PAPERS OX GEOGRAPHY AND GEOLOGY 169 



inches or more may be precipitated, while in years of 

 least rainfall the total may not be more than twenty-four 

 inches or even less. It is in these years of least rainfall 

 that the need of conservation of water becomes most ap- 

 parent, and when the effects of pollution become most 

 acute and dangerous. 



Drift formations of various depths cover most of our 

 State and the yellow clays, sands and gravels absorb 

 water which is diffused through their layers. Below the 

 yellow clay lies. a blue or bowlder clay that as a rule is 

 impervious to water. Part of the rain sinks through the 

 soil and subsoil into this yellow clay, which thus over a 

 large area is a water bearing stratum. Water is re- 

 tained in it because of blue clay underneath. This under- 

 ground reservoir is not by any means inexhaustible, al- 

 though our State report properly calls it our largest and 

 most valuable supply, and surface wells over much of the 

 State extend into it. The level in this formation has been 

 lowered, and it can no longer be relied upon, as in pioneer 

 days, to meet the increasing demands. Professor J. A. 

 Udden in one of the early Bulletins says: "The general 

 level of the ground water is being lowered." As early 

 as 1908, the Bulletin spoke of the insufficiency of the yel- 

 low clay supply. 



Another important consideration is that the wide- 

 spread tile drainage of the surface has accelerated the 

 run off so that not so large a quantity of water as for- 

 merly reaches the yellow clay. In a general way one can 

 figure this run off at nearly one-third of the precipitation. 

 It is estimated that nearly one-third is evaporated and 

 at some seasons the proportion is greater. This leaves 

 about a third to sink into the water bearing clays, sands 

 and gravels, and a portion of this seeps out along the 

 edges of the blue clay into the streams or breaks out in 

 springs. Or very great economic importance, therefore, 

 is what becomes of the portion that seeps into the clays 

 or runs into the streams. It is on this that man and all 

 other animal and even vegetable life must depend for 

 most convenient supplies. 



In the State Bulletin of 1913 it is stated that "With 

 very few exceptions there are no sources of water supply 



