34 ILLINOIS STATE ACADEMY OF SCIENCE 



W. Currier has made numerous observations which tend to 

 prove that the present occurrence of the fluorspar is due to 

 a replacement of crystalline calcite v^^hich first closed up the 

 open spaces along fault planes. 



Not all of the faults of southern Illinois v^^ere associated 

 with the deep seated igneous intrusion, for certain of them 

 are far from any known igneous dikes, and there must have 

 been other stresses of the earth's crust at this time which 

 were relieved by faulting. In general the more continuous 

 faults in southeastern Illinois have a northeast-southwest 

 direction. To the southwest they pass beneath the embay- 

 ment deposits, as has been previously indicated, but it is per- 

 haps significant that the projection of this belt of faulting 

 in a southwesterly direction follows very closely the western 

 border of the area occupied by the embayment deposits, and 

 the suggestion may be offered that this whole embayment 

 area may be the result of a downward dislocation of the 

 crustal block lying southeast of the continuation of the 

 faulting which is exposed across Pope and Hardin Counties, 

 Illinois, but which is hidden to the southwest. Certain 

 structural features as far southwest as northern Louisiana, 

 may possibly be associated with this same line of defor- 

 mation. 



Coming from the Ozark region west of the Mississippi 

 river there is a belt of faulting which crosses the river in 

 the vicinity of Grand Tower, Illinois. These faults have an 

 east-west or northwest southeast direction, with the down- 

 throw side on the north, and it is not unlikely that the east- 

 ward extension of some portion of this fault belt may be 

 responsible for the uplifted Ozark ridge across southern 

 Illinois. 



The widespread distribution of the faults of southern 

 Illinois establishes the fact that this area has been, in the 

 geologic past, one of great crustal disturbance. Lines of 

 weakness once established in the earth's crust repeatedly 

 give way under the accumulating stresses, and it is not 

 unlikely that movements of greater or less magnitude have 

 taken place along certain of these fault lines at intervals 

 since they were first established. It is quite possible that 



