APPLICATION OF PHYSICO-CHEMICAL PRINCIPLES. DAS) 
sequently the temperature may become so high that the tendency for 
reactions to take place which result in the development of heat is less 
dominant. However, at moderate depth, under ordinary conditions, 
say at 9,000 meters, the temperature is not very high, probably in the 
neighborhood of 300° C. Thus the tendencies for reactions to take place 
under the first part of van’t Hoff’s law, rather than the second part, 
would generally still control for a very considerable depth if it were not 
for the enormous pressure. This may become the dominating factor, 
especially in places of mass dynamic action, and reactions take place 
which result in the production of less volume. The compression may 
be accomplished by the driving off of a substance, as water or carbon 
dioxide, by the replacement of one substance by another, as magnesium 
for calcium, or by the more regular arrangement or greater complexity 
of the molecules, as in the case of devitrification. All of these conden- 
sations result in the evolution of heat. If, in order to produce the con- 
densation, the chemical reactions are of such a character as to occur 
under the second part of van’t Hoff’s law (and, as subsequently seen, 
this is commonly the case), they result in absorption of heat. The net 
result as to absorption or evolution of energy will depend upon the rela- 
tive values of these opposite factors. 
RELATIONS OF THE TWO PHYSICO-CHEMICAL ZONES 
Where expansion of volume is the rule, energy is absorbed (1) in 
increasing the volume of the rock affected by the reaction, and (2) in 
lifting the overlying rock in order that space shall be available for the 
expansion. Where contraction of volume occurs, energy is developed 
(1) by the decrease in the volume of the rock affected by the reaction, 
and (2) by the sagging of the overlying material. Below the extreme 
outer film of the earth the second factor is of vastly greater importance 
than the first, and its relative importance increases with depth. This is 
more broadly true in the case of expansion than in the case of contrac- 
tion. The importance of the second element in the case of expansion is 
illustrated by the frequent rapid hydration or slacking with great ex- 
pansion and rapid disintegration which follows when a partly hydrated 
rock, buried but a few feet, is brought to the surface.* Apparently, when in 
place, the tendency for hydration and development of heat was not suffi- 
cient to lift the superjacent material. When this necessity was removed 
by relieving the material from pressure, the process went on to comple- 
tion with great rapidity. In the case of contraction in the zone of frac- 
ture, the strength of the rocks may be sufficient to prevent subsidence 
* Disintegration of the granitie rocks of the Distriet of Columbia, by G. P. Merrill: Bull. Geol. 
Soe. Am., vol. vi, 1895, p. 332. 
