COMPOSITION AND TESTING OF WELL WATERS 161 



and gravels, where the grains are often nearly pure quartz (silica), 

 only slight amounts of mineral matter are dissolved, some of the 

 waters from such materials having only a few parts per million of 

 dissolved solids. Elsewhere, however, where more soluble miner- 

 als are present, as in the alkaline or calcareous deposits of desert 

 regions, the amount dissolved from sands and gravels is often 

 very large. 



In clays, because of the fineness of the grain, the water is 

 brought into much more intimate contact with the material, and 

 the amount of mineral matter dissolved is considerably greater, 

 being usually several times as much as in sands and gravels. 

 Many of the clay waters are decidedly alkaline or calcareous and 

 are unpalatable and otherwise unfit either for drinking or for use 

 in boilers. 



In sandstones the waters are somewhat more mineralized than 

 those of sands and gravels for the reason that the waters, owing to 

 their slower movement, are in more intimate contact with the 

 grains and remain longer in contact with the particles. Likewise, 

 slate waters are usually more mineralized than those of their un- 

 consolidated counterparts, the clays. 



In crystalline rocks the mineral content of the water may be 

 even less than in sandstone, since not only are rocks of this type 

 relatively insoluble, but the water moves almost solely along open 

 joints, etc., and is brought into intimate contact only with very 

 limited surfaces. 



In marly clays and in limestones the waters dissolve large 

 amounts of lime and magnesium which give the water the peculiar- 

 ity known as hardness. Many waters from the softer limestones 

 carry the offensive sulphureted hydrogen gas and are known as 

 sulphur waters. 



Next to the composition of the water-bearing bed, time is the 

 most important element in determining the amount of mineral 

 matter in ground waters. The longer the water is in contact 

 with the rocks the more mineral matter will be dissolved. As, in 

 general, long periods of time are required, under the laws of cir- 



