222 PROCEEDINGS OF THE ACADEMY OF [1888. 



ON THE FAUNA OF THE LOWER COAL MEASURES 

 OF CENTRAL IOWA. 



BY CHARLES R. KEYES. 



The carboniferous rocks of the region in the immediate vicinity 

 of Des Moines have, until quite lately, yielded only fragmentary 

 remains of fossils. Recent investigations, however, have disclosed 

 a rich fauna embracing, as hereafter enumerated, more than 35 

 genera and nearly 60 species, the majority of them in a most perfect 

 state of preservation. In Iowa the lower coal measures present, 

 lithologically, a marked contrast with both the under-(subcarbonif- 

 erous) and the over-lying (middle and upper coal measures) strata 

 which are pre-eminently calcareous, while the lower coal measures 

 are characterized by an almost total absence of the calcareous 

 divisions, which are represented only by a few thin bands of impure 

 limestone, local in distribution. A section of the rocks at Des 

 Moines presents : 



Drift 20 feet. 



Loss 15 " 



Middle coal measures 40 " 



Lower coal measures . . . . . 160 " 

 St. Louis limestone (not exposed in Polk county.) 



The superficial deposits have been quite thoroughly studied by 

 McGee and Call,^ but the palaeozoic rocks have in Polk county 

 received but a passing notice. Though economically of far greater 

 importance than any other formation in the state, the lower coal 

 measures have received comparatively little geologic attention ; and 

 the two attempts at an exhaustive and detailed survey of this 

 formation in Iowa, and a correlation of the different coal horizons 

 was unfortunately rendered abortive by circumstances entirely 

 beyond the control of those engaged in the study of the Des Moines 

 valley region. In Iowa the lower coal measures probably have a 

 maximum thickness of more than tAvo hundred feet, but notwith- 

 standing the fact that at Des Moines the entire formation underlies 

 the city, which is situated just at the eastern border of the middle 

 coal measures, this maximum is nowhere, in Polk county, attained. 

 The base of the middle coal measures as characterized by Mr. St. 

 John^ and as is clearly shown in several localities in the immediate 

 vicinity of Des Moines, is composed of variegated clays and shales, 



1 Vide Am. Jour. Sci., vol. xxiv, Sept. 1882. 

 ^ White's Geol. Iowa, vol. I, p. 272. 



