392 J. S. DILLER — GEOLOGY OF THE TAYLORVILLE REGION. 



Grizzly on the opposite side of Indian creek, just where we would expect 

 the fault to occur. Furthermore, the Jura fault has produced exactly 

 the same kind of effects, different only in degree from those we seek to 

 explain. 



The fault surface and some of the strata beneath have been exposed 

 by erosion on the southwestern slope of mount Jura, but the immediate 

 r< suit of this faulting was to narrow the belt of Triassic exposures and 

 cover them up to the northeastward of mount Jura by shoving over upon 

 them the Jurassic formations from the southwestward. 



The completely brecciated quartz-porphyry or quartz-porphyrite which 

 occur- so abundantly on the northeastern slope of mount Grizzly over- 

 lying the fault was. in many cases at least, brecciated at the time of its 

 eruption, but in other cases it more closely resembles a fault breccia and 

 its genesis may then properly be attributed to the displacement. 



The position of the fault on the lower slope of Grizzly lias not been 

 definitely traced out, as it has on the southwestern slope of mount Jura. 

 It gradually sinks to the northward with the crest of Grizzly, reaching 

 Indian creek Mime distance above the bridge. Continuing in the same 

 direction, near Chapman's it cuts off a small portion of the northwestern 

 corner qf mount Jura ; thence it crosses the northern arm and follows the 

 eastern slope of the curved ridge between Cook canyon and Indian valley 

 toward Mountain meadows. 



On the western side of the northern arm the quartz-porphyry so 

 abundant on the slopes of mount Grizzly is shoved far over to the north- 

 eastward upon the Foreman beds, so that the Jurassic formations, if they 

 extend northwestward beyond the northern arm, are chiefly or wholly 

 covered up by the fault. To the southward along the slope of Grizzly 

 the position of the fault has not been definitely traced. 



From the relative positions of the Hosselkus limestone, as seen in the 

 Long section and further to the westward, we can get some idea of the 

 amount of displacement in the Taylorville fault. This limestone crops 

 out about one and one-half miles northeast of the axis of the northern 

 arm synclinal, so that its position in the other arm of the synclinal would 

 be underneath the middle portion of mount Jura. The 'strike of this 

 limestone, at its outcrop on the slope of mount Grizzly southwest of 

 Genesee, carries it beneath mount Jura near the middle. If, now, the 

 Shoo Fly limestone is Triassic i Hosselkus), as there is some reason for 

 supposing, the thickness of the strata between the Hosselkus limestone 

 and the Silurian on the southwestern side of the Grizzly arch is about 

 15,70 » feet, and the Silurian should he expected below the fault nearly 

 five miles southwest of the Hosselkus limestone underlying the middle 

 of mount Jura. This would make the displacement of the Silurian 



