308 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. . 



crust a much more considerable thickness. Cordier assuming 100 ° , 

 Wedgewood as the melting point of lava, determined the depth 

 at which everything is in a fluid state, from his observations : — 



At Canneaux, to be 148 English geographical miles. 

 At Littry 84 do. 



At Decise 64 do. 



And he finally draws the conclusion that the average thickness 

 of the solid crust of the earth cannot well exceed 56 English geo- 

 graphical miles.* Sartorius von Waltershausen's estimate will be 

 referred to when we come to take into consideration the density 

 of the earth. Naumann remarks as follows on the subject :■ " the 

 " temperature of the fused lava may certainly in the depths of 

 " the earth be estimated as at least 2000 ° C. If the increase of 

 ** temperature follows the law of an arithmetical progression,, then' 

 " such a temperature would be reached at a depth of 200,000 feet } . 

 " or nineGerman,(=36 English) geographical miles. But since 

 " it is more probable that the distance corresponding to an in- 

 " crease of 1 ° Centigrade augments with the depth, we are jus- 

 M tified in assuming a much greater depth, and in supposing it not 

 K at all impossible that the seat of the fluid lava is to be found at 

 " a depth of twenty and perhaps even upwards of thirty geogra- 

 " phical miles (=80 or 120 English geographical miles,)"f There 

 are not wanting observations to prove that the temperature of 

 the earth's crust increases less rapidly towards the interior. Thus- 

 from a comparison of several observations, Fox deduced the 

 result that within the first 600 feet, the temperature increases 

 more quickly than in the following 6O0 feet. Henvvood obtained 

 similar results within the first 950 feet, and Rogers found in Vir- 

 ginia a notable enlargment with the depth, of the space corres- 

 ponding to 1° increase. In the artesian well at Crenelle the 

 temperature observed at 



70 feet depth was , 20.00° O. 



1555 " , 26.43° G. 



The thermometer in the cellar of the Paris observatory shews a 

 constant temperature of 1 1 .7 ° C. Calculating from this depth of 

 86 feet, the distance cot responding to one degree y s increase of tem- 

 perature within the first 677 feet is 81.6 ieet; and within the 



* Naumann, Geognosie, I. 74. 

 t Naumann, Geognosie, I, 67. 



