26 A. P. VINOGRADOV 



The available data as to the amounts of H2O, COj, HF, HCl, S, HBO3 and 

 a number of other elements or compoimds in the crust of the Earth, have led 

 to the unanimous opinion that they cannot be products of rock weathering of 

 the crust of the Earth. Some other source must be found. 



If we leave aside details of minor importance we may take it that two different 

 sets of ideas prevail as to how the crust of the Earth and oceans and the atmos- 

 phere could have been formed. The first is that the crust of the Earth was formed 

 during the primary process of differentiation of the substance of the Earth from 

 the hot material of the protoplanet. The second is that they were formed from 

 the cold material of the earth by melting. They are directly opposed. 



I have presented these two sets of ideas in the form of comparative tables 

 (Table i). I do not pretend that I have been perfectly objective in constructing 

 them. It seemed to me that, on both schemes, the formation of the crust of the 

 Earth should show some sort of logic. 



THE 'HOT' THEORY OF THE FORMATION OF 

 THE CRUST OF THE EARTH 



If we assume that the crust of the Earth was formed from hot material of the 

 protoplanet, the phenomena which are most difficult to explain are (i) the ab- 

 sence from the Earth of all inert cosmic gases while H2O, CO2 and other light, 

 volatile chemical compounds were retained; (2) the absence of a thick crust of 

 the Earth and (3) the appearance of intense volcanic activity, especially at the 

 beginning of the geological history of the Earth. 



On the stars and the nebular aggregations of matter in space there is a very 

 considerable amount of the heavy inert gases Ne, A, Kr, Xe. For example, the 

 amount of Ne and A is of the same order as that of O2. In our atmosphere, on 

 the other hand, the concentration of the light and heavy inert gases is negUgibly 

 small and constitutes some thousandths or tens of thousandths of their relative 

 concentration in the universe as a whole. The inert gases which are found to the 

 greatest extent in the atmosphere of the Earth, viz. "^^A and -^He are of secondary 

 origin, arising from the radioactive decay of ^°K and U and Th either by the 

 spontaneous fission of U and Th, or as a result of secondary nuclear reactions 

 occurring on a modest scale on the Earth. The isotopic composition of these 

 inert gases in the atmosphere of the Earth is different from the isotopic compo- 

 sition of the stars and cloud-like aggregates. If we do not make the assumption 

 that these inert gases were absent from the protoplanetary cloud, then it is hard, 

 on the basis of the 'hot' theory of the formation of the Earth, to explain why 

 N2, CO2, HoO, and other gases and vapours were retained on the surface of the 

 Earth while the heavier Ne, A, Kr and Xe disappeared. 



Now for a few words about the thickness of the crust of the Earth. If the 

 siliceous material of the Earth became differentiated at a high temperature, there 

 must have occurred a complete and far-reaching melting out heating those parts 

 of it which fuse most difficultly. Calculations and actual fusion of the ultrabasic 

 rocks (dunites) have shown that, in them, this process is not finished. From the 

 relevant data it might be expected that from a substance having a composition 



