and the Earth's Thermal History. 107 



rocks. Chamberlin believes that the temperature gradient in the 

 outer part of the earth was such as to preserve "equilibrium between 

 solidity and liquefaction according to the conditions present at each 

 depth". 1 The oldest Archaean rocks imply that at the time of their 

 formation the equilibrium between solidity and liquefaction began at 

 no great depth from the surface. 



A further criticism made by Daly against the view of a solid 

 exterior during the later stages of growth is based on a comparison 

 of the average densities of the earth and the outer planets and sun. 

 The low densities of the outer planets may be due to high temperature 

 or to the absence of heavy elements. As in the case of the sun, the 

 interpretation of astrophysics is that the temperature is high. If the 

 larger planets are still in a condition of high temperature it seems 

 likely that the earth has passed through an essentially similar stage. 



The contraction of the earth evidenced by the phenomena of 

 mountain building is not invoked in this discussion because there 

 are sources of contraction other than cooling, notably mechanical 

 compression, chemical readjustment, and physical change of state. 

 If cooling were admitted as the chief cause of contraction, then the 

 universality of orthogneisses among the earliest igneous rocks and of 

 cleavage, foliation, crumpling, and distortion among the earliest sedi- 

 ments, and the gradual restriction of these features in later geological 

 time to narrow belts on the earth's surface, points at once to more rapid 

 cooling in the outer part of the crust during the earlier of the Pre- 

 Cambrian periods, and therefore to a higher temperature from which 

 to cool. It is doubtful, however, whether this argument is valid. 

 It is certainly true of contraction, but not necessarily true of cooling. 



Summing up we may conclude that considerations based on — 



(1) the existing salinity of the ocean; 



(2) the absence of ancient sediments or schists with significant 



peculiarities of composition ; 



(3) the possibility of heat generation being greater than radiation 



during the later stages of the earth's growth ; 



(4) the probability of a strongly athermanous primitive atmosphere ; 



(5) the density stratification of the earth, particularly in its outer 



shell ; . 



(6) the universality of plutonic igneous rocks and their metamorphic 



equivalents at no great depth in early Archaean times ; 



(7) the probable high temperatures of the larger sister planets, 



all point to the improbability of a solid exterior during the earth's 

 later stages of growth, while conversely, they all favour the proba- 

 bility that magmatic temperatures existed at or immediately below 

 the surface throughout the period of growth. 



10. Mathematical Treatment. 

 The effect of radio-activity on earth temperatures and the rate of 

 cooling has been ably discussed mathematically by Messrs. L. E,. 

 Ingersoll and 0. J. Zobel, of the University of Wisconsin, in their 

 recent book on the Mathematical Theory of Heat Conduction (Ginn& Co.. 

 1913). They assume that only a fraction, l/n, of the total annual 

 1 Joum. Geol., p. 686, 1911. 



