UNCONFORMITIES IN THE CARBONIFEROUS. 205 



ness known in the state mentioned. The Ste. < lenevieve limestone, which 

 Shumard differentiated from the St. Louis deposits near the mouth of 

 Aux Vases river, a few miles below the old village of Ste. Genevieve, 

 appears to he merely the upper part of the main group of strata ; and the 

 fossils contained fully substantiate this view. 



The unconformity of the St. Louis rocks upon the underlying strata 

 in Iowa and the adjoining portions of the neighboring states has been 

 fully explained by White* The thinness of the limestone has been 

 alluded to already. This is due partly to the thinning out of the strata 

 northward and partly to the subaerial erosion prior to the deposition of 

 the Coal Measures of the region. 



Toward its present northern limits the upper part of the St. Louis is 

 composed of soft, plastic, highly fossiliferous marls, which are well ex- 

 posed nt Fort Dodge, in the northern-central part of Iowa, and at Harvey, 

 in the central portion of the state, besides numerous other localities im- 

 mediately to the southward of the last named place. At Elk cliff, a few 

 miles from Harvey, as well as elsewhere, the marl has been removed 

 entirely down to the hard limestone upon which rests directly the strata 

 of the Coal Measures. Nor is this all : the uneven configuration of the 

 ancient land surface is further shown by the presence of more than 100 

 feet of clays and shales, represented a short distance down the stream 

 (Des Moines river), before the level of the summit of the old limestone 

 elevation is reached. f 



Over all the northern area of the St. Louis a characteristic brecciated 

 rock is observable. But south of the Missouri river evenly bedded lime- 

 stones are present, with occasional extensive beds of oolite. In places at 

 Ste. Genevieve the oolitic limestones present perfect cross-bedding, such 

 as is commonly seen in sandstones, a. fact which is very suggestive in its 

 bearing upon the origin of certain oolites. 



The fauna! features of the St. Louis are peculiar in many respects, and 

 quite distinct from those of both the overlying and underlying strata. 

 particularly from the latter. 



K askaskia 01; "Chester" I5ki>s. 



Aux Vases Sandstone. — In southern Illinois and southeastern Missouri 

 the Kaskaskia comprises extensive beds of limestone and shale. Every- 

 where over this -district these calcareous poi-t ions, which greatly predomi- 

 nate in the lower part of the group, are underlain by a line grained fer- 

 ruginous sandrock. This sandstone is recognizable above the city of St. 



Geology of Iowa, vol. i. 1870, pp 22i>-229. 

 I K' ■>'■- : Bill Goo). Soc. Am., vol. a, 1890, p, 287. 



