ECOLOGICAL CROSS SECTION OF THE MISSISSIPPI. 145 



limestone 10 to 12 feet thick . . . quarried for build- 

 ing stone. In the river bluffs at the old Pittsburg mines 

 it is only about 4 feet thick. 



Referring to the western boundary of the coal measures^ 

 mentioned in the passage just quoted; Bowman* points out that 

 "at Granite City the river sediments rest on the massive 

 bed of the Mississippian limestone, while at Monk's Mound 

 and near Peters they rest respectively on a shale and a sand- 

 stone which are undoubtedly coal measures strata.'' He 

 expresses the opinion that the line of contact between the 

 Mississippian and Pennsylvanian formations in the Ameri- 

 can Bottoms, while not known with exactitude, is probably 

 somewhere between the eastern edge of East St. Louis and 

 Monk's Mound. As has been pointed out previously, it is 

 probable that the Pennsylvanian formation extended across 

 the flood plain of the Mississippi, since "coal measures rocks 

 are found not only in the bluffs from Centerville around to 

 North Alton and in sections of deep wells near Monk's Mound 

 and Peters, but beyond the Mississippi in the City of St. 

 Louis and districts to the west and northwest. This was 

 probably the condition in prcglacial times, since geologists 

 are agreed that the river trimmed the present bluff line 

 previous to the glacial epoch." 



The subcarboniferous limestone, which in the southern 

 portion of the adjoining county of Randolph is at least 600 

 feet thick, has thinned out, before reaching the southern 

 part of St. Clair county, to an aggregate of less than 100 

 feet, and includes only the lower sandstone and a thin bed of 

 limestone. These beds outcrop around the southwest bord- 

 ers of the coal field in St, Clair county, commencing in the 

 river bluffs, about 2 miles below Centerville station. Worthen 

 describes it in substance as follows: 



The limestone is a coarse-grained, thin-bedded gray or 

 brown limestone, with partings of argillaceous material and 

 contains numerous characteristic fossils. The entire thick- 

 ness may be 20 to 30 feet. It is overlaid by the shales and 

 argillaceous limestones of the Coal Measures, inclosing a 



* loc, ciL 



10 



