Underground Water in South- Eastern Bechuanaland. 255 



Experience has shown that a well sunk within or upon the edge of 

 a pan usually gives a good supply of water at a shallow depth. The 

 reasons for this are threefold : firstly, the underlying rock, being 

 kept moist, tends in consequence to decompose and become more 

 porous ; secondly, as the level of the pan is often at quite an 

 appreciable depth below the surrounding country, moisture gravitates 

 towards the depression; and thirdly, owing to seepage from the pan 

 itself, the level of the water-table beneath and immediately around 

 the pan is usually higher than elsewhere. 



For abundance of supply it would be difficult to beat the little 

 depression on the farm Water Pan between Vryburg and Genesa ; 

 the underlying rock consists of a very decomposed and highly porous 

 granite. 



Brak pans, which are so characteristic of that portion of the 

 Kalahari north of Upington, are practically unrepresented in this 

 district ; the exceptions are Groot and Klein Chwaing. 



(5) The junction of tivo formations. — It is rather surprising that so 

 little advantage is taken of the occurrence of water at the junction 

 of two dissimilar sets of rocks. For example, the base of the Black 

 Eeef Series is markedly water-bearing, and fine springs occur at 

 Vryburg and at Motiton. 



There is one drawback to boring, namely, the excessively hard 

 formation, which is almost impenetrable with ordinary drills. 



Effect of the formation. — Over most of Bechuanaland the soil is 

 sandy and of considerable depth, yet there appears to be but little of 

 the water stored in it that can be made available. Consequently it 

 is to the underlying rock that one has to turn, and as the nature of 

 it may vary it is of great importance to discover the effect of the 

 geological formation. 



This will be briefly considered below : — 



(a) Granite and gneiss. — This formation is hardly ever well 

 exposed, and is usually covered with a mantle of reddish 

 sandy soil. 



As a rule the rock is a well-foliated muscovite-granite or 

 muscovite-gneiss, with the foliation planes dipping at a high angle. 

 The rock may be veined by pegmatites or traversed by quartz-reefs, 

 both of which will probably influence the movements of under- 

 ground water. 



The granite and gneiss are more or less decomposed, and this 

 alteration extends below the surface to a rather variable depth ; the 

 more micaceous gneissic varieties are usually altered to a greater 

 degree than the compact unfoliated granites. 



