[12 
ECONOMIC GEOLOGY 
diabase had become cold and been fractured. At the Beaver 
Mine, e.g. the veins are almost in the middle of the diabase 
sill, so that it must have been solid, and relatively cool 
throughout. The association of the veins with faults is 
established throughout the field (cf. Whitman, California 
Univ. Public., xiii, 1022, pp. 263-5, 209) ; many of them have 
a throw of only a few feet, but they formed impermeable 
sheets, which blocked the drainage, like the crushed bands 
with the silver veins of Annaberg. The diabase was the 
toughest rock in the area, and when the country was folded, 
shearing took place along the margins of the sill producing 
many small compression fractures, and planes of slipping. 
The veins at Cobalt, as at Annaberg, in spite of their great 
richness therefore have a limited range in depth, as the fis- 
sures were formed beside horizontal or gently inclined shearing 
planes, and not as great vertical fractures. Some of the 
greater faults, such as Cobalt Lake Fault with a throw of 
500 feet, must be deep-seated and may have served as channels 
for solutions from below. They were at first comparatively 
cool and deposited calcite veins, which filled any fissures 
whatever their inclination. Subsequently nickel and cobalt 
sulphides and arsenides were brought from a greater depth 
and were deposited mainly in the steeper fissures, because 
the high gas pressure forced the solutions along the most 
vertical course. New cracks were formed in the old veins, 
and native silver deposited after the sulphides, arsenides, and 
antimonides. According to C. R. Van Hise (¥. Can. M.1., 
x, 1007, p- 53), S. F. Emmons (Types of Ore Dep., 1911, 
p. 151), J. M. Bell (Econ. Geol., xviii, 1923, p. 604), and E. S. 
Bastin (Econ. Geol., xii, 1917, pp. 225-8), the rich silver 
ores are secondary. Some authorities hold that the silver 
was derived from an upward extension of the lodes; but the 
arguments by W. L. Whitehead (Econ. Geol., xv, 1920, 
pp. 127-30) against this view appear conclusive. The native 
silver though secondary in origin, was probably, as urged 
by E. S. Bastin, introduced by magmatic waters as acid 
sulphides during the last stage of mineralization. The deep- 
seated origin of the ores has been maintained by J. B. 
Tyrrell (Tr. I.M.E., xxxv, 1908, pp. 494-5), and by Spurr 
(Eng. and Min. Journ. Press, cxvi, 1923, pp. 709-12), who 
describes the lodes as *f veindikes.”’