Moreover, the quantity of material which has been furnished by organic causes is by no means small. The coal-beds are the product of vegetable growth exclusively. We not unfrequently find strata of great extent consisting almost entirely of the shells of molluscous animals, of the stems of encrinites, or of the shields of microscopic animalcules.
But the most abundant rock which can be regarded as the product of animal organization is the coral formation. It consists of immense walls of coral limestone, separating either an atoll or the land of an island or continent from the open sea. The base of this wall has a width varying from a hundred feet to a mile or more, and the outer edge of it is at such a distance from the shore as to give a depth not much exceeding a hundred feet. Over this area of the bed of the sea, which forms the base of the wall, the coral polyp commenced its work. Attaching itself in immense numbers over this area, it deposits calcareous matter from its under surface, and thus, by degrees, elevates itself towards the surface of the water, till it reaches a level a little above low-water mark. The height of the wall would not, with these conditions, exceed one hundred feet; but some hundreds of the islands surrounded by coral walls are gradually subsiding. The depositions of the polyps keep pace with the subsidence, so that this wall has reached an elevation from its base of a thousand feet, and in one instance of two thousand feet. (See Figs. [61], [62], [63], [64].)
Most of the islands of the torrid zone are thus surrounded with coral reefs, except a few where the cold polar currents reduce the temperature too low to admit of their growth. In one instance, along the north-east coast of New Holland, there is a coral reef, some twenty-five miles from the land, which has a continuous extension, excepting occasional inlets of no great depth, of a thousand miles. The reef along the island of New Caledonia is four hundred miles long. A large number of other reefs have a nearly equal extension. There is thus an area of several thousands of square miles covered to a great depth with this coralline limestone. Some limestone formations of great extent among the older rocks were the work of similar animals. These lower forms of organization have, therefore, always been important geological agents, both in collecting the carbonate of lime from its solution in the waters of the ocean, and in depositing it as solid rock.
Water is, next to heat, the most important geological agent. All the stratified rocks are aqueous deposits, and their total amount is in some respects a measure of the influence which this agent has exerted. The materials have been obtained from the destruction of preëxisting rocks, transported by water, and deposited in layers.
When the first strata were formed, the sediment must have been obtained entirely from igneous rocks, because only those rocks existed; but now it is obtained from every kind of rock which is exposed to abrading or decomposing agencies. Hence, many of the later formations contain fragments, and sometimes within the fragments well-characterized fossils, of earlier formations.
The sediment which is ultimately to become stratified rock is deposited on the beds of the ocean, and other great reservoirs of water. The formation of most of the aqueous rocks, therefore, as well as of the igneous rocks, is deep below the surface; and neither of these operations, on the large scale, is directly exposed to our observation. We may, however, learn by observation, how the sediment is furnished to the waters and transported by them, and we can form some correct ideas of the manner in which it will be laid down on the bed of the ocean, and solidified.
I. The Furnishing of Sediment.
1. Almost all the minerals which occur in the geological formations are, to some slight extent, soluble in water. Hence, rain water, by passing through a stratum of earth or rock and reäppearing as a spring, loses the insipidity which it had as pure water, and becomes palatable. It is then found to hold in solution some small proportion of earthy substances, upon which this change of taste depends. Although the proportion of dissolved matter is very small, yet the surface of earth upon which this distilled water is shed is one-fourth of the surface of the globe, and solution below all that surface is constantly taking place. No inconsiderable amount must thus have been furnished, from the existing rocks of each period, towards the formation of the strata of a later period.
There are some substances which are soluble in water, in large quantities. Rock-salt is an example. It is not found in any very large proportion in rocks generally, but a very large aggregate amount has been taken up by the waters which have filtered through the strata. The ocean gathers into itself, by degrees, all the soluble substances which are thus taken up. It receives supplies of water charged with these substances from springs, rivers and lakes. It returns as much water as it receives; but it is always in the form of vapor, and is therefore pure water. Hence the saline properties of the ocean, and of those inland seas which have no outlets. There is thus gathered the materials for the rock-salt deposits.