


ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 351 
different levels, in mountain regions and low grounds alike; 
not infrequently, indeed, copious springs of fresh water 
coming from such regions issue on the floor of the sea. The 
various divisional planes of rocks—joints, bedding-planes, 
faults, etc., naturally constitute the chief underground water- 
ways. In the case of soluble rocks these water-ways become 
widened by the chemical and mechanical action of the 
running water, until very considerable tunnels may be 
worked out—giving passage to torrents, streams, and rivers. 
Relatively insoluble rocks are not, of course, traversed by 
subterranean channels of this kind, but, if sufficiently porous, 
they frequently contain enormous stores of water. When 
such beds are inclined and underlaid by impermeable strata, 
the water they contain naturally makes its way downward in 
the direction of dip. Should the strata be horizontal, under- 
ground flowage nevertheless does not cease—the water under 
hydrostatic pressure being forced to percolate through the 
rocks. Such movements, indeed, necessarily take place 
even in amorphous rocks which are neither bedded nor 
jointed. 
In the following case (Fig. 126) we have, say, an 
amorphous mass of sand and gravel (a) resting upon a 


FIG. 126.—HEAPING UP OF WATER IN SUPERFICIAL DEPOSITS. 
horizontal surface of impervious clay (0). Rain falling on 
the surface of @ is greedily absorbed, and gradually sinks 
until the bed becomes saturated up to a certain limit (w), when 
the frictional resistance to its passage outwards is overcome 
by the hydrostatic pressure. It is then forced to flow along 
the surface of the underlying clay, and escapes to the light of 
day at s s asa line of seepage marking the junction between 
the porous and impervious beds; but should there be irregu- 
larities in the surface of the clay, it may issue in the form of 
definite spriggs. Springs of this kind are of common occur- 
