32 ILLINOIS STATE ACADEMY OF SCIENCE 



not been mapped in detail except for a short distance near 

 Shawneetown on the Ohio river, where east-west faulting 

 has been observed, but this faulting may not extend across 

 the State. On the southern slope of the Ozark hills a re- 

 markable series of faults has been mapped in Hardin, Pope 

 and Johnson Counties. 



The exact time when all of this faulting occurred cannot 

 be determined with certainty. The deformation must have 

 taken place after Pennsylvanian time for the strata of Penn- 

 sylvanian age are involved. The other limit which can be 

 established is determined by the age of the gulf embayment 

 deposits which stretch from the Gulf of Mexico coast line to 

 southern Illinois. These deposits overlie the faulted rocks 

 under consideration in an undisturbed condition. The age 

 of the oldest of the embayment deposits is probably Cretac- 

 eous, so the age of the faulting would fall between the Penn- 

 sylvanian and the Cretaceous, a very long period of time, 

 even to the Geologist. Within these time limits falls the 

 close of the Paleozoic era, a time when much deformation 

 was in progress in many parts of the world, and it is com- 

 monly assumed that the faulting in southern Illinois was 

 accomplished, in the main, at that time. 



In Hardin County it is significant that associated with 

 the faults are found the remarkable deposits of fluorspar 

 for which southern Illinois is famous, and also there are 

 present in the same region numerous dikes of igneous rocks 

 which penetrate the limestones and other formations of 

 Mississippian and Pennsylvanian age. It is believed that 

 there is some definite connection between the three phe- 

 nomena, the faults, the mineral veins, and the igneous dikes. 

 There is probably a considerable area of southeastern Illi- 

 nois and the adjoining portion of Kentucky which is un- 

 derlain by a great mass of igneous rock. This was in effect 

 the site of a great volcano in the long distant past, most of 

 whose lava was injected into and between the sedimentary 

 rock strata. If any of the lava was ever extrusive on the 

 surface, it has long since been removed by the processes of 

 erosion. Numerous dikes, some of which may have ex- 

 tended to the surface when they were formed, are known to 

 l)e present, and at least one mass in northern Hardin County, 



