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GEANITE AND ARCHAEAN EOCKS. 355 



dered improbable by the fact that no erratic boulders are now found there, 

 too large to have been brought down by the force of a mountain torrent, 

 though the waters of this lake might have levelled off or dispersed the accu- 

 mulations of finer moraine material. In the little secondary glacier-basins, 

 on the northern side of Lone Peak and of the ridge which ex,tends eastward 

 from this peak, large fields of n^v^ ice, often with little glacier-ponds within 

 them, may still be found even after midsummer. 



The geology of this region may be described in a few words as fol- 

 lows: — A body of Archaean slates and granite is surrounded, and partly 

 covered, on all sides except the west, by a conformable series of sedimentary 

 rocks, of an aggregate thickness of over 30,000 feet, extending in age from 

 the Cambrian to the Jurassic inclusive. The granite mass, though eruptive, 

 has not been protruded through this immense thickness of overlying rocks, 

 but their beds were deposited around and over a submerged mountain- 

 range of granite surrounded by Archsean rocks; and subsequent elevation, 

 flexure, dislocation, and erosion have produced the conditions represented 

 on the map, where it will be seen that of this conformable series, now bent 

 and twisted, different horizons from the Cambrian up to the Middle Coal- 

 Measures are at different points in contact with the granite body. Of the 

 immense arch which once covered this body, the western half has been 

 faulted down, while the top of the arch, with its thickness of 30,000 feet of 

 rock masses, has been broken up and worn away by atmospheric agencies. 



Granite and Arch^an Rocks. — The Cottonwood granite body forms 

 the main mass of the Lone Peak Ridge, toward the eastern end of which it 

 disappears beneath the slates and quartzites, the base of the Twin Peak Ridge, 

 and a northwestern spur of this peak which juts out toward the mouth of Big 

 Cottonwood Canon, and the bed of Little Cottonwood Canon, from half a 

 mile above its mouth nearly up to the town of Alta. Here it disappears 

 under the Wahsatch limestone, but re-appears at the head of Big Cottonwood 

 Canon, forming the head of this and of the Snake Creek Canon, and the mass 

 of Clayton's Peak. The best exposures of this body are found in the lower 

 part of Little Cottonwood Canon, where it shows a conoidal structure, that 

 is, has a tendency to weather in curved surfaces on a large scale, while the 

 prevailing planes of cleavage have a dip of about 50° to the westward. 



