June] HUNT — ON VOLCANIC ACTION. 171 



about one degree for each sixty feet in depth. If, however, we 

 go back to a period in the history of our globe when the heat 

 passing upwards through its crust was sufficient to raise the 

 superficial temperature twenty times as much as at present, that 

 is to say, one degree of Fahrenheit, the augmentation of heat in 

 descending would be twenty times as great as now, or one degree 

 for each three feet in depth (Geol. Journal, viii. 59.) The 

 conclusion is inevitable that a condition of things must have 

 existed during long periods in the history 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 pressure 

 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 principle 

 extends to that liquefaction which constituted solution ; where, as 

 is with few exceptions the case, the process is attended with 

 condensation or diminution of volume, pressure will, as shewn by 

 the experiments of Sorby, augment the solvent power of the 

 liquid.* Under the influence of the elevated temperature, and 

 the great pressure which prevail at considerable 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 temperatures 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 con- 

 sequently very far below that required for simple igneous fusion. 

 The intervention of water in giving liquidity to lavas, has, 



* Sorby, Bakerian Lecture, Royal Society, 1863. 



