282 VAN HISE—METAMORPHISM OF ROCKS AND ROCK FLOWAGLE. 
that the heat absorbed is more than 683 K — 238 K, or 345 K, for in the 
above equations a larger amount of water is combined with the FeS than 
with the FeO.* 
Another set of reactions of the most fundamental importance and wide- 
spread character, in which the first part of van’t Hoff’s law of chemical 
reactions and the law of pressure stand opposed to each other, and which 
occur in an opposite sense in the two physico-chemical zones, is the 
mutual replacement of carbon dioxide and silicon dioxide. Near the 
surface carbon dioxide replaces silicon dioxide, with great development 
of heat and expansion. The general fact of the carbonation of the sili- 
cates under these conditions the world over is well known. 
It is in the outer of the two belts of the upper physico-chemical zone, 
(see page 278) that of weathering, in which the process of carbonation goes 
on with greatest rapidity. Simultaneously with the deposition of the 
carbon dioxide much of the silica replaced is taken into solution and is 
carried downward by the percolating waters. In the inner of the two 
belts of the upper physico-chemical zone the silica is deposited upon an 
enormous scale. This deposition is probably accompanied by consider- 
able absorption of heat, under the law that the negative value of the heat 
of solution is greater, the more insoluble the substance (see page 277). 
Of course, carbonation in the upper zone may take place without re- 
placing some other compound,as in the case of the union of carbon dioxide 
with iron protoxide of magnetite, thus producing iron carbonate; but 
in this case the liberation of heat and expansion in volume are even 
ereater than in the replacement of silica by carbon dioxide. 
In the lower physico-chemical zone, and especially under mass dy- 
namic conditions, silica replaces carbon dioxide upon the most extensive 
scale with great absorption of heat and with condensation. As illustra- 
tions of this may be mentioned the formation of wollastonite from pure 
limestone, of tremolite from dolomitie limestone, and of actinolite and 
ertinerite from ankerite or from siderite. In the impure limestones under 
deep-seated conditions, where numerous bases are present, various com- 
plicated silicates form, such as other pyroxenes and epopnbales and 
tourmaline, chondrodite, etcetera. 
The physico-chemical principles cited (pages 275- 277) give reasons for 
the existence of the above reverse sets of reactions in the two zones. We 
can now state a cause why hydration takes place in the first and dehy- 
dration in the second. and so on for other reactions characteristic of each 
of the physico-chemical zones. The depths at which the reactions re- 
verse for different compounds and for the same compound under differ- 
* Lehrbuch allgemeinen Chemie, von W. Ostwald: Zweiter Band, 1892, pp. 296, 301. 
