40 Transactions of the South African Philosophical Society. 



nature of volcanic action help us in the understanding of the South 

 African occurrences, but it is in accord with a large number of facts 

 which have previously been the cause of much speculation, such as 

 the small depth of the origin of earthquake tremors previous to 

 volcanic eruptions, and the production of mud-volcanoes vomiting 

 boiled fish. It also explains the situation of volcanoes like those 

 forming the Galapagos Islands on the points of intersection of two 

 sets of folds acting at right angles to each other, the tearing and 

 rending being naturally greatest at these structural nodes. 



The well-known fact that volcanoes do occur on structural 

 displacements can now receive a new interpretation ; no longer do 

 we conceive these folds opening great cracks that penetrated to the 

 molten interior of the earth, thus violating all the laws of Isostacy, 

 or what was known before the Americans called the equilibrium 

 of the earth's crust by this name, namely, the closure of fissures at 

 great depths owing to the pressure that the weight of rock exerts, 

 but we look to these structural lines as the actual seat of the fusion 

 of the rock, the motion and friction of the bending and shearing 

 being translated into heat, and the volcanoes thus naturally forming 

 •on these lines. 



The question of whether the sedimentary rocks and the older 

 crystalline series in their immense variety can, when fused together 

 in different proportions, give rise to the various kinds of igneous 

 rocks, is too large a one to go into here, and is a matter of 

 compilation of rock analyses, which, with skilful arrangement, could 

 be made to prove or disprove the theory. The only difficulty that 

 I have come upon in thinking the matter over is the great amount 

 -of titanium in our Kimberley and Sutherland pipes, but the difficulty 

 is of the same order as the presence of manganese at Wellington 

 and Hout's Bay, which has formed from solution and redeposition 

 from waters soaking out from the Table Mountain sandstone, a rock 

 that is stained blue with iron sulphides. 



There is a fairly general concensus of opinion that the view of the 

 interior of the earth as set forth by Prof. Judd in his edition of 

 Scrope's "Volcanoes," and endorsed by Prof. Suess," is correct, 

 namely, that the interior of the earth is composed of a heavy metallic 

 centre, surrounded by a siliceous slag. If volcanoes do, therefore, 

 penetrate to the great depths, would they not bring great masses of 

 metallic substances with them to the surface like the nickeliferous 

 iron of Ovifak in Greenland ? Not a single oceanic island composed 

 of volcanic rocks has yielded a mine of any sort of metal. 



That volcanoes have nothing whatever to do with the molten 

 * Nature, vol. lxiv. p. 629. 



