336 GEOLOGICAL SURVEY OF THE TERRITORIES. 



of feldspar over a foot in length. This' general character of the granite 

 is retained all the way westward to the boundary, but changes in Gun- 

 nison CaQou and its immediate vicinity. There we have granite analo- 

 gous to the one found at Badger Creek in section a, a middle- to coarse- 

 grained porphyritic granite, with white to yellowish oligoclase, gray, 

 white, and pink quartz, black mica, and large Carlsbad twins of ortho- 

 clase, continuing through the whole canon, with a few exceptions. On 

 the heights inclosing this canon, the granitic rocks weather in bold but 

 rounded outlines. It was her§ that a good opportunity was offered to 

 observe the often-asserted fact that even without the action of moving 

 water, without any motion on part of the rocks, bowlders and rounded 

 cliffs will form high up in the mountains, sometimes on their very sum- 

 mits. As one of the most eifective agents, expansion by heat has 

 been mentioned, the result of which would be a scaling off of the outer 

 crust, and, in course of time, an approximation to the spheroid form. It 

 seems to me, however, that the action is more easily explained, and in 

 itself is certainly simpler, if we take into consideration the mechanical 

 retention of water by the mica and by the frequently-cracked feldspar, 

 combined with the expanding action that cold temperature has upon 

 water in causing it to freeze. Thus scales of the rock will gradually be 

 carried off comparatively rapidly, as in the high altitudes of that coun- 

 try frost occurs almost every night for the greater portion of the year. 

 Other atmospheric influences certainly have their share in i^roducing 

 the noticed effect ; and sudden changes of temperature, even without 

 freezing, cannot occur without leaving their traces in the course of time. 

 An interesting point to study the behavior of granite occurs at station 

 2, Mount Ouray. This mountain is composed of hornblendic rock, but at 

 a distance shows a stratified appearance on its eastern side. Approach- 

 ing closer, however, it is seen that this is owing merely to the fact that 

 narrow and some wider seams of granite are interstratified, i. e., intrud- 

 ing between the strata of the hornblende-rock. The afflxed cut will 

 illustrate this phenomenon, having been taken from a point near the 

 summit of the peak, where a seam of pegmatitic granite about one foot 

 in width intruded between the hornblende-strata, continuing for a con- 

 siderable distance. Numerous occurrences of this kind were noticed 

 on this mountain ; rarely, however, was the granite-seam as small as 

 in this instance. The very top of the peak is composed of granite, con- 

 taining reddish feldspar, white to gray quartz, and white mica, at 

 several points changing to a true pegmatite, with its agglomerated 

 quartz-crystals contained in the orthoclase-matrix. Invariably the 

 junction between the granite and hornblende-rock is perfectly clear and 

 sharp, without any transition of one into the other. In the resume of 

 section c, the relation of these two rocks, as regards their age and, 

 partly, genesis, will be considered. Entirely different from all the gran- 

 ites thus far treated of as occurring in this section, we find those of 

 the southern end of the Sahwatch range, the granite of the high range 

 bordering upon the Arkansas Valley. Generally it is of a light-gray 

 color, middle-grained, with either one or two kinds of feldspar, and, as 

 far as noticed, always black or dark-brown mica. As a rule, this gran- 

 ite weathers in sharp, well-defined fragments, which character is imparted 

 to the whole mountains composed of it. Mount Princeton, situated a lit- 

 tle north of Chalk Creek, at an elevation of 13,997 feet, tapers off' to a very 

 fine point, as do a number of the other peaks in that range. Its granite 

 presents a stratified appearance. The steep descent toward the valley 

 is rather surprising, inasmuch as the comparatively low land on the 

 east side of the river is composed of an entirely different granite. 



