CHEMICAL CHANGES IN BELT OF CEMENTATION. 603 



are of vastly greater importance than in the belt of weathering. At the 

 surface or within the belt of weathering, igneous rocks rapidly lose their 

 heat, and thus produce a comparatively short-lived effect. Within the belt 

 of cementation the igneous rocks retain their heat for a very long time. 

 By their introduction they produce both a direct effect, due to their own 

 action, and — far more important — an indirect effect by heating the 

 solutions, and thus very greatly increase their efficiency both as to speed 

 of reaction and as to quantity of material which can be held, as is fully 

 explained elsewhere. (See pp. 79-81.) 



The range of temperature at which the solutions work is from 0° C. to 

 the critical temperature of water, which, as we have seen (pp. 566-569), is 

 365? C. The amount of work which the solutions accomplish in a given 

 time at very high temperatures is almost indefinitely greater than that 

 which is accomplished at lower temperatures. (See p. 79.) 



The pressure at which the solutions work varies from that of an 

 atmosphere to the pressure of a column of water to the bottom of the 

 zone of fracture. Supposing the bottom of this zone of fracture to be at a 

 depth of 10,000 meters, the maximum pressure would be 1,000 kilograms 

 per square centimeter. Therefore the range of pressure upon the solutions 

 within the belt of cementation is very great. 



By observation we know that in the belt of cementation the important 

 chemical reactions which take place as a result of the action of the various 

 agents are oxidation, carbonation, hydration, and solution and deposition. 

 These reactions are the same as those in the belt of weathering. Just as in 

 the belt of weathering, the reactions of oxidation, carbonation, hydration, 

 and possibly solution preponderate over the reverse reactions. However, 

 the relative importance of these various reactions in the two belts is very 

 different. 



All of these reactions, as has been seen, are of very great consequence 

 in the belt of weathering. Oxidation and carbonation are of much less 

 consequence in the belt of cementation; but this statement can not be made 

 of hydration, solution, and deposition Hydration, because of the decreased 

 prominence of oxidation and carbonation, becomes the chief one of these 

 three processes in the belt of cementation. Solution and deposition are both 

 of fundamental importance; but the first, solution, does not dominate as 

 it does in the belt of weathering. All of these reactions have been fully 



