40 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



ages of all of tbe canon rocks. During the short interval between 

 storms I had a good opportunity of studying the interesting features of 

 the opposite side of the cahou. The relations of the rhyolite sheets to 

 the Tertiary conglomerates and sandstones are evident at a glance. In 

 the nortii wall of Jasper Creek and in the promontory that overlooks 

 tbo Yellowstone between Jasper and Agate Creeks there are some inter- 

 esting phenomena (Plate XX). The great sheet of rhyolite is from 300 

 to 400 feet thick and forms the escarpments of the walls on the north 

 side of Jasper Creek. At the top, for a thickness of about 100 feet, the 

 ledge is broken and partly tree-covered. Beneath this follows a magnifi- 

 cent wall, some 300 feet in height, in which we have a series of the most 

 perfect columns, straight, regular, and generally vertical. In one place 

 the columns are exposed to the full height of the wall. At the base of 

 the steeper part of the wall is a dark recess that is probably a cave, the 

 result of the crumbling away of the soft deposits at the base, which 

 here seems- to consist of a dark band of porous pitchstone underlaid by 

 white, bedded tuffs similar to those of Mount Evarts. The rhyohtes are 

 yellowish and reddish-gray in color. These igneous rocks rest upon the 

 rather uneven surface of the Tertiary strata. The latter formation 

 extends down to the river in dark, deeply lined and buttressed slopes. 



Back in the canon of Jasper Creek, a fourth of a mile or more, there 

 occurs a most interesting feature which, more than anything else I 

 have seen, throws light upon the topographical character of the surface 

 upon which the rhyolites were flowed. It is clear at a glance that at 

 that period a cailon existed where Jasper Creek now runs. The rhyolite 

 flows have, on reaching tbe brink of the old canon, poured down its 

 sides and filled in the valley, the line of contact being now plainly 

 exposed, sloping at an angle of nearly forty-five degrees. The drawing 

 (Plate XX) will make my meaning clear. As would be expected, there 

 is a tendency on the part of the columns, which are characteristic of 

 tliis rhyolite, to form at right angles to the plane of the base of the sheet. 

 Whether or not this flow entirely filled the old cahon it is diiScult to 

 say, but the fact that the upper surface of the rhyolite sheets has such 

 a uniform general level, tends to show that all irregularities of the old 

 surface were totally obliterated, and that the subsequent drainage has 

 probably been formed without regard to the preceding, and that the 

 occurrence of a modern river on the site of an ancient channel is merely 

 an accident. Following the face of the sheet of rhyolite from Jasper 

 Creek around to the end of the promontory overlooking the Yellow- 

 stone, we can see the same tendency to curve down the slopes of the 

 canon, although the erosion has been carried back so far as to obliterate 

 the inclined part of the sheet. On another occasion, while making 

 examinations on the Amethyst Mountain side of the cafion, I examined 

 closely a similar case of inclined flow, which 1 will describe further on. 

 The walls of the caiion on the east side, above the mouth of Jasper 

 Creek, are so broken as to allow of the growth of considerable forest. 

 There are numerous outcrops of rocks, howevei?, and along the middle 

 of the slope there is a ledge that has the appearance of basalt. It is 

 possible that it is a fragment of the same flow that I had just examined 

 on the west side, but may be interbedded with the rhyolites as others 

 appear to be farther down. 



When I had emerged from the caiion it was already late, and I had to 

 make all haste to reach camp on Elk Creek. Following the brink of the 

 canon I encountered two troublesome gulches or side caiions before reach- 

 ing Tower Creek. One of these was Antelope Creek, which I crossed 

 pretty high up so as to avoid the cauoned part. The rocks, so far as they 



