1906.] GEOLOGY IN RELATION TO WATER SUPPLY. 285 



eration succeeded generation and the population increased, the 

 soil beneath barns, houses, and fields became charged with cer- 

 tain poisonous products which found their way into the wells. 

 The surface wells of most thickly settled regions are now sub- 

 ject to contamination, but the danger is not readily recognized, 

 for certain kinds of filth increase the clearness and sparkle of 

 the water and add an agreeable taste. The old oaken bucket 

 is no longer a symbol of purity. Windmills, steam pumps, and 

 iron pipes are much less poetic, but far more conducive to 

 health. At the present time the nature of the water supply is 

 being carefully studied and the tendency everywhere is to sink 

 w^ells of small diameter and of great depth and wherever a 

 sufficient number of inhabitants makes it profitable to store up, 

 the water in reservoirs and to keep it constantly under the sur- 

 veillance of trained scientists. 



SOURCES OF WATER SUPPLY. 



The water supply of the earth is included in rivers, lakes, 

 and ground water. In the first instance, the world's entire 

 supply is the rainfall, which is controlled in its amount and 

 periodicity by the temperature and prevailing winds, so that 

 there are distributed over the earth regions of no rainfall, arid 

 regions, humid regions, and regions of excessive rainfall. The 

 rainfall measured during one year in Indio-California was less 

 than i/io of an inch, and in Northeast India, at Assam, 905 

 inches fell in a year. 



Rivers. Rivers form the first natural water supply. They 

 develop on land surfaces wherever there is a rainfall and the 

 rocks are not so porous as to take up all the water. The size 

 of the stream, its system of tributaries, and the shape of its 

 valley depend upon several factors, and the development of a 

 complete river system is a long and complicated process. 

 When water falls on a new land surface it is a stranger — no 

 rain-drops have been there before it and no rills are marked 

 out to rivers and thence to the ocean. It flows down any of- 

 fered slope in simple channels — about equally spaced. It pays 

 no attention to the character of the rock floor beneath — hard 

 rock and soft are equally acceptable. As the streams enlarge, 

 however, they soon learn to select the softer and less resistant 

 rock for their channels, and as time goes on they become more 



