The Origin of the Biosphere 27 



similar to that of stony meteorites some amount of a lighter fraction could be 

 melted out. In fact, the thickness of the crust of the Earth is extremely small, 

 about I 100 of that of the mantle. This question, however, requires separate 

 discussion. We will leave it on one side for the moment. Finally, we will consider 

 the reasons for the intense volcanic activity which occurred at the begiiming of 

 the formation of the crust of the Earth. The fact that this phenomenon took place 

 in the remote past is beyond question and is geologically well documented. It 

 is enough to point out that, at present, there are more than 500 active volcanoes 

 on the Earth. Naturally, the mechanism of volcanic activity cannot be considered 

 apart from the mechanism of the differentiation of the Earth into layers. Accord- 

 ing to the views of the very high-standing speciahsts who have concerned them- 

 selves with this question from the point of view of the 'hot' theory, such as 

 Chamberlin, Buddington, Bullen, Jeffreys and others, the crust of the Earth 

 was formed during a, geologically speaking, very short period of about 10^- 

 10'^ years. Further thermodynamic changes tended towards the loss of heat and 

 the cooling of both the Earth as a whole and its crust. During this process the 

 liquid magma retained the cosmic gases and vapours which had been in equili- 

 brium with it at the temperature of solidification, and became solid. By far the 

 greater part of the gases and vapours of volatile substances were present, accord- 

 ing to this hypothesis, at the periphery of the planet, forming its primary heavy 

 atmosphere which, only later, came down to the cooling surface of the Earth. 

 Naturally, the question arises as to why, at that time, the process of formation 

 of the sohd crust of the Earth should have been accompanied by volcanic acti- 

 vity which continued, though less strongly, throughout the whole later history 

 of the Earth. To explain this effect one would have to postulate, not a fall but a 

 rise of temperature within the Earth, or else to admit that there was a secondary 

 melting of the substance of the Earth, a fresh rise of temperature within the 

 planet. According to the 'hot' theory, however, such a supposition is manifestly 

 absurd. 



THE HYPOTHESIS OF THE MELTING OUT OF THE 

 CRUST FROM A COLD EARTH 



The hypothesis that the crust of the Earth was formed from cold material 

 involves difficulties of its own. The solid, cold, heterogeneous material of which 

 the planet was formed did not retain any gases or other compounds which were not 

 chemically bound in it. This helpful concept first occurred to H. Brown [2]. This 

 plausible assumption provides a general answer to the question of the reasons 

 why all the inert gases in the Earth fall far short of their cosmic abundance. 



When the Earth reached a certain size there took place an adiabatic process 

 in which its substance became heated owing to the accumulation of radiogenic 

 heat generated by the radioactive breakdown of U, Th and K, and perhaps 

 also, to some extent, owing to gravitational compression. Under the influence of 

 this heat the substance of the Earth differentiated and the light and low-melting 

 fractions separated outwards to the surface. It is very hard to explain this pro- 

 cess in physico-chemical terms. I will therefore allow myself a digression which 

 seems to be suggestive. I drew attention to the fact that the chemical composi- 



