278 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGE. 
opposing tendencies. The changing factors in these two physico-chem- 
ical zones are temperature and pressure. Both of these increase with 
depth. 
UppmR PHYSICO-CHEMICAL ZONE 
The chemical reactions which occur within the upper zone of observa- 
tion of the earth are at the lower temperatures referred to in van’t Hoft’s 
law. Hence near the surface the reactions usually, if not always, take 
place with the development of heat, according to the first part of van’t 
Hoft’s law. Therefore in this zone the occurrence of a reaction in the 
alteration of rocks is favorable to further reaction and alteration, for the 
heat developed by the first reaction is retained by the adjacent material, 
at least for a time, and this promotes a subsequent reaction, etcetera; but 
this tendency would be reversed if the temperature became too high. 
The pressure near the surface is small, and therefore the law of chemical 
reactions with the liberation of heat in the outer zone is the dominating 
factor. ; 
Hence an alteration may take place which works with or against 
pressure. In the first case, both the chemical reaction and the compres- 
sion in volume result in the liberation of heat; in the second case, the 
heat liberated is that developed by the chemical reaction minus that ab- 
sorbed as a result of the work done in expanding the volume. 
In the treatise from which this paper is taken it will be shown that 
the upper physico-chemical zone is divisible into two parts, the reactions 
within which strongly contrast: (1) an upper belt. mainly above the 
level of underground water, which is generally known as the belt of 
weathering, where disintegration, decomposition, and solution are the 
rule, and (2) a lower belt of greater thickness, in which cementation of 
openings is the rule, and therefore a belt in which induration is one 
of the most characteristic features. 
The material dissolved in the upper belt is abundantly deposited 
in the lower belt, and thus there is a constant downward transfer of 
material. The total amount of material which has been deposited in 
the thick lower belt is only in small part derived from the thin belt of 
weathering which exists at a given time; but, as a result of erosion, the 
belt of weathering is constantly migrating downward and encroaching 
upon the upper part of the belt of induration, and therefore there is 
never lack of soluble material in the upper belt, which may be dissolved 
and transferred to the lower belt, where it may be deposited. 
LOWER PHYSICO-CHEMICAL ZONE 
If one imagines himself as passing from the surface to considerable 
depth below the surface, the temperature ever becomes higher, and con- 
