226 HAROLD L. ALLING 
respect to each other they will tend to separate into distinct and 
separate phases. The course of separation is indicated upon the 
basal plane of the diagram, by the lines IF, and IG, F, 
and G marking the limit. As a matter of fact we find in natural 
specimens every gradation between these two limits. That is, 
the diagram of one specimen may be properly represented by a 
cross-section of the solid model cut parallel with and near the 
back plane while another may be indicated by a section nearer to 
the front plane. It is obvious that the diagram, Figure 6, is highly 
diagrammatic and would be subject to modifications if the prin- 
ciple of dimorphism of the components were introduced. 
Specific Gravities of the Potash-Soda Feldspars.—As has been 
noted before, Allen and Day have shown that the specific gravities 
of the plagioclase feldspars change uniformly from 2.605 for 
artificial albite, to 2.768 for artificial anorthite, the plot of their 
values assuming a straight line as in Figure 3. If, however, a 
break or cusp occurs in specific gravity curves it signifies a discon- 
tinuity in physical, crystallographic, and chemical properties, 
resulting from the development of new modifications or the forma- 
tion of compounds. In the potash-soda series, the formation of 
compounds need not be considered, but the possible existence of 
dimorphous forms must be. 
Hintze’ gives the following data for the construction of such 
curves: : 
CoMPOSITION 
PERCENTAGE SPECIFIC 
MINERAL Or-AB Ratio ; 
(A) Or (B) K:0 GRAVITY 
Percentage Percentage 
Adularia....... Or 16 OO) 05 2.56 
Adulaniayeeneecr Or, Ab; 13 80 77 2.57 
Amazonite..... Or; Abz IO 60 60 2.58 
Rerthitex sane Or, Ab; 7 50 41 2.60 
Loxoclase...... Or, Ab, 4 20 24 2.61 
In graphic form these data are ambiguous. ‘The differences can 
be clarified by the examination of another system which has been 
studied experimentally: that of FeSO,-7H,O and MgSO,-7H.O. 
t Carl Hintze, Handbuch die Mineralogie, Il, p. 1358. 
