68 t)r. H. Franklin Parsons on Earth Temperatutes. 



increase with depth is slower than where the beds are level. In 

 some cases local irregular increases of heat in mines appear to 

 be due to the oxidation of pyrites. Under high ground, as in the 

 St. Gothard and Mont Cenis tunnels, the temperature rises, but 

 not with a gradient so steep as that of the surface. The mean 

 deduced by the Committee from the thirty-six sets of observations 

 was 1° increase in 64 ft., or over 82° per mile. 



At this rate, at a depth small in comparison to the diameter 

 of the earth, say, 100 miles, a temperature would be reached at 

 which all substances which we know would be melted or volati- 

 lised, and from the fact of the temperature increasing with the 

 depth, the conclusion was formerly drawn that the interior of the 

 earth must be a mass of molten matter, enclosed like an egg 

 within a thin shell of solidified rock. Mathematicians, however, 

 say that the rigidity of the earth must be equal to that of steel, 

 or the equatorial bulging caused by its rotation on its axis would 

 be greater than it is. Hence the physical condition of the 

 interior of the earth must be looked on as a problem which is 

 not yet solved, and perhaps never will be. The enormous pres- 

 sure in the interior of the earth is a factor altogether beyond our 

 experience. 



The geo-isotherms or planes of equal temperature in the earth's 

 crust cannot be everywhere equally distant from the centre ; 

 they will be depressed under the oceans and polar regions, and 

 elevated under mountains and high plateaux, in volcanic regions, 

 and in the tropics. At Yakoutsk, in Siberia, the earth is perma- 

 nently frozen at a depth of 540 ft., whereas at Trevandrum, in 

 India, the mean temperature at a depth of 12 ft. is found to 

 be 86°. 



The heat of the interior of the earth, coupled with great pres- 

 sure and the probable presence of superheated water acting 

 during the ages of geologic time, must powerfully modify the 

 physical and chemical structure of the materials of the earth ; 

 and when we remember that the more ancient rocks which are 

 known to us must have been brought to the surface by move- 

 ments of the earth's crust after having been at one time covered 

 by strata several miles in thickness which have subsequently 

 been denuded, we need not wonder at these early rocks having 

 lost almost all traces of their original structure, and become 

 profoundly altered and metamorphosed into crystalline materials. 



The temperature of the ground at any given depth is mainly 

 derived from two sources, viz., the rays of the sun and the 

 earth's internal heat, the temperature of the superficial layers 

 depending almost entirely upon the former source, while the 

 latter becomes appreciable at the greater depths explored by 

 geologists. There may be other subsidiary sources, such as 

 chemical changes and the heat produced by unequal pressures. 



