CHAPTER V. 



General VIEW of the geography and geology oe the Elk Mountains. — Eruptive 



GRANITES. — KhYOLITES AND DYKES. — EROSION ON A GRAND SCALE. — LOCAL DIUFT- 

 DEPOSITS. 



The Elk-Mountain group is one of the most remarkable ranges in our 

 western Territories, and, so far as my own explorations have extended, 

 is unique in form and structure. For this reason a small party was 

 organized in the summer of 1874 under my immediate direction, with 

 Mr. Holmes as assistant geologist, and Mr. Chittenden as topographer, 

 for the special study of this curious and most interesting group. The 

 numerous sections and maps which are given in this report form a por- 

 tion of the results of this specific study. Much attention was given to 

 this region the previous year, and the results printed in the annual 

 report for 1873. As our explorations are extended to the westward of 

 this range, we hope to be able to present a more complete geological as 

 well as topographical view of this region. 



The Elk-Mountain group lies immediately west of the great Sawatch 

 range, which forms the water-divide of the continent. It occupies an 

 area of about 800 square miles, between meridians 106° 45' and 107° 15', 

 and parallels 39° 30' and 39°. 



The Sawatch group is one of the loftiest and most symmetrical ranges 

 in the West. It extends from the Mountain of the Holy Cross to the north, 

 latitude 39^28', longitude 106° 28', southward to the San Luis Valley, a 

 distance of over 80 miles. For this entire distance the range literally 

 bristles with lofty points, about ten of which rise above 14,000 feet, and 

 many more are 13,000 feet above sea-level. The uniformity of this great 

 mountain-mass is a remarkable feature. Standing on some high peak 

 and glancing along its pointed summits from north to south there seems 

 to be comparatively little variation either in form or height. On either 

 side of the Sawatch range there are several somewhat lower parallel 

 ranges which are undoubtedly portions of an immense anticlinal, of 

 which the main granitic mass is the central nucleus. Between each of 

 the parallel portions of the anticlinal are valleys at intervals of greater 

 or less width. Immediately west of the granitic nucleus is the valley of 

 the Gunnison, and on the east, the valley of the Upper Arkansas, and 

 east and west of them are comparatively low granitic ranges capped 

 with sedimentary rocks. Both the metamorphic and sedimentary rocks 

 incline at vari^ous angles from the great central mass. 



There are here represented two quite distinct types of mountain ele- 

 vation, though the forces have influenced each other's results to a greater 

 or less degree. The Sawatch or main range presents an example of a 

 long-continued, uniform movement upward, which, but for the inter- 

 vention of side-forces, would have produced a remarkably symmetrical 

 mountain-group with the main granitic core or central mass, and on 

 either side parallel valleys and ranges, each becoming lower and lower 

 until the ridges faded out in the plains. The sedimentary rocks would 

 have inclined at various angles east and west from either side, until 

 they became horizontal in the plains. On the east side of the main 

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