34 Transactions of the South African Philosophical Society. 



tinder water, so the parallelism between the two rocks is further 

 brought out from the history of the theories regarding their origin. 



I have brought these facts together to show that in the Tonga 

 Islands, in Tristan d'Acunha, and in Ascension there are signs 

 of volcanic tuffs made up of non-volcanic material like the Cave 

 Sandstone, and that the dust of which they are composed must 

 have been torn off cold from the throat of the volcano, and that 

 the rocks which surround the throat are of ordinary continental 

 types of rock. 



To pass these facts in review, we find that we can commence with 

 an example like the Island of Trinidad or the Falkland Islands, 

 which stand, without doubt, on a continental shelf. Then we have 

 an island like South Georgia, or New Caledonia, which, though sur- 

 rounded on all sides by open water of 1,000 to 2,000 fathoms depth, 

 yet bear such unmistakable signs of belonging to a continental 

 area that there has never been any doubt of the fact. Then we have 

 the Fiji group, an isolated collection of volcanic peaks separated 

 from New Caledonia by a wide stretch of deep ocean, still unmis- 

 takably showing their continental origin from the nature of the base 

 on which they stand. Then we have the Tonga group, as truly 

 oceanic as any island can be, still affording evidence of having a 

 continental base. And, finally, the Lesser Antilles, showing no direct 

 evidence of continental origin, yet standing on a ridge, the two ends 

 of which come to the surface at Trinidad and Puerto Eico, and show 

 that the whole is composed of continental types of rock. 



With such evidence before us, can we still assert that the oceanic 

 islands are volcanic cones rising from the abysmal depths of the 

 sea, and that the ocean floor from which they spring is made of a 

 type of rock different from that which forms the continents ? For 

 my part I think that we have sufficiently clear 'proof that they all 

 spring from continental ridges, and that the outer crust of the earth 

 is uniform in composition throughout the continents, and throughout 

 the area covered by the ocean. 



What, after all, are volcanoes ? If we fix our attention on those in 

 eruption, or which are still in the condition in which they were when 

 they were active, we can easily be led to believe that they are vents 

 that reach down to the molten, or potentially molten, interior of the 

 earth. But when we come to examine the volcanoes of an earlier 

 age, and those which have been dissected by long continued pro- 

 cesses of denudation, we come to a deep-seated base of plutonic 

 rocks, granites, gabbros, &c, and we come to regard the lavas as the 

 more easily melted scum that is driven off from an immensely 

 greater area that remains behind to re-solidify slowly in the great 



