THE CHESTER SERIES IN ILLINOIS 403 



but numerous observations seem to establish a limit of about forty 

 feet for the interval between the underlying and overlying sand- 

 stones in which it must be contained, which seems to establish a 

 nearly uniform thickness in both of the two Chester areas of the 

 state. As the formation is traced eastward, in the eastern part of 

 Pope and in Hardin County, the Clore seems to become thinner 

 even than the forty feet of the region to the west. 



The fossils of the Clore formation are numerous in places, and, 

 Hke the faunas of all the other limestones of the Chester series, it 

 possesses certain rather distinctive features. The general composi- 

 tion of the fauna is similar to that of other Chester horizons, 

 being made up largely of bryozoans and brachiopods, with the 

 axes of Archimedes and numerous species of FenestelHds being 

 conspicuously present. The most significant feature of the fauna, 

 however, is the abundance of small, cyHndrical, branched bryozoa 

 of the genus Batostomella, B. nitidula apparently being a conspicu- 

 ous species. The fossils are most abundant upon the surfaces of 

 thin limestone layers imbedded in some of the shales, and one 

 such horizon is rather persistently present near the middle of the 

 formation throughout most of the southern belt, and perhaps also 

 in the Mississippi River counties as well. It is from this layer 

 that the bryozoan-covered slabs of the Clore can be collected in 

 great numbers. Among the brachiopods of the Clore fauna, large 

 forms of Spirifer increbescens and Composita subquadrata, similar 

 to those in the Menard limestone, are common in some localities. 



Degonia sandstone. — During the earlier field studies in Randolph 

 County, it was believed that the massive sandstone formation 

 which was found to overlie the Clore limestone was the basal' 

 member of the Pennsylvanian, but during the progress of the 

 field mapping of the Campbell Hill Quadrangle in the summer of 

 1919 by J. M. Weller, it has been found that this sandstone is 

 succeeded by a still higher Chester limestone, which in turn is 

 overlain by the true lower Pennsylvanian Pottsville sandstone 

 filled with pebbles in many places. This sandstone, now known 

 to be Chester, is especially well exhibited in the walls of several 

 tributaries to the Mississippi River in Degonia Township of Jackson 

 County, Illinois, and the name has been chosen from these exposures. 



