PAPERS ON GEOGRAPHY AND GEOLOGY 203 



shales and limes, and as most if not all of these were ma- 

 rine, the local supply of organic material was perfectly 

 adequate. 



Present conceptions of artesian circulation, in a basin 

 such as our Illinois Paleozoics form, favor the idea that 

 marginal movements are much more rapid than are those 

 in the deeper part of the basin. Such circulation doubt- 

 less has been changed mail} 7 times since the Illinois 

 Paleozoics were deposited as rock debris, but most of 

 this circulation has probably moved southward. During 

 Pennsylvanian time the LaSalle anticline formed a 

 structural high, along both sides of which there may have 

 been relatively rapid movement of meteoric water. If not 

 too rapid this would favor oil accumulation in suitable 

 reservoirs. With an oil content in some of the shales of 

 12 to 16 gallons per ton 3 the adequacy of material is no 

 problem. If this hypothesis should prove correct, the 

 oil now developed is local in origin and large areas to the 

 west of the fold may justify test drilling where struct- 

 ural and textural conditions can be determined as favor- 

 able. Similarly, reserves in the Mississippian and lower 

 rocks were developed under physiographic conditions 

 unlike those of the Pennsylvanian, and for obvious rea- 

 sons may be expected to conform more closely to the 

 larger structural features. 



'Ashley, G. H., Bulletin 641, U. S. Geol. Survey, pp. 314-319. 



