MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 437 
drawing (enlarged 11:1) of a hypersthene-norite from Messel 
(about 10 km. from Arendal, Norway), containing about 20 per cent 
labradorite (Ab,An,), nearly 80 per cent hypersthene (consisting, 
according to the determination of the optical character and of the 
Fic. 13.—Norite from Skougen, Norway. The hypersthene, to a great extent, 
has idiomorphic outlines against the labradorite, and the hypersthene in several 
places shows synneusis structure. The biotite is inclosed in the exterior parts of 
the hypersthene individuals or is grown on these. (24: 1.) 
optic axial angle, of about 0.25 FeSiO,:0.75 MgSiO,), a little horn- 
blende, and a little pyrrhotite, but no biotite, diallage, and oxidic 
iron ore. . 
In gabbros and norites, containing somewhat more plagioclase 
(labradorite), say 30, 35, 40, or 45 per cent, and correspondingly 
less pyroxene, we still find the pyroxene individuals to greater or 
lesser extent with idiomorphic contours against the plagioclase, 
while the latter lacks idiomorphism. 
