604 A TREATISE ON METAMORPHISM. 



considered in Chapter VI on "The belt of weathering," and they will here 

 be taken up only so far as their work is different in the belt of cementation 

 from that in the belt of weathering. The modifications are easily made for 

 oxidation, carbonation, and hydration and their reversals, but are complicated 

 for solution and deposition. 



In the tipper part of the belt of cementation, that near the level of 

 ground water, the active agents in the solutions are very similar to those 

 in the belt of weathering, and therefore the reactions are closely allied to 

 those of the belt of weathering. But as depth below the belt of weathering 

 increases, the distinctions between the reactions which are of controlling 

 importance in the two belts becomes more and more clear, and for the 

 major part of the belt of cementation the relative importance of reactions is 

 very different from that in the belt of weathering. The depth to which all 

 the reactions of the belt of weathering extend into the belt of cementation 

 depends very largely upon whether the waters for the particular area are 

 ascending or descending. At places where large amounts of descending 

 waters are converged, and where the variations of topography are marked, 

 so as to give a very high head, all the reactions of the belt of weathering 

 may be quantitatively important to depths of 800 or 1,000 meters or more, 

 although such depths as these are very unusual. But where the descend- 

 ing currents are driven by a low head, some of the reactions of the belt 

 of weathering may extend only to a very moderate depth into the belt 

 of cementation. In areas where the water currents are ascending, and so 

 continue to the belt of weathering, some of the reactions of the latter belt 

 may extend scarcely at all into the belt of cementation. Oxidation, car- 

 bonation, hydration, and solutioii and deposition commonly do not take 

 place separately, but occur at the same time upon the same materials; 

 although, as pointed out on page 611, in the upper part of the belt of 

 cementation, carbonation and oxidation are to some extent exclusive of 

 each other. But for the purposes of analysis it is necessary to treat each 

 separately. 



OXIDATIOX. 



Ordinarily when water passes from the belt of weathering into the belt 

 of cementation it contains oxygen in solution. The amount of this oxyg*en 

 may be as great as can be dissolved under atmospheric conditions of pres- 

 sure and temperature, or the water may be saturated. From this the 



