SHELLS. 
201 
to establish those divisions which are called geological forma¬ 
tions. . . . Thus it appears, that the more perfect forms of 
animals become gradually more abundant, as we advance 
from the older into the newer series of depositions, whilst 
the more simple orders, though often changed in genus 
and species, and sometimes losing whole families, which 
are replaced by new ones, have pervaded the entire range 
of fossiliferous formations. . . 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 some¬ 
times 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, ten 
thousand- four hundred and fifty-four microscopic cham¬ 
bered 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. . . . Similar accumulations 
of microscopic shells have been observed also in various 
sedimentary deposits of fresh-water formation. A strik¬ 
ing 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, now inhabiting 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 laminae into 
which this clay is easily divided, are often entirely covered 
with them as with small seeds. The same shells occur 
also in the Hastings sand and sandstone, in the Sussex 
marble, and in the Purbeck limestone, all of which were 
deposited during the same geological epoch in an ancient 
lake or estuary, wherein strata of this formation have 
been accumulated to the thickness of nearly a thousand 
feet. ... In the case of deposits formed in estuaries, the 
