and the Earth's Thermal History. 103 



source of heat generation is of practically no importance except 

 locally, where for a time it may succeed in appreciably steepening the 

 temperature gradient. The heat liberated by weathering is probably 

 balanced by the absorption of heat at greater depths, which is involved 

 in the processes of metamorphism. Of much greater importance is the 

 question whether the earth can still be regarded as possessing an 

 ancient heritage of heat dependent on its origin. 



According to the Laplacian hypothesis the earth began as a hot 

 gaseous spheroid, and its subsequent history has been largely dependent 

 on the gradual cooling from that initial state. The Meteoric 

 hypothesis of Lockyer leads to an earth having a similar thermal 

 history. In each of these cases original heat is regarded as by far 

 the most important source of the earth's thermal energy. -Recently, 

 however, both of these hypotheses have been discredited on dynamical 

 and other grounds, and Professor Chamberlin has introduced his 

 Planetesimal hypothesis as an alternative which successfully over- 

 comes the difficulties encountered by the older views. According to 

 Chamberlin 1 (writing in 1905) there are four main sources of heat 

 arising from the mode of origin postulated by the Planetesimal 

 hypothesis. (1) The original earth nucleus about which the plane- 

 tesimals gathered may have been hot, the heat being partly original, 

 partly the result of condensation. (2) By the infalling of planetesimals 

 heat would be produced. If the infalling were sufficiently rapid, the 

 heat newly generated would be retained and the surface might be 

 kept in a state of fusion. It is considered, however, by Chamberlin 

 that in the later stages the rate of accretion became so slow that most 

 of the heat generated by impact was radiated away. (3) The chief 

 source of heat is assigned to mechanical compression in the interior 

 of the growing earth. (4) Another effect of compression would 

 be a molecular re-arrangement by which denser compounds would be 

 produced. It is thought probable that this re-combination would be 

 attended by the liberation of thermal energy. 



Dr. A. C. Lunn 2 has discussed mathematically the thermal con- 

 dition of the earth on the basis of the Planetesimal hypothesis. He 

 takes into account only heat derived from gravitational energy, 

 remarking that the heat obtained from other sources might be 

 relatively important, and that if it were of sufficient magnitude it 

 would completely alter the thermal process. 3 According to the 

 assumptions made, which are admitted to be uncertain, Lunn finds 

 the temperature gradient near the surface to be only 330° C. in 

 200 miles (0-00001°C. per cm.). The actual present gradient is about 

 0-00032° C. per cm. Such a result, obtained before the geological 

 significance of radio-activity was realized, appears to be in complete 

 harmony with the fact that radio-activity seems to leave little room 

 for any appreciable temperature gradient .due to an original store of 

 heat. Writing in 1911 Chamberlin pointed out this co-operation 

 of planetesimal and radio-active agencies, 4 and he also indicated the 



1 Chamberlin & Salisbury, Text Book of Geology, vol. ii, p. 100, 1906. 



2 Carnegie Inst., Washington, Pub. No. 107, 1909, p. 169. 



3 Lunn, loc. cit., p. 230. 



4 Journ. Geol., p. 673, 1911. 



