THE ARTESIAN WATER SUPPLY OF EASTERN FLORIDA. 19

in the accompanying diagram. It consists of a pervious layer (a), out-cropping at the surface on either side and sagging at the middle, 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 enters 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 exposed edges of the stratum. As a matter of observation, it is fLound in all cases to rise nut quite -so high as the exposed edge of the stratum, the lo ss being due to the f riction 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 f ree through the coarse material, and hence meeting with less f riction than through fine. Whether or not wells put down in the basin will obtain flowing or non-flowing artesian water, depends 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 arrangemlent of strata is not the only possible structure resulting in artesian pressure. The same result may among other ways be brought about quite effectively by an -inclined porousI 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 drawinga.'These conditions are often met with in the strata of

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