368 GEOLOGY OF THE EUREKA DISTRICT. 



character has wholly disappeared, but which still retains, besides some partially altered 

 mica, apatite and zircon in a perfectly fresh condition. 



An interesting example of altered hornblende-mica-audesite and at the same 

 time of local accessions of porphyritical quartz is the occurrence west of Glen Dale 

 Valley, south of Hoosac Mountain (49, 50). The feldspar of the rock is completely 

 replaced by calcite, quartz and colorless potash-mica, which are also the residual 

 products of the decomposition of the grouudmass. Hornblende with dark border is 

 very abundant in thin section 49. It is altered to yellowish green, coarsely fibrous 

 chlorite, which polarizes strongly and extinguishes parallel to the fibers, and contains 

 small, yellow, highly refracting grains, possibly epidotc. The mica has become color- 

 less, but retains its negative, apparently uniaxial character, and is rich in the most 

 beautifully developed microscopic crystals, that are in part tetragonal pyramids of a 

 colorless mineral with high index of refraction, apparently anatase, in part slender 

 prisms of epidote ( ?) and thin plates of hematite, besides smaller grains in lines perpen- 

 dicular to the sides of the mica plate and apatite prisms with glass inclusions (49). 

 Quartz is sparingly present in rounded macroscopic grains in the variety rich in horn- 

 blende, but is very abundant in the purple variety (50), poor in hornblende, which in 

 the field appears as a local modification of the former. The quartz occurs both in 

 rounded fragments and in dihexahedral crystals and contains glass inclusions". 



Andesitic Pcariite and Dacite. The third and most interesting form of andesite found in 

 the district unites under its numerous and varied forms characters both of the 

 pyroxeue-andesite and hornblende-mica-audesite, and presents, as an extreme 

 variety, daeite. Its connection is most intimate with the hornblende-uiica-andesite, 

 which indeed is found passing into it in an outcrop back of the windmill pump east 

 of Secret Canyon road (74), and also east of Hoosac Mountain (73, 75a, 75b, 76). It 

 is again found in association with liornblendevinica-andesite in Sierra Canyon and in 

 the gulch south of Carbon Ridge. Its resemblance to pyroxene-andesite will be seen 

 to be confined to a variety with inicrolitic, felt-like groundmass, and to the presence 

 of pyroxene, which is lacking in the hornblende-mica-andesite just described. In the 

 two principal localities where it has come to the surface, at Dry Lake and in the 

 vicinity of Sierra Canyon and South Hill, it presents so great a variety of form that 

 the extremes of the series would scarcely be suspected of belonging to the same geo- 

 logical body, but this is evidently the case in the occurrence south of Dry Lake. The 

 following able of comparison is arranged to show the parallelism in the forms from 

 which thin sections have been made. The series commences with the most crystalline 

 variety and that most clossly allied to the horublende-mica-andesite, and finishes 

 with the most glassy, porous, and quartzose form, or dacite 



