But this conclusion does not rest upon antecedent probability alone. We have proof unquestionable that a process such as we have described is actually going on. In the dredging of the river Thames large masses of solid Conglomerate are found from time to time, firmly compacted together by a ferruginous cement. And there is internal evidence that the process of solidification has been effected by natural causes within historic times; for it happens not unfrequently that Roman coins and fragments of pottery are found embedded in the solid block of stone. Similar discoveries were made in deepening the bed of the river Dove in Derbyshire, about the year 1832. Thousands of silver coins were found about ten feet under the surface, firmly cemented into a hard Conglomerate. Several of these coins bear dates of the thirteenth and fourteenth centuries; and therefore the pebbles which form the rock must have been deposited and converted into a solid mass since that time. But we must not suppose that so long an interval is necessary for the consolidation of rocks. In the early part of the present century a vessel called the Thetis was wrecked off cape Frio on the coast of Brazil. A few months afterward, when an attempt was successfully made to recover the dollars and other treasures which had gone to the bottom with the wreck, they were found completely enveloped in solid masses of quartzose Sandstone. The materials of the newly-formed stone were in this case manifestly derived from the granite rocks of the Brazilian coast.[38]
In many parts of the Mediterranean, and along its shores, this process is known to be going on with equal rapidity. “The new-formed strata of Asia Minor,” writes Sir Charles Lyell, “consists of stone, not of loose, incoherent materials. Almost all the streamlets and rivers, like many of those in Tuscany and the south of Italy, hold abundance of carbonate of lime in solution, and precipitate Travertine, or sometimes bind together the sand and gravel into solid Sandstones and Conglomerates; every delta and sandbar thus acquires solidity, which often prevents streams from forcing their way through them, so that their mouths are constantly changing their position.”[39] In the Museum at Montpelier is exhibited a cannon embedded in a crystalline calcareous rock which was taken up from the bed of the Mediterranean near the mouth of the Rhone.[40]
To these examples of the solidification of rock within recent times we are tempted to add one more, taken from a Memoir published by the late Dr. Paris in the Transactions of the Royal Geological Society of Cornwall. “A sandstone occurs in various parts of the northern coast of Cornwall, which affords a most instructive example of a recent formation, since we here actually detect Nature at work in converting loose sand into solid rock. A very considerable portion of the northern coast of Cornwall is covered with calcareous sand, consisting of minute particles of comminuted shells, which in some places has accumulated in quantities so great, as to have formed hills of from forty to fifty feet in elevation. In digging into these sand-hills, or upon the occasional removal of some part of them by the winds, the remains of houses may be seen; and in places where the churchyards have been overwhelmed, a great number of human bones may be found. The sand is supposed to have been originally brought from the sea by hurricanes, probably at a remote period. It first appears in a state of slight but increasing aggregation on several parts of the shore in the Bay of St. Ives; but on approaching the Gwythian River it becomes more extensive and indurated.... It is around the promontory of New Kaye that the most extensive formation of Sandstone takes place. Here it may be seen in different stages of induration, from a state in which it is too friable to be detached from the rock on which it reposes, to a hardness so considerable that it requires a very violent blow from a sledge to break it. Buildings are constructed of it; the church of Cranstock is entirely built with it; and it is also employed for various articles of domestic and agricultural uses.”
No reasonable doubt can therefore remain that the loose beds of gravel, sand, and clay, which, as we have already seen, are deposited from day to day, and from year to year, and from century to century, beneath the waters of the ocean, may be converted in the course of time by natural agents into solid rocks of Conglomerate, of Sandstone, and of Shale. But this is not enough. It yet remains for us to explain how these solid rocks come to be arranged in a series of distinct layers or strata. The reader will remember that the supply of materials in any given area of the ocean is not fixed and continuous, but, on the contrary, variable and intermittent. During the periodical rains within the tropics, and during the melting of the snows in high latitudes or in mountain regions, the rivers become enormously swollen, and carry down a far greater quantity of sediment than at other seasons. The waste of cliffs, too, by the action of the waves, is much greater in winter than in summer. Thus, while at one season a particular river or current may be comparatively free from sediment, at another it will carry along in its turbid course an almost incredible freight of mineral matter. We have a notable example in the case of the Ganges. The bulk of earthy matter which this river discharges into the sea during the four months of rain, averages about 50,000,000 of cubic feet per day; whereas the daily discharge during the three months of hot weather is considerably less than one hundredth part of that amount.[41]
Besides this variety in the quantity of materials carried, there is also a great variety in the velocity both of rivers and of currents; and therefore they will not always carry the same materials to the same distance; for the less rapid the stream, the sooner will the sediment fall to the bottom. We may add that currents, as is well known, often change their direction from various causes, and thus at different times they will carry the waste of the land to different parts of the ocean.
From these considerations two conclusions may be fairly deduced: First, that the process of deposition may often go on very rapidly for a time over a given area, and then altogether cease, and after an interval begin again. In this way time may be allowed for one deposit to acquire more or less consistency before the next is superimposed; and thus a succession of distinct beds will be produced. Secondly, we may infer that the same precise materials will not always be deposited over the same area; at one time it will be sand, at another gravel, at another clay, at another some combination of these or other mineral substances. And thus it may happen that the strata deposited in successive periods of time shall not only be distinct one from the other, but composed of different materials;—that there shall be, in fact, as we so often see that there are, beds of Conglomerate, Sandstone, Clay, Marl, and other rocks, succeeding one another in every variety of order.
CHAPTER VI.
STRATIFIED ROCKS OF MECHANICAL ORIGIN—FURTHER ILLUSTRATIONS.
Impossible to witness the formation of stratified rocks in the depths of the ocean—On a small scale examples are exhibited by rivers and lakes—Alluvial plains—Their extraordinary fertility—Great basin of the Nile—Experiments of the Royal Society—The Mississippi and the Orinoco—Some rivers fill up their own channels—Case of the river Po—Artificial embankments—Large tract of alluvial soil deposited by the Rhone in the Lake of Geneva—Deltas—The delta of the Ganges and Brahmapootra—Delta of the Nile.