GEOLOGY OF LITTLE ROCKY MOUNTAINS 427 
country—a type that is well known because it prevails at the 
famous Cripple Creek district of Colorado. While the deposits 
are as yet but little developed, they promise to be actively 
exploited when the mineral lands which are now within the 
limits of the Fort Belknap Indian reservation shall be declared 
open to location. The gold ores are tellurides associated with 
fluorite, and occur in the altered porphyry. This character of 
ore, and its association with phonolitic rocks, is of such interest, 
that it seems appropriate to record here the association of tel- 
luride ores with phonolitic rocks which observation shows to 
prevail not only at the Cripple Creek region but in the Black 
Hills of Dakota and in the Judith Mountains of Montana. 
The mineralized zone, which extends to the vicinity of Indian 
Peak in a northeast direction across to the north slope of Granite 
Mountain, is probably about 2000 feet or more in width. Within 
this area the rock is generally bleached and rotted, white, rusty or 
pink in color, and cut by veins in which the rock is seamed by 
quartz stringers and quartz films, and with cavities filled with 
rusty ore. The pitch of the ore body is steep, some 80° perhaps, 
and the rock is generally broken by fracture into some angular 
bits or blocks a foot or so long. 
The ores carry gold and occasionally silver. They consist 
of brecciated or shattered country rock impregnated, coated, and 
replaced by quartz, often associated with fluorite and carrying 
small amounts of telluride, pyrite, and possibly other minerals. 
The ores do not occur in well defined fissure veins with definite 
mineral walls. The gold occurs both as a telluride and as free 
gold. In the altered ore forming the surface of the ore depos- 
its and the “float” of the mineral belt, the gold can be seen to 
be free, but in many cases the gold can only be seen after 
roasting the ore. A characteristic ore of the district consists of 
an intimate mixture of quartz and fluorite, whose brilliant purple 
color makes it readily recognizable. 
Superficial alteration of the deposits has caused the oxida- 
tion, hydration and leaching of the ore, which consists of a 
granular, friable, vesicular quartz more or less incoherent and 
