118 MICROSCOPIC SHELLS OF CYPRIS. 
example of this kind is found in the abundant 
diffusion of the remains of a microscopic crusta- 
ceous 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 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 1000 feet. (See Dr. 
Fitton's Geol. sketch of Hastings, 1833, p. 68.) 
We have similar evidence of the long duration 
of time, in another series of Lacustrine forma- 
tions, more recent than the chalk, viz. in the 
great freshwater deposits of the tertiary period 
in central France ; here the district of Auvergne 
presents an area of twenty miles in width, and 
eighty miles in length, within which strata of 
gravel, sand, clay, and limestone have been 
accumulated by the operations of fresh water, to 
the thickness of at least seven hundred feet. 
Mr. Lyell, in his Principles of Geology, 3rd ed. 
voL iv. p. 98, states that the foliated character of 
