AMERICAN GEOLOGY— SURVEYS UNDER HAYDEN. 



601 



liase line «,«»> ft. 



Fi». 1. Section across Northern Gr 

 7 miles 



j£\* 



In describing the Sierra La Sal south of the Gunnison in the report 

 for 1875, Peale compared them in structure with the Elk Mountains — 

 that is, as of eruptive origin. "By this," he wrote, "] mean that 

 the sedimentary strata have been lifted up by eruptive rock which has 

 broken through them in some places, and in others is seen only as the 

 result of subsequent erosion.' 1 As illustrative of this, he gave the 

 figure reproduced here. (Fig. 106.) 



The idea thus advanced seems to have been contagious, for in 

 the same report Holmes described the Sierra El Late as formed 

 of a number of distinct 

 bodies of trachytic rock 

 that had reached their 

 present horizon through 

 closely associated vents, 

 frequently bending up e 

 the sedimentary rocks at 

 a high angle around the 

 borders, the upturned 

 strata including the 

 lower part of the Middle 

 Cretaceous shales and 

 portions of the Dakota 

 sandstone. His obser- 

 vations tended to show, 

 to quote his own words, 

 ""that there had been a 

 sort of absorption, so to 

 speak, of the shales, and 

 that at least half of the 

 space through which the 

 trachyte is distributed is 

 occupied by the crushed 

 and metamorphosed 

 fragments of shale. As a consequence the height of the arch — such as 

 may once have existed — would not equal the height of the trachytic 

 mass, as only the higher layers of shale extend entirely over it. His 

 idea regarding the formation of this mountain can be best understood 

 by reference to tig. 107, copied from Plate l»i in the report for 1875." 

 (See further on p. 622.) 



«The views regarding this: method of mountain formation were subsequently 

 summed up by Peale in an article, On a Peculiar Type of Eruptive Mountains in 

 Colorado, which was published in No. 3 of the Bulletins of the Geological Survey, 

 May 15, 1877. 



Fig. 



Section across Mfddle Group. 

 7 miles 



Ilakola 



'"■Fig. 3. Section across Southern Group. E 



7 miles 



us |3gl .Jura Trias L^J Paleozoic I I Trachyte 



Sections acros.s the SIERRA LA SAL. 

 for lines see map on Plate V. 



Fig. 106.— (After A. C. Peale.) 



£K. 



