MancJiester Memoirs, Vol. xliv. (1900), No. 7. 3 



mind, a case of occluded gases. It is assumed that, 

 because air, carbonic acid, and other gases occkided by 

 water, are driven out by heat, steam would be driven out 

 of quartz or granite while fluid, rather than that it should 

 separate out when they are in a plastic, or perhaps even 

 in a solid state; but we have the well-known cases of silver 

 and copper absorbing oxygen while in a molten con- 

 dition and violently ejecting this gas when solidifying. 

 Gases are also given off by steel when cooling, forming 

 blow-holes in the castings. It is therefore possible 

 that molten silica has a natural inclination to occlude 

 steam and that, if the pressure is sufficiently great, the 

 weight of steam occluded is proportionately great. The 

 large percentage of water contained in some quartzes 

 seems to indicate that silica when fluid can, under 

 pressure, absorb a large volume of steam. Possibly the 

 expulsion of steam takes place at some definite tempera- 

 ture, and not neccssaril\- at that of freezing. It may 

 be well to point out here that the measurements made on 

 the sizes of the cavities in the quartz, and the amount of 

 water they contain, do not necessarily indicate a definite 

 temperature as suggested by Mr. Sorby. At least, it is 

 very near that of the critical temperature of steam, and as 

 at high temperatures, the product of volume and 

 pressure of steam is proportional to the temperature, 

 and the temperature of the earth's crust to the depth 

 below the surface, it follows that the relative volumes of 

 the cavities and the steam they contain will be constant 

 for every temperature of formation. We are, therefore, 

 not limited to any definite temperature as being the only 

 one at which granite has been formed, which would have 

 to be the case if the above view were strictly true. 



Turning now to the previously mentioned differences 

 of melting temperatures of the minerals in granite, it 



