MISSISSIPPIAN SEAS IN ILLINOIS 577 



material contains a considerable number of glacial bowlders of 

 various types and sizes. Some of them are of igneous origin, others 

 carry fossils of Niagaran age. Those containing the Mississippian 

 fauna were observed only in the southeastern part of the pit. The 

 bowlders have been piled into many small heaps by the workmen, 

 several of which are made up almost exclusively of those of Missis- 

 sippian age. The collections which have been made were largely 

 secured from these piles, but similar bowlders may be found 

 scattered over the southeastern part of the floor of the pit. 



Lithologic character of the bowlders. — In their lithologic character 

 these Mississippian bowlders are dolomitic limestones, which 

 closely resemble the Niagaran dolomite of the Chicago region in 

 general appearance; when fresh they are bluish gray in color and 

 very hard, but on weathering they take on an earthy, yellowish- 

 brown color and become very soft. 



Physical condition of the bowlders. — The great majority of the 

 bowlders have well-weathered surfaces, and in many examples the 

 outer surface to the depth of an inch or more is decomposed to a 

 soft, yellowish, more or less porous rock as a result of weathering. 

 Some large pieces are completely weathered to the center. This 

 weathered condition of the Mississippian bowlders is, in general, 

 in marked contrast to that of the bowlders of other ages found in the 

 pit. Many of the Mississippian bowlders are rather angular, and 

 none have well-worn faces ; they vary in size from small fragments 

 up to irregular blocks containing two or more cubic feet; in general, 

 they lack the characteristic appearance of typical glacial bowlders. 

 If their location were not known, they might easily be mistaken for 

 residual fragments of weathering. 



So far as now known, these bowlders are confined to the south- 

 eastern section of the Carey clay-pit. A few small masses with the 

 same lithological character were found on the surface of the moraine 

 near the pit, but as none of these fragments have afforded determin- 

 able fossils they cannot be identified with certainty. 



Criteria by which the bowlders may be recognized. — The Mis- 

 sissippian drift bowlders commonly may be recognized by their 

 dirty, yellowish, weathered surfaces and by their two most marked 

 paleontological characteristics, namely, (a) an abundance of 



