CLIMATIC VARIATIONS GREGORY. 347 



cicled with periods of emergence ; and it would be quite inapplicable to 

 the Upper Paleozoic glaciation of Australia, of which the glacial 

 deposits were in places submarine. 



THE OBLIQUITY OF THE ECLIPTIC. 



Nor, in spite of the fresh use made of it by Ekholm and Dickson, 

 does the variation in the obliquity of the ecliptic appear to help mate- 

 rially; for all the influences of this agency are open to the funda- 

 mental objection that variations in obliquity recur at what, geolog- 

 ically speaking, are short and frequent intervals; whereas ancient 

 glaciations happened but seldom, and were apparently irregular in 

 their time of return. 



VARIATIONS IN THE CARBONIC ACID CONTENT OF THE ATMOSPHERE. 



The view that now seems most popular explains the major climatic 

 changes by variations in the powers of selective absorption of heat 

 by the atmosphere. The change is attributed either to variations in 

 the amount of aqueous vapor as urged by de Marchi °' or of carbon 

 dioxide as advocated by Svante Arrhenius,^ and recommended to us 

 by the brilliant advocacy and high authority of Prof. T. C. Cham- 

 berlin.*' 



The aqueous vapor theory has been adequately disposed of by 

 Arrhenius, whose alternative is especially attractive, as it demands 

 comparatively small differences of temperature and very modest vari- 

 ations in the amount of carbonic acid. Thus he calculates that an 

 increase of the carbonic acid from 0.03 to 0.09 per cent would give the 

 polar regions a temperate climate, by a rise of from 12° to 1G° F. 

 Nevertheless, this theory — that former colder periods were due to a 

 reduction of the carbonic acid in the air and warm periods to an in- 

 crease in its amount — is faced by objections which I venture to think 

 still inadequately answered. 



No one is likely to deny the possibility of great variations in the 

 former composition of our atmosphere. The theories of Koene 

 (185G), Phipson (1893-94), or Stevenson (1902),'^ that the primitive 



° De Marchi, " Le Cause dell'era Glaciale," Pavia, 1895. 



^ S. Arrbenius, " On the influence of carbonic acid in the air upon the tempera- 

 ture of the ground," Phil. INIag., ser. 5, Vol. XLI, 1896, pp. 237-27G. 



'^ T. C. Chamberlin, "A group of hypotheses bearing on climatic changes," 

 Journ. Geol., Vol. V. 1897, pp. 676-683 ; " The influence of great epochs of lime- 

 stone formation upon the constitution of the atmosphere," ibid., Vol. VI, 1898, 

 pp. 609-621. 



'^ J. Stevenson, " The chemical and geological history of the atmosphere," 

 Phil. Mag., ser. 5, Vol. L, pp. 312-323, 399-407 ; also Pt. II, " The composition 

 and extent of the atmosphere in very primitive times," Phil. Mag., ser. 6, Vol. 

 IV, 1902, pp. 448-451. 



