Geological History of the Atmosphere. 101 



oxidation of vegetable remains would be very greatly in- 

 creased also, and that to an extent probably not much less 

 than the ratio of the increase in the luxuriance of vegetation 

 itself. The difference between the amount of carbonic acid 

 annually decomposed by vegetation and the amount annually 

 restored by eremacausis &c. would no doubt be greater in 

 such a case than it is at present ; but it would not necessarily 

 be enormously greater, and indeed it is not at all likely that 

 it would go bevond the limits that a large increase in the 

 supply of telluric carbonic acid (but still an increase only 

 to the extent limited by the general conditions of the 

 problem and the probabilities of the case) would be quite able 

 to overtake. 



We may thus consider that a sufficiently clear case has 

 been made out for the variability of the amount of atmospheric 

 carbonic acid within very wide limits, limits wide enough 

 to make the theory that the great climatic changes of geolo- 

 gical history are due to variations in the amount of atmospheric 

 carbonic acid quite a practicable, if not also a highly probable 

 one. An interesting point that now suggests itself is whether 

 the theory can be tested by an appeal to known geological 

 facts; that is to say, whether there is evidence to the effect 

 that there was great and unusual volcanic activity during 

 or just before warm geological epochs, and a marked decrease 

 in volcanic activity during cold or glacial epochs. In this 

 connexion it is at once recalled that there was great volcanic 

 activity in Scotland during the Tertiary epoch or part of it; 

 but still it would not do for us to lay much stress on the 

 evidence of a high degree of volcanic activity in one locality 

 only. In order to make this inquiry of any practical use, it 

 would be nece-sary for us to have some knowledge of the 

 general conditions as regards volcanic activity over the whole 

 world during each epoch ; the reason for this being that car- 

 bonic acid evolved in any particular region is quickly diffused 

 through the whole atmosphere of the earth. The percentage 

 of carbonic acid in the atmosphere is practically the same in 

 all latitudes and longitudes : and probably this uniformity 

 has always held good no matter what tin 1 exact percentage 

 for the time being may have been. For the same reason also 

 the theory of the course of climatic change that is based on 

 variations in the percentage of atmospheric carbonic acid is 

 general or universal in character : that is, if climatic changes 

 are brought about in this way at all, they must take place in 

 all parts of the world and in both hemispheres alike no matter 

 where the sear of special volcanic activity (in the ease o£ an 

 increase in the amount of carbonic acid) maybe. This feature 



