150 WARREN. 



tive to those of composition i. Further lowering of the temperature 

 would result in an unmixing, either gradually along the lines iN and 

 gM or with a break at temperature C, with the development of a 

 perthitic intergrowth, just as has been explained previously. 



If we assume that the equilibrium is not continuously adjusted 

 during the first crystallization interval, as perhaps might be expected 

 in the case with magmas whose earlier crystallizations formed with 

 relative rapidity, developing phenocrysts of some size, it seems proba- 

 ble that, in the case of mixtures like w, for example, zonal growths 

 might be developed — alternati\'e crystallizations of the two phases of 

 variable composition — until the solidification had been completed. 

 The results of some such crystallization as this are probably repre- 

 sented by the zoned feldspar of many granites of coarsely porphyritic 

 habit as, for example, certain phases of the RapakJAvi (Finland) granite 

 and those from near Jonesport, Me., U. S. A. At lower temperatiires 

 the unmixing would occur in amount corresponding to the particular 

 compositions of the different zones. With relative rapid cooling, prob- 

 ably accompanied by undercooling as in the case of the porphyries, 

 metastable mixed-crystals might be formed, at least during the earlier 

 part of the period, and these would later unmix more or less completely 

 forming intergrowths. 



From a melt having the composition of the eutectic a similtaneous 

 crystallization of the two mixed-crystals of compositions i and q 

 would result, and while these might, under very favorable conditions, 

 develop as separate crystallizations, it seems much more probable that 

 intergrowths would result. Furthermore, undercooling would be much 

 more probable to occur in the case of melts of eutectic composition. 

 The amount of the undercooling and the resultant eft'ect on the rate 

 of crystallization and the equilibrium adjustment would doubtless de- 

 termine whether a homogeneous metastable mixed-crystal, or a cryp- 

 toperthite, or perhaps a perthite would form. In any case further 

 cooling of the solid crystals would tend to bring about an unmixing 

 which would change the metastable mixed-crystals into an inter- 

 growth, or coarsen any intergrowth already formed, the full or partial 

 accomplishment of such a process depending on local conditions. 

 Moreover mixtures of compositions near to the eutectic, perhaps even 

 somewhat removed from it, would in all probability behave in much 

 the same manner, particularly where rapid cooling obtained, and 

 little or no distinction could be made between the final products so 

 far as structure is concerned. The general structure of the crypto- 

 perthites and their approximation to a constancy of chemical composi- 



