THE INTRUSIVE DOLERITES AND ALLIED ROCKS 285 



tillite into regular columns traverse boulders and matrix 

 alike without deviation. 



The larger intrusive sheets of dolerite frequently show 

 columnar structure. Many examples of this can be 

 seen in the sheets which crown the Nieuweveld escarp- 

 ment in Beaufort West. Other good examples are 

 Tafel Berg (Beaufort West), Theebus near Steynsburg, 

 and Donker Hoek's Berg near Sterkstroom. 



Owing to the strong tendency to spheroidal weather- 

 ing that is characteristic of the dolerite, the intrusions 

 give rise to great piles of boulders. Thousands of 

 square miles in the Upper Karroo are covered with 

 boulders such as are seen in the foreground in Plate 

 XXI., which is reproduced from a photograph of typical 

 dolerite country behind the Nieuweveld escarpment. 

 Not infrequently the dolerite decomposes to a coarse 

 yellow-brown crumbly material which can be easily ex- 

 cavated. This is particularly the case wherever the 

 rock forms wide flat tracts of country, as around Kim- 

 berley for example. 



Occasionally portions of such a sheet weather much 

 more slowly than the rest, and a peculiar type of scenery 

 is produced in which masonry-like masses of jointed 

 dolerite rise in mounds from a plain of rotten crumbly 

 dolerite. This can well be seen at Pampoen Poort 

 between Victoria West and Carnarvon. Wells and 

 bore-holes in such decomposed dolerite usually yield good 

 supplies of water. 



The country occupied by the dolerite sheets is, as a 

 rule, more fertile than that formed by the sedimentary 

 rocks alone, for the dolerite contains valuable food 



