670 /. H. L. VOGT 



Rosenbusch {op. ciL, p. 51) maintains the view that the forma- 

 tion of muscovite in the granite must be explained as a pneumato- 

 lytic process' "worauf auch seine weite Verbreitung in den Pegma- 

 titen deutet." This may perhaps to a certain extent be adequate 

 for the muscovite, which belongs to miarolitic druses, but it cannot 

 be applied to the common muscovite evenly distributed in the 

 granite. The structure proves that the muscovite has crystallized 

 from the magma at a very early stage, in part at the same time and 

 in part somewhat earlier than the biotite. 



Whether hypersthene, biotite, or biotite and muscovite shall 

 crystallize in acid granites with ca. 75 per cent SiOj does not depend 

 upon the presence in the magma of those compounds that are shown 

 by the quantitative chemical analysis, nor upon certain variations 

 with regard to pressure, time of cooling, etc. When to this are 

 added the facts that muscovite as a primary formation in igneous 

 rocks is limited to granite-pegmatite dikes and to some granites, 

 and that muscovite is characterized by relatively much H2O, 

 biotite by a smaller percentage of H^O, and hypersthene by no 

 H2O at all, we must conclude that the decisive factor is the varying 

 content in the magma of H^O (eventually also other volatile com- 

 pounds) . 



In the two-mica-granites examined by me, there is up to about 

 twice as much muscovite as biotite. If we turn to the granite- 

 pegmatite dikes, however, we find at times, though very rarely, as in 

 the case of some districts of Smaalenene in Norway, muscovite 

 only, without any biotite at all, and the quantity of muscovite 

 may here rise to as much as about 10 per cent. 



The great majority of alkali-granites are characterized by bio- 

 tite. Two-mica-granites, in Norway as elsewhere in the world, 

 are rare, and hypersthene-granites, so far as yet known, are still 

 more rare. 



The content of H2O in the granite-magma thus in most cases 

 must have been lying within the interval that gives biotite. As a 



' While H2O dissolved in the magma appears in the same manner as the other 

 components, I do not find it natural or right to extend the meaning of the term pneu- 

 matolysis to include the formations of minerals, as biotite or muscovite where the 

 magmatic dissolved H2O was co-operant. 



