PEALE ON ERUPTIVE MOUNTAINS IN COLORADO. 559 



are seen dipping away from tbe ernptive nucleus. Compared with the 

 Sierra La Sal, there is, in the Sierra Abajo, a tendency to more rounded 

 summits. There is a general uniformity in height, and some of the 

 mountains are even flat toi)i)ed. Tlieir general height above the sand- 

 stone of the Dakota group being so nearly the thickness of the Creta- 

 ceous shales, leads to the supposition that the trachyte never reached the 

 surface, but was stopped by the massive sandstone at the base of the 

 Upper Cretaceous, which doubtless once prevailed over the whole area. 

 The rock of the Abajo is a porphyritic rock, like that of the Sierra La 

 Sal. Dr. Newberry visited the eastern side of the Sierra in 1859, and 

 also considered it to be locally upheaved.* 



SAN JUAN AREAS. 



Under this head E arrange the groups lying between the San Juan 

 Mountains and the San Juan River. They were studied by Mr. Holmes 

 in 1875 and 1876, and are as follows: San Miguel, La Plata, El Late^ 

 and Carriso. The San Miguel area is nearest to the Elk Mountains,, 

 from which it is distant nearly 100 miles. 



SAN MIGUEL AREA. 



This area is some sixty miles east of the Sierra Abajo, and consists 

 of four or five groups of comparatively simple structure. They are the 

 Mount Wilson group, the Bear Creek group, the Dolores group, and the 

 Lone Cone group. They lie between the heads of the San Miguel, Do- 

 lores, and Animas Rivers. The Mount Wilson and Bear Creek groups 

 were visited by Dr. Endlich in 1874. t He recognized them as isolated 

 eruptions. The eruptive rock is a greenish paste, with innumerable 

 white crystals of oligoclase, which are set off to advantage by accompa- 

 nying black crystals of hornblende. In general, the igneous rock rests 

 on the sedimentaries, with, in some places, intruded sheets between the 

 strata. The western groups were visited by Mr. Holmes in 1876. The 

 rock is similar to that already described, and is generally found resting 

 on the Cretaceous shales, wliich are usually metamorphosed. There are 

 numerous dikes in the region, and fragments of shales are often included 

 in the igneous rock, as at Lone Cone. The sources of the erui)tive ma- 

 terial are generally obscured, but the probability is that it came through 

 fissures and spread out in the shales. 



LA PLATA AREA. 



The La Plata Mountains, lying from 20 to 30 miles south of the San 

 Miguel Mountains, are of more than ordinary interest, on account of 

 the relations of the igneous rocks to the sedimentaries and the degree to 

 which the metamorpiiism of the latter lias been effected by contMct with 

 the erupted material. Mr. Holmes's report for 1875 describes the area in 

 detail. The mountains are composed in part of eruptive material and 



* Exi>l()iing Expedition to Junction of Grand and Green Rivers, p. 100. 



t Annual Report United States Geological Survey for 1874, I87(i, pp. 207-208. 



