nouiEs.] SOPEIS PEAK. 61 



this region are connected, and, generally speaking, not tbe result of 

 complicated causes, may be pretty conclusively shown. 



It must be noticed, in the first place, that on the east side the sedi- 

 mentary strata lie up against the granite of the Sawatch range, and that 

 on the west they have been carried high up on the arch of the Elk 

 Mountains, leaving the synclinal depression between the ranges. In the 

 second place, that the axes of the two ranges are not j)arallel; that they 

 approach each other toward the south and sej>arate toward the north, 

 giving an included angle of some 30°. In the vicinity of Italian peak 

 the granites of the two ranges are in contact, or nearly so, as seen at a 

 in section 2. On station 3, a few miles farther north, a fragment of the 

 Paleozoic rock is caught up and held, as in a vice, between the masses 

 of ernptive and metamorphic granite, b, section 3. iSTorth of this, down 

 the valley of Castle Creek, the sedimentary area widens rapidly. The 

 edges facing the Casile group are bent up at a sharp angle, but as the 

 fold widens it also flattens, so that 30 miles north of Italian Monnt- 

 ain, near the line of section 10, the belt of strata is 25 miles wide and 

 has nowhere a dip greater than 10° or 12°. 



In the plate I have indicated the two g'ranites by different symbols, 

 the metamorphic by short broken lines and the Elk Mountain granite 

 by dots. The points of contact, as shown at a, b, c, &c., are, of course, 

 only given to indicate a probable contact line. That such a separation 

 really exists, however, is evident from the fact that when obsi^rved in 

 close contact, near Italian Mountain, they are totally distinct in appear- 

 ance and in reality. In all its general features, the geology of the val- 

 ley of Koaring Fork and of the eastern slope of the Elk Mountains, 

 seems simple enough, and I shall hasten on to the north and west. 



GEOLOGY OF SOPEIS PEAK AKD YICmiTY. 



Having ascended Sopris Creek for some five or six miles, we turned 

 abruptly to the right and crossed the low divide that connects Sopris 

 peak with an outlying triangular spur, and descended by a deep gulch 

 into the valley of Eock Creek. We reached this creek at the point 

 where the upturned edges of Cretaceous No. 1 (Dakota group) cross, 

 and found that our descent had been almost with the strike of the beds, 

 !N. 40° TV. This is on the west slope of the Eoaring Fork synclinal, and 

 the dip is therefore to the east. The creek passes out into the Cretaceous 

 shales and reaches the river some eight or nine miles below. The 

 crossing of jSTo. 1 here marks the foot of the caBon of Eock Creek. 



Beneath l^o. 1, on the south side, a very beautiful section of the 

 Jurassic is exposed. Near the summit of the bluff, about 200 feet of the 

 Lower Cretaceous measures are exposed, consisting principally of com- 

 pact yellowish sandstones. Some thin beds of shale are interstratified 

 with the sandstones, and near the base there is an irregular stratum of 

 moderately coarse conglomerate. The Jurassic section, beginning at 

 the top, is as follows : 

 20 feet shales, containing seams of greenish and purplish quartzite. The 



shales weather like fire-clay. 

 6 feet thinly laminated, fine-grained, flinty quartzite. 

 10 feet yellowish quartzite. 

 80 feet shales and calcareous sandstones. 

 40 feet sandstones and sand shales. 

 40 feet yellowish sandstone with layers of gypsum. 

 Red shales and red sandstones of indefinite thickness. 



From the trail, near the creek-bed, a very fine view of this cliff is 



