380 ELEMENTARY GEOLOGY 
and placer gold of the west, of Russia and Australia. In the 
Malay peninsula, tin is obtained from the same source; and all 
the platinum that is mined comes from similar gravels. Aside 
from these three metals, there is little ore of value gained from 
such sources. 
Chemical Ore Deposits. It is impossible to consider 
all the ways in which ore deposits are accumulated by 
means of chemical change. The three chief varieties 
are: — 
(1) Zrue veins, or those accumulated in fissures of 
various kinds; (2) those formed by replacement of other 
minerals; (3) concretionary deposits. 
TruE VeIns. These, which are the most valuable 
of all the ore deposits, are formed in fissures or other 
cavities in the earth. There are various kinds of cavi- 
ties in the rocks, such as (1) joint planes, (2) gas cavi- 
ties, caused by the expansion of steam in lava rocks, 
(3) caves produced by the solvent action of water upon 
soluble substances, as carbonate of lime, and (4), most 
important of all, the cavities formed by faulting. 
As water passes through the earth, it is dissolving 
here, changing there, and depositing elsewhere. It is 
by this action that ores are accumulated in cavities. 
Ordinarily the substances carried are other than metal- 
lic minerals; and so in such cavities we most commonly 
find quartz, calcite, and similar minerals of no value, 
though sometimes ores are thus gathered into beds. 
