CHAPTER III. 



CARBONIFEROUS ROCKS OF SOUTHERN AND WESTERN IOWA. 



SECTION I. 



THEIR LITHOLOGICAL CHARACTER. 



This system of rocks admits, in Iowa, of three distinct subdivisions, namely : a 

 great calcareous formation at the base, coal-bearing strata in the middle, and heavy 

 beds of sandstone on the top. 



Of these subdivisions, the lower consists mostly of limestones ; shales and argilla- 

 ceous beds predominate in the central ; while gritstones almost entirely compose 

 the upper. 



With a view to determine the exact position of the workable beds of coal, of the 

 best building materials, and of other strata of economical value, distributed through 

 this carboniferous system, the elements of stratification of these rocks have been 

 closely studied, and numerous sections and measurements made at many localities. 



The combined Table and Section No. 1 D, to No. 40 D, minutely exhibits the 

 most persistent lithological characters of the various beds composing the lower sub- 

 division. The former has been constructed from more than a hundred measured 

 sections, taken at various points on the Mississippi, Des Moines, Skunk Rivers, 

 and their tributaries. 



This subdivision consists of a lower and upper series. The former, down to 

 within sixty feet of its base, presents an almost unbroken series of limestones, 

 with occasional thin bands and reticulations of chert. Beneath an oolitic bed, 

 which lies about sixty feet from the base of this formation, the limestone beds are 

 less pure, being, to some extent, argillaceous and siliceous. 



In the lower series six distinct members may be recognised. Commencing at 

 the base we have : 



1. Earthy, marly, and impure calcareous rocks; crumbling and disintegrating 

 by atmospheric influences, so that they do not afford good building materials, and 

 are, for the most part, concealed in the slopes by a talus of rubbish, or covered with 

 soil and vegetation. 



