nsome J METAMORPHISM OF THE COUNTRY ROCK. 115 
rough chemical reactions, undergoes a partial or complete change 
its chemical constitution. Rocks or aggregates of minerals are 
metasomatic" if any or all of the constituent minerals have under- 
did such changes. 1 
In Silver Lake Basin the simple fissure veins which there predomi- 
ite, carrying galena, sphalerite, chalcopyrite, pyrite, and sometimes 
trahedrite, in a gangue of quartz with frequently some barite, are not 
eompariied by any very evident or striking alteration of the imme- 
ate wall rock. 
As elsewhere pointed out, the rocks of Silver Lake Basin belong to 
e Silverton scries. No attempt will be made in this report to give 
e petrographical characteristics of the series as a whole (for which 
e reader is referred to the forthcoming report of Mr. Whitman 
oss); it is enough to say that the prevailing rock of the basin, and 
e one which constitutes the greater part of the country rock met with 
the workings of the Silver Lake and Iowa mines, is a firmly con- 
lidated, homogeneous, andesitic breccia. This differs from theolder 
n Juan breccia in its greater homogeneity and in the frequent 
Durrence of more or less orthoclase. Indeed, it is not unlikely that 
e whole should be referred rather to the latites than to the andes- 
is proper. With the breccia are associated massive andesites and 
iles. The breccia is in general somewhat altered, even when not 
>se to an ore-bearing lode, a result that is to be expected when the 
Drough Assuring which the rocks have undergone and the extent to 
iich they must have been permeated by ore-bearing solutions are 
M^en into account. 
A specimen of this rock taken on the trail from Arrastra Gulch up 
(the basin, at an elevation of about 11,800 feet, may be regarded as 
oresentative of the relatively unaltered facies. It is a tough, 
3enish-gray rock with pale lilac-gray mottlings, and, although show- 
:£ a few feldspar phenocrysts, is generally of rather compact texture. 
lie breccia structure, not very apparent on fresh fracture, owing to 
if homogeneity of the component fragments, becomes visible when 
3 rock is breathed upon or has been slightly weathered. Under the 
croscope its clastic structure is evident. -Fragments of general 
desitic character lie in the matrix of fine andesitic detritus, which 
lot always easily separable from the larger fragments. This clastic 
utrix contains broken phenocrysts of andesine or labradorite, and 
i occasional somewhat rounded grain of quartz, in a fine feldspathic 
d obscure ground mass. There is some magnetite present, but fer- 
nagnesian constituents are lacking. If formerly present, they have 
3n destroyed by secondary alteration. The feldspars are largely 
inged to sericite (with perhaps some kaolinite) and to calcite, while 
nches of calcite and flecks of chlorite are thickly disseminated 
ough the rock. Some epidote is frequently present, although not 
1 Lindgren; Trans. Am. Inst. Min. Eng., Vol. XXX, 1900, p. 580. 
