and the Earth's Thermal History. 61 



problem is that relating to the depth within the earth's crust at 

 which temperatures are attained such that, under suitable conditions 

 of pressure, molten rock magmas may exist. The determination of 

 the minimum depth of possible rock fusion is a first essential to any 

 adequate theory of vulcanism, and indeed of igneuus activity in 

 general. It is not sufficient, however, to ascertain that depth for 

 present conditions alone. Its variation during the earth's geological 

 history must also be investigated ; for if, as is generally believed, the 

 earth is a cooling body, the depth must be slowly increasing, and in 

 former periods it must necessarily have been nearer the surface 

 than it is now. In the limiting conditions both of position and time, 

 the depth of fusion may have been at, or so near as to be for all 

 practical purposes at, the surface itself. That is to say, at the 

 beginning of geological history the earth may have been in a molten 

 condition at, or immediately below, the then existing surface. 



The necessity of investigating this problem afresh arises from the 

 discovery, within the last decade, of the widespread distribution of 

 radio-active elements among the rocks of the earth's crust. 1 Although 

 numerically the actual proportion of the radio-elements in the rocks 

 is exceedingly small, yet owing to the fact that their disintegration 

 is accompanied by a spontaneous generation of heat, it happens that 

 their presence is fraught with a significance that cannot be Over- 

 emphasized. If each gram of the earth's substance were as rich in 

 the radio-elements as are the rocks which have been examined, the 

 earth's total output of heat from this source alone would, in any 

 given period, be about 300 times as great as the amount actually lost by 

 conduction to the surface and radiation into space. 2 If for ' gram ' in 

 the above statement, we substitute ' cubic centimetre', then the heat 

 income would be more than fifty times the heat expenditure. 



This astonishing result pulls us up sharply, for it is manifestly 

 absurd to believe that our planet is becoming hotter at the appalling 

 rate implied in these figures, or, indeed, that it is becoming hotter 

 at all. There are two possible ways by which the dilemma may be 

 avoided. The radio-active elements may be largely confined to the 

 crustal rocks, leaving the main body of the interior free from the 

 embarrassing results of their energetic decay; or, on the other hand, 

 the disintegration to which the heat owes its origin may be inhibited 

 under the conditions prevailing in the earth's interior. In the 

 immediate problem before us, it matters little which of these views be 

 accepted, for in each case the radio-active generation of heat is restricted 

 to a superficial shell. It should,, however, be pointed out that there 

 is a strong consensus of experimental evidence pointing to an actual 

 concentration of the radio-active elements in the earth's crust, whereas 

 there is no evidence whatever that physical conditions can influence 

 the rate of disintegration and heat production. 3 The experiments 

 already made have reached temperatures up to 2500° C, and pressures 



1 Strutt, Proc. Eoy. Soc, A, vol. lxxvii, p. 475, 1906; Holmes, Science 

 Progress, No. 33, p. 15, 1914. 



2 Holmes, Science Progress, No. 33, p. 21, 1914. 



3 My Science Progress paper (No. 33, 1914) deals fully with this question. 



