PERTHITIC FELDSPARS. 145 



of Mic. 85-90 percent; Ab + An, 15-10 per cent. The albite set 

 free by such a change — unmixing — might be expected to withdraw 

 more or less completely from the microcline, and orientate itself in 

 positions such as those occupied by the albite lamellae in the perthite 

 intergrowths. This unmixing would also doubtless coarsen the tex- 

 ture in the case of the cryptoperthite feldspars. 



The present writer believes that there is little doubt but that in the 

 compositions of the homogeneous alkali-feldspar phenocrysts of our 

 effusive or intrusive rocks we have evidence that the potassic and sodic 

 feldspars form what amounts to a continuous series of mixed-crystals. 

 He has also pointed out in the case of the alkaline-granites and their 

 associated cover rocks of porph^Titic texture, occurring in the region 

 of the Blue Hills and Quincy, Massachusetts,^^ that, whereas, in the 

 relatively quickly chilled (quenched) porphyries, the phenocrysts are, 

 or were originally, a homogeneous soda-potash-feldspar, in the under- 

 lying and more slowly cooled granites, the feldspars, of substantially 

 the same composition, are a microcline-microperthite. It was also 

 pointed out in the paper alluded to, that the phenocrysts of the 

 porphyries were easily altered and recrystallized, wholly or in part, 

 and appeared to have been generally quite unstable as originally 

 formed. The conclusion was drawn that the alkali-feldspars probably 

 form at relatively high temperatures a complete series of homogeneous 

 mixed-crystals, but at lower temperatures, probably as a result of 

 an inversion, they formed a broken series of mixed-crystals with a 

 eutectic point, as was proposed by Vogt. The phenocrj^sts would, 

 therefore, represent the earlier formed phase caught in the rapidly 

 chilled and consolidated magma and prevented from unmixing by 

 the great inertia to molecular rearrangements characteristic of the 

 solid state — an unstable phase in other words. The granitic feld- 

 spars on the other hand, crystallizing under more fa^'orable condi- 

 tions to the adjustment of equilibria, underwent the unmixing more 

 or less completely, and being crystallographically very similar, finally 

 crystallized in parallel position as a perthitic intergrowth. 



The writer suggested that the feldspars might first crystallize as a 

 series of mixed-crystals belonging to type I of Rooseboom's classifica- 

 tions of binary systems. Dittler ^^ has offered on the other hand 

 experimental evidence to show that these feldspars belong to type III 

 of Rooseboom's classification, viz. a continuous series of mixed-crystals 



17 These Proceedings, 40, No. 5, 817-323 (1913). 



18 Dittler, E. Die Schmeltzpunktkurvc von Kalnatronfeldspathen, T. M. 

 P.,B. B. 3, 513-522 (1912). 



