LTDENBITRG AND DE KAAP, TRANSVAAL, SOUTH AFRICA. 039 



shales and sandstoues through which it often passes ; and this fact 

 gives rise to definite physical and surface-features. 



Very prominent objects in the immense plains of South Africa, as 

 also indeed amongst the mountains, are the long narrow ridges of 

 what at first sight would appear to be water worn boulders, but 

 which are really cores from the concentric weathering of diorite 

 dykes. Beneath them the whole of the dyke is often entirely 

 disintegrated into a soft but compact argillaceous mass, passing 

 gradually down into hard unchanged diorite. I have seen a good 

 illustration of this where one of the head streams of the Krokodil 

 Eiver passes through a deep gorge, the almost vertical sides of 

 which expose a good section of decomposed diorite, capped by a 

 mass of rounded cores that spread out beyond the walls of the dyke 

 on either side to some distance. I think the rock weathers more 

 rapidly below the surface ; it gets washed away beneath the harder 

 cores, which settle down and accumulate along the line, repre- 

 senting probably several hundred feet of the dyke, gradually 

 weathered and removed by denudation. 



When no line of cores has been left, which frequently happens 

 on sloping ground, the course of a dyke may still be traced merely 

 by changes in the vegetation : a greener tint in the grass — which 

 is sometimes quite verdant, whilst that on the shales has been 

 parched to a yellow or brown colour — by lines of bushes, by 

 the growth of diflPerent kinds of plants, and so on. Single or 

 parallel lines of dykes may sometimes be detected, even at a great 

 distance. 



Where a dyke crosses a watershed from one valley to another, a 

 tributary stream usually cuts its course along the weathered 

 surface into each main valley until the two nearly meet at the 

 watershed, leaving only a narrow " neck '^ or " pass " between. 

 The rain-collecting area of such necks being reduced to a minimum, 

 they are now subject, as ridges often hundreds of feet above the 

 valleys below, to the least possible action of denudation. A good 

 illustration of this phenomenon is the neck, about a hundred yards 

 wide, at the head of two very deep gorges, one opening north, the 

 other south, just east of the townlands of Lydenburg. On the road 

 to Spitzkop there are four or five such necks (or weathered dykes) 

 almost close together, being perhaps a hundred yards apart, many 

 hundred feet above the ravines, and just wide enough for a waggon- 

 road, with small rounded hills between them ; these are called the 

 " DeviFs Knuckles." 



