THE HILLSIDE SPRING AND ITS WORK. 31 



condensed and rained down again. Also, part of that which 

 soaks in the ground returns when the surface becomes dry, 

 and is evaporated. But, not to be too precise, let us attempt 

 to follow the water which soaks in. First of all, it must have 

 dissolved some substances with which it came in contact at 

 the surface. These substances must be, to a limited extent 

 certain mineral constituents of the Drift; but the Drift has 

 been so many thousand years exposed to rains, that all its 

 readily soluble constituents have been dissolved away from 

 the surface. The chief agencies which supply soluble matters 

 to the surface are man and animals. The underground waters, 

 therefore, carry with them a certain amount of solutions of 

 organic and inorganic origin, and are not absolutely pure, 

 like carefully distilled water. They may even be poisonous 

 and unsanitary. 



Following these waters in thought, beneath the surface, 

 we see them percolating through the sands and gravels, which 

 we have found to make up the principal part of the upper 

 Drift. Through layer after layer they continue to descend. 

 If any obstruction is encountered, they are quickly deflected 

 around it, and so continue to settle toward the impervious 

 Bowlder Clay at the bottom of the Drift ; or, if that is , 

 absent, the waters settle to the bed-rock. We will not attempt, 

 at present, to follow them in the rocks. 



Now, we know that the Drift contains sheets of impervious/ 

 clay. Of course, then, these intercept the descending water. 

 The water arrested by a clay-bed saturates the overlying sand, 

 and gradually flows along the surface of the clay to a lower 

 level. But we have seen that all these Drift beds are of 

 quite limited extent. The water, therefore, soon reaches the 

 edge of the clay-bed and escapes down to a lower level. 

 Probably it is again intercepted by a deeper clay-bed. Along 

 this it flows in a similar way, and so continues always ap- 

 proaching nearer and nearer to the lower limit of the Drift. 

 Some of the clay-beds are concave upward, and thus form real 

 dishes or cisterns, which remain full. 



Suppose we dig a well. While passing through the sandy 



