MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 633 
with a somewhat larger quantity of magnetite (and ilmenite) the 
sequence of crystallization, on the contrary, is different. Here we 
find idiomorphic crystals, especially octahedrons of magnetite, 
appearing in the silicate minerals first crystallized. In gabbroidic 
rocks with much hypersthene, diallage, or olivine, the magnetite 
seems already to have commenced its crystallization at about 4 per 
cent Fe,O,, and about simultaneously with the first individualized 
Fic. 33.—Hyperitic-structured olivine-gabbro from Langé, near Krageré, Nor- 
way. Contains about 25 per cent olivine (Ol.), ca. 1-2 per cent magnetite with 
ilmenite (black), 60 per cent labradorite, 12 per cent diallage, 2 per cent brown horn- 
blende, o.or-o.02 per cent spinel (Sp.) in the magnetite, very little apatite. Drawn 
partly from photograph and partly from nature. 
Magnetite (with ilmenite), younger than the olivine. Olivine with reaction 
rims against the labradorite. (In the inner zone hypersthene, in the exterior zone 
green hornblende and spinel.) Between the magnetite and the labradorite is a 
reaction rim consisting chiefly of light-brown hornblende. (28:1.) 
ferromagnesian silicate. This boundary, 4 per cent Fe,O,, is, how- 
ever, quite approximate, and it is to a great extent dependent 
upon the composition of the magma. 
In gabbro rocks, rich in plagioclase, and in anorthosite gabbros, 
_the boundary seems to lie somewhat higher, viz., according to 
