598 JOHN JOHNSTON AND PAUL NIGGLI 
Let us assume further that the minerals X, Y, Z are stable both 
above and below this inversion point, that they are made up of 
other components and do not affect the equilibrium relations of 
ABCD. We could then have the rocks XYZABC (I), XYZABD 
(II) on the one hand, XYZCDA (III), XYZCDB (IV) on the other. 
These four types are the ideal ones in the temperature, pressure and 
concentration region characterized by the relation 1,A-+1,B=> 
n,C-+n,D. Now if a rock of type I comes into the temperature 
and pressure regions of types II or III, then D must be formed and 
either A or B must disappear. 
Now the relations in actual metamorphic processes are compli- 
cated by a number of factors: that the rate of reaction is often 
small, that metastable forms occur, or that false or partial equili- 
brium only is attained. Hence we cannot know if a relation 
observed in the study of thin sections of a series of rocks is really 
characteristic of the temperature and pressure conditions which 
obtained during the process of metamorphism. Nevertheless, 
although the whole transformation may be metastable, it may be 
profitable to consider the ideal relation involved. 
On the other hand, the formation of a variety of combinations 
is favored by the circumstance that pressure and temperature 
change during the metamorphic process and that the co-ordinates 
of the invariant point of neighboring and chemically very similar 
rocks may be different. Indeed in geologic units one seldom finds 
the production by metamorphism of a single type of mineral com- 
position; instead of this there is usually a whole series, the indi- 
vidual members of which are closely interrelated. To take a few 
examples which have been investigated, there are: the hornblende 
porphyroblastic schists of Tremola’ (Gotthard, Switzerland); the 
glaucophane schists of Syra? (Greece) and of the Val de Bagne? 
(Wallis, Switzerland); the chloritoid schists of the Garvera* region 
(Gotthard); the eclogites of the Otztal’ (Tyrol), etc. 
™L. Hezner, Neues Jahrbuch f. Min., Beilage Band XXVIII (1908), 157. 
2 H.S. Washington, Am. Jour. Sci. (4), XI (1901), 35. 
3T. Woyno, Neues Jahrbuch f. Min., Beilage Band XXXIII (1911), 136. 
4P. Niggli, Beitrdge geol. Karte Schweiz, N.F., XXXVI (1912). 
sL. Hezner, T'sch. Min. Petr. Mitteil, XXII (1903), 437, 505. 
