556 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



in appearance with that of Mount Guyot. In a dike extending south 

 of east from the peak, it is trachytic. 



White Bock area. — This area is named from White Rock Peak, and 

 is the most irregular, from the fact that on the center of the mass a 

 portion of the Palaeozoic strata still remain. The igneous rock is ex- 

 posed around the base of the stratified mass. The Palaeozoic rocks 

 have been carried up with the eruptive mas;^, and now rest upon it in 

 almost horizontal position, forming the Castle group of peaks. Castle 

 Peak has an elevation of 1.415 feet, while White Rock Peak, the high- 

 est peak of igneous rock in the area, has an elevation of 13,357 feet. 

 West of the Castle group, the- connection with the beds that dip to the 

 northeast from the mountains is not broken. On the south and west 

 sides of the area, the general dip of the strata is to the southwest, 

 although near it there is considerable confusion in the beds from the 

 presence of the fault-fold, to which I have already referred. They are 

 penetrated by dikes and sheets of igiieous rock, and in places are com- 

 pletely overturned. In the synclinal between the White Rock and 

 Snow Mass areas, the strata are of Carboniferous age. There are nu- 

 merous dikes, and the beds are considerably metamorpho'sed. The 

 peaks into which these sedimentaries are eroded attain as great an 

 elevation as do the eruptive peaks in the adjacent areas. The eruptive 

 rock is generally granitic in appearance. 



Snoii' Masa area. — This is by far the largest of the eruptive areas of 

 the Elk Mountains, containing about forty square miles. On the south 

 and southwestern sides of the area, the stratified rocks are much 

 disturbed.* On the southwest, the Cretaceous is in contact with the 

 eruptive granite. On the north and east, the Palaeozoic rocks rest on it. 

 East of the area is a dike or sheet of igneous rock between layers of the 

 Palaeozoic formations. It was once doubtless connected with the mass of 

 the main area. Erosion of the valley of Snow Mass Creek has cut 

 across it and broken the connection. Between the Snow Mass area 

 and Sopris area is a synclinal of sedimentaries. 



Sopris area. — This is the most northern and most symmetrical of the 

 Elk Mountain areas. The sedimentaries are seen dipping away from it 

 on all sides. On the west, erosion has removed a portion of them, so 

 that a tongue of the igneous rock extends westward from the mass of 

 which Sopris Peak is the center. In the areas just described, the rocks 

 are generally compact and fine-grained, like the granite (?) of Mount 

 Guyot. In the dikes radiating from the central niasses, the trachytic 

 character is prevalent. In some of them, however, the rock cannot be 

 distinguished from that of the central eruptive areas. 



West Elk groups. — Southwest of the Elk Mountains are a number 

 of isolated mountain groups, those nearest the main range being gen- 

 erally included with it. As we recede from the Elk Mountains, they be- 



* See sections D and E oa sheet opposite p. 72, Annual Report of United States Geo- 

 logical Survey for 1874. 



