CHAPTER I. 



SUEPACE GEOLOGY — VALLEY OF EAGLE RIVER. 



Eagle Kiver is a branch of Grand Eiver, one of the forks uniting to 

 form the Colorado. It rises immediately opposite the head of the 

 Arkansas, and is about sixty -four miles in length. At its head it is 

 formed by two main branches, one having its source in the Park range, 

 and the other rising in the Sawatch range, \Yhich terminates in the 

 Mountain of the Holy Cross. The Sawatch range, on the western side 

 of the valley of the Arkansas Eiver, forms the continental divide. North 

 of the Holy Cross the range falls off, the water-shed or divide crossing 

 to the eastward at Tennessee Pass, between the heads of Eagle Eiver and 

 the Arkansas. 



Eagle Eiver flows around the northern end of the Sawatch range. Its 

 general course, at first, is a little west of north. Ten miles north of the 

 Holy Cross it bends more to the westward, and the general course for 

 nearly fourteen miles is north 64° west. It then turns abruptly and 

 flows south 78"^ west, which course it holds quite uniformly for about 

 twenty miles, to its mouth. 



The greater part of its drainage is from the south. The entire area 

 drained by the southern branches is a little over five hundred square 

 miles. The opposite side of the river was in Mr. Marvine's district, and 

 will no doubt be fully treated of in his report. 



The river is a very rai)id stream throughout its entire length. The 

 average fall is 67.2 feet per mile. From Tennessee Pass to the mouth of 

 Eoche-Moutonnee Creek, the rate is 150 feet, and from here to the head 

 of the second canon 49.4 feet, while from the latter place to the mouth 

 it is 32.4 feet. 



The upper part of Eagle Eiver was partially described in the last 

 annual report (1873), our division having followed it as far as Eoche- 

 Moutonnee Creek, for the purpose of ascending the Mountain of the 

 Holy Cross. In order that this report may be complete I will have to 

 repeat a portion of the notes on my work of the previous year. For a 

 distance of about three miles from Tennessee Pass the river is in a 

 canon-like valley, the hills on either side being comparatively low and 

 rouuded. The rocks are granitic, with occasional dikes of volcanic ma- 

 terial. From this canon the stream emerges into a broad meadow-like 

 valley of about four miles in length, in which it is joined by the branch 

 rising in the Park range near Quandary peak. 



The valley is three miles in width, the hills on either side of granitic 

 rock being capped with sedimentary formations, which will be referred 

 to in more detail in another part of the report. Leaving this valley, 

 the river flows immediately into a canon with steep sides, the trail leav- 

 ing and crossing to the western branch. A line of outcrop of quartz- 

 ites crosses the river and follows the summit of the ridge between 

 the two branches. These beds are, in all probability, primordial. Car- 

 boniferous beds outcrop on the eastern side of the eastern branch, but 

 1 defer tUeir description for the present. 



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