RELATIONS OF SOLUTION AND DEPOSITION. 487 



Consequent upon these various reactions, combined with solution, 

 material is continuously abstracted from, the belt of weathering, and thus 

 the openings tend to increase in size. This does not appear in the case of 

 the unconsolidated materials at the surface, for gravity brings the particles 

 together as fast as material is dissolved; but in the rocks which have 

 sufficient strength to hold together, the openings are often numerous and 

 large, and constitute a considerable percentage of the volume of the rocks. 

 The best illustrations of rocks with large openings are the limestones, 

 which, above the level of ground water, are commonly intersected with 

 numerous open joints, are often porous throughout, and not infrequently 

 contain caves. On account of the continuous abstraction of material by the 

 ground waters the belt of weathering might be called the belt of solution. 



DEPOSITION. 



Later it will be seen that concurrently with solution, favored by it 

 and in direct proportion to it, deposition is constantly going on. Thus 

 it will be seen that the substances dissolved in greatest quantity are those 

 deposited in greatest quantity. As just noted, in limestone regions solution 

 takes place on an enormous scale. But while this is going on deposition 

 of CaC0 3 is occurring on a large scale; for instance, in caves travertine, 

 stalactites, and stalagmites are forming. Where evaporation is not easy, 

 solution will be the rule; where evaporation is easy, deposition. Thus a 

 cave may be simultaneously enlarged at the bottom and decreased in size 

 at the top. Solution and deposition are correlative processes, but in the 

 belt of weathering the first is dominant. The absolute amount abstracted, 

 estimated by Reade at 96 tons per annum per square mile, represents the 

 difference between the results of solution and deposition. 



GENERAL STATEMENTS. 



On the preceding pages it has been seen that the processes of oxidation, 

 carbonation, hydration, and solution are characteristic of the belt of weath- 

 ering, and that all take place on a great scale. However, it has also been 

 seen that to some extent the reverse of these processes — deoxidation, 

 silication, dehydration and deposition — also occur. This is what one 

 would expect from the laws of physical chemistry. All chemical reactions 

 are reversible; and while the first set of the above processes greatly 



