246 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



in the Na:Cl ratio in sea water, but the buried (connate) sea waters 

 have been exposed to change, and the rate of change of the ratio of 

 sodium to chlorine has been irregular. 



GEOTHERMAL GRADIENT, THE COOLING EARTH 



There is another progressive activity which has been used to estimate 

 geologic time which goes back to the great name of Kelvin (Sir William 

 Thomson). This was so much discussed at one time that it should 

 be mentioned here, especially as it has recently been studied by 

 Spicer.^^ Tliis method depended upon the cooling of the earth. 

 Obviously, if the earth is losing energy from the interior, as it actually 

 is, and if we know the rate at which it is escaping — and this Kelvin 

 estimated from the increase of temperature with increasing depth in 

 mines — we can estimate how long it has been escaping, if we know the 

 initial condition. 



Unfortunately, Kelvin made assumptions which are not likely to be 

 true. In the first place, he assumed a practically instantaneous drop 

 of the surface temperature at the time of consoUdation of the crust to 

 practically the present temperature instead of the critical temperature 

 of water, which might have been the surface temperature if the ocean 

 was then all or mainly in the atmosphere. On the other hand, if there 

 were then no ocean and less temperature, the surface temperature 

 might have been down to 200° lower than at present, and have risen 

 slowly as water and gas emerged from the interior. In the second 

 place, he assumed a uniform temperature at the beginning from surface 

 to center. In the third place, he assumed an increase of temperature 

 of about 1° F. in 60 feet depth in the earth. Later observations in 

 Africa and Canada make it possible that the normal rate of tempera- 

 ture increase is only one-half or one-third as much, and we know noth- 

 ing of what it is under the ocean — that is, for three-quarters of the 

 globe. Finally, he made no allowance for the then unknowTi effects 

 of radioactive disintegration which in earlier time would have been 

 even greater than now. The result is that it is not difficult to select 

 possible geothermal data to fit a wide range of ages.^* 



However, the geothermal gradient has also been used to estimate 

 the time since the last ice age. For the wave of heat which started 

 down when the ice retired does not seem to have reached the bottom of 

 the deeper mines and wells. Tliis is shown in such a well as that of 

 figure 3, after C. E. Van Orstrand. The decrease of temperature 

 toward the surface from the lower levels of Michigan mines would 

 indicate a temperature near freezing near the surface. The post- 

 glacial wave of heat seems to have reached a depth of only between 



" Bull. Geol. Soc. Amer., vol. 48, pp. 75-92. 1937. 



" Lane, A. C, Schaeberle, Becker and the cooling earth. Science, n. s. vol. 27, pp. 689-592, 1908. 



