Louis V. (Louis Valentine) Pirsson.

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

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Online LibraryLouis V. (Louis Valentine) PirssonRocks 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 text (page 16 of 35)
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tions which represent the chemical changes involved.

Calcite Quartz Wollastonite Garb. diox.
CaCO 3 + SiO 2 = CaSiO 3 + CO 3

Dolomite Quartz Pyroxene

CaMg(CO 3 ) 2 + 2 SiO 2 = CaMg(SiO 3 )2 + 2 CO 8

Calcite Clay Quartz Garnet Carb. diox. Water

3 CaCO 3 + H 4 Al 2 Si 2 O 9 + SiO 2 = C a3 Al 2 Si 3 O 12 + 3 CO 2 + 2 H 2 O

Calcite Clay Anorthite

CaCO 3 + H 4 Al 2 Si 2 O, = CaAl 2 Si 2 O 8 + 2 H 2 O + CO 2

In some cases the rock is thus entirely changed into
silicates or mixtures of them, but usually it consists of
impure marble or altered limestone containing the min-
erals aggregated into lumps or bunches. Carbonaceous
material which may be present is often changed into
graphite. In addition to those minerals mentioned, a
variety of others, whose origin depends on the mineral-
izing vapors given off by the igneous rock, may also be
formed, such as mica (phlogopite), chondrodite, horn-
blende, vesuvianite, epidote, tourmaline, etc. In these
cases the main materials are those already in the rock;
the vapors furnish the volatile components, the hydroxyl,
boron, fluorine, etc., needed for their composition. Such
minerals furnish transitions to the more typical cases of
pneumatolytic contacts mentioned below. It should not
be forgotten also that many of these minerals contain


oxide of iron, ferrous or ferric or both, and this must come
from the limonite or other hydrated iron oxides mixed in
with the impure marly beds and deposited with the other
material at the time of their formation. Perhaps of the
minerals mentioned garnets, pyroxenes and vesuvianite
may be taken as among the most typical of such occur-
rences in altered limestones. Many instances of such
contacts are known in various parts of the world and some
of them have become famous for the variety and beautiful
crystallizations of the minerals which they afford and
which are to be found in all mineral cabinets.

In the case of clay shales and slates variable effects are
produced, but usually ones that are well marked and
characteristic. While such rocks consist mostly of
microscopic fragments of quartz, granules of clay, mica,
etc., there is considerable variability in their composition
and accordingly a difference in the result of the meta-
morphism. Sometimes they are baked into a dense, hard
rock with conchoidal fracture, of a black or very dark
stone color, called hornstone, which closely resembles trap
or basalt. Sometimes they are like the hornstone in
hardness, texture and fracture but differ in color, being of
a light gray to green-gray or greenish and are known as

In other cases where the beds are more rich in kaolin, a
mineral, andalusite, is apt to develop according to this

Kaolin Andalusite Quartz Water

H 4 Al 2 Si 2 O 9 = Al 2 SiO 5 + SiO 2 + 2 H 2 O

At the contact a rock composed largely of this, often in
recognizable grains and crystals, and mixed with a brown
biotite in glimmering specks, forms a granular rock,
generally dark in color and much resembling an igneous
rock in texture. All visible evidence of bedding of
sedimentary character is lost. This would be termed an
andalusite hornfels. Further from the contact the rock
begins to lose its granular texture; it becomes more schist-






like or perhaps slaty and is dotted with the andalusite
prisms. These very frequently gather up the dark
organic matter of the rock and arrange it within them-
selves in the manner characteristic of this mineral. They
then appear dark on a lighter background, as seen in
Fig. 2, Plate 12, and this variety of rock is known as
" Fruchtschiefer " (fruitschist) by the Germans. The
rock has much the character of a fine-textured mica-

Still further from the contact the effects of meta-
morphism are less and less marked, the beds show more
and more of their original sedimentary nature; in this
part the most evident effect is a spotting of the shales or
slates from collection of organic matter or minerals into
more or less well defined points or knots. Such a develop-
ment of knots is one of the most characteristic features
of moderate contact metamorphism and, when encoun-
tered in the field, should always lead to search for more
intensive effects and the possible nearness of intrusive
igneous rock bodies. The latter may of course be below
and not yet exposed by erosion.

Just as all kinds of variations between sandstones,
limestones and shales are found in nature, so do the
different varieties of rocks produced by contact meta-
morphism, as described above, vary and grade into one

In the nature of things the already existent igneous
rocks are less altered by the contact metamorphism of
following intrusions than the sedimentary ones. This is
particularly true of the granites and other very feld-
spathic rocks. The ferromagnesian ones, those containing
feldspars rich in lime, and especially those composed
chiefly of pyroxene, show at times considerable effects.
The pyroxene is converted into hornblende and the rock
becomes an amphibolite and even at times a hornblende

Pneumatolytic Contacts. It was mentioned above in


connection with the changes observed in limestones that
minerals appeared whose origin was due to the mineraliz-
ing vapors given off from the igneous mass. At times in
contact zones the outer rocks may be converted into
masses of such minerals, testifying to the abundance and
energetic action of the excluded vapors. Such minerals
as tourmaline, topaz, fluorspar, vesuvianite, mica (mus-
covite), etc., ones containing hydroxyl, fluorine and
boron are characteristic of these occurrences. The
masses thus formed are not widespread and regular
around the contact but appear here and there, espe-
cially near fissures, sometimes in isolated areas in the
other rocks, sometimes in large, sometimes in smaller
lumps and masses, following the irregular escape of
the gases.

Contact Zones and Ore Bodies. In the contact zones of
igneous rocks, the passage of the vapors and the move-
ment of heated solutions in them, combined often with
their own chemical composition, which causes them to
react with the solutions, have made them especially
favorable places for the deposit of ores. Their loss of
volatile substances causes a reduction of volume, they
become more porous, if not too deeply buried, and permit
more easily the circulation of fluids. As a result of this
we find many valuable deposits of the ores of gold, silver,
lead, copper, etc., from magmatic waters, in such contact
zones. In places in the mining regions of the Rocky
Mountains the contact between sedimentary beds and
intrusive masses of granite, porphyry, etc., from some
elevated point may be followed with the eye for miles by
the successive mines, pits, and heaps thrown out from
prospects. So well is this known that contacts between
porphyry and limestone are eagerly sought by every pros-
pector. Any adequate treatment of this subject would
carry us far beyond the limits of this work and further
information should be sought in those treatises which
deal with the origin of ore deposits.


Classification of Igneous Rocks.

Introductory. There is probably no subject in the
domain of natural science concerning which there has been
and is to-day less agreement than in the classification of
igneous rocks. The reason for this is that there are no
distinct boundary lines drawn by nature itself. The
igneous rock masses of the earth possess certain features
which may be used to distinguish and discriminate them,
one from another, such as their geologic mode of occur-
rence, their mineral composition, their texture, and their
chemical composition, which nearly represents that of the
original magma. A very brief inspection serves to show,
however, that in each of these features gradations exist
without hard and fast lines. If we classify them accord-
ing to mode of occurrence and divide them into intrusive
and extrusive rocks, then, for example, it is clear that every
lava flow is (or was) prolonged into depths below by an
intrusive continuation in the form of a dike or volcanic
neck. We should have to separate the intrusive from
the extrusive at some point by an arbitrary plane; above
this the rock would receive one name, below it another,
though it is clear that the material just above and that
just below would be absolutely alike. The same is true
when we consider the other features of rocks mentioned;
they are found to grade into each other mineralogically,
chemically and texturally, and where lines are drawn it
must be done arbitrarily. It is due to these facts that
so much diversity of opinion has existed regarding their
classification, some laying stress on one feature, some on
another. By general common consent among petro-
graphers, especially since the use of the microscope has
served to reveal the composition of dense rocks, a large
number of different kinds or types of igneous rocks are
recognized, based primarily on the kinds and relative
quantities of their component minerals and on their
texture, but as to the manner in which these recognized


kinds shall be grouped in a classification there is, as stated
above, wide diversity of opinion. It would not be proper
to go into the discussion of this subject further, but it
should be clearly understood at the outset, that what-
ever method of classification of igneous rocks is used, the
boundary lines must be artificial ones and in many cases
just where a rock should belong must be a matter of
opinion, which each must decide for himself.

Older Megascopic Classification. Before the micro-
scope came into use in studying rocks, they naturally
divided themselves into two groups, those whose com-
ponent mineral grains were large enough to be seen and
recognized and those which were too compact to permit
this. The former group was divided into different kinds
according to the mineral varieties composing them, the
latter according to the color, texture, luster and other
physical properties they presented. In this manner by
common usage a megascopic classification, extremely
useful for geologic and common purposes, came about,
which gave rise to such terms as granite, diorite, porphyry,
greenstone, basalt, etc.

Effect of the Microscope. When the microscope came
into use it was discovered that the dense rocks could be
studied and their component mineral grains determined,
nearly as easily as the coarse-grained ones, and the result
of these studies showed that a vastly greater diversity
existed among them than had been suspected. And
among the coarser-grained ones it was also found that
many minerals, until then not known in them, existed,
and that great variations among the minerals known to
compose them could be seen, as well as differences in tex-
ture, etc. To express these differences among the rocks
and to connote the ideas regarding them which they
engendered, not only have a whole host of new rock names
arisen, but the old megascopic terms have been defined
and redefined by various authorities, until they have
nearly all lost their original significance.


This has been an unfortunate phase of the history of
the development of petrography as a science, because
these former megascopic or field names, as we should
term them now, served a very useful and necessary pur-
pose, which the more exact and scientific nomenclature
of modern petrography cannot replace. The person who
desires to deal with rocks and name them from the mega-
scopic, field point of view, such as the field geologist, the
engineer, the architect, etc., is left without any equipment
for doing so. A single illustration will suffice. The
old term granite meant any granular igneous rock and
then later one composed of quartz and feldspar. Now, as
used by modern petrographers, it is only a granite in case
the feldspar is chiefly alkalic, while if it is dominantly
soda-lime feldspar, the rock is termed a quartz diorite,
a distinction which ordinarily cannot be made without
microscopic study.

The redefinition and specializing of these useful general
field terms is very much the same as if the botanists had
redefined such terms as bush, tree, vine, shrub, etc., and
had made them the names of particular species or genera,
so that if tree, for instance, were properly used, it would
designate only oaks, or even quercus alba.

In the meantime in the world at large, where rocks are
commercially dealt with, as in mining, architecture, etc.,
the use of rock names in the old way has gone on
quite regardless of the petrographers, but the geologist
or engineer who has endeavored to keep up with the de-
velopment of the science and use its terms megascopically
has carried an ever increasing load until finally he has
been compelled to become a petrographer or else give up
in large part any independent use of rock names. With-
out doubt it is largely due to this fact that every advance
in the definiteness and completeness of petrographic
scientific nomenclature has raised a wave of protest among

Present Need in Classification. It is clear that the


parting of the ways has long been reached and it ought
to be definitely recognized that the further development
of petrology and of the classification and nomenclature of
rocks from the scientific standpoint must be left largely
to petrographers, while those who have occasion to deal
with them in the purely megascopic manner must have a
method of classification and a set of terms of a totally
different scope and usage. They must in large measure
revert to that which was in vogue before the microscope
came into use.

It matters little whether such a classification is com-
pletely based on all the principles underlying scientific
petrology which the study of rocks has revealed or not; to
be useful it must be practical and to be practical it
must be based entirely on the evident megascopic char-
acters of rocks, such as can be seen by the eye or pocket
lens or be determined by simple means at every one's

Classification used in this Work. As it is the object of
this work to treat rocks from this point of view the follow-
ing method of classification has been adopted.* First,
the rocks are considered according to their texture and
from this it will be found that they divide naturally into
three classes, grained f, dense, and glassy.

A. Grained Rocks. By this is meant those rocks
whose component mineral grains are large and distinct

* This is essentially that proposed by the author and several
other petrographers. "Quantitative Classification of Igneous Rocks,"
by Cross, Iddings, Pirsson and Washington, University Chicago
Press, 1903, p. 180.

t The term " grained " is here used instead of " granular " for
two reasons. First, because granular (from granule a little
grain), strictly speaking, means fine-grained, while the rocks included
may be coarse, medium or fine-grained. Second, because granular
is used by many petrographers in a technical way as an equivalent
to " even-granular " and opposed to porphyritic, while grained
rocks may be either. Phanerocrystalline, macrogranular, mega-
granular, etc., have much the same meaning but it is better to use
& simple English word than a compound Latin or Greek one.


enough to be seen and recognized by the eye alone or
with the lens. No hard or fast line can be here drawn as
to the size of grain; it will vary with the kind of mineral,
their association together, and on the experience and skill
of the observer. In general it may be said that it includes
rocks whose average size of grain is as large or larger than
that of ordinary loaf sugar.

B. Dense Rocks. This will include those which are
nearly or wholly of stony appearance and texture but
whose minerals cannot be determined, because the con-
stituent particles are too minute. They may even appear
homogeneous but are generally microcrystalline.

C. Glassy Rocks. Includes those wholly or in part
made up of glass, as shown by their vitreous or pitchy
luster, conchoidal fracture, and other characters and

Treatment of Porphyries. Reference is had in the
above to rocks whose average size of grain is uniform or
nearly so. But, as explained in the description of the
porphyritic texture on page 156, many igneous rocks are
porphyries, that is, they contain distinct crystals or
phenocrysts much larger in size than that of the average
grain of the groundmass in which they lie embedded. It
is assumed that in general the size of the phenocrysts is
such that they can be distinctly seen and the particular
kind of mineral composing them can be recognized or
approximately determined. In classifying porphyries
they at once naturally fall into two classes; first, (D) one
in which not only the phenocrysts, but also the mineral
grains of the groundmass are large enough to be determined
and second, another (E) in which the groundmass is
either too dense to be made out or (F) glassy. In the
former case (E), two subdivisions can be made, one (a) in
which the phenocrysts are very abundant and a good idea
of the mineral composition of the rock as a whole may be
had and another (&) in which the amount of groundmass
is predominant and this cannot be done.


For a clearer understanding these divisions of porphyries
may be shown in tabulated form.

D. Groundmass grained, recognizable.

E. Groundmass dense, unrecognizable.

a. Phenocrysts very abundant and recognisable.
6. Phenocrysts not very abundant or rare,
groundmass very abundant.

F. Groundmass glassy.

It would be logical to make the same subdivisions a and 6 under
class F as are made in E but cases where glassy groundmasses are
filled with abundant recognizable phenocrysts dominating in amount
over the groundmass though known, are not sufficiently common to
make worth while the subdivision for practical purposes.

No sharp lines can be drawn between these divisions;
they pass into one another gradually, except as to whether
the rock is glassy or stony in texture.

It is to be observed that since the rocks belonging in
division D have their mineral constituents determinable
they belong in the same category as those in A of the
evenly granular ones previously mentioned, so far as this
particular is concerned. Likewise in division E, sub-
division a, if half or 'more of the rock is composed of
recognizable phenocrysts its general mineral character
can be determined and it falls in the same category. But
in the remaining divisions not enough of the mineral
characters can usually be told to safely identify the rocks
on this basis and such rocks are evidently to be classed
with division B, dense rocks and C, glasses which are
not porphyries and which cannot be subdivided according
to mineral composition.

Subdivisions of Class A. The igneous rocks having
been divided into classes on the basis of texture it now
remains to show on what grounds these classes can be
further subdivided and the individual kinds of rocks,
from the megascopic standpoint, obtained. This is done,
as already suggested, in the even- and porphyritic-grained


rocks by considering their mineral composition. First
we may broadly divide them into two main groups.

a. Rocks in which the feldspars or feldspars and
quartz predominate.

6. Rocks in which the ferromagnesian minerals (py-
roxene, hornblende, olivine, etc.) predominate.

As a rule, the rocks of the first group are light-colored,
white, red or gray, but this is not an absolute rule since
the feldspars are sometimes very dark from an included
pigment. In general the rocks of the second group are
dark in color to black but this is also not an invariable
rule since some, like those composed wholly of olivine, are
rather light.

The first group a may be further subdivided on the
basis of the relation of quartz to the feldspars. Those
which contain an appreciable amount of quartz with the
feldspars fall in one division and are termed granite, when
even granular in texture, and granite porphyry, when of
porphyritic texture, while those in which quartz is absent
or is present in inappreciable quantity are called syenite
and syenite porphyry respectively. Further division of
these into varieties on the basis of particular mineral
characters will be considered in the description of them
in the succeeding chapter.

The second group 6 is subdivided on the basis of the
relation of the feldspars to the ferromagnesian minerals,
into those which contain feldspar, subordinate in amount
to the ferromagnesian minerals, and those in which it is
wanting. Thus we have as follows:

c. Rocks with predominant ferromagnesian minerals.

feldspar subordinate.

d. Rocks consisting wholly of ferromagnesian minerals.

Group c is subdivided according to the nature of the
predominant ferromagnesian mineral present. For
practical purposes there need be only two considered


here; if it is hornblende the rock is diorite, if it is pyroxene
it is gabbro. The means for distinguishing between horn-
blende and pyroxene are discussed in the description of
these minerals in a preceding part of this work. In many
cases, especially in the finer-grained rocks of this group,
it may not be possible to distinguish between hornblende
and pyroxene and the rock may then be termed dolerite.
This name would then mean that the rock consisted chiefly
of indeterminable predominant ferromagnesian minerals
with subordinate feldspar.*

Porphyries occur in this group but they are relatively
of less importance than in the preceding ones; they are
treated in the descriptive part. Rocks in which ferro-
magnesian minerals, other than hornblende and pyroxene,
predominate over feldspar are known but are of little
practical importance in a megascopic scheme of this
character and are therefore omitted. They will be
mentioned later.

The last group d, consisting wholly of ferromagnesian
minerals, is divided according to the kinds of these minerals
present. The most common and prominent mineral in
the group is pyroxene but this is usually associated with
olivine and the rock is termed peridotite. This is the
most common member and may be used as a general term
for the group. If olivine is absent and the rock consists
wholly of pyroxene it is pyroxenite, if of hornblende,
hornblendite. Varieties are described under peridotite.

* In this usage of dolerite the author adopts and follows that
proposed by Chamberlain and Salisbury (Geology, vol. 1, p. 431,
1904) which is already obtaining considerable vogue and from such
authority is likely to become general. In Germany the term is
restricted to certain coarse-grained basaltic rocks; in England it
has had a certain use for all coarse-grained basalts and for rocks
termed elsewhere diabases; in America it has been little employed
and may well be revived as a field name in the sense suggested.
With this meaning it is a very useful term. The word is from the
Greek, meaning deceptive, with the idea that the pyroxene cannot
be distinguished from the hornblende.


Porphyries in this group rarely occur and are of no prac-
tical importance.

In summation, in considering the classification of the
group of grained rocks whose constituents are determinate,
one should consult what has been said regarding the
chemical composition of igneous magmas and the
variations in mineral composition beginning on page 141.
It is not possible, however, to classify them entirely, for
megascopic purposes, by the diagram given on page 145,
for, in general, we cannot discriminate between the different
kinds of feldspars. Thus the rock there shown as quartz
diorite must be classified under the head of granite, while,
as compared with the diagram, the diorite and gabbro
mentioned above broadly overlap. Still, in a general way,
bearing these exceptions in mind, the classification, dis-
tinguishing between the feldspathic and the ferromagne-
sian rocks, brings out the ideas there expressed. The rocks
of this class are nearly always intrusive, rarely extrusive.

Subdivisions of Class B. In considering the second
class of rocks, B, whose texture is so fine or dense that the

Online LibraryLouis V. (Louis Valentine) PirssonRocks 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 text (page 16 of 35)