176 A TREATISE ON METAMORPHISM. 



by carbonic acid at ordinary temperatures.'"* But he says that, since 

 carbonic acid does not combine with alumina or peroxide of iron, the 

 silicates of these compounds are not decomposed by carbonic acid. b How- 

 ever, we now know that the process of carbonation takes place with all the 

 natural silicates. It will be shown in Chapter VII that this process of 

 carbonation goes on throughout the entire zone of katamorphism, but it is 

 in the upper of the two belts of the zone of katamorphism, that of weath- 

 ering, in which the process of carbonation goes on with greatest rapidity 

 and is especially characteristic. Simultaneously with the substitution of 

 the carbon dioxide for the silica much of the silica separates as colloidal 

 silicic acid, is taken into solution, and is carried downward to the belt of 

 cementation by the jjercolating waters. In this belt the silica is deposited 

 on an enormous scale. The carbon dioxide is furnished in solution, being 

 mainly derived directly or indirectly from the atmosphere. When carbon 

 dioxide replaces silicon dioxide the volume would be decreased, provided 

 all of the silicic acid were abstracted in solution. But it is probable that 

 the larger portion of the silica set free in the zone of katamorphism by 

 carbonation is deposited in the belt of cementation, and therefore the 

 volume of the zone of katamorphism as a whole, so far as this reaction is 

 concerned, is increased. The deposition of silica in the belt of cementation 

 is probably accompanied by a considerable absorption of heat, under the 

 law that the negative value of the heat of solution is greater the more 

 insoluble the substance. 



Carbonation in the zone of katamorphism may take place without 

 replacing silica, as in the case of the union of carbon dioxide with iron 

 oxide in magnetite, thus producing iron carbonate. In this case the 

 liberation of heat and the increase in volume are both great. 



In the zone of anamorphism, and especially under mass-mechanical 

 conditions, silica replaces carbon dioxide in the carbonates on the most 

 extensive scale. So far as I am aware, Bischof was the first to realize that 

 under proper conditions the process of carbonation of the silicates could be 

 reversed. He shows by experiment that carbonates of calcium, magnesium, 

 and iron are decomposed by silica at a boiling temperature, and cor- 



a'Bischof, Gustav, Elements of chemical and physical geology, translated by Paul and Drummond, 

 Harrison & Sons, London, vol. 1, 1854, p. 2. 

 6 Bischof, cit., vol. 1, pp. 4-5. 



