ORE DEPOSIT THEORIES. =, Bis 
of leaching by the circulation of water through extremely 
minute openings is lessened in proportion to the activity of 
the force which we may suppose has produced ‘circulation. 
Speaking generally, it may be also said that the general 
facts of the occurrence of ore deposits point to a compara- 
tively short period, or to a succession of comparatively short 
periods, of deposition rather than to a continuous process 
going on with equal vigour since the beginning of geological 
time. If, therefore, the materials for ore deposits have been 
concentrated by the leaching action of meteoric waters, 
then it is almost certain thatthe heat engendered by intru- 
sive masses of eruptive rock is the principal force producing 
effective circulation. 
The Plutonic Theory. 
The plutonic school look upon the eruptive rocks as the 
main source of the metallic contents of ore deposits. In 
this they differ not very greatly from the meteoric school, 
for the latter also believe that the metallic contents of many 
deposits may be wholly derived from eruptive rocks. The 
distinguishing feature between the two theories is not so 
much the source of the metals, as the method by which they 
have been extracted. The meteoric school hold that this 
has been effected by the leaching action of the meteoric 
waters. The plutonic school, however, suggest two other 
methods of concentration, namely, “ magmatic differentia- 
tion’ and “ magmatic extraction.” Both of these methods 
have been thoroughly established in the case of certain 
classes of ore deposits, and with regard to these there is now 
very little difference of opinion. But there are other very 
large classes of ore deposits upon which definite proof either 
way has not yet been obtained, and in these classes there is 
a very wide difference of opinion. 
Ore Deposits produced by Magmatic Differentiation. 
It is now a thoroughly established fact that during the 
consolidation of certain eruptive rocks important concen- 
trations of ores may take place through the process known 
as magmatic differentiation. The most important of these 
deposits consist of concentrations of titanic iron ores in 
gabbro, of chrome iron ores in peridotite, and of nickeliferous 
and cupriferous pyrrhotite in gabbro. The concentration 
of these ores is merely one phase of the general phenomenon 
of magmatic differentiation, or the process by which rocks 
of varying composition solidify from one and the same 
molten magma. As an example of the more general 
