38 Transactions of the South African Philosophical Society. 



We have at least some evidence to go upon when we say that the 

 heating of the Karroo rocks produced an expansion which became 

 checked along the outer ridge of the structural basin, that is to say, 

 on the outside of the Karroo, where a zone of old schists, slates, and 

 granites exists. The lateral thrust expending itself against this buffer 

 forced up the strata just within it in a series of folds which, on the 

 south, form the folded mountain ranges of the colony. The thrust 

 could not have come from the south, as the granite bars the way for 

 the transmission of thrust in this direction. 



At the other end of the expanding strata, where they abutted 

 against the old pre-Cape rocks of Prieska and Griqualand West, 

 there was no buckling, but the immense pressure, relieved though it 

 was to the south by the formation of folds, resulted in some re- 

 adjustment of the strata, the transmission of the stress not being 

 perfectly free ; and it seems that it was the buffer of old sedimen- 

 taries and granite that was crushed and not the Karroo rocks 

 themselves (see Fig. 3). 



The tearing and rending of the rock masses resulted in the ultimate 

 fusion, the production of gases from the volatilisation of the water 

 and carbonates held in the rocks, and the consequent production of 

 the vertical pipes of Kimberley and neighbourhood. Very little truly 

 molten matter resulted from the action of the Kimberley volcanoes ; 

 in Sutherland Mr. Eogers has recorded a good deal of melilite-basalt 

 as occurring in the vents, but, in the great majority of cases, the pipes 

 are filled with material that has merely been blown to fragments 

 without fusion. 



Looked at, therefore, in the light of actual evidence, Tristan 

 d'Acunha with its volcano bringing up materials which show that 

 it rests on a base of continental rocks goes far to add one more nail 

 in the coffin of the theory of the Permanence of Ocean Basins, wilich 

 the researches of Lester in the Tonga Islands and Woolnough in the 

 Fiji Islands had already assailed along similar lines, a theory which 

 has long held the field from the brilliancy of its advocacy by Mr. 

 Wallace. 



Looked at from the theoretical standpoint which our researches on 

 the volcanoes of South iVfrica have led us to, namely, that volcanic 

 action is not a deep-seated one, but the resultant of earth move- 

 ments, the intensity of which, concentrated upon certain fusible 

 portions of the strata affected, such as the limestones and dolerites 

 and schists heavily charged with iron, gneisses and hornblende 

 schists, melted these in some cases with the production of lava, and 

 simply blew them to pieces at others with the formation of such 

 rocks as the Cave Sandstone. Not only does this conception of the 



