ECONOMIC GEOLOGY 483 



In well sinking advantage is commonly taken of 

 what are known as " aars," experience showing that 

 water is usually got at shallower depths there than 

 elsewhere. " Aar " is the name given to any feature on 

 the surface which is very long compared to its breadth ; 

 thus it may be applied to the outcrop of a dyke, to a 

 low ridge of limestone, to a slight depression probably 

 marked by a different soil from the surrounding veld, or 

 to a line of country characterised by denser vegetation 

 or by a particular kind of bush. In the Karroo beds 

 the dolerite dykes are of great utility, especially where the 

 formation contains thick close-grained sandstones ; the 

 value of the kimberlite dykes as a source of water supply 

 has already been referred to. 



It has always been a rule to avoid boring in dolerite, 

 nevertheless in the Northern Karroo many of the horizon- 

 tal or slightly inclined dolerite sheets are decomposed to a 

 friable sandy material and produce no marked feature. 

 Wells in such dolerite commonly give good supplies, 

 and the water is almost invariably of better quality 

 than that obtained from the adjoining Karroo beds. 

 Owing to the nature of the partly decomposed igneous 

 rock it is more satisfactory to put down a well than to 

 attempt a bore-hole. 



In the great dry sandy region of the Kalahari the 

 scanty ground-water is so held by the deep sand that it 

 is not available for collection in wells, and supplies can 

 only be expected in places where the sand is thin, as in 

 the neighbourhood of the German border, and in the 

 tufa-choked beds of the Molopo and Kuruman Rivers.. 



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