MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 529 
cretion of an olivine-bearing hypersthenite (or olivine-augite-horn- 
blende-hypersthenite) appears locally (at Nonaas), with a chemical 
composition: 
about 47-48 per cent SiO, 
about o.5-1 per cent TiO, 
about 5-6 per cent Al,O, 
about 2. 5-3 per cent Fe.O, ee 
about 12-13 per cent FeO 
about 17-18 per cent MgO 
about 11-12 per cent CaO 
about o. 5-1 per cent alkali 
This rock, which is quite fresh, consists mineralogically of about 
75 per cent pyroxene, viz., about 50 per cent hypersthene (optically 
negative, 2V=ca. 80°, that is to say, with stoechiometric 25-30, 
closest at about 27 per cent FeSiO,) and about 25 per cent mono- 
clinic pyroxene (extinction-angle 39-40°); on an average about 
12 per cent olivine (optically negative, 2V=ca. 83°, consequently 
with stoechiometrict about 30 per cent Fe,SiO,); about 12 per cent 
intensively greenish-brown, primary hornblende; and in addition 
locally o.5 per cent biotite and about o.5 per cent plagioclase, the 
latter only here and there as mesostasis of quite subordinate 
importance. Oxidic iron ore is entirely lacking. Apatite and 
pyrites appear in small quantity. On the photomicrograph, 
Figure 25, the light-streaked mineral chiefly consists of hypersthene 
and a few individuals of monoclinic pyroxene. Uppermost to 
the right we see a hypersthene individual, cut at right angle to 
the c-axis. 
The mineral which on account of the strong absorption of light 
shows dark-gray in the photograph is hornblende, and the mineral 
which appears white is olivine, quite fresh, without any sign of 
serpentinization or other change. ‘The pyroxene individuals show, 
as well in length as in cross-section, quite good idiomorphism 
against the olivine. The same, though not quite so distinctly, is 
the case with the hornblende, which belongs to a somewhat later 
stage of crystallization than the hypersthene (see above). On the 
other hand, the olivine does not show even a sign of idiomorphism 
against the hypersthene and the other minerals, but forms—in 
™The optical determinations have been undertaken by Docent Carstens and 
myself in collaboration. 
