This magnificent fossil is in the Petrified Forest of Arizona; and it affords one of the most striking examples known of the solidification and petrifaction of material by the infiltration of mineral salts. The trunk is now not merely encrusted with stone: it is permeated by silica, and is, in fact, itself a stone as hard as flint.
CHAPTER V
RECORDS LEFT BY THE SEA
We have already spoken of the story which the sea writes in the annals of geology. It is a story with two plots. In the first place, the sea is always wearing away the land. In the second place, it is arranging on its own bed the materials which it takes from the land, either directly or indirectly. As a sequel to both stories, the materials all neatly ranged, packed, and folded are revealed when the sea subsides from them, or when, in process of one of those great geological changes, the origin of which we have already attempted to account for, the sea bottom is raised to become the land of a continent. The first part of the sea's belligerent story is written so plainly for all eyes to see that one scarcely need dwell on it. Every strip of coast around these islands bears witness to it.
Off Shetland masses of rock twelve or thirteen tons in weight have been cut out from the cliff seventy feet above the smooth-water level. The sea's battering-rams are the masses of shingle, gravel, and loose blocks of stone which it carries with it; but it has subtler methods in the corrosive action of its salts, for just as it rusts or wears away iron, so its salts and acids must eat their way into many rocks.
But, after all, the coast-line of the world is a small fraction of the whole land surface of the globe; and a smaller fraction of the sea's own wide area. On that area are flung all the records and treasures which the sea has wrested from the land. The rivers, as we have already several times repeated, are the chief carriers of deposits to the sea. By their deltas they may be known. The deltas of the Ganges and Brahmaputra cover an area as large as that of England and Wales. The delta has been bored through to a depth of nearly five hundred feet, and has been found to consist of numerous alternations of fine clays, marls, and sands or sandstones, with occasional layers of gravel. In all this accumulation of sediment there are no traces of marine animals; but land plants and the plants and animals of the river and of the surrounding land have been discovered in quantity. The sea most often destroys land; but it sometimes deposits beaches; and, we might almost say, silts up the land. At Romney Marsh, for example, a tract of eighty square miles which was marsh in Julius Cæsar's time is now dry land, and has become so partly by the natural increase of shingle thrown up by the waves. The coarsest shingle usually accumulates towards the upper part of the beach, and the rest arranges itself generally according to size and weight, that which is finest being nearest to low-water mark.
It is often long before the stuff brought down by the rivers settles on the floor of the ocean. The finer particles may be carried out to sea for three hundred miles or more before they settle. Within this three-hundred-mile zone the land-derived materials are distributed over the floor in orderly succession. Nearer the land we shall find coarse gravel and sand. Beyond there will be tracts of finer sand and silt, with patches of gravel here and there. Still farther off will come fine blue and green muds, which are made of the tiny particles of such materials as form the ordinary rocks of the land. But when we are once past this zone of land material we come upon deposits which are the ocean's own freehold—materials which it does not derive from the continents, but which may be called oceanic in origin. First there are vast sheets of exceedingly fine red and brown clay. Whence comes it? It is by far the most common deposit in all the deeper parts of the ocean. It may either be the dust of volcanic fragments washed away from volcanic islands, or (which is much more likely) it may be supplied by eruptions under the sea. For it must be remembered that the sea floor is two to five miles nearer the hot rocks that are in the interior of the earth than the land surface is, and that consequently the water coming into contact with them may cause explosions arising from the action of steam. This is a question we shall have to consider later, and for the present we must ask the reader to accept the fact—and read on.
There is one very curious thing about this red clay, and it is that the accumulations of it appear to be built up very slowly. Where it occurs farthest from land great numbers of sharks' teeth with ear bones and other bones of whales have been dredged up from it. Some of these relics are quite fresh; others are coated with a crust of brown peroxide of manganese. Some are covered with this material and hidden in it. One haul of an ocean dredge will bring up the bones in all these states, so that they must be lying side by side. The bones are probably those of many generations of animals, and it must take a long time to cover them with the manganese deposit. But the clay is deposited even more slowly than the manganese, so that it must fall very slowly indeed.
But besides these things the bottom of the sea receives deposits of the remains of all kinds of shells, corals, and all sorts of marine creatures, great and small. As the countless myriads of the animals of the sea die, the shells with which they are covered, or the bones which form their framework, fall continually to the bottom of the oceanic gulfs in which they dwell. Then the ocean floor is covered with the remains of tiny animals incomparably more numerous than the stars of the sky; and this grey slimy ooze of organic matter hardens by pressure into sedimentary rock. In the course of ages, when the slow decline of the water lays it bare, it may become part of the land on which men dwell. But it is always forming, has always been forming, since life first appeared on the earth. It is on this ocean floor that man to-day lays his telegraph cables. Mr. Rudyard Kipling, in his verses "The Deep Sea Cables," has drawn a vivid picture of the bed of the deep ocean:—
The wrecks dissolve above us: their dust drops down from afar—