248 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
rock, although all much altered, shows no change either in hand 
specimens or under the microscope. It all appears to have been 
originally rhyolite. 
The ore, consisting of galena and sphalerite, occurs replacing and 
impregnating the rhyolite in the vicinity of two or more fractures, 
which have a general strike of about N. 27° E. and dip southeast at 
a high angle. These fissures contain crushed country rock and ore, 
and there lias evidently been late movement along them since the ore 
was deposited. They are probably merely a portion of a pronounced 
fault zone, which here follows the general trend of the creek. The 
ore bodies are found by following these fractures, but are not confined 
within their walls. They extend irregularly into the rhyolite for 30 
or 40 feet to the Avest, the low-grade galena, associated with consider- 
able sphalerite, sometimes forming large masses wholly replacing the 
rhyolite, but more often impregnating it thickly in small nests and 
isolated particles. There are no walls to the ore bodies, and they are 
followed into the rhyolite until the proportion of ore to gangue 
becomes too small for profitable working. Very little ore has thus 
far been found to the east of the fissures, although the workings have 
as yet revealed no change of country rock in that direction. 
The rhyolite has been altered in connection with the ore deposition 
to a finely crystalline mass of sericite, kaolin, and quartz in varying 
proportions, serving as gangue for the ore. 
The principal fractures of this mine appear to be comparable to the 
so-called ore break in the Guston mine. It is very probable that the 
zone of fractures extending up Mineral Creek and passing just east 
of Summit forms part of the Assuring found in the Lake and National 
Belle mines near Red Mountain. 
The ore of the Silver Ledge is concentrated in a small mill run by 
steam. It is equipped with 1 Gates crusher, 2 sets of wells, 4 three- 
compartment jigs, 10 stamps, and 4 double-deck and 2 single-deck 
Wilfley tables. The capacity is about 60 tons a day. The principal 
output of the mine is a low-grade galena, which is shipped in the form 
of concentrates. A little free gold, associated with a telluride sup- 
posed to be calaverite, was found on the 300 level as a thin skin on a 
slickensided fracture surface in the usual impregnated rhyolitic coun- 
try rock. This was the only occurrence of free gold known in the 
mine. 
The future development of this mine is likely to be of great interest. 
It is evident that if the interpretation of the rhyolite as a down-faulted 
sheet or flow be correct, then an entirely different set of conditions 
will be encountered when the bottom of the rhyolite is reached. In 
all probability the ore deposit will become a simple fissure vein or a 
series of parallel veins. Moreover, further exploration of the country 
east of the shaft may reveal ore bodies yet unknown. The crosscut 
already begun in this direction might perhaps be extended with 
advantage until the andesitic rock of the Silverton series is reached. 
