344 GEOLOGICAL SUEVEY OF THE TEERITOKIES. 



their usual character, frequently show a compact texture. The audesite: 

 on station 23 has a very fine-grained, homogeneous paste of maroon 

 color, with numerous crystals of oligoclase, few of orthoclase, and some 

 of chlorite embedded in it. So great, however, is the variation that 

 a short distance from the top the paste turns buff, the oligoclase has 

 disappeared, sanidite occurring instead, and fine needles of hornblende 

 replace the chlorite. Whereby these changes are produced that are 

 only local, and sometimes limited to a very small portion of the rocks 

 only, I cannot say, but strongly suspect that diifereuces in the process 

 of cooling, regarding both the method of, and time consumed in, cool- 

 ing, may have produced these effects. All over the mountain, however, 

 that rises considerably above timber-line, the rocks weather in thin 

 slabs, from one inch iu thickness to several feet, giving a submetallic 

 sound when struck with the hammer. Some of the mountains between 

 Ouray's Peak and station 23 have a slight color, from light red to yel- 

 lowish, owing, probably, not to a change of material but to a difference in 

 the weathering. On station 21 the character of the audesite is very sim- 

 ilar to that of station 23, showing the same compact texture, and the same 

 minerals, with the exception of orthoclase and chlorite, which latter is 

 replaced by crystals of mica. Sanidite is contained iu numerous crystals. 

 Magnetite occurs in considerable quantity. As before, the same changes 

 of color occur, due this time more to weathering, as the composition of 

 the rocks undergoes no change. Taking the general aspect of the rock, 

 it resembles trachyte to some extent ; but its geological features and 

 mineral constituents make it out as and-esite. Leaving this interesting 

 point and proceeding westward, we find the character of the country 

 changing somewhat, the high, massive mountains disappear along 

 Saguache Creek, and instead we find low table-hills, with steep sides 

 and level tops. Frequently the sides of these tables have a stratified 

 appearance, and the last upper stratum presents a steep, rugged face. 

 A very characteristic feature of the mountains can be observed all along 

 Saguache Creek, and also across the continental divide, that is, the 

 terrace-formation on their sides. Evidently the tuff that forms these 

 hills has at one time flowed along in such a manner as to form a stratum j 

 another flow has succeeded it, perhaps after a lapse of considerable time ; 

 and the conditions at present are such that the hill-side will not weather 

 off so as to form an unbroken slope, but every one of these larger flows 

 is designated by a sharp terrace of barren rock, which is well illustrated 

 by the annexed cut. Frequently three, four, and even more terraces are 

 found, one above the other, parallel to each other, presenting light colors 

 mostly, varying from white to yellow, reddish, and light brown. 



Throughout the southwestern portion of the section, the occurrence 

 of the trachorheites is uniform and very similar to those just de- 

 scribed. Station 32 is located on the edge of a trachytic ridge, where the 

 rock is red, slightly vesicular, certainly enough so to give it the charac- 

 teristic roughness, changing at times into black, then becoming more 

 compact. Crystals of sanidite and brown mica are dispersed all through 

 the rock, and h;f alite is found coating the sides of any small fissures 

 that may occur. Hyalite is found very frequently in these volcanic 

 rocks, as is also jasper, the latter varying in color from white to yellow, 

 red, brown, green, and black. This trachyte contains but a trace of 

 magnetite, while an audesite found near by contains a considerable 

 quantity. Station 33 to the southwest of 32 is located upon a high, 

 prominent point composed of audesite. The lithological character of 

 this rock is so constant that, although a great many miles apart, no 

 specific differences are found, and iu this instance again we have a rock 



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