formations, give further evidence not only of the lapse of time, but also of important changes in the physical condition and climate of the ancient earth.
Besides these more obvious remains of Testacea and of larger animals, minute examination discloses occasionally prodigious accumulations of microscopic shells that surprise us no less by their abundance than their extreme minuteness; the mode in which they are sometimes crowded together, may be estimated from the fact that Soldani collected from less than an ounce and a half of stone found in the hills of Casciana, in Tuscany, 10,454 microscopic chambered shells. The rest of the stone was composed of fragments of shells, of minute spines of Echini, and of a sparry calcareous matter.
Of several species of these shells, four or five hundred weigh but a single grain; of one species he calculates that a thousand individuals would scarcely weigh one grain. (Saggio Orittografico, 1780, pag. 103, Tab. III. fig. 22, H. 1.) He further states that some idea of their diminutive size may be formed from the circumstance that immense numbers of them pass through a paper in which holes have been pricked with a needle of the smallest size.
Our mental, like our visual faculties, begin rapidly to fail us when we attempt to comprehend the infinity of littleness towards which we are thus conducted, on approaching the smaller extremes of creation.
Similar accumulations of microscopic shells have been observed also in various sedimentary deposites of freshwater formation. A striking example of this kind is found in the abundant diffusion of the remains of a microscopic crustaceous animal of the genus Cypris. Animals of this genus are enclosed within two flat valves, like those of a bivalve shell, and now inhabit the waters of lakes and marshes. Certain clay beds of the Wealden formation below the chalk, are so abundantly charged with microscopic shells of the Cypris Faba, that the surfaces of many laminæ into
