SOURCE OF CARBONATES. 479 



to over 50 per cent. But carbonation with or without desilication occurs 

 on far the most extensive scale in connection with hydration. "When the 

 great majority of the abundant silicate minerals are carbonated they are 

 also hydrated. (See pp. 397-399.) Finally carbonation with desilication 

 occurs rather extensively in connection with both oxidation and hydration. 

 (See p. 399.) In all these combinations, provided all the compounds 

 formed remain as solids, there is an increase in volume. This increase 

 rarely falls below 5 per cent; it runs as high as 75 per cent, or even higher, 

 but the more common range is between 15 and 50 per cent. It is well 

 known that the processes carbonation, carbonation combined with hydra- 

 tion or oxidation,- and carbonation combined with both hydration and oxi- 

 dation, liberate much heat, but the data are not at hand from which the 

 amount can be calculated. 



The process of carbonation just considered is one of paramount impor-' 

 tance in the belt of weathering. Although the process is not so extensive 

 as hydration, if one Avere to pick out a single chemical process especially 

 characteristic of this belt and of great significance in geology it would be 

 carbonation. Carbonation has continued through all geological time since 

 land areas first arose above the sea. Moreover, it has continued in all the 

 land areas of the globe; but the process is very slow in the frigid zones, 

 becomes of importance in the temperate zones, and is of great consequence 

 in the torrid regions. In the warm regions the process is rapid in propor- 

 tion as there is high humidity and - consequently abundant life. In the 

 arid regions carbonation is comparatively slow. 



There is no reason to suppose that the carbonates existed as original 

 rocks or as original minerals of the igneous rocks. If this be so, all the 

 carbonate formations which now exist and which have existed at any time 

 in the past have been produced by carbonation. The precipitation of 

 carbonates in the sea is accomplished largely through organic agencies. 

 These carbonates are essential to the existence of great classes of sea ani- 

 mals, and certain it is had not these carbonates formed by the process of 

 carbonation and been contributed to the ocean, the evolution of life upon 

 the globe would have followed entirely different lines from those that have 

 been followed. 



