254 EOCENE PERIOD. [Ch, XVIII. 



\vide geographical range, as, for example, Lucina divaricata, 

 and are therefore fitted to exist under a great variety of cir- 

 cumstances. On the other hand, the great proportion of the 

 Eocene marine testacea which have become extinct sufficiently 

 demonstrates that the loss of species has been due to general 

 laws, and that a sudden catastrophe, such as the invasion of a 

 whole continent by the sea a cause which could only anni- 

 hilate the terrestrial and fresh- water tribes, is an hypothesis 

 wholly inadequate to account for the phenomenon. 



Strata with and without organic remains alternating. Be- 

 tween the gypsum of the Paris basin and the upper marine 

 sands a thin bed of oysters is found, which is spread over a 

 remarkably wide area. From the manner in which they lie, it 

 is inferred that they did not grow on the spot, but that some 

 current swept them away from a bed of oysters formed in some 

 other part of the bay. The strata of sand which immediately 

 repose on the\>yster-bed are quite destitute of organic remains ; 

 and nothing is more common in the Paris basin and in 

 other formations, than alternations of shelly beds with others 

 entirely devoid of them. The temporary extinction and 

 renewal of animal life at successive periods have been inferred 

 from such phenomena, which may nevertheless be explained, 

 as M. Prevost justly remarks, without appealing to any such 

 extraordinary revolutions in the state of the animate creation. 

 A current one day scoops out a channel in a bed of shelly sand 

 and mud, and the next day, by a slight alteration of its course, 

 ceases to prey upon tjie same bank. It may then become 

 charged with sand unmixed with shells, derived from some dune, 

 or brought down by a river. In the course of ages an in- 

 definite number of transitions from shelly strata to those with- 

 out shells may thus be caused. 



Concluding remarks. It will be seen by our observations 

 on Auvergne and other parts of Central France, and on the 

 district round Paris, that geologists have already gained a con- 

 siderable insight into the state of the physical geography of 

 part of Europe during the Eocene period. We can point to 



