628 



/. H. L. VOGT 



The influence of pressure with regard to the minerals amphi- 

 bole, biotite, and muscovite must then be indirect, since the forma- 

 tion of these minerals depends essentially on the magmatic content 

 of H2O, etc., which further, to a certain extent, depends on the 

 pressure. 



By great differences in pressure, H2O content, etc., a magma 

 otherwise of the same or almost the same chemical composition 

 may give rise to the crystallization of totally different minerals or 

 combination of minerals. P. Niggli, in his work (1920, p. 207) 

 cited below, mentions as a typical instance of this the two rocks, 

 (i) a durbachite (rich in biotite and hornblende and further ortho- 

 clase, plagioclase, a little quartz and titanite bearing boundary 

 fades of granite), and (2) a glassy leucite basalt. 



I. Durbachite from Durbach, Schwarzwald. A. Sauer, Mitt. Badische Geol. 

 Landesanst., II, 1892 (1890). 2. Leucitebasalt from Gaussberg, Kaiser Wilhelm II 

 Land. R. Reinisch, Deutsche Siidpol-Exposition II (i), 1906. 



Undoubtedly, on account of the high content of water, biotite 

 and hornblende, and further orthoclase, have been formed in the 

 durbachite, while on the other hand in leucite basalt which solidified 

 at lower pressure leucite was formed instead of orthoclase and K2O- 

 rich biotite. 



RS dissolved in silicate melts crystallizes at a pressure of one 

 atmosphere as monosulphid (oldhamite, alabandine, sphalerite, 

 pyrrhotine or troilite, etc.) , but may at high pressure by pneumatoly tic 

 processes (in the crystallization of magmas as described by Brogger* 



^Zeits.f. Kryst. Min., XVI (1890), I, p. 161. 



