42 CONTEMPORANEOUS ORIGIN OF ROCKS [Ch. IV, 



those inhabiting the adjoining parts of the Mediterranean, 

 although the two seas are only separated by the narrow isthmus 

 of Suez. We shall show, in a subsequent chapter, that calca- 

 reous formations have accumulated, on a great scale, in the lied 

 Sea, in modern times, and that fossil shells of existing species 

 are well preserved therein ; while we know that, at the mouth 

 of the Nile, large deposits of mud are amassed., including the 

 remains of Mediterranean species. Hence it follows that if, 

 at some future period, the bed of the Red Sea should be laid 

 dry, the geologist might experience great difficulties in en- 

 deavouring to ascertain the relative age of these formations, 

 which, although dissimilar both in organic and mineral charac- 

 ters, were of synchronous origin. 



There might, perhaps, be no means of clearing up the ob- 

 scurity of such a question, yet we must not forget that the 

 north-western shores of the Arabian Gulf, the plains of E^vpt. 



5*7 * 



and the isthmus of Suez, are all parts of one province of terres- 

 trial species. Small streams, therefore, occasional land-floods, 

 and those winds which drift clouds of sand along the deserts, 

 might carry clown into the Tied Sea the same shells of fluviatilc 

 and land tcstacea, which the Nile is sweeping into its delta, 

 together with some remains of terrestrial plants, whereby the 

 groups of strata, before alluded to, might, notwithstanding the 

 discrepancy of their mineral composition, and marine organic 

 fossils, be shown to have belonged to the same epoch. 



In like manner, the rivers which descend into the Caribbean 

 Sea and Gulf of Mexico on one side, and into the Pacific on 

 the other, carry down the same fluviatile and terrestrial spoils 

 into seas which are inhabited by different groups of marine 

 species. 



But it will much more frequently happen, that the coexist- 

 ence of terrestrial species, of distinct zoological and botanical 

 provinces, will be proved by the specific identity of the marine 

 organic remains which inhabited the intervening space. Thus, 

 for example, the distinct terrestrial species of the south of 

 Europe, north of Africa, and north-west of Asia, mi "-lit all be 



