CAEBOXTFEEOES SYSTEM. 223 



of them, and by these sandstones that formation gradually 

 merges into the Lower coal-measures, instead of the latter 

 resting directly upon the St. Louis limestone as they do in 

 Iowa. 



The Sub-carboniferous group has another lithological pecu- 

 liarity, not entirely unknown in other rocks it is true, but its 

 prevalence and periodical occurrence are remarkable, as well 

 as the effect it seems to have produced upon animal life in the 

 waters in which the strata were deposited. This is the 

 presence of the silicious material which forms the beds of 

 passage from one of its limestone formations to another, and 

 has been before referred to. The silex, except in a few 

 unimportant instances, is not in the form of sand, but has 

 evidently been precipitated from the waters of the seas in 

 which it was at first held in solution, the increasing preva- 

 lence there of which seems to have been incompatible with 

 the continuance of the forms of marine life that existed so 

 abundantly during the deposition of the limestones. It 

 seems to have been especially uncongenial to the echino- 

 derms, for from and after the first material increase of silex 

 in the limestone strata they began to diminish in numbers 

 and became wholly extinct when the strata became mainly 

 silicious ; but they were introduced again in great abundance 

 under other specific forms as soon as the limestone of the 

 next epoch began to accumulate. 



These peculiar silicious deposits, which have already been 

 described, prevailed periodically, from the epoch of the 

 lower Buiiingt on limestone to the Keokuk limestone inclusive r 

 and then ceased: at least in their distinctive form after the 

 last named epoch. 



The rocks of the Sub-carboniferous period have in other 

 countries and in other parts of our own country, furnished 

 valuable minerals and even coal ; but so far as we know them 

 in Iowa, the economic value of the group is confined to its 

 stone alone. 



It has been shown upon previous pages that the Lower 

 Silurian, Upper Silurian, and Devonian rocks of Iowa are 



