PRINCIPLES UNDERLYING METAMORPHIC PROCESSES 499 
sure which varies enormously with the temperature. Unequal 
pressure moreover affects such dissociations to a still greater extent 
than it influences melting-points. Transitions involving a vapor 
phase therefore can be employed as reference points when, and only 
when, we can accurately define the character and magnitude of the 
compression—a condition unlikely to be fulfilled in any instance 
of geologic importance at any time in the immediate future. 
Occurrence of reactions in systems solid-solid.—This subject has 
been treated at some length in a recent communication from this 
laboratory;' so only the general conclusions will be presented here. 
Reaction may proceed at the surfaces of contact of adjacent grains 
of the constituents; but it by no means follows that reaction occurs 
in all systems where we might on other grounds judge that the 
stable state of the system would be that which obtains after reaction 
has taken place. The extent of the reaction will be increased by 
renewal of the surfaces of contact, and this may be effected by any 
agency which produces a kneading or grinding of the mass. Hence 
the reaction will be furthered by application of non-uniform 
(unequal) pressure (or in other terms, shearing stress), which more- 
over may also act in another way, the net result of which, however, 
is again to bring together new surfaces of contact. This will 
happen, namely, if the unequal pressure is of such character as to 
cause any of the original constituents or of the products of reaction 
to melt at the temperature of experiment; for then it will obviously 
further the reaction. Furthermore, non-uniform pressure can pro- 
mote reaction by bringing adjacent grains into good contact; the 
reaction can then progress by diffusion across the area of contact. 
The effect of uniform pressure on reaction between solids is limited 
to this réle, and therefore its effect is in general slight. 
Interdiffusion of solids is appreciable in some systems: for 
instance, gold traveled into lead some 7 mm. in four years at room 
temperature, and about as far in as many weeks at 160°.” All the 
evidence indicates, however, that interdiffusion goes on only in 
tJ. Johnston and L. H. Adams, Am. Jour. Sci. (4), XXXV (1913), 205; Z. anorg. 
Chem., LXXX (1913), 281. 
? Roberts-Austen, Phil. Trans. Roy. Soc. London, A., CLXXXVII (1896), 383. 
For full information on this subject see “‘Report on Diffusion in Solids” by C. H. 
Desch, British Assoc. Report (Dundee, 1912); see also the recently published Geolo- 
gische Diffusionen by R. E. Liesegang (Dresden: Steinkopf, 1913). 
