2 vredenburg: recent ARTESIAN EXPERIMENTS IN INDIA. 



of the conditions essential to their occurrence. Briefly considered 

 they depend upon the following phenomena. The water that is 

 poured over the land in the shape of rain is disposed of in three 

 ways : part of it is returned to the atmosphere by evaporation ; 

 part of it runs over the surface to form rivers flowing towards 

 the sea or towards some inland drainage basin ; while the remain- 

 der sinks into the ground. It is from this subterranean source 

 that artesian wells and also ordinary shallow wells derive their 

 supply. 



That portion which soaks into the ground usually keeps the 

 soil and rocks moist up to a certain distance from the surface. 

 Beyond a certain depth, which varies locally according to the amount 

 of rainfall and the nature of the rocks, this moisture gives place to 

 complete saturation. The water thus occupying the interstices of 

 the rocks forms an underground reservoir, the amount stored away 

 depending on the porosity of the rock. When the interstices take 

 up a considerable proportion of the volume occupied by the rock, 

 the amount of water that can be stored is proportionately great. 

 The rock is then said to be " porous " or " permeable," while, if the 

 opposite conditions prevail, the rock is more or less " impermeable." 

 The degree of coarseness of texture is also to be considered, for if 

 the interstices are very small, the retarding effects of friction and 

 capillarity come into greater play, and in this way certain fine- 

 grained rocks, such as clay, act as highly impermeable strata not- 

 withstanding the fact that they can absorb a certain amount of water. 

 On the other hand it may also happen that rocks of an impermeable 

 texture may act as porous masses owing to their fissured or cavern- 

 ous conditions. Sandstones and conglomerates constitute the most 

 usual instances of porous formations. Some kinds of non-indurated 

 calcareous rocks, such as chalk, are also fairly permeable. Clay is 

 a well-known example of an impermeable medium, while the same is 

 usually the case with crystalline sandstone or " quartzite," crys- 

 talline limestone, and the generality of igneous and metamorphic 



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