CARBON ATION GENERAL IN BELT OF CEMENTATION. 611 



the silicates are undoubtedly those of dominant importance. The action of 

 carbonic acid upon the silicates results in the production of carbonates of the 

 bases and the liberation of silica, which may remain in situ as quartz or 

 go into solution as colloidal silicic acid. Monoxides of iron and manganese 

 originally present in this form or produced by the oxidation of sulphides 

 may unite with the carbon dioxide and form carbonates. 



Since the carbon dioxide of the belt of cementation is derived from 

 above, from below, and, by reactions of oxidation, from within the belt, 

 one would expect carbonation to take place throughout the belt of cemen- 

 tation. In the upper part of the belt of cementation the main source of 

 supply of carbon dioxide is that from the belt of weathering; but, as has 

 been noted, where the carbon dioxide is abundant oxygen is apt to be 

 deficient, so that where one finds active carbonation in the upper part of 

 the belt of cementation one would not expect to find active oxidation. 



The majority of the carbonates formed, and those originally present in 

 the belt of cementation, are rather soluble. Commonly, therefore, the 

 waters issuing- from the belt of cementation contain very large amounts of 

 carbonates and carbon dioxide. Average amounts of carbonates given by 

 Peale from many mineral springs from various portions of the United States 

 are as follows : " 



Grains per gallon. Milligrams per liter. 



North Atlantic States ...., 32.88 56.27 



South Atlantic States 13.70 23.45 



North Central States 45.35 77.62 



South Central States 18.30 31.32 



Western States 139.59 238.90 



Carbonic acid is a very much less active agent than oxygen. The 

 process of carbonation therefore takes place slowly, and the carbonic acid 

 does not all succeed in getting united with the bases. In many instances 

 the amount which has failed to unite with the bases is large. This is 

 illustrated by the abundance of uncombined carbon dioxide in the waters 

 of many of the famous springs of the world, as those of Carlsbad and 

 Shasta, and also by the great abundance of uncombined carbon dioxide 

 noted in connection with the rising waters of many mines. 



It has been explained under "The belt of weathering" that the 

 increase in volume in the process of carbonation of the silicates, provided 



oPeale, A. C, Lists and analyses of the mineral springs of the United States: Bull. U. S. Geol. 

 Survey No. 32, 1886, pp. 1-235. 



