THE FAULT SYSTEMS IN SOUTH UF SOUTH AFKICA. 3/1 



frequently represented by fissures, which have subsequently 

 become filled in with vein-material and now form ore-veins. 

 The larva-lilled fault-tissure of the Zuurljerg is a special case, 

 which I shall refer to separately. 



The Worcester-Swellendam fault is also interesting in illus- 

 trating the effect of material of different toughness in responding 

 to the tearing forces developed along the fault. As long as the 

 fault traverses normal sediments, slates and sandstones, the 

 fault runs more or less in a straight course, but at Robertson 

 there is a big dome of granite which has proved too massive 

 for the fault to tear through, and consequently the line nms 

 round this obstruction, showing on the west side an interesting 

 splintering in the rocks of the down-throw side. The effect of 

 the Robertson granite is that of a knot in a piece of plank that 

 is being split for firewood. 



In contrast to these clean-cut faults, which show no trace 

 on the surface except in the different geological formations 

 which require a practised eye to notice, vrc have the Baviaans 

 Kloof faults. Here the width of the strip let down is very 

 narrow, a couple of miles or so, and in some cases less. Most 

 of the faults have their counter fault in the form of a monocline. 

 It is obvious, therefore, if a great block of the earth's crust is 

 bent downwards in the line of the break, the top end of the 

 block will butt against the opposing side of the fault-plane, and 

 the lower portion will leave an open space. To illitstrate what 

 I mean, place a thick book edgewise against the wall and then 

 bent it downwards ; the top cover will scrape the wall, and the 

 lower cover will not be in contact with it. The consequence of 

 this movement is that along the fault-planes of Baviaans Kloof 

 we find immense zones of broken-up rock, where the earth- 

 block has scraped along the fault-plane. 



In the Zuurberg, at Mimosa, on the Port Elizabeth railway- 

 line, the result of a similar tilting of the sinking block is shown 

 in a band of lava which occupies the fault-plane together with 

 two small volcanic necks. When the block sank, the lower por- 

 tion separated from the stationary rocks on the north side, and 

 an open space resulted. Now, in the earth's crust no spaces can 

 long remain open. Small fissures are filled in by the deposit 

 of minerals, which constittite the ore-veins ; larger ones are 

 gradually closed by the operations of the law of viscid sub- 

 stances, for the earth's crust is, in spite of the apparent hard- 

 ness of the rocks of the surface, perfectly plastic to long-- 

 continued stresses. If, however, there is molten material ready 

 to hand, the molten rock rushes up the fissure, and either solidi- 

 fies as a dyke, or if the fissure is sufiiciently near the surface of 

 the earth, the up-rushing molten material forces an opening, and 

 a volcano results. I am still inclined to think that the frictional 

 heat caused by the grinding of the two segments of the earth's 

 crust against each other was the determining cause of the 

 Mimosa volcanoes, not. perhaps, causing the entire heat requisite 



