WATER SUPPL,Y OP PASTERN AND SOUTHERN PRORIDA. 141 
artesian basin. 
A variety of conditions in the arrangement and structure of 
the underlying deposits may bring about artesian pressure. The 
simplest, although probably not the most common, is that of a 
basin-like arrangement of successive relatively pervious and 
impervious strata. This typical structure, known as an artesian 
basin, is shown 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- 
Fig. 1.—Illustrating Structure of an Artesian Basin. 
tight confining layer (c), and below which is also an impervious 
layer (b). Water enters the pervious layer at its surface ex¬ 
posures 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 found in all cases to 
rise not quite so high as the exposed edge of the stratum, 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, 
depends upon the surface elevation of the mouth of the well. The 
diagram illustrates a basin in which flowing artesian wells may 
be obtained. 
