84 GEOLOGY OF ILLINOIS. 



Nos. 2 and 3 vary in thickness from two to three feet, while No. 1 was not met 

 with in the county sufficiently developed to be of any economical value. 



St. Louis Group. The outcrop of the Lower Carboniferous limestones in 

 this county, is restricted to the valleys of the principal streams, and -to the Illi- 

 nois river bluffs, between the mouth of Sugar creek and the south line of the 

 county. The St. Louis group, which comprises the upper division of the series, 

 consists of a gray concretionary limestone of variable thickness, ranging from five 

 to twenty feet, forming the upper member of the group, below which we find a 

 brown magnesian limestone, sometimes quite massive, and in regular beds, and 

 at other localities, intercalated with shales, or passing into a thin bedded or 

 shaly limestone. 



The concretionary limestone is not very regular in its development, but often 

 occurs in isolated patches or outliers, and is a rough gray limestone, presenting 

 no regular lines of bedding, but usually concretionary or brecciated in its struc- 

 ture. It outcrops at intervals, along the bluffs of Crooked creek, through its 

 whole course in this county, and also along the bluffs of the Illinois river, as 

 far north as the vicinity of Browning, where it disappears. It was also found 

 on Sugar creek, as far up as McKee's mill, on section 17, township 2 north, 

 range 1 east. The only fossils that were obtained from this limestone, was the 

 Lithostrotion canadense, a silicious coral that abounds in it almost everywhere, 

 and is found weathered out in the beds of the streams, in masses, often of con. 

 siderable size, which, from the polygonal form of the single corallites that go 

 to form the mass, are often called petrified honey comb. In the vicinity of Bir- 

 mingham, we found this limestone eighteen feet thick, and overlaid by the 

 conglomerate sandstone of the Coal Measures. It is underlaid by a bed of cal- 

 careous sandstone, and also a magnesian limestone about ten feet thick, which 

 forms the base of the St. Louis group at this locality. 



The magnesian limestone is far more regular in its development than the 

 concretionary limestone, and is usually of a rusty brown color on the surface, 

 from the oxydation of the iron which it contains. It contains a few species of 

 fossils, among which are Productus Altonensis, Archimedes Wortheni, Spirifer 

 Keokuk, Rhynchonella mutata, and a large Conularia, perhaps C. Missouriensis 

 of Swallow. This limestone occurs at the base of the bluff at Frederick, and 

 also in the vicinity of Schuyler City, which is the most northerly point where 

 we found it exposed in the river bluffs. 



Keokuk Group. Only the upper portion of this group is exposed in this 

 county, and its greatest development appears to be in the vicinity of Birmingham, 

 in the northwest corner of the county. The greatest thickness exposed here is 

 about fifty feet, of which the lower fifteen feet is a thin bedded limestone, con- 

 taining many of the characteristic fossils of this group, above which there is 

 about thirty-five feet of calcareo-argillaceous shales, containing geodes of quartz 



