D2 
ECONOMIC GEOLOGY 
on its intrusion. Some of the lode-fissures were due to 
shrinkage of the cooling monzonite and dykes. The sulphides 
in the original lodes were sparse; but as the country was 
cut down by denudation the primary sulphides were concen- 
trated into bodies of chalcocite which formed the wealth of 
the Butte Field. 
OVER-DISSEMINATED AND REPLACEMENT Bopies—Rocky 
Mountains, Congo—The copper-fields of the Rocky Moun- 
tains, in Arizona and Utah, contain secondary ores concen- 
trated from low-grade primary sulphides. = The mining 
districts consist of ancient granitic rocks covered by Palaxo- 
zoic and Mesozoic limestones; these rocks were invaded 
by Lower Kainozoic granite-porphyry and monzonite, beside 
which are contact bands of lime-silicates, including garnet, 
tremolite, vesuvianite, diopside, and epidote, and of calcite 
mixed with chalcopyrite, bornite, and pyrites. Lenticular 
and tabular deposits of the same ores pass from the porphyry 
into the limestone, which also contain scattered grains and 
thin veins of the sulphides. The primary ores contain about 
5 per cent. of copper, and mining is dependent on the large 
secondary enrichments of chalcocite and carbonates, At 
Bisbee (cf. Ransome, U.S.G.S., Prof. Pap., No. 21, 1905), 
6 miles N. of the Mexican frontier, the mines are in Car- 
boniferous Limestone; the ore is oxidized to the depth of 
1400 feet, and caves contain the beautiful blue azurites for 
which the Copper Queen and other mines were famous. 
At Clifton-Morenci in S.E. Arizona (cf. W. Lindgren, ibid. 
No. 43, 1905), the granite and monzonite-porphyry intrusions 
and many dykes are bordered by contact ores; the ores to 
the depth of from 50 to 200 feet were mainly oxidized ; 
from the depth of 100 to about 400 feet lay secondary en- 
richments of chalcocite with from 3 to 4 per cent. of copper ; 
below 400 feet are low-grade primary ores of pyrite, chal- 
copyrite, and blende. The mines at Bingham, in Utah 
(Butler, 4tid., No. III, 1020, pp. 340-62), in Palmozoic 
limestones are at the margin of monzonite and djorite. 
porphyry, and contain masses of chalcocite concentrated 
from the primary ores. 
Most of the vast quantities of low-grade ores of Katanga 
in the Belgian Congo appear to be also alluvial in origin (as 
suggested by Lindgren, Econ. Geol., vi, 1911, P- 575: cf.