GEOLOGICAL PAPERS. 93 



upper rocks are characterized by coals, thin hmestones, and shale. 

 They are shown to be thickest in the south-central part of the 

 State along the axis of the main basin or syncline. From this 

 region they become thinner as the formations rise towards the 

 borders of the State. This due partly to surface erosion and 

 partly to variation in original deposition. 



The massive Pottsville sandstones underlie the "Coal Meas- 

 ures'' proper and are a part of the Pennsylvanian series. The 

 Pottsville sands are often interbedded with shales and hence the 

 top is difficult to identify, owing to the merging of the sends with 

 overlying shaly rocks. The correlations in the cross-sections 

 were based, for the most part, upon the top of the thick sand, 

 immediately underlying the conspicuous shaly rocks. There is 

 also a usual absence of limestone strata in them, thus differing 

 distinctly from the underlying Mississippian rocks. The several 

 sections show that the Pottsville is practically absent west of an 

 irregular line drawn from Springfield through Carlyle to Coul- 

 terville. It is also absent north of Springfield, except possibly in 

 the vicinity to the northeast. A thickness of 450 feet along the 

 C-C section, presented later, decreases to 300 feet along the A-A 

 section, and to 50 or 100 feet along the E-E. These sandstones 

 are very productive in the main fields, and are called the Bucha- 

 nan from the name of the farm on which oil was first found in 

 that sand. 



The Mississippian series lying in the Carboniferous, next be- 

 low the Pennsylvania, contains the most widely productive oil 

 sands in the State. The upper part of the Mississippian includes 

 the Chester rocks, characterized by a succession of limestones, 

 red shales, and sandstones. The top of these rocks is marked by 

 the first limestone below the Pottsville. The red shales are im- 

 portant horizon markers with the oil men, signifying the ap- 

 proach to such sands as the Kirkwood, Tracey and McClosky. 

 The Kirkwood sand of Lawrence County, the Benoist sand of 

 Marion County, Lindley sand of Bond County, and the Sparta 

 sand of Randolph County, all belong to the same horizon and 

 underlie the Chester red shales. Oil men scarcely ever drill be- 

 low the Chester rocks and into the massive St. Louis limestones, 

 the "big line." In several cases, however, deep wells have been 

 drilled in search of the Trenton or Niagara. 



The oil of the main fields comes from the following sands : 



