OF CREATION. 



Ill 



ance, and of large size, offers a characteristic of consi- 

 derable interest. 



I have been the more anxious to mark the exist- 

 ence of this break in the general continuity of the 

 various strata, because, unlike that occurring after the 

 deposit of the chalk, it is by no means distinctly visible 

 to every one, nor is it altogether understood even by 

 many Geologists. It is, however, almost equally im- 

 portant with respect to the great standard of com- 

 parison, that of organic life, and its conditions have 

 been worked out carefully, although they have only 

 lately been so satisfactorily proved as to admit of 

 confident and direct assertion. One reason of this 

 may perhaps be, that of the disturbances ultimately 

 so effective we see in some cases the first small com- 

 mencement, and are enabled to trace the gradual 

 change in the general character of the deposits, and 

 perceive the mineral structure of the beds insensibly 

 adapted to the new state of things. Another reason 

 undoubtedly is, that the changes produced on the 

 older rocks after the deposit of the carboniferous sys- 



Fiff. 42 



Fig. 43 



HETEROCERCAL TAIL. 

 (Platysomus.) 



HOMOCERCAL TAIL. 



(Pristipoma.) 



