TiiE FAULT SYSTEAIS IN THE SOUTH OF 

 SOUTH AFRICA. 



Bv Professor Ernksi' II. 1.. S( iiw.\rz, A.R.C.S.. F.G.S. 



If it were asked in what partictilars the work of pioneers 

 of S<uith African geology like Andrew Bain and E. J. Dunn 

 differed from that done by the Geological Surveys, it might be 

 said that, apart from tlie greater detail of the latter, the syste- 

 matic survey takes more fully into consideration the question 

 of faults. It is very easy to mark a fault on the geological map; 

 in fact, it is a general rule in the field that when in difficulty put 

 in a fault, and it can be said that the full recognition and proper 

 use of this principle were first apjjlied in South Africa by 

 the Geological Survey of Cape Colony, established in 1896. 

 The fault system has now been more or less completely mapped 

 in the Cape Province, and the fractures occiUTing in the various 

 localities have been described, but so far there has been no 

 general review of the faults taken by themselves, nor of the 

 interrelationshi]) between faults of various periods and between 

 the faults and the fold system. 



A few general remarks are necessary in introducing the 

 subject in order to make clear in what sense we are using the 

 various terms employed. In the first place, a fault is here 

 understood to be a normal fault caused by a sinking of a part 

 of the earth's crust along a certain plane : it is due to a tangen- 

 tial stretching of the earth's crust, whereby a block falls down, 

 and it is the direct opposite of a fold, which is produced b>' 

 tangential pressure resulting in the squeezing upwards of a ridge. 

 A fold may break along the axis, and one portion may be 

 pushed over the other, but such reversed faults are more pro- 

 perly called thrusts, and have nothing in common with the 

 normal tension fault. 



In the second place, a fault being the result of a sinking 

 of the ground, somewhere in the neighbourhood of any given 

 fault there must be another fault ; that is to say, if we come 

 across a fault runm'ng east and west with a down-throw to the 

 south, somewdiere to the south of the line of outcrop, we must 

 find a second fault with down-throw to the north. The simplest 

 case is in the Great Rift Valley of Central Africa, in which 

 one looks across from the one plateau to the other, and in be- 

 tween there is a strip of country let down vertically several 

 thousand feet, between two faults, yet having all the surface- 

 features the same as those on the plateau of which it once formed 

 a part. Now, this second fault need not necessarily be a direct 

 break, which is the essential feature of a fault. Supposing the 

 .strip of earth'.s crust sinks along a certain line, it may happen 

 that instead of breaking away on the far side, the strip may 

 simply bend without breaking, hingeing, as it were, on a line 



