TllANSACTIONS OF SECTION C. 591 



cooled down the water separated from the substance of the heryl and formed the 

 cavities in which we now find it imprisoned. 



If this be so, it follows that when the beryl crystallised out of the magma, 

 the latter was in a fluid condition, and held a considerable amount of heated 

 water in solution. The temperature of the magma must have been above that 

 of red heat, and the potential energy of the water held in a fluid state by pressure 

 must have been great. When therefore in the course of the earth movements 

 which accompany or in some cases are caused by the intrusion of eruptive igneous 

 masses, pressure was temporarily relieved by the rupture and faulting of rocks, 

 the superheated water contained in the magma would be ready to flash into 

 steam with almost explosive violence. 



It must also be borne in mind that water under great pressure, at or above a 

 red heat, has a powerfully solvent action on most minerals, even on so refractory 

 a mineral as quartz. When therefore granite in the molten and fluid condition 

 of the 8atlej granite was erupted along a line of faulting, fissure, or weakness, 

 the superheated water or steam, bearing with it touch mineral matter in solution, 

 must have acted with great chemical energy on the rocks into which it was 

 intruded. 



I have spoken of water carrying mineral matter in solution, and of a magma 

 carrying water in solution. These two conditions may rapidly succeed each other 

 under varying conditions of temperature and pressure. To use the words of Van 

 Hise, * under sufficient pressure and at a high temperature there are all gradations 

 between heated waters containing mineral material in solution and a magma con- 

 taining water in solution.' 



The condition of the beryl crystals, crowded as they are with liquid cavities, 

 shows how high a proportion of superheated water was contained in the fluid 

 granite magma at the time of their formation. 



Sorby estimated that the fluid cavities in the quartz of granites sometimes 

 amount to more than ten thousand millions to the cubic inch. As quartz, how- 

 ever, is usually the last mineral of a granite to consolidate, it may be thought that 

 the water contained in it is a residuum left by the felspar and muscovite on their 

 separation from the magma ; but the case of the beryl above quoted shows clearly 

 that the amount of water difl'used through the magma before the mica, felspar, 

 and quartz began to consolidate must have been very considerable. The amount 

 of water held in solution by a granite, during the time of its aqueo-igneous 

 fusion, cannot be estimated by the amount of water given in the analysis of 

 consolidated and dried hand-specimens of that rock. A considerable proportion 

 of this liquid must necessarily have been lost during the gradual cooling of the 

 rock, and in the course of its intrusion into neighbouring sedimentary strata as 

 sheets, dykes, and veins. Sorby, as the result of other lines of investigation, 

 came to the conclusion that the amount of water present in granite, though limited, 

 is considerable. 



We must now turn for a few minutes to consider the important question of 

 the porosity of minerals, and their permeability by heated water and gas at high 

 pressure. 



The fact that solid substances are built up of molecules having interstitial 

 spaces between them hardly needs demonstration nowadays. 



But have we all qiiite realised that the molecules of rock-forming minerals 

 and crystals are not inert particles of matter, but that they vibrate or revolve or 

 are endowed with other orderly movement that may be likened to the motion of 

 the planets round the sun ? 



Far, far away in space the solar system would, to an eye formed like our own, 

 in all probability present a nebulous appearance, because the eye would not be 

 able to see the individual members of our system. 



So, too, the molecules of which crystals are built up may have their appro- 

 priate motions, but we cannot see them with the eyes of sense because the mole- 

 cules are beyond the highest powers of the microscope. 



We can, however, I think, perceive them with the eye of the scientific ima- 

 gination ; and the hypothesis that the molecules of minerals are separated from 



