Ch. IV.] PROVED BY ORGANIC REMAINS. 41 



above-mentioned formations are simultaneously in progress, 

 may be considered as one zoological province ; for, although 

 certain species of testacea and zoophytes may be very local, and 

 each region may probably have some species peculiar to it, still 

 a considerable number are common to the whole sea. If, 

 therefore, at some future period, the bed of this inland sea 

 should be converted into land, the geologist might be enabled, 

 by reference to organic remains, to prove the contemporaneous 

 origin of various mineral masses throughout a space equal in 

 area to a great portion of Europe. The Black Sea, moreover, 

 is inhabited by so many identical species, that the delta of the 

 Danube and the Don might, by the same evidence, be shown 

 to have originated simultaneously. 



Such identity of fossils, we may remark, not only enables us 

 to refer to the same era, distinct rocks widely separated from 

 each other in the horizontal plane, but also others which may 

 be considerably distant in the vertical series. Thus, for ex- 

 ample, we may find alternating beds of clay, sand, and lava, 

 two thousand feet in thickness, the whole of which may be 

 proved to belong to the same epoch, by the specific identity of 

 the fossil shells dispersed throughout the whole series. It 

 may be objected, that different species would, during the same 

 zoological period, inhabit the sea at different depths, and that 

 the case above supposed could never occur ; but, for reasons 

 explained in the last volume*, we believe that rivers and tidal 

 currents often act upon the banks of littoral shells, so that a sea 

 of great depth may be filled with strata, containing throughout 

 a considerable number of the same fossils. 



The reader, however, will perceive, by referring to what we 

 have said of zoological provinces, that they are sometimes sepa- 

 rated from each other by very narrow barriers, and for this 

 reason contiguous rocks may be formed at the same time, dif- 

 fering widely both in mineral contents and organic remains. 

 Thus, for example, the testacea, zoophytes, and fish of the Red 

 Sea, may be considered, as a group, to be very distinct from 



* Chap. xvii. p. 280. 



