MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 637 
was also later indicated by Posnjak, Allen, and Mervin? at the 
Laboratory in Washington. 
That this is really so, appears theoretically through the fact 
that the process: 
FeS-++ SZFeS, 
is reversible. FeS in solid solution is able to absorb as much as 
23 per cent FeS,. The solubility of FeS in FeS, is, however, nil or 
minimal. 
For the discontinuous binary mix-crystal system, only two 
cases exist, viz., either type V (with a eutectic) or type IV (with a 
bending-point). By the following observations we may determine 
to which of these two types the system FeS:FeS, belongs. 
1. The pyrrhotite has a relatively low melting-point not 
only at the pressure of one atmosphere but, as appears from the 
statement given below, also at the high pressures prevailing in 
deep-seated magmas. The pyrite, on the other hand, crystallized 
in the deep-seated magmas at a very early stage and, consequently, 
at a relatively high temperature. The pyrite, accordingly, at high 
pressure may exist in the solid phase at a much higher temperature 
than the melting-point of the pyrrhotite. The genesis of the 
intrusive pyrite deposits proves that FeS, at very high pressure 
may also occur in the liquid phase. Pyrite consequently (at 
high pressure) has an even much higher melting-point than pyrrho- 
tite. 
2. Pyrrhotite (FeS with FeS, in solid solution, with as much as 
23 per cent FeS,:77 per cent FeS) has a higher melting-point 
(G on Fig. 36) than FeS. This already proves that the system 
belongs to type IV.’ 
3. We have a confirmation of this in the fact that in the ore 
deposits of pyrite and pyrrhotite, formed by magmatic differentia- 
tion, we always find, irrespective of the proportion by weight 
between the two sulphides, the sequence of crystallization, (1) pyrite 
(2) pyrrhotite, but, on the other hand, never the inverse sequence 
of crystallization, (1) pyrrhotite, and (2) pyrites. Regarding the 
t Amer. Jour. of Sci., Vol. XXXVI (z915). 
2See my résumé-treatise, “Die Sulfid-Silikatschmelzlésungen” (1917). 
