The Origin of the Biosphere 29 



residues and consider the accumulation in the crust of the Earth (granites and 

 basalts separately) of such elements as the alkali metals, U, Th and other ele- 

 ments, then we find that the thickness of the mantle of the Earth involved for 

 this purpose will be hundreds of kilometres, in some cases more than a thousand*. 

 It follows from this that in order to achieve a considerable concentration of 

 many of the chemical elements in the rocks of the crust of the Earth a very 

 great thickness of the mantle would have to have been involved. This makes it 

 obvious why the crust of the Earth is so thin. 



If we assume that the crust of the Earth has been growing at a steady rate for 

 5 X 10^ years, then its thickness has increased by i x lO"^ cm/year. If it has 

 been growing for 5 X lo^ years it has grown i cm thicker each year, and only 

 if the process was completed within 5 x lo^ years would the rate of growth 

 have been 10 m/year. 



THE PRIMARY ATMOSPHERE 



From what has just been said it may be seen that I support the hypothesis of 

 the heating of the cold substance of the Earth and of the formation of the pri- 

 mary atmosphere as a result of the liberation, during the melting of the material 

 of the Earth, of volatile compoimds which had been chemically bound to it 

 until that time. 



The composition of the volcanic gases of the present day gives a general indi- 

 cation of the nature of this primary atmosphere. The main bulk of it consisted 

 of H2O, next in amount come CO2, CO, HCl, HF, H2S, N2, NH4CI and maybe 

 CH4. No great concentration of inert gases has been observed. The high con- 

 centrations of 40A which are sometimes observed in volcanic gases, like the 

 relatively high concentration of it in the atmosphere, is mainly to be attributed to 

 the radioactive decay of ^OK -^^o^ Xhe time which would have been required 

 for the accumulation in this way of the amount of ^OA which is now present in 

 the atmosphere if it arose from the amount of formations exposed to superficial 

 weathering would seem to be about 5 x 10^ years, i.e. it corresponds to the age 

 of the Earth as estimated by the lead isotope method. f 



The amoimt of ^He, another inert gas which is present in the atmosphere and 

 which seems to be a product of the radioactive decay of U and Th, is less than 

 would be expected from the available data as to the amoimts of U and Th in 

 the crust of the Earth. This indicates that He, and, of course, H which is still 

 Hghter, are continually leaving the Earth. The loss of H by this means is greater 

 than that of D. The idea which we have accepted about the formation of the 

 primary atmosphere does not account for the preservation of gaseous N2. N2 is 

 present in volcanic gases but its source is unknown. It is very Hkely that N2 was 

 present in the form of compounds, the general view being that these were nitrides 

 of Fe, Ti and other metals or ammonium salts. The nitride Fe5N2 has been 

 found as an efflorescence on the lavas of Etna and Vesuvius, while osbornite, 



* In making t±iis calculation I have assumed that the basalts are 20 km thick and that 

 the average thickness of the granites over the whole surface of the Earth is 10 km [21]. 



t I shall deal in detail with the isotopic composition of the gases of the atmosphere 

 in another place [24, 13]. 



