PHYSIOGRAPHICAL GEOLOGY. 177 



of temperature ; generally speaking, they show an increase of 

 i C. in grades of about 30 to 34 metres (104 to 118 feet). 

 The results yielded by borings have been confirmed by observa- 

 tions in the great Alpine tunnels. 



The Italian geologist, Giordano, published in 1870 exact 

 observations made in the Mont Cenis tunnel, and the German 

 civil engineer, Stapff, published those in the St. Gothard tunnel 

 (1877-80). In the middle of the Mont Cenis tunnel the rock 

 has a temperature of 29.5 C. 



In spite of the numerous local variations in the exact rate 

 of increase of temperature, there can be no doubt that the 

 temperature of the ground increases so far as depths below 

 the surface have yet been reached ; the probability is that at 

 still greater depths still greater increase of temperature takes 

 place. Hot springs in many cases rise from great depths, and 

 cannot be shown to have connection with volcanoes or with 

 any particular geological formation. 



Calculations have been made with respect to the probable 

 rate of progression in the increase of temperature at depths 

 still unattained, but the results cannot be regarded as trust- 

 worthy. Thus, although all geologists agree that the rise of 

 temperature in the earth's crust is due to the internal heat of 

 our planet, we have not yet sufficient data to determine either 

 the prevailing inner temperature or the thickness of the earth's 

 crust. 



At the same time, the hot springs and geysers indicate 

 temperatures that reach the boiling-point in the earth's crust, 

 and the wide distribution of volcanoes demonstrates still higher 

 degrees of temperature in the crust. The scientific authorities 

 in the first half of the nineteenth century regarded it as an 

 accepted fact that the earth's nucleus was molten, and was 

 surrounded by a comparatively thin crust. Humboldt and 

 Elie de Beaumont valued the thickness of the earth's crust at 

 40 to 50 kilometers, and this result almost agrees with the 

 more recent work of the Rev. O. Fisher, who valued the thick- 

 ness at 25 English miles. But the calculations made by 

 various authorities differ very considerably, some calculations 

 giving a result of only 14 English miles for the thickness of the 

 earth's crust, others a result as great as 75 English miles. 



The great chemist, Sir Humphrey Davy, did not believe 

 in the original molten condition of the earth's nucleus. He 

 believed that the earth's nucleus was originally composed of 



T2 



