222 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



Atlantic, embraced both continents ; for cretaceous deposits 

 are spread over a considerable part of North America, as well 

 as Europe. The. subsidence of the Wealden must necessarily 

 have taken place before the lowermost of the chalk-strata 

 (the neocomian) were deposited ; but the destruction of the 

 islands, or continents, from whose spoils the Wealden deposits 

 were derived, must have been gradual, since remains of their 

 fauna and flora are found sparingly distributed in the cre- 

 taceous sands. 



It is obvious, that the period through which the chalk-ocean 

 extended, with but little modification in its character for the 

 organic remains of the formation maintain a very general 

 correspondence throughout must have been of great dura- 

 tion, however rapid may have been the reproduction of those 

 infinitesimal animal forms (the Foraminifera) of which the 

 strata of white limestones are so largely composed. 



Til. Tertiary Epoch. The close of the cretaceous era 

 appears to have been followed by elevatory movements, which 

 broke up the bed of the ocean, and slowly upheaved large 

 areas ; and as the elevation continued, the deposits which had 

 been formed in the profound depths of the sea were brought to 

 the surface, and became exposed to the destructive action of 

 the waves. 



These masses of cretaceous rocks were gradually disinte- 

 grated and swept away, and in some places the Wealden beds 

 gradually emerged above the waters, and, finally, the petrified 

 forest of Portland rose in the midst of the sea, and became 

 dry land. At length some portions of the strata attained an 

 elevation of several hundred feet, and a group of islands was 

 formed ; but in the depressions or basins of the chalk still 

 covered by the waters, sediments derived from the destruction 

 of the sea-cliffs, and the degradation of the surface of the 

 land, were slowly deposited. 



Herbivorous and carnivorous mammalia of numerous genera 

 and species now, for the first time, appeared, and inhabited 

 the islands and continents formed by the elevated masses of 

 the former ocean-bed ; and in the new (tertiary) deposits 

 then in progress, the bones of the animals, and the remains 

 of the trees and plants, became imbedded. 



IV. Post-tertiary Epoch. The Tertiary epoch in its turn 

 also passed away the elevatory movements continued other 



