10 ECONOMIC GEOLOGY
alluvial grains of blende being deposited in limestone, any
galena having been separated during the process; the zinc
was concentrated and altered to silicate and oxide during
the thermal metamorphism of the area.
The Sullivan Mine, near Fort Steele, British Columbia, the
greatest zinc mine in Canada, is a huge replacement deposit,
in places 240 feet thick, in slate and quartzite.
SILVER
SILVER (Ag; at. wt. 107-7; sp. gr., 10°5 ; melting-point,
(850° F.) is a white metal of a beautiful lustre and useful for
jewelry, plate, and currency, as it does not oxidize at ordinary
temperatures, and is hardened by addition of copper. It is
the best conductor of heat and electricity, and is inferior
only to gold in malleability. It was mainly used for coinage,
and as its abandonment as the legal standard of value by
many countries coincided with increased production, its
price-fell from about 5s. an oz. between 1860 and 1870 to
from 2s. to 2s. 6d. from 1900 to 1915. After the War, it
rose to 17s. 6d. an oz., but has again fallen in 1927 to 2s. 1d.
Silver is seldom mined independently, and most is obtained
from ores of lead, copper, and gold. The chief silver-pro-
ducing countries are the United States, Mexico, and Canada.
Smaller supplies are obtained in Australia, Peru, Chile,
Bolivia, Japan, Spain, and Portugal. The primary ore is a
sulphide associated with lead and zinc, and as the problems
of its ores are those of the metals with which it occurs, no
special reference to them is necessary.
Cosarr FreLp—Silver, however, occurs in some veins
which are worked for it alone, or also for cobalt and nickel,
The historic mines of this type are in the gneiss of Annaberg
and Joachimsthal in Saxony. The most important now are
at Cobalt in Ontario; ? the veins were discovered casually
in a railway cutting in 1903 and regarded as ores of copper.
In recent years the field has had an output of silver smaller
tW. G. Miller, Ont. Bur. Mines, xix, pt. 2, 1913; Miller and C. W.
Knight, Eng. and Min. Journ., xcv, 1913, Pp. 1129-33; J. M. Bell,
Tr. LM. and M., xxxi, 1922, pp. 304-32, for S. Lorraine; W. H. Collins,
G.S. Canada, Map 155 A; C. W. Knight, Ann. Rep. Ont. Dep. Mines,
xxxi, 1922, pp. 321-58, gives summary of the literature. Microscopic
study of the Ore, F. N. Guild, Zcon. Geol., xii, 1917, pp. 297-353, pls.
X-XXV.
