384 ELEMENTARY GEOLOGY 
Other Ore Deposits. Besides these, there are beds of ore 
formed at the contact of intruded igneous rocks, through the 
direct action of these heated masses. There are also deposits 
precipitated from solution, such as the iron ore which sometimes 
forms in the bottom of swamps. Other classes less common 
would be included in a complete study of the ores. 
Conditions favoring Ore Deposits. — Considering only 
the ores stored up in fissures through the action of 
heated water, which include the most valuable ore 
treasuries, we see that the places 
where such deposits are most likely 
__\ to occur are in regions of mountain 
growth, and particularly among 
mountains recently formed. This 
is due partly to the fact that moun- 
tains of this nature have many fault 
Fig. 232. 
urea Ge thas ee planes, and hence cavities in which 
placement of a quartz ores may be deposited, and partly 
grain (Q) by iron (J). ; 
The grain formerly ex. tO voleamic action. Heat is fur- 
ee to outer boun- nished by the intruded lava which 
also supplies the ore for the water, 
for it will be remembered that igneous rocks contain 
ores. These are the main reasons for the importance 
of the Cordilleras of the west, where gold and silver 
and many other ores occur in veins wonderfully rich, 
making this region the greatest mining district of the 
world. 
