
ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 361 
the outcrops of the pervious beds which are overlaid and 
underlaid by impermeable strata, percolates in the direction 
of the bedding-planes, and accumulates until each porous 
stratum becomes saturated up to its outcrop. It is obvious 
that this imprisoned water must be under hydrostatic pres- 
sure, which necessarily increases with the depth and reaches 
a maximum in the centre of the basin. Were a boring made 
at 1 through the uppermost impervious stratum into the 
subjacent water-logged bed, an uprush of water to the sur- 
face would ensue. At first, the water might form a tall 
fountain—the height of which would be determined not only 
by hydrostatic pressure, but by the amount of frictional 
resistance to be overcome by the water. If the passage of 
the water through the porous bed were favoured by open 
fissures, the fountain might reach a height not very much 
below that of the outcrop of the bed. Shortly, however, it 
would begin to decrease in height until it reached a level 
determined by the average rainfall of the district. It would, 
ietact, behave like a perennial spring. A boring sunk at 
2 would tap a deeper stratum, and cause a still stronger 
outflow owing to the greater head; while the beds tapped 
bpyeasand 4 would for a similar reason send. yet more 
powerful uprushes of water to the surface. 
The geological conditions represented in the diagram 
are, of course, ideal. Each pervious stratum is supposed to 
retain all the rain-water which soaks into it at its outcrop. 
In point of fact, however, such conditions rarely if ever do 
obtain. So-called impervious strata are only relatively non- 
porous, while continuous joints and other lines of fissure, 
traversing all the beds of a series alike, are so seldom 
absent, that the water in deeply buried pervious beds must 
in less or greater degree escape towards the surface. Hence, 
when an artesian well is sunk, the water does not always 
rise so high as might have been anticipated, even after 
allowance has been made for frictional resistance. It must 
not be supposed that a basin-shaped arrangement of the 
strata is essential for the formation of artesian wells. Any 
series of impermeable and permeable beds dipping continu- 
ously in one direction for some considerable distance, may 
contain abundant supplies which, under certain conditions, 
