478 A TREATISE OX METAMORPHISM. 



The process of carbonation with the liberation of silicic acid is contin- 

 uous, cumulative, and takes place on a vast scale. Notwithstanding this, 

 there is, as already seen (p. 474), only a comparatively small amount of 

 carbon dioxide in the belt of weathering at any one time — from- 38 to 500 

 parts in 10,000. Carbonates may be present; if so, they represent the 

 action of earlier carbon dioxide. The continuous process of carbonation 

 may be understood when the law of mass action and the time factor are 

 considered. Under the law of mass action an acid anywhere existing will 

 unite with some of the bases present. The compounds thus produced are 

 in part transported elsewhere by underground and overground drainage. 

 The carbon dioxide used is resupplied by the oxidation of organic material, 

 and thus the reacting agent continues its work; and so the process goes 

 on unceasingly through geological time. 



While this cumulative process goes on without cessation in the humid 

 regions where the carbonates are largely removed by solution, in the semi- 

 arid and arid regions the carbonates, especially the alkaline carbonates, 

 may become so abundant in the soil as to be unfavorable to vegetation, and 

 thus check the process. (See p. 543.) Where the alkalies, especially 

 sodium carbonate, exceed a certain very small amount, vegetation can not 

 exist, and the region becomes a desert." 



"The amount of soluble salts which plants can stand depends upon 

 the character of the salt, the character of the soil, and the kind of plant- 

 Hilgard states that few plants can bear as much as 0.1 of 1 per cent of 

 sodium carbonate, or about 3,500 pounds per acre to a depth of 1 foot 

 [about 3.9 kg. per sq. meter to depth of 30 cm.] ; of sodium chloride, about 

 0.25 per cent; and of sodium sulphate most plants can grow with 0.45 to 

 0.50 per cent present, and are affected by even less salts in the sandy lands 

 than on heavy clay or gumbo lands." 6 



The process of carbonation with desilication just described may take 

 place without other reactions with a number of minerals. (See p. 396.) 

 The change involves an increase in volume ranging from about 13 per cent 



a Means, Thos. H., and Gardner, Frank D., A soil survey of the Pecos Valley, New Mexico: 

 Field operations of Div. of Soils, U. S. Dept. of Agric, 1899, No. 64, 1900, pp. 53-58. 



i Whitney, Milton, and Means, Thos. H., The alkali soils of the Yellowstone Valley: Bull. Div. 

 of Soils, U. S. Dept. of Agric, No. 14, 1898, p. 10. 



