spencer] ENCAMPMENT COPPER REGION, WYOMING. 159 
exception of the diorite, all the rocks mentioned are of sedimentary 
origin, having been originally deposited as horizontal beds or strata, 
and afterwards thrown into east-west folds by comprehensive forces 
acting in a north-south direction. 
The diorites are igneous rocks which were intruded into the sedi- 
mentary series in a molten state after the greater part of the folding 
and compression had taken place. They occur in dikes from a few 
feet up to half a mile in width, frequently extending along the strike 
for several miles. They are of almost universal occurrence through- 
out the quartzite belt, and are also found cutting the granites, gneisses, 
and schists which occur both to the north and to the south of the 
quartzite area. 
The schists of the region are mostly hornblende-schists, which may 
be seen in typical development upon the north slopes of the conti- 
nental divide in the heads of Jack Creek and North Spring Creek, 
and also in the region of Huston Park. The other rocks of the min- 
eral belt may be classed under the general names of granite and 
granite -gneiss, and all of these appear to have been formed since 
the hornblende-schists, though they are probably older than quartz- 
ites and associated formations. The formations represented in the 
Sierra Madre are of pre-Cambrian age and belong to the most ancient 
series known within the Rocky Mountain province. In general, the 
formations are well exposed and easily accessible for examination, 
though locally they are covered by overwash or by glacial debris. 
The topography of the region is more than ordinarily smooth for a 
mountainous country reaching elevations above 10,000 feet, a fact 
which allows the building of wagon roads to almost any desired locality 
at comparatively slight expense. 
ECONOMIC GEOLOGY. 
The ore deposits of the Encampment region have not, as a rule, 
been developed to a sufficient depth to afford opportunity for an 
exhaustive study. Hence, while those of the few mines in the dis- 
trict which have been opened to a considerable depth have been care- 
fully studied, the information obtained with regard to the occurrence 
of economic deposits as a whole is largely based upon general geolog- 
ical relations ascertained from surface examinations. 
Ores. — Copper is the predominant metal of value in the ores of the 
district, though there are a few deposits carrying values in silver, 
and gold occurs alone in quartz veins, or in variable but always small 
amounts accompanying the copper ores. The ores of copper comprise 
the sulphides, chalcopyrite, chalcocite, bornite, and covellite and 
their usual alteration products, malachite, azurite, chrysocolla, and 
the oxides. The silver-bearing ores are argentiferous galena, occur- 
ring with sphalerite and pyrite in fissures with a gangne of quartz, 
together with calcite, or the carbonate of iron, siderite. 
