PEALE.] GEOLOGY — TRACHORHEITES — GUNNISON RIVER. 169 



stratified, and there are huge castle-like forms, abrupt walls, and spires 

 and towers. Station 31 is situated in the midst of a mass of hills of 

 this material, their summits being generally broad and rounded. The 

 thickness of the breccia at station 31 is about 3,000 feet. These hills 

 slope toward the Gunnison in long, gentle spurs. The breccia north of 

 station 31 rests against the edge of the hills of porphyritic trachyte, 

 already described. It rests for the most part on Cretaceous sandstones. 

 The line of junction is seen on the west branch of Ohio Creek and on 

 the Gunnison at various points. It is probable that in the center there 

 may be shales between the sandstones and the breccia, as at station 73, 

 on the Gunnison. The valley of the Gunnison, on the north side from 

 Ohio Creek as far as station 71, has been subjected to considerable erosion, 

 and the breccia forms the basis of the hills. It probably rests partly on 

 the schists, with an occasional patch of sandstone between, as indicated 

 by an outcrop in the bluff opposite the mouth of Cochetopa Creek. 

 Fig. 4, Plate XIV, represents a section across the Gunnison, in the 

 meadow below the mouth of the creek. Fig. 3, on the same plate, is 

 a section through station 71. Here the sandstones appear and the 

 breccia is capped with a white rhyolitic rock, which is probably under- 

 laid by obsidian and tuffa, as we see farther down the river. Under 

 station 71 the line of junction is concealed. West of the station, below 

 the caiion which the river cuts in the schists, there is a steep dip of the 

 sandstones to the west or southwest, which is the reason they do not 

 outcroj) in the section in Fig. 2, which is made below the caiion. Bor- 

 dering the valley in which this section is made, on the north side, are a 

 number of buttes capped with obsidian and trachyte. Back of them the 

 mesas extend toward the hills to the northward. These mesas are 

 exposed on both sides of the Gunnison. At station 73 I made the fol- 

 lowing section of the volcanic layer: 



Section Xo. 20 — Gunnison Eivei-, near station 73. 



Tliickness. 

 Base. Peet. 



d 1. Breccia 400 



(2. Light piokish-white tufa ") 



g I 3. Gray laminated trachyte I • .^ 



I 4. Hard obsidian porphyry f 



1^5. Soft spheruhtic and porphyritic obsidian J 



Purple vesicular rhyolite 50 



Bluish-gray rhyolite 30 



Top. 



Total 520 



This section goes as far as the top of the bluff, but as we go back 

 there is a greater thickness, and probably a repetition of the upper por- 

 tion of the section, that is from the obsidian upward. The letters in 

 the section above correspond to the letters in Fig. 1, Plate XIY. As 

 the rocks of this section are typical of the rocks in these mesas, I will 

 describe them more in detail. The breccia I will pass by for the pres- 

 ent. The tufa, which rests immediately upon it, is almost white in color, 

 and appears to be made up mainly of feldspathic material, with parti- 

 cles of quartz and mica interspersed. It is soft and very fine in texture, 

 having a sandy feel when crushed in the fingers. Above station 73 this ^ 

 tuff i.s about five feet thick. Layer No. 3 is a dull, purplish-gray rock, 

 in which there are numerous particles of quartz and obsidian, with crys- 

 tals of sauidine porphyritically imbedded. There are also a few crys- 

 tals of black mica. Under the glass this rock has a vitreous appear- 

 ance. It is probably rhyolitic. It is from two to four feet thick. Layer 

 •No. 4 is very hard, and breaks into square blocks^ with very smooth, 



At 



