GEOCHEMICAL INVENTORIES 119 



Now the trouble with geochemical inventories always is that they 

 rest on quite a number of assumptions of a rather hypothetical 

 character. We know only the constitution of the uppermost level of 

 the crust, through oilwells and mines, up to a couple of kilometers. 

 Old cores of eroded mountain chains may give us an insight into 

 the local constitution of parts of the crust which were formerly buried 

 perhaps as deep as 30 km. But global inventories must use rather 

 far-stretched extrapolations from these meagre data. The structure 

 and the composition of the crust under the continents and the oceans 

 is, for instance, very different at present. A geochemical inventory 

 will therefore differ accordingly, if the author adheres to one of a 

 group of theories which postulates permanence of oceans and con- 

 tinents, or if he accepts the possibility of large parts of the con- 

 tinents foundering to oceanic depths. 



Geochemistry consequently is not in a position to give well founded 

 answers to many of the question marks it ought, in theory, be able 

 to answer. I will follow here a recent summary by Engelhardt 

 (1959), which is based on the assumption of a certain permanence of 

 oceans and continents. 



According to Engelhardt the question about the quantity of carbon 

 dioxide present in atmosphere and hydrosphere cannot be answered 

 by geochemistry. The reason is that carbon dioxide does not form a 

 closed system on the surface of the earth. There is constant injection 

 of carbon dioxide into the atmosphere from volcanic activity. It is 

 one of the commonest gases, not only during eruptions proper, but 

 also during the decline of volcanic activity, and in hot springs and 

 fumaroles. Its rate of production through volcanic activity has, how- 

 ever, not even been estimated with any reliability at present. Far less 

 do we know of its production in the geological past. The only thing 

 we know is that there have been strong variations in volcanic activity 

 in geologic history. Hence we may assume that the volcanic produc- 

 tion of carbon dioxide has been sufficient to account for the free 

 oxygen present now, but we have no data upon which to base an 

 inventory. 



We have had more luck with the other side of the process: the 

 production of carbon by the plants during organic photosynthesis. 

 After careful consideration of the amount of fossil carbon present in 

 the sediments of the crust of the earth, Engelhardt reaches the con- 



