98 EEPOET UNITED STATES GEOLOGICAL SUEVEY. * 



From the sections which are given below, the relative position of this 

 Prozoic granite will he seen. So far as could he determined, this most 

 ancient range was but very little affected, locally, by the great dynamic 

 disturbances which succeeded the close of the Cretaceous era. 



During the early Palaeozoic period this low range, which then occupied 

 a, much higher relative position than at the present time, formed an 

 effective barrier to the sonthwestward progress of the waters. We lind, 

 therefore, tha^ on the southern slope of the granitic groups the older 

 •sedimentary formations occupy a very subordinate position, except at 

 ■such points where the low elevation of the range i)ermitted the passage 

 •of waters in which the sediment was deposited. We may regard the 

 ■strata deposited during the older sedimentary period north of this range as 

 the terminal edges of the extensive areas to the north and northeast. 

 An exception to this occurs, however, along the former depression be- 

 tween the granites of the western end of the Sweetwater Hills and those 

 of the Granite Hills. There free access was afforded, and as a result we 

 tind the sedimentary beds of the eastern Sweetwater and the Seminole 

 Bills. 



jMETAMORPHICS. 



In the preceding chapter it has been stated that the metalliferous me- 

 tamorphics extended for some distance down on the Sweetwater. Their 

 last appearance on this river is found in the Little Canon west of Saint 

 Mary's ranch. At this point they are directly overlaid by Potsdam quart- 

 jzite. We observe that the metamorphic granite, which is elsewhere inter- 

 posed between these schists and the Silurian strata, is entirely eliminated. 

 This may be accouuted for most readily by the assumption that the old 

 •schists here reach their greatest horizontal and vertical development. In 

 consequence of this the extreme border of the granites does not reach so 

 far south. The nearest point where the granite crops out again is south 

 •of Sheep Mountain, on the line of the anticlinal upheaval, where Pots- 

 dam quartzites are tound resting upon it. To the south of the river the 

 •schists are covered by Tertiary beds, determining the limits of the group. 

 Within our district this schist does not again make its appearance. 



Metamorphic granite is found in the Sweetwater and Seminole Hills. 

 Ft is composed of orthoclase, white and yellow quartz, muscovite, and 

 .some oligoclase. The exposures we find in these hills are, in my opinion, 

 .& continuation of the youngest granites appearing on the eastern slope 

 .of the Wind Eiver Eange. After the continuity of the granite in that 

 region is broken we first find it again south of Sheep Mountain. There 

 it occupies its present position by virtue of the anticlinal fold which 

 passes through in a direction south of east. Again we find a small 

 exposure near the Sweetwater, about 12 miles below Saint Mary's ranch. 

 From that point we must travel in a more easterly direction, about E. 

 -20° S., in order to find another outcrop. It is once more met with in the 

 Sweetwater Hills west of Elkhorn Gap. Ascending the mountain 

 directly west of the gap, we find the entire summit and the higher por- 

 tions of both the north and south slopes composed of granite. It is here 

 too much weathered to show any definition of stratoid arrangement, but 

 from the shape of the hill and its trend we may infer that we stand upon 

 the crest of an anticlinal uplift. Proceeding westward on this fiat-topped 

 hill we find the granite gradually pinching out, until we reach a group 

 of sedimentary beds dipping steeply toward the south. Several of the 

 formations from the Silurian upward are here exposed, while the granites 

 have sunk down low and are covered by the strata of the Sweetwater 

 Group. 



