E. H. L. Sckwarz — Hot Springs. 255 



the earth, however, things are in equilibrium ; enormous pressures 

 do exist, but they are produced by the superincumbent masses of 

 rock, and the heat that there is at great depths only exists as a mode 

 of energy which helps in balancing the stress. In other words, 

 looked at from an isostatic point of view, there is no surplus energy 

 in the earth's interior to expel the large quantities of water that come 

 up in hot springs ; these must rise in accordance with hydrostatic 

 laws. The argument from the permanency of such springs, which 

 would seem to imply that they are independent of supplies from the 

 surface, can be met with the counter argument that, as they are hot, 

 and must therefore come up from great depths, the area with which 

 they are in hydrostatic connection is sufficiently large to ensure 

 ^ constant average. 



The third point is that raised in my paper on the deformation of 

 rocks, referred to above. It is that though lavas do undoubtedly 

 contain a large quantity of water-vapour, nevertheless this 

 water-vapour is held up in occlusion, and is unavailable unless 

 the lava has cooled down to a certain point. My inference was 

 drawn from a study of the Drakensberg lavas, in which there are 

 very large vesicles in the shape of branching pipes ; these occur 

 ■only on the bottom of the lava-flows, the topmost portions having 

 got rid of most of their water-vapour and show only the normal 

 rounded vesicles. This seemed to me a sufficient proof that the 

 water- vapour was held in actual occlusion, just as gases are held 

 in occlusion in furnace slag.^ For when the lava was flowing, the 

 pressure on the surface being reduced from that of many atmospheres 

 in the chimney of the volcano to that of one atmosphere, one would 

 naturally expect that all the water- vapour would explosively escape ; 

 this, however, was not the casOj or else the vesicles would not have 

 been formed. What happened was this : the topmost layers of the 

 lava-flow cooled down to the expulsion point, and then only gave 

 off their occluded water-vapour ; later, the lower layers cooled 

 down and endeavoured to get rid of their water-vapour, but the 

 upper layers had already cooled below the expulsion point, and 

 were therefore unable to absorb and pass on that which the lower 

 layers tried to get rid of; the consequence was that the water- vapour 

 was obliged to come out from the body of the molten rock, and 

 had to force this apart in the shape of the very large vesicles in 

 order to accommodate itself. From a study of the microscopic 

 characters of the lavas I was led to infer that the temperature of 

 the extrusion of the water-vapour was a little above the melting- 

 point of labradorite, that is to say, somewhere in the neighbourhood 

 of 1200° C. The whole of this question is one that can be settled 

 by laboratory experiment, and I am earnestly hoping that there will 

 be early opportunities of doing such work. 



My fourth point is in connection with the moon. That body was 

 separated from the earth at a time when the water now existing 

 on the surface of the earth formed part of the atmosphere. It has 



^ Sir LowtMan Bell: Journal of the Iron and Steel Institute, No. 11, 1881. 



