

358 STRUCTURAL AND FIELD GEOLOGY 
Wells.—The foregoing rapid sketch of the conditions that 
determine the underground circulation of water and its dis- 
charge at the surface will suffice to indicate what course we 
should follow in searching for a subterranean water-supply. 
From the very earliest times, men have dug for water—a 
common well being simply a hole sunk below the water-level, 
into which percolation from the surrounding rocks takes place. 
In these advanced days, we now imitate Nature on a bolder 
scale, and by means of our boreholes produce more or less 
deep-seated perennial springs, Water is doubtless very 
generally distributed through the superficial parts of the 
earth’s crust, but all rocks, as we have learned, are not 
equally absorbent, and the depth of the water-level from the 
surface is very variable. It is obvious, therefore, that before 
proceeding to sink a common well, we should first ascertain 
whether the geological conditions are favourable or not. If 
the rocks of the district be highly jointed and pervious, we are 
unlikely to succeed ; but if they be less fissured, there is some 
hope of reaching the water-level at a moderate depth. It 
is needless, however, to say that much will depend upon 
the climatic conditions of the region, for the position of 
the water-level is necessarily largely determined by the 
rainfall. 
The superficial accumulations of this and many other 
countries not infrequently contain large quantities of water, 
either derived directly from the rainfall or introduced into 
them by natural springs—while in other cases, it has filtered 
into them from streams. Sheets of sand, for example, which 
are underlaid and perhaps overlaid by impervious clay, 
usually hold water. Again, the recent alluvia of our rivers, 
and the more ancient flats and terraces of similar materials 
which occur so frequently at various levels in our valleys, may 
yield copious supplies. The water obtained from the younger 
alluvia has obviously percolated into them from the adjacent 
stream or river. The older alluvia, on the other hand, have 
usually derived theirs from the valley slopes—a very superficial 
supply—but now and again the water flows into them from 
true springs issuing into or underneath the deposits. Common 
wells, dug in superficial deposits of the kind referred to, not 
infrequently yield a good supply of potable water, but they 
