146 WARREN. 



having a minimum melting point. He determined the melting inter- 

 vals of some nine soda-potash feldspars ranging in composition from 

 Or., 6.0 per cent to Or., 67.6 per cent, the majority of them lying in the 

 range Or., 40.0 per cent to 50.0 per cent. The melting intervals were 

 determined upon crushed material under the microscope by the method 

 of Doelter. To the writer the data given by Dittler are not conclusive. 

 The number of points determined seem rather too few; and in view 

 of the very considerable errors that are associated with this method of 

 melting point determination, fully pointed out by A. L. Day,^^ it 

 seems doubtful if Dittler's melting intervals have much significance. 



Makinen,^° following Vogt, has also expressed the opinion that the 

 perthites are the result of an unmixing, and suggests an analogy in the 

 behavior of the system NaCl-KCl studied by H. Brand. ^^ He has 

 also pointed out that the unmixing process must have taken place 

 somewhat above the temperature 575° C.^^ He finds that in the 

 Tammela pegmatites, muscovite has clearly replaced the feldspar after 

 the albite bands were formed, and as this muscovitization took place 

 during the pneumatolitic period, during which only a quartz was 

 formed, the temperature of the albite crystallization is at least fixed 

 above the inversion temperature of the quartz, viz. above 575° C. 



Further consideration of the problem of the relations of these feld- 

 spars has led the writer to abandon the idea suggested by him in a 

 former paper and above referred to, that the soda-potash feldspars of 

 the porphyries represent a series of mixed-crystals having a definite 

 range of stability at relatively high temperatures. This assimiption 

 leads to serious difficulties when one attempts to apply it to the crystal- 

 lization of the ordinary granites which are characterized by the pres- 

 ence of the two feldspars separately crystallized, difficulties from which 

 the theory put forward by Vogt is in large measure free. In its general 

 features the latter seems to the writer to be well in accord with the 

 observed facts and to lend itself well to the interpretation of the 

 textural relations of the two feldspars as they occur in their rocks. 



To account for the development of the homogeneous alkalic feld- 

 spars which are intermediate in composition between the two mixed- 

 crystal phases of Vogt, anorthoclase etc., it seems sufficient to the 

 author to consider them as entirely metastable crystallizations. They 

 represent the first crystallizations of feldspar from a magma chilled 



19 Fortschrift der Min., Kryst, u. Pet. 4, 134-137 (1914), 



20 Op. cit. 75. 



21 Neues Jahr. fur Min. Gcol. u, Pal., 32, 638 (1911), 



22 Op. cit., 74. 



