256 Transactions of the South African Philosophical Society. 



The reason is to be sought for in the different rates of expansion 

 and contraction of the mineral constituents, by which planes of 

 weakness are developed in the rock. 



The very well-banded gneisses, with regular layers of quartz, 

 felspar, mica, or hornblende, may have a high degree of fissility 

 imparted to them through this process, aided by mineral decomposi- 

 tion. This condition is favourable for the penetration and retention 

 of rain-water. 



In boring, as long as the core brought up shows cavities and 

 fissures, or the felspars are clouded or kaolinised, there is always a 

 possibility of obtaining water. If, however, the core shows a 

 perfectly sound, fresh, compact granite, it may be advisable to 

 stop. In some cases the apparently solid rock has contained 

 numerous minute cavities, hardly visible to the naked eye, and 

 the borehole has yielded a considerable supply. 



The more compact varieties of granite — and in this category we 

 may include quartz-porphyry — will in most cases yield but little or 

 no water. A formation of quartz-porphyry, such as that between 

 Vryburg and Genesa, should as far as possible be avoided. 



In boring or sinking spots should not be chosen where the granitic 

 rock forms marked outcrops, and by preference sites should be 

 selected where there are pans on the formation. 



Generally speaking, the granite is a most uncertain rock in which 

 to bore for water, and failures may continually be expected. 



A diamond drill is the best for the purpose, as a jumper will often 

 be unable to penetrate the less weathered varieties unless with great 

 difficulty. 



(b) The diabase formation. — This commonly forms flattish ground, 

 and from it excellent supplies of water are obtained as a rule, some- 

 times from remarkably shallow depths. Especially is this so in 

 depressions and pans and along stream-courses. 



The formation, which is composed principally of igneous material, 

 consists of layers of differing composition and texture, while much of 

 the lava is amygdaloidal. 



Down to a certain depth the diabase is full of cavities and fissures, 

 but beyond that, at a distance of a few hundred feet below the 

 surface, the rock becomes massive, and the finer-grained varieties 

 may be absolutely waterless. We have, for example, a dry bore- 

 hole over 500 feet deep in this formation at Vryburg Station. 



(c) The dolomite. — This, as a rule, provides the best water supply, 

 and fortunately is a formation of great thickness and one covering a 

 vast area. 



Water is usually met with not many feet below the surface, and 



