/ 



98 MICROSCOPIC SHELLS OE CYPRIS. 



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 accu- 

 mulated 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 formations, more recent than 

 the chalk, viz. in the great fresh-water deposites of the ter- 

 tiary 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, 3d edit. vol. iv. p. 98, 

 states that the foliated character of many of the marly 

 beds of this formation is due to the presence of countless 

 myriads of similar exuvias of the Cypris which give rise to 

 divisions in the marl as thin as paper. Taking this fact in 

 conjunction with the habit of these animals to moult and 

 change their skin annually, together with their shell, he 

 justly observes that a more convincing proof of the tran- 

 quillity of the waters, and of the slow and gradual process 

 by which the lake was filled up with fine mud cannot be de- 

 sired. 



Another proof of the length of time that must have elapsed 

 during the deposition of these tertiary fresh-water forma- 

 tions in Auvergne, is afibrded near Cleremont by the occur- 

 rence of beds of limestone, several feet in thickness, almost 

 wholly made up of the fossil Indusisc, or Caddis-like cover- 

 ings, resembling the cases that enclose the larvae of our 

 common May-fly. 



Mr. Lyell states that a single individual of these Indusiae 

 is often surrounded by no less than a hundred minute shells 



