PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 593 
The provisional diagram given by Shepherd and Rankin is 
reproduced in Fig. 1; we shall make no attempt to discuss it here, 
except in regard to two points, viz., (1) the occurrence of resorption, 
and (2) the formation of very different end-products from systems 
of very similar original composition. 
er 
A203 S10. 
ANORTHITE 
2-——- + 
Av20; SiO; 
<a 
3CAO AL.O. ea CROVAO: 
Fic. 1.—Provisional Diagram! for System.CaO - AL,O; « SiO, 
Consider a mixture the composition of which is represented by 
a point on the line PQ, and suppose it to be cooling. Lime crystals 
separate, the composition of the melt moves along the line POR 
until the point R on the boundary curve R-17 is reached. At this 
point the solid phase (3CaO-ALO,) begins to separate, which is 
possible only by resorption of CaO; the composition of the melt 
* The position of several of the points which were still doubtful at the time this 
diagram was published has now been more definitely established by further investiga- 
tion, the results of which will appear in a forthcoming paper by Rankin. These 
changes, however, do not affect any of the deductions which appear in the paper of 
Shepherd and Rankin (loc. cit.). 
