558 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



it is the result of one uplift, the sedimentary saddles separating three 

 eruptive centers each having a much greater elevation than the sur- 

 rounding country. Compared with the Elk Mountains, its structure is 

 simple. 



As we approach the mountains, the sedimentary strata are seen rising 

 on their bases, and in the northern group the red beds (Trias?) dipping 

 eastward 50° to 60° form a portion of some of the outlying peaks. In 

 the middle group, remnants of Cretaceous strata are found, resting hori- 

 zontally on the highest peak. In the southern group, the sandstones of 

 the Dakota group dip steeply from the east, west, and south sides, and 

 curve over the summits of several of the lower peaks. 



The peaks of the Sierra La Sal have an elevation of from 11,000 to 13,000 

 feet, rising from 7,600 to 9,000 feet above the level of Grand River, which 

 lies on the north, and from 7,300 to 8,500 feet above the Dolores, which' 

 is on the east. These rivers flow through caiions that are from 1,500 to 

 2,000 feet below the level of the surrounding country. The area occu- 

 pied by the mountains is about 100 square miles, only a small part being 

 of the eruptive rock. The rock is composed of a light gray feldspathic 

 matrix, with crystals of feldspar and acicular hornblende, porphyriti- 

 cally imbedded. The igneous peaks are sharp, and connected by nar- 

 row ridges separating amphitheaters which head the drainage of the 

 mountains. 



The Sierra La Sal has been formed by the upheaval of an igneous 

 mass, which has broken through the lower strata, carrying up their 

 broken ends. When the Cretaceous beds were reached, the resistance 

 seems to have been small enough to allow portions of the strata to be 

 carried to the top of the mountains. In some of the peaks of the mid- 

 dle group, there are included fragments of the shales. As far as noted, 

 there are no radial dikes in the Sierra La Sal. 



SIERRA ABAJO. 



The Sierra Abajo is a little west of south from the Sierra La Sal and 

 about forty miles distant. It was visited during the summer of 1876 by 

 Mr. Holmes, and from his report I condense the following description: — 



The mountains consist of a number of small groups of volcanic sum- 

 mits. The trend is not sufficiently marked to make a well-defined range. 

 The individual groups are more like small ranges, butcollectively they are 

 rather a group. The eruptive material, althcnigh probably from the same 

 nucleus, seems to have forced its way through the lower sedimentary 

 strata by a number of channels. When the yielding Cretaceous shales 

 were met with, it appears to have carried them up, and to have intruded 

 sheets between the strata. Portions of the shales are still found in all 

 parts of the group, having been caught in the eruptive material. The 

 low saddles between the groups are invariably composed of metamor- 

 phosed Cretaceous shales. 



On the slopes of the mountains, the Dakota group and overlying shales 



