12 
ECONOMIC GEOLOGY 
gold, but tellurides and blende. The average gold contains 
about I per cent. of silver, and becomes finer with increasing 
depth owing to the greater solubility of silver, The Champion 
Lode is in places 40 feet thick but it narrows to a mere lode- 
track. It was worked in prehistoric times and has been 
mined to the depth of 6000 feet. The lode-quartz, when 
released from pressure in the mine workings, is apt to fly 
to pieces in fata] explosions or * rock-blasts.” 
Vorcanic FiELDs—Rocky Mountains anD New ZEALAND 
—In many volcanic areas gold-quartz veins occur along 
intersecting fractures due to earth-movements or to the 
shrinkage of the rocks. Typical examples occur in the Rocky 
Mountains of Colorado and the adjacent States, These 
mountains consist of Palzozoic, Jurassic, and Cretaceous 
rocks, in places covered by Eocene conglomerates. After 
deposition of the conglomerate volcanic eruptions discharged 
a volcanic breccia and vast lava flows of andesite and rhyolite, 
The volcanic areas were afterwards faulted and gold lodes 
deposited along the faults. Thus the Smuggler Vein of the 
Telluride Goldfield is a fissure vein which runs for many 
miles N. and S. and has been worked to the depth of 3500 
feet; it consists mainly of quartz, with rhodocrojsite 
(MnCO0,), calcite, siderite, and barite. It is cut across by the 
Revenue Vein, a lead-silver lode, and both of them are cut 
by the Pandora Vein of gold-quartz along a fault that throws 
the southern part of the Smuggler Vein to the E. 
In the Camp Bird Field in Ouray County, Colorado, the 
volcanic breccia ig traversed by a five-mile-long fissure vein,1 
which intersects numerous earlier lead-zinc lodes and contains 
shoots of pyritic gold-quartz ore that extend along the lode 
for over 1000 feet, and are about 500 feet high ; they appear 
to have been fed by pipes of ore that go 800 feet deeper. 
The gold ore encloses fragments of the older lead-zinc ores; 
it shows no crustification, and appears to have formed by 
a single filling of the fissure, 
The lodes in the volcanic areas are not themselves vol- 
canic, and are more recent than the eruptions. The ores 
were deposited by solutions rising along fractures due to the 
For latest account of the lode, cf, Spurr, who calls it a compound 
veindike, Zeon, Geol, xx, 1925, pp. 114-52.