PAPERS OF GENERAL INTEREST 33 



near Sparks Hill, seems to be an ancient volcanic neck, 

 or outlet. The most western of these dikes which has been 

 obsen^ed is in Pope County about one mile west of Golconda. 

 They have been observed at numerous localities in Hardin 

 County, and are known to be present in some of the coal 

 mines near Harrisburg. They have also been obsei'\'ed at 

 many localities in Livingston and Crittenden Counties. Ken- 

 tucky. Doubtless many other dikes are present within this 

 region wholly covered by the sui-ficial mantle rock, some of 

 which may be discovered in the future. The depth beneath 

 the present surface, of the great body of igneous rock with 

 which these dikes must connect, is unknown, for it has never 

 been pentrated either in mining operations or by deep 

 drilling. 



The presence of such an intrusion is believed to be respon- 

 sible for at least some of the faulting of the region. When 

 the mass was injected into the strata of the crust, the beds 

 overhing it were necessarily bowed up, and in this process 

 of bowing the beds were stretched and great fractures were 

 fonned. The amount of movement on opposite sides of 

 these fractures was not the same so that faulting resulted. 

 The faults which were fonned during this upbowing pro- 

 cess, however, probably were not the most complicated ones. 

 While the deeply buried m.olten mass was still very hot and 

 remained in a more or less plastic or viscous condition, the 

 enormous weight of the overlying sediments resting upon it 

 must have had a tendency to squeeze it out laterally so that 

 the original dome would become lower and broader. With 

 the readjustment of the crustal blocks in connection with 

 this settling of certain arch-like segments of the dome, there 

 was a virtual collapse of the strata in certain areas, occas- 

 ioning extremely complicated faulting. 



The fluorspar veins of southeastern Illinois are along cer- 

 tain of the faults in the more complexly faulted areas, or 

 in rather close association with faults. The fluorine content 

 of the mineral is a product of igneous rocks and doubtless 

 was originally given off from the igneous rock which pre- 

 sumably underlies the entire fluorspar region. A deteraii- 

 nation of the actual genesis of the ores as they are found at 

 the present tinie is complicated by many factors, but Mr. L. 



