206 PROCEEDINGS OF SECTION C. 
science of petrology, they have evolved an ultra-ascension 
theory, which ascribes the majority of ore deposits either to 
direct differentiation from eruptive magmas, or to “ after 
actions ”’ tonnected with them dufing the process of consoli- 
dation. We may call this the “ plutonic school.” 
it is, of course, not to be supposed that the members of 
the opposing schools advocate their own theory to the entire 
exclusion of the other. The members of the meteoric 
school will admit the applicability of the plutonic theory 
to some deposits, and wice versd; but among the extreme 
members of each school there is always a tendency to mini- 
mise the applicability of the opposing theory almost to the 
point of extinction. 
The Meteoric Theory. 
In the meteoric theory of the origin of ore deposits it is 
supposed that ores are concentrated into veins or other 
forms of ore deposits from the minute metallic ingredients 
of either sedimentary or. eruptive rocks by the action of 
circulating meteoric waters. The metallic ingredients may 
come from the rock immediately adjoining the deposit, or 
from rocks at. great distances from them, the distinguishing 
feature, from the point of view taken up in this paper, being 
not so much the source of the metal, but the method by 
which it is supposed to have been concentrated, for 1t 1s in 
this respect that the two theories chiefly differ. 
Of the rain which falls upon the surface of the earth, a 
portion sinks down through the pores, cracks, and fissures 
of the rocks into the crust of the earth, to issue again after 
a longer or shorter journey as a spring. This underground 
water is of two kinds. Firstly, that which circulates near 
the surface, flowing from higher and very numerous points 
of ingress to lower and fewer points of egress after a com- 
paratively short journey. Such surface waters are usually 
oxidising in their action, and though they exert a most 
important influence over the upper portions of ore deposits 
which are already in existence, and produce local concentra- 
tions in them, they are not believed to be capable of 
producing any considerable primary concentration from the 
minute metallic constituents of the rocks. Secondly, that 
which takes part in the deep underground circulation. The 
motive force producing this circulation is no doubt in part 
the same as that producing circulation in the upper zone, 
namely, differences in the levels of ingress and egress, but 
it is probably chiefly due to differences in temperature 
between the ascending and descending columns. The deep 
underground waters have usuallv lost their free oxygen, and 
