612 JOHN JOHNSTON AND PAUL NIGGLI 
flow of rocks has been studied experimentally, notably by F. D. 
Adams and his collaborators;* it resulted always in the production 
of a kind of cleavage. - 
It is characteristic of those cases of metamorphism where 
stress has been the predominant factor that the system, taken as a 
whole, is not in simple equilibrium, but rather in a stationary con- 
dition only. Now since, as we have seen, unequal stress acts 
mainly by depressing the melting-point and by raising the solu- 
bility, it follows that its effect on the transformation solid-solid 
is very small. Consequently we might expect that the forms pro- 
duced under unequal stress would often be forms which cannot be 
obtained from ordinary melts; and this indeed is often observed. 
Let us compare the mineral composition of the crystalline schists 
(which have very often been formed under stress) with that of erup- 
tive rocks or with metamorphic rocks of the Katazone, making 
use of the summary given by Grubenmann? (see p. 613). 
From this summary it is evident that many of the minerals 
occurring in rocks which have been subjected to unequal stress 
occur also as ordinary products of the secular alteration of mag- 
matic minerals at low temperatures. The water content of many 
of them determines in large part their behavior when heated or 
melted under ordinary conditions. Nevertheless the paragenesis 
of the stress minerals is, as Grubenmann has pointed out, char- 
acteristic of relatively low temperatures. Stress has increased 
the “‘activity,”’ so that in stressed systems some reactions readily 
take place which otherwise would go very slowly. Thus unequal 
stress may be regarded as a kind of catalytic agent, though in 
thinking of it in this way one must remember that it has frequently 
a characteristic influence on the nature of the products of a reac- 
tion, in addition to its influence on the rate of formation of those 
products. 
In spite of the fact that in stressed systems we are not dealing 
with simple equilibrium conditions, we may apply principles similar 
to those set forth on p. 597 to ideal typical systems. In doing 
* Phil. Trans. Roy. Soc. London, A, CXCV, 363; Am. Jour. Sci., XXIX (1010), 
465; Jour. Geol., XVIII (1910), 489; XI Congrés geol. internationale, II (1910), g11. 
2 Die kristallinen Schiefer, 2. Aufl., 1910. 
