80 GEOLOGICAL SURVEY OF THE TERRITORIES. 



The western branch is in reality the continuation of the main river, 

 being twelve miles long. It rises in the Sawatch range, and drains the 

 country for six* miles south of the Mountain of the Holy Cross. Its 

 course at first is north 40° east, but in the lower two miles it flows almost 

 at right angles to this, being parallel to the eastern branch. On the 

 western side of the stream the hills are gneissic, the sedimentary cap- 

 ping have been removed by erosion. All the streams joining the river 

 above the mouth of the Piney on the south and west present abundant 

 evidence of intense glacial action. They are parallel to each other, the 

 general course being north 40° east. The glaciation was described in the 

 report for 1873, and I, therefore, simply refer to it here. 



Before uniting with the western branch, the eastern fork is joined 

 by a branch of considerable size having its source in the Park range, 

 opposite Ten-Mile Creek, one of the tributaries of Blue Eiver. The 

 geology about the head of this stream has never as yet been fully inves- 

 tigated, but I am inclined to think that all the formations, from the 

 Carboniferous to the Eed Beds, inclusive, will be found along its course. 

 The Cretaceous beds would scarcely appear until we reach a point 

 farther north, near Mount Powell. As I mentioned in last year's re- 

 port,* I think it probable that a fault extends along the western edge 

 of the Park range, west of Blue River. 



After the union of the two forks. Eagle River enters a deep and inac- 

 cessible canon of about four miles in length, cut in dark-colored gneissic 

 rocks, from which it emerges just above the mouth of Roche-Moutonn6e 

 Creek, The trail keeps high (800 to 1,000 feet) above the level of the 

 river, on the hills on the eastern side, near the edge of the sedimentary 

 formations, vhich are exposed on both sides of the canon. On the 

 western side there are only patches of quartzite, remnants of the Pots- 

 dam group. On the eastern side there are other beds, probably of Silurian 

 age, upon which rest Carboniferous layers, and possibly the Devonian, 

 although it seems to be altogether wanting, there being no positive evi- 

 dence of its existence here. These beds all dip about 10° to 20° to the 

 northeast, the inclination gradually changing more to the north as we 

 follow the river. In the bluffs on the right-hand side of the river, oppo- 

 site the mouth of Roche-Moutonn6e Creek, formations from those of Pri- 

 mordial age to the Permian, or Permo-Carboniferous, are exposed. 

 This, of course, includes the Devonian doubtfully, for that formation 

 has not, as yet, been positively identified in Colorado. Just above the 

 mouth of the creek gneiss is seen on the edge of the river, but as we go 

 down, higher and higher sedimentary beds gradually form the base of the 

 bluffs, and below the Piney the line of outcrop of the Carboniferous 

 crosses, and still farther down even the Cretaceous shows on both sides, 

 the strike curving around the end of the range and continuing along 

 the western side to the Elk Mountains, in our last year's district. On 

 the west side of the river, as far as the Pine.y, there are long spurs, or 

 ridges, sloping gently at an angle of about 10°, toward the river. 

 These ridges are capped with quartzite, which I have considered to be 

 the equivalent of the Potsdam group. These quartzites terminate 

 within a short distance of the center of the range. Erosion has re- 

 moved the beds formerly resting upon them, their hardness preserving 

 them. They are shown in Section B, Plate I. 



The creeks, separating these ridges, have their origin in beautiful 

 meadow-like parks, nestling immediately below the peaks in the range, 

 from whose snow banks they derive their supply of water; thence they 

 flow with a comparatively uniform descent to within a short distance of 



'Seventh Annual Report, 1873, page 242. 



