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 con- 
ditions 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 moun- 
tain system; they are not found in any quantity in con- 
nection with the Alpine-Himalayan movements. Copper