MOVEMENTS AND DEFORMATIONS 363 



If the effect of radioactivity be superposed on that of compression, 

 it modifies the curve of Fig. 288, by raising its outer part, making 

 it accord with the observed temperatures shown in deep ex- 

 cavations. 



Observed temperatures in deep excavations. As the earth is 

 penetrated below the zone of seasonal changes, by wells, mines, 

 tunnels, and other excavations, the temperature is almost invari- 

 ably found to rise. The rate of rise, however, is far from uniform. 

 If we set aside as exceptional the unusually rapid rise near volcanoes 

 and in other localities of obvious igneous influence, the highest rates 

 are more than six times the lowest, the range being from 1 F. in 

 20 feet, to 1 in 130 feet, with an average of 1 in 50 to 60 feet. The 

 more recent deep borings in which the temperatures have been care- 

 fully recorded indicate a slower rate of rise, say 1 for 80 feet. It 

 is sot probable that the average rates of increase as determined 

 by observation continue to the center of the earth. ^One degree 

 in 60 feet, continued to the earth's center would give a temperature 

 of 348,000 Fahr., and 1 Fahr. in 100 feet would give 209,000 

 Fahr. It is much more probable that the rate of rise diminishes 

 below the superficial zone, and that the temperatures cited are 

 far in excess of those actually existing at the center. 



The amount of loss of heat. The amount of loss of interior heat 

 may be estimated by that which is observed to be passing outward 

 through the rock, or by computing the amount which should be 

 conveyed outwards with the estimated temperature gradients, 

 and with the known conductivity of rock. On this basis estimates 

 of the loss of heat in 100,000,000 years range from 10 C. (18 Fahr.) 

 (Tait *) to 45 C. (81 Fahr.') , for the whole earth. This is an exceed- 

 ingly small result, and emphasizes the low conductivity of rock. 



The amount of shrinkage from loss of heat. With this amount 

 of cooling, the shrinkage from this cause may be calculated, the 

 average coefficient of expansion of rock being known. This has been 

 done, both experimentally and theoretically. For a loss of 10 C. 

 of heat, the circumferential contraction is calculated to be 1.6 to 

 2.35 miles; for a loss of 45 C., 7.27 to 10.5 miles. These results 



*Tait, Heat, p. 225. 

 2 Daniell's Physics, p. 407. 



