360 DESCRIPTIVE GEOLOGY. 



many microscopical impurities, the latter, particularly, colored plates of red 

 and black oxide of iron. 



Resting- on a spur of the granite mass, overlooking the lake at the head 

 of Cottonwood Canon, is a curious garnet rock, which may represent a por- 

 tion of the Archsean body exposed at this point, if, as might naturally be 

 supposed, the Archsean rocks surround the granite body. It is a coarse- 

 grained quartz mass, of a greenish tinge, through which are disseminated 

 abundant crystals of dark-brown garnet, with local concentrations of specu- 

 lar iron, often in large tabular crystals, and dark-green hornblende. Under 

 the microscope, the greenish tinge is found to be due to a fibrous green epidote, 

 and the garnet crystals to present a schistiform structure as if formed by 

 the continuous superposition of layers. 



In Big Cottonwood Canon, about two miles below the bend, is a small 

 exposure of granite, somewhat resembling the Clayton Peak granite, which 

 is in places very much decomposed on the surface, crumbling into a gran- 

 itic sand. It is a fine-grained admixture of quartz, feldspar, black mica, 

 hornblende, and titanite; the hornblende being generally fibrous and some- 

 what decomposed. In the feldspars, plagioclase is so largely predominant 

 that the rock might almost be classed as a diorite. It is difficult to say 

 whether this rock should be considered as part of the main granite body, 

 which it does not resemble very closeh^, or with the later outbursts of 

 granite-porphyries and diorites, which are found intersecting the sediment- 

 ary rocks of this region. These dikes of porphyry and diorite are very 

 frequent, especially around the Clayton Peak mass and in the region where 

 the mineralization of the beds has been most developed ; they are mostly 

 of too limited extent to be represented on the map, and only a few typical 

 localities have been represented, which will be now described. Near this 

 granite or diorite body of Big Cottonwood Caiion, in one of the beds of 

 the uj)per part of the "VVahsatch limestone, is a dil^e about 20 feet wide, of 

 syenitic granite-porphyry, so classed by Zirkel, which resembles the granite 

 in general appearance, having a rather coarse crystalline groundmass, in 

 which plagioclase can be recognized by the naked eye, and containing, 

 besides, quartz, orthoclase, mica, and hornblende ; its whole mass is impreg- 

 nated with iron pyrites, which, as revealed by the microscope, penetrates 



