214 PROCEEDINGS OF SECTION C. 
have the most ample ground for believing that on the whole 
the upward flow exceeds the downward very considerably 
in velocity’ 
The above statement of the theory of underground flow- 
age is necessarily condensed, and simply aims at a general 
statement of what seems to be the necessary conditions 
governing this circulation. I think, however, that the fol- 
lowing conclusion has been fairly demonstrated :—That, as 
a rule, the descending currents of the deep underground 
circulation pass through a large number of small openings, 
and the ascending currents pass at a relatively higher 
velocity through a small number of large openings. 
General Conclusions. 
The circulation which is produced by the want of hor- 
zontality of the surface of the ground water level and by 
the natural increase of heat as depth is gained in the earth’s 
crust, are general causes, comparatively feeble in their 
action, but which are everywhere and always present, and 
they must have the effect of producing a universal under- 
ground water circulation. Still, it seems highly probable 
that this universal underground circulation has had little 
to do with the leaching out of the materials for ore deposits. 
No doubt it must be the means of producing vast alterations 
in the composition and structure of rocks, and is largely 
responsible for the more general phenomena of induration 
and regional metamorphism; but its very universality is a 
strong argument against its being one of the prime causes 
of ore deposits, for ore deposits are far from being universal 
phenomena even among rocks which belong to the same 
geological horizon. 
There are many reasons for believing that the main causes 
of effective leaching, if leaching is an important factor in the 
formation of deep-seated ore deposits, must be the heat 
produced by intrusive masses of igneous rock, for here 
we have a certain cause of vigorous circulation. Every- 
one can understand that the introduction of a mass of 
molten rock into a region saturated with stagnant or 
slowly-moving water must have the effect of producing 
violent circulation. It is a notorious fact that ore deposits 
are found in regions where vulcanism has been active; 
eruptive rocks are almost always found in the vicimity of 
ore deposits, and in the few well-established instances where 
eruptive rocks are apparently absent there is always the 
possibility that they are present below the surface, but have 
not yet been exposed by denudation. Moreover, the diffi- 
culty which we encounter when we try to explain the process 
