FEAix.] GEOLOGY AECH^AN ROCKS OF GUNNISON AND GRAND. 107 



I have already said that the canon of Eagle Eiver is due mainly to 

 erosion. The course was probably determined by a liue of fracture of 

 which all evidence has been removed. There is no evidence of glacial 

 action on Eagle River, although all the branches coming from the Sawatch 

 range show it abundantly, especially Roche-Moutonnee Greek. 



The valley of this creek is filled with masses of gneissic rock, beauti- 

 fully rounded and smoothed by the glacier which once filled its valley. 



Grand River. — A little more than three miles below the mouth of the 

 Eagle, the Grand enters a canon in which it is more than probable 

 there are outcrops of metamorphic rocks, probable gneiss and schists. 

 To verify this opinion the bottom of the canon ought to be followed. 

 The rocks at the entrance of the cahon are limestones, which from their 

 position were supposed to be of Carboniferous age. The course of the 

 river at this point is a few degrees south of west, whiJe the limestones 

 have a dip which in general is east. On the north side of the river 

 it probably changes to south of east, and as we go up the creek that 

 joins the Grand just above the caiion it changes more to the north. As 

 we go down the caijon, therefore, the sedimentary beds rise and the 

 Archflean rocks ought to be seen beneath. This opinion is confirmed by 

 Mr. Marvine, in whose report this caiion will be fully described. He 

 was on the northern side and had a much better opportunity to study 

 its features. 



He says that on comparing the thickness of the sedimentary rocks 

 with the depth of the caiion it is evident that the gneiss must show be- 

 neath. The area, he thinks, is limited, for some five miles down the 

 canon there is either a fault or an abrupt fold which brings the sedi- 

 mentary beds once more to the bottom of the canon. With the excep- 

 tion of this exceedingly limited area, I believe there are no Archaean 

 rocks shown along the course of the Grand, from the mouth of the Eagle 

 to the mouth of the Gunnison. 



Gunnison River. — The head of the Gunnison River is entirely in meta- 

 morphic rocks, which were described in the reports of last year. Below 

 the mouth of East River, it flows through a belt of Cretaceous rocks, 

 from which it again enters gneiss and schists, in which it keeps almost 

 ' entirely from the mouth of Ohio Creek to the mouth of the North Fork. 

 The only exceptions are where the river flows through broad, meadow- 

 like valleys, and even here there are schists underlying the river-drift 

 and alluvium. There are but few of these meadows, and all are above 

 the Lake Fork of the Gunnison. 



This belt of gneiss and schists is narrow for the most part, extending 

 but a short distance from the edge of the river, except on the lateral 

 branches where the metamorphic rocks are exposed some distance from 

 the Gunnison, forming long, tongue-like areas. These lateral branches 

 are generally in canons, and the Gunnison itself cuts a caiion in the 

 gneiss. 



Below the cattle-camp near the mouth of Cochetopa Creek, the pre- 

 vailing rocks on the south side of the Gunnison, for a distance of more 

 than six miles, are schists and gneiss. Receding from the river, vol- 

 canic rocks will probably appear on top. On the north side, resting on 

 the gneissic rocks, is a heavy layer of volcanic breccia, underlaid in 

 places by sandstones, as evidenced by an outcrop not far below the cat- 

 tle-camp. At station 71, the Archaean belt narrows, forming the walls of 

 a canon, through which the Gunnison winds with rather a tortuous 

 coarse. A section from station 71 to the river is shown in Fig. 3, Plate 

 XIV. On the opposite side of the river the rocks are identical. Imme- 

 diately on the gneiss the8andstonesrest,asshownintheiilustration. They 



