200 CHEMISTRY OF THE EARTH. 



above, llie ruins of the crust of anhydrous and primitive igneous roclc. This, 

 it is conceived, iimst, by contraction iu cooling, have become porous and 

 permeable, for a considerabk^ depth, to the waters afterward precipitated 

 upon its surface. In this way it was prepared alike for mechanical disin- 

 tegration, and for the chemical action of the acids, which, as shown in § 10, 

 must have been present in the air and the waters of the time. It is, more- 

 over, not imj)rol)able that a yet unsolidified sheet of molten matter may 

 then have existed beneath the earth's crust, and may have intervened in 

 the volcanic phenomena of that early period, contributing, by its extra- 

 vasation, to swell the vast amount of mineral matter then brought within 

 aqueous and atmospheric influences. The earth, air, and water thus made 

 to react upon each other, constitute the first matter from which, by 

 mechanical and chemical transformations, the whole mineral world known 

 to us has been produced. 



§ 39. It is the lower portions of this great disintegTated and water-im- 

 pregnated mass which form, according to the present hypothesis, 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 iu the earth's crust is due to the transmis- 

 sion and escape of heat from the interior, Mr. Hopkins showed mathe- 

 matically that there exists a constant proportion between the effect of 

 interualheat at the surfjice and the rate at which the temperature in- 

 creases in descending. Thus, at the present time, while the mean tem- 

 perature at the earth's surface is augmented only about one-twentieth of 

 a degree Fahrenheit, by the escape of heat from below, the increase is 

 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 ol our globe when the 

 heat passing upwards through its crust was suthcient to raise the super- 

 ficial 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, {Geological 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 Avould have been sufiflcient to give rise to all the phenomena 

 of metamorphism, vulcanicity, and movements of the crust, whose origin 

 Herschel has so well exi)lained. 



§ 40. Coming, in the next place, to consider the influence of pressure 

 upon the buried materials derived from the mechanical and chemical dis- 

 integration 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 constitutes solution ; 

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

 condensation or diminution of volume, pressure will, as shown by the ex- 

 periments of Sorby, augment the solvent power of the liquid. Under the 

 influence of the elevated temperature, and the great pressure which pre- 

 vail at considerable 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 approaching red- 



