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



posited by Indian creek and its tributaries in canyons and valleys cut to 

 the depth of 2.000 feet directly across the bed of the ancient stream by 

 which the Johnson gravel was deposited. The character of the fossil 

 plants found in the auriferous gravels at Mountain meadows and else- 

 where in the same district, as well as the topography of the region, indi- 

 cates that the northern end of the Sierra Nevada at the time the John- 

 son gravel was deposited had a much gentler relief and lower altitude 

 than at the present time. As already shown.* the region was greatly 

 affected by a post-Miocene upheaval. 



DEFORM A 770 V. 



Structure of Mount Jura. — Having considered the four unconformities 

 or structural breaks which occur in the suite of rocks of the Tavlorville 

 region, attention may he turned to the structure displayed by the youngest 

 system, i. c, the Jurassic. We may expect this system to be affected by 

 the same kind of movements and other changes which took place in the 

 older strata but, on account of its youth, to have been affected in a less 

 degree, and therefore in certain respects to furnish better examples for 

 study. 



The strike of the rocks in mount Jura, with comparatively moderate 

 variation, is northwest and southeast, while the dip is uniformly south- 

 westward. On the western slope of the mountain the continuity of the 

 stratified rocks is greatly interrupted by eruptive masses, but the strati- 

 graphic order of the rocks is clearly defined. Near the base, next to the 

 siliceous eruptive, occurs the Hardgrave sandstone, followed up the slope 

 by porphyrite and the Thompson limestone, which dips beneath the 

 Hardgrave sandstone and overlies the Mormon sandstone. Continuing 

 up the slope, as shown in the section of mount Jura, we next come to a 

 zone of porphyrite on which there are a number of small areas of strati- 

 fied rocks, especially of the Hardgrave sandstone. This is succeeded, 

 near the summit of the spur where the section crosses, by an acid erup- 

 tive and the Hardgrave sandstone, which are followed in order down 

 the eastern slope toward Hinchman ravine by the Thompson limestone, 

 Mormon sandstone. Bicknell sandstone and Hinchman tuff. This suite 

 composes the smaller section down the western slope of Hinchman 

 ravine, figure 5 j but on the north the Thompson limestone and a por- 

 tion of the Mormon sandstone are replaced by an eruptive. The order 

 of the stratification on the western and eastern slopes of mount Jura is 

 the same. In both cases the oldest stratum is on top and the youngesl 

 at the bottom of the suite, and it is evident that the whole mass has 

 been overturned. 



* U. s. Geol. Surv.. Eighth Ann. Rept . 1686'87, pp. W9-422. 



