44 GEOLOGICAL SURVEY OF THE TERRITORIES. 



over the speckled tracbyte, quite compact, witbout lines of bedding, 

 and much cbanged, sbowiug the effect of tbe igneous rocks below. We 

 can see here that tbe igneous rocks must be of comparatively modern 

 origin, and must bave been intruded between the strata, forming a con- 

 spicuous agent in the uplift. 



At the bead of Iowa Gulch, on the south side of the stream, the mas- 

 sive granites rise up 1,000 to 1,200 feet, with a cap of tbe quartzites. 

 It is from this exposure that tbe great masses of granite scattered over 

 the lower portion of Iowa Gulch were derived. At the head of the 

 gulch the strata are greatly faulted. Tbe varieties of texture in the 

 grauites are numerous, and it appears more massive and older than any 

 before seen in this range. It is much broken by irregular jointing, but 

 no lines of bedding can be seen. The great amount of broken masses, 

 most of them more or less rounded, which are scattered over the sides 

 and bottom of these gulches, is marvelous. There is less of this glacial 

 evidence in the California Gulch than in any of the gulches on either 

 side. This granite nucleus, or uplift, seems to extend across the heads 

 of the gulches in a north and south course, while the streams that flow 

 down the west side of the range cut through this granite nucleus and 

 have their origin in the stratified rocks that incline from the east slope. 

 This, however, is not an uncommon occurrence. There is a constant 

 tendency in these mountain- streams to wear these gorges back beyond 

 the true axis on either side. Each one rises in a sort of amphitheater 

 which has been formed by tbe breaking down of the sides of the gorges 

 by tbe water and ice in the fissure, and the melting of tbe snow sweeps 

 the fragments slowly down into the gulch. This gnawing process, as 

 it might be called, at the mass of the mountain has been going on for 

 ages, so that in many places tbe crest is a single sharp ridge between 

 the amphitheaters on either side. In many places this crest is worn 

 through, and many of the low passes were once high walls wholly inac- 

 cessible. 



The great mass of sedimentary stratified rocks of which this range is 

 largely composed inclines away south of east from this granite nucleus, 

 showing clearly its anticlinal structure. The granite, as seen in Iowa 

 Gulch, is a true " wedge," the quartzites capping the summits inclining 

 in opposite directions, a portion dipping west at an angle of 10° or 12°, 

 Although, in general terms, many of the mountains are illustrations of 

 the single wedge structure, I have never seen a clearer exhibition than 

 this, where the granite is capped with the quartzite strata, and the 

 eroding agents bave sliced it down vertically, so that tbe relations are 

 seen perfectly. The amphitheater at the head of Iowa Gulch is about 

 10,500 feet. In passing over the crest into Horseshoe Gulch we are 

 obliged to climb an almost vertical wall for 1,500 feet. Resting upon 

 the granites at tbe bottom are about 200 feet of quartzites distinctly 

 stratified ! Above the quartzites are 800 to 1,000. feet of uustratified 

 sombre-gray r>orphyritic trachyte, breaking off in vertical columnar 

 masses. Over the trachyte comes quartzite and arenaceous limestone, 

 passing up into about 50 feet of very cherty, partially metamorphosed, 

 limestone, in which the galena mines are located ; above this are alter- 

 nations of quartzites with beds of limestone, 2,500 to 3,000 feet in the 

 aggregate, in the range. The summit of the divide is about 12,500 feet ; 

 the high peaks, north or south, rise upward of 13,000 or 14,000 feet. 

 The summits and sides of the range are covered with fragments or M- 

 hris of broken trachyte mingled with quartzites, the former predomi- 

 nating. 



At the head of Empire Gulch the granites rise up nearly to the sum- 



