326 J. H. L. VOGT 
following assertion is applicable: ‘The saturation-boundary 
between binary mix-crystals as well as in general the equilibrium 
between two solid phases of a binary system do not change by 
contact with other phases and components, when these new entering 
components do not form solid solutions or stoechiometric com- 
pounds with the former solid phases.” The proportion between 
An and Ab in the segregated plagioclase mix-crystal will in this 
manner be the same whether the crystallization takes place in a 
pure An+Ab melt or in a silicate melt (or magma) which besides 
plagioclase also delivers, for instance, magnetite, olivine, etc. 
In order to investigate the laws of crystallization of the principal 
components of the magma, we may generally leave the components 
which are present only in subordinate quantity out of consideration, 
provided that the latter do not form solid solutions or enter into 
mix-crystal combinations with the principal components. We 
must, however, take into consideration that when A, B, and C 
form a ternary eutectic, and C only is present in small quantity, 
the simultaneous crystallization between A and B, along the 
eutectic boundary between A and B, will not be identical quanti- 
tatively with the composition of the binary eutectic between A 
and B. If C, however, is present in minimal quantity, the dif- 
ference between the point in question on the eutectic boundary 
between A and B and the binary eutectic A:B will be so incon- 
siderable that it practically may be left out of consideration. 
THE FELDSPARS, AB: AN, OR: AB-+-AN. 
The binary system Ab: An (with melting-point Ab = 1100+ 10° 
and An=1550°+ 2°) belonging to mix-crystal type I has been 
studied in detail by N. L. Bowen’ of the Geophysical Laboratory 
of the Carnegie Institution of Washington at the pressure of one 
atmosphere (and with chemically pure substances). We reprint 
Bowen’s graphic exhibit as Figure 2, where the great horizontal 
difference between the liquidus and solidus curves is shown. 
As an example we may mention that from a molten mass, 
Ab,An,, the first mix-crystal, separating at 1450° (without super- 
t Amer. Jour. Sc., Vol. XXXV (1013). 
