ROCK-FOLDS 573 



which accentuate the angular outcrop. Keeping our illustration 

 in view, this granite would represent the sides of the sink, 

 and the Neo-afric beds the layer of lead ; the Pal-afric beds 

 also would represent the wooden bottom of the sink, for they 

 pass under the Neo-afric series. The disturbing cause, the hot 

 and cold water of our illustration, is represented by a very 

 heavy intrusion of dolerite in the form of sheets in the central 

 portion of the Neo-afric region. The disturbance is represented 

 by several ranges of closely folded mountains which follow the 

 lines of the granite external to them. 



It was for a long while a matter of dispute as to whether 

 the force which buckled up these mountains came from the 

 interior or from the side of the ocean. But it has recently 

 been found that on the southern coast, a little to the east of 

 George, where the granite ends, there is a significant change 

 in the character of the folds. Where the granite bars the way 

 to the sea the folds in the mountains internal to it are closely 

 huddled together, and follow the granite outcrop in trending 

 east and west. Where the granite ends, the folds widen out, 

 and curve seawards. 



This feature on the south coast might be explained by the 

 granite having been thrust northwards as an immense ram into 

 the yielding strata, but then the same features exist on the 

 west coast, where the granite runs north and south. It is 

 unthinkable that two extended masses of granite should have 

 travelled, the one northwards, and the other eastwards, and have 

 clamped the later rock-formations between them. It has taken 

 ten years' continuous field work to prove this point, but it has 

 been accomplished. As regards the disturbing cause, the 

 dolerite intrusions, it is hard to convey any sense of the 

 magnitude of these. They cover an area of over 70,000 square 

 miles, and the separate sheets are ranged one above another, 

 sometimes forming laccolites, when the whole country for 

 miles is one black mass of dolerite ; more extensively they 

 occur as sheets, one of which is exposed in the western Karroo, 

 having a superficial area of 3,000 square miles, but the buried 

 portion of this was probably four times as much again. The 

 sheets, also, may be from 200 to 300 ft. thick. With so 

 great an amount of molten material thrust into the Karroo 

 rocks some effect must have been produced by the expansion, 

 letting alone the space occupied by the intrusions themselves ; 



37 



