MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 525 
and (basic) plagioclase, it has already been indicated by A. Harker* 
. that of these two minerals, the one present in quantity above a 
certain limit first commences crystallizing. This is also in accord- 
ance with my own investigations. ; 
I find it superfluous to discuss the plagioclase rocks with con- 
siderable olivine, and as a consequence with crystallization of an 
Fic. 23.—Photomicrograph (20:1) 
essential part of the olivine before the commencement of the 
solidification of the plagioclase. But I am going to discuss the 
inverse proportion, much plagioclase and little olivine, and choose 
as an example an olivine-bearing labradorite rock from the Ekersund 
field. This consists of nearly 90 per cent labradorite (Ab,An,), 
about 7 per cent olivine (optically negative; 2V=ca. 85°, conse- 
quently with about 0.25 Fe,SiO,:0.75 Mg.SiO,), besides a little 
diallage. (See photograph Fig. 23 and drawing Fig. 24.) 
t The Natural History of Igneous Rocks, 1909, Pp. 170. - 
