COMBINATION OF PROCESSES. 653 



COMBINATIONS AND RELATIONS OF MECHANICAL WORK, CHEMICAL WORK, 

 AND IGNEOUS WORK. 



For the sake of clearness of analysis, each of the mechanical processes 

 of consolidation — strain within the elastic limit, and fracturing; the chemical 

 processes of cementation and metasomatism; and the igneous process of 

 injection — has been treated separately. As a matter of fact, any one of 

 the processes may be dominant in an individual case, as, for instance, the 

 cementation of a sandstone to quartzite. Commonly, however, they do not 

 occur independently in the rocks, but two or more take place simultaneously. 

 The joint work of different combinations will be briefly considered in order 

 to make clear the interaction of the processes. 



The mechanical, chemical, and igneous processes often occur together, 

 one frequently promoting the other. The chemical processes are promoted 

 by the mechanical processes. Fracturing gives water a much larger surface 

 of contact; it promotes water circulation; it allows easy injection. The 

 mechanical processes induce a state of strain in the mineral particles where 

 it does not fracture them. Therefore fracturing very notably promotes 

 cementation, metasomatism, and injection. Indeed, in the belt of cementa- 

 tion, where the rocks are massive and originally contained no openings, 

 cementation, metasomatism, and injection would take place with difficulty 

 and slowly if it were not for mechanical action. 



It has been explained that the processes of oxidation, carbonation, and 

 hydration, characteristic of the belt of cementation, can take place only 

 with expansion of volume. Where there are no openings available, in 

 order that these reactions shall occur, it is necessary that much material 

 shall be dissolved to compensate for this expansion; and in the massive 

 rocks the water circulation is so slow that the material can be dissolved and 

 transported from the belt of cementation only very slowly indeed. This, 

 therefore, is the explanation of the small amount of alteration in the belt 

 of cementation where the rocks are not porous and have not been fractured. 



However, where the massive, originally nonporous rocks have been 

 fractured, the fracturing increases the total volume of the rocks by the 

 amount of the openings produced. Later the openings are closed by 

 cementation, metasomatism, and injection, and thus there is absolute 

 increase in the volume of the material. This increase has been estimated 



