582 SCIENCE PROGRESS 



has been raised above its supporting strength, that is, the 

 weight of material piled vertically above the sandstone has been 

 sufficient to crush it. The rock having been wet, the pressure 

 has been accommodated by folding, not on a horizontal plane 

 such as we are familiar with, for instance, in the Jura Moun- 

 tains, but on a vertical plane, and as a result we have a 

 miniature mountain range standing, as it were, on end. Seen 

 from the point of view from which the sketch is taken, the 

 folding appears to have been actually vertical, but on either 

 side one may see similar folds whose planes lie inclined to the 

 vertical and fan out from a centre near the level of the river. In 

 the minor folds there are evidences of horizontal faults which look 

 like thrust-planes due to tangential pressure, but I regard them 

 as slip planes produced normal to the pressure, in the same 

 way as slaty cleavage develops, that is, by compressional flow. 



In offering the above illustrations I have been guided by 

 the desire to select examples which have more than a local 

 or purely geological interest. The study of rock-flow has a 

 very important bearing on the true understanding of the 

 strength of materials, and our South African folded mountains 

 contain a vast number of instructive examples. The geological 

 survey of the Cape Colony is carried out on too broad lines 

 for details such as these to be adequately studied, but the 

 questions of rock-flow have been so prominently brought to 

 the notice of geologists by the writings of Van Hise, now 

 collected together in that author's treatise on Metamorphism, 

 that even while occupied in tracing boundary lines the examples 

 in the Zwartberg Mountains could not be overlooked. 



