678 A TREATISE ON METAMORPHISM. 



than silication of the carbonates, with decarbonation, takes place. In many 

 of the ancient formations the process of transformation has been complete; 

 in others it is far advanced, and it is uncommon to find any carbonate 

 formation that is buried in the zone of anamorphism in which the process 

 has not taken place to an important extent. 



It has been explained (pp. 665-667) that a portion of the carbon 

 dioxide and water freed by silication probably escapes to the belt of cemen- 

 tation and thence to the surface. It will be seen on pages 970-971 that one 

 of the important sources of carbon dioxide for the process of carbonation 

 is the vast quantity which comes to the surface through ground waters. 

 Illustrating this is the startling case furnished by Lecoq, who says that the 

 mineral springs in Auvergne district annually give off 7,000,000,000 cubic 

 meters of carbon dioxide ; a and the immeasureably greater amount which 

 reaches the surface for the world as a whole is mainly carbon dioxide freed 

 by the process of silication in the zone of anamorphism. This idea, that 

 the carbon dioxide which reaches the surface from subterranean sources is 

 that derived from the carbonates produced in previous geological ages, was 

 first suggested by Bischof. 6 



However, a part of the carbon dioxide and accompanying water does 

 not escape, and this is believed to largely explain the innumerable cavities 

 partly filled with water and carbon dioxide which are so generally found 

 in the sedimentary rocks metamorphosed in the zone of anamorphism. 

 Such inclusions are comparatively rare, although not unimportant, in the 

 original igneous rocks. (See p. 969.) Silication is thus offered as a cause 

 which explains a large proportion of the liquid carbon dioxide inclusions 

 in the rocks. 



Commonly when the silicated rocks have reached the surface, so that 

 thin sections may be cut, liquid carbon dioxide and water do not entirely 

 fill the cavities, a part being occupied by the gases of these compounds. 

 Doubtless in the deep-seated zone where the rocks were altered the carbon 

 dioxide and often the water were above their critical temperature, and were 

 altogether in the form of gas. Possibly if accurate measurements ' were 

 made of the volumes of water, liquid carbon dioxide, and water and carbon- 

 dioxide gas, and the total volume of many of the openings in a rock, so as 



" Blake, R. F., and Letts, E. A. The carbonic anhydride of the atmosphere: Sci. Proc. Royal 

 Dublin Soc, new series, vol. 9, pt. 2, 1900, p. 159. 



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

 Harrison & Sons, London, vol. 1, 1854, pp. 237-241. 



