PRESSURE IN FORMATION OF ROCKS AND MINERALS 747 



some period the rock was permeated by very mobile solutions from 

 which crystals subsequently separated while the still volatile com- 

 ponents passed on. It was presumably in some such way that the 

 pegmatites formed, for there are other reasons for believing that 

 they formed at about 600°, a temperature at which all their con- 

 stituent minerals would long have been solid had there been no 

 volatile components present. 



SUMMARY 



In the foregoing pages is presented a brief discussion of some 

 aspects of the influence of pressure on the formation of rocks and 

 minerals. In general it would seem that the importance to geology 

 of the effects of pressure upon so-called physical changes (e.g., the 

 melting-point of a single pure substance) has been overestimated 

 relatively to that of the influence of pressure upon chemical changes 

 — in other words, upon equilibrium in polycomponent systems. 

 Change of effective pressure will in general change the configuration 

 of the various fields of stability in a system, acting thus in a way 

 precisely analogous to change of temperature, or of gross composi- 

 tion; but in the case of pressure the effect will usually not be espe- 

 cially marked unless one or more of the components is volatile — 

 that is, unless the concentration of one or more of the components 

 really changes appreciably with change of pressure. This is merely 

 an example of the general rule that the magnitude of the effect of 

 pressure on a system depends upon the difference in compressibility 

 of the several phases present, being greatest when this difference 

 is greatest, and conversely. 



Accordingly we must, in any discussion of the course of crys- 

 tallization from a complex magmatic system, take into account the 

 mode in which the effective pressure varies as well as the mode of 

 cooling. For change of pressure, like change of temperature, may 

 affect the order of crystalHzation — and even the character — of the 

 minerals which separate; this result of course depending merely 

 upon the circumstance that the saturation limits (solubilities) of 

 the several solid phases which could possibly separate out are not 

 all affected equally by change of conditions. 



January 29, 1915 



