

364 STRUCTURAL AND FIELD GEOLOGY 
Nevertheless, even in such cases, there will in all probability 
be some closer grained or relatively impervious bed or beds 
to stay the downward passage of the water. An extended 
survey of adjoining districts may even show that the pervious 
beds, as they range beyond our region, are underlaid by 
or interosculate with impervious beds; or that, in the direction 
of the dip, they are eventually interrupted by faults, dykes, 
or other barriers, and may not, therefore, be so barren of 
water at a moderate depth as appearances in our own 
neighbourhood might have led us to infer. Should the whole 
area be more or less deeply mantled with impervious super- 
ficial accumulations, such as boulder-clay, the prospects of 
obtaining water by boring would be considerably increased. 
7. It goes without saying that the water obtained from a 
bore-hole, although sufficiently abundant, may yet be unsuit- 
able for the purpose the engineer has in view. A careful 
survey of the catchment area should, therefore, be made 
preliminary to any boring. It may be that some of the 
pervious beds contain deleterious ingredients, which must 
unfavourably affect the quality of the water; while, at other 
horizons, water-bearing strata of a satisfactory character 
occur. Should it be necessary to pass through an undesirable 
source of supply to reach a more promising source at a lower 
level, the water coming from the higher level can be prevented 
from contaminating that of the lower level, by simply tubing 
it off. Another danger is the infiltration of foul liquids from 
the surface. These may not penetrate sufficiently far to 
affect the water at the relatively deep level from which it is 
drawn. But unless the bore-hole is properly tubed for some 
distance down from the surface, it is likely enough to be 
invaded by pollutions. 
Drainage.—In planning a scheme of drainage either for 
town or country, it is not enough to consider only the natural 
slope of the ground, and the ease with which the waste 
products of a community can be discharged elsewhere. It 
must be remembered that the external configuration of the 
ground may not coincide even approximately with the 
internal or geological structure. The surface may slope in 
one direction and the subjacent strata in quite another. 
There is always a danger, therefore, of polluted liquid finding 

