522 SHE VOGH, 
a) As there is such an exceedingly great difference between the 
melting-points of the two components, it must a priori be taken for 
granted that the binary system Mg,SiO,:Fe,SiO, belongs to type I 
(without a maximum or minimum). 
b) In zonal olivine we find, according to a number of investi- 
gators (Sigmund, Becke, Stark),’ Mg.SiO, concentrated in the 
1400 
1500 
1200 
1100 
100 My, SiO, 0M14, SiO, 
OFe, SiO, 100 Fe, SiO, 
Fig. 22.—Schematic melting-diagram Mg,SiO,: Fe,SiO, (percentages by weight) 
kernel, and Fe,SiO, in the peripheral zone. Stark found in a 
zonal olivine from basalt containing 19.5 per cent Fe.SiO, in the 
kernel and 34.5 per cent in the exterior zone, consequently a 
marked difference, but not by far so great a difference as between 
Ab:An in the plagioclase of corresponding rocks. ‘These investi- 
t See Tscherm. min. u. petr. Mitt., Vols. XVI, XVII, and XVIII. 
