CAUSES OF CEMENTATION. 631 



We are now prepared to suggest causes which may combine to explain 

 the apparently contradictory facts that a vast amount of openings in the 

 belt of cementation is cemented and that emerging waters may contain 

 more material in solution than entering waters; or that, of the two processes 

 of deposition and solution, the latter is the preponderant one. 



EXPANSION REACTIONS. 



The first cause which will be given for cementation — and I believe the 

 one of greatest importance — is that of expansion reactions. It has been 

 explained (p. 603) that the chief reactions of the belt of cementation are 

 oxidation, carbonation, and hydration, and that of these three the latter is 

 of greatest consequence. Further, it /has been seen that, provided all of the 

 compounds formed remain as solids, the average volume increase in con- 

 sequence of these reactions varies from 15 to 50 per cent or more. So far 

 as these expansion reactions take place — and they undoubtedly occur on a 

 most extensive scale — they tend to fill the openings, and thus cement and 

 consolidate the rocks. 



In the following section it is explained that the process of metasoma- 

 tism, or change of the minerals within the body of the rocks, has taken place 

 upon a vast scale in the belt of cementation. Indeed, in extensive ancient 

 formations, especially in rocks of a porous character, it has frequently hap- 

 pened that, with the possible exception of quartz, scarcely a vestige of any 

 original mineral remains. Many of the rocks, especially the igneous rocks, 

 are largely or almost wholly composed of silicates. Even in the acid rocks 

 the amount of quartz and other compounds, aside from the silicates, together 

 is rarely as great as 50 per cent, leaving 50 per cent or more of silicates. 



We shall now consider the expansion due to alteration of a rock which 

 contains 50 per cent of silicates. If we suppose the silicates to be com- 

 pletely altered by the processes of oxidation, carbonation, and hydration, 

 and that in these processes the expansion is 25 per cent — which is less rather 

 than more than the average amount — if all of the materials were deposited 

 the resultant expansion would be sufficient to furnish material to fill pore 

 spaces to the amount of 12 J per cent of the total original solids of the rock. 

 Therefore, in the case of an igneous rock of exceptional acidity and excep- 

 tional porosity, the expansion reactions would be adequate to entirely fill 

 the openings of a rather porous rock. But it may be said that many of the 



