[Footnote 55: Lyell's _Principles of Geology_. Ninth edition, p. 252.] 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. [Footnote 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.