The alluvial matter deposited by the Rhone at its entrance into the Lake of Geneva suffers perhaps no change when it once reaches the bottom. Layer after layer accumulates tranquilly, without disturbance from surface currents or other causes, so that the renovating effects of the stream have here every advantage. It is otherwise, however, where a delta gathers at the mouth of a river upon the sea-margin. There tides and currents are ever demolishing what the stream has piled up. Often, too, owing to the prevalence of high winds from seawards, the river is dammed up for leagues, and the waters of the ocean encroach far on the delta, mingling in this way marine remains with those that are fluviatile or terrestrial. But with these modifications the process of delta-formation remains essentially the same, both in lakes and at the sea. The vast quantities of sand and gravel transported by rivers during the flood-season sink to the bottom as soon as the motion of the water will permit. This takes place at the shore, where eventually wide tracts of low alluvial land encroach upon the sea, covered with marshes and overgrown with vegetation. A section of any of these deltas, obtained in boring for water, shows a succession of sands and clays, with occasionally a few calcareous beds and quantities of peaty matter formed of vegetation either drifted or that grew on the spot.[56] If, now, a sufficient amount of matter were piled over these loose incoherent strata, they would eventually become as hard and compact as any of our ordinary building stones. The sand would subside into a firm compact sandstone; the clay, in like manner, would consolidate into fissile shale; the peat would become chemically altered into coal; the calcareous seams would take the form of layers of limestone; while the leaves, twigs, branches, and trunks, dispersed through all the beds, would get black and carbonized, so as precisely to resemble the lepidodendra, calamites, stigmariæ, &c., of the carboniferous rocks. And thus might a mass of fossiliferous strata, thousands of feet deep and thousands of square miles in extent, be amassed by the prolonged operation of a single river.
[56] The structure of maritime deltas, especially their relation to the growth and entombment of forests, will be more fully alluded to in a subsequent chapter, when we come to inquire into the origin of a coal-field.
It often happens that a delta is prevented from extending further seawards owing to the prevalence of some marine current that comes sweeping along the coast-line and cuts away the accumulations thrown down by the river. The sediment thus removed is often carried to great distances, and eventually settles down as a fine mud along the floor of the sea, entombing any fucoids, infusoria, shells, corals, fish-bones, or other relics that may lie at the bottom.
He who has witnessed a storm along a rocky coast-line, has marked the breakers battering against the weather-bleached cliffs, and heard the thunder-like rattle of the shingle at the recoil of every wave, needs not to be told how vast an amount of sediment must in this way be formed. The pebbles of the beach are ground down still smaller, the sand produced by their friction finds its way to a lower level, while the finer particles taken up by the water are borne out to sea, and if a current traverse the locality may be transported for leagues, till they at last settle to the bottom. The floor of the sea is consequently always receiving additions in the form of fine mud—the waste of the land—derived either from breaker-action, rivers, or icebergs, so that a series of marine deposits exactly similar to those we find among the rocks of our hills and valleys, must be constantly in the course of formation. If circumstances be favourable, the shingle of the beach may eventually either be covered over or reach a part of the sea undisturbed by currents or waves, and then consolidate into what we call conglomerate or pudding-stone. The sand, as before, becomes sandstone, and the mud laminated shale or hardened clay. These deposits may go on forming for thousands of years, until at last some slow elevation or some sudden upheaval of the ocean bed brings them to the light of day as part of a new continent. Thus exposed they would differ in no respect from rocks of a similar kind now visible, and the geologist, in tracing out their origin and history, would have no hesitation in ranking them among the ordinary marine formations of the globe.
In fine, we cannot quit the subject without being convinced that these ceaseless changes afford one of the grandest examples of that continuous series of mutations—cycle and epicycle—which has been already alluded to as a distinguishing feature in all the operations of Nature. We are accustomed to think and speak of "the everlasting hills." We look on the solid lands whereon we dwell as the emblem of all that is stable and steadfast, and on the boundless ocean as the type of all that is unsteady and changeful. The traveller who stands on those plains where the human race was cradled, marks still the same valleys with their winding rivers, still the same rocks and hills, still the same blue sky overhead. The dust of centuries has gathered over the graves and the dwellings of the early races, yet the covering is but thin, and if we could conjure from their resting-place some of these venerable patriarchs, they might perhaps see little or no change on the haunts of their boyhood. We feel it otherwise, however, when we contemplate the ocean. In sunshine and in storm its surface never rests. The wave that now breaks against some bald headland of our western shores may have come sweeping across from the coast of America, and the broad swell that rolls into surf along the shores of Newfoundland may have travelled from the frozen seas of the North Pole. And so it has ever been; the "far resounding sea" of Homer is the "far resounding sea" still; and the "countless dimpling of the waves," invoked in his agony by the chained Prometheus, remains restless and playful as ever.
"Firm as a rock," and "fickle as the sea," have therefore become proverbs of universal acceptance. Yet when we investigate the matter as we have done in this and the preceding chapter, it appears that an exactly opposite arrangement would be nearer the truth. It is the sea that remains constant—
"Time writes no wrinkle on its azure brow;"
while the land undergoes a continual change. Hills are insensibly mouldering away, valleys are ever being widened and deepened, rocky coasts and low alluvial shores suffer a constant abrasion, while even within the bowels of the earth the process of decomposition uninterruptedly proceeds. And thus, in place of remaining unchanged from the beginning, we know of nothing more mutable than the land on which we dwell, so that if the waste everywhere so apparent were to go on unchecked or unmodified, island and continent would eventually disappear beneath the waves. Here, however, another principle comes into operation. The debris removed from the land, as we have seen, is not annihilated. Slowly borne seawards, it settles down at river mouths or on the floor of the ocean as an ever-thickening deposit, which eventually hardens into rock, as solid and enduring as that whence it was derived. But it does not always remain there. Owing to the action of subterranean agencies with which we are but slightly acquainted, different parts of the sea-bottom are continually rising. Sometimes this process goes on very slowly, as along the shores of Sweden, where the coast has been ascertained to emerge in some localities at the rate of about thirty inches in a century; sometimes with prodigious rapidity, as on the coast of Chili, where the land was upheaved from two to seven feet in a single night. There can thus be no doubt that the mysterious agency which produces earthquakes and volcanoes on the land affects equally that portion of the earth's crust covered by the waters of the ocean, and must be ceaselessly employed in elevating large areas of sea-bottom into new continents, that will ere long become clothed with vegetation and peopled with animals. In contemplating, therefore, the constant decay in progress on the surface of the land, we see not a mere isolated process of waste, but a provision for future renovation. The sandstone cliffs of the shore are battered down and their debris carried out to sea, but when sea-bottom comes to be land-surface, they may be sandstone cliffs again, lashed once more by the breakers, and once more borne as sediment to the depths of the sea. And thus, in what may seem to us sublime antagonism, land is ever rising in the domain of ocean, and ocean ever encroaching on the regions of land. No sooner does a new island, or mountain peak, or wide area of continent, appear above the waves, than the abrading agencies are at work again. Rain, air, frost, rivers, currents, breakers, all begin anew the process of destruction, and cease not until the land has utterly disappeared, and its worn debris has sunk in mid-ocean to be in process of time once more dry land, and suffer another slow process of obliteration.
Such is the economy of nature around us now, and that such will continue to remain the condition of things in the future, we can affirm with probability from a consideration of the history of the past. The geologist can point to masses of rock several miles in thickness, and occupying a large area of the globe, formed entirely of the worn debris of pre-existing formations. The very oldest rocks with which he is acquainted are made up of hardened sediment, pointing to the existence of some land, even at that early period, worn down by rivers or wasted by the sea. During all the subsequent ages the same principles were at work, and now well-nigh the only evidence of the geological periods is to be gathered from the layers of sediment that successively settled down at the sea-bottom. The records which it is the task of the geologist to decipher, are for the most part written in sand and mud—the deposits of the ocean, for in by far the larger number of formations into which the stratified part of the earth's crust has been divided, and which form his only guide to the history of the past, he can detect no trace of land. Hill and valley have alike disappeared, and the character of their scenery and inhabitants he can often but dimly conjecture from the nature of the sediment and of the drifted terrestrial relics that may chance to be found among strata wholly marine.