PERTHITIC FELDSPARS. 147 



with relative rapidity. It seems reasonable to suppose that the two 

 alkalic feldspars, at least, possess, in the neighborhood of their crystal- 

 lization interval, a strong tendency to crystallize together in all pro- 

 portions on account of the close similarity in crystal structure and 

 volume which characterize them. The rate of the process which tends 

 to separate them into two types of mixed crystals, one potassic and the 

 other sodic, is probably a slow one, possessing relatively little energy, 

 much less rapid and energetic than the tendency of the two feldspars 

 to crystallize from the rapidly chilled and probably undercooled 

 magma. Under such conditions the latter tendency dominates and 

 the two feldspars crystallize together either as a homogeneous (so far 

 as can be told) crystal or as an exceedingly fine intergrowth (crypto- 

 perthite). Once formed in this fashion, though in a metastable con- 

 dition, the inertia of the solid state, doubtless especially great in 

 substances like these, indefinitely postpones further change in the 

 direction of the establishment of a true equilibrium, and the crystals 

 often persist as formed. The case is quite different when a longer 

 period of time is allowed for crystallization, as in the case of the 

 magmas which consolidate at relatively deeper levels or even in the 

 case of the groundmass of the porphyries. ^^ Here we would have 

 slower cooling, less undercooling, and probably also more abundant 

 liquid or vapor phases. The separation of the two mixed-crystal 

 phases can then take place with less or greater perfection according 

 to the perfection of the equilibrium adjustment permitted by the 

 rate of cooling and other conditions. The result would be either 

 the formation of a perthitic or microperthitic intergrowth, or, under 

 still more favorable conditions, of separate unorientated crystalli- 

 zations of the two phases. 



The process of feldspar crystallization may be further considered 

 with the aid of a diagram representing the kind of equilibrium which 

 is believed to hold for these minerals. The diagram. Figure IV, is sub- 

 stantially that used by Vogt. The true position of the curves is of 

 course not known but the various points on the eutectic line and along 

 the base may be considered as located approximately. The positions 

 of the points N and M have been located in accordance with the figures 

 obtained for the compositions of the mixed-crystals in the present 

 investigation, and lie at or somewhat outside Vogt's lower estimate 

 (about 10% of Or.) for the corresponding points. The points i and q 



23 See in this connection the"Jdescription of the^groundmass feldspars in the 

 Blue Hill porphyries, Op. cit.;.249 and 322.* 



