T,C)^ THE FAL'LT SYSTEMS IN SOUTH OF SOUTH AFRICA. 



which, with a greater tension, would become a second fault. 

 >uch a fold, having all the effect of a fault in that it lets down 

 ^i strip of the earth's crust, is called a monocline, because, un- 

 like other folds, it has an inclination in one direction only. 

 What I want to emphasize here is the fact, often lost sight of 

 in practice, that a fatilt must have a companion, or counter- 

 fault, or a monocline in the near neighbotirhood. 



If the earth is pulled apart, and two slips are faulted down 

 with a zone lietwecn them that remains tmatifected, this zone 

 will become relatively higher than the rest; it will constitute 

 a block-motuitain, or horst. This peculiar structure is common 

 in the plateaux of Utah and Colorado, and the gridiron country 

 of von Richthopen in China, and is exemplified by the fault- 

 block of Madagascar, which has a fault in the east and a mono- 

 cline in the west. In the area with which we are dealing, there 

 are no examples of block-motmtains. 



In the third place, faults have a limited horizontal exten- 

 sion, which is self-evident, as otherwise they would go right 

 roiuid the world. The way a fault dies away, however, is not 

 so obvious. At Worcester, for instance, Ave tind the l^cca beds 

 brotight down on a le\el with the Alalmesbur\- clay slale-~ ; at 

 Robertson the Dwyka Conglomerate, luiderlying the Ecca, 

 touches tlie Aialmesbury beds ; further east, the Witteberg 

 underlying the Dwyka Conglomerate, and still further cast, the 

 Bokkeveld luiderlying the \\'ittel)erg, and then the Table Mnuii- 

 tain Sandstone underlying the Bokkeveld, in turn come level 

 with the :\Ialmesbur\- Clay Slates ; finally, at Swellendam the 

 fault dies out with the Table Mountain Sandstone on the south 

 of the line of fault-arching over the Malmesbury beds, and 

 joining the Table ^fountain Sandstone on the north of the line. 

 The downthrow at W<^rcester is two miles vertical on the south 

 side, and this is reduced gradually in the east to nothing. It 

 will be found in the secjuel that the post-Cretaceous faults have 

 this pro}:)erty of disappearing very markedly developed. A 

 fault in this system, after l)eginning (juite abruptly along a cer- 

 tain line, lets down the faulted area to its maximum within a 

 short distance of its commencement, then as abruptly jjeters out. 

 leaving the grotmd quite ttnaffected for a certain distance ; then 

 the fault appears again, the same downthrow occurs, and the 

 fatilt again peters out. and so on repeatedly. This may be, per- 

 haps, better illustrated by taking the (ireat Rift \'alley again; 

 then the floor is sometimes very deep with great lakes, such as 

 Tanganyika, in the bottom, and further on the bottom rises, and 

 the rift apparently dies out, only to recommence after an interval. 



The relationship between faults and folds is never rigidly 

 uniform ; generally we can state the relationship in terms of 

 time and space. For the first, we may say that a fault usually 

 follows a fold. W'hen pressure accumulates in the earth> 

 crust, and a folded mountain range is produced, the resulting 

 fabric is usually greater than the pressure warranted, and as a 



