THE ARTESIAN WATER SUPPLY OF EASTERN FLORIDA. 
109 
in the accompanying diagram. It consists of a pervious layer (a), 
out-cropping at the surface on either side and sagging at the mid¬ 
dle, above which is an impervious or water-tight confining layer 
Fig. 6.—Illustrating Structure of an Artesian Basin. 
(c), and below which is also an impervious layer (b). Water en¬ 
ters the pervious layer at its surface exposures at the sides. The 
water collecting in the central part of the basin is under pressure 
from the weight of the additional water entering from the sides. 
Therefore, a well put down to the water stratum in any part of the 
basin will obtain artesian water, or water which will rise in the 
boring. The rise in the boring is determined by the elevation of the 
in-take area, and can in no case rise above the elevation of the ex¬ 
posed edges of the stratum. As a matter of observation, it is 
found in all cases to rise not quite so high as the exposed edge of 
the stratums, the loss being due to the friction of movement through 
the rock. This loss of head due to friction necessarily varies with 
the texture of the stratum through which it passes, the passage 
being more free through the coarse material, and hence meeting with 
less friction than through fine. Whether or not wells put down in 
the basin will obtain flowing or non-flowing artesian water, de¬ 
pends upon the surface elevation of the mouth of the well. The 
diagram illustrates a basin in which flowing artesian wells may be 
obtained. 
ARTESIAN SLOPE. 
The basin arrangement of strata is not the only possible struc¬ 
ture resulting in artesian pressure. The same result may among 
other ways be brought about quite effectively by an inclined 
porous stratum wedging out between two impervious strata. 
This condition is illustrated by the accompanying simple sketch, 
in which the pervious stratum (a) is represented as pinching out 
and disappearing between impervious strata. A pervious stratum 
grading into an impervious or less pervious condition resulting 
in artesian pressure is represented by (b) of the same 
drawing. These conditions are often met with in the strata of 
the coastal plain. Not infrequently, a sandstone forma¬ 
tion grades off shore into a finer sandstone, and ultimately into 
a shale. This condition comes about naturally through the sort- 
