General C. A, McMahon's Address. 461 



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 Satlej granite was 

 erupted along a line of faulting, fissure, or weakness, the superheated 

 water or steam, bearing with it much 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 

 containing water in solution." The condition of the beryl crystals, 

 crowded as they are with liquid cavities, shows how high a pro- 

 portion 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, however, 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 diffused 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 quite realised 

 that the molecules of rock-forming minerals and crystals are not 

 inert particles of matter, but that they vibrate or revolve or are 



