BAIN.] GEOLOGY. 31 



GEOLOGIC STRUCTURE. 



GENERVL RELATIONS. 



Tlie stimetnre of this district stands in marked contrast with that 

 of the remaining- portion of Illinois. The widespread presence of ex- 

 tensive fanlting and the intrusion of igneous rocks set it oif sharply 

 from the general surrounding region. If the Kentucky-Illinois dis- 

 tj-ict be considered as a whole, it seems to represent a portion of an 

 extensive northwest-southeast monocline or, possibly, the northeast 

 half of a dome whose remaining portion has disappeared beneath 

 the Tertiary deposits of the Mississippi embayment. To the north 

 and east the Mansfield sandstone dips sharply away and becomes 

 deeply buried beneath the Coal Measures strata. To the south and 

 west the beds pass, with little dip and no uniformity, beneath the 

 later sediments. Loughridge gives data suggestive of faulting along 

 the contact," and it may be that beneath' the Cenozoic l)eds of that 

 region there is deeply buried a structural remnant corresponding 

 to that remaining on the other side of the district. Be that as it may, 

 there are within the district many evidences of local disturbance. 

 These include a sharply defined little dome near Hicks, in Hardin 

 County, and numerous faults throughout the territory. (PI. IV.) 



HICKS DOME. 



This is a nearly complete uplift about 7 miles in diameter. Since 

 the black shale of the Devonian is brought up to the topographic 

 level of the upper Birdsville within 3 miles, a vertical uplift of at 

 least 1,000 to 1,500 feet has taken place. To the southeast the dome 

 is cut off by a normal fault of 1,000 feet or more, by which Mansfield 

 sandstone is brought into contact with St. Louis limestone. Around 

 the edges of the dome, as in the vicinity of the Empire mines, there 

 are numerous normal faults, though none of them, so far as has been 

 determined, have much tlirow% Their presence none the less renders 

 it improbable that the dome was raised by ordinary lateral thrust 

 and indicates rather vertically acting forces. This conclusion is reen- 

 forced by the fact that normal faulting, almost exclusively, occurs 

 in the district as a whole. 



Around the edge of the dome the rocks have quaquaversal dips of 

 high angle. Along Hicks Branch, in sec. 26, T. 11 S., Pv. 8 E., the 

 dips are 10° to 20° to the south and Avest. On the northeast the dip 

 in section 17 is 5°. On the southeast, in section 31, it is 11°, and on 

 the southwest, in section 35, the dip is 8°. The dip decreases from 

 the center outward. The occurrence of the St. Louis along Hicks 



" Geol. Survey Kentucky, Jackson Purchase Region, 1888, p. 267. 



