REVIEWS 439 
It is furthermore pertinent to inquire, with Le Conte, if it be held 
that the waters from which the ores were deposited do not originally 
come from the surface, and if the ‘“‘ascending currents” are not fed by 
equivalent ‘‘descending currents,’ what is the nature of this water, 
and whence its source? It is admitted by practically all students of 
the problem that the deposits in question are the work of waters of 
some sort. 
The question whether the waters derive their motive force from 
gravitative stress is one which must be answered in each case with an 
eye to the facts of the particular case. Admittedly the friction between 
the moving fluid and the walls of the conduit decreases the pressure, 
and, supposedly at least, the friction might become so great as to 
wholly stop the flow. In the case of water moving through super- 
capillary tubes this cannot on the average be true, so long as the dis- 
tance measured is vertical, and the tubes are approximately regular, 
since the pressure increases favz passu with the friction. The flow is 
further increased presumably by decreasing viscosity resulting from 
heat. It may be stated at once that probably, as it seems to the 
reviewer, the major flow concerned in the formation of ore deposits is 
that through supercapillary tubes. In the matter of flow through 
capillary tubes, there is some doubt, since the laws of such flow under 
high pressure seem not well understood; but if Poisenille’s law be 
applicable, the result must be the same, since the flow is proportional 
to the pressure itself, which increases directly with the depth. Wher- 
ever the underground waters are in motion it is difficult to conceive 
of their not flowing in obedience to gravitative forces; except in very 
especial and particular instances where the little understood gaseous 
pressure may become operative. 
In regard to the second question, whether ores are not more 
directly dependent upon the phenomena of vulcanism than would be 
inferred from a general reading of Van Hise’s paper, it is but fair to 
state that he evidently recognizes these forces as important, though he 
‘devoted more space to less generally recognized phases of the prob- 
lem. It has been tacitly assumed by all parties to the discussion that 
heated waters under pressure have been the main agents in collecting 
the material which makes up the ore bodies. The underground waters 
may become heated by (1) contact with volcanic rocks recently injected 
into the outer crust of the earth, (2) by contact with rocks heated by 
dynamic action, (3) by penetrating to a sufficient depth to absorb heat 
