ORE DEPOSIT THEORIES. 2k 
seems to be a reasonable hope that a comprehensive theory 
of magmatic differentiation will be evolved which will not 
only account for the concentration of ores in the magma, 
but will also account for the pneumatolitic extraction of the 
metals which are found in deposits formed by gaseous 
emanations from igneous magmas. It is to this class of ore 
deposit that we must now turn our attention. 
Magmatic Extraction. 
Eruptive magmas are known to contain magmatic water 
in varying amounts. This is proved directly by our ex- 
perience of volcanic eruptions. These may be divided into 
two classes, namely, quiet eruptions and explosive eruptions. 
In the former, lava-flows predominate; in the latter, 
showers of ashes and great quantities of steam are the 
principal products of eruption. The fusing point of an 
eruptive rock is dependent largely upon the amount of mag- 
matic water present in the magma. Thus, Daubrée has 
shown that all silicious rocks and glass mixtures in the 
presence of superheated water will become more or less 
liquid at temperatures much below that necessary to pro- 
duce true fusion. At 400° F. such rocks become pasty, at 
800° completely fluid. The same takes place at even lower 
temperatures if a little alkali be present. True igneous 
fusion of the same materials only takes place at a tem- 
perature of 2500° to 3000°. It has further been shown 
that the temperature of quiet eruptions is generally much 
greater than the temperature of explosive eruptions. There 
is no doubt that the same difference exists in the case of 
magmas which solidify underground. This is shown by the 
difference in the character of the metamorphism which 
various rock masses have produced in the adjoining rocks. 
Sometimes the latter have been completely fused by contact 
with the eruptive rock. In this case the metamorphosed 
zone is always very narrow, often not more than a few 
feet in width. In other cases the adjoining rock has not 
been fused at all, and shows little sign of having been sub- 
jected to high temperatures, but here the zone of contact 
metamorphism is often extremely large, and the formation 
of new minerals very abundant. The former kind of con- 
tact metamorphism is believed to have been produced by 
an igneous rock at a high temperature with little magmatic 
water; the latter by an igneous rock at a relatively low 
temperature with much magmatic water. In the former 
case the agent producing metamorphism is heat; in the 
latter case it is believed to be magmatic water, which is 
