390 Transactions of the SoutJi African Philosophical Society. 



crevices in the rock are bound to close at no very great depths owing 

 to the immense pressure of the superincumbent mass. Prof. Suess"^' 

 has been led from these difficulties to postulate that w^ater never 

 finds its way from the surface to the depths were high temperatures 

 exist, and he maintains that the water in hot springs, like those of Brand 

 Vlei near Worcester, and geysers, are the occluded vapour that 

 is being given off by the cooling globe, so that every fresh volcanic 

 eruption bringing up its immense quantities of water-vapour, and 

 every hot spring, increases the quantity of water available at the 

 surface of the globe. The Eev. O. Fisherf has explained how the 

 water-vapour may have originally got into the molten rock. He 

 supposes that when the earth was still a liquid body, the atmosphere 

 consisted of the volatile elements, for the most part water, that are 

 now found cooled on the earth's surface. Taking the present bulk of 

 water and volatilizing it, it is found that the gaseous envelope thus 

 formed would produce a pressure of 327 atmospheres at the surface 

 of the liquid globe, a pressure quite sufficient to force a considerable 

 amount of water-vapour into the liquid rock, on the principle of 

 forcing carbonic acid into water with a sparklet. Even when the 

 pressure is released in aerated water, a considerable amount of 

 gas still remains held up in the liquid, and this is much more so 

 in liquid rock. ~ . 



I had occasion to notice this latter phenomenon in the lavas 

 of the Drakensberg which have recently been mapped by the 

 Geological Survey (pi. vi.). Here there are many thousands of feet of 

 lavas piled up one upon another, and nearly all characterised by a 

 great abundance of ^steam-holes. It is well known that lava, when 

 ordinarily poured out on the surface, gives off water- vapour freely, 

 but usually the lava solidifies into a more or less solid mass ; it is 

 only in particular cases, when the molten substance is cooled rapidly, 

 that the water-vapour is retained in the rock, and one gets all 

 varieties of cavernous lavas from pumice to the so-called amygdaloids. 

 In the Drakensberg lavas, however, the top layer of any particular 

 flow is usually fairly free of these blow-holes, showing that the 

 water-vapour originally contained in it had been got rid of. The 

 deeper layers, however, which cooled more slowly, are full of little 

 hollows, and the bottom of most of the lava-flows are permeated 

 with long tubular vesicles of the thickness of a lead pencil, and 

 closely crowded together. This indicates that the water-vapour 

 must have been occluded in the molten magna, and was only 

 extruded on the rock reaching a certain temperature on cooling ; as 



* Geo<iraphic(d Journal, Vol. xx., 1902, p. 520. 

 t " Physics of the Earth's Crust," 1889, p. 148. 



