OF CREATION. 229 



sand;" and this again gives place to the lower chalk. 

 The upper greensand is generally barren of fossils, 

 and was most likely formed rapidly in a shallow sea. 

 After it was deposited the sea-bottom probably sunk 

 more rapidly, previously to the great change indicated 

 in the chalk beds above. 



The depression of the sea-bottom, which probably 

 went on, with few alternations of elevation, during 

 the greater part of the deposit of the chalk, was a 

 very important phenomenon, and altered, as may be 

 supposed, the general character of the marine fauna 

 in almost every important respect. We at once lose 

 the species usually predominant in shallow water and 

 on a coast-line, and either obtain deep sea or free- 

 swimming animals, or else find the deposit singularly 

 barren of all organic remains. But, as it is not to be 

 supposed that a continued depression, amounting in 

 the whole to many hundred fathoms, could take place 

 without considerable effect being produced on the 

 beds beneath the surface, and since such depression 

 was perhaps accompanied by a partial upheaval in 

 some other subterranean tract, it will not be thought 

 extraordinary if the effects of submarine volcanic 

 action are also met with connected with this period. 

 Among such effects may have been the outpouring of 

 large quantities of silica, either in solution in hot 

 water, or thrown out in the state of vapour ; and in 

 this way we may possibly account for the layers of 

 flint alternating with the pure white carbonate of 

 lime of which the chalk mainly consists. 



In the early part of the cretaceous period the ani- 

 mal kingdom was represented by species which ex- 

 hibit striking differences when compared with those of 



