Composing that part of the earth which is accessible to us,—or as it is generally called the earth’s crust,—there are rocks of various kinds, of which some have been formed by igneous agency, either as lavas or beds of ashes, or in the form of molten magmas which gradually cooled and became crystalline below a mass of superincumbent strata. With these rocks we need not concern ourselves.
A large portion of the earth’s crust consists of such materials as sandstones, limestones, shales, and similar strata which have been formed in precisely the same manner as deposits are being accumulated at the present day. The whole surface of the earth is continually exposed to the action of destructive agencies, and suffers perpetual decay; it is the products of this ceaseless wear and tear that form the building materials of new deposits.
The operation of water in its various forms, of wind, changes of temperature, and other agents of destruction cannot be fully dealt with in this short summary.
A river flowing to the sea or emptying itself into an inland lake, carries its burden of gravel, sand, and mud, and sooner or later, as the rate of flow slackens, it deposits the materials in the river-bed or on the floor of the sea or lake.
Fragments of rock, chipped off by wedges of ice, or detached in other ways from the parent mass, find their way to the mountain streams, and if not too heavy are conveyed to the main river, where the larger pieces come to rest as more or less rounded pebbles. Such water-worn rocks accumulate in the quieter reaches of a swiftly flowing river, or are thrown down at the head of the river’s delta. If such a deposit of loose water-worn material became cemented together either by the consolidating action of some solution percolating through the general mass, or by the pressure of overlying deposits, there would be formed a hard rock made up of rounded fragments of various kinds of strata derived from different sources. Such a rock is known as a Conglomerate. The same kind of rock may be formed equally well by the action of the sea; an old sea-beach with the pebbles embedded in a cementing matrix affords a typical example of a coarse conglomerate. Plant remains are occasionally met with in conglomerates, but usually in a fragmentary condition.
From a conglomerate composed of large water-worn pebbles, to a fine homogeneous sandstone there are numerous intermediate stages. A body of water, with a velocity too small to carry along pebbles of rock in suspension or to roll them along the bed of the channel, is still able to transport the finer fragments or grains of sand, but as a further decrease in the velocity occurs, these are eventually deposited as beds of coarse or fine sand. The stretches of sand on a gradually shelving sea shore, or the deposits of the same material in a river’s delta, have been formed by the gradual wearing away and disintegration of various rocks, the detritus of which has been spread out in more or less regular beds on the floor of a lake or sea. Such accumulations of fine detrital material, if compacted or cemented together, become typical Sandstones.
In tracing beds of sandstone across a tract of country, it is frequently found that the character of the strata gradually alters; mud or clay becomes associated with the sandy deposit, until finally the sandstone is replaced by beds of dark coloured shale. Similarly the sandy detritus on the ocean floor, or in an inland lake, when followed further and further from the source from which the materials were derived, passes by degrees into argillaceous sand, and finally into sheets of dark clay or mud. The hardened beds of clay or fine grained mud become transformed into Shales. As a general rule, then, shales are rocks which have been laid down in places further from the land, or at a greater distance from the source of origin of the detrital material, than sandstones or conglomerates. The conglomerates, or old shingle beaches, usually occur in somewhat irregular patches, marking old shore-lines or the head of a river delta. Coarse sandstones, or grits, may occur in the form of regularly bedded strata stretching over a wide area; and shales or clays may be followed through a considerable extent of country. The finer material composing the clays and shales has been held longer in suspension and deposited in deeper water in widespread and fairly horizontal layers.
In some districts sandstones occur in which the individual grains show a well marked rounding of the angles, and in which fossils are extremely rare or entirely absent. The close resemblance of such deposits to modern desert sands suggests a similar method of formation; and there can be no doubt that in some instances there have been preserved the wind-worn desert sands of former ages. Aeolian or wind-formed accumulations, although by no means common, are of sufficient importance to be mentioned as illustrating a certain type of rock.
CALCAREOUS ROCKS.
The thick masses of limestone which form so prominent a feature in parts of England and Ireland, have been formed in a manner different from that to which sandstones and shales owe their origin. On the floor of a clear sea, too far from land to receive any water-borne sediment, there is usually in process of formation a mass of calcareous material, which in a later age may rise above the surface of the water as chalk or LIMESTONE. Those organisms living in the sea, which are enclosed either wholly or in part by calcareous shells, are agents of limestone-building; their shells constantly accumulating on the floor of the sea give rise in course of time to a thick mass of sediment, composed in great part of carbonate of lime. Some of the shells in such a deposit may retain their original form, the calcareous body may on the other hand be broken up into minute fragments which are still recognisable with the help of a microscope, or the shells and other hard parts may be dissolved or disintegrated beyond recognition, leaving nothing in the calcareous sediment to indicate its method of formation.