288 HAROLD L. ALLING 
We are not greatly interested in Figure 17 for it was introduced 
as a stepping-stone to Figure 18 with which we shall have much 
to do. Intermediate between the two diagrams already discussed 
is a type of diagram shown in Figure 18 (Roozeboom’s Type V) 
which represents a condition where limited solubility between the 
two components prevails in the solid condition. The lines FD, 
and GD, are the assumed solubility lines, showing the limits of 
solubility. Again, going through the same procedure as before, 
we take a melt composed of 60 per cent of N and 4o per cent of M 
and allow the temperature to fall to T, indicated by the point A 
on the liquidus. At this point the composition of the crystals 
separating into solid form is A. This is not pure WV as it would 
be in the previous Figure 17 but it is a solid solution, 91 per cent 
N and g per cent M, the percentage being found by dropping a 
perpendicular line from A; to the “Percentage Composition” 
scale at the bottom of the Figure 18. The liquid remaining 
has an approximate composition of A. As the temperature falls 
the change in the composition of the crystals is represented by the 
liquidus curve from A through B and C toward H. When the 
eutectic point £ is reached the eutectic mixture freezes, being com- 
posed of solid solution D,, 80 per cent NV and 20 per cent M, and 
solid solution D,, 20 per cent N and 80 per cent M. Thus the 
resulting solid mineral is composed of crystals—solid solutions— 
having a range in composition represented by the curve A,D, sur- 
rounded by the eutectic mixture which is composed of two solid. 
solutions, D, and D,. 
The discussion of these diagrams has been merely for the pur- 
pose of attempting to explain the meaning and the use of what are 
known as thermo-equilibrium diagrams. 
CONCLUSIONS 
1. The application of the phase rule and thermo-equilibrium 
diagrams to the feldspar system enables the mineralogist and the 
petrographer to secure a much better conception of the true 
physical-chemical nature of these minerals. This method of 
investigation throws considerable light upon the character of many 
other mineralogical systems. 
