

350 STRUCTURAL AND FIELD GEOLOGY 
the water is pumped from some point above a town, while in 
other cases it is drawn from the higher reaches of the river 
and brought in open courses or in pipes. This source of 
supply is seldom desirable, but not infrequently no other 
source is available, in which case the only thing to be done 
is to look well after the filtering, which doubtless minimises 
the danger of pollution, but cannot always be implicitly relied 
upon to protect the population. It is remarkable, however, 
how rapidly rivers seem to purify themselves from the pollu- 
tions poured into them by the villages and towns upon their 
banks. Soon the water begins to clear—a foul-smelling mud 
settling upon the stones and gravel of their beds, and gather- 
ing here and there in extra quantities along their margins. 
Exposed to sunlight and the action of the atmosphere, the 
various organic impurities become broken up—a process in 
which numerous minute forms of life play a not unimportant 
part—until eventually the river may become bright and 
sparkling as at first. All such waters, however, are properly 
held suspect, and ought never to be used for domestic 
purposes before they have been carefully examined and 
declared safe. 
Underground Water.—The proportion of the rainfall 
that sinks into the ground naturally varies according to the 
character of the underlying rocks. But, whatsoever the 
nature of the rocks may be, they are commonly charged with 
water up to a certain limit known as the water-level, the 
depth of which from the surface is determined by the amount 
of rainfall, the configuration of the surface, and the geological 
conditions. In some districts, the level in question may be 
reached at only a few yards down; in other places it may 
sink to great depths, and it usually fluctuates with the rainfall. 
Owing to these several conditions, a constant underground 
circulation is kept up—gravitation and hydrostatic pressure 
forcing the water through the pores and fissures of the rocks 
until it can escape at the surface in the form of springs. In 
regions composed chiefly of highly pervious rocks of great 
thickness, springs are of infrequent occurrence, and are apt to 
appear only in the deeper depressions of the land. But in 
countries where the rocks are of variable character and 
structure, underground water may be discharged at many 

