A _rock_ is any mineral, or mixture of minerals, occurring in masses of
considerable size. This distinction of size is the only one that can be
made between rocks and minerals, and that is very indefinite. A rock,
whether composed of one mineral or several, is always an aggregate; and
therefore no single crystal or mineral-grain can properly be called a
rock.
Before proceeding to study particularly the various kinds of rocks, a
little more preliminary work should be done. As already intimated, the
more important characteristics of rocks may be grouped under two general
heads,—_composition_ and _texture_.
_Composition of Rocks._
Rocks are properly defined as large masses or aggregates of mineral
matter, consisting in some cases of one and in other cases of several
mineral species. Hence it is clear that the composition of rocks is of
two kinds: chemical and mineralogical; for the various chemical elements
are first combined to form minerals, and then the minerals are combined
to form rocks.
Of course those minerals and elements which can be described as
principal or important rock-constituents must be the common minerals and
elements. Now it is a very important and convenient fact that although
chemists recognize about sixty-five elementary substances, and these are
combined to form nearly one thousand mineral species, yet both the
_common_ elements and the _common_ minerals are few in number.
So that, although it is very desirable and even necessary for the
student of lithology to know something of chemistry and mineralogy, it
by no means follows that he or she must be master of those sciences. A
knowledge of the chemical and physical characteristics of a few common
minerals is all that is absolutely essential, though it may be added
that an excess of wisdom in these directions is no disadvantage.
Chemical Composition of Rocks.
The elementary substances of which rocks are chiefly composed, which
make up the main mass of the earth so far as we are acquainted with it,
number only fourteen:—
_Non-Metallic or Acidic Elements._—Oxygen, silicon, carbon, sulphur,
chlorine, phosphorus, and fluorine.
_Metallic or Basic Elements._—Aluminum, magnesium, calcium, iron,
sodium, potassium, and hydrogen.
The elements are named in each group in about the order of their
relative abundance; and to give some idea of the enormous differences in
this respect it may be stated that two of the elements—oxygen and
silicon—form more than half of the earth’s crust.
Silicon, calcium, and fluorine, although exceedingly abundant, are also
very difficult to obtain in the free or uncombined state, and specimens
large enough to exhibit to a class would be very expensive. With these
exceptions, however, examples of these common rock-forming elements are
easily obtained.
My purpose in calling attention to this point is simply to suggest that
the proper way to begin the study of minerals and rocks with children is
to first familiarize them with the elements of which they are composed.
The most important thing to be known about any mineral is its chemical
composition; and when a child is told that a mineral—corundum, for
example—is composed of oxygen and aluminum, he should have a distinct
conception of the properties of each of those elements, for otherwise
corundum is for him a mere compound of names.
It is very important, too, if the pupil has not already studied
chemistry, that he should be led to some comprehension of the nature of
chemical union and of the difference between a chemical compound and a
mechanical mixture. For this purpose a few simple experiments (the
details of which would be out of place here) with the more common and
familiar elements will be sufficient. Mrs. Richard’s “First Lessons in
Minerals” should be introduced here.
Mineralogical Composition of Rocks.
The fourteen elements named above are combined to form about fifty
minerals with which the student of geology should be acquainted; but not
more than one-half of these are of the first importance. It is desired
to lay especial emphasis upon the importance of a perfect familiarity
with these few common minerals. There is nothing else in the whole range
of geology so easily acquired which is at the same time so valuable; for
it is entirely impossible to comprehend the definitions of rocks, or to
recognize rocks certainly and scientifically, unless we are acquainted
with their constituent minerals.
With one or two exceptions, these common rock-forming minerals may be
easily distinguished by their physical characters alone, so that their
certain recognition is a matter of the simplest observation, and
entirely within the capacity of young children. Furthermore, being
common, specimens of these minerals are very easily obtained, so that
there is no reason why teachers should not here adopt the best method
and place a specimen of each mineral in the hands of each pupil. Typical
examples, large enough to show the characteristics well, ought not to
cost, on the average, over two cents apiece.
A MINERAL is an inorganic body having theoretically a definite chemical
composition, and usually a regular geometric form.
THE PRINCIPAL CHARACTERISTICS OF MINERALS.—These may be grouped under
the following general heads:—
(1) _Composition_, (2) _Crystalline form_, (3) _Hardness_, (4) _Specific
gravity_, (5) _Lustre_, (6) _Color and Streak_.
1. _Composition._—This, according to the definition of a mineral, ought
to be _definite_, and expressible by a chemical formula. When it is not
so, we usually consider that the mineral is partially decomposed, or
that we are dealing with a mixture of minerals. It is well to impress
upon the mind of the pupil the important fact that the more fundamental
properties of the elements, such as specific gravity and lustre, are not
lost when they combine, but may be traced in the compounds. In other
words, the properties of minerals are, in a very large degree, the
average of the properties of the elements of which they are composed;
minerals in which heavy metallic elements predominate being heavy and
metallic, and _vice versa_.
To fully appreciate this point it is only necessary to compare a mineral
like galenite—a common ore of lead, and containing nearly 87 per cent.
of that heavy metal; or hematite (specimen 13), containing 70 per cent.
of another heavy metal, iron—with quartz (specimen 15), which is
composed in nearly equal parts of oxygen and silicon, two typical
non-metallic elements. Many minerals contain water, _i.e._, are
hydrated. Now water, whether we consider the liquid or solid state, is
one of the lightest and softest of mineral constituents; and it is a
very important fact that hydrated minerals are invariably lighter and
usually softer than anhydrous species of otherwise similar composition.
Other striking illustrations of this principle will be pointed out in
the descriptions of the minerals which follow.
2. _Crystalline form._—A crystal is bounded by plane surfaces
symmetrically arranged with reference to certain imaginary lines passing
through its centre and called axes. Crystals of the same species are
always constant in the angles between like planes, while similar angles
usually vary in different species; so that each species has its own
peculiar form.
“Besides external symmetry of form, crystallization produces also
regularity of internal structure, and often of fracture. This regularity
of fracture, or tendency to break or cleave along certain planes, is
called cleavage. The surface afforded by cleavage is often smooth and
brilliant (see specimens 17, 18, and 21), and is always parallel with
some external plane of the crystal. It should be understood that the
cleavage lamellæ are not in any sense present before they are made to
appear by fracture.”—(Dana.)
Crystals are arranged in six systems, based upon the number and
relations of the axes, as follows:—
Isometric System.—Three equal axes crossing at right angles. Example,
cube.
Tetragonal System.—Two axes equal, third unequal, all crossing at right
angles. Example, square prism.
Orthorhombic System.—Three unequal axes, but intersections all at right
angles. Example, rhombic prism.
Monoclinic System.—Three unequal axes, one intersection oblique.
Example, oblique rhombic prism.
Triclinic System.—Three unequal axes, all crossing obliquely. Example,
oblique rhomboidal prism.
Hexagonal System.—Three equal axes lying in one plane and intersecting
at angles of 60°, and a fourth axis crossing each of these at right
angles and longer or shorter. Example, hexagonal prism.
By the truncation and bevelment of the angles and edges of these
fundamental forms a vast variety of secondary forms are produced. The
limits of the guide will not permit us to follow this topic farther; but
it may be added that for the proper elucidation of even the simpler
crystalline forms the teacher should be provided with a set of wooden
crystal models and Dana’s “Text-Book of Mineralogy.”
The crystallization of a mineral may be manifested in two ways: first,
by the regularity of its internal structure or molecular arrangement, as
shown by cleavage and the polarization of transmitted light; and,
second, by the regularity of external form which follows, _under
favorable conditions_, as a necessary consequence of symmetry in the
arrangement of the molecules.
When a mineral is entirely devoid of crystalline structure, both
externally and internally, it is said to be _amorphous_.
Perfect and distinct crystals are the rare exception, most mineral
specimens being simply aggregates of imperfect crystals. In such cases,
and when the mineral is amorphous, the _structure_ of the _mass_ may
be:—
Columnar or fibrous.
Lamellar, foliaceous, or micaceous.
Granular.—When the grains or crystalline particles are invisible to the
naked eye the mineral is called _impalpable_, _compact_, or _massive_.