MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 517 
the remaining part of the final product will commence to crystallize 
only after all in all 90 per cent plagioclase and yr onenC have 
solidified. ; 
As is well known, a little orthoclase or microcline often appears 
with the quartz in the mesostasis of the gabbro rocks. The 
explanation of this fact is quite simple. If the magma contains 
somewhat more Or (KAISi,Os) than is absorbed by the plagioclase, 
the small surplus of Or will be concentrated in the final magma, 
and consequently will crystallize together with the quartz and some 
plagioclase and ferromagnesian silicate in the mesostasis. 
_ In the chapter in Part II on the “Oligoclase-Granite Dikes”’ 
_of many gabbros and norites, we will show that these dikes represent 
the final magma, resulting at a very late stage of the crystallization . 
of gabbroidic magmas which contain a little quartz. 
ON THE DIFFERENCE BETWEEN THE SOLUBILITY OF THE FERROMAG- 
NESIAN SILICATES IN ACID (GRANITIC) AND BASIC 
(GABBROIDIC) MAGMAS 
We shall commence by mentioning some binary eutectics. 
Synthetic determinations, according to Hae Geophysical Labaratory, Wash- 
ington: 
Qu (crystobalite, ca. 1650°):An Geen per cent Qu:52 per cent 
An (1353°). 
Qu (crystobalite):Diops (1391.5°)=16 per cent Qu:84 Diops (1362°). 
An: Diops= 42 per cent An:58 per cent Diops (1270°). 
_Ab (ca. 1100°):Diops=about 97 per cent Ab:3 per cent Diops. (The 
last statement is according to extrapolation by Bowen.) 
Analytical (see above) Qu: Ab=about 28 per cent Qu:72 per cent Ab. 
‘For the Na,O-rich granites containing pyroxene instead of the 
usual biotite we must assume that Qu:Ab:Diops (at high pres- . 
sure) form a ternary eutectic. The same must be assumed also 
for Qu:An:Diops. The latter system, however, is of sub- 
ordinate interest petrographically. For the ternary systems 
Qu:Ab:Diops and Qu:An:Diops, with regard to which we 
know all the binary eutectics between the individual substances, 
we shall construct the approximate ternary systems, Figure 10. 
We here especially fix our attention on the fact that the ternary 
eutectic Qu:Ab:Diops can contain no more than a few per cent, 
or probably not even so much, of diopside. On the other hand, 
