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 2 of 35)
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Even when much finer grained than this, some minerals
may be distinguished by certain characters they possess
as explained in the chapter on the rock-making minerals.
Even when they are so dense that the component grains
can no longer be discriminated from each other, the color,
the hardness, the style of fracture under the hammer, the
specific gravity and the behavior of fragments or of the
powdered rock under the action of acids, are all impor-
tant characters which serve to distinguish different kinds
of rocks.

Implements and Apparatus. The first requisite is a
suitable hammer for obtaining material. It should be a
square-faced geological hammer of the
type shown in the adjoining figure. It is
convenient to have one end wedge shaped.
The steel should be tempered as hard as
possible without making it too brittle,
otherwise the edges wear off very rapidly.
If made to order it is a great convenience
to have the hole as large as possible,
consistent with strength, and tapered some-
what; the handle may then be somewhat
larger at the hammer end and thrust Fig. a. Geological
through the hole until brought up in the
taper by the enlarged end. This device, which is the
familiar one used in securing the handles of picks, is a great
convenience as it prevents the head coming off when the

Pirsson and Washington (Chicago University Press). Elemente der
Gesteinslehre, by H. Rosenbusch (Stuttgart). Petrology for Students,
by A. Harker (Cambridge University Press). Igneous Rocks, by
J. P. Iddings (Wiley & Sons, New York).


handle shrinks. The hammer should be of good weight,
about two and one-half pounds for the head, to enable
good-sized pieces of rock to be readily broken up and
fresh material within to be secured. Of course anything
in the way of a hammer or sledge may be used on occa-
sion, but this implement will give the best service for
general use.

If, in addition to procuring material for examination,
it is desired to trim and shape it into specimens for the
collection a small trimming hammer will be found
convenient. It should be double-headed,
square-faced, and of very hard steel, and
the head may weigh about six ounces. Hand
specimens for collections are usually about
4X3X1 inches in size and are made by
selecting a suitable large flake or spall obtain-
ed by the large hammer and knocking small
chips from it along the edges first on one side
.a. Trimming and then on the other until trimmed to the
required shape and size. A well-made
specimen should show hammer marks only on the edges
and never on the faces.

A pocket-lens is also essential; one of the apochromatic
triplets now made by several makers of optical instru-
ments is best, but much cheaper ones will serve the
purpose. One with a focal distance of one inch is most
convenient for general use.

In addition to the above, which are for use in the field,
a small amount of the apparatus used in the laboratory
for the determination of minerals will often prove of
great service. This would include a blowpipe and plati-
num tipped forceps for testing fusibility, pieces of quartz,
calcite and ordinary window glass for testing hardness, a
simple apparatus for determining specific gravity, a
magnet, a few test tubes, dilute acids and a Bunsen gas
burner or alcohol lamp for testing solubility, and a glass
funnel, filter paper and a few reagents, such as solutions


of ammonia, silver nitrate and barium chloride, for
making tests by chemical reactions. A small agate or
steel mortar is needed for grinding a fragment of the rock
or mineral to powder for making chemical tests. This
list might be increased almost indefinitely into the full
equipment of a mineralogical laboratory, but most
chemical laboratories contain all the apparatus and
reagents necessary for the determination of minerals and
rocks mentioned in this book and, where such a laboratory
is not available, the material which has been named above
will cover nearly all necessary demands and may be used
almost anywhere.



Composition of the Earth's Interior. The origin and
history of the rocks composing the solid crust of the earth
are of necessity bound up with the history and origin of
the globe itself. Beyond that history, however, which is
revealed in the sedimentary rocks, our ideas on this sub-
ject, as regards the earth, must, with our present knowl-
edge, be largely of the nature of pure speculation. Below
a relatively very shallow depth the same is true with
respect to the character and condition of its interior. We
do not know what it is like, and it is of course possible that
we never shall. The view which is most generally held
is that the earth was once a molten mass, the outer shell
of which solidified through cooling to a solid crust, while
the interior, though excessively hot, also solidified through
the enormous pressures exerted upon it by the overlying
portions; between the two is either a zone of liquid because
the pressure is not there sufficient to solidify it, or of
heated material which will become liquid if for any cause
the pressure in the crust above is diminished; this zone is
regarded as the seat of volcanic and other important
geological activities.

While this view has been long and is still widely held
and has been of great service in explaining many geological
phenomena, certain objections to it have been advanced,
and recently Chamberlain has propounded another.

According to this the earth is regarded as never having
been liquid but always a solid which has been gradually
built up by the infall and accretion of relatively small
solid bodies termed planetesimals. Through the enor-



mous pressures exerted under the influence of gravity,
contraction has ensued and gaseous matters have been
expelled, giving rise to the atmosphere and water on the
surface. This contraction is held to be the source of the
interior heat, and to the issuance of the heated gases is
attributed the origin of volcanic activity.

Still another view has been advanced in recent years by
Arrhenius according to which the interior is neither in a
solid or liquid but in a gaseous condition. It is assumed
that all substances if sufficiently heated must be in the
state of a gas; experiment teaches us that if any gas is
heated to or above a certain degree called its " critical
temperature " it cannot be reduced to a liquid or solid by
pressure alone, and it is held that this will be true even
though the pressure be enormous enough to contract the
gases to a density far beyond that which the substances
would have if in the solid condition. It is assumed that
the temperatures reigning in the earth's interior are so
great that all substances must be in a gaseous condition
and above their critical temperatures, but that the
pressures are also so enormous that they are reduced to a
state of density far greater than that of solids at the
surface, and that on account of this condensation their
internal viscosity or resistance to flowage is so great that
they possess also a greater rigidity, one sufficient to meet
the demand which astronomical investigations have shown
that the earth as a whole must possess.

Following this view then there is, first, a solid outer
crust, then a zone of molten liquid or of solid material so
greatly heated that it is capable of becoming liquid if the
pressure above is in any way lightened, and then finally
the great interior mass consisting of heated gases in a
condition of enormous condensation.

The three hypotheses presented above will serve to
show how widely divergent are the views in regard to this
subject among scientific men at the present time and how
purely speculative our ideas must be.


Facts which are known. On the other hand it must
not be assumed that nothing is known of the earth beyond
that which we can see at the surface. We know, for
instance, that there is a considerable increase in heat as
we go downward in the crust. We know also that there
are bodies of molten material, which, though they may
be relatively small as compared with the size of the earth,
are yet absolutely large, and we see the upward prolonga-
tion to the surface of these masses in active volcanoes.
We know that such bodies not only exist in the earth's
interior now but have also in past geological ages, as
shown by the way in which they have been forced upward
into its crust or poured out upon its surface. We know
that upon the land surfaces wherever the deepest seated
rocks, which underlie all the stratified and metamorphic
ones which have accumulated upon them, are exposed by
erosion they present the general characters of igneous
rocks, and thus lead us to infer that they were at one time
in a state of fusion. As the sedimentary and metamorphic
rocks are secondary or derived from previously existent
ones, this leads to the natural assumption that they came
from material originally similar to these deep-seated ones
and that their substance had at some previous stage
passed through a state of fusion.

Rock material then having been wholly or at least very
largely in a molten condition, it is evidently a matter of
importance that we should know something of the nature
and properties of the molten fluids which have formed
it. We can do this to some extent in active volcanoes
where we see some of the properties of these fluids exhib-
ited, but those which are most important in rock forma-
tion we can best learn by study of the igneous rocks which
are the result of the direct solidification of these molten
masses, and this subject is, therefore, treated in the
chapter upon them. There are, however, certain aspects
of it which can well be considered here, and one of these
is the general chemical composition of the earth's crust.


Chemical Composition of the Earth's Crust. During
recent years several thousand chemical analyses have
been made of rock specimens from visible parts of the
earth's crust. The great majority of them are from
Europe and the United States, but enough have been
made from other parts of the world to show, in con-
junction with the microscopical studies of other specimens,
that the essential facts which these analyses teach are
almost beyond question of general application. One of the
most important general truths learned by these investiga-
tions may be thus broadly stated the general chemical
composition of the earth's crust is everywhere similar.

The statement thus broadly made demands explanation.
It does not mean that one portion of the rock crust is
composed of exactly the same chemical elements in
exactly the same proportions as any other portion. It
means that it is composed of the same elements and that,
although these may vary greatly in proportions from
place to place or from one kind of rock mass to another,
if we take large areas involving many kinds of rocks the
average of such areas will be very nearly alike. Thus
the composition of the average rock computed from all
the analyses made of specimens from the United States
is essentially the same as the average computed from the
analyses of the rocks of Europe. The average rock of
New England is essentially that of the Rocky Mountains
region. On the other hand a large part of Quebec Province
is composed of one kind of rock which extends with
monotonous sameness over a vast area; the composition
of this has not the same proportions as the average rock,
and if we were considering this particular part of the
continent we should have to increase greatly our area to
obtain an average. Some parts of the continental areas
are covered with limestone which is essentially carbonate
of lime alone, but is a relatively thin, concentrated coating
of a special substance we should have to balance it with
large masses of other rocks.



The average rock has been computed from the analyses
by Clarke and by Washington and the results are shown
in the table below in Column A.


93 Per Cent


7 Per Cent
The Ocean.


Average with


47 07


49 77



26 08



7 34



4 11







2 24




2 33




2 28



















Sulphur ..... . .




All others







Clarke has also calculated that if we assume that the
crust has this composition to a depth of ten miles and
add in the water of the oceans, the atmosphere and an
amount of sedimentary rocks in proper proportions, the
general average of the whole will be that shown in Column
C, of the above table. These assumptions are reasonable,
and correspond with the facts so far as known. Even if
these results are not very accurate they must be approxi-
mately so and they are of value in showing the relative
proportions of the elements in the outer part of the earth.
From them important deductions can be drawn.

The Elements of Geological Importance. From the
table just given we see that the first eight elements are
present in quantity, and are, therefore, of geologic impor-
tance. Oxygen forms about one half of the outer part of


the earth, and the quantity in the atmosphere and in the
ocean is small, compared with that locked up in the under-
lying rock. Silicon comes next and forms about one
fourth and after it are aluminium and five other metals, of
which iron is the most important, the others being calcium,
magnesium, sodium and potassium, in the order of their
importance. After these comes a small group of four
elements, which, although of secondary rank in quantity,
demand mention: they are, hydrogen, titanium, carbon
and chlorine. Of these, titanium is a rather inert element
from the geological standpoint, plays no important part
in geological processes or results, and may, therefore, be
dismissed. The hydrogen and carbon, on the contrary,
are of great importance, they are of great activity in
geological processes, produce results of petrologic interest,
and must therefore be considered with the primary group
first mentioned. All the other elements, however impor-
tant in special cases, or for organic life or civilized activities,
are from the standpoint of general geology and petrology
of relatively little interest.

Combinations of Chemical Elements. Except oxygen,
carbon, and possibly to an unimportant extent iron, the
elements mentioned above do not occur alone, or native;
they are always combined in some form producing com-
pounds known as minerals. We may state this chemically
by saying that they are either in combination with oxygen
as oxides, or these oxides are in combination as salts.
Two, carbon and silicon, are negative elements their
oxides C0 2 and Si0 2 are anhydrides of acids; the others,
leaving hydrogen aside, are metals, or positive elements
whose oxides act as bases. The oxide of aluminium,
A1 2 3 , acts sometimes as a base and sometimes as a weak
acid, especially in combination with strong positive bases,
such as soda, Na 2 O, and potassa, K 2 O, and in combination
with silica, SiO 2 . Fe 2 O 3 acts as an acid in spinels.

From what has been said we have to deal with these
oxides which are of chief petrologic importance.



SiO 2 , silica, in combination and free as a solid.
CO 2 , carbon dioxide, in combination and free as gas.


AljOa, alumina, in combination and free solid, sometimes acidic.

Fe 2 O 3 , ferric oxide, in combination and free solid.

FeO, ferrous oxide only in combination in solids-

MgO, magnesia, in combination and free, solid.*

CaO, lime only in combination in solids.

Na,,O, soda only in combination in solids.

K 2 O, potassa only in combination in solids.

The above table is given in order of decreasing acidity
and increasing basicity from top to bottom. To this list
we should also add water, H 2 O, which occurs free in the
gaseous, solid and liquid states, and in combination.

Since we are considering rocks it is evident that of
these oxides and their combinations we need to regard
only those which form solids. They are then the com-
pounds of silicic and carbonic acid or silicates and car-
bonates and the oxides of silica, alumina and iron.

Ice, the solid form of water, may also be regarded as a rock,
but as such, needs no further consideration in this work. Com-
binations of the oxides of aluminium, iron and magnesium and
of the silicates with water also occur. Combinations with sulphur
as sulphides and sulphates and of phosphorus as phosphates and
of chlorine as chlorides are at times of local importance though
never having the general interest of those mentioned above.
They receive attention in their appropriate places.

* The solid MgO is the mineral periclase, excessively rare, and of no petro
logic importance.




SINCE all rocks, with the exception of a few glassy ones
of igneous origin, are composed of minerals it is of first
importance in their study and determination that a good
knowledge of the important rock-making minerals, of
their obvious characters and properties, should be had.
This is so indispensable, that before taking up the rocks
themselves, the following part of this work is devoted:
first, to a general account of those properties of minerals
which are of value in megascopic determination; second,
in a succeeding chapter, to a description of the minerals
individually; and third, to methods for their determination.

Minerals defined. A mineral is defined as any inorganic
substance occurring in nature which possesses a definite
chemical composition. To this, for petrographic purposes
we should add, that it is also a solid and usually it has a
definite crystalline structure. The word is also used in two
ways with different meanings: in one, which may be termed
the abstract chemical way, we refer to a compound having
a certain composition, as in speaking of calcite we mean
the compound CaCO.3, carbonate of lime; in the other
when we speak of the minerals of a rock we refer to the
actual crystal grains, the minerals as distinct entities or
bodies which compose that rock.

Crystals defined. Most chemical compounds when
their molecules are free to arrange themselves in space and
the conditions are proper for them to assume solid form,



as for example when they solidify from solutions, appear
in crystals. That is, the molecules arrange themselves in
a definite geometric system, characteristic of that com-
pound, and governed by mathematical laws, which give
the solid a definite internal structure and an outward
form bounded by planes which are always placed at
certain angles to one another. Thus minerals crystallize
in cubes, octahedrons, prisms, etc. The conditions in
rock formation are sometimes such that a mineral can
assume outward crystal form, and it is then bounded by
distinct planes; more commonly, however, the growing
crystals interfere with one another and have no distinct
form, or, as in the sedimentary rocks, they are fragments
only of former crystals, or their plane surfaces have been
worn off by attrition. Since, however, they possess the
inward characteristic structure we still call such bodies
crystals, though lacking the outward form. Thus, when
we speak of the crystals or crystal grains composing a
rock, we do not necessarily imply that these have plane
surfaces which give them geometric forms. Such grains
are sometimes called anhedrons (from the Greek, meaning,
without planes).

List of Properties. The chief properties of the rock-
making minerals by which they may be known are their
crystal form, color, cleavage and associations; these are
perceived by the eye, and in addition we have their hard-
ness, specific gravity, and their behavior before the blow-
pipe and with chemical reagents, properties which demand
some form of testing with apparatus.

Crystal form. The mineral grains which compose
rocks do not, as a rule, possess good outward crystal form
as mentioned in a preceding paragraph. The reason for
this is that in the igneous and metamorphic rocks, the
growing minerals interfere with one another's develop-
ment, and thus, while they may roughly approximate to
a certain general shape the mineral has endeavored to
assume, the outer surface is not composed of smooth


definite planes; while in the sedimentary rocks the grains
are either broken fragments or rounded by rolling and
grinding. It may happen, however, that in a liquid
molten mass when crystallization begins, one or more
kinds of minerals may commence growing in crystals
scattered through it and complete their period of growth
before the others which will compose the general mass of
the rock have commenced. In this case they have not
been interfered with, and they may exhibit good outward
crystal forms bounded by distinct planes. This is well
shown in those kinds of igneous rocks which are described
elsewhere as porphyries. Likewise in the metamorphic
rocks, certain minerals often appear in such well-bounded,
distinct crystals, as to indicate that they are of later origin,
and although formed by molecular rearrangement of
materials in a solid or somewhat plastic mass the con-
ditions were such that they were not interfered with
during their period of growth. This is illustrated by the
excellent crystals of garnet, often seen in the rock known
as mica schist.

Thus in general the outward crystal form or shape of
minerals in rocks is wanting and cannot be used as a
means for determining them, but in many special cases it
may be well developed in rock-making minerals and it
can then be very useful. The shapes in which each
mineral is most apt to occur are described under the
heading of that mineral in the descriptive part.

Color. The color of minerals, when used with proper
precautions, is also a very useful property for helping to
distinguish them. The color of minerals is dependent
upon their chemical composition, in which case it may be
said to be inherent, or it may be due to some foreign sub-
stance distributed through them and acting as a pigment,
and their color may then be termed exotic. It is because
the color of the mineral grains of rocks is frequently exotic
that precaution must be used in employing it as a means
of discrimination.


In regard to inherent colors, neither silica nor carbonic
acid in combination as silicates and carbonates has any
capability for producing color, and so far as they are con-
cerned, such compounds would be colorless, or, as will be
presently explained, white. So, silica alone, as quartz, is
naturally colorless. The same is also true of the metallic
oxides alumina, lime, magnesia, and the alkalies soda and
potash. Thus, carbonate of lime or calcite, carbonate of
lime and magnesia or dolomite, oxide of alumina or corun-
dum, silicates of lime and magnesia, silicates of alumina
and the alkalies or feldspars are all inherently colorless
minerals. The metallic oxides which chiefly influence
the color of rock minerals are those of iron, chromium
and manganese, and the only one of these which is of
wide petrographic importance is iron, especially iron as
ferric oxide. The minerals containing iron as a prominent
component are dark green, dark brown or black, and
these colors may ordinarily be regarded as indicative of
this metal.

With respect to the exotic colors which minerals fre-
quently exhibit, this may be due to one of two causes.
It may happen that a minute amount of some compound
of an intensely colorative character is present in chemical
combination. Thus a minute amount of manganese
oxide in quartz is supposed to produce the amethyst
color, traces of chromic oxide sometimes color silicates
green, and probably copper does also. Or the color may
be due to a vast number of minute bodies dispersed
through the crystal as inclusions. These minute specks
may have a distinct color of their own and thus act as a

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 2 of 35)