The _mechanical_ influence is very apparent in the sands and marls, which are evidently the spoils of islands and continents, washed down by currents and floods of fresh water, and deposited over an ancient ocean bed. The _chemical_ composition of the flints or concretionary nodules, which give such a remarkable character and appearance often to the chalk, is equally demonstrable; from fifty to a hundred beds of chalk, pure and beautifully white, will sometimes be seen alternating with as many bands of dark-colored flints, all regularly arranged as cannon-shot of all sizes on a floor, and presenting, for miles along the cliffs of the sea-shore, lines of beautifully defined fortifications. The _organic_ agent is visible in the nucleus of these round masses, which consists of an animal or vegetable substance, as a coral, a shell, a piece of flustra, or sponge. The nodules assume various shapes, that seem to be molded according to the cavities of the matrix in which they are imbedded, but are actually the forms of the bodies or organic substances to which they are attached. The explanation given is, that a chemical attraction has taken place between the vegetable or animal remains, strewed abundantly through the waters, and the silicious matter held in solution. The silex in solution gradually incrusts, or incorporates with, the organized substance,—and thus were produced at once the flinty concretions and the wonderful petrifactions contained in them. Break any one of these nodular masses, and minute drops of moisture will, if immediately inspected, be seen to ooze out from its pores: thus clearly furnishing a proof of the state of solution in which it originally existed, and the watery menstruum in which it was produced. The mineralogical history and arrangement of the chalk group of rocks are therefore in many respects very interesting. The chalk overlies the wealden, which was a mere delta at the river’s mouth. The bed of the river suddenly disappears, and now there rests upon it a deep sea formation. How stupendous and overwhelming the forces of nature through all her operations! How vast her affluence and prodigality, which could so thoroughly alter all her exterior and interior arrangements, and fill the seas with this new matter.