SALMON — ON THE FOllMATION OF ORE-VEINS. 
431 
greatest objection to this hypothesis is probably that it involves the 
theory of a cooling globe, and thus places itself in antagonism to the 
LycUian doctrines, now rising to popularity. But it seems to me 
that this antagonism is more ap])arent than real ; and that the hypo- 
thesis might be made to harmonise with the Lyellian philosophy ac- 
cepted in the moderated form in which it is most popular. 
The investigations of Sorby and others now lead us to believe that 
many of the crystalline rocks are hydato-pyrogene rather than purely 
pyrogene — that is, were formed at a high temperature, but in con- 
nection with water. This discovery is of the very highest im- 
portance in the theory of ore-veins, for in them we constantly find 
occurrences which can be only accounted for by the action of water, 
which in fact seems to have been the carrier from the metallic source, 
where it took up the ore in solution, conveying it to, and subse- 
quently precipitating it in, the vein. 
I have already referred to metalliferous deposits originating by 
metamorphism, or by replacement and change of constituents, without 
any original fissure. I am not going to refer to these again here ; 
but I wish to point out that almost every vein-deposit, no matter 
how found, must have been subject to some such similar meta- 
morpliic action, from the very hour of its original filling, tending to 
modify its contents in a greater or less degree. Never-ceasing 
change is as much the law in the interior of the earth as at the 
surface ; the changes may be slow, but in Nature nothing is absolutely 
stationary. Hence it does not follow that the minerals we now find 
in any vein are in the same state, either as to form, or combination, 
in which they were originally placed there. Many instances occur 
where it is most important to bear this in mind. 
In following out inquiries into the origin of metalliferous deposits, 
we must never fail to bear in mind the caution of Prof. Cotta that 
they are not to be regarded as " an isolated phenomenon." This 
caution may be un-necessary for geologists ; but unfortunately it is 
not so for " practical" men. No one can usefully approach this 
subject, with the object of generally investigating it, -without a 
thorough knowledge of the principles of Geology, an intimate know- 
ledge of rocks, and an accurate acquaintance with mineralogy. 
