682 IDAHO MINING DISTRICTS. 



The rock is composed of gray quartz, white or reddish feldspar 

 partly orthoclase, partly an acid soda-lirne-feldspar and small biotite 

 flakes, the incipient decomposition of which usually gives the rock a 

 rusty aspect. Hornblende is rarely found. Muscovite often occurs 

 in the more acid varieties. Of accessory constituents, which are best 

 studied by washing the decomposed sandy granite, there are: Ilmen- 

 ite in often perfect crystals (though little or no magnetite appears to 

 be present), apatite, zircon in extremely sharp, slightly brownish 

 crystals, small garnets, titanite, and brownish or yellowish imperfect 

 crystals of monazite. For description of the monazite sands, see pp. 

 677-679. 



DIKES ASSOCIATED WITH THE GRANITE. 



The granite is traversed by 'dikes, which in some places become 

 very numerous and large. Dikes of granite-porphyry and aplite are 

 common, though rarely very long and wide. Many such dikes, to- 

 gether with others of pegmatitic character, occur; for instance, along 

 the road southwest of Idaho City and on the hill, with an elevation of 

 6,200 feet, due south of the town. 



Dark-colored, lamprophyric dike rocks, which generally belong to 

 the minettes, were noted in a few localities. These dikes are, as a 

 rule, narrow, and their occurrence is closely connected with that of 

 the veins. From the Sub-Rosa and Forest King mines dark-gray, fine- 

 granular dike rocks were collected, generally rich in black inica. 

 In thin section the former appears as typical minette, consisting of 

 biotite, augite, magnetite, and orthoclase, with pauidiomorphic struc- 

 ture. The feldspar crystals show a tendency to radial or spherulitic 

 arrangement. A similar minette, consisting of biotite, hornblende, 

 and orthoclase, was collected at the Gold Dollar tunnel, 1,000 feet east 

 of the pass leading from Placerville to Garden Valley. The dikes 

 evidently antedate the veins. 



The Bois"e Ridge south of Quartzburg contains scattered dikes of 

 granite-porphyry and diorite-porphyrite, but near the latter locality 

 begins a very important belt of dikes intimately connected with the 

 Quartzburg belt of gold deposits. The rocks are in all respects simi- 

 lar to those which appear at Willow Creek mining district; the latter 

 may in fact be regarded as the westward extension of the Quartzburg 

 belt, having the same direction and lying in its continuation to the 

 west-southwest, but a distance of 8 miles, barren of mineral deposits 

 and dikes, separates them. The dikes do not follow the mineral 

 deposits in detail and are very irregular, sometimes only a few hun- 

 dred feet wide, then again expanding to a width of over a mile. 

 Owing to unusually deep residuary soil on the divide toward Payette 

 River, the contact of granite and porphyries is generally difficult to 

 trace, and the areal extent indicated on PI. XCVI must be regarded 

 as only approximately correct. 



