32 FLUOJiSI'AK DEPOSITS OF flOTTTHEKN ILLINOIS, f bull. 255. 



Branch, and aloiio- a branch of Big Creek in section 20 (PI. Til), 

 indicates a local irregularity in the dome. 



There are no known occurrences of igneous rock connected with the 

 Ilicks dome, but the presence of unusual quantities of mica in the 

 streams at one or two points lends color to the suspicion of their 

 presence. 



FAULTS. 



Normal faults are found throughout the two counties. A number 

 of these are represented upon the accomiianying mai)s, and their 

 particular relation to the ore deposits will be discussed later. The 

 throw varies from an imperceptible amount to 1,000 feet or more. 

 All the Carboniferous formations are affected. East of Cave in 

 Rock the Mansfield sandstone and the Ste. Genevieve limestone are 

 brought together. On Hog Thief Branch (sec. 30, T. 11 S., R. 9 E.) 

 the St. Louis limestone is brought into contact with the Mansfield 

 and Avith either Cypress or Birdsville sandstone. In the region 

 beween Elizabethtown and Karbers Ridge there are many faults, 

 and rapid changes in strata may be noted in traversing au}^ of the 

 roads. In Pope County, from the Ohio and the Bay Bottoms to the 

 encircling escarpment of Mansfield sandstone, faulting is a common 

 phenomenon, and in Gallatin County (sec. 21, T, 12 S., R. 7 E.) the 

 Mansfield itself is cut by faults in numerous directions. 



There is no regularity as to the trend of the fault planes, though 

 a majority of those so far studied have a general course north of east. 

 The individual fault planes are rarely traceable more than 1 or 2 

 miles, but along the same general course faulting occurs for 12 to 15 

 miles. For example, from the Empire mine (sec. 34, T. 11 S., R. 7 

 E.) to the Wright prospect (sec. 3, T. 13 S., R. 5 E.) there is a suc- 

 cession of prospects and evidence of disturbance of the beds, though 

 there is small probability of a continuous fault. From the Rosiclare 

 and Fairview mines the general course of the faulting may be pro- 

 jected southwest to Bay City, and along the line frequent evidence 

 of disturbances will be foimd, while at Bay City there is also min- 

 eralization. 



On the other hand, there is usually clear evidence that the indi- 

 vidual faults are of limited length. They occupy en echelon posi- 

 tions and are by no means exactly parallel. This is illustrated on 

 the special map of the Rosiclare district, wdiere the various fault 

 planes are offset as indicated, and vary in direction nearly 30°. 

 This tendency of fault planes to take a course at a slight angle with 

 what ap])ears to mark the line of major stress is believed to be the 

 expi-ession of a general law. The reason is not well understood, but 

 seems to lie in the character of the rocks and the direction of the 



