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Show 42 CONTEMPORANEOUS ORIGIN OP ROCKS (Ch. IV. those inhabiting the adjoining parts of the Medite~ranean, althouO'h the two seas are only separated by the narrow Isthmus of Sue~. We shall show, in a subsequent chapter, :hat calca· reous formations have accumulated, on a great scale, m the Red Sea, in modern times, and that fossil shells of existing species are well preserved therein; while we know that, ~t the .mouth of the Nile, large deposits of mud are amassed, mcludmg the remains of Mediterranean species. Hence it follows that if, at some future period, the bed of the Red Se~ shou~d ~e laid dry, the geologist might experie~ce great difficulties m. 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 clearmg 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 Egypt, and the isthmus of Suez, are all parts of one province of terrestrial species. Small streams, therefore, occasional land-floods, and those winds which drift clouds of sand along the deserts, might carry down into the Red Sea the same shells of fluviatile and land testacea, which the Nile is sweeping into its delta, toO'ether with some remains of terrestrial plants, ·whereby the gr~ups 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 coexistence of ter·rest·rial 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~ ~nd nor~h-west of 1\s~a~ might all be Ch. IV.] PROVED BY ORGANIC REMAINS, 43 shown to have been contemporaneous, if we suppose the rivers flowing from those three countries to carry the remains of different species of the animal and vegetable kingdoms into the Mediterranean. In like manner, the sea intervening between the northern shores of Australia and the islands of the Indian ocean, contains a great proportion of the same species of corallines and testacea, yet the land animals and plants of the two regions are very dissimilar, even the islands nearest to Australia, as Java, New Guinea, and others, b~ing inhabited by a distinct assemblage of terrestrial species. It is well known that there are calcareous rocks, volcanic tuff, and other strata in progress, in different parts of these intermediate seas, wherein marine organic remains might be preserved and associated with the terrestrial fossils above alluded to. As it frequently happens that the barriers between different provinces of animals and plants are not very strongly marked, especially where they are determined by differences of temperature, there will usually be a passage from one set of species to another, as in a sea extending from the temperate to the tropical zone. In such cases, we may be enabled to prove, by the fossils of intermediate deposits, the connexion between the distinct provinces, since these intervening spaces will be inhabited by many species, common both to the temperate and equatorial seas. On the other hand, we may be sometimes able, by aid of a peculiar homogeneous deposit, to prove the former coexistence of distinct animals and plants in distant regions. Suppose, for example, that in the course of ages the sediment of a river, like that of the Red River in Louisiana, is dispersed over an area several hundred leagues in length, so as to pass from the tropics into the temperate zone, the fossil remains imbedded in red mud might indicate the different forms which inhabited, at the same period, those remote regions of the earth. It appears, then, that mineral and organic characters, although often inconstant, may, neverthelessJ enable us to· establish the |