480 A TREATISE ON METAMORPHISM. 



Notwithstanding the vast scale and dominant importance of the 

 process of carbonation, it is mainly accomplished, as has already been seen, 

 through the small amount of carbon dioxide continuously concentrated in 

 the belt of weathering by means of organic material. It is therefore clear 

 that a very slow but continuous~action, extending over the globe throughout 

 geological time, has produced stupendous results. But the positive side 

 of the process of carbonation is of scarcely less importance than the nega- 

 tive side. It has been pointed out that carbonation largely takes place 

 through the decomposition of the silicates or by desilication. The silica 

 set free, as already noted, largely separates as colloidal silicic acid. The 

 amount of silica thus liberated approximates to the molecular equivalent of 

 the carbon dioxide which unites with the bases combined with silica; or, 

 since the molecular weights of carbon dioxide and silica are 44 and 60.4, there 

 is about one and one-third times as much silica released from the silicates 

 as there is carbon dioxide combined in the carbonates. The stupendous 

 results of the process of carbonation are therefore matched by the results 

 of the process of desilication. As will be seen subsequently, while a por- 

 tion of the silica of the colloidal silicic acid is precipitated in the belt of 

 weathering, probably by far the greater part of it is carried by the ground 

 waters to the belt of cementation, and is there largely precipitated. 



While carbonation, with correlative desilication, is of such fundamental 

 importance in the belt of weathering, under exceptional conditions silication 

 ma}' occur. Where silica is very abundant and in a readily soluble form, 

 the law of mass action may be so effective as to result in the formation of 

 abundant silicates rather than carbonates. This has not been observed as 

 a general process in the belt of weathering, doubtless because of the lack 

 of close observation. However, in many cases in ore deposits above the 

 level of ground water the silicates of the metals are formed. So far as I 

 know, this is better illustrated in the lead and zinc district of Missouri than 

 elsewhere. Here silica as chert, partly amorphous, is very abundant. The 

 zinc was originally in the form of sulphide. As the level of ground water 

 descended owing' to denudation, and the sulphide arose into the belt of 

 weathering, the zinc sulphide was decomposed, the zinc being oxidized. 

 Simultaneously with this process it united, upon a somewhat extensive 

 scale, with the silica, producing silicate of zinc. 



