CAUSES OF CEMENTATION. 635 



free. This material is taken into solution by the descending waters and 

 transmitted to the belt of cementation. As already pointed out, by the 

 processes of denudation and downward migration of the belt of weathering, 

 an abundant increment of silicic acid is ever contributed to the belt of 

 cementation. If this material could be there deposited, this would account 

 for the very general cementation by quartz. It has been seen that the 

 silica largely separates from silicates as colloidal silicic acid. This com- 

 pound is unstable, and tends to slowly decompose into water and silica. 

 During the long and very slow journey of the water in the belt of 

 cementation there is ample time for this process. It is likely that the 

 principle that insoluble products are apt to be precipitated has a bearing 

 at this point. Quartz is an extremely insoluble compound, and therefore 

 tends to separate from the solutions bearing colloidal silicic acid. In the 

 precipitation of the silica from the solutions the previous almost universal 

 presence of quartz is also of great importance; for it has been shown (pp. 

 120-122) that the presence of a mineral of a certain kind favors the abstrac- 

 tion of like material from a solution and the addition of that material to 

 the nuclei. Hence a large part of the most abundant substance contributed 

 by the solutions from the belt of weathering is precipitated in the belt of 

 cementation. As already pointed out, an additional supply of silica is fur- 

 nished by the carbonation of the silicates within the belt of cementation. 

 Finally, silica is furnished by emanations from intrusive rocks. Thus 

 is explained the universal dominance of quartz as a gangue and cementing 

 material, and the formation of great masses of sandstones and quartzites by 

 this cementation process,, as fully explained (pp. 864-868). 



It is highly probable that the solutions are partly or fully compensated 

 for the silica abstracted by the addition of calcium, magnesium, sodium, 

 potassium, iron, etc., obtained by alteration of the minerals within the belt 

 of cementation. It has been noted (pp. 609-610) that carbon dioxide is 

 added to the belt of cementation in four ways. This carbonic acid unites 

 with bases, producing carbonates. Further, it has been explained (pp. 

 624-625) that carbonates are very important compounds added to the belt 

 of cementation. Thus the solutions of the belt of cementation have two 

 sources of these compounds. It is therefore to be expected that undergound 

 solutions which issue at the surface and in springs would bear as their most 

 abundant compounds the carbonates of the alkalies, alkaline earths, and iron, 



