
362 STRUCTURAL AND FIELD GEOLOGY 
can be reached by boring. Some of these conditions may 
therefore be briefly considered. 
A well-jointed porous bed, or series of beds, underlaid 
and overlaid by impermeable strata, may ,thin-out gradually 
in the direction of dip, and when such is the case they 
become water-logged (Fig. 137). Or the descent of water 
along the bedding-planes may be interrupted, not by the 
thinning-out of beds, but by such barriers as have already 
been referred to—faults, dykes, and other discordant junctions. 
Or, in the absence of any underground dams, the descending 
water may be stopped at extreme depths by the increasing 
temperature —the hydrostatic pressure being eventually 
counterbalanced by the tension of superheated steam. It 
is under such conditions as these that many natural springs 

FIG. 137.—WATER-BEARING BEDS WEDGING OUT DOWNWARDS. 
originate—the imprisoned water seeking to escape pressure 
tends to rise through joints or other lines of weakness towards 
the surface. Sometimes, however, it is prevented doing so, 
or is only partially successful, and immense stores are thus 
often retained underground. Such hidden sources are not 
difficult to discover, especially in regions the geological 
structure of which can be readily ascertained from a study 
of the rocks exposed at the surface. 
The more important points to be considered in the 
search for an artesian water-supply may be summarised as 
follows :— 
1. Having ascertained that the strata over a wide region 
have a dominant dip in one direction, we must endeavour to 
acquire as complete a knowledge as possible of the various 
rocks and rock-groups. It is essential for success that 
pervious beds should occur interstratified with impermeable 
strata, the best kinds of water-bearing beds being sand and 

