668 J. E. L. VOGT 



The formation of the minerals in both the anorthosites and the 

 olivine rocks — with at least, respectively, ca. 90 per cent plagioclase 

 (labradorite-bytownite) and 85-90 per cent olivine (poor in iron) — 

 thus indicates a crystallization of a magma very poor in H2O. 



As to the relation between hyperstkene, biotite, and biotite-\-mus- 

 covite in the alcali granites, we shall as a beginning cite a very instruc- 

 tive statement by H. Rosenbusch {Mikroskop. Phys. d. Massigen 

 Gesteine, II, i [1907], p. 71): "Die Analyse eines Hypersthen- 

 granites und eines gewohnlichen normalen Alkaligranite sind nicht 

 sicher zu unterschieden." 



In granites with composition 



73~77 psr cent SiOz 



ii-iS 



1-5-3 

 o . 2-0 . 7 

 0.25-1.4 

 5-8 



ALO3 



Fe203+FeO 



MgO 



CaO 



K.O+NaA 



(with varying proportion of K2O and NajO) we find, in some cases, 

 though rather rarely, hypersthene — in by far the most cases biotite 

 — and at times biotite +muscovite. 



In the hypersthene-granite from Birkrem and environs in the 

 Ekersund Soggendal-field, the hypersthene (opt. neg.), according 

 to my determination, shows axial angle ca. 70°, the composition is 

 thus ca. 0.64 MgSi04:o.36 FeSi04 (equivalent to about 23 per cent 

 MgO and 18 per cent FeO). The quantity of hypersthene in this 

 rock (with ca. 73-75 per cent SiOz) according to microscopical 

 examination is quite small, about i per cent, corresponding to ca. 

 0.2-0.25 per cent MgO+0.2 per cent FeO. An analysis published 

 by C. F. Kolderup^ shows 73.47 per cent SiOa, 0.12 TiOz, 15.42 

 AI2O3, 1.02 Fe203 (including FeO), 0.20 MgO, 1.35 CaO, 5.57 Na^O 

 and 3.64 K2O, thus stoechiometric 0.70 NaaO 10.30 K2O. The three 

 analyses of charnockite (hypersthene-granite) from Madras^ (India) 

 with 75.3-77.5 per cent SiOj, on the other hand show in part a middle 



' Das Labradorfelsgebiet bei Ekersund und Soggendal, Bergens Museums Aarbog 

 (1896), p. 96. 



' Cited from H. S. Washington, Chemical Analyses of Igneous Rocks, 1884-1913 

 (1917, pp. 88 and 956). 



