Sedimentary rocks are such as give evidence of having been formed by
successive deposits of sediment in water. They include sandstones or
freestones, limestones, clays, etc. The material for these must have
been derived from some original source, and in many instances this may
be traced to the disintegration of older rocks. Thus gneiss appears to
be formed by the disintegration of granite. The great class of
sedimentary rocks may be divided into three smaller divisions. These
divisions, with the chief rocks of each division, may be tabulated as
follows:
(a) Mechanically formed rocks from detrital sediments:
Conglomerates, sandstones, clay, and shale.
(b) Organically formed rocks from animal and plant remains:
Limestones, chalk, coral, peat, and coal.
(c) Chemically formed rocks from material once in solution:
Limestones, stalactites, gypsum, rock-salt and sinter.
Most of the stratified rocks contain fossils; and since each group
contains certain kinds peculiar to itself, it is by means of these
organic remains that their relative ages have been determined.
Although the lowest stratified rocks are more ancient than those which
have been deposited above them, the layers or beds do not always retain
a horizontal position. Were such the case, it could only be by deep
cuttings that we should arrive at the older strata. We however find
that, owing to some convulsion of nature, stratified rocks have been
thrown out of their original position, and thus crop out to the surface.
Not only is facility thus afforded us to become acquainted with the
nature of the lower rocks, but many of the most valuable products of the
earth are by this means rendered accessible to man.
[Illustration: =HOW THE HISTORY OF THE EARTH IS EMBEDDED IN THE ROCKS=
A million years ago, a little stream trickled down a mountain-side,
carrying with it grains of sand and stones which fell to the bottom of
the sea. In the sea swam a great and wonderful creature called an
ichthyosaurus. One day the great creature died, or probably it was
killed in battle with another strange monster, and its body fell to the
bottom of the sea among the shells and seaweed. Meanwhile, the stones
and sand brought down by the stream continued to fall upon the bed of
the sea until at last the great reptile’s body was buried, and the lower
layers became pressed into hard rock by the weight on top. One day an
elephant going to the river to drink broke off his tusk, and this was
carried down by the river and sank in the sea. Another day a bird was
drowned, and this, too, fell upon the ocean-bed. Dead fishes and shells
also sank, and all were buried by the never-ceasing shower of mud and
earth and sand and stones. Ages after the ichthyosaurus died, men began
to live on the earth, and one day a man who had made a boat went out to
fish. Trying to spear a big fish, the head of his harpoon broke off and
fell to the bottom of the sea. In course of time this also was buried in
the mud. The bottom of the sea crept higher and higher, till at last it
became dry land. Then one day men began to dig, and the world’s
wonderful story was revealed as we read it here. First the spear-head
was found, then the tusk, the bird’s skeleton, the shells, the fish, and
at last the skeleton of the great sea reptile, all turned to stone and
become _fossils_, a word that means “something dug up.”]
The greater number of these beds contain organic remains, i. e., the
remains of animals and plants, which are termed fossils. Among these the
most numerous are the remains of marine animals, and in some instances
shells and corals occur in such abundance as to form the principal part
of extensive beds. Every part of the earth exhibits similar, or nearly
similar formations; and not only are marine fossils met with in the
interior of continents, and at great elevations above the sea, but a
vast variety of plants, corals, shells, fish, reptiles, etc., are found,
of species dissimilar to any at present on the land or in the waters.
Besides rocks, we meet with earthy formations on the surface. These
include such loose materials as are disintegrated or worn away from
rocks, and form, when combined with decayed animal and vegetable matter,
the soil of meadows and arable lands.
IGNEOUS, OR UNSTRATIFIED ROCKS are such as appear to be of igneous
origin, or to have been formed by the action of fire or intense heat.
They are called unstratified, because instead of having been deposited
in successive layers, like the stratified rocks, they seem to have been
formed by the fusion or melting of the materials of which they are
composed, and the subsequent cooling and hardening of the melted matter
into one great mass. Granite, basalt, lava, etc., are examples of this
class of rocks, and represent respectively the sub-classes of plutonic,
trap, and volcanic rocks. Plutonic rocks are those which have cooled
under the pressure of overlying rocks; trap rocks, those which have
cooled under that of deep water; and volcanic rocks, such as have cooled
in the air.
Though granite is the most useful of the igneous rocks, basalt is
probably the most interesting because of the wonderful formations it
discloses. It is a dense basic lava of a dark color, that breaks with a
conchoidal or shell-like fracture, and shows a finely grained or
hemi-crystalline texture in a glassy base. The basalt rocks are found
both as intrusive masses and as sheets that have been poured out on the
surface. Many of these lava sheets of basalt in slowly cooling and
solidifying acquired a columnar structure, the columns often having a
more or less hexagonal shape, though the number of sides varies. Fine
examples of these columnar basalts occur at Fingal’s cave in the island
of Staffa, at the Giant’s Causeway in the north of Ireland, and on the
shores of Lake Superior.
METAMORPHIC, or Transformed rocks, include altered rocks of either
sedimentary or igneous origin, in which the acquired are more prominent
than the original characteristics. Igneous rocks have, in many cases,
forced their way up through stratified rocks. These igneous formations,
while still in a molten state, in coming in contact with the aqueous or
stratified rocks, have usually changed the character of those portions
immediately near them. The chief changes of structure effected by
metamorphic action are crystallization and foliation. Examples of
metamorphic rocks are marble, quartzite, slate, gneiss, and the schists.