‘MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 433 
(t) hypersthene in great quantity (stage i); (2) biotite, in the 
exterior part of the orbs simultaneously with the crystallization 
of the rest of the hypersthene (stage ii), then followed some biotite 
alone, as scales or thin shells on the outside of the orbs (stage iii); 
CS 
— ——— —— 
. 
a= 
Ve Ba ey 
Ai it l Zz i aa 
jen Yo 
Fic. 8.—Section through the orbicular quartz-norite from Romsaas, Norway. 
Natural size. Lightly shaded mineral is hypersthene, dark is biotite, and white is 
feldspar and quartz. 
' 
if 
(3) after the orbs were formed, the intervening mass had the 
composition of a biotite-quartz-diorite (analysis No. 45), and the 
crystallization became that usual in these rocks, viz., at first 
biotite (the close of stage iii), then plagioclase (stage iv), and finally 
also quartz (stage v) solidified. 
