228 C. J\ T . Fenner — Crystallization of a Basaltic Magma 



brium, i. e., the proportions in which various mineral compounds 

 exist in the solution, depends upon the concentrations of the 

 various oxides present. 



If we imagine that in a solution of this nature in which 

 equilibrium has been attained, crystallization begins, it is seen 

 that a disturbing factor is introduced ; for the removal by 

 crystallization of one or more compounds is constantly chang- 

 ing the relative proportions of the substances left in the 

 mother-liquor. It is evident that certain substances which 

 may have been present in small amount in the original magma 

 may reach a high degree of concentration in the residual dregs 

 of mother-liquor. The conditions of equilibrium are com- 

 pletely shifted. New compounds are formed, while those 

 compounds which were present in preponderant amount in the 

 original magmatic solution may be entirely broken up and 

 destroyed, As a result the crystals first deposited become 

 unstable in contact with the mother-liquor and are either taken 

 up by the solution or react with it to form zones of new min- 

 erals. This consequence of the principle of mass-action is well 

 understood. Pirsson has thus explained reaction-rims of ensta- 

 tite and biotite surrounding olivine crystals and separating 

 them from alkali feldspar in the shonkinite of the Little Belt 

 Mountains, M on tan a. * 



While it is possible to account for certain phases of the phe- 

 nomena of resorption in this manner, there are others which 

 cannot be brought into line with this explanation. Magmas 

 consolidating at depth frequently exhibit a different association 

 of minerals from magmas of the same composition consolidat- 

 ing on the surface; and a magma in which intra-telluric crys- 

 tallization has begun may, when erupted at the surface, attack 

 the minerals first deposited and take them more or less com- 

 pletely into solution, and in the final consolidation these min- 

 erals may not re-appear. In the Watchung magma olivine 

 ciwstals of the intra-telluric period were attacked and in most 

 cases almost completely resorbed before any notable develop- 

 ment of the three final products of surface crystallization 

 appeared. In this case it is obvious that the phenomenon 

 cannot be explained by reference to a concentration of material 

 in a residual mother-liquor. To account for such occurrences 

 the author would offer another explanation, based upon the 

 displacements of equilibrium which take place within a solu- 

 tion under conditions of simultaneous diminution of pressure 

 and temperature. 



Displacements of equilibrium within a solution effected by 

 changes of temperature and pressure —vanH Hoff^s law. — The 

 extension of the law of mass-action to include changes of tem- 

 *L. V. Pirsson: 20tn Ann. Eept. U. S. Geol. Sur., 1898-99, p. 482. 



