518 THe Ls VOGT 
the eutectic boundary-line between diopside (or pyroxene in 
general) and anorthite, bytownite, and labradorite (see above) 
contains as much as about 55, 45, and 35 per cent pyroxene respec- 
tively, and these figures will only be displaced to a slight degree 
by the presence of a lesser quantity of quartz, as 2, 5, or 10 per cent. 
In magmas of granitic composition, with AD as the plagioclase, 
diopside must consequently commence crystallizing earlier than 
plagioclase (albite), even if only as little as a few per cent of diopside 
are present. In magmas of gabbroidic composition (with labra- 
Dions. 
Au. An. Qu. Le Ab. 
Proncin 
Fic. 19.—Diagrams of the individualization fields Qu:Diops:An, and Qu: 
Diops: Ab. 
dorite or bytownite), on the other hand, plagioclase commences 
crystallizing earlier than the pyroxene, even if as much as 35-40 
per cent of pyroxene is present. 
This result with regard to the slight solubility of CaMgSi,0¢ 
in acid magmas, deduced from physico-chemical foundations, is 
verified by the petrographical investigation of granitic igneous — 
rocks. And this slight solubility in the acid igneous rocks Fe 
applies not only to diopside, but also to the Mg, Ca-, Mg, Fe, Ca-, 
or Mg, Fe-silicates in general. 
It is apparent that— 
t. The crystallization of the silicates in the granites (with 
about 70-76 per cent SiO,) commences with the crystallization of 
the ferromagnesian silicates (biotite, hornblende, augite, hyper- 
sthene) when the latter is present in a quantity of at least a few 
