PAPERS ON GEOLOGY AND GEOGRAPHY 227 



out the Chester, indicates gentle earth movements that 

 may be considered introductory to the big earth movements 

 which took place at or near the close of Chester time. These 

 big earth movements are believed to have been the first 

 to produce any structure in the southern half of the LaSalle 

 anticlinal zone of a magnitude comparable to that of the 

 present day. 



PENNSYLVANIAN TIME 



The Pennsylvanian is found along the La Salle anticline 

 resting uncomformably on rocks of all geological ages from 

 Ordovician to Chester. To the north it is very thin, and 

 there is no doubt that the northern part of the anticlinal 

 zone and even the crest of the fold farther south must have 

 been outside the area of deposition during part or all of 

 Pottsville time. 



The base of the Pennsylvanian varies in elevation from 

 500 feet above sea level to 1,000 feet below sea level. Its 

 thickness varies from a few feet up to 1,400 feet. The 

 hiatus represented by the uncomformity at the base of the 

 Pennsylvanian is very great, as in the extreme instance 

 all beds that might intervene between the Ordovician and 

 Pennsylvanian are missing. Above the Pottsville the 

 amount of Carbondale and McLeansboro deposited varies 

 slightly in most cases. Wherever these formations thicken 

 east and west off the anticlinal crest, the thickening is 

 chiefly due to the coming in of lower beds rather than to 

 the thickening of upper members. This evidently indicates 

 continuance of movement after the beginning of and during 

 early Pennsylvanian time, and is responsible for a greater 

 accentuation of folding in rocks older than the Pennsyl- 

 vanian than in the Pennsylvanian rocks themselves. Prob- 

 ably the difference in elevation caused by late and post- 

 Pennsylvanian folding is in places as much as 500 feet, as 

 shown by present elevations of late Pennsylvanian beds. 

 This latest folding is itself capable of producing in the dif- 

 ferent beds sufficient "structure" to account for the known 

 oil pools on minor structures along the La Salle anticline. 



It seems probable then that in the southern half of the 

 La Salle anticline at least, the arch did not exist except in 

 a very minor way until late Chester or more probably Potts- 



