VI.] THE PROBABLE SEAT OF VOLCANIC ACTION. 65 



of the cooling globe when the accumulation of comparatively 

 thin layers of sediment would have been sufficient to give 

 rise to all the phenomena of metamorphism, vulcanicity, and 

 movements of the crust, whose origin Herschel has so well 

 explained. 



Coming, in the next place, to consider the influence of press- 

 ure upon the buried materials derived from the mechanical and 

 chemical disintegration of the primitive crust, we find that, by 

 the presence of heated water throughout them, they are placed 

 under conditions very unlike those of the original cooling mass. 

 While pressure raises the fusing point of such bodies as expand 

 in passing into the liquid state, it depresses that point for those 

 which, like ice, contract in becoming liquid. The same prin- 

 ciple extends to that liquefaction which constitutes solution ; 

 where, as is with few exceptions the case, the process is at- 

 tended with condensation or diminution of volume, pressure 

 will, as shown by the experiments of Sorby, augment the solv- 

 ent power of the liquid.* Under the influence of the elevated 

 temperature and the great pressure which prevail at consider- 

 able depths, sediments should, therefore, by the effect of the 

 water which they contain, acquire a certain degree of liquidity ; 

 rendering not improbable the suggestion of Scheerer, that the 

 presence of five or ten per cent of water may suffice, at temper- 

 atures approaching redness, to give to a granitic mass a liquidity 

 partaking at once of the character of an igneous and an aqueous 

 fusion. The studies by Mr. Sorby of the cavities in crystals 

 have led him to conclude that the constituents of granitic and 

 trachytic rocks have crystallized in the presence of liquid water, 

 under great pressure, at temperatures not above redness, and 

 consequently very far below that required for simple igneous 

 fusion. The intervention of water in giving liquidity to lavas 

 has, in fact, long been taught by Scrope, and, notwithstanding 

 the opposition of plutonists like Durocher, Fournet, and Riviere, 

 is now very generally admitted. In this connection, the reader 

 is referred to the Geological Magazine for February, 1868, page 

 57, where the history of this question is discussed. 



* Sorby, Bakerian Lecture, Koyal Society, 1863. 



