PRESSURE IN FORMATION OF ROCKS AND MINERALS 745 



small amount of water may be quite large; for it would seem that a 

 lowering of 100° could easily result from the presence of water vapor 

 at a pressure less than 20 atm. Incidentally it may be pointed out 

 that the circumstance that pure water has a critical point at 370° 

 and 200 atm. is of secondary importance in the discussion of systems 

 belonging to this type.^ 



The great similarity exhibited by these two systems shows that 

 there is no real difference between melting-curves and solubility 

 curves; a simple and well-known illustration of this is the fact that 

 the melting-point of CaCl26H20 to a liquid of its own composition 

 is merely a particular point on the graph representing the solubility 

 of CaClz in water. In general it is much simpler to consider all 

 such phase diagrams as solubility diagrams, as by so doing it is 

 easier to grasp their full significance and to predicate what will 

 happen under specified circumstances. This point was clearly 

 stated thirty years ago by Guthrie, the first investigator of eutec- 

 tics, who wrote as follows:^ 



[My experiments show] that water at a high temperature may not only 

 play the part of a solvent in the ordinary restricted sense, but that there is in 

 many cases no limit to its solvent faculty; in other words, that it may be 

 miscible with certain rocks in all proportions: that solution and mixture are 

 continuous with one another. And this continuity, as my experiments prove, 

 is established in some cases — and these indeed with bodies having no chemical 

 affinity with water — at temperatures not above the temperatures of fusion of 

 those bodies per se. 



Moreover, Guthrie saw the geological significance of these 

 results, for he wrote: 



Just as in the selective formation of salt-alloys we may have the artificial 

 type of the genesis of many primary rocks and metamorphic modifications, so 

 in the wonderful solubility in or miscibiHty with water of such alloys and of 

 some salts at high temperatures we may have a no less clear type of the forma- 

 tion of certain volcanic rocks and an explanation of some of their peculiarities. 

 .... Obsidian melted and under pressure will, I presume, mix freely with 

 water. When this pressure is gradually removed, water vapor escapes, and 

 although it takes with it a large amount of heat, the temperature of the obsidian 



^ With regard to this point see Moray and Niggli, Jotir. Am. Chem. Soc, XXXV 

 (1913), 1089 ff. 



^Phil. Mag., XVIII (1884), 117. 



