PERTHITIC FELDSPARS. 153 



tain conditions, of albite or microcline alone, perhaps of a composition 

 very near to that of the pure end members of the system. Such, 

 probably, are the crystallizations v.hich we find about the pockets or 

 elsewhere in certain pegmatites. 



We feel therefore justified, at least for the purposes of discussion, 

 in assuming that the same general relations obtain between the potassic 

 and sodic feldspars during their crystallization in the pegmatites as 

 in the granites, and in looking upon the figures here obtained for the 

 chemical composition of the two mixed-crystal phases, fron, a study 

 of the pegmatitic perthites, as holding true, approximately of course, 

 for the granitic feldspars. Much further work must be done on the 

 feldspars and their rocks before the problem can in any way be looked 

 upon as settled, but it is believed that the theory elaborated above 

 may at least serve as a reasonable working basis. 



Summary. — The results of a micrometric and chemical study of 

 six specimens of perthitic feldspars from granite pegmatites are 

 recorded. To these have been added the results of a similar study by 

 Makinen of the perthites from the granite pegmatites of Tammela, 

 Finland. Preliminary studies of certain other perthites are also 

 referred to. Taken together the feldspars studied represent widely 

 separated localities and quite a diversity of composition. It is believed 

 that they may be considered representative of the perthitic feldspars 

 of granite pegmatites in general. 



The methods of study employed and the characteristics of the inter- 

 growths are briefly described. The quantitative results are collected 

 in three tables which show the chemical compositions of the various 

 feldspars studied, the relative amounts of the two mixed-crystal 

 phases which constitute them and the chemical composition of each 

 of these phases. These results and their probable precision are briefly 

 discussed. 



The older theory that the perthitic structure is due to the introduc- 

 tion of albite from without into a previously formed microcline or 

 orthoclase crystal is discussed and held to be untenable. The more 

 recently proposed theory of Vogt that the potassic and soda-lime 

 feldspars form a broken series of mixed-crystals with a eutectic point 

 between them is discussed with particular reference to the alkalic 

 members and is held to offer a satisfactory l)asis on which to explain 

 their relationships so far as these can be judged from such evidence, 

 chemical and petrographical, as is at present available. The progress 

 of crystallization in mixtures of various composition is considered 

 with the aid of a diagram, a), for the case where a perfect and continu- 



