kanso.me.] MISCELLANEOUS MINERAL RESOURCES. 143 
The facility with which the rhyolitic rocks of this region become 
to a greater or less extent metasomatically replaced by ore has 
already been noted. In most cases such replacement is a minor 
accompaniment of the deposition of ore in fissures. But at the Silver 
Ledge mine replacement is the dominant mode of occurrence. The 
ore, consisting of galena and sphalerite, occurs metasomatically, 
replacing rhyolite in the vicinity of a zone of fracturing and faulting 
trending about N. 20° E. These fractures contain crushed country 
rock and ore, and there lias evidently been movement along them 
since the ore was deposited. The principal ore bodies, however, are 
not found within the fissures, but as irregular replacements of the 
rhyolite which forms their walls. In some instances bodies of ore have 
been followed into the country rock for 30 or 40 feet from the main 
fissures. The replacement is sometimes complete, resulting in solid 
masses of ore. More often it takes the form commonly described as 
impregnation; i. e.. the ore is more or less thickly scattered through 
the rhyolite in specks and small bunches. In this case there is no 
definite boundary to the ore body, and only so much of it is removed 
as can be profitably worked. 
The deposition of* the ore has been accompanied by metasomatic 
change ami a complete recrystallizat ion of the rhyolite. The result- 
ing material, which forms the gangue of the ore, is ;i nearly white, 
minutely crystalline aggregate of sericite, kaolin, and quartz, with 
generally a little rutile and occasionally a small amount of calcite. 
The proportions of the three principal constituents vary, some facies 
of the rock consisting chiefly of sericite and kaolin, while others are 
largely sericite and quartz. 
MISCELLANEOUS MINERAL RESOURCES. 
Compared witli the extraction of gold, silver, copper, and lead, all 
other mineral productions of the quadrangle are insignificant. At 
present no deposit is being worked for zinc, but sphalerite ores are 
abundant. As a rule, ore bodies showing much sphalerite with little 
galena have been little prospected, on account of the smelter penalty 
aft aching to an excess of zinc in ores smelted for other metals. But 
with the increasing demand for zinc ores it is not unlikely that some 
of the sphalerite lodes may ultimately be worked for this metal. 
This, however, may be some years hence, owing to the natural disad- 
vantages under which mining is carried on in so elevated a region. 
The occurrence of hubnerite on Cement Creek has aroused expec- 
tations of the profitable exploitation of the deposits. The mineral is 
easily concentrated from its gangue, but it is very doubtful whether 
it occurs in deposits of sufficient size or constancy to justify extensive 
operations. 
