HOLiiEs] THE 'GRAND CANON. 41 



are exposed, appear to be rhyolites. On the opposite side of the canon 

 the capping- of rhyolite could be traced with ease. At Agate Creek it is 

 from 300 to 400 feet thick. The upper surface is quite flat, and, extend- 

 ing back to the slopes of Amethyst Mountain, there are broad meadows. 

 The rhyolite sheet seems to thin out toward the northwest. The Tertiary 

 formations, which form the greater part of the caiion wall, rise higher 

 and higher until, at the point of the promontory, opi:)osite the mouth of 

 Tower Creek, they reach the top of the wall and the sheet of rhyolite 

 feathers out and falls back from the caiion brink. It is exposed, how- 

 ever, on the north side, facing East Fork. At one place a mile or more 

 above the mouth of Tower Creek, there is a break in the sheet or sheets 

 of rhyolite. A later sheet seems to have flowed over the eroded edges 

 of a former one so as to give the appearance of a non-conformity. The 

 rhyolites extend down to the mouth of Tower Creek. They may be 

 seen both above and below the falls. They appear in the bed of the 

 creek near its mouth, and in the highest point of the short ridge that over- 

 looks the falls on the east. Strangely enough the rhyolites do not reach 

 across the creek, the western bank being formed of basalts and Tertiary 

 strata, mostly conglomera,tes. At the falls these rocks occur on both 

 sides and form most of the high towers for which the locality is noted. 

 These rocks also extend down to the bed of the stream below the falls. 

 The strange relation of the rhyolites and conglomerates I was not able to 

 make out for want of time. From what I could see, the creek must flow 

 along the line of an old Tertiary cliff against which the rhyolites flowed, 

 otherwise we have here a point of outflow, and the rhyolites that occur in 

 the creek bed low down occupy the old rents. The i)resence of rhyolitic 

 breccias and tuifs at the Devil's Hoof confirm this. This locality has 

 often been described, and 1 have but little to offer that is new. The 

 character of the conglomerates and their relations to the volcanic rocks 

 can be studied here to great advantage. 



On a previous day, while engaged in the study of Amethyst Mountain, 

 I descended to the brink of the caiion on the north side near the mouth 

 Agate Creek. In descending from the ridge that extends to the west- 

 ward from the main summit of Amethyst Mountain, I found that the 

 conglomerates reached to the base of the steep slopes, and that the 

 table-land which extends from the caiion to the base of the mountain 

 is composed entirely of the rhyolites. To all appearances these rocks 

 pass beneath the conglomerates of the mountain. In descending the 

 caiion wall I i)assed down over about 200 feet of light, yellowish rhyo- 

 lite, and found below the well-known Tertiary conglomerates; these 

 extend down to the river in a rather gentle slope, which is broken mid- 

 way by a partially wooded terrace. At my left, and inclining with the 

 slope at an angle of 50°, I found a tongue of rhyolite (c/, Plate XXI) 

 extending down over the conglomerates, precisely as I saw them on 

 Jasper Creek. The base part of the mass was dark, resembling a pitch- 

 stone, and identical with the lower part of the great sheet which caps 

 Mount E varts. Beneath this, and separating it from the conglomerates, 

 is a bed of finely- grained white and gray tufaceous sandstone (e, Plate 

 XXI), which also is precisely like the similar formation in Mount 

 Evarts. This locality is, perhaps, two miles below that previously de- 

 scribed opposite the mouth of Jasper Creek. The mist which prevailed, 

 at the time prevented me from making satisfactory observations of the 

 opposite walls of the canon. The upper third evidently consists of the 

 coarsely columnar rhyolite. Beneath this, appearing through the i)retty 

 dense coN'ering of trees, were two ledges of dark, very perfectly colum- 

 nar rock that may be basalts. The geologist who attempts to study the 



