HORIZON OF THE GRIZZLY QUABTZTTE. 391 



of the arch, the middle or lowest stratum of the arch being exposed on 

 the steeper slope in the direction of the overturning. From this point 

 of view the oldest formation in the Grizzly anticlinal is apparently the 

 ( rrizzly quartzite. 



The older strata in the crest of the Grizzly anticlinal are depressed to 

 the northward. In the summit of Grizzly mountain just north of the 

 40th parallel they have an elevation of 7.700 feet. From this point they 

 gradually sink 4,200 feet in three and a half miles to the bridge across 

 Indian creek, one-fourth of a mile east of Taylorville, where they pass 

 beneath Indian valley at an elevation of 3,500 feet. 



The western limb of the anticlinal embraces all of the formations lying 

 between the northern extension of the crest of Grizzly mountain and 

 Spanish creek. Beginning with the oldest, lying near the middle of the 

 arch, they occur in the following order: Grizzly quartzite, Montgomery 

 limestone, Taylorville slates. Arlington beds and Shoo Fly beds, together 

 having a total thickness of over 16,000 feet. 



The eastern limb of the Grizzly anticlinal was much contracted and 

 obscured by the overturning, and none of the formations occurring in 

 the western arm save the Hosselkus limestone, which is beyond the sec- 

 tion, have been recognized on its eastern side. The obscurity is greatly 

 increased by the presence of the Jura, which reposes on the older strata 

 unconformably and covers them up. It seems most probable also, as 

 will be shown in the sequel, that the case is still further complicated by 

 faulting such as affected mount" Jura. 



Taglorville Fault. — The terminal portions of the long section are com- 

 paratively simple. Its greatest complexity lies near the middle, in the 

 vicinity of the western base of mount Jura. As seen in considering the 

 northeastern arm of the Grizzly anticlinal and the southwestern arm of 

 the adjoining synclinal, this limb, which is an element of both, must he 

 regarded as involving all the strata of the Taylorville region from the 

 Silurian to the Jurassic, inclusive, having a total thickness of 24,500 

 feet. In considering this, however, we should reduce the total amount 

 by 2,001) feet, the thickness of the Jurassic, which lies upon the older 

 rocks unconformably, which would leave 22.500 feet for the section from 

 the Silurian to the Triassic, inclusive. The actual thickness of the rocks, 

 measured from the Grizzly anticlinal to the middle of the northern arm 

 synclinal, is only about 9,000 feet, so that 13,500 feet <>f strata have sud- 



deilly disappeared from the middle portion of the section. 



The structure of mount Jura at once suggests thai the disappearance 



of this large body of strata may he due to a profound fault along tin 



northeastern slope of Grizzly mountain. The fault which we have found 

 distinctly marked in mounl Jura leaves its southwestern slope for mount 



