146 ECONOMIC GEOLOGY OF SXLVERTON QUADRANGLE, [bull. 182. 
country rock of the mines to the depth now worked, belong to the 
Silverton series and illustrate its typical development. They are pre- 
vailingly breccias, in which the angular fragments and the interstitial 
matrix are composed of the same materials, with practically no admix- 
ture of foreign particles. The induration of these breccias has pro- 
duced rocks which can with difficulty be distinguished from massive 
lavas. A careful inspection of weathered surfaces, or a microscopical 
examination, will, however, usually reveal their true clastic nature. 
The general character of these breccias is andesitic. But they differ 
from normal andesite, such as occurs in the San Juan formation, by 
the presence of occasional porphyritic crystals or phenocrysts of 
orthoclase and quartz, and in the occurrence of orthoclase in the 
groundmass. The abundance of the phenocrysts of orthoclase feldspar 
and quartz varies in different varieties or facies of the breccia. As a 
whole these rocks appear to be most closely related to the latitic group 
of intermediate rocks. Associated with the breccia are massive rocks 
of the same general type as the breccias, which have been intruded 
as sheets and dikes, and possibly in part as contemporaneous lava 
flows. As examples of these may be cited the masses forming the 
summit of Round Mountain and the hard sill that holds up the waters 
of the lake at its northern end. The latter, a dark-gray rock of splin- 
tery fracture, shows abundant fresh-looking phenocrysts up to about 
a centimeter in length, and smaller crystals of hornblende, lying in a 
compact groundmass. A thin section shows, under the microscope, 
phenocrysts of labradorite feldspar, brown hornblende, and pale- 
greenish augite, in a feldspathic groundmass. The augite is partly 
decomposed to calcite and chlorite, and certain areas of chlorite appear 
to represent former phenocrysts of biotite. The groundmass, while 
somewhat obscured by calcite, chlorite, and other products of altera- i 
tion, appears to consist chiefly of plagioclase, orthoclase, and glass. 
The accessory minerals are magnetite and apatite. The rock from 
the summit of Round Mountain is very similar, but the presence of 
orthoclase in the groundmass is more clearly shown and the augite is 
fresher and more abundant. 
The lodes of Silver Lake Basin are large and prominent, and on 
this account were discovered and located at an early stage of thei' 
mining development of the region. But it was not until the erection 
of the Silver Lake mill, about 1890, that they were worked on thf 
scale necessary for the profitable and permanent extraction of low->; 
grade ore. They are nearly all fissure veins of comparatively simple 
form, and have clearly defined walls. The wall rock has been chera j 
ically altered by the solutions traversing the fissures and, to a sligh 
degree, impregnated with ore minerals, but it is not regarded as on 
and is always readily distinguished from the vein filling. 
The lodes wholly or partly within the basin may be divided into tw< j 
groups. The first group comprises at least two very strong and per I 
