32 GEOLOGY OF ILLINOIS. 



this group form the bed of the creek, and the overlying shales form the main 

 portion of the bluffs on the creek, throughout its course in this county. 



The limestones which constitute the lower division of this group, occupy the 

 lower portion of the river bluff, about half a mile above Griggsville Landing, 

 and from thence to Chambersburg. Their entire thickness probably does not 

 exceed sixty feet, and at some points the beds are quite massive, and compara- 

 tively free from chert, and form an excellent building stone. This is the char- 

 acter of the rock at some points on the south forth of McGee's creek, between 

 Perry and Griggsville. 



In the northwest part of the county this limestone is exposed on Hadley's 

 creek, and, in the vicinty of Huntley's coal bank, where the coal abuts directly 

 upon it, we found some of the characteristic fossils of this formation, among 

 which were Spirifer Keokuk, S. neglectus, and some teeth of fossil fishes. 

 Fossils were not found very abundant at any of the localities where we found 

 this limestone exposed, and as but few quarries have been opened in it, there is 

 but a limited field for the collector among the outcrops of this limestone in this 

 county. At Perry Springs we obtained two specimens of Agaricocrinus Amer- 

 icanus, and one of Archimedes Owenana.. The same species also occur in the 

 thin bedded limestones in the bed of the creek at Chambersburg. 



St. Louis Group. We saw no indications of the presence of this group any 

 where in the county, except on the breaks of McGee's creek, in the northeast 

 part of the county, and on the south fork of the same creek in the vicinity of 

 Perry. The beds exposed here consist of brown magnesian limestone and 

 shales, and range from twenty to thirty feet in thickness. One mile and a-half 

 northwest of Perry, quarries have been opened in the brown magnesian lime- 

 stone of this group, which is there about eighteen to twenty feet thick, and 

 directly overlies the geodiferous shales of the Keokuk group. About three miles 

 north of Perry Springs, and near the north line of the county, these magne- 

 sian beds are also exposed, and are overlaid by some shaly beds, the whole at- 

 taining a thickness of about twenty feet. No exposure of the gray concretion- 

 ary limestone, which usually forms the upper member of the group, was met 

 with in this county. 



Coal Measures. The coal formation occupies but a limited area in the cen- 

 tral and northern portions of this county, underlying the whole of town 4 south, 

 range 4 west, and a portion only of the four surrounding townships. The en- 

 tire thickness of the formation, as it appears in this county, probably does not 

 exceed sixty feet. The following are the principal points where coal has been 

 dug in this county : 



Huntley's old bank, on the northwest quarter of section 15, town 4 south, 

 range 5 west. Coal sixteen to twenty-four inches thick, overlaid by about six 

 inches of black shale. 



