MK3R0SC0PIC CHAMBERED SHELLS. 97 



rent formations, give farther evidence not only of the lapse 

 of time, but also of important changes in the physical con- 

 dition 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 minute- 

 ness ; the mode in which they are sometimes crowded toge- 

 ther, 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 calca- 

 reous 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 Orjttografico, 1780, pag. 103, Tab. III. fig. 22. H. 

 1,) He farther slates that some idea of their diminutive size 

 may be formed from the circumstance that immense num- 

 bers of them pass through a paper in which holes have been 

 pricked with a needle of the smallest size. 



Our mental, hke 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 fresh-wa- 

 ter formation. A striking 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 laminse into 



VOL. I. — 9 



