ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 305 



reached at which water boils. It is a matter of much difficulty 

 however to determine, with any degree of precision, the depth at 

 which this heat is attained. If we assume that the same increase 

 of 1 ° Centigrade for every 100 feet depth, which takes place at 

 the surface, continues to greater depths, the calculation is very 

 simple. The temperature of the Mondorff artesian well was 34 ° 

 Cent, at a depth of 2066 feet. If we add 100 feet for each 

 of the remaining 66 ° C, we have a temperature of 100 ° C, at 

 a depth of 8666 feet. It will however be shewn in a subsequent 

 part of this paper, that we are not justified in assuming that the 

 increase of temperature follows such a regular progression, that 

 the rapidity with which the temperature increases, diminishes with 

 the depth, and that consequently the depth at which a constant 

 temperature of 100° C. reigns, is much more considerable than that 

 above stated; that it is at least 10,000 feet, and probably even as 

 much as 20,000 feet.* It is quite possible that under the great 

 pressure which must exist at this latter depth the boiling pojnt 

 of water may be higher than 100 ° C, but then however, this 

 might be it could not retain this higher temperature until it 

 reached the surface. Because however rapidly it might ascend, 

 its temperature would on the way decrease with the removal of 

 the pressure, steam being at the same time generated. It is not 

 improbable that the waters of the Geyser and the Strokkr have 

 at their sources a much higher temperature than 100 ° C, and 

 that the eruptions observable at these springs ' are caused by the 

 generation of steam in the Carial of egress, owing to the removal 

 of the pressure. This view is supported by the observations made 

 on the temperature of these springs. The water of the Geyser at 

 the surface has a temperature of 76 ° to 89 ° C, but at a depth 

 of twenty-two meters it is from 122 ° to 127 ° C. The water of 

 the Strokkr is continually boiling at the surface, and has, at a depth 

 of forty-one feet, a temperature' of 114 ° C.f But although it is 

 possible for water to exist at a much higher heat than 100 ° C, 

 at such great depths, it is nevertheless also evident that at still 

 greater depths, and increased temperatures, it can only exist in 

 the form of steam. We can moreover readily conceive a depth 

 and temperature to which it would be impossible for water to 

 penetrate. If the temperature of the earth's crust continues to 



— * Naumann, Geognosie, I, 66. — ~ 



t Krugvon Nidda, in Kars ten's Archiv fur Mineralogie, &c, ix, 247. 

 Can Nat. 20 Vol. VIII. 



