MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 327 
saturation), has a composition nearly exactly Ab..Ang., or Ab,;An,. 
Even before Bowen’s investigation (1913), I had (see especially 
Tscherm. Mitt., Vol. XXIV [1905]), on the basis of the well-known 
zonal structure of the plagioclases, and further on the basis of the 
proportion between Ab: An in the total plagioclase calculated from 
the analysis of the rocks and in the first segregated crystal, decided 
that the system Ab:An belongs to type I, and, furthermore, that 
in the crystallization of plagioclase in eruptive rocks, especially in 
t00An 80 60 AO 20 oAn 
OA 20 40 60 80 100A6 
Fic. 2.—The melting-diagram, An: Ab, after Bowen 
dike and effusive rocks, there is a very great horizontal difference 
between the liquidus and the solidus curves. 
The binary diagram for Ab:An, at the pressure of one atmos- 
phere and for too per cent Ab+An, may with unessential modifica- 
tions of the horizontal difference between the two curves (see 
a following chapter) be transferred to magmas, crystallizing at 
high pressure, which besides Ab and An contain other components. 
The investigation of the mix-crystal system and the proportion 
between the liquidus and solidus curves for Ab:An may conse- 
quently also be applied to rocks, which besides plagioclase also 
