
Parr Il. Suor.ii.§2] SPRINGS. 345 
along which it will flow until it issues either as springs or in a 
general line of wetness along the side of the valley (6). The second 
sandy bed (c) will serve as a reservoir of subterranean water so long 
as it remains below the surface, but any valley cutting down below 
its base towards or beyond 6 will drain it. 
Except, however, in districts of gently inclined and unbroken 
strata, springs are more usually of the second class, where the water 
has descended to a greater or less distance from the surface and has 
risen again to the surface in fissures, as in so many syphons. Lines 
of joint and fault afford ready channels for subterranean drainage 
(Fig. e) Powerful faults which bring different kinds of rock against 

Fic. 96.—DExEp-SEATED SPRINGS RISING THROUGH JOINTS (ss) AND A Fau.t (f )}. 
each other are frequently marked at the surface by copious springs 
(f. Fig. 96). So complex is the network of divisional planes by which 
rocks are traversed that water may often follow a most labyrinthine 
course before it completes its underground circulation (Fig. 97). 
In most districts rocks are permeated with water below a certain 
limit termed the water-level. Owing to varying structure and 
relative capacity for water among rocks, this line is not strictly 
horizontal like that of the surface of a lake. Moreover, it is liable 
to rise and fall according as the seasons are wet or dry. In some 
places it lies quite near, in others far below, the surface. A well is 
an artificial hole dug down below the water-level, so that the water 
Fic. 9'7.—INTRICATE SUBTERRANEAN CoursE OF PEeRCOLATING WATER. 
may percolate intoit. Hence, when the water-level happens to be at a 
small depth wells are shallow, when at a great depth they require 
to be deep. 
