1889-90.] NATURAL HISTORY OF GROUND WATERS. 157 



as Hofmann calls them, different zones, viz., the upper or evaporating 

 zone, which may even hold the rainfall of a whole year, and becomes the 

 receptacle of all kinds of organic impurities, the culture medium for germs 

 of every sort. Hygienically, this is of all the most interesting and 

 important. The second, or intermediary zone, always humid, reached 

 at varying depths, may give passage to waters very slowly, and only gives 

 passage to the surplus over saturation derived from the upper layers. 

 While, generally speaking, this area is beyond the influence of surface 

 operations, yet it will readily be seen that in the degree that the super- 

 ficial layer becomes by the leaf mould of forests, drainage and cultiva- 

 tion, more pervious, the greater the amount of water that is absorbed, and 

 the more readily will the underlying zones be influenced from it. Drain- 

 age and the cultivation of grasses, now that the forest areas have disap- 

 peared, are essential to the conservation of the level of ground-waters, 

 since through their agency perviousness of soil is maintained, and the 

 retention of pluvial waters where they fall is made possible. 



The third zone in compact soil is only a fev/ inches thick, increasing 

 in depth where upper soils are more pervious. This is the supersaturated 

 layer along and through which flows the underground stream. The 

 depth of this stream or the thickness of this zone is slowly or rapidly 

 influenced according to the compact or porous character of the upper 

 soils, the extent of the watershed above any given point, and the inclina- 

 tion or dip of the more or less impervious stratum or hard-pan along 

 which this underground stream flows. An illustration as in the instance 

 of the London West flood in 1883 demonstrates this, while on the other 

 hand the greater hygroscopy and retentiveness of clays with their closer 

 texture, as already shown, will prevent surface influences readily showing 

 themselves. 



The remaining cause influencing these subterranean streams is that 

 already referred to as existing here and there in those 'instances where 

 the surface stream, as some river or lake, has waters at the same level as 

 the subterranean waters. The rise and fall of these surface waters caused 

 by heavy downfalls of rain, and by winds raising or lowering the level at 

 different parts along our lakes {e.g., at Owen Sound, Goderich, Lake St. 

 Clair, Moulton Township), shows how the ordinary conditions governino- 

 these underground streams may have local extraordinary influences con- 

 travening the general law. 



Returning to the consideration of the conditions which govern the 



existence and multiplication of microbes in the soil, we have to recollect 



the inconceivable richness of the superficial layers of organic matters in 



microbes. Two millions to a centigramme of soil, as Duclaux gives it, but 



12 



