The Bocks of Tristan cVAcunha. 37 



stone boulder, and the rest of the material was granite and crystalline 

 schists, as witness the nature of the Cave Sandstone, the mass 

 resulting from the fusion together of the basic and acid types of rock 

 resulted in the production of moderately basic types of igneous rocks, 

 which are the lavas of the Drakensberg. 



In the Sutherland and Kimberley pipes I have said that they are 

 probably connected with the intrusion of the dolerite. My line of 

 argument is that, during the deposition of sediment in Karroo times, 

 the floor on which they rested was obliged to sink. In doing so, 

 the deeper layers stretched, and being already consolidated, they 

 tended to crack. The rock at such great depths is in an unstable 

 condition, half too refractive to melt under the pressure, and half 

 sufficiently fusible to do so, hence the material that became molten 

 was forced up into that which had fractured under the pressure, and 

 the dolerite of the Karroo w T as thereby injected into the strata along 

 pre-existing cracks formed by the bending of the sediments. The 

 age of this intrusion is fixed between fairly narrow limits; it was 

 after the deposition of the Ecca beds," for they are found south of 

 the folded mountain region in Robertson, and took place before the 

 close of the Cretaceous period, for borders of dolerite occur in the 

 coarse conglomerates of that formation in Pondoland.f 



The whole series of Karroo rocks now became heated by the 

 addition of so much intensely-hot material, and there are exceedingly 

 interesting phenomena connected with this stage, as, for instance, 

 the distillation of coaly material into vertical cracks at Leeuw River's 

 Poort and in the Cambdeboo Mountains. J Heated rock, like every 

 other kind of material, expands, and, as a result, the rocks exerted a 

 tremendous lateral push. 



There is a great deal of doubt as to how far a lateral stress can be 

 communicated along a mass of rock. In experimental examples the 

 most interesting is Mr. Reade's observation on terra-cotta copings 

 set in cement. These, forming the top of a wall, were exposed to 

 the sun's rays, and expanded, pushing the neighbouring one aside or 

 riding over it. § The same effect is shown by the same author in his 

 book on the "Evolution of Earth Structure," in the case of a sheet of 

 lead, where prolonged alternations of heat and cold resulted in the 

 raising of an anticlinical ridge, the whole effect of the expansion being 

 transmitted along the sheet of flexible metal which was not held 

 down by any cement, yet the relief took place along one line only. 



* Ann. Kep. Geol. Comm., 1896, p. 28. 



f Ibid., 1902, p. 44. 



\ Ibid., 1902, Cape Town, 1903, p. 16. 



§ Geol. Mag., 1888, p. 26 ; " Evolution of Earth Structure," 1903, p. 204. 



