346 GEOLOGICAL SUKVEY OF THE TERRITORIES. 



tained in the shape of small fragments, which have been turned to semi- 

 opal in the altered tuff. The cleavage of the latter has become con- 

 choidal ; it is hard, resembling jasper or massive porcelain more than any- 

 thing else, and the change can be .traced as far as the obsidian. As- 

 cending to the summit of the hill, however, we again find andesite, 

 identical with that of station 26. From the point where it covers the 

 Tertiary beds the ash extends downward to Saguache Creek, and there 

 follows along the base of the bluffs for a short distance. 



On station 2G the audesite shows a phenomenon that is rarely equaled, 

 and by which I was led to term it phonolithic andesite in contradistinc- 

 tion to other rocks of that group. When striking the large bowlders, even 

 those of many cubic feet in size, they give a clear ringing sound, closely 

 resembling that from a bell. The Einging Hills at Pottstown, Pa., are 

 similar. 



In the southern portion of section c several times caves were found in the 

 tuffs, a feature that seems to be characteristic to them. Crowning with a 

 steeply-edged stratum some sloping hill, the caves had their entrance at 

 the base of the rocky precipice, and extended inward sometimes for 

 considerable distances. In several instances the shape of the entrance 

 was so well cut that the idea of its having been done by human hands 

 suggested itself. 



Not unfrequently colummar structure was seen in different species of 

 the trachorheites, as well in the andesite as in the tuffs, due most 

 likely to particular circumstances by which the cooling may have been 

 accompanied. 



Besides this immense area of volcanic rocks, we have two more in 

 section c, excepting those small remnants of overflows occurring in 

 Tomichi Valley. Extending a little north, and about six miles west of 

 station 45, is another accumulation of these rocks. It seems to be 

 mainly trachyte, with probably some propylite, having a light-green 

 paste, white to yellowish oligoclase, and a greenish hornblende. On the 

 north side of the Gunnison another rock occurs that belongs to this 

 group, covering the Carboniferous of station 38, composed of a light- 

 grayish, almost white, microcrystalline paste, containing crystals of 

 oligoclase and needles of hornblende, besides a very small quantity of 

 magnetite. It is a trachyte, quite closely allied to propylite, and extends 

 over but a small area to the southeast and east of the station. Station 

 42 is located on a trachyte of similar constitution, covering the Carbo- 

 niferous rocks, which extends north and northeast into Dr. Peale's 

 district, where he has found it, and gives a more definite description of it. 



Dikes occur in considerable numbers throughout the granitic country, 

 having a course of about north to south, and are composed of the same 

 mineral constituents that characterize them in the other sections ; epi- 

 dote and hornblende mostly replacing mica, while at other times mica 

 predominates. 



Drift covers a considerable area along the western side of the Arkansas. 

 A belt of about five miles in width runs along it, keeping parallel* in its 

 course and narrowing out toward the south. This drift is comi)osed, so 

 far as I have been able to learn from examinations at certain compara- 

 tively isolated points, of material that was brought down by the river 

 mainly; secondarily, by the different creeks striking the river from the 

 west. Although glacial action seems to have had considerable eifect in 

 transporting drift- material higher up on the Arkansas, I have not recog- 

 nized any such means of conveyance in that portion of our district. 

 Taking into account the shifting of streams parallel to their own general 

 course, it becomes evident that the material deposited immediately 



