ORIGIN OP ERUPTIVE AND PRIMARY ROOKS. 303 



At Rouen 90- 8 Feet. 



AtMondorff 91. 1 " 



At Riidersdorf 92. " 



At Neusalzwerk 92.27 " 



AtGrenelle 95. « 



At St. Andre (Eure) 95. 3 « 



These results shew a remarkable coincidence, but there are 

 others which shew extraordinary differences, such as the follow- 

 ing:— 



AtLaRochelle 60.6 Feet. 



At Pitzbuhl near Magdeburg 80.0 " 



At Artem in Thiiringia, 120. " * 



These latter results, as well as those differing widely from each 

 other, which have been obtained in mines, are not to be regarded 

 as at all invalidating the general result. These differences may 

 be caused by variations in the conducting capacity of the various 

 rocks ; by the neighborhood of subterranean water courses ; but 

 especially by the greater or lesser distance of the point of obser- 

 vation from the source of the internal heat ; in other words by the 

 varying thickness of the earth's crust; 



We have thus seen that actual observations have been made 

 as to the temperature of the crust at various depths beneath the 

 surface, sometimes as much as 2000 feet, and the result of these 

 has been to prove that an increase of temperature takes place 

 with the depth, amounting on the average to about 1 ° Cent, for 

 every 100 feet. We have next to enquire as to whether any increase 

 of temperature takes place at still greater depths. We have 

 abundant prook that this further increase does take place, in the 

 temperatures of the thermal springs so widely distributed over 

 every part of the surface of the globe. These temperatures are 

 much higher than those which have been observed in mines or 

 artesian wells. The waters of these springs rush with extraordi- 

 nary force out of the ground, from which circumstance we may con- 

 clude that they ascend from their sources, with a rapidity which 

 does not permit them to cool very considerably in their passage 

 through the upper and colder strata. Although we are ignorant 

 of the exact depth from which the waters of these springs 

 rise, we are nevertheless justified in assuming that they come 

 from greater depths than those of mines or artesian wells. 



Naumann's Geognosie, I, 48. 



