CHAPTER III. 
CARBONIFEROUS ROCKS OF SOUTHERN AND WESTERN IOWA. 
SECTION I. 
THEIR LITHOLOGICAL CHARACTER. 
Tus system of rocks admits, in Iowa, of three distinct subdivisions, namely: a 
creat 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 oilitic 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  wilabers may be recognised. Commencing at 
the base we have: 
1. Earthy, marly, and impure calcareous rocks; crumbling and dintetbouibedine 
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. 
