BAIN.] GEOLOGIC HISTORY. 35 



DEFORMATION AND INTRUSION OF IGNEOUS ROCKS. 



At some time after the deposition of the Mansfield sandstone, and 

 presumably after the deposition of a considerable thickness of coal 

 measures, the region was uplifted and exposed to erosion. This may 

 well have been merely a recurrence of uplift along a line marked out 

 by previous deformation, since in the total unlikeness of the pre-St. 

 Louis Carboniferous beds of this district to those along the Missis- 

 sippi to the northwest there is evidence of an old barrier between the 

 two provinces. However that may be, the j^articular uplift mentioned 

 was either in the Carboniferous or later. It was accompanied or suc- 

 ceeded by fracturing and diverse displacement, and since this form of 

 deformation probably occurs only when the beds are lightly loaded it 

 took place either before any great thickness of coal-measure shales 

 was deposited over the region or after they were eroded. If account 

 be taken of the large amount of erosion which the Mississippi Valley 

 has undergone in Mesozoic and Cenozoic time, it seems probable that 

 this uplift occurred after a considerable thickness of rocks — later 

 than any now known in the area — was deposited. In central and 

 southern Illinois there are 1,000 feet or more of coal measures which 

 are unrepresented in Pope and Hardin counties. As these beds are 

 nowhere preserved in the fault blocks of the latter area, there would 

 seem to have been first an uplift of the district as a whole, followed 

 by erosion, and this in turn by fracturing and displacement of the 

 individual blocks of strata. 



While the hypothesis is not perhaps susceptible of absolute demon- 

 stration, the intrusion of the igneous material seems to have accom- 

 panied the fracturing and displacement. The relations of the dikes 

 to the faults are not clear. In general the dikes seem to occupy frac- 

 tures along which there is practically no faulting, and the deej) fault 

 planes are not occupied by igneous material, though they are fre- 

 quently the locus of veins. The occurrence in the north flank of 

 Downey's bluif of a fault cutting the small sheet of peridotite of 

 course merely proves that some faults are later than some intrusions. 



No attempt to fix more definitely the age of the dikes and deforma- 

 tion will be made, as any suggestion would rest wholly on analogy 

 with neighboring regions showing similar phenomena, and there 

 are none sufficiently close to give much weight to such suggestions. 



PERIOD OF EROSION. 



Since the deformation, the faulting, and the intrusion of the 

 dike material there has been prolonged erosion of the area. It is 

 extremely probable that this erosion has occurred in two or more 

 cycles, but of the earlier ones there is now no demonstrable evidence. 



