Van Hise — Metamorphism of Bocks and Rock Flowage. 79 



" In genera], in comparing substances which are chemically 

 analogous, and soluble with difficulty, the heat of precipitation 

 (= the negative value of the heat of solution) is greater the 

 more insoluble the substance is." 



Finally, the relations between heat, pressure, and chemical 

 action in a solution may be generally expressed as follows : 

 " Every change of one of the factors of an equilibrium occa- 

 sions a rearrangement of the system in such a direction that 

 the factor in question experiences a change in a sense which is 

 contrasted with the original change." 



Application of Physico-Chemical Principles to the Earth's 



Crust. 



It is evident from the foregoing principles that within the 

 superficial zone of rocks in which reactions take place directly 

 under our observation, and within the deeper-seated zone in 

 which reactions have taken place and later have been brought 

 within our observation, there may be opposing tendencies. 

 The changing factors in these two physico-chemical zones are 

 temperature and pressure and consequently chemical action. 

 Both of these increase with depth. 



Upper Physico-chemical Zone. 



The chemical reactions which occur within the upper zone 

 of observation of the earth are at the lower temperatures 

 referred to in van't Hoff's law. Hence near the surface the 

 reactions usually, if not always, take place with the libera- 

 tion of heat, according to the first part of van't Hoff's law. 

 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 compression in volume result in the liberation of heat ; 

 in the second case, the heat liberated is that developed by the 

 chemical reaction minus that absorbed as a result of the work 

 done in expanding the volume. 



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 disintegra- 

 tion, 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. 



