MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 329 
perthite. By the crystallization of acid plagioclase, the latter 
absorbs up to about 10 per cent (or 12 per cent) Or, which on 
cooling may give anthiperthite. Basic plagioclase (labradorite- 
bytownite) seems to absorb a somewhat smaller quantity of Or. 
Concerning this matter we refer to the chapter on the anorthosites 
in Part IT. 
By the above-mentioned crystallization in the acid magmas 
of Or and Ab (or Ab+,,) there result two minerals: orthoclase 
about 0.7 Or + 0.3 Ab (or Ab+a,), and acid plagioclase, about 
o.9 Ab (or Abya,) + 0.1 Or. 
Orthoclase and albite have almost exactly the same melting- 
point, and almost exactly the same atomic weight, probably also 
about the same latent melting-heat, etc. Granted a binary eutectic 
system (type V), the eutectic must lie almost exactly midway 
between 0.7 Or + 0.3 Ab and o.9 Ab + o.1 Or, consequently 
at o.4 Or:o0.6 Ab, just as we found above. 
As An has a far higher melting-point than Or (and also than Ab), 
it is likely that the eutectic boundary between orthoclase and 
plagioclase with decreasing Or is characterized by increasing An 
in the plagioclase. In this manner we shall probably find the 
individualization-boundary between orthoclase and labradorite at 
Bens | OLIO455 Ons AD an. 
This explanation is, however, not explicit, as we have not taken 
into consideration that in certain rocks or under certain conditions, 
the details of which we are not acquainted with, the two independent 
minerals, orthoclase and albite (or some other acid plagioclase), 
do not appear, but instead we find the mineral anorthoclase. 
In my treatise in Tscherm. Mitt., Vol. XXIV (1905) I pointed 
out the fact that a whole series of analyses of anorthoclase shows 
telations about o.4 Or:o0.6 Ab (or about 0.42 Or:o0.58 Ab+an) 
and I set forth the hypothesis that the anorthoclase might be defined 
aS a microscopical or submicroscopical eutectic intergrowth of 
orthoclase (microcline) and albite (or some other acid plagioclase). 
This hypothesis is, however, quite dubious, and the physicochemical 
interpretation of the anorthoclase is still an open question. 
