PAPERS OX GEOGRAPHY AND GEOLOGY 357 



to be of great value in determining many of the above 

 factors. The large oil companies have established labor- 

 atories and employ men to study the cuttings not only 

 after they have been brought to the laboratory, but also 

 to collect and in some cases study them as they come 

 out of the wells. It is of interest to note that perhaps 

 the greatest impetus has been given to this work by com- 

 mercial enterprises. It seems to be a paying proposi- 

 tion with them on a dollar and cents basis. 



The problem which presents itself in Illinois and the 

 need for microscopic study are understood readily when 

 it is considered that the bed rock of at least two-thirds 

 of the State is obscured by glacial drift or loess, and the 

 determination of structure is therefore very difficult and 

 unsatisfactory. The rocks that do outcrop occur along 

 the margins of the State, along the Mississippi and Ohio 

 rivers and the adjoining territory, and in the northern 

 part of the State along the larger streams and in artificial 

 exposures. In some cases these beds continue into the 

 central part of the State with the same lithological char- 

 acter- they display at the outcrop, but in others they 

 change very markedly, due to differences in position with 

 reference to the shore lines of the seas in which they were 

 deposited. One group of rocks in particular, which out- 

 crops in the southern part of the State, exhibits this phe- 

 nomenon. This is the Chester group, one of the import- 

 ant oil-producing groups of rocks in Illinois. In outcrop. 

 the sequence of the Chester formation has been well es- 

 tablished, but as the rocks pass to the north and into the 

 central part of the State they become covered and also 

 change markedly. In tins central area key beds are not 

 numerous, and. such phases as red rocks and colored 

 shales are used for the lack of better evidence on which 

 to base correlations. 



It is obviously impossible to use the same large fossils 

 in identifying the cuttings from wells that it is possible 

 to use in identifying the beds at the outcrop, but it may 

 be possible to identify in the cuttings from wells the same 

 small fossils which may be used to identity the beds at 

 the outcrop. With this in mind approximately 1,800 sam- 

 ples of the Chester rocks have been obtained very care- 



