An Unrecognisiid Agent in the Deformation of Bocks. 391 



the top layers had already reached this temperature, and had there- 

 fore become impervious to water- vapour, this in the lower layers of 

 the lava was forced to occupy spaces which it cleared by its own 

 expansion. 



Sir Lowthian Bell has described a similar occlusion of gas in 

 ordinary furnace slag even at ordinary atmospheric pressure ; on 

 the slag cooling to a certain point, the gas is given out suddenly, 

 exactly in the manner I have supposed the water-vapour in the 

 Drakensberg lavas to be." It is evident, therefore, that the rock 

 sufficiently deep in the earth's crust to be in a state of potential 

 fusion, must contain a very large amount of occluded water- vapour, 

 but from the observations on the Drakensberg amygdaloids it is also 

 very probable that this vapour is held in permanent occlusion and is 

 not available for extrusion in hot springs. In spite of the high 

 authority of Prof. Suess, I must conclude that such are fed by water 

 from the surface. Further, I think the facts prove that all water that 

 permeates deep-seated rocks not in the zone of potential fusion, 

 is derived from the surface, or according to Posepny's nomenclature, 

 is vadose, and not hypogene. 



The difficulty to explain the creeping of water down to great depths 

 against pressure is cleared up by Prof. Daubree'sf beautiful experiment 

 on the capillarity of rocks. Prof. Daubree placed a flag of sandstone 

 on a vessel in such a way that the sandstone separated the lower from 

 the upper half. The interior of the lower half communicated with a 

 pressure gauge, and the upper was open to the air. On pouring a 

 little water on the sandstone, and heating the vessel to 160° C, the 

 pressure in the lower half was observed to increase. A stop-cock 

 was now opened to the air, allowing the pressure in the lower half to 

 become the same as that in the upper, and was then closed, this 

 operation being repeated several times, the heating going on all the 

 while. Every time the pressure of the lower half increased, showing 

 that the water was drawn through the sandstone by means of the 

 capillarity exerted by the minute interspaces and against the pressure 

 exerted by the heated air in the lower half, which tended to force its 

 way in the opposite direction. Substituting the flag of sandstone for 

 the cooler portion of the earth's crust, and the moderate heat of the 

 laboratory for the immense heat in the interior of the earth, we can 

 easily see how the water in a similar way would be drawn down into 

 these great depths ; the differences in Prof. Daubree's experiment 

 and the actual state of affairs in Nature being one of degree and not 

 of kind. If Prof. Daubree's experiment is reversed, and water poured 



* Journal of the Iron and Steel Institute, No. 11, 1881. 

 t " Geologie Experimentale," 1879, p. 258. 



2G 



