CRUSTAL DEFORMATION AS THE CAUSE OF CLIMATIC CHANGES. 259 



reached on the basis of the changes during the past two or three thousand years. He can 

 not accept the carbonic-acid theory of glaciation, for two reasons: In the first place, glacial 

 periods seem to come on quickly, whereas changes in the carbonic-acid content of the air 

 must be very slow. In the second place, glacial epochs alternate with interglacial epochs 

 in a way which demands that the amount of CO2 shall have varied much more rapidly 

 than seems possible. Finally, no glacial period seems to have followed the enormous 

 locking up of carbonic acid in the vast limestone deposits of the Cretacic, while strong 

 glaciation followed the much smaller locking up of CO-, during the Miocene and Pliocene. 

 Moreover, the conclusions set forth in earlier portions of this book add still another strong 

 argument against the carbonic-acid theorj', for they show that the climatic changes of 

 historic times appear to be of too long duration to be explained on purely meteorological 

 grounds and yet are far too rapid to be due to changes in the amount of CO2 in the air. 

 Having excluded the carbonic-acid theory on these grounds, Professor Schuchert's final 

 conclusion is that changes in the form and size of the continents and seas, together with 

 the uplifting of mountain ranges upon the land and the diversion of oceanic currents and 

 winds from one area to another, and the subsequent changes in the amount of aqueous 

 vapor contained in the air, have been the chief factors in producing the marked climatic 

 variations which characterize the geological record. "Briefly then," as he puts it, "we 

 may conclude that markedly varying climates of the past seem to be due primarily to 

 periodic changes in the topographic form of the earth's surface, plus variations in the 

 amount of heat stored by the oceans. The causation for the warmer interglacial climates 

 is the most difficult of all to explain, and it is here that factors other than those mentioned 

 may enter." 



Let us now sum up the evidence as to the various climatic hypotheses. Professor 

 Schuchert's conclusion, being based upon the well-verified agreement of two distinct types 

 of related facts, is much more weighty than any conclusion based upon purely theoretical 

 grounds, and there seems to be good reason to accept it as in large measure correct. Yet 

 it lacks finality in several respects. In the first place, the theory of crustal deformation 

 makes no attempt to explain the small climatic changes now in progress. Secondly, it 

 can not explain such occurrences as the marked changes which culminated about the time 

 of Christ, about 1000 A. d., and about 1350 a. d. Thirdly, it can not explain interglacial 

 climates. And lastly, it does not explain why mountain-making and continental uplift 

 are usually accompanied by cool climates even at sea-level, although the law of chances 

 would indicate that part of the time the uplifting of the land should be as potent in causing 

 parts of the sea to become warmer as in causing them to become cooler. The volcanic 

 hypothesis appears to be a useful supplement to the hypothesis of crustal deformation, 

 but it fails to account for many of the most striking phenomena, and would seem to occupy 

 a position of only secondary importance. In the first place, the occurrence of pronounced 

 volcanic activity during geologic times does not appear regularly to coincide with pro- 

 nounced glaciation. In the second place, although volcanoes can be shown to have had 

 a distinct effect upon terrestrial temperature in the period since measurements of the sun 

 began to be made with accuracy, the effect is sporadic. It appears to be by no means so 

 important as the effect which seems to be exerted by changes in the sun, if we may judge 

 from the agreement of the sun-spot curve with the curve of the earth's temperature. Our 

 third hypothesis, that of carbonic-acid gas, seems to be unsatisfactory because it can not 

 account for the rapidity with which chmatic changes take place, and because times of 

 maximum glaciation do not regularly follow times when the maximum amount of CO2 is 

 withdrawn from the atmosphere. This does not mean that we reject the idea that carbonic 

 acid is an important cause of climatic changes, but merely that it seems safer to assign to 

 it a contributory role. Variations in the amount of carbonic-acid gas in the atmosphere 

 from year to year, apart from human manipulation, probably occur, though this has never 

 been demonstrated by actual observation. 



