Ch. IX.] OF AQUEOUS EOCKS. 99 



which they depend, and that difference of longitude as well as latitude is 

 generally accompanied by a dissimilarity of indigenous species. 



As different seas, therefore, and lakes are inhabited at the same period, 

 by different aquatic animals and plants, and as the lands adjoining these 

 may be peopled by distinct terrestrial species, it follows that distinct fossils 

 will be imbedded in contemporaneous deposits. If it were otherwise — if 

 the same species abounded in every climate, or in every part of the globe 

 where, so far as we can discover, a corresponding temperature and other 

 conditions favorable to their existence are found — the identification of 

 mineral masses of the same age, by means of their included organic 

 contents, would be a matter of still greater certainty. 



Nevertheless, the extent of some siugle zoological provinces, es- 

 pecially those of marine animals, is very great ; and our geological 

 researches have proved that the same laws prevailed at remote periods ; 

 for the fossils are often identical throughout wide spaces, and in de- 

 tached deposits, consisting of rocks varying entirely in their mineral 

 nature. 



The doctrine here laid down will be more readily understood, if we 

 reflect on what is now going on in the Mediterranean. That entire sea 

 may be considered as one zoological province ; for, although certain 

 species of testacea and zoophytes may be very local, and each region has 

 probably some species peculiar to it, still a considerable number are com- 

 mon to the whole Mediterranean. 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 contem- 

 poraneous origin of various mineral masses scattered over a space equal 

 in area to half of Europe. 



Deposits, for example, are well known to be now in progress in this 

 sea in the deltas of the Po, Rhone, Nile, and other rivers, which differ 

 as greatly from each other in the nature of their sediment as does the 

 composition of the mountains which they drain. There are also other 

 quarters of the Mediterranean, as off the coast of Campania, or near the 

 base of Etna, in Sicily, or in the Grecian Archipelago, where another 

 class of rocks is now forming ; where showers of volcanic ashes occa- 

 sionally fall into the sea, and streams of lava overflow its bottom ; and 

 where, in the intervals between volcanic eruptions, beds of sand and clay 

 are frequently derived from the waste of cliffs, or the turbid waters of 

 rivers. Limestones, moreover, such as the Italian travertins, are here 

 and there precipitated from the waters of mineral springs, some of which 

 rise up from the bottom of the sea. In all these detached formations, 

 so diversified in their lithological characters, the remains of the same 

 shells, corals, Crustacea, and fish are becoming inclosed ; or, at least, a 

 sufficient number must be common to the different localities to enable the 

 zoologist to refer them all to one contemporaneous assemblage of 

 species. 



There are, however, certain combinations of geographical circum- 

 stances which cause distinct provinces of animals and plants to be sepa- 



