178 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
of fracturing, and the mineralizing solutions have penetrated and 
altered the rock between the fissures, converting and changing the 
rock to what is conveniently called pyritized granite, since the horn 
blende and mica are altered to pyrite. 
The deep development work of many of the mines shows a decided 
change in the amount of mineralization of the fractures. There is 
an increasing number of small veins of quartz and pyrite separated 
by altered granite. Some of the large lodes whose entire width is 
workable pass downward into a cluster of small veins of quartz and 
pyrite separated by altered granite. In other words, the replacement 
of inter- vein material by ore decreases with depth. There is also a 
decided increase in the number of small fissures devoid of ore and 
filled by friction breccia, but showing trifling displacement. This is 
particularly noticeable in the levels 1,600 feet or more below the sur 
face. On the other hand, some of the newer fault veins that show 
little or no ore in the upper levels contain pay ore below, because the 
open nature of the fault material permitted a deeper seepage than 
usual of descending waters. 
Secondary enrichment. — The enormous bodies of copper glance 
which have made the Butte district famous are probably the largest 
and best examples of secondary enrichment known. The fracturing 
of the veins has permitted the access of meteoric waters, which, dis 
solving the copper from the lean ores of the oxidized zone, deposit it 
by reaction with pyrite, in the depths. These deposits were greatest 
in the upper level of the mines and have gradually lessened with 
depth. In some of the veins the lower limit of enrichment has been 
reached, in others the deepest workings still show these enrichments. 
In general there is a marked association of faulting of the veins 
with bodies of rich ore, and these faulted areas are wet, so that the 
miners say: "A dry and tight vein is barren: a wet and crushed one 
is rich." This is particularly marked where the veins contain much 
pyrite, though the glance is more conspicuous in white quartz. In 
the deeper levels newly deposited quartz occurs with the glance. In 
the deepest levels, 2,000 feet or more below the surface, rounded 
masses of glance 2 and 3 feet across occur in crushed quartz contain- 
ing relatively little pyrite. 
Change of character of mineralization i villi, depth. — The most nota- 
ble change in mineralization with increasing depth is the greater 
abundance of enargite. In the eastern part of the copper area, in the 
Rarus Hill and its vicinity, this ore extends upward to the oxidized 
zone, sometimes very nearly to the surface. West of here there is a 
notable increase of enargite in depth, the mineral occurring for the 
first time in the very deep level of some mines (i. e., 1,800 to 2,200 
feet), an association that also prevails in some of the later veins, such 
as the Blue, as well as in the older ones. 
Influence of country rook.— There is a distinct association of the 
