THE MINERALOGRAPHY OF THE FELDSPARS 287 



The same conclusion was reached by Johansson^ and Vegard.^ 

 The suggestion that the diagram of the potash-Soda series is a 

 series of solid solutions with a minimum melting point, is one that 

 Warren formerly held^ in explanation of the homogeneity of the 

 soda-rich phenocrysts of the porphyries in the region of the Blue 

 Hills and Quincy, Massachusetts. Warren regarded this form of 

 diagram as portraying the conditions prevailing at high tempera- 

 tures, but with faUing temperature exsolution took place. He 

 later abandoned this view because it is not applicable to the crystal- 

 lization of gram'tes, and considered the homogeneous alkalic 

 feldspars, such as anorthoclase, to be metastable crystallizations. 

 It seems to the present writer that Warren in seeking a single dia- 

 gram for the potash-soda series abandoned an idea well worth con- 

 sidering. If a series of diagrams, as later proposed, is correct, then 

 we can retain Warren's idea as applicable to such metastable solid 

 solutions, but not to feldspars crystallizing under plutonic conditions. 

 Dittler's diagram is open to the criticism that he determined the 

 melting point interval, the area between the soHdus and liquidus, 

 by Doetler's microscopic method. Serious errors are known to be 

 associated with this method, as pointed out by Day.^ 



The difficulty with the D it tier diagram is that it applies to 

 rapidly cooled metastable feldspars and not to others. The sugges- 

 tion is therefore offered that a diagram which combines the diagrams 

 of Vogt and Dittler is more likely to approach the truth than either 

 one alone. 



COMBINATION OF THE TWO EXTREMES 



In Part I the combination of these extremes was attempted in 

 Figure 5 (p. 225). The two binary diagrams are shown related to 

 each other through the degree of equilibrium, the solid solution 

 diagram with a minimum being the unstable one, while the eutectifer- 

 ous diagram represents the system when perfect equilibrium obtains. 



'H. E. Johansson, "Om faldspaternas sammansattning och bildnings forhallan- 

 den," Diese Zeitschr., XXVII (1905), 338. This paper the writer has not seen, as it is 

 reported by Makinen ("Uber die Alkalifeldspate," Sonderahd. aus. Geol. Foreningens., 

 I (1917), to occur only as a "separate" or abstract. It is not generally known. 



^ Vegard, "Die Konstitution der Mischkrystalle," PAy^. Zez^^c/zr., XVIII (No. 15, 

 1917), 33, 93-96. 



5 C. H. Warren, Proc. Amer. Acad. Arts and Sci., XL (No. 5, 1913), 317-23. 



'' A. L. Day, Fortschr. der Min. Kryst. u. Pet., IV (1914), 134-37. 



