194 REPORT UNITED STATES GEOLOGICAL SURVEY. 



eastern group in 1875, and has described its structure in the report for 

 that year. This is one of the finest groups of summits in the Rocky 

 Mountains, and viewed from the north presents a magnificent panorama. 

 The geologic structure of the western half is quite simple. All of that 

 portion of the summits that rises above 1,200 feet is of trachyte. This 

 is underlaid by Cretaceous shales, in a horizontal position. Interbedded 

 with the shales are a great number of sheets of trachyte. These are 

 somewhat variable in thickness and manner of occurrence, and seem 

 to decrease in number and importance in descending from the main 

 trachyte mass. 



The divide between the eastern and middle groups has an altitude of 

 11,500 feet, and seems only to be preserved at its present height by a 

 heavy bridge-like mass of trachyte, which connects across from group 

 to group and prevents the degradation of the soft shales beneath. In 

 the heads of the valleys north and south of this divide there is a suc- 

 cession of irregular shelves or steps produced by outcropping edges of 

 sheets and masses of trachyte. 



The Dolores group is a small, compact cluster of summits, a number 

 of which rise to the height of 13,000 feet. From the central peak three 

 narrow crests radiate, one to the north, one to the east, and the other 

 to the southwest. Dolores Peak is the outer point of the eastern ray, 

 and is directly connected by the saddle mentioned above with the Wil- 

 son group. The geological structure is identical with that group. The 

 northern arm falls off abruptly to the north and terminates in a sharp 

 trachytic butte which overlooks the caiiou of the Rio San Miguel. The 

 southwest arm connects with the low masses of the Lone Cone group. 

 The conical peak called Lone Cone is a very prominent landmark, over- 

 looking, as it does, all the great plateau region to the south and west. 

 The summit rises to the height of 12,500 feet, and in shape resembles a 

 triangular pyramid. The pyramidal part is a bare mass of grayish tra- 

 chyte, which rests on a base of horizontal shales. On the southeast and 

 northeast sides; the shales have fallen away from beneath the trachyte 

 so that vertical faces, from 100 to 200 feet in height, have been formed 

 by the breaking down of the trachyte. On the west the long slides 

 of trachytic debris make the summit easily accessible. The shales 

 about the base of the pyramid seem to be somewhat metamorphosed, 

 and there is doubtless considerable interbedded trachyte, but a dense 

 growth of pine timber completely covers the middle slopes and makes 

 investigation very difficult. 



From the southeaf<t angle of the pyramid a high, timbered ridge ex- 

 tends out to the most southerly summit of the group. This peak is 

 conical in shape, but is inferior in height to Lone Cone. The summit 

 is of trachyte, but the timber reaches so high that the shales, which 

 very probably lie beneath, are not visible. In the ridges or saddles 

 which connect this point with the Dolores group and Lone Cone there 

 are occasional outcrops of shales and trachytes, as in the saddle be- 

 tween the Dolores and Eastern groups. South of this point there is 

 another large mass of trachyte, the relations of which to the surround- 

 ing rocks 1 could not clearly make out. The trachyte caps two low, 

 massive, partially severed ridges, that extend down toward the Dolores. 

 From the south, as seen in 1875, it seems to rest upon the Lower Cre- 

 taceous sandstones, and probably at the southeast upon the Red Beds. 



Thus it appears that the San Miguel Mountains comprise at least five 

 nearly distinct masses of trachyte, four, at least, of which rest upon 

 horizontal Cretaceous shells, and at a uniform level above the sea. It 

 seems probable, therefore, that they all belong to the same flow, which 



