ECONOMIC GEOLOGY 481 



a "water-bearing" layer. All rocks are traversed by 

 joints, surfaces separating the rocks into large or small 

 blocks, and along some of these are developed cracks 

 which permit water to flow along them. Some of the 

 rain that falls on the ground soaks into it and flows 

 slowly along joints, perhaps escaping as a spring where 

 the surface cuts a joint at a low level. The success of 

 a well or bore-hole depends upon its intersecting these 

 joints below the level of the ground-water of the neigh- 

 bourhood. The surface of the ground-water varies in 

 position according to circumstances ; it rises after rain, 

 and sinks during drought in consequence of leakage 

 from springs, evaporation from the ground, and the 

 daily need of plants ; it is brought locally nearer the 

 surface by comparatively impervious bodies of rock, such 

 as dykes of dolerite or large veins of quartz, and this 

 fact is made use of by boring on the supply or up-hill 

 side of such obstructions. There is no ground for sus- 

 pecting the existence of a large body of water available 

 by deep boring (i.e., over 500 feet or so) in the interior 

 of the Colony, but there is no doubt that the prevention 

 of waste of rain-water, by obstructing sluits and the 

 encouragement of plant growth which will hinder the 

 flow of water over the surface during rain, will increase 

 the supply of ground-water to the benefit of both springs 

 and wells. This process is of the greatest importance 

 in the region occupied by the Karroo formation, where 

 the general shape of the country is well adapted for its 

 success. 



An interesting and extremely valuable collection of 



facts about the composition of spring and well waters 



31 



