ARTESIAN WELLS. 
157 
of the upper dish, the water will rise through it. In such an experi- 
ment the saucers represent the impervious and the layer of sand 
(between them the pervious beds in an artesian basin. These condi- 
tions are indicated in the following ideal cross section of an artesian 
basin, in which the pervious bed A is water charged from the rain fall- 
ing on its exposed rim and percolates downward until the bed is sat- 
urated to the impervious rocks B. A hole drilled at any point, as D 
or E, within the basin will permit the water to rise, owing to the pres- 
ence of water at higher levels, and to overflow at the surface, provid- 
ing the elevation where the hole is made is below the level of the 
lowest point in the. rim of the basin by means of which the water is 
enabled to overflow. The upper limit of saturation in the pervious 
bed is termed the artesian head. In the diagram this horizon is indi- 
cated by the line OF, and water will rise and overflow within the basin 
at any locality where the surface is below this level. 
Fig. 4.— Section of an artesian basin. 
Artesian basins are usually produced by a gentle downward bending 
of rocks containing alternating beds of pervious material, such as sand 
or sandstone, and impervious beds, such as clay and shale. There are 
jnany qualifying conditions, however, as has been shown by Chamber- 
lin in the article referred to above. 
The essential condition which causes water to rise and overflow in 
the case of an artesian well is that it shall be under sufficient pressure. 
In order to insure this condition, however, a complete basin need not 
be present. This is indicated in fig. 5, showing wedge-shaped pervi- 
ous beds so exposed at the top as to become water charged and inclosed 
below in impervious material. Such an occurrence might be termed 
an u artesian wedge." 
1) c 1 b 
Fig. 5. — Section of an artesian wedge. 
In each of the two illustrations just given the pervious beds are 
saturated and the water contained in them is essentially at rest; that is, 
hydrostatic pressure is what causes a portion of the water to rise. In 
order that a well may continue to flow with undiminished pressure, it 
is evident that the supply must be renewed. 
Hydrostatic pressure in a saturated stratum, or reservoirs, is not the 
only cause, however, by which wells may flow. Another method of 
flow is illustrated by what is known as an artesian slope. 
