PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 597 
therefore, we have in equilibrium BCDL, where D stands for the 
solid phase (CaCl,-4H,O). Thus the reaction (CaCl,-2MgCl,- 
12H,O) + (CaCl,-6H,O) = (CaCl,-4H.O) + solution now takes 
place, and instead of III (BCL) we then have the combinations DLC 
or DLB. For instance, at 28°2 (CaCl,-4H,O) and (CaCl,-6H,0) 
are in equilibrium with solution of the composition 100H,O, 14.4 
CaCl, 1.37MgClL. The relation between P and T for the 4-phase 
equilibrium CDBL is not known; but by analogy we may consider 
that its general course is similar to that of the curve for ACBL. 
All of these relations are 
exhibited in the subjoined 
figure (Fig. 3), which repre- 
sents a part of the ideal 
figure. From it we see again 
how relatively small tempera- 
ture changes and large pres- 
sure changes alter the type of 
the 3-phase, and also of the 
2-phase, combinations. 
Consideration of an ideal 
case.—Let us suppose that ea az a aie 
between the four minerals Fic. 3.—Portion of PT projection for 
ABCD. whose compositions system H,O - MgCl - CaCh, representing the 
B stable 3-phase groupings and two 4-phase 
may be expressed in terms jjnes. 
of the three components A, 
L, and M, the relation 2,A-+7,B=n,C-+n,D exists;' it being 
assumed further that there are no false equilibria or unstable 
combinations. At arbitrary pressure and temperature only three 
of these can coexist; let the right-hand side be favored by lowering 
of temperature and of pressure. Then at low temperatures and 
small pressure, we should have the stable combinations ABC or 
ABD; at high temperatures and pressures, CDA or CDB. 
™ This equation is analogous to the equation already given for the system CaCl.- 
MgCl.-H.O. The complete analytical presentation of the relations for ternary systems 
is given by Schreinemakers in Heterogene Gleichgewichte, III, 210 f., which the reader 
desirous of fuller information with regard to ideal cases is advised to consult. (This 
book forms the third volume of the treatise, of which Vols. I and II only were completed 
by Roozeboom.) 
