358 DESCRIPTIVE GEOLOGY. 



ever, sliowing- any very distinct bedding-planes. These rocks, though 

 essentially quartzites, have a very distinct habit from the Cambrian quartz - 

 ites of Big Cottonwood Canon. They contain mica in very varying quan- 

 tity, and, where this is abundant, have a laminated structure and approach 

 a true mica-schist ; toward the mouth, the mica is replaced by hornblende, 

 which gives to the rock a greenish tinge. 



In the ravines on the west face of Twin Peak, between the mouths of 

 Big and Little Cottonwood Canons, a greater thickness of Archaean rocks 

 is found, probably not less than 2,000 feet, though, owing to the limited expos- 

 ures and irregulay position of the beds, it is not possible to make an accu- 

 rate estimate. The strata are much contorted, and in one case completely 

 surround a little knob of granite, which projects from a western spur of Twin 

 Peak. In the lower beds is found a light-green hornblende-schist, made np 

 largely of quartz, the mass being penetrated by small bluish-green horn- 

 blende prisms which give to the rock its schistose structure, and a little 

 brown mica. With these are associated various slates and mica-schists, 

 while the upper beds consist of light and darker-colored quartzites, among 

 which a greenish schistose rock contains apparently a considerable amount 

 of chlorite, and shows the effects of great pressure. A typical mica-schist 

 is found in direct contact with, and in places penetrated by, the granite 

 body already mentioned, which is exposed in the little side-canon at the 

 mouth of Big Cottonwood, and which marks the dividing line between the 

 Archaean rocks and the Cambrian slates and quartzites of Big Cottonwood 

 Canon. The latter have a strike at right angles to them, or in a northwest 

 direction, and dip steeply northeast, while the Archseans dip west and south 

 and strike northeast.^ 



The Cottonwood granite body preserves the same mineralogical char- 

 acter throughout the greater part exposed ; but, while that at the mouth of 

 Little Cottonwood is composed mainly of large feldspar crystals, with 



' Since the above was printed, Mr. J. E. Clayton lias reported the discovery by 

 him, in the Archaean schists of the first small cailon north of Little Cottonwood, of a 

 white mica-schist, containing well-defined cruciform crystals of staurolite, which 

 would resemble the paragonite-schists of Eed Creek, Uinta Range, together with a 

 similar hornblende rock, which render more complete the resemblance of these two 

 Archaean bodies. 



