MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 671 



rare exception only, the quantity of H^O may have been so small, ' 

 that hypersthene has been formed (or, principally in somewhat 

 more basic granites, a monoclinic pyroxene, besides or instead of 

 hypersthene), and relatively seldom only the content of H2O, 

 etc., was somewhat higher, so that muscovite was formed together 

 with biotite. In granite-pegmatite dikes of analogous chemical 

 composition the content of H2O, etc., throughout must have been 

 considerably higher, as we here never find hypersthene or diopside, 

 but mica and not seldom muscovite together with biotite, occasion- 

 ally even muscovite alone without biotite. 



And if we further draw a parallel between the granites (with ca. 

 75 per cent SiOz) on the one hand and norites, gabbros, anorthosites, 

 etc. (with ca. 50 per cent SiOz), on the other, we will find that mica 

 plays a predominant part in the first-mentioned acid rocks, while in 

 the last-mentioned basic rocks biotite is much more subordinate 

 than the pyroxenes, and muscovite is entirely lacking. 



By three different methods of investigation, viz., in the study of 

 the distribution of miarolitic druses, in the study of the magmatic- 

 epigenetic formations of ore deposits, etc., connected with igneous 

 rocks, and in the study of the relation between mica (biotite, even- 

 tually also muscovite), and in part also hornblende, and pyroxene, 

 we thus get a confirmation of the theoretically drawn conclusion 

 that the granites on an average contain the highest percentage of 

 H2O, etc. 



Then we come to the question of how much H2O, etc., there may 

 have been present at the start of the crystallization in the magma 

 and especially in the granite-magma. 



In the muscovite granite-pegmatite dikes there may occur about 

 10 per cent muscovite containing 5-8 per cent H2O. In proportion 

 to the entire magma there thus entered into the early crystallizing 

 mica ca. 0.5-0.8 per cent H2O. But still there must have been 

 present so much H2O that the later separated minerals might also 

 obtain the pegmatitic structure. This quantity of H2O (with COa, 

 etc.) escaped, except those small quantities that entered into the 

 microscopic pores or was occluded in the separating minerals. This, 

 including the content of H2O in the mica, will only make i per 

 cent by weight of the entire magma. The gas that escaped in the 



