Geological History of the Atmosphere. 449 



ancient times, there could not be any carbonates present such 

 as we imd now, and therefore the carbonic acid now found 

 in them in the combined condition must have existed at that 

 time in the free condition, unless of course the cjirbon and 

 oxygen composing it existed in some other forms of chemical 

 combination. The total quantity of these carbonates is very 

 great. Dr. Sterry Hunt, as already referred to, in his 

 papers on the Geological Relations of the Atmosphere, 

 •calculated that there was enough C0 2 present on the earth 

 in the form of carbonates to form, if set free, an atmosphere 

 200 times greater in extent than our present atmosphere. 

 Now it is obvious that an atmosphere of C0 2 of this extent 

 would have a very great influence on many terrestrial 

 phenomena. The terrestrial radiation of heat would be 

 much affected by it, and the temperature of the surface of the 

 earth would be considerably higher than with our present 

 atmosphere. Prof. Arrhenius, of Stockholm, calculates (Phil. 

 .Mag. April 1896, p. 26S) that if the present small quantity 

 of carbonic acid in the atmosphere (*03 per ceut.) were 

 increased 2'D to 3 times, the temperature in the arctic regions 

 would rise about 8° or 9° C, an amount sufficient to cause 

 very great climatic changes. And if such a (comparatively) 

 small addition of carbonic acid to the atmosphere is likely 

 to cause such a change in the temperature, what are we to 

 infer regarding the influence of a quantity several hundreds 

 of thousands of times greater ? Clearly an atmosphere con- 

 taining such a quantity of carbonic acid or even a considerably 

 less quantity, say one tenth or one hundredth of the above 

 amount, would have a marked effect in retarding the radiation 

 of heat from the earth, and it is therefore quite possible that 

 the rate of the secular cooling of the earth was considerably 

 lower in very earlv oeolooical times than it is now. 



Further, an atmosphere containing much carbonic acid 

 must have had a much more powerful solvent or disin- 

 tegrating action on siliceous rocks than our present one, and 

 therefore the rate of the formation of carbonates and prob- 

 ably also the rate of general denudation must have been 

 much greater than they are now. The rate of denudation 

 would also be increased, as compared with that of modern 

 times, by the much greater rainfall which we should expect 

 1o take place as a result of the greater extent of the at- 

 mosphere and the greater quantity of aqueous vapour that it 

 would contain. 



It is worth noting here, that the formation of carbonates 

 from silicates (which are anhydrous, or nearly so, in the case 

 of igneous and crystalline rocks) is usually accompanied 



