;J80 J. S. DILLER GEOLOGY OF THE TAYLORVILLE REGION. 



section (figure 3). Those relations may be the result of either uncon- 

 formable deposition or of displacement, or of both. 



The sandstone near the contact is much fractured and the pieces in 

 in many cases are bounded by slickensides. The fossils, which are well 

 marked in some cases within a rod of the contact, gradually disappear 

 in that direction as the slickensides increase. The Mormon sandstone 

 is not the bottom member of the Jura but the third counting from be- 

 low upward, the Hardgrave sandstone being lowest and the Thomp- 

 son limestone next. The absence of the lower beds of the Jura and the 

 presence of numerous slickensides near the plane of the contact render 

 it probable that there is displacement at this point. Furthermore, a large 

 mass of the Triassic slates and sandstones, which are younger than the 

 Hosselkus limestone, do not appear between it and the Mormon sand- 

 stone ; but this fact may find its explanation either in the displacement 

 of the beds or in post-Triassic folding and erosion previous to the uncon- 

 formable deposition of the Jura. The Mormon sandstone in Peters 

 ravine lies at least in large part between the Carboniferous and the Trias, 

 a feature which is forcibly repeated in mount Jura, and is fully described 

 in a subsequent paragraph. Whatever may have been their relative 

 position at this place originally, it is evident that great changes have been 

 wrought in it during the folding of the rock at a later epoch. 



N.t. 



Piouke 3. — Jura- Trias Un co nfo rm i ty . 

 6 = Mormon sandstone ; 11 = Hosselkus limestone; 13 = Robinson beds; E = Eruptive rocks. 



Strong evidence of an unconformity by deposition between the Jurassic 

 strata and the older rocks is obtained by a general survey of their areal 

 relations. In connection with the eruptives associated with them the 

 Jurassic strata form the whole of mount Jura and, with a few exceptions, 

 are limited to its slopes. The belt in which they. occur is two and one- 

 half miles in width, with a length parallel to the strike of about five miles. 

 Although the Jurassic rocks, full of fossils, are well exposed along the 

 southern side of the northern arm for one and one-half miles, with a 

 strike to the northwestward, yet it has not been definitely proven that 

 any of them appear on the other side of the valley, only a short distance 

 away. 



At the southern base of mount Jura the Jurassic rocks cross Indian 

 creek to the slope of Grizzly mountain, hut the exposures are small, em- 

 bracing only detached masses of the Hardgrave sandstone, the Thomp- 



