540 A TREATISE ON METAMORPHISM. 



cavities may be lined with crystals of nearly the composition of the sur- 

 rounding rock, thus producing geodes; the joint, fault, and irregular open- 

 ings may be partly or completely filled with carbonates, producing veins; 

 but geodal filling and veins commonly form in the belt of cementation. In 

 the caves the floor is ordinarily covered with stalagmites and from the roof 

 stalactites depend. The precipitation of the carbonate in the openings is 

 commonly caused by the lessened pressure, by the escape of some of the 

 dissolved carbon dioxide, or by evaporation, any one of which or any 

 combination of which may produce a condition of supersaturation and 

 consequent deposition. 



Another compound which is very abundantly precipitated in the belt 

 of weathering is silica, as opal, chert, or quartz. This again is largely a 

 consequence of its abundance, but also is a function of its solubility. (See 

 p. 480.) By the decomposition of the silicates silicic acid, probably 

 largely as colloidal silicic acid, is set free in almost incredible quantities. 

 (See pp. 480, 516-517.) The larger amount of this material probably 

 joins the belt of cementation, and thus is a source of the silicification which 

 is so dominant a process of that zone. (See pp. 622-623.) But by chemical 

 change, which is a factor in its precipitation as well as in its abundance, 

 a very considerable amount is partly or completely dehydrated and thrown 

 down in the belt of weathering as opal or quartz. Like calcite and dolomite, 

 it is deposited in the belt of weathering in geodal cavities, in caves, and in 

 veins, and as siliceous sinter. ~ 



(b) The more insoluble a compound is, the more likely is it to form 

 a precipitate in the belt of weathering. This is almost self-evident, and 

 only needs cases illustrating the principle. The best illustrations are 

 furnished by the iron and aluminum oxides and their mixtures. It has 

 already, been seen that these are the two compounds which are most 

 frequently taken as constant in weathered rocks, and on this hypothesis are 

 used to determine the amount of loss of the other constituents. Much of the 

 iron in the original compounds is in the ferrous form, but under the con- 

 ditions of the belt of weathering it is oxidized to the ferric form, and much 

 of it simultaneously precipitated. On account of the insolubility of the 

 oxide of iron and oxide of aluminum, as compared with other constituents, a 

 large amount of iron, as hematite, limonite, and other hydrated forms, and 

 of alumina, both hydrous and anhydrous, and of mixtures of iron oxide 

 and aluminum oxide, bauxite, accumulates at the surface. It is well known 



