116 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
seen in this particular slide. Such of the massive andesites and 
latites as were examined in thin section show a similar development 
of secondary chlorite, calcite, and sericite, indicating an alteration 
that is essentially propylitic in character. 
Within the Silver Lake and Iowa mines the amount of alteration in 
the country rock which can properly be referred to the action of ore- 
bearing solutions moving along any given fissure is slight. The 
breccia usually preserves its greenish color and general appearance 
close up to the fissure walls. In the Iowa mine specimens were taken 
in the main crosscut on the fourth level at various distances from 
the Iowa vein, for the purpose of studying the alteration of the 
country rock dependent upon the formation of this lode. All of these 
specimens are pale greenish-gray rocks of one kind — a breccia of the 
type already described. 
At points 150 feet east of the lode the country rock is fine grained 
and faintly mottled, showing only a few pale, minute phenocrysts of 
feldspar and an occasional tiny grain of quartz. Under the micro- 
scope the rock reveals the character of a much-altered andesitic tuff 
or fine breccia. The feldspars have been completely altered to aggre- 
gates of sericite and calcite, while areas of calcite and chlorite prob- 
ably represent former phenocrysts of augite. The groimdniass is a 
rather indistinct aggregate of secondary quartz, sericite, and chlorite, 
with a little apatite and rutile. The rock is thus practically wholly 
recrystallized into a secondary aggregate while retaining the grosser 
structure of the original. 
At a distance of 100 feet from the vein the country rock is mega 
scopically like that just described. Under the microscope it shows 
less alteration than the preceding. Chlorite and calcite are abun- 
dant, but much of the plagioclase is still recognizable. Sericite and 
quartz are not such prominent constituents as in the other rock. 
At places 50 feet from the vein the country rock is still of the same 
general appearance as that already described. In thin section under 
the microscope it shows a general andesitic structure, but is almost 
wholly recrystallized. The feldspar phenocrysts have been changed 
to aggregates of calcite and sericite, while areas of chlorite and cal- 
cite, with sometimes rutile, are all that remain of the phenocrysts of 
augite or biotite. The groundmass, also, while preserving the out- 
lines and in small part the substance of former lath-shaped feldspars, 
is now an aggregate consisting chiefly of quartz, chlorite, sericite, 
and a little rutile and apatite. The thin section does not show 
clastic structure, and is probably from a fragment in the breccia. 
At a distance of 2 feet from the vein the only megascopical change 
in the country rock consists of the presence of an occasional speck of 
galena. Rather curiously, finely disseminated pyrite, which is a com- 
mon feature of most country rock near a productive lode, does not in 
this case accompany the galena as a megascopical constituent. Under 
