20

ECONOMIC GEOLOGY

Magmatic water from the ore-zone rises through the overlying
plutonic rocks and carries metals upward along fractures
and fissures. Ore formation is naturally often associated
with igneous intrusions, because they rise most readily in
fractured areas, and as the intrusion of an igneous mass would
rupture the adjacent crust. The predominance of lodes in
the older rocks is natural, because these rocks are nearest the
ore-zone and have had the longest time for impregnation from
it. Ores occur in later igneous rocks where the ascent of
materials has been rendered possible by increased gas pressure

Depth in
Miles

)
2

0)

0)

Bm

~ 687° Critical
Temperature

2600° Dissociation
Temperature

=

50

F16. 8.—DIAGRAMMATIC SECTION OF ThE EARTH'S Crusr.
Diagrammatic section of the earth’s crust showing a rise of the barysphere
(dotted) above its normal level of about 40 miles to less than 30 miles,
whereby it carries the ore-zone (xxx) above the 20ne of the dissocia.
tion of water at the depth of 30 to 40 miles. The rising water-vapour
converges into the main ascending currents and condenses to water
on reaching the critical temperature of water at the depth of from 6-12
miles.

due to heat and the existence of channels due to fractures
and fissures.
From the ore-zone there is an ascending sequence, successive
metals being deposited as they come under suitable conditions
 of pressure and temperature. Gold ores are most
abundant in the pre-Palzozoic rocks. Tin and tungsten
generally occur in and near the plutonic rocks which are
exposed in the roots of the upper Pal@ozoic or Altaid mountain
 system; they are not found in any quantity in connection
 with the Alpine-Himalayan movements. Copper