ROCK-FOLDS 575 



the Zwartberg Mountains ; they form the innermost of the 

 coast ranges of the south of Cape Colony, and extend from 

 Ladysmith, in the Colony, past Oudtshoorn and Prince Albert, to 

 Willowmore and Uniondale ; that is to say, due east and west for 

 a length of 170 miles. We can obtain some idea of the nature of 

 the broad folding by laying an S on its back, thus : s. en n., the 

 syncline being on the south side and the anticline on the north. 

 External to the anticline there are added, in places, several sub- 

 sidiary folds which broaden out the range. On the south side 

 there is a subsidiary anticline which, however, is only seen at the 

 ends; in the middle, in the greater portion of the range, this 

 anticline has been denuded away and the southern limb is found 

 at a distance of several miles away. The rock is mostly coarse 

 sandstone, the Table Mountain sandstone, bedded in various 

 degrees of thickness ; in one part, right in the heart of the 

 mountains and almost inaccessible, there is a narrow valley filled 

 in with the overlying Devonian shales. The ends plunge down 

 beneath younger strata. 



(a) Folds in which the beds fail to adhere in the angle. — Where 

 well-bedded rocks are sharply folded there are frequently found 

 spaces which are sometimes gaping, at other times filled in with 

 veinstone and ore-bodies, as in the Bendigo goldfields and the 

 Broken Hill mine, in Australia; but the feature I wish to illus- 

 trate here is one in which a series of beds have been bent, and 

 have, as it were, acted as a nut-cracker on the rocks internal to 

 them. This pre-supposes a certain tensile strength for the rock, 

 which is a remarkable feature, considering the material is being 

 folded at the same time that it is exerting this property. If we 

 take a bar of iron and heat it red-hot in the centre so that we 

 can bend it, we reduce its cohesion, but in these rock-folds the 

 cohesion remains apparently the same during as well as before 

 and after the movement. 



A magnificent illustration of this is shown on the east side 

 of the north end of Meiring's Poort, a tremendous gorge in the 

 mountains. The fold is a gigantic "knee-bend," that is, a fold 

 with the limbs straight, instead of being curved ; in this case 

 one limb is horizontal and the other is turned under through an 

 angle of 135 ; that is to say, the whole forms an overfold. The 

 face exhibiting the feature is practically vertical, and is, roughly, 

 2,000 ft. high. It will be seen that there has been a strong 

 tendency for the beds to adhere to one another, and to retain 



