CARBON ATION AND VEGETATION. 477 



ordinary temperatures and pressures." A great variety of silicates were 

 thus decomposed. The alkaline silicates and carbonates of sodium, potas- 

 sium, calcium, magnesium, and iron were formed. Not only do the alkaline 

 carbonates decompose silicates in the belt of weathering, but other carbon- 

 ates accomplish the same result. This has been shown by observations on 

 the alterations in the soil by Hilgard. 6 Of these other carbonates, that of 

 calcium has been found to be the most important. Hilgard states that 

 the decomposition of the silicates is much more active in calcareous soils 

 than in noncalcareous soils that are otherwise similar. He says " this is 

 seen when we contrast the analyses of calcareous clay soils of the humid 

 region with the corresponding noncalcareous ones of the same. In the 

 former the proportions of dissolved silica and alumina are almost invariably 

 much greater than in the latter, so far as such comparisons are practicable 

 without assured absolute identity of materials.'"' 



Since the carbonate sediments are mainly or solely produced by the 

 process of carbonation of the silicates of an earlier period, Hilgard's state- 

 ment is equivalent to saying that the process of carbonation to-day is 

 promoted by the carbonation of past g-eological ages. 



It is well known that soils containing carbonates are fertile, and 

 therefore are favorable to abundant vegetation. This is well illustrated by 

 the limestone soils. As already seen, vegetation promotes carbonation, 

 and thus produces carbonates. It therefore appears that there is constant 

 action and reaction through veg;etation and carbonation in promoting the 

 process of carbonation. Vegetation results in the process of carbonation, 

 and this produces the carbonates. Carbonates in turn result in further 

 carbonation and furnish favorable conditions for vegetation. Carbonation 

 provides carbonates, and therefore promotes vegetation. The relations of 

 the three may be represented by the following diagram, the arrows 

 indicating the directions of action and reaction. 



, Vegetation. 



Carbonation =» Carbonates. 



«Bischof, G., Elements of chemical and physical geology, translated by B. H. Paul and J. Drum- 

 mond, Harrison & Sons, London, 1854, vol. 1, pp. 8-11. 



''Hilgard, E. W., Report on the relations of soil to climate: Bull. Weather Bureau, U. S. Dept. 

 of Agric., No. 3, 1892, pp. 36-38; cited, Merrill, p. 371. 



c Hilgard, tit., p. 37. 



