Dr. T. Sterry Hunt— On Volcanic Action. 249 



the semi-liquid layer supposed to intervene between the outer solid 

 crust and the inner solid and anhydrous nucleus. In order to obtain 

 a correct notion of the condition of this mass, both in earlier and 

 later times, two points must be especially considered, the relation of 

 temperature to depth, and that of solubility to pressure. It being 

 conceded that the increase of temperature in descending in the 

 earth's crust is due to the transmission and escape of heat from the 

 interior, Mr. Hopkins showed mathematically that there exists a 

 constant proportion between the effect of internal heat at the surface 

 and the rate at which the temperature increases in descending. 

 Thus, at the present time, while the mean temperature at the earth's 

 surface is augmented only about one-twentieth of a degree Fahren- 

 heit, by the escape of heat from below, the increase is to be found 

 to be equal to 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 con- 

 clusion 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 pressure 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 be- 

 coming liquid. The same principle extends to that liquefaction 

 which constitutes solution ; where, as is with few exceptions the 

 case, the process is attended vnth condensation or diminution of 

 volume, pressure will, as shown by the experiments of ISorby, aug- 

 ment the solvent power of the liquid,' under the influence of the 

 elevated temperature, and the great pressure which prevail at con- 

 siderable depths. Sediments should, therefore, by the effect of the 

 water which they contain, acquire a certain degree of liquidity, ren- 

 dering not improbable the suggestion of Scheerer, that the presence 

 of five or ten per cent, of water may suffice, at temperatures ap- 

 proaching 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 con- 

 clude 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 

 ^ Sorby, Bakerian Lecture, Eoyal Society, 1863. 



