CARBONIFEROUS SYSTEM. 203 



chert together with nodular masses of flint ; the whole mixed 

 with a smaller proportion of calcareous matter. The exis- 

 tence of these silicious beds of passage from one calcareous 

 division to the other, itself suggests the propriety of regarding 

 the Burlington limestone as two distinct formations, and this 

 suggestion is strengthened by some well marked palseonto- 

 logical differences also, especially in its crinoidal remains. 

 Both divisions of the Burlington limestone are eminently 

 crinoidal, that is, they are prominently characterized by a 

 profusion of crinoidal remains, all of which in each divisions 

 are specifically distinct from all of those in the other, so far 

 as yet ascertained. It seems that the accession of silicious 

 material to the waters of that epoch resulted in or at least 

 was followed by the extermination of all the species of 

 crinoids then existing, and although they flourished in just 

 as great profusion when the calcareous condition of the 

 waters was restored, they were all of new species ; these being 

 all in turn exterminated by the accession of the silicious 

 material which we find to mark the close of the full epoch 

 of the Burlington limestone.* 



The objection to the separation of the two divisions lies in 

 the fact that the area over which this distinction has been 

 recognized is quite limited, and the interests of geology can 

 hardly be served by multiplying names that have only a local 

 or very limited application. The two divisions will therefore 

 be considered as one formation in this report, and the 

 thickness of it as given in the table and section on preceding 

 pages includes both divisions of the limestone together with 

 both series of silicious beds which separate and terminate 

 them respectively. 



A characteristic outcrop of the Burlington limestone is 



* From the fact that the extinction of the forms of life, that characterized each of the 

 limestone epochs of the Sub-carboniferous group, was commenced with and gradually 

 accomplished during the deposition of those silicious beds of passage, the latter are 

 regarded as the upper portion and the limestone as the base and middle portions of 

 the formation. Therefore, the silicious beds above all the Burlington limestone are 

 included with it, and the " geode bed " is for the same reason included with the Keokuk 

 limestone. 



