^/i< THE FAULT SYSTEMS IN SOUTH OF SOUTH AFKUA. 



three lines of fault-pits running in parallel courses, generally 

 east and west, with a southerly turn on tb.e eastern extremity. 



The direction of the linear series is east and west, a course 

 determined by the grain of the country — that is to say, by the 

 general direction of the folds of the older rocks. That there 

 should be a tendency to crack along these lines, and that the 

 cracks should be in pairs, so that they throw down slips of 

 ground between parallel faults, is a consequence depending on 

 the general fracturing of the Indo-African Continent. This is 

 too large a subject to be treated casually here, but we can say 

 without fear of contradiction that these fault-troughs have a 

 family resemblance to the great rift-valleys of Central Africa, 

 and hence we can assume, without launching too far into the 

 earth-movements that produced the rift valleys. I have stated 

 realm of speculation, that they were caused by the same set of 

 al)ove that there was a tendency to crack in this manner and in 

 this direction, but the tendency was only translated into actuality 

 when the lines of potential faulting were crossed b\- another 

 series of potential faults running in a north-easterly direction, 

 or in a direction parallel to the straight coast-line on the eastern 

 side of South Africa. These north-easterly faults, or rather 

 potential faults, were also trough faults akin to the rift-valleys. 

 ^Ve might put the case differently : each set of trough-faults 

 was in itself not powerful enough to tear through the structure 

 of the earth's crust, but where the two crossed each other, the 

 strength of the earth's crust Avas so reduced by the double strain 

 that it was unable to resist, and the fault-])it resulted. 



The feebleness of the fault-lines in the south-west of Cape 

 Colony indicates that they lie on the outer fringe of the great 

 fractures. The existence of such potential or feebly developed 

 fractures in the neighbourhood of the great fractures was 

 beautifully shown in Daubree's experiment in compressing a 

 column of semi-plastic material. The column was made of a 

 mixture of plaster of Paris, beeswax and resin, and was twice 

 as long as in cross section ; this was placed in a hydraulic press 

 and compressed in the direction of its length. The column 

 broke with diagonal fractures, the one large one starting from 

 the top left-hand corner to the bottom right-hand corner, the 

 other starting from the bottom left-hand corner and meeting 

 the first about the centre of its course. \Miere the two frac- 

 tures meet, there is shown this fringe of smaller cracks and 

 lines of distortion without actual break, running in two series 

 parallel to the main fractures, and drop])ing rectangular blocks 

 between them.* The main fractures of the Tndo-African 

 fractures occurred just after the Eocene l)ut liefore the Miocene 

 Period, and these smaller fractures in the soulh-west of Cape 

 Colon)'^ are of the same age. The two main trotighs or lines 

 of fault to which these latter are subsidiary, are represented 



* A Daubree, " Etudes de Geol. Experimentale," Paris, (1879) [2], Fig. 3. 



