44 UNIVERSITY OF MISSOURI STUDIES 



Strike N. 75° E. The lower member dips about 20° N. W. along 

 the fault plane, but on the bluff of Big River, a quarter of a mile 

 south, the beds dip very little. None of the above faults have any 

 apparent effect upon or relation to the mineral deposits. 



Altho the faults discussed above are the only ones actually 

 determined, it is believed that there was a period of faulting 

 much earlier than that in which these faults were developed. The 

 Potosi dolomite is very variable in thickness in the area, ranging 

 from an apparent thickness of a little more than 100 feet in the 

 region about the Calico, to nearly 300 feet in the southern part 

 of the area. In the Fertile region also it does not have anything 

 like its maximum thickness. It is believed by all who have 

 studied in the region that there is an unconformity between the 

 Potosi and the Proctor formations. That the deformation which 

 produced this unconformity was accompanied by faulting is not 

 improbable; nor is it improbable that in the subsequent erosion 

 all surface evidences of the faulting disappeared. This sequence 

 of events would cause the Potosi formation to have a variable 

 thickness. If such faulting occurred, it certainly preceded the 

 deposition of the Proctor dolomite, for this formation is nearly 

 uniform in thickness. As shown in discussing the formation 

 (page 37), there are apparent discrepancies in the bedding in the 

 Potosi formation, as well as in thickness, which suggests that 

 there had been faulting and subsequent erosion. 



Time of the faulting. — There may have been two periods of 

 faulting. If there is an earlier one it accompanied the deforma- 

 tion at the close of the Potosi, while the last one is certainly post- 

 Gasconade. As a matter of fact it is probable that faulting took 

 place as late as the last uplift in Tertiary times, tho it may have 

 occurred earlier. The later faults are unmineralized, so they 

 must have occurred after the period of mineralization. Spurr* 

 believes that there have been two periods of faulting or fractur- 

 ing in the Lead Belt of southeastern Missouri, one of which is 

 distinctly older than the period of mineralization. It is evidently 

 impossible to fix the time any closer than is suggested above. 



*Spurr, J. E., Econ. Geol., vol. 10, p. 472. 1915. 



