

ECONOMIC ASPECTS OF GEOLOGICAL STRUCIURE 353 
geological structure is obviously quite unfavourable to the 
outflow of underground water, the water, as in the previous 
cases, making its way in the direction of dip (Fig. 129), and 
therefore away from the valley. 
We have been considering the flow of water through 
porous rocks as being conducted along the planes of bedding, 
but we must not forget that sedimentary strata are traversed 
by joint-planes, and that the presence of these naturally 
influences the circulation. When all the porous beds in a 
series of strata are fully saturated, the water will follow the 
normal direction. But when continued dry conditions have 
cut off the supply from above, and the discharge by springs 
begins to diminish, water seeks its way down through joints 
and fissures from one porous bed to another. Hence the 


BE 
LEZ 
FIG. 129..—DRAINAGE IN ANTICLINAL STRATA. 
springs issuing from the deepest beds will continue to yield 
their usual supply long after the highest lying springs have 
failed. The exhaustion of the springs maybe still further 
delayed by the exuding of water from the less porous beds— 
all of which, although spoken of as impermeable, are yer 
capable of absorbing and giving out water in less or greater 
degree. 
Springs are not less characteristic of massive eruptive 
rocks than of sedimentary beds, but while the underground 
drainage of the latter is conducted principally through porous 
strata, and therefore follows a determinate direction, that of 
the former keeps to the clefts and fissures, and as these vary 
in width and trend, and may be numerous in some places 
and far apart elsewhere, one never can tell where springs 
are likely to appear at the surface. Rain falling upon a 
eranite mass finds its way down through innumerable 
fissures, and after a relatively short downward course may, 
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