KNDLicii.] SWEETWATER VALLEY PKOZOIC ROCKS. 97 



Stiulyinf? the detail-features of the hills we find that they are composed 

 mainly of a coarse red granite. The effect produced by atmospheric 

 influences has been to destroy the red color in some instances, and wo then 

 find it gray. U]ion reaching the base of a hill the first impression ob- 

 tained is that of an enormous pile of huge granitic bowlders heaped 

 together. Orthoclase, quartz, and muscovite compose the main portion 

 of the rock. Toward the northern limits of the group some hornblende 

 and chlorite may be observed. There, too, we find some of the younger 

 granites. At several points the prevalence of orthoclase is so great as 

 to produce apparent stratification. It will be found, however, that the 

 divisions simulating strata all run in curved, short planes. Numerous 

 dikes of basaltic rock were observed within these granites, varying in 

 thickness from two feet to several hundred. In some instances the dikes 

 exhibit columnar structure. At right angles to the enclosing walls the 

 jaaterial has segregated into smooth-sided but irregular prisms. An oc- 

 currence of this kind may be seen at the I>e\il's Gate. It will be remem- 

 bered that this euphonious title has been given to a vertical opening in 

 the granites, about 5 miles west of Eock Independence. A narrow 

 gi'anitic ridge extends to the southward from the main groui), and is cut bj" 

 .1 deep gap. The walls of this gap are about 400 feet above the Sweet- 

 water Eiver, which flows through it. From some remnants we can see 

 that originally this fissure was filled with erui)tive material which was 

 segregated into columns. Due to this latter fact is the greater power 

 that erosive agents could exercise upon the enclosed mass, and it was 

 gradually removed. Dr. Hayden supposes, and I agree Avith him, that 

 formerly the river curved to the southward, floAving around the granitic 

 ridge, until finally it found its way into the gap and aided in the destruc- 

 tion and removal of the dike. 



Both the analogy of composing minerals and the characteristic occur- 

 rence of dikes connect the Granite Hills with the outcrops at the west- 

 ern end of the Sweetwater Hills. Although a number of instances were 

 observed where the dikes i^enetrated granite which was o\'erlaid by 

 younger rocks, the erupted material invariably stopped with the granite, 

 shoAving that its age is very great. The area occupied by the Granite 

 Hills has approximately an oval shape. Both west and north we find 

 isolated outcrops, rising to relative elevations of 600 to 700 feet. To- 

 ward the east the hills assume a southeasterly trend, and are crossed, 

 by way of the Devil's Gate, bj^ the SweetAvater River. One of the 

 isolated exjiosiu-es of this granite is formed bj' Rock Indeiiendence. Dr. 

 Hayden, in 1870, made some measurements of this historic block. He 

 found the circumference of the granitic rock to be 1,550 yards ; the height 

 of the north end 193 feet ; that of the southern 167 feet. Near the mid- 

 dle, between these two high pohits, there is a depression, so that there 

 the relative elevation is not more than 00 feet. Beyond Rock Independ- 

 ence the granite i)asses out of our district. 



Connecting the three exposures Avhicli I regard as belonging to one 

 series, we obtain an old granitic range of considerable length. It 

 shows a slight S-ahaped ciu've upon horizontal projection. Between 

 the eastern end of the subsidiary range and the western beginning of the 

 Sweetwater Hills there was a decided depression, permitting the granites 

 to be hidden from sight by the succeeding sedimentary fornuitions. A 

 similar depression existed in the si)ace intervening between the last- 

 named outxirop and the Granite tTillH. From all tlnit I have seen 

 throughout this region I have come to the conclusion that the continuity 

 of these three groups may be regarded as having formerly existed, and 

 that the granite composing them is the oldest rock in oiu- district. 

 7 G s 



