68 GEOLOGY OF ILLINOIS. 



sandstone outcrops in a continuous mural bluff, from fifteen to twenty feet in 

 bight, and when the lower coal (No. 1,) is developed at all, it will be found 

 immediately above this sandstone. 



From the preceding sections, a general idea may be had of the thickness, 

 and lithological character of the Coal Measures, as they are developed in this 

 county, and it only remains now to speak of the extent of surface which they 

 underlie. Originally, they covered the entire area of the county, but in the 

 subsequent excavation of the valleys of the Illinois river, and its main tributa- 

 ries, the whole thickness of Coal Measure strata have been cut away, down to 

 the underlying Lower Carboniferous limestones, into which all the principal 

 streams have cut their channels, along the lower portion of their courses. 

 Hence, the Coal Measures are now found only beneath the surface of the high- 

 lands, and in the valleys of the smaller streams, but they underlie nearly all the 

 uplands in the county, except a limited area in township 2 south, range 2 west, 

 in the vicinity of Versailles, where the hills consist of Loess. In this vicinity 

 the Coal Measure strata have been removed by the same agencies that scooped 

 out the main river valley, and the bluffs here are formed by the Quaternary 

 deposits, that were subsequently deposited in, and now partially fill this ancient 

 valley. 



The principal coal seam developed in this county, is No. 2, or the Colchester 

 seam of McDonough county, and it outcrops on most of the small streams, and 

 may be reached by shafts almost anywhere on the uplands, in the central, north- 

 ern, or western portions of the county, at a depth varying from one hundred 

 to one hundred and fifty feet. 



St. Louis Group This group forms the upper division of the Lower Car- 

 boniferous series in this portion of the State, and consists of a hard gray con- 

 cretionary limestone, varying from five to ten feet or more in thickness, which 

 constitutes its upper division, and a brown magnesian limestone, and calcareous 

 sandstone, with some intercalations of blue clay shale, which form the lower 

 division of the group. Its entire thickness in this county, may be estimated 

 at about forty feet. We found the upper division well exposed on the Dry 

 Fork of McGree's creek, six miles south of Mount Sterling, at Tucker's old 

 mill. The rock is here an irregularly bedded gray limestone, a portion of 

 which is stained a deep rusty brown color, by the decomposition, or oxydation 

 of the crystals of iron pyrites which it contains, and it also contains irregular 

 seams of green marly clay. We obtained a few fossils from the beds at this 

 locality, among which were Lithostrotion proliferum, Archseocidaris Wbrtheni, 

 and Granatocrinus cornutus. The last named species has not been found at any 

 other locality in the State. In the bluffs of McGee's creek, about a mile and 

 a half below Jaqueth's mill, there is an exposure of about thirty feet of buff 

 and brown magnesian limestones and shales, which belong to this group, and 



