MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 669 



proportion of Na^O to KaaO, and in part predominantly K^O, viz., 

 almost 0.7 K20:o.3 NaaO. We shall group the contents of MgO 

 and K2O in the just-mentioned analyses of hypersthene-granite 

 with 73.5-77.5 per cent SiOa: 



per cent MgO .... o . 20 o . 43 o . 69 o . 60 

 K2O . . . .3.64 4.14 3.34 6.13 



It appears from this that the formation of hypersthene in these 

 alkali-granites is not dependent on any especially high percentage 

 of MgO (or MgO-|-FeO) nor on an especially low percentage of K2O. 



According to my examinations of some Norwegian "white gran- 

 ites," relatively rich in acid plagioclase (according to the nomen- 

 clature of V. M. Goldschmidt^ " Trondhjemite ") from the north 

 of Norway carrying both Muscovite and biotite, the muscovite occurs 

 exactly in the same way as the biotite. Especially it is to be empha- 

 sized that the muscovite-individuals frequently are congested in 

 small aggregates — they thus show "together swimming structure" 

 (synneusis structure) indicating formation at a very early stage — 

 and they are in more places deposited on the small apatite-crystals 

 which serve as "Fixkorper." Some individuals show idiomorphous 

 contours parallel to 001, as well as perpendicular to 001, against 

 the quartz and the feldspar. Most frequently occur the usual 

 lobed outlines, however, as in most of the individuals of biotite 

 in granite. 



In some of these muscovite-bearing granites, as, for instance, in a 

 rock from Narvik — with twice as much muscovite, in leaves up to 

 3 mm. in size, as biotite — ^we observe crystallographically parallel 

 growths, at times in alternating strata of the two mica-minerals, 

 as described by Rosenbusch (op. cit., p. 57). 



In other samples, however, as in a rock from Fustervand near 

 Mosjoen, with nearly twice as much biotite as muscovite — the last 

 one in leaves only ca. i mm. in size — we observe individuals of 

 muscovite with idiomorphic contours inclosed in the biotite, 

 indicating that the muscovite at least for a great part was formed at 

 an earlier stage than the biotite. 



' "Geologisch-petrographische Studien im Hochgebirge des Siidlichen Norwegens," 

 Ges. d. Wiss., Kristiania (1916). 



