We have here a solid mass of stone, composed of several recent species of shells, corals, &c. It is a fragment of a large block, dredged up from the British Channel, off Brighton. Similar masses have been obtained at different soundings along this part of the Sussex coast; and in some specimens numerous other species of recent shells, as oysters, mussels, whelks, &c. enter into the composition of the consolidated rock. The shelly and coralline limestones and sandstones, so abundant in the ancient strata of England have been formed in a similar manner; and when the modern conglomerate of Brighton shall have been permeated with crystalline matter, and subjected to great pressure by superincumbent deposits, through countless centuries, and at length be elevated above the waters, it will constitute beds of shell-marble, in some mountain range, and become an interesting, perhaps the only memento, of the races of mollusca and polypiaria of the present seas, when all record and traces of Great Britain and its inhabitants shall be destroyed. [Illustration: Lign. 124. Shell-Limestone; from the mouth of the Thames. Fig 1.--A mass of Cockle-shells and Whelks, consolidated into a coarse limestone. 2, 4.--One of the shells, Cardium edule, extracted from the block. 3.--A slice of the rock, polished, the markings on the surface being derived from sections of the shells. ] [Sidenote: MODERN SHELL-LIMESTONE.] Off the Kentish coast, near the mouth of the Thames, a bank of consolidated shells, chiefly of one species, is in the progress of formation, from which blocks may be obtained of great firmness and solidity (_Lign. 124_); these, when cut and polished (_fig._ 3), display a variety of markings, produced by the sections of the shells. Extensive shoals of loose shells, composed almost wholly of the _Cardium edule_, exist in several localities, near the embouchure of the Thames; and these are continually shifting with the changes of the wind and tide; it is only in a few places that consolidated blocks occur, like that of which a fragment is figured in _Lign. 124_. These examples of shelly limestones and sandstones now in progress of formation will familiarize the student with the nature and origin of those ancient deposits of a similar character, which contain extinct species and genera of mollusca.