254 
EOCENE PERIOD. 
[Ch. XVIII. 
■wide geographical range, as, for example, Lucina divaricate^ 
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 oyster-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 the 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 
