SUEVEY OF COLORADO AND KEW MEXICO. 77 



iu oue of tlie loftiest peaks in Colorado. This peak has a large depres- 

 sion on the east side, which may once have formed a portion of the crater. 

 At the junction of the forks commences oue of the most remarkable 

 examples of what appear to be igneous rocks I have ever seen in the 

 West. On the east side of the creek we have the steep slopes, and on 

 the west the projecting edges. We have here eight hundred to one 

 thousand feet of eruptive rocks with a somber hue, but with a stratiii- 

 cation as perfect as in any sedimentary rocks. It is composed of layers 

 never over one to four inches in thickness, inclining south of west forty- 

 five degrees. Some of the layers would make good flagging stones. 



A little further down we come to the gneissic rocks, incliniug north- 

 west fifty to sixty degrees. Some of the black-banded gneiss has zig- 

 zag seams of feldspar and quartz running through them. 



About three miles before reaching the Arkansas there is a curious junc- 

 tion of the massive red feldspathic granites, inclining northeast seventy 

 degrees, with the dark-banded gneiss, inclining northwest twenty-five 

 degrees. At the point of synclinal junction all is confusion ; the two 

 kinds of rocks are crushed together, and yet there is no break iu the 

 mountain. As we emerge from the pass to the South Arkansas we have 

 the finest exhibition of banded gneiss I have seen in the West. The 

 rocks are of various colors, red, yellow, Avhite, and black, and the layers 

 are quite thin, and their appearance is very picturesque. The general 

 course of the Poncho Creek, from its source in the snow peak to the 

 Arkansas, is north. 



The gneiss is very varied in its texture ; some of it contains garnets ; 

 some of it is very close feldspathic, micaceous, or whitish quartzose. 



On the east side of Poncho Creek, about one hundred and fifty 

 feet above the Arkansas, on the side of the mountain, is a hot spring- 

 surrounded with a large tufaceous deposit. There is also near the foot 

 of the pass, on the side of the mountain, an extensive deposit of the 

 yellowish marl, filled with water- worn boulders. 



Between the South and ISTorth Arkansas there are some remarkable 

 terraces or benches extending the whole breadth of the valley from moun- 

 tain to mountain. On the north side of the South Arkansas are three 

 terraces, beside the rounded hills near the base of the mountains, which 

 rise in succession like steps. 



The high eruptive range which seems to cross the South Arkansas, 

 and to pass up along the west side of thel^orth Arkansas, aiDi)ears to be 

 composed of a series of enormous dikes in a chain merging into each 

 other, and having a strike about northeast and southwest. The general 

 trend of the aggregate is about north and south. 



On the west side of the Arkansas Valley the recent tertiary beds run 

 up to and overlap the margins of the mountains. They are composed 

 mostly of fine sands, arenaceous clays, and pudding-stones, cream-col- 

 ored arenaceous clays, and rusty yellow marls, fine sand predominating. 

 These beds weather into peculiar architectural forms, somewhat like the 

 " Bad Lands" of Dakota. Indeed they are very nearly the same as the 

 Santa Fe marls, and were doubtless cotemi)oraneous, and dip at the 

 same angle, three to five degrees, a little west of north. The tops of 

 the hills have ail been planed down as if smoothed with a roller. I have 

 called this grouj) the Arkansas marls. They occupy the entire valley of 

 the Arkansas. This valley is about forty miles in length, and on an 

 average about five to ten miles in width. It might j)roperly be called a 

 park, for it is completely surrounded by mountains. On the west side 

 is one of the grandest ranges of eruptive mountains on the continent. 

 On the east side is also a lofty range with a metamorphic nucleus, but 



