SOME ASPECTS OF GEOLOGIC TIME 261 



sea might have been larger than it now is. This suggestion, 

 at first sight, seems probable. The erosion of the chalk hills 

 and their conveyance to the sea in solution by the rivers is 

 certainly occasioned mainly by the carbon dioxide which falls to 

 the ground in the rain. The same cause is an important factor 

 in all erosion. For that reason the factor must be briefly 

 considered. Here it is hardly possible to dogmatise either way. 

 Nothing is easier than to make rash and unfounded theories. It 

 is certainly difficult to imagine causes which would enormously 

 increase the carbon dioxide in the air for a particular geologic 

 period. Where it would come from, and why it should vanish, 

 are, at least, problems which require careful consideration. It 

 will suffice, however, to make two comments. 



In the first place we must note that we have, in the sea, 

 an enormous reservoir which acts as a giant fly-wheel on the 

 composition of the atmosphere. Those who accept this theory 

 must account, not only for the production of the carbon dioxide 

 to fill the atmosphere, but also for that enormously greater 

 amount which would dissolve in the ocean. The amount of 

 carbon dioxide in the atmosphere and in the ocean is in approxi- 

 mate equilibrium, and the amount in the atmosphere is only 

 a small fraction of that contained in the sea. In the next place, 

 we must note that the suggestion only affects the time necessary 

 to evolve the limestone from igneous rock in so far as it affects 

 nearly all the recognised methods of estimating geologic time. 

 It is, of course, true that a more acid rain would more rapidly 

 dissolve the lime from the igneous rock, and so increase the total 

 mass of terrestrial limestone, but the same factor would hasten 

 all the processes of erosion and deposition. Rock would more 

 quickly be crumbled, and carried away in sediment by the rain. 

 The dissolved sodium would more quickly reach the sea. Thus, 

 if this hypothesis seek to harmonise any discrepancy that may 

 be supposed to exist between the evidence of limestone and that 

 supplied by other methods of denudation, the suggestion will 

 utterly fail. It cannot too strongly be emphasised, in all geologic 

 speculation, that it is necessary to try to disentangle the full 

 bearing of many correlated factors. 



It must be admitted, however, that, for a special period, 

 which would not greatly affect general averages, the factor 

 might not be without its effect on the rate of formation of 

 particular deposits, such as those we have noted at some length 



