ORE DEPOSIT THEORIES. 209 
The flowage of water through capillary openings is, however, 
influenced to a very considerable extent by temperature, 
for it has been shown to be inversely proportional to the 
viscosity of the liquid. Now, the viscosity of water at 90° 
C. is one-fifth of what it is at 0° C., consequently five times 
as much water will flow through a capillary opening of 
given size at 90° temperature than will flow through ‘the 
same opening under otherwise similar conditions at 0° C. 
This fact must be of very great importance in promoting 
the circulation of water in depth and in the vicinity of hot 
eruptive masses. 
When the cracks become so small that the molecular 
attractions of the solids extend across the opening, we have 
what Van Hise calls sub-capillary openings; for sheet 
openings, such as cleavage spaces, this class includes all 
openings under -0001 mm. wide. Concerning the flowage 
of water through these openings, Van Hise says—‘‘ The 
water is held as a film glued to the wall by the adhesion 
between the water and the rock. There is no free water in 
such openings as these... The flowage must be exceedingly 
slow or nil.” 
These facts seem to point to the conclusion that an effec- 
tive circulation through a dense rock such as granite or 
gabbro must require an enormous motive force for its pro- 
duction, but it is hardly possible to draw any definite 
conclusions as to the amount of force required, or as to the 
rate at which the circulation will take place, or, indeed, 
whether it can become an effective circulation at all. 
Assuming that all rocks are to some extent porous (a fact 
that has been definitely ascertained by experiment), and 
that some force or forces exist urging the water to pass 
through them, then it follows, that no matter how small the 
force may be, circulation will take place. It is then a 
question of time only for the leaching process to become 
complete. 
The Motive Forces producing Circulation. 
The motive force which produces underground circulation 
may be conceived as arising from several causes, or from a 
combination of these causes. In the first place, the surface 
of the deep underground water is not horizontal, but forms 
an undulating plane, rising with the hills and falling with 
the valleys. From this fact it follows that there will be a 
tendency for the water to flow from the elevated regions to 
the depressed regions. In doing so, according to well-known 
laws of hydro-dynamics, it must utilise the whole of the 
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