476 A TREATISE ON METAMORPHISM. 



resistant of all the minerals and rocks tested. Apatite was found to be as 

 readily soluble as the more soluble silicates. Perhaps one of the most 

 interesting results in reference to the silicates is the comparative readiness 

 with which the hydrous silicate, serpentine, is attacked. From this fact 

 Mueller makes the point that this silicate does not represent an end product 

 of alteration." 



Johnstone later experimented upon the micas, including muscovite, 

 biotite, and lepidomelane, and found that they are attacked by water solu- 

 tions of carbon dioxide.'' As one would expect from the above reactions, 

 Struve" found that water containing carbon dioxide at ordinary pressure 

 attacked basalt, phonolite, gneiss, granite, clay slate, and porphyry. 



Combining the experimental fact that carbon-dioxide solutions decom- 

 pose the silicates and the observed -fact of the abundance of C0 2 where 

 vegetation is plentiful, one would expect that the process of carbonation 

 would be more rapid in regions of abundant vegetation than elsewhere. 

 Belt many years ago made observations which confirm this expectation. 

 He says that the decomposition of rocks in tropical America is largely con- 

 fined to the forest regions, and ascribes it to the action of water charged 

 with acids derived from the decomposing vegetation. 1 ' Where vegetable 

 matter is abundant it has also been observed that the amount of dissolved 

 silica contained in underground water is much greater than where veg- 

 etation is sparse or absent. 6 This observation is direct evidence that the 

 reactions of carbonation and desilication are correlative, and are very largely 

 due to the concentration of carbon dioxide in the belt of weathering by the 

 oxidation of organic matter. 



Since the reaction of carbon dioxide upon the silicates and other com- 

 pounds forms carbonates, in a soil there may be present both sedimentary and 

 secondary carbonates. Now, it has been shown experimentally by Bischof 

 that the alkaline carbonates are capable of decomposing the silicates at 



"Mueller, Richard, TJntersuchungen ueber die Einwirkung des kohlensiiurehaltigen Wassers 

 auf einige Mineralien und Gesteine: Tschermak's mineral. Mittheil., vol. 7, 1877, pp. 25^18, especially 

 pp. 39,46^8. See Merrill, G. P., Rocks, rock-weathering, and soils; Macmillan Co., New York, 1897, 

 pp. 192-193. 



b Johnstone, A., On the action of pure water and of water saturated with carbonic-acid gas on 

 the minerals of the mica family: Quart. Jour. Geol. Soc. London, vol. 45, 1889, pp. 363-368. 



cStruve, F. A. A., Ueber die Xachbildung der naturlichen Heilquellen, Pogg. Ann. vol. 7, pp. 

 341-372-429-150. 



tf Belt, Thomas, The naturalist in Nicaragua, 1874; cited in Merrill, p. 175. - 



"Hunt, T. Sterry, Chemical and geological essays, 1875, pp. 149-152. 



