GEOLOGY — CANON OF MPTO-LY-AS RIVER. 45 



The subordinate mountain ranges which divide the Des Chutes basin are usually low, having 

 an altitude rarely greater than 1,000 feet above the level of the plateaus which border them. 

 They are composed of trap or metamorphic slate, sometimes forming groups or clusters of 

 conical hills ; at other times continuous chains, of which the trends do not harmonize with each 

 other, nor with those of the mountain systems which border either side of the great area in 

 which they are situated. 



LOCAL GEOLOGY. 



The plateau forming the eastern base of the group of mountains, of which the Three Sisters are 

 the most prominent peaks, has a nearly uniform altitude of about 4,000 feet, and extends from 

 the base of the Cascades thirty miles eastward, with no considerable interruption. It is here 

 everywhere covered by a thick layer of trap, which is cut through only by the canon of the 

 Des Chutes, which we did not visit, but of which the dark and perpendicular walls were visible 

 from the summit of the Cascades. Near our depot camp, on Why-chus creek, a rounded hill 

 rises some three hundred feet above this plain, composed of trap, or red and frothy scoria. This 

 hill has the appearance of having been formed by an eruption from below, subsequent to the 

 consolidation of the plain on which it stands. A few miles northwest from this point a detached 

 mountain rises from the plain, more accurately conical in outline than any other I have seen. 

 It is wooded to the summit, though exhibiting many bare surfaces composed of scoria. There 

 can hardly be a question that this is one of the lateral vents of the great volcanic chain which 

 passes but few miles to the westward of it. 



East of this conical mountain we crossed a succession of ridges of trap, evidently formed by 

 streams of lava poured down from the Cascade range. Approaching Mount Jefferson, we one 

 morning found our progress suddenly arrested by a canon 1,950 feet in depth. The southern 

 wall on which we stood was composed of metamorphic slate, dark gray in color, silicious and 

 somewhat crystalline in structure. The opposite side of the canon was- formed by the slope of 

 Mount Jefferson, which rose, almost unbroken, to its summit, far above the line of perpetual 

 snow. On the side of Mount Jefferson was plainly discernible a stream of black and ragged 

 lava, which, issuing from a point near the snow line and following the course of a mountain 

 torrent, had descended nearly to the Mpto-ly-as river. Picking our way down the wall of the 

 canon which I have described, we found all parts above the talus which covered its base, 

 composed of the same metamorphic slate, very homogeneous in texture, and nowhere exhibiting 

 any intruded minerals. This slate was inclined at a high angle, dipping toward the southeast. 



From this point we followed down the Mpto-ly-as river, for nearly twenty miles, along the 

 immediate banks of the stream. The walls of the canon on either side continued as high as 

 where we struck it till we emerged from the hills which form the eastern base of Mount Jeffer- 

 son, and came upon the plateau of the Des Chutes. This canon, where cut through the 

 hills, exposes nothing but volcanic rock, generally dark, vesicular trap, with sometimes vol- 

 canic conglomerate. In some places where this last formed the north wall of the canon, the 

 fragments which it included were of large size, cemented by a tufaceous base which was readily 

 eroded by the action of the weather. The portions of this material which here underlie these 

 larger masses of inclosed trap were protected by them from the erosion which wore away the 

 surrounding rock, and they were left perched on pinnacles sometimes twenty or thirty feet 

 in height, and having a less diameter at the summit than the rock which they sustain. 



The canon, as far as we followed it, seemed to be of uniform character, precipitous walls rising 



