630 A TREATISE ON METAMOKPHISM. 



of pressure has not been sufficiently appreciated. The experimental work of 

 Barus" shows that at temperatures above 185° C. water and soft glass are 

 miscible in all proportions. Further, it is held that water and rock are mis- 

 cible in all proportions at proper temperatures and pressures. (See p. 723.) 

 These conclusions throw new light upon the importance of the third 

 and fourth causes of precipitation. If these conclusions as to the misci- 

 bility of water and rock be true, at the high temperatures and pressures 

 which prevail in the deep seated circulation the water must be very rich in 

 mineral content. It is clear that ascending waters which continuously fall 

 in temperature and pressure must throw down great quantities of this 

 material before they reach the surface. It is notable that large channels 

 are most favorably situated with reference to all of these causes of 

 precipitation, for in the large channels mingling of solutions is most 

 likely to take place, along them reactions with the wall rock are important, 

 and in them solutions are most likely to be ascending'. (See pp. 582-584.) 

 Therefore all the causes for precipitation combine to explain the great 

 amount of material deposited in the large openings. 



While all of these statements are undoubtedly true, in view of the fact 

 already pointed out, that waters issuing from the belt of cementation 

 may contain more material in solution than those entering that belt, none 

 of them, nor their combinations, explain the general cementation in large 

 and small openings alike. Before we can satisfactorilv assign causes for 

 general cementation it is necessary also to recall the cementing' minerals 

 and to indicate their relative importance. Of the substances deposited in 

 the belt of cementation quartz is undoubtedly the one which dominates 

 over all others. The one great process in the belt of cementation is 

 silicification. Of very great importance is the deposition of the silicates 

 and carbonates. Both of these classes of minerals are deposited on a 

 great scale, but which of these is quantitatively the more important it is 

 hard to say. It has been noted that of the various silicates deposited the 

 important ones are the zeolites, the chlorites, the serpentines and talcs, and 

 the epidotes. The deposition of oxide of iron, as hematite, limonite, etc., 

 and of aluminum, as gibbsite and diaspore, is rather important, but quan- 

 titatively very subordinate to that of the classes of minerals already 

 mentioned. 



«Barus, C, Hot water and soft glass in their thermo-dynamie relations: Am. Jonr. Sci., 4th ser., 

 vol. 6, 1898, p. 270; and vol. 9, 1900, pp. 167-168. 



