362 ORIGIN OF RAND T.AXKETS. 



panying it as a second phase of intrusion. At Komgha anci 

 Kentani these form great parallel dykes which, weathering more 

 readily than the surrounding rocks, score the land as if by a 

 furrow ; the depression of these diorite dykes is known as the 

 Transkei Ciai). '^ '^^ eastern dolerite, again, is characterised by a 

 little black mica, whereas the western dolerite is frequently 

 accompanied by quartz, granophyrically intergrown with the 

 felspars. The difference in petrographical character, however, 

 is not sufficient lo found any argument upon. 



The eastern portion of the Karroo laccolite is pierced by the 

 Drakensberg volcanoes, whereas the western portion is pierced by 

 a large number of volcanic chimneys containing for the most 

 part ultra-basic lavas; such as the Kimberley and Sutherland 

 pipes. The Transvaal laccolite is similarlv pierced by the vents, 

 as yet undiscovered, from which were poured out the volcanic 

 ash of the Springbok Flats and the lavas of the lUishveld. which 

 are held to be equivalent to the Cave Sandstone and Drakensberg 

 lavas. There is something, however, about the relationship of 

 the dolerite to the Karroo sediments which has not yet been 

 explained ; wherever one finds the Karroo sediments, there is 

 dolerite, and this often far beyond the possible area of the 

 Karroo laccolite. For instance, in German South-West Africa 

 the Karroo sediments were probably formed in a separate basin 

 altogether, yet the dolerite sills and dvkes accomj^any the sedi- 

 ments. Tt appears as if the peculiar mud out of which the 

 Karroo sediilients .'are formed ran casilv into a molten condition 

 as dolerite.'' ;,:■' , -■ 



The 'basitlfe S:>|: the Drakensberg is of the same composition 

 as the 'f^artoii, dolerite, :possibly because it represents the same 

 matelrial melted up but the fact brings in a good deal of con- 

 fusioi*. as the lavas are traversed bv dolerite dykes indistin- 

 guishable from the earlier dolerite of the great laccolite, yet 

 clea 1*1%' belonging to a subsequent date. 



l^i'e- 'dolerite belonging to the great laccolite occupies a 

 superficial- area of some 700 miles long Ijy 200 miles broad; the 

 individual sills are some lOO to 500 feet thick, and these, with the 

 connecting dykes and laccolites. would, if melted up, form a ball 

 50 miles in diameter. Is all of this new material which has been 

 thrust in, or is some of it, the sedimentary rock.^ assimilated with 

 a proportion of original igneous rock? T think there can be no 

 doubt that the latter is what has happened, because the dolerite 

 sheets and dykes never show any signs of iTaving thrust aside the 

 rocks. A dyke a hundred feet or more in thickness may come 

 up at a high angle and then bend over to follow the bedding 

 plane of the sediments as a sill; now if the dolerite were entirely 

 new material that had not melted out its cavity, there should be 

 visible above tlic bend of the dyke to the sill some continuance 

 of the crack up which the dolerite came, but there is none, as far 

 as T have seen, in the Karroo. Dykes mav occui:)y fault-planes. 



