300 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



ture of the air, water or rock there occurring. Those obtained from 

 observations made on the rock plainly deserve most confidence.* 

 Not only in European mines,but in those of South America, Mexico, 

 the United States, and the East Indies, observations have proved 

 that the temperature increases with the depth, and shewn that it 

 remains invariable at one and the same depth, provided no disturb- 

 ing causes are at work. In 1740, Gensanne instituted experiments 

 at Giromagny in the Vosges which gave the following results : — 

 At a depth of 



339 feet the temperature wag 12.5 ° . Centigrade. 



634 " " " 13.1° " 



948 " " " 19.0° " 



1333 " " " ........22.7° " 



Saussure obtained the following results at Bex in the Canton 

 Waadt, in a shaft in which no one had been for three months 

 previously. 



Depth. Temperature. 



322 14.4 ° Centigrade. 



564 15.6 ° " 



en 17.4° " 



Similar observations were afterwards made in the mines of 

 Freiberg by d'Aubuisson, Von Humboldt, and Von Trebra ; in 

 the mines of Cornwall by Forbes, Fox, and Barkam, and in the 

 Anzasca valley by Fontanetti. The most comprehensive and 

 exact observations were however those made at the instance of 

 the government mining officials of Saxony and Prussia, in the 

 mines of those countries. The observations in the Prussian mines 

 led to the following results.f 



1. That a decided increase of temperature takes places with 



increase of depth. 



2. That the temperature at every greater depth is invariable, 



since the annual oscillations were at the most only 1 ° . 



3. That the depth corresponding to an increase of temperature 



of 1 ° , differs extremely in different localities, varies from 

 48 to 355 feet, and on an average amounts to 167 feet. 



4. That the temperature increases twice as rapidly in coal mines 



as in ore mines. 



* Naumann, Lehrbuch der Geognosie, I, 49. 

 f Poggend. Ann ; vol. xxii, 1831, p. 497. 



