CONDITIONS OF COAL DEPOSIT. 361 



vegetation to have been acquired. Under what circum- 

 stances are we Ukely to find this vegetation fossilized ? 

 Large surfaces of land impfy large rivers with their accom- 

 panying deltas ; and are liable to have lakes and swamps 

 These, as we know from extant cases, are favourable t( 

 rank vegetation ; and afford the conditions needful for pre- 

 serving it in the shape of coal-beds. Observe, then, that 

 while m the early history of such a continent a carbonif- 

 erous period could not occur, the occurrence of a carbonif- 

 erous period would become probable after long-continued 

 upheavals had uncovered large areas. As in our own sedi- 

 mentary series, coal-beds would make their appearance only 

 after there had been enormous accumulations of earlier 

 strata charged with marine fossils. 



Let us ask next, in what order the higher forms of ani- 

 mal life would make their appearance. We have seen how, 

 in the succession of marine forms, there would be some- 

 thing like a progress from the lower to the higher : bring- 

 ing us in the end to predaceous molluscs, crustaceans, and 

 fish. What are likely to succeed fish ? After marine crea- 

 tures, those which would have the greatest chance of sur- 

 viving the voyage would be amphibious reptiles : both be- 

 cause they are more tenacious of life than higher animals, 

 and because they would be less completely out of their 

 element. Such reptiles as can live in both fresh and salt 

 water, like alligators ; and such as are drifted out of the 

 mouths of great rivers on floating trees, as Humboldt says 

 the Orinoco alligators are ; might be early colonists. 



It is manifest, too, that rej^tiles of other kinds would 

 be among the first vertebrata to people the new continent. 

 If we consider what will occur on one of those natural 

 rafts of trees, soil, and matted vegetable matter, sometimes 

 swept out to sea by such currents as the Mississippi, with a 

 miscellaneous living cargo ; we shall see that while the 

 active, hot-blooded, highly-organized creatures will soop 

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