GRANITE AND ARCH^AN ROCKS. 359 



hornblende and mica in general rather widely disseminated, giving to the 

 mass a beautifully white appearance, and all these crystals are remarkably 

 fresh and iindecomposed, toward the east the rock becomes gradually finer- 

 grained and darker, owing to the closer arrangement and larger proportion 

 of hornblende and mica, and the component crystals, especiall}^ the feld- 

 spars, are duller and more decomposed. The structure-planes also, which 

 below are widely spaced and not prominent, about half-way up the canon, 

 at Tannersville, have become more frequent, and so regular as to give the 

 appearance of lines of stratification. 



Along the steep northern wall of the canon, the dividing line between 

 the darker Cambrian slates, which form the upper 2,000 feet of Twin Peak, 

 and the white granite beneath, can be distinctly traced by the eye as one 

 ascends the canon. The slates are not conformable with this dividing line, 

 but stand at an angle of 45°, dipping to the northeast; hence the ends of 

 different series of beds are constantly seen as one ascends, and the dividing 

 line reaches lower on the wall, until just below Alta the quartzites of the 

 upper Cambrian descend to the bed of the canon, and, crossing to the 

 southern side, curve round and form the main ridge between this and Ameri- 

 can Fork Canons as far west as the third high peak east of Lone Peak, 

 whose summit is formed of metamorphic slates, while its northvf estern face 

 is in granite. 



In the bed of the canon above Alta, the granite has been again denuded, 

 and in the divide between Big and Little Cottonwood Canons, just east of 

 this town, the ridge is formed mainly of granite, more or less covered by 

 the Wahsatch limestone, whose continuity it breaks. This granite, as has 

 been already remarked, closely resembles in general appearance and com- 

 position that at the mouth of the canon. The ver}^ summit of Clayton's 

 Peak, however, is made up of a rock which differs in external appearance very 

 much from this rock : it is a dark, fine-grained, syenitic-looking rock, made 

 up of quartz, orthoclase and plagioclase feldspars, mica, and hornblende ; 

 the latter is very abundant, and of a lighter green, and less distinctly crys- 

 tallized than in the other granites. Under the microscope, it is seen to con- 

 tain also titanite, of a darker brown than in the other granites, apatite in 

 flattened crystals, and magnedte, while the quartz and feldspars contain 



