6i6 



/. E. L. VOGT 



The error made by transferring the determination of the compo- 

 sition of the eutectic on a pressure of one atmosphere, to be applied also 

 to the pressure prevailing during the crystallization of the eruptive 

 rocks, will accordingly be rather unimportant provided that the minerals 

 involved are formed independently of pressure. 



I have demonstrated this fact by some petrographic instances 

 in my publication in Tscherm. Min. Petrogr. Mitt., XXV (1906), 

 and XXVII (1908). I will here only point out that the analyses 

 of the final eutectic quartz: feldspar-product from igneous flows, 



formed at a comparatively low 

 pressure, accord within the 

 limits of error exactly with the 

 corresponding final product from 

 granitic deep-seated rocks and 

 also with the graphic granite in 

 the Archean granite-pegmatite 

 dikes formed at very great depths 

 (see the analysis No. 1-40) . And 

 I deem it right to employ the 

 experimentally determined eu- 

 tectic Qu:An, at a pressure of 

 one atmosphere, for parallellism 

 with the eutectic Qu : Ab in deep- 

 seated rocks or granite-pegmatite 

 dikes (Fig. 3). 



The same general reasoning as 

 to the inconsiderable influence of 

 pressure on the dislocation of the 

 melting point and on the eutectic may be transferred, in all essen- 

 tials also to mix-crystal systems, or at any rate, to the continuous 

 mix-crystal systems of type I. 



For example, let us consider Ab : An. Even if the melting point 

 of An should, at high-pressure, rise a little more or a little less than 

 of Ab, it must be presumed a priori that the course of the liquidus 

 and the solidus curves, and, what is petrographically of the greatest 

 importance, the horizontal distance between the liquidus and the solidus 

 curves, will remain almost the same (Fig. 51) at a high pressure 



Fig. 51. — Illustrating the imessential 

 dislocation of the horizontal difference be- 

 tween the liquidus and the solidus curve of 

 the An : Ab-system by low and high pres- 

 sure. 



T , T= smelting point by respec- 

 tively low and high pressure. 



