ORIGIN OF LIMESTONES. 791 



CARBONATE ORDER. 



CALCIUM-MAGNESIUM CARBONATE FAMILY AND METAMORPHOSED EQUIVALENTS. 



The calcium-magnesium carbonate family, with its metamorphosed 

 equivalents, comprises limestone, dolomite, marble, cherty limestone, cherty 

 dolomite, and cherty marble, silicated marble, and silicate rocks. Each of 

 these will be considered in turn. 



L1MESTOKES. 



Limestones comprise all those rocks which are mainly composed of 

 calcium carbonate (CaC0 3 ). 



SOURCE OF MATERIAL OP LIMESTONES. 



The original dominant source of the calcium and magnesium of the 

 calcium-magnesium carbonates is mainly the many alkaline earth-bearing 

 silicates. The silicates are decomposed in the zone of katamorphism, and 

 very largely in the belt of weathering, by the processes of carbonatiou, 

 hydration, and oxidation. So far as the process is due to carbonatiou the 

 carbonates are formed directly. In so far as the bases are set free by 

 other reactions they may later unite with the carbon dioxide. The carbon 

 dioxide immediately concerned in the process of carbonation is mainly 

 derived directly from the atmosphere, from that produced by the oxidation 

 of the organic compounds, by the liberation from rocks during their decom- 

 position of inclosed carbon dioxide, and from the deep-seated waters rising 

 from the zone of anamorphism. (See pp. 461-465, 473-475, 608-610, 667, 

 677-678.) The relations of these sources of carbon dioxide to one another 

 and the replenishment of the supplies in order to carry on the process of 

 carbonation are fully discussed in Chapter XI, under the heading " Carbon." 



The calcium and magnesium unite with the carbonic acid formed and 

 form carbonates. The original carbonates are solely produced by the 

 reactions of the zone of katamorphism, and chiefly by reactions of the belt 

 of weathering. 



The calcium carbonate formed as above described, after transportation 

 to the sea or to lakes with no outlets, is thrown down as organic precipi- 

 tates and as chemical precipitates, and thus limestone is formed. 



