254 Transactions of the South African Philosophical Society. 



As far as possible ridges and watersheds should be avoided, for the 

 area that can be drained by a well in such a position is invariably 

 small, and the proportion of rainfall absorbed by the soil is much 

 lower than on the flats. A troublesome strip of country is the main 

 watershed, and the wells along it are considerably deeper than else- 

 where. The normal scarcity of water is intensified by the existence 

 of a belt of compact quartz-porphyry. 



(2) Dykes, faults, &c. — It is not uncommon to find shrubs clustered 

 thickly together along a certain line with such regularity as to produce 

 a narrow belt resembling an artificial plantation. These "aars," as 

 the Dutch farmers call them, may extend in straight lines for miles, 

 and are due to several causes. 



Basic igneous dykes may give rise to these phenomena, and in 

 most cases wells sunk on their outcrops give good supplies of water. 

 The effect may be produced by quartz-reefs, but they are not very 

 plentiful here, and the underground circulation of water is therefore 

 not appreciably influenced by them. 



Sometimes, on sinking, no foreign rock is met with, but there is a 

 zone of crushed and decomposed material along which the water 

 makes its way ; this is not unusual in the granite area. 



In the dolomite region such " aars " form narrow ridges a few feet 

 higher than the surrounding country ; they are generally capped 

 with calcareous tufa, and support a thicker vegetation than round 

 about. 



(3) Calcareous tufa. — Patches of soil are very frequently found 

 covered by a deposit of calcareous tufa of greater or less thickness. 

 In many cases this points to the presence of water at no great depth. 



In the dolomite area, and again in that occupied by diabase and 

 amygdaloid, the carbonate of lime is derived by direct solution from 

 the former rock, and by the decomposition of the minerals in the 

 latter. Moisture containing the carbonate in solution is drawn up 

 to the surface by capillary forces, and by evaporation the tufa is 

 formed. In many cases the rims and floors of pans are composed of 

 a similar deposit. 



Sometimes, however, the carbonate of lime is brought down by 

 rivers which take their rise in a dolomite area, and the tufa so formed 

 is deposited upon any kind of rock, and may therefore be no indicator 

 of underground water. 



(4) Pans and vleys. — These depressions are extremely numerous 

 throughout the district, and vary from a few yards to over a mile in 

 diameter ; they exist on nearly every formation. 



These pans hold water for a certain period during the rainy season; 

 all of them dry up during the winter. 



