42 Transactions of the South African Philosophical Society. 



Dr. Sterry Hunt was led by an altogether different train of 

 reasoning to come to much the same result, namely, that volcanic 

 lavas are re-melted portions of sedimentary beds ; he considered the 

 water in volcanic rocks to be that which they had originally held 

 when deposited in the ordinary way," and that they were rendered 

 fluid by the under surface being forced into the region of rock fusion. 



In conclusion, I have brought forward evidence to show that 

 a continental ridge crossed the Atlantic from Africa to South 

 America; it probably extended from Cape San Eoque, through 

 St. Paul's rocks to Sierra Leone on the north, and from the south 

 of Brazil, through Tristan d'Acunha to the Cape of Good Hope on 

 the south ; it probably existed from Devonian to late Tertiary times. 



I have shown that oceanic islands are not exclusively volcanic, 

 but that the materials found among their ejactamenta contain 

 certain types of rock that are known as continental. 



I have shown that volcanoes are not caused by pipes that reach 

 to the profound depths of the earth and tap a problematical molten 

 magma, but are more probably due to the heat caused by the 

 crushing of rocks along structural lines. 



I have shown that the true normal original rocks of the earth's 

 crust are profoundly different from the basic rocks of volcanic 

 eruptions. It follows, then, that the Tristan d'Acunha group, 

 though volcanic, stands on a continental ridge, and the volcanoes 

 of which it is composed are the result of a fusion of continental 

 types of rock. 



And in regard to the permanence of ocean basins I have brought 

 forward clear evidence that continental types of rocks underlie the 

 great oceans, and that, therefore, the confinement of the waters 

 to their present area throughout geological time must be considered 

 to be unlikely. 



Tristan d'Acunha. 



Slide A dense stony nodule, slaty black, and mottled on the 



No 1139 . ... 



Basalt. ' o^side with little white rounded spots. 



Under the microscope : — 

 A very fine crystalline matrix remarkably even in texture, com- 

 posed of minute laths of felspar, all perfectly formed and terminated 

 by definite crystalline boundaries. The laths are all extended in 

 one direction, very few lying diagonally; they average '01 milli- 

 metres in breadth. 



The augite is in very fine granules, but frequently shows crystalline 



* Trans. Cambridge Phil. Soc, vol. xii. p. 414. 



