MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 629 
chiefly magnetite, giving in this manner a form of structure nearly 
identical with graphic granite or with coarse-grained granophyre. 
This must be due to a simultaneous crystallization of the oli- 
vine and the iron ore, especially the magnetite, and the propor- 
tion by weight in the intergrown individuals gives about 0.2 5 
Fic. 28.—Photomicrograph (25:1). ‘‘Titanomagnetite”’-olivinite from Fiskaa, 
Séndmére, Norway. White=olivine, black=titanomagnetite. The small gray 
grains in the photograph are spinel. 
magnetite:o.75 olivine, this representing a point or a line on a 
quite complicated eutectic boundary curve. 
Magnetiie in gabbro, norite, anorthosite, etc.—As is well known, 
and still believed by many petrographers, Rosenbusch made the 
assertion that the oxidic iron ores, magnetite, ilmenite, etc., always 
belong to the oldest segregations, and crystallized before the 
solidification of the silicates. ‘This assertion, however, is not cor- 
rect with regard to the gabbroic rocks, for when only little magne- 
tite (and ilmenite) is present, the crystallization of iron ore does not 
