COMBINATION OF PROCESSES. 655 



Irving further points out" that, after an igneous rock has solidified, as 

 it continues to cool it contracts, and therefore at the contact of the igneous 

 rock with the intruded rocks openings are likely to be produced. Fre- 

 quently the contraction openings are not, confined to the contacts, but 

 extend into both the injecting and the injected rock. Such openings, being 

 within the intrusive rocks or near the contacts of the intrusive and intruded 

 rocks, furnish trunk channels which are likely to receive solutions from both 

 the injecting and the injected rocks. Thus are explained the very frequent 

 trunk channels for circulation and vein formation within and adjacent to 

 igneous rocks. The openings along the borders of igneous rocks and 

 within them are of very great consequence in connection with ore deposits. 

 (Seep. 1116.) 



Concluding, it is clear that while one or two of the processes of cemen- 

 tation, metasomatism, and injection may occur without the other one or 

 two, it is rarely that one works alone. Of the processes, that which is most 

 likely to occur without the others is cementation. Metasomatism rarely, if 

 ever, takes place without cementation. Cementation and metasomatism 

 may take place on an extensive scale without injection. Injection is invari- 

 ably accompanied by cementation and metasomatism. 



CHANGES OF CHEMICAL COMPOSITION. 



The amount of change in the chemical conrposition of the rocks In the 

 belt of cementation is likely to vary with the porosity. Where the rocks 

 are dense, chemical analyses appear to show that the average composition 

 of the rocks does not greatly change, except by oxidation, carbonation, and 

 l^dration. The ox} 7 gen and carbon dioxide are added directly or indirectly 

 from the atmosphere, and the water from the hydrosphere. In the opening 

 pages of Chapter II it has been explained that all chemical changes involve 

 molecular mechanical action, even where the average chemical composition 

 remains the same. There may be interchange on a great scale between the 

 minerals within short distaiices (for instance, glasses may Avholly devitrify), 

 but in the dense rocks the migrations of material are ordinarily confined 

 within somewhat narrow limits and the average change in chemical 

 composition is not great. 



"Irving, A., Chemical and physical studies in the nietarnorphisni of rocks, Longmans, Green &Co., 

 London, 1889, pp. 82-84. 



