PEALE.] 



GEOLOGY SECTION NO. 20. 237 



As we g'O up the limestones gradually thin out, and the sandstones be- 

 come coarser and coarser, pointing" to the existence of shallower waters 

 during their deposition. I was unable to define the line between the 

 Carboniferous and the beds that I consider to be of Permian age. 



Going south from Weston's Pass we find that the range ends rather 

 abruptly in Buffalo Peaks. South of this the divide between South Park 

 and the Arkansas is comparatively low. Buffalo Peaks consist of two 

 high conical points of about equal elevation, viz, 13,365 feet. Between 

 the two there is a ridge bounding an amphitheater, which faces to the 

 northeast. On the face of this we find, going from the top toward 

 the base, the following beds: 



Section No. 20. 



a. 1. Trachyte, weathering very dark from the amount of iron it contains. 

 There are crystals of hornblende and sanidine. On fracturing 

 the rock the matrix is seen to be light-colored. Tiiis is found 

 on the western summit of the peak, and extends across along the 

 ridge to the eastern summit. Below we have — 



&. 2. Trachyte. The upper part of the bed has a bluish color, which 

 becomes red below. This is also hornblendic, and contains crys- 

 tals of sanidine. The rock is more compact and finer-grained 

 than that on the summit ; 100 feet. 



c 3. Eeddish breccia. The included masses are highly porphyritic, 100 

 feet. 



d. 4. Light gray tufa. I could not see the line between it and the one 



above. There are light-yellow pebbles in this rock. In the lower 

 part there is a soft black layer, thin ; 5 feet. 



e. 5. Columnar layer of very hard black rock, containing a large quantity 



of hornblende and obsidian. There are, also, a few included 

 pebbles. The columnar form is quite distinct in places ; 50 feet. 

 [ 6. Light tufacious rock, very solt, mostly of a white color, but pink in 

 ^ 1 some places and yellow in others, 150 feet. 



•'' 1 7. Gray breccia,which reaches to the base of the peak as far as can 

 ( be seen ; 200 feet. 



This section corresponds with Fig. 8, Plate YI. 

 I was extremely sorry that I was unable to work to the eastward of 

 the peak. Until that is done the opinions in regard to these rocks must 

 be to a great extent conjectural. On the west side of the west cone, g 

 in Fig. 8, Plate VJ, resting on the granite, is a bed of white and rusty- 

 looking quartzite, dipping toward the east at an angle of about 10°. 

 Going toward the north we find above the quartzite, and dipping in the 

 same direction, a bed of limestone very much changed. In one place 

 there is a flinty layer, over which I found coatings of chalcedony. These 

 beds are probably a continuation of those seen in Weston's Pass at the 

 point e in Fig. 1, Plate XII. They seem to have been caught here by 

 the volcanic rock. As we go south the beds continue uninterruptedly, 

 as we shall see on Trout Creek. The rock on the summit of Buff- 

 alo, although lighter in color, is very nearly like that found on the 

 tops of the buttes near the salt-works. They are probably identiical, 

 the differences being caused, perhaps, by the difference in length of 

 time of cooling. In both places the trachyte is micaceous. The peaks 

 are evidently a center of eruption, and the overflow was probably toward 

 the southeast in thedirection of the salt-works. Fig. representsan ideal 

 section through the peaks from east to west, the dotted lines representing 

 the planes of deiiosition of the various beds given in the section above. 



