90 



ILLINOIS ACADEMY OF SCIENCE. 



character of the strata, were published by Bain*, and are modi- 

 fied sHghtly to agree with later conclusions. 



Overlying the consolidated rocks of the State except in the 

 extreme southern and the northwestern counties, there is a vary- 

 ing thickness of glacial deposits or "drift." These clays, sands, 

 gravels, etc., are commonly encountered in drilling before hard 

 rock is reached. Locally, they contain gas, but the pressure is 

 usually slight and the life of the individual wells is short. While 

 it is not possible in every case to absolutely exclude the possibil- 

 ity of these wells representing leakage from lower reservoirs, a 

 sufficient explanation of them is believed to be found in the de- 

 cay of woody material buried in the drift itself. 



The stratigraphic section for southern Illinois is most import- 

 ant in the study of oil possibilities. The formations promising 

 best production are indicated by italic and occur chiefly in the 

 Carboniferous system. Possible oil "sands" are suggested also 

 in the Ordovician and Silurian systems, especially in central and 

 southern Illinois. 



Northern Illinois Section. 

 This section is intended to be representative for that portion of the 

 State lying north of Rock Island, La Salle and Kankakee. 



•Bain, H. Foster, Petroleum fields in Illinoii in 1907: Bull. Ill State Geol. 

 Survey No. 8, pp. 273-312. 



