438 CAUSE OF THE GLACIATION 



The theories can be grouped under three classes, 

 astronomical, geographical and meteorological. The 

 first seeks an explanation through change of position of 

 the earth's axis of rotation or a decrease of the heat re- 

 ceived from the sun ; the former seems to be quite in- 

 admissible, and the latter concerns a subject of which 

 very little is known. 1 The geographical explanations 

 are of two kinds, those which postulate the existence of 

 extensive and high mountains, and those depending on 

 the effect which would be produced by certain distribu- 

 tions of land and water ; there is absolutely no support 

 for the former hypothesis in South Africa, and the ex- 

 tensive area affected, together with the great probability 

 that the country in Dwyka times stood not far above sea 

 level, would seem to condemn it; the second class of 

 geographical explanations is closely connected with the 

 meteorological group, and though it is exceedingly diffi- 

 cult for others than physicists to estimate the value of 

 the many hypotheses and calculations put forward, it 

 seems that of all the explanations offered these alone 

 are of any value. The chief condition to be accounted 

 for is low temperature in moderate and low latitudes ; 

 this has been explained by supposing that there was 

 less carbon dioxide in the air than now, for that gas 

 hinders the loss of heat from the earth's surface by 

 radiation into space ; and by showing that the oceanic 

 circulation might have been so arranged that warm 

 water from equatorial regions did not greatly mitigate 



1 For a discussion of the possible variation of the amount of heat, re- 

 ceived from the sun see E. Dubois, Arch. Mus. Leyden, 1901, p. 311, 

 and 1902, p. 73. 



