66o 



/. H. L. VOGT 



applied to the combination H2O : silicate, the water at a low tem- 

 perature will only keep dissolved a trifle of silicate. The eutectic 

 point (E in Fig. 52) of the water solution will consequently practi- 

 cally coincide with Sa which is illustrated by the schematic Fig. 53, 

 copied from Th. Vogt's treatise where, under the signature B, we 



keep together the different sili- 

 cate components, for instance, in 

 a granite. 



We may here distinguish 

 between three temperature 

 stages: 



I . From Sb (the melting point 

 of the silicate) to Q. 



We here get a crystalliza- 

 tion of the silicate by continu- 

 ously decreasing temperature, 

 the quantity of H2O increasing 

 in the mother liquid (the rest- 

 magma). There is at last pro- 

 duced a magma relatively 

 strongly enriched in H2O, of the 

 composition Q. 



At the same time some of the 

 light volatile compounds escape, 

 so we get a gas phase which can 

 effect pneumatolytic formations. 

 Concerning this we refer to the explanation on the fundamentals 

 given by Th. Vogt and Niggli in Beyschlag-Krusch-J. H. L. Vogt, 

 Erzlagerstdtten, II, second edition (1921), pp. 555-61. 



The gas pressure (the pressure of the escaping gas phase) at Sb 

 (thus for pure B) is very little and increases with the quantity of 

 the light volatile compounds dissolved in the magmatic solution (up 

 to Q). The gases escape when the gas pressure exceeds the 

 external pressure. At a relatively low external pressure, as for 

 the crystallization of a magma at a little depth, the light volatile 

 compounds will consequently escape at higher temperatures, thus 

 also at a relatively earlier stage than at higher pressure, by the 



Fig. 52. — The quantitative proportion 

 between A and B as abscisse and the 

 temperature as ordinate. 



