Miscellaneous. 



443 



of nature to give countenance to our theories, but let us frame those 

 theories on nature as it is. 



Following, therefore, the train of reasoning which we have adopted, 

 we are struck with amazement in stepping over the boundary which 

 separates the secondary from the tertiary periods. We no longer 

 encounter those monstrous reptiles which haunted the shores and 

 rivers of former lands ; we do not now wonder at the vast difference 

 of the former state of things, but feel a difficulty in believing we 

 are not looking upon a world similar to our own. We have seen 

 that the forms of animal life either gradually run into one another 

 in the various strata, or that these were separated by a distinct line 

 of demarcation, and characterised their own peculiar class of fossils. 

 With respect to land animals, we have also seen that when any 

 great addition of forms was effected, evidences exist of causes having 

 been in operation to remove carbonic acid from the atmosphere. 

 Now, whence do we derive the evidences of this withdrawal in the 

 era between the secondary and the tertiary periods? Nothing can 

 be more striking than the difference of organic remains between the 

 infra and supracretaceous beds. The supracretaceous deposits are 

 characterised by the wonderful similitude of organic remains to types 

 now existing — the infracretaceous, by their utter dissimilarity. On 

 this account we are disposed to lay more weight to our former ex- 

 planation of the manner in which the cretaceous beds may have been 

 formed; viz., by a decomposition of the sulphate of lime existing 

 in sea water by means of carbonates formed through the agency of 

 decaying plants. Such, at all events, may have been a partial cause, 

 and would account, in a great measure, for the small coherence and 

 chalky nature of the limestone ; a character of a precipitate. 



There are many circumstances which countenance this idea. The 

 recent experiments of Kuhlman on the preparation of artificial stones 

 by means of silicate of potash, are powerful advocates of its truth. 

 There cannot be the slightest doubt that silicate of potash formed 

 an important ingredient in the sea of former times; every rock 

 of marine origin proves this. Now, as the carbonic acid was evolved 

 by the decay of the marine vegetation, it would decompose the 

 silicate of potash, forming carbonate of potash, and depositing silica. 

 This carbonate of potash again meeting with the sulphate of lime 

 in solution, would occasion the double decomposition of which we 

 have spoken. And that these chemical changes did take place, the 

 composition of the chalk shews us ; for it every where contains both 

 potash and silica. The organic remains converted into silica owe 



