44 



pressed down from above the coal have been given on a pre- 

 ceding page. In this manner, also, the slickensiding was ac- 

 complished by the slipping of the clay in the fissure, and not 

 by the movement of the walls of the fissure upon each other. 



It is thought that the principle of unequal contraction in the 

 dilTerent parts of the coal seam, during the progress of its 

 consolidation, applies also in the formation of the more com- 

 mon types of rock rolls in the top of coal seams in Illinois. 

 The character and sequence of the beds above the coal seam are 

 considered the chief factors in determining whether rolls or 

 clay seams will be formed in the adjustment of the strains 

 arising from such unequal contraction. 



It seem^ probable that clay seams have been formed in dif- 

 ferent ways in different areas, under varying conditions of 

 roof and floor, and varying degrees and kinds of strains to 

 which the strata were subjected. It is not possible that earth- 

 quake phenomena or general crustal strains, such as produce 

 wide-spread faulting, could be concerned in the formation of the 

 fissures of the clay seams in the Springfield region. Fractures 

 from the above-mentioned causes would not be limited to a few 

 feet in vertical height, or to one particular coal seam, as in the 

 area under discussion. 



