430 GEOLOGY. 



{ CaO.Al203.2Si02 

 ( Na20.Al203.6Si02. 



Assuming the two molecules represented by this formula to be equally 

 abundant, and allowing the whole to be acted on by HjO and CO2, we have 



CaO.Na20.2Al203.8Si02+ 4H2O + 2CO2 



= CaC03+ Na2C03+ 2(2H20.Al203.2Si02) + 4Si02. 



When waters charged with carbonates descend into the earth they 

 are likely to precipitate a portion of their burden, forming calcite and 

 other crystalline carbonates, and hence these are among the most 

 common minerals found in veins and rock cavities. Carbonates are 

 also deposited when carbonate-charged waters come to the surface 

 and evaporate or lose a part of their carbon dioxide. 



Decarbonation also takes place, but it is, at least at the surface, a 

 much less common process, and its conditions are less well understood. 

 Sufficiently high heat will drive off the carbon dioxide, as in the artificial 

 process of burning lime, but this is rarely observed in nature. Even 

 lava intrusions do not usually reduce limestone to caustic lime at any 

 appreciable distance from the contact. It is beheved, however, that in 

 the deeper zones, where high pressure and heat prevail, carbonates are 

 changed into sihcates, thus in a way reversing the process that prevails 

 at the surface, and setting free again a portion of the carbon dioxide 

 that had become locked up in the formation of the carbonates. To this 

 action some of the carbon dioxide of deep-seated thermal springs is 

 assigned. 



The carbonation of the sihcates takes place at the expense of the 

 carbon dioxide of the atmosphere and hydrosphere, and hence in 

 proportion as the igneous rocks are changed into carbonates, the 

 atmosphere and hydrosphere are depleted of carbon dioxide, new 

 supphes being neglected. As plants are dependent on carbon dioxide 

 for their principal food, and as animals are dependent on plants for their 

 food, directly or indirectly, the process of carbonation has a profound 

 bearing on the life-history of the earth, and will often invite attention 

 in the historical chapters. It is sufficient here to note that carbonation 

 is one of the chief processes in the alteration of igneous rocks and fur- 

 nishes, directly and indirectly, a larger percentage of the mineral 

 substances dissolved in the waters that flow from the land, than any 

 other single process. 



