MINUTE FOSSIL SHELLS. 199 



Similar accumulations have been found in deposits 

 of fresh-water formation. Animals of one genus {Ci/- 

 pris) are enclosed in two flat valves, and now inhabit 

 the waters of marshes and lakes. Some clay beds 

 are so abundantly charged with minute shells of one 

 species, that the surfaces of parts into which the 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. 



The Altain chain of mountains in Siberia is flanked 

 on each side by a chain of hills enclosing marine shells. 

 On a comparison of these shells in form and substance, 

 no difference can be detected between several varieties 

 of them and those which still inhabit the sea. At 

 Tourraine, in France, a hundred miles from the coast, 

 and about nine feet beneath the surface, a bed of fossil 

 shells has been found, nine leagues in length, and about 

 twenty feet in thickness. Such beds exist in various 

 parts of Europe, and in South America they are said 

 to be very frequent. 



In the limestone rocks of Dovedale, in the county of 

 Derby, and in the calcareous region which forms so 

 large a portion of the district called " The Peak," 

 marine shells are continually found incorporated. The 



