ansome.1 METAMOKPHISM OF THE COUNTRY ROCK. 119 
luces an argentite ore. It is the usual alteration seen in the mines 
if Savage Basin, Sultan Mountain (inmonzonite), and in many other 
odes where metamorphism of the country rock other than that of prop- 
/litic nature is not a conspicuous phenomenon, and where no special 
microscopical study of wall rock was made. 
The alteration of the country rock in Silver Lake Basin may be 
iccounted for on the assumption that the solutions which filled the As- 
sures contained alkaline carbonates. But that the alkaline carbon- 
ttcs were present in small amount seems to be indicated by the uni- 
*orm presence of chlorite (except in the actual walls of the vein, where 
t as well as calcite may be absent); for, as Lindgren 1 has pointed 
nit, it is probable thai chlorite can not exist under the action of 
strong solutions of alkaline carbonates. The immediate wall rock of 
he fissures, being more exposed to the action of solutions, has in 
most cases suffered the removal or furl her alteration of the chlorite 
iiid carbonates which were probably first formed. Connected with 
his later stage is the accumulation of the insoluble sericite, derived 
from labradorite, biotite, and probably other minerals, and very likely 
bhe direct addition of quartz in place of the removed calcite. Although 
no chemical analyses have been made of these wall rocks, it is rrob- 
able that the abundance of sericite (derived in great part from lime- 
soda feldspars), which is common in the rock in contact with the vein 
filling', indicates a direct addition of potash to the substance of the 
rock, as was found by Lindgren 2 to be the case in the Grass Valley 
and Nevada City veins. Kaolinite was not certainly identified in the 
altered wall rocks of Silver Lake Basin, but in the Dives mine (on the 
North Star lode) it apparent ly occurs with sericite and quartz, as seen 
in an altered andesil ic horse in the lode. Metamorphism in connec- 
tion witli the Dives ore body is similar to t hat just described in detail 
for the Silver Lake mines; but in its greater intensity, in the presence 
of kaolinite, and in the more evident silicification of portions of the 
wall rock it is intermediate in character between that metamorphism 
and the kind next to be described. 
Of a somewhat different kind from that which has just been dis- 
cussed is the metamorphism observed in connection with the ore 
deposits of Engineer Mountain and of the Red Mountain district. As 
the ore of Engineer Mountain occurs in lodes, while that of the Red 
Mountain district is prevailingly in stocks, and as the metamorphism 
presents some phases of difference in the two modes of occurrence, 
they will be separately treated. 
The Polar Star lode, which has produced some rich silver ore carry- 
ing argentite and proustite, may be considered as a type of the Engi- 
neer Mountain deposits. Unfortunately, its workings are no longer 
1 Trans. Am. Inst. Min. Eng., Vol XXX, 1900, p. 610. 
2 The gold-quartz veins of Nevada City, etc.: Seventeenth Ann. Rept. U. S. Geol. Survey, 
Pt. II, 1896, p. 148. 
