GEOLOGY DIAGRAM 20 



217 



either because the rock is too hard, or because it is clay, and all 

 this water flows at the surface of the impenetrable layer 

 which stops it, and flows out in the neighbourhood forming a 

 spring. 



Warm Springs. Warm springs, possessing medical pro- 

 perties for the cure of diseases, are often met with in mountain- 

 ous countries. They are called mineral waters, or thermal springs, 

 thermal being derived from a Greek word meaning warm. 

 Sometimes these waters contain a large quantity of sulphur, and 

 smell strongly of rotten eggs. 



Wells, Artesian Wells. The wells which are dug to obtain 

 water are designed to strike one of these springs of subterranean 

 water which is supposed to exist at a moderate depth. AVhen 

 it is found, the water is seen to filter into the well from all sides 



Subterra- 

 nean 

 Spring 



Section of strata pierced by Artesian wells. 



of its surface, and it is raised from the bottom by buckets or 

 a pump. 



But it may also happen that a spring of water enters between 

 two layers of impenetrable soil, resting on the slope of a moun- 

 tain, and then one part of the spring of water will soon be much 

 higher than the other, and will form a kind of raised reservoir, 

 like those constructed to give more force to water, and make it 

 rise in a fountain in ornamental basins. Artesian wells are 

 designed to pierce to a great depth in the ground, to attempt to 

 strike a spring of this kind, for then there is no occasion to 

 draw the water, for it will rise to the surface of the ground of 

 itself. The name of these wells comes from the district of Artois, 

 where the first were dug. 



