jS. F. Emmons — Little Cottonwood Granite. 141 



been a matter of wonder to me that so few mistakes were 

 made. 



The problem which had the most important bearing ii]3on 

 the earlier history of the A^asatch Rano^e proved to be that with 

 regard to the age of the Little Cottonwood granite. This is a 

 huge, homogeneous body, somewhat in the shape of a half dome, 

 which forms the mass of Lone Peak and the lower two-thirds 

 of Twin Peak, and is well exposed in the deep glacial trough 

 of Little Cottonwood Canyon that runs in a nearly straight 

 west line between the two. Around this mass and in general 

 dipping away from it on the north, east, and south wrap the 

 series of Paleozoic sediments, which curve in strike from north- 

 west on the north through north-south to northeast on the 

 south. The lowest member of this series consists of 12,000 

 feet of Cambrian beds, mainly quartzites, which alone come in 

 contact with the Cottonwood granite body. On the west face 

 of the granite rest a series of westerly dipping crystalline 

 schists of assumed Archean age. These beds are uncon- 

 formably overlaid by the Cambrian beds, which differ from 

 them nearly 90° both in strike and dip. That the granite was 

 intruded into and hence later than these Archean beds is 

 readily evident, the contacts lying along the valley slope of 

 the range, w^here they can easily be seen. The contacts of the 

 granite with the Cambrian beds, on the other hand, are less 

 easy of access, being mostly high up on the steep mountain 

 slopes and covered by brush and talus, so that in the time 

 allotted for exploring this region few were actually observed. 

 Two alternative solutions of the problem presented themselves, 

 Avhich are indicated by Mr. King'^ as follows : 



''It is very evident that the granite is either an intrusive 

 mass or else an original boss on which the sedimentary material 

 was deposited." The close lithological resemblance of this 

 granite to the Jurassic granites of the Sierra Nevada had at 

 first suggested its later intrusion, but after careful weighing of 

 all the evidence it was concluded that the preponderance was 

 in favor of its pre-Cambrian origin. The Clayton Peak mass 

 farther eastward, at the head of Big Cottonwood Canyon, 

 though separated at the surface by several thousand feet of 

 easterly dipping quartzites and limestones, and of somewhat 

 different structure, was assumed to be part of the same boss. 

 This assumption necessitated in Mr. King's reconstruction of 

 the Archean surface, on which the Paleozoic sediments were 

 deposited, the existence in this part of the original range of a 

 steep cliff of about 30,000 feet elevation. 



While there is abundant evidence in other parts of the 40th 

 Parallel region to support Mr. King's statement that the pre- ■ 



*40tli Parallel Eeports, vol. i, " Systematic Geology," p. 48. 



