CHAPTER Ux XITT 
ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE— 
continued 

Water-supply. Lakes and Impounded Streams, Reservoirs. Supply 
from Rivers. Underground Water—the Water-level; Natural 
Springs as illustrating the course followed by Subterranean Water ; 
Surface and Deep-seated Springs. Common Wells and Driven 
Wells. Artesian Wells. Considerations to be kept in view in the 
search for an Artesian Water-supply. Drainage. Distribution of 
Disease in relation to Geological Conditions. 
Water-supply.—Superficial and underground sources of 
supply alike depend upon the amount of precipitation and 
the physical aspect and geological conditions of a country. 
But the relative amount of water circulating above and below 
ground respectively is determined mainly by the character of 
the rocks and the mode of their arrangement. Two regions, 
for example, may have the same amount of rainfall, and, 
nevertheless, the one may be little better than a dry desert, 
while the other may rejoice in numerous streams and rivers, 
and be conspicuous for its fertility. There are many lands 
that consist of rocks so highly pervious that rain and melting 
snow at once descend below the surface, and streams and 
rivers are impossible—all the drainage being conducted 
underground. On the other hand, we may encounter else- 
where the opposite extreme—namely, a country built up 
of impermeable rocks which absorb so little water that 
practically the entire rainfail flows in superficial courses to 
the sea. Between these two extremes there are many 
gradations—most lands being composed partly of porous 
and partly of impermeable rock-formations. 
Jt will be convenient to treat of the water-supply derived 
347 

