MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 571 



total is probably an appreciable rise in temperature. It has even been 

 conjectured that the heat of volcanic action is due to chemical com- 

 bination in the lower reaches of water circulation, but this is obviously 

 an over-estimate. 



Differences in the conductivity of rock are an obvious source of varying 

 underground temperature gradients. If an outer formation conducts 

 heat more freely than those below, it tends to lower the gradient within 

 itself and to cause a relative rise in the gradient just below. If a lower 

 formation is more conductive than that above, it tends to lower the 

 gradient within itself, and to raise it in the one above, because it carries 

 heat to the outer one faster than the latter carries it away. 



The compression to which rocks have been subjected affects their 

 temperature.. At the surface the variation from this source is chiefly 

 dependent on the lateral thrust suffered. 



When allowances are made for all these and other known causes 

 of local variation of temperature, it is still not clear that a uniform 

 average gradient remains as the true conception. If the earth were 

 once a molten spheroid, there would be a strong presumption that, 

 aside from local variations, there would be a normal curve applicable 

 to all regions. On the other hand, if the internal heat has arisen chiefly 

 from compression, and if the compression has varied in different regions, 

 as the inequahties of the surface render probable, there would be no 

 such definite normal curve in the accessible zone of the earth, but 

 rather a varying rate in different regions. In either case, the later 

 movements, compressions and strains of the crust, must modify the 

 original thermal gradients. 



Gradients projected.— It is not probable that these gradients, even 

 when corrected for local variations, continue unmodified to the center 

 of the earth. If they did, 1° F. in 60 feet continued to the earth's 

 center would give 348,000° F., and 1° F. in 100 feet would give 209,000° F. 

 It is much more probable that the rates of rise fall away below the super- 

 ficial zone. If w^ater circulation in the fracture zone is the most effi- 

 cient agency cooperating with conductivity in the outward conveyance of 

 heat, as seems probable, the gradient in that zone should rise at an 

 abnormal rate, and hence the average gradient in the deeper portions 

 not affected by this circulation should be lower. It will be recalled 

 that the central temperature deduced from an extension of Barus' 

 fusion curve is 136,800° F. (76,000° C), which, high as it is, gives a 



