336 JOH. Ey VOCE 
Qu:75-70 Ab (or Or), just as we have in reality derived from the 
analyses of the graphic granites. 
As the melting-points as well as the binary eutectics (as will be 
shown in a following chapter) are only very little displaced by 
pressure, we are justified in drawing a parallel between the eutectic 
SiO,:An, determined for low pressure, and the eutectic SiO,:Ab 
(or SiO,: Or), calculated for high pressure. The case is somewhat 
100An OAn 
or Ab or Ab 
OSi0, 100 SiO, 
Fic. 3.—Melting-diagram, An:SiO, (after Bowen), and Ab:SiO, (schematic 
after Vogt). ; 
complicated, however, by the fact that SiO, in one case is tridymite 
but in the other quartz. | 
The melting-point for the binary eutectic Equa» (or Eou-or) 
must, according to the nature of the case, lie considerably lower 
than the melting-point of pure Ab (or Or), consequently consider- 
ably lower than 1100° and certainly somewhat lower than 1000”. 
As an estimate we set it at 975°, which should be approximately 
correct. 
