The composition of the Chalk, and the prevalence throughout that rock of the relics of animals that can only live in salt-water, prove incontestably that the chalk and flint were deposited in the sea; and that our beautiful South Downs, now so smooth and verdant, and supporting thousands of flocks and herds, and the rich plains and fertile valleys spread around their flanks, were once the bed of an ocean. It is also evident not only that such must have been the case, but also that the Chalk was deposited in the basin of a very _deep_ sea--in the profound abyss of an ocean as vast as the Atlantic. [Sidenote: AMMONITES AND NAUTILI.] From the absence of gravel, shingle, and sea-beach, it is certain that the white chalk-strata were formed at a great distance from sea-shores and cliffs; and this inference is confirmed by the swarms of shells termed _Ammonites_ and _Nautili_, which we know from their peculiar structure were, like the recent pearly Nautilus, inhabitants of deep waters only. For these are chambered shells; that is, are divided internally by thin transverse shelly septa or plates, into numerous cells; the body of the animal occupied only the outer compartment, but was connected with the entire series of chambers by a tube or siphuncle, which passed through each partition. This mechanism constituted an apparatus which contributed to the buoyancy of these animals when afloat on the waves; for the Ammonites and Nautili were able to swim on the surface, or sink to the depths of the ocean at pleasure. The fragile Nautilus that steers his prow, The sea-born sailor of his shell canoe, The Ocean Mab, the fairy of the sea, O'er the blue waves at will to roam is free. He, when the lightning-winged tornadoes sweep The surf, is safe, his home is in the deep; And triumphs o'er the Armadas of mankind, Which shake the world, yet crumble in the wind. Byron, _The Island_. [Sidenote: WHITBY SNAKE-STONES.] [Illustration: Lign. 7:--Ammonite from Whitby.] The Ammonites, so called from the supposed resemblance of their shells to the fabled horn of Jupiter Ammon, are only known in a fossil state; but they must have swarmed in the ancient seas, for several hundred species have been discovered in the Chalk and antecedent strata, though none have been found in any deposits of more recent formation; at the termination of the chalk epoch the whole race, therefore, appears to have perished. The Ammonites are commonly termed _snake-stones_, from the origin ascribed to them by local legends; those of Whitby are well known (see _Lign. 7_).[G] [G] Note III. _Whitby Ammonites._ Thus Whitby's nuns exulting told-- How that of thousand snakes, each one Was changed into a coil of stone, When holy Hilda prayed: Themselves, within their sacred bound, Their stony folds had often found. Scott's _Marmion_.