Underground Water in South- Eastern Bechuanaland. 253 



water ; curiously enough, water appears to be absent in certain 

 reaches and is found again at points further down the valleys. 

 Why this should be so is not evident ; possibly the water may 

 make its way beneath one or other of the banks either along an 

 old and now-buried channel or along cracks and fissures in the 

 rocks. 



According to Penning," a somewhat analogous phenomenon exists 

 in the lower portion of the Molopo Eiver north-westward from 

 Kuruman; his notes on the underground water of this part of the 

 Kalahari, and also of the region still further to the west, afford very 

 considerable and interesting information. 



A rather curious feature, confined to the granite area, is the 

 occurrence of what are known as " sand-wells." By sinking a pit 

 in the sand a little water collects at the bottom ; on deepening the 

 excavation the water-level does not remain stationary, but falls con- 

 siderably, and the supply is soon exhausted. These " sand-wells," 

 curiously enough, are often productive at points quite high up on the 

 sides of granitic ridges. 



Ground water. — From the soil the seepage gradually makes its 

 way downwards into the underlying rock, and the depth to which 

 the water will penetrate will depend upon the nature of the rock, its 

 porosity, degree of decomposition, the existence of joints, &c. 



In addition there is the slow but regular movement of the ground 

 water towards those parts of the country having a lesser elevation. 

 In this the movement is aided by fissures, channels, or " veins " of 

 varying width, along which the water can more readily travel than 

 through the pores of the rock. Though it is an open question 

 whether these " veins," which make water-boring in many cases so 

 uncertain an operation, can be located by human agency, there are, 

 nevertheless, various surface indications that may influence a person 

 in making a selection of a site. 



(1) Topography. — It is usual, of course, to choose a site at as low 

 a level as will be convenient, yet sometimes, paradoxical as it might 

 appear, this may not be the best situation. For example, supposing 

 we have the common case of a wide flat crossed by a deepish river- 

 valley, it is not unusual to find that a well sunk near the centre of 

 the flat will give a better supply, and that also at a shallower depth, 

 than will be obtained at points on the slope not far from the river- 

 bed. This is due to the rapid fall of the water-table towards the line 

 of drainage below the river channel. 



* W. H. Penning, "Gold and Diamonds," chapters iv. and v., London, 1901. 



