218 Observations upon the Geology and Paleontology 



hand-specimens cannot readily be distinguished from it. 

 Some parts of this bed also are, to a great extent, com- 

 posed of the remains of crinoids, nearly all the species of 

 these fossils being peculiar to it, and generally ranging- 

 throughout its entire thickness. The limestone of this 

 bed, even in its purest parts, contains a considerable 

 admixture of silicious matter, the imbedded fossils being 

 frequently quite silicified, especially near the exposed sur- 

 faces. It gradually becomes cherty in its upper parts, and 

 finally passes into the chert bed of passage to the Keokuk 

 limestone. 



The following table will show the vertical range of some 

 of the most conspicuous shells found in the series of rocks 

 just enumerated, but more particularly those of the Che- 

 mung rocks. 



The difterent beds are represented by columns, with 

 numbers at their heads which correspond to those used in 

 the preceding section. Asterisks in these columns oppo- 

 site the name of each fossil show that it has been recog- 

 nized in the beds which they represent. Their absence 

 indicates only that it has not been so recognized. 



The dotted line indicates the division between the lower 

 and middle portions of No. 7. This dotted line represents 

 in reality an imaginary line between the true Devonian 

 and Carboniferous rocks ; a line where Devonian species 

 ceased only to predominate, and upward from which the 

 Carboniferous species flourished in full force ; the latter 

 being often found below, and the former above, this line — 

 that is, the Devonian species gradually die out, and are 

 as gradually supplanted by Carboniferous species. So 

 that this line between the two great systems is not so dis- 

 tinct as those between the different beds of the series here 

 exposed, because the sudden lithological change which 

 marks their separation is wanting here. A necessity exists 

 for recognizing a line of this kind, and to which we must 



