
ECONOMIC ASPECTS OF GEOLOGICAL STRUCTURE 359 
are not always to be trusted. Near towns and villages, and 
even in the vicinity of isolated dwellings, they are liable to 
contamination, impurities being readily filtered into them, 
especially during wet seasons. The wells sunk in an old river- 
terrace may, under ordinary conditions, yield excellent water, 
more particularly when the parent source of the supply is a 
spring discharging into the deposits. Jn times of heavy 
flood, however, when the adjacent river rises to the level of 
the terrace, the gravel is rapidly saturated, and impurities 
may be washed into the wells. The fact that when the rivers 
are in flood, outbreaks of typhoid fever often occur in riparian 
districts supplied from such wells is sufficiently suggestive. 
The water, which under ordinary conditions is quite whole- 
some and suitable for all domestic purposes, is, perhaps, never 
suspected, and may continue to be used until the next con- 
siderable flood repeats the work of its predecessor. 
In certain districts deeply covered with loose deposits of 
gravel, sand, clay, etc, it is often difficult or practically 
impossible to sink common wells for a local water-supply. 
When such is the case, engineers often have recourse to 
driven wells. These are made by forcing down a strongly 
pointed iron pipe, pierced with holes round the bottom to 
admit the water. The advantages of this system are obvious, 
for not only can the pipe be driven to depths much below 
those reached by any ordinary well, but the water-supply 
obtained is protected from impurities coming from the surface. 
In populous districts, however, even driven wells may in time 
become polluted, for the pipe, subject to the corrosive action 
of foul liquids descending from the surface, may eventually 
yield admission to the enemy. 
When good springs are not available, common wells are 
often the only source of supply in country districts. In 
sinking these it is always advisable to take the geological 
conditions into consideration. Remembering that under- 
ground water finds its way in the direction of dip, care 
should be taken to sink wells in such positions that impure 
surface water cannot reach them by percolating along the 
bedding-planes. It is absolutely necessary, moreover, that 
wells should be placed as far away as possible from dwelling- 
houses, cesspools, drains, etc., and every possible source of 
