39© Report S.A.A. Advancement of Science. 



similar result, are where the water enters the coarse stratum at its 

 outcrop on the surface and flows down the dip of the beds, being 

 confined to the porous strata by impervious material above and 

 below, until it reaches an obstruction to its course, such as an igneous 

 dyke, or a portion of strata which has graded into a more compact 

 rock whicii gives no passage to the water, or a fault cutting it off 

 by an impervious rock barrier, so that being tapped at a point where 

 the surface is lower than at the catchment outcrop, the water rises 

 almost to the same level as the intake. In the remaining two-thirds 

 of the artesian wells these conditions either do not exist or are so 

 modified that the pressure is not sufficient tO' cause the boreholes to 

 flow at the surface, and windmill pumps or other means are required 

 to raise the water. 



WATER-BEARING HORIZONS. 



The question of underground water supply has so recently 

 received attention in South Africa that our knowledge of this great 

 and important subject, in its relation to this country, is naturally at 

 present limited, but as the number of artesian wells increases so will 

 the knowledge of water-bearing horizons in the various geological 

 formations become enlarged, so that there is a strong hope these may 

 be determined definitely in certain regions of the Colony. 



SELECTION OF SITES. 



In most cases the selection of sites for boring is restricted, and 

 in some there is nothing that can be termed selection at all. Water 

 supplies for Public Buildings and Institutions are usually required on 

 the premises, and it often happens that if the site could have been 

 shifted a few yards a much less depth would have sufficed to reach 

 the water; or if the position of the borehole could have been moved 

 down the dip of the strata a short distance, flowing water would have 

 been tapped and pumping entirely avoided. Even on farms, where 

 the space is not so limited, the really good site is often just over the 

 boundary fence, and it is seldom that, in picking out sites for the 

 shallower bores, under 500 feet in depth, there is sufficient choice of 

 ground to give an adequate opportunity of applying the principles 

 of hydrogeology tO' advantage. These principles would suggest the- 

 careful examination of the surface indications — like the lines of 

 land drainage, the existence of bush or forest which impede the 

 flow off of the rain, the areas where percolation is probable, and the 

 existence of springs shewing where the surplus water below finds 

 access to the surface — before taking into< consideration the denuda- 

 tion and erosion the original land has undergone, and the deposition 

 of the resulting drift or alluvium ; or the effect which dip, strike, and' 

 structural jointing of the stratified rocks would have on the course ol 

 subterranean drainage, and that which the position and capacity of 

 the water-bearing formations would have on the lines of saturation.. 



