l88 LACCOLITES AND BYSMALITES. 



Beaufort West ; here the crack comes up vertically for a distance, 

 then follows the bedding planes horizontally, and finally strikes 

 upwards and pinches out. Such a cavity possibly started before 

 the dolerite came up, but whereas a narrow fissure of this kind, a 

 few feet wide, produces no perceptible alteration in the beds, 

 when it becomes enlarged to a hundred or more feet wide to 

 accommodate one of the great dykes, there would have resulted 

 a considerable disturbance if the dyke merely pushed aside the 

 country rock. The dykes come up with remarkably parallel 

 sides, and pass off into sheets, sometimes even stopping dead at 

 a particular horizon with a square termination. Only once has 

 a dyke been seen coming up along the plane of a small fault, but 

 this was in Matatiele. where the sandstones play a far more 

 important part than in the Karroo proper. The best evidence, 

 however, is afforded by the laccolites of the Transkei. where the 

 sedimentary rocks are seen abutting directly against the sides of 

 the laccolites as if they had been shorn off. In the Quizzyhota 

 laccolite along the Kei River, near the railway bridge, the vertical 

 side of the laccolite facing the river is 1,500 feet high, and the 

 shales of the Karroo system lie quite unaltered up the vertical 

 face; a certain amount of thermal alteration has gone on in the 

 immediate neighbourhood of the dolerite, but there is no dis- 

 turbance of the stratification. In this particular laccolite the 

 mass of igneous rock has an elevated ridge running parallel to 

 the river front, and a short way from the vertical face the main 

 mass of the laccolite runs into the hills with horizontal upper 

 surface some 80 feet below the elevated crest. The strata lying 

 upon this lower portion are horizontal, and in one of the kloofs 

 there is a sheer cliff exhibiting a section of where these strata 

 abut upon the rising dome of the elevated crest, and one can see 

 perfectly plainly that the ends of the strata abut on the face of 

 the dolerite as if they had been cut away to receive it. 



If it is possible, then, that the dolerite melts out the cavity 

 which it subsequently occupies, what becomes of the surplus 

 material ? It is out of the question to suppose that the sedi- 

 mentary rocks should spontaneously melt and change to igneous 

 rock, for the dolerite, which is the commonest dyke rock in South 

 Africa, is invariably more basic than the shales into which it is 

 intruded. 



Compare an average shale and an average dolerite : — 



Slate. Dolerite. 



Silica 54 per cent. 50 per cent. 



Alumina 13 T4 



Iron oxides 9 \y 



Lime 7 10 



Magnesia 6 6 



Potash 1 1 



Soda 2 2 



Carbon dioxide 4 



Water 4 



