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

mineral grains cannot be determined, we have little with
which to classify them for field purposes except the
color. They are thus divided into two groups, the light-
colored and the dark-colored. Of course if the rock is either
white or black there can be no difficulty in assigning it to
one or the other of these two divisions, but all gradations
of color exist and it is often a matter of pure choice to
which a particular rock should belong. Evidently some
closer definition of the terms is needed. We may do this
as follows. The term dark includes rocks that are very dark
gray, very dark green or black; all other colors, white, red,
purple, yellow, brown, light and medium gray, light and
medium green are light. The latter are known under the
name of felsite, while the former or dark rocks are basalt.
The division thus made also expresses in a general way an
important fact concerning their composition, for the
former are derived from magmas, which, under different.



200 ROCKS AND ROCK MINERALS

physical conditions producing coarser-grained rocks,
would crystallize as granites and syenites. On the other
hand, the basalts represent the diorites, gabbros and
peridotites in dense or fine-textured forms. While many
exceptions will be found, this general rule holds true and
the light rocks as defined above are chiefly feldspathic,
the dark are mainly ferromagnesian.

While the rocks of this group are often of homogeneous
texture and aspect, they are also very often porphyritic.
If the amount or bulk of phenocrysts in relation to the
fine or dense (aphanitic) groundmass is very large, say
half the mass of the rock or more, such porphyries pass
back into class A, of grained rocks as previously explained.
But if the amount of phenocrysts is less to much less than
the groundmass then we have felsite porphyry and basalt
porphyry respectively, according to the color of the ground-
mass. It has also been suggested that they may be
called leucophyre (light-colored porphyry) and melaphyre
(dark-colored porphyry), respectively.* Further sub-
divisions of these porphyries can be made according to
mineral character of the prominent phenocrysts. Thus
we might have quartz- felsite-porphyry; feldspar- felsite-
porphyry; hornblende-felsite-porphyry or quartz-, feldspar-
and hornblende-leucophyre, and similarly we have augite-
basalt-porphyry, mica-basalt-porphyry, feldspar-basalt-por-
phyry or augite-, mica- and feldspar-melaphyre. Many
combinations of this kind can be made but the above will
suffice as examples. The rocks of this class are some-
times intrusive, sometimes extrusive.

Subdivisions of Class C. The rocks of the third class,
C, those wholly or partly of glass, are distinguished by
their glassy or resinous luster and want of stony texture.
They may be classified as follows:

OBSIDIAN, luster strong, bright, glassy; color usually
black, sometimes red, more rarely brown or
greenish.

* Quantitative Classification of Igneous Rocks, p. 184.



GENERAL PETROLOGY OF IGNEOUS ROCKS 201

PITCHSTONE, luster resinous or pitch-like; colors

various, as above, but black less common.
PERLITE, glassy rock with perlitic structure, produced

by small spheroidal fractures; usually gray in

color.
PUMICE, highly vesicular glass (see page 158), usually

white or very light-colored.

Any of these may be porphyritic or not; though cases
of porphyritic pumice are much less common than in the
other three. When porphyritic a general name for them
is vitrophyre (glass porphyry) and different varieties may
be distinguished, as in the porphyries of the class above,
according to the kind of predominant phenocrysts; thus
quartz-vitrophyre, feldspar-vitrophyre, etc. The rocks of this
class are practically wholly confined to extrusive lavas.

Class D. In addition to the three main classes of
igneous rocks described above we may add as an appendix
in a fourth class, D, the fragmental material thrown out
in volcanic eruptions and already mentioned on page
140 as tuffs and breccias.

Such material serves as a connecting link between the sedimentary
and igneous rocks. For, as it falls through the air, it becomes assorted
as to size, and successive outbursts thus produce rough but distinct
bedding. Or it may fall into water and become perfectly stratified.
Falling on the land it may cover vegetation and contain fossil
imprints of plants, leaves, etc. ; or if into water, of marine organisms.
Thus if we classify volcanic ash beds as igneous rocks we cannot
say that a distinguishing feature of igneous rocks is that they never
contain fossils. See remarks on page 133.

Classification Tabulated. The classification which has
been adopted and described in the foregoing may now be
shown, for convenience of reference, in tabulated form on
the following page.

Classifications based on Microscopic Research. In the
classification previously described, the color and texture
of rocks play a prominent part, and mineral composition
can be used only in an approximate manner. But where



202



ROCKS AND ROCK MINERALS





5




























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gnizable. Mostly Intrush


b. Ferromagnesian rock
to black.








ecognizable. Intrusive an


b. Dark colored to black
magnesian


BASALT.


BASALT-PORPH


>f Glass. Extrusive.


irlite, Pumice, etc.


1 Pitchstone-porphyry).


Extrusive.


u

43


With subordinate
Feldspar.


DIORITE.
GABBRO.
DOLERITE.


| DIORITE-PORPHYRY.


Constituent Grains Reco


4*
_bC

"3

3

CO

js **


Without Quartz.


SYENITE.
a. Syenite.
b. Nephelite Syen-
ite.
c. Anorthosite.


SYENITE-PORPHYRY.


ts Nearly or Wholly Unn








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3IDIAN, Pitchstone, Pe!


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GENERAL PETROLOGY OF IGNEOUS ROCKS 203

rocks are studied in thin section under the microscope
texture becomes of much less importance; all of the
minerals and their exact characters can be discovered and
their relative proportions made out. In this more exact
work the kinds of rocks that are recognized by petrog-
raphers are based primarily on the kinds and to some
extent the relative proportions of the component minerals.
This makes a great number of kinds of rocks which have
been named. Generally they are grouped first, according
to minerals and second, according to texture; some petrog-
raphers lay weight also on their mode of occurrence,
whether extrusive or intrusive, while others add to this
the genetic relations or groupings which they show in
nature. Classifications have also been proposed in which
their chemical composition plays the most prominent
part.

Quantitative Classification. Recently several petrog-
raphers, including the author, have proposed an exact
scientific classification of igneous rocks based on their
chemical composition, expressed, however, in terms of
minerals of definite composition, called standard minerals.
For this purpose a chemical analysis of the rock is neces-
sary but, where this cannot be obtained, an approximately
correct result may be achieved by measurement of the
minerals under the microscope, computing from this their
relative bulk and weight, and, their composition being
known, reckoning from this the chemical composition of
the rock as a whole, as if obtained by chemical analysis.

The chemical composition is then computed, according
to a set plan, into the relative amounts of standard
minerals. These standard minerals are divided into two
main groups; one characterized by the presence of alumina
and silica, such as the feldspars, nephelite, corundum and
quartz, but without iron or magnesia, the second charac-
terized by iron and magnesia but without alumina, such as
olivine, diopside, hypersthene, aegirite and iron ores. The
complex ferromagnesian minerals which contain alumina,



204 ROCKS AND ROCK MINERALS

such as hornblendes, biotite, augite, etc., are not treated
as standard minerals because it is better to consider them
as compounds of simpler molecules of the two preceding
groups. The first of these is called the salic (Si and Al)
the second the femic (Fe and Mg) groups of standard
minerals and the composition of the rock computed in
quantities of them is called its norm, which may thus,
when hornblende or biotite are really present in it, differ
considerably from its actual mineral composition or mode.
All igneous rocks may be expressed in salic and femic
minerals and according to the relative amount of each
group as compared with the other they are divided into
five classes, persalane, nearly or entirely composed of salic
minerals (sal: fern > 7: 1); dosalane, mostly salic

(sal: fern < 7:1 > 5:3);

salfemane, equal or nearly equal quantities of each
(sal: fern < 5:3 > 3:5);

dofemane, mostly femic minerals (sal: fern < 3:5 > 1:7);
and lastly perfemane, nearly or entirely femic

(sal: fern < 1:7).

Up to this point it is possible to use this classification in a
megascopic manner. The classes thus obtained are sub-
divided into orders on the relations of the salic minerals,
quartz, feldspars and feldspathoids (generally nephelite),
to one another in the first three classes and on somewhat
similar relations among the femic minerals in the last two.
More minute consideration of the mineral oxides divides
the orders into rangs and the rangs into grads. The
proportions by which they are thus divided are always the
same as that by which classes are made.

Further details regarding this and other systems of
classification founded upon results obtained by micro-
scopical research are to be found in the list of works
mentioned on page 10.



CHAPTER VH.
DESCRIPTION OF IGNEOUS ROCKS.

Grained Igneous Rocks.

As explained under the section on classification the
grained igneous rocks are those whose mineral grains are
approximately of equal size and large enough to be
identified with eye or lens, aided when necessary by
chemical or physical tests. Those rocks whose grain is
too fine to permit this will be found under the heading of
the dense igneous rocks. The porphyries, the major part
of whose constituent minerals can be distinguished, are
described in the following section.

GRANITE.

Composition. Granites are granular rocks composed
of feldspars and quartz. Sometimes they consist of these
minerals alone but generally there is more or less mica
present and often hornblende.

The feldspar is the predominant mineral and is readily
recognized by its appearance and cleavage. Sometimes
only one kind of feldspar is present but generally there
are two, orthoclase and soda-lime feldspar. They may
sometimes be distinguished by their colors; if one feldspar
is flesh-colored to red while the other is white, gray or
yellow, it is pretty certain that the first is orthoclase, the
second soda-lime feldspar (plagioclase). Close inspection
of a cleavage surface of the latter with a lens may show
the twinning striations (see page 38) but the grains are
rarely coarse enough to permit this. Rocks in which the
amount of plagioclase is greater than the orthoclase are
called quartz diorites by petrographers and are placed in a



206 ROCKS AND ROCK MINERALS

separate family, but this cannot be done in megascopic
determination and they are all here classed as granites.
They have also been called granodiorites in the western
United States.

Quartz normally occurs as formless material, filling the
interstices between the other minerals and hence without
definite shape. The normal color is white to dark, smoky
gray; sometimes it is red from included hematite, more
rarely a bluish color. In the finer-grained granites the
color is usually light or white, especially in those of a
sugar granular texture. It is recognized by its oily,
greasy luster and conchoidal fracture. In porphyritic
granites it sometimes occurs in large dihexahedral crystals
or round grains.

The mica may be either the light or colorless muscovite
or black biotite or both kinds may be present. Cases
where muscovite is the only mica are rare. Hornblende
occurs in black to dark green grains or prisms. It is
sometimes the only dark mineral but is more usually
accompanied by biotite. These are the chief minerals,
but if the rock is fairly coarse-grained, close inspection
will commonly show occasional metallic-looking specks
or grains of iron ore. Sometimes other minerals may be
seen, brassy crystals of pyrite, dark red grains of garnet,
etc., but these are occasional and are not of importance in
determining the rock.

Texture. In ordinary normal granite the texture is an
even granular one and alike in all directions through the
rock. From this type insensible gradations, sometimes
in the same mass, may be observed into the texture of
gneiss which becomes noticeable through the linear
arrangement of the components, especially the micas.
Thus the rock passes into granite gneiss. The normal
texture is shown on Plate 13. In other cases a tendency
may be noted for some of the orthoclase crystals to be
larger than the average grain and of more distinct crystal
form. In this way the rock becomes porphyritic and



PLATE 13.




A. COMMON GRANITE.




13. PORP11YRITIC GRANITE.



DESCRIPTION OF IGNEOUS ROCKS 207

when this is pronounced it is the porphyritic granite
described below. Often the dark minerals tend to group
or bunch together in spots.

Color. The general color of the rock depends largely
on that of the feldspar and in the proportion of this to the
dark minerals. Thus the color shades from white into
gray to dark gray, resulting from the mottling by the
biotite, etc. Such types are very common wherever
granites are abundant, as in New England. More rarely
the quartz and feldspar are themselves gray to dark gray
and thus determine the color. An example of this is the
granite of Quincy, Mass., largely used as a building stone.
Another very common type of coloring is one in which the
rock is flesh-colored, pink to red and even deep red. Such
red granites are found in Maine, Missouri, Colorado,
Scotland and other localities and are largely quarried and
used for building.

Varieties. The varieties of granite depend on the
relative proportions of the light and dark minerals, the
color and the texture. The relative amount of the biotite
(or hornblende) to the quartz and feldspar may vary
widely; it may be entirely wanting or it may be present
in large amount and make the rock quite dark. Such
extreme cases are less common. The grain may become
as coarse as large peas or even larger. These variations
combined with those in color produce distinct types of
granite which have often received local names. Some
other varieties are described in following sections.

Porphyritic Granite. As mentioned above the feldspar
may partly occur in large distinct crystals or phenocrysts.
Strictly speaking this would cause the rock to become a
granite porphyry but where the groundmass in which these
lie is as coarse as an average granite it is the custom to
speak of it as porphyritic granite, laying stress on the
character of the groundmass rather than on the por-
phyritic quality. The feldspar phenocrysts are of ortho-
elase and have the forms shown under feldspar, page 35.



208



ROCKS AND ROCK MINERALS



Reflection of light from the cleavages of these on the rock
surface often shows they are in twin halves, due to Carls-
bad twinning. The size of these phenocrysts is some-
times quite large, an inch long and broad or even more.
An illustration of this type of granite is seen on Plate 13.
Such rocks occur in New Hampshire and other localities
in New England, in Colorado, in the Sierra Nevada
Mountains, in England (Dartmoor and elsewhere), in the
Black Forest region and other places.

Chemical Composition. The mass compositions of a
few selected granites are shown in the analyses given here
to illustrate the kind of magma from which such rocks
have crystallized.

ANALYSES OF GRANITES.





I


II


III


IV


V


SiO 2


77,6


74.4


71.2


68.0


66.3


A1 2 3


12.0


13.1


13.7


17.2


16.0


Fe 2 3


0.6


0.7


1.7


3.1


1.8


FeO


0.9


0.9


1.0


0.4


1.9


MgO


trace


0.4


0.8


1.2


1.1


CaO


0.3


1.3


2.3


2.9


3.7


Na ? O


3.8


2.6


3.6


3.2


4.1


K 2 O


5.0


6.1


3.8


3.9


3.5


H 2


0.2


0.3


1.7




0.5


XyO*


0.2


0.4


0.2




0.9


Total .


100.6


100.2


100.0


99.9


99.8



* XyO = small quantities or traces of other oxides.

I, Hornblende Granite, Rockport, Mass.; II, Biotite Granite,
Crazy Mountains, Montana; III, Granite, Conanicut Island, Rhode
Island; IV, Granite, Kirkcudbright, Scotland; V, " Granodiorite or
quartz diorite," Mariposa County, California.

The large percentages of silica, alumina and alkalies
explain the predominance of feldspars and quartz. With
the increasing lime in the last two, the alkalic feldspars
give place in precedence to plagioclase; the increasing



PLATE 14.




A. EROSION OF GRANITE IN THE HIGH ALPS.
(After Duparc.)




B. EROSION OF GRANITE IN OLD AND LOW MOUNTAIN

REGIONS, STONE MOUNTAIN, GEORGIA.

(Georgia State Geological Survey.)



DESCRIPTION OF IGNEOUS ROCKS 209

iron and magnesia show increasing amounts of the dark
minerals; coincidently with this the silica falls; the amount
of free quartz is less and such rocks approach the next
class, the syenites.

Physical Properties. The specific gravity of granites
varies with the kinds and relative amounts of the com-
ponent minerals; from 2.63-2.75 is the ordinary range,
those containing more ferromagnesian minerals being the
heavier. The average weight of a cubic foot of granite is
about 165 pounds. Usually the porosity of such granites
as are quarried for building purposes is very small, the
percentage of water absorbed, compared with the weight
of the dry rock, being about 0.15 of one percent. Thus a
cubic foot of average granite if completely saturated would
absorb about 4 ounces of water. The strength of granites
in resistance to crushing is very great and probably far
greater than any load they would be called upon to bear
in architectural work. A series of Wisconsin granites
tested by Buckley showed crushing strengths varying from
15,000-40,000 pounds per square inch; some of these were
very high, and from 15,000 to 20,000 is perhaps the aver-
age. As the pressure at the base of the Washington Monu-
ment is 342.4 pounds per square inch, it will be seen there
is an ample reserve in most cases.

Uses of Granite. As is well known, on account of its
great strength and durability, granite is extensively used
for architectural purposes. Its pleasing colors and the
high polish it takes cause it to be employed as an orna-
mental stone in interior work, in monuments, etc. In
one respect, however, many granites have a defect which
somewhat impairs their value for use in buildings in
large cities. This defect is that they do not resist fire well,
but crack, scale and sometimes crumble under great
heat. One reason for this is that the quartz grains are
very commonly filled with minute bubbles containing
water or liquid carbonic acid gas (CO 2 ) or both.* They
are so minute that they are often only to be detected with

* The different rates of expansion of quartz and feldspar are another cause.



210 ROCKS AND ROCK MINERALS

high powers of the microscope in thin sections but they
may absolutely swarm in the quartz and constitute an
appreciable fraction of its bulk. They represent material
taken up or included at the time of its crystallization.
Under the action of heat the pressure on these sealed
crystal flasks becomes enormous; each quartz grain
becomes, so to speak, a veritable tiny bomb and eventually
it must crack in all directions and crumble and thus injure
the strength and resisting capacity of the stone. Feld-
spars practically never suffer from liquid inclusions, like
quartz, nor do the other ordinary rock minerals, so that
rocks like syenite or diorite, in which quartz is absent or
only sparingly present, make in respect to resisting fire
much better stone than granite.

Jointing in Granite. Granite tends to a block jointing
on a large scale in the great stocks. There generally
tends- to be three distinct sets of joints, two of which
approximate to the perpendicular, the third to the hori-
zontal. Sometimes these are nearly at right angles pro-
ducing cubes, more often at angles which make rhomboidal
blocks. Sometimes the horizontal one is most pronounced
and the mass has a sheeted or layer-like character sug-
gesting bedding. In dikes the joints are much more
numerous and the mass breaks into small blocks, plates,
etc. This jointing of granite is a matter of much impor-
tance in work of excavation, in mining, tunnelling,
quarrying, etc., in facilitating removal of material, but it
also explains why every granite mass is not suited to
furnish material in blocks of sufficient size .for construc-
tional purposes. Quarries like those in Finland, in the
so-called Rapakiwi granite, from which the base, a
cube of 30 feet, and the shaft, 100 feet high by 15 feet in
diameter, of the Alexander monument in St. Petersburg
were taken and those in Egypt from which the great
obelisks were cut are not common. Compare Plate 10.

Erosion Forms of Granite. The jointing of granite
largely conditions the work of erosive agencies on the




A. Craftsbury, Vermont.



B. Kortfors, Sweden.



C. Stockholm, Sweden.
ORBICULAR GRANITES.



DESCRIPTION OF IGNEOUS ROCKS 211

mass but the topographic forms produced also depend
greatly on the severity with which these act. In the
high mountain chains and wherever they are very ener-
getic, spires, needles and castle-like forms are produced,
but in the lower massive and older ranges and where
glaciation has been pronounced the granite stocks form
more smoothly modeled, rounded or dome-shaped masses
with gentle slopes and broad valleys, such as are seen in
the hills and mountains of New England and in parts of
Great Britain. The views on Plate 14 are illustrative of
this.

Orbicular Granite. It sometimes happens that the
component minerals of a granite, instead of being
uniformly distributed in grains of about the same size, are
collected in some spots in an unusual way and arranged in
ovoid or spherical bodies. Thus in a granite from Crafts-
bury, Vermont, called " pudding granite," the rock is full
of nodules, varying from the size of a pea to that of a nut,
composed almost entirely of agglomerated leaves of black
mica, as seen on Plate 15. More commonly the bodies
are composed of several minerals and consist of a nucleus
with a concentric outer shell or shells. The component
minerals are the same as those in the main body of the
rock but their proportions differ in the nucleus and in the
shells, sometimes consisting mostly or entirely of salic
minerals, while some shells consist mostly of ferromag-
nesian ones. Their appearance is shown on Plate 15.

The bodies are round, ovoid and often lenticular or
spindle shaped, as if drawn out. It was formerly thought
that they represented pebbles and were a proof of the
metamorphic origin of granite from conglomerates, but