pjj^LE.] GEOLOGY ELK MOUNTAINS. 257 



3. Then tbere is a space reaching to the bed d, which seems to be filled 

 entirely with black shales. At 6? is a bed of white sandstone, quartzitic 

 in places. It is beyond doubt the jS^o. 1 Cretaceous. Above it 

 are variegated shaly beds. The colors are very bright. They are 

 green, purple, yellow, and white. They extend for 300 or 400 feet, and 

 are succeeded by red-beds which extend for nearly 1,000 feet, grading 

 into maroon-colored beds, beyond which are yellow beds, probably rep- 

 resenting the Silurian. Above No. 1 Cretaceous, therefore, is a com- 

 plete series from the top of the Jurassic to the base of the Silurian. At g 

 the eruptive granite is seen. The illustration shbws also how the beds 

 farther along, at a, are in their natural position. On the other side of 

 the valley, just above Crested Butte, the dip is northeast, and at this 

 point, therefore. East Eiver flows in the axis of a synclinal fold. At 

 the base of Gothic Butte the dip is the same as on the opposite side of 

 the valley, viz, toward the southwest. On Slate Eiver the rocks are 

 all Cretaceous, and there are some very fine terraces cut into these rocks. 

 The general dip, I think, is to the northeast or east. I hope to be able 

 to visit this region again in continuing the work westward, and to be 

 able then to determine some points that are as yet somewhat indefinite. 

 The Elk Mountain region is sufiicient of itself to furnish several years' 

 work. Slate Eiver formed the western limit of our work for the season 

 at this point, although more to the north we worked considerably farther 

 to the westward. Fig. 1, Plate XVIII, represents a section (partly 

 ideal) through Gothic Mountain and across the granitic area west of 

 White House Mountain. It is an interesting fact that the beds from h 

 to c are right side up, while a short distance down the river they are 

 inverted, as seen in Plate XVII, and also on Eock Creek, as shown in 

 Fig. 2, Plate XVIII. It is opposite Gothic Mountain that they are in 

 the normal position. It is fair, then, to infer that the presence of Gothic 

 ]\Iountain has something to do with it. Either the force of upheaval of 

 the latter counteracted the force of elevation of the main range, or the 

 egress of the trachyte acted as a relief to the latter force. The two 

 elevations were probably contemporaneous, at least in part. The erup- 

 tion of Gothic Mountain must have occurred at the same time that the 

 inversion of the beds, both below and above it, took place. At the same 

 time the beds were compressed laterally, and the fold between c and d 

 was formed. As we look down East Eiver, from Bellevue Mountain, 

 we have a fine view of these beds. They are so folded between c and d 

 as to form a semi-quaquaversal. As shown in the illustration, subse- 

 quent erosion has broken the connection between the beds, and a little 

 beyond the point where the section was made the granite is seen in the 

 center. At e, on the north side of the granite area, the sedimentary 

 beds are seen dipping toward the northeast. It is probable that the 

 mass of granite shown at/ and the trachyte forming Gothic Mountain 

 are connected. It would be interesting if we could trace the trachyte 

 down to that connection and learn whether or not there is a gradual 

 transition into the granite. The Elk Mountains present many complex 

 problems, and it will require years of close and arduous work to master 

 all the geological details. Each stream, with all the numerous small 

 branches, will have to be followed to the very source, and almost every 

 ridge and prominent peak will have to be ascended before all the ob- 

 scure points can be made clear. It is one of the most interesting regions 

 in the entire extent of the Eocky Mountains. I am confident, however, 

 the observations made during the season of 1873 are in the main cor- 

 rect, and although much remains yet to be done, future observers will 

 find but few errors in the work already recorded. 

 17 U s 



