MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 537 
If we deduct 3.88 per cent Fe,O, in the diallage and, at an estimate, 
2.0 per cent Fe,O, in the hornblende, we get: 
Diallage, No. 52c, 1 MgO: 0.46 FeO; 
Hornblende, No. 52d, 1 MgO: 0.42 FeO, 
consequently, as emphasized by Kiichler, about the same MgO: 
FeO proportion in both minerals. 
HYPERSTHENE AND BIOTITE 
The two analyses from a dacite (No. 53) show: 
Hypersthene, No. 530, 1 MgO: 1.00 FeO; 
Biotite, INOM53¢, te Vig@ rir o2) He®: 
consequently exactly the same MgO:FeO proportion in both 
minerals. | . 
If for the Romsaas quartz-orbicular-norite, which in the entire 
rock only contains about 0.5 per cent Fe,O,, we assume as an 
estimate 1 per cent Fe,O, in the biotite, we get: 
Hypersthene, No. 41, 1 MgO : 0.39 FeO; 
Biotite, No. 42, 1 MgO: 0.27 FeO. 
Even if the last figure is not quite exact, relatively somewhat less 
FeO appears in the biotite than in the hypersthene. 
When there is a simultaneous appearance of biotite and hyper- 
sthene in the same rock, the biotite seems throughout to carry 
considerably more TiO, than the hypersthene. 
The summary, here briefly stated,. verifies the earlier con- 
clusion, especially by A. Merian? (1884), W. Wahl? (1906), and 
Kiichler (Joc. cit., 1914), viz., that the composition of the ferro- 
magnesian silicates depends quite simply upon the composition of 
the entire rock or magma, and further that the relations between 
MgO and FeO (or Mg-and Fe-silicate) in two from the same magma 
crystallizing ferromagnesian silicates such as olivine: orthorhombic 
t “Analysen gesteinsbildender Pyroxene,”’ Neues Jahrb. f. Min., etc., Beil., Bd. 
IIT (2884). 
2 Die Enstatitaugite (dissertation), Helsingfors, 1906; Tscherm. Miit., Vol. XXVI 
(1907). 
