
360 STRUCTURAL AND FIELD GEOLOGY 
pollution. Frequently, indeed, it may be found necessary ~ 
to line them with water-tight walls, especially in the case 
| of wells that are sunk in more or less unconsolidated deposits. 
| Even after every precaution is taken, however, no surface- 
ij well can be considered perfectly safe—although there are 
| some obviously more liable to be contaminated than others. 
| Wells sunk in river alluvia, in prox- 
imity to and upon the same level as 
dwelling-houses, are the most danger- 
ous of all. 
_ Artesian Wells.—The driven wells 
referred to above are often true arte- 
sian wells on a small scale, for an 
artesian well is simply a borehole 
sunk to some permeable stratum in 
which the water is under such high 
pressure that when it is reached it 
rises towards the surface—the upper 
limit reached by it being determined 
by the height of its head or source 
above the mouth of the well, and the 
amount of frictional resistance it has 
to overcome. Should the latter be 
very great, there may be no rapid 
rise of water in the well. The rock 
may be porous enough in texture, but 
if it be not traversed by more or less 
open joints and fissures, the hydro- 
static pressure may barely suffice to 
keep up a gentle circulation. Fortu- 
nately, such planes of division are 
/ seldom or never absent, and in cer- 
ae eg! tain rocks, as we have learned, they 
ety Wi y ‘ae are often relatively wide and open. 
The accompanying diagram (Fig. 
136), will serve to indicate the geological conditions under 
which an artesian water-supply is obtained. The section is 
supposed to be taken across a broad area, throughout which 
the strata, consisting of pervious (#) and impervious (277) 
| beds, are arranged in a basin-shaped: form, Rain falling on 

ae SS SS ROE Se Rm Se ee 
SS 
Fic. 136.—ARTESIAN WELLS. 

