

ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 349 
is at this stage of his work that he will find a knowledge of 
structural geology most helpful. He should carefully in- 
vestigate the rocks occupying the proposed site, in order to 
satisfy himself as to their soundness. Should they be very 
porous and much shattered and jointed, the conditions will 
be unfavourable, and a better site, if possible, should be sought 
for. This will be all the more advisable should the strata in 
question be inclined in the same direction as the valley, for 
under such conditions leakage is almost certain to take place 
—much water escaping along the bedding-planes, not to 
reappear at the surface, perhaps, till after an underground 
course of many miles. Should the inclination of the strata, 
however, be in the opposite direction, there is not the same 
danger of considerable loss, since any water that finds its way 
below the surface may possibly be discharged again further 
up the valley. It is needless to say, however, that no engineer 
would think of forming a reservoir over an area of highly 
jointed and pervious rocks, if he could avoid doing so. 
Unfortunately, the bottom of a valley is frequently covered 
with thick alluvial deposits, and the engineer, unless he knows 
something of geological structure, may not be aware of the 
nature and arrangement of the underlying rocks—even after 
he has tested the ground by means of boreholes. 
The selection of a site for his embankment demands the 
greatest care. Sometimes there is no difficulty—the bed of 
the valley may be deeply filled with tough, homogeneous clay, 
than which no better foundation could be obtained. It is 
always well, however, to make sure by a series of borings 
that no water-bearing beds are present in or underneath the 
clay, for in either case these would be sources of danger— 
allowing of leakage, and thus threatening the stability of the 
embankment. As foundations for an embankment, highly 
jointed rocks of any kind—certain igneous rocks, limestones, 
and loose shattery shales especially—are to be avoided. When 
the engineer has no choice of sites, but must, if at all possible, 
build his embankment upon rocks, the character and structure 
of which are quite unfavourable, the difficulties he must 
encounter will add greatly to the cost of the undertaking. 
Rivers.—Some towns and cities draw their water-supplies 
from the rivers on which they are situated. In certain cases 
