The processes of deposition upon the land and beneath the water, which we have so far been studying, result, for the most part, only in the bringing together of great masses of loose and incoherent material, which in the case of marine deposits are saturated with salt water. If such masses are properly to be compared with the hard rocks of the earth's crust, it will be necessary to show that loose sediments may be consolidated and rendered hard and firm, like the latter. This is not difficult, for we have abundant evidence to prove that such consolidation actually does take place, and in a variety of ways.

(1) Consolidation By Weight Of Sediment

When deposited on a sinking sea-bottom, sediments often accumulate in masses of great thickness, and in such cases the lower portions must tend to consolidate from the weight of the overlying masses. Of course, such a process cannot be directly observed in modern accumulations, because only the surface of them is accessible, but from the analogy of observed facts we may safely infer that this weight is not without effect.

(2) Consolidation By Cement

Sediment is often penetrated by percolating waters, which carry in solution various cementing substances, such as Si02, CaC03, FeC03, etc., and the deposition of these materials in the interstices of the loose sediment will bind the particles into a firm rock. This process we have already had occasion to observe in several instances, as in the coral reefs, the drift-sand rock of Bermuda, the modern sandstones on the coast of Asia Minor and Brazil, and many others. In all of these cases the cementing substance is CaC03, but other modern rocks are known in which Fe203, formed by the oxidation of FeC03, plays the same role, as in Florida where the waters from ferruginous springs bind the grains of calcareous sand into a hard rock, the modern date of which is proved by the presence in such rock of the bones of Indians. Both of these substances are very common as cements among the ancient rocks. The deposition of silica in the interstices of sand has also been observed, where the original sand grains can with difficulty be detected with the microscope, the rock appearing to be a mass of crystalline quartz grains.

A cementing effect may also be produced by reactions within the mass of the sediment itself, as is seen in the solidification of volcanic ash mingled with water to form tuffs.

(3) Consolidation Through Heat

This may be local, as in the neighbourhood of volcanoes, or general and due to the internal heat of the earth. For sediment to reach great thickness it must subside, and this subsidence brings the lower parts of the mass deep down into the crust, where they are invaded by the earth's interior heat, and baked as bricks are burnt in a kiln. This process is likewise one which cannot be directly observed, but the effects of molten lava upon loose sediments may be watched, and the consolidating power of heat has been tested experimentally.

(4) Consolidation By Lateral Pressure

This is probably the most widely acting and important agency of consolidation. Though it acts so gradually and at such depths that we cannot see it in operation, yet the inference is, none the less, a safe one. We shall see later that very many of the stratified rocks are no longer in the nearly horizontal position in which they were first laid down, but have been folded and fractured through the operation of great lateral pressures. The more intensely folded and compressed any rock has been, the harder has it become, not only through the mechanical pressure, but by the heat and the chemical changes which such compression generates. In addition to this, we know from experiment that loose materials may be consolidated by powerful compression. Certain exceptional rocks of very ancient date are known, which are almost as incoherent as when first accumulated, but these all retain their original horizontal position and have not been compressed. It must not be supposed, however, that only compressed sediments have become hard, for great areas of scarcely disturbed rocks are found, which are perfectly solid and firm; here some other solidifying agent has been at work.

There are certain other features in which the loose modern sediments differ from the older and harder rocks, such as joints, and cleavage which divides many rocks into thin plates, independently of the planes of stratification. These may be shown, however, to be structures which the rocks have acquired after their formation, and therefore need not be discussed here.

The parallel is now complete between the sediments which we may observe to-day in the process of accumulation, and the hard stratified rocks which make up by far the largest part of the dry land. For all these ancient rocks we may find a counterpart in sediments now forming, and we may conclude with perfect confidence that the ancient rocks were formed by the same agencies as the modern accumulations. Every rock contains a more or less legible record of its own history.