
monly than hornblende. In size both are apt to be small,
compared with feldspar. Biotite as a phenocryst, is in six-
sided tablets with fine basal cleavage, black to bronze-
brown in color. In these rocks its crystals are apt to be
small.

General Properties. The chemical composition of these
porphyries is similar to that of the corresponding kinds of
granular rocks previously given and need not be repeated.
Their specific gravity and modes of alteration and conver-
sion into soil are the same. The jointing depends largely
on the mode of occurrence ; it is apt to be platy or small
cuboidal, or to form small parallelopipedons with acute
angles, in the feldspathic porphyries of dikes and sheets,
and larger blocks in the greater intrusions; the doleritic
porphyries tend to columnar jointing.

Occurrence. The porphyries of this class are commonly
found in the minor intrusions; in dikes, intrusive sheets
and laccoliths, sometimes in volcanic necks. They are
also not uncommon as marginal phases of intrusive stocks
and bathyliths of granite, syenite, etc.; they here represent
an endomorphic contact modification and in traversing
areas of such rocks, if it is observed that they are becom-
ing porphyries with finer grain, approach to the contact
should be suspected. They may also occur in extrusive
lava flows, especially if these are very thick and massive,
but in this mode of occurrence they are generally replaced
by the denser felsite and basalt porphyries described
beyond.

These rocks are far too common to give any list of
localities; they are everywhere found where erosion has
exposed the older crystalline rocks and where igneous
activity, has displayed itself. Where larger stocks and
intrusions have occurred they are especially apt to be
present, sometimes cutting them as dikes, sometimes
extending from them in apophyses, and sometimes in
dikes, sheets, etc., as satellites grouped about them.



DESCRIPTION OF IGNEOUS ROCKS 247

Perhaps the most notable instances of the occurrence
of these rocks are to be found in the great laccoliths of the
Rocky Mountains' region, in Colorado, Utah, Wyoming
and Montana, which are generally composed of granite or
syenite porphyries. Some of these masses are 'a mile in
thickness by several in breadth, though often smaller. In
these regions they often form powerful intruded sheets,
several hundred feet in thickness. It is in the contact
zones of these intrusions, especially with limestone, that
a large proportion of the valuable ore deposits, such as the
silver-lead ones, which have made these regions famous
for their mining industries, are found. Thus to the
western miner the word " porphyry " is always of sugges-
tive significance.

Dense Igneous Rocks.

In the preceding groups of igneous rocks it is assumed
that all the component grains of the rock, or those forming
the greater part of it, can be determined and the mineral
constitution safely established. In the present group it
is assumed that the texture of all of the rock, or of the
greater part of it, is so dense that this cannot be done.
No definite line can be drawn between the two groups; in
many cases, whether a given rock should belong to the one
or the other, is largely a matter of opinion, dependent
upon the experience of the observer, his knowledge of
rocks and minerals, his power of observation, keenness of
eyesight, and the excellence of his lens. In this respect
we are also limited by our size ; if we were ants, instead of
men, who were studying rocks, it is probable that few
would be placed in this group.

As has already been explained, under the section treating