376 Cause of Glacial Epochs. 



been many guesses, and which, perhaps, are only now 

 beginning to be understood. 



It is not very many years, since a great difference 

 in the geographical distribution of land and sea was 

 regarded as a possible or even a probable cause of the 

 occurrence of important changes of climate during 

 Geological Time. If, said Lyell, in his earlier writings, 

 all the continental lands, were gathered in tropical 

 regions, and the rest of the globe were mainly covered 

 by sea, the climates of the world would be tropical and 

 temperate according to their latitudes, and if all the land 

 were mainly massed round the poles, even in the tropics 

 there would be no tropical heat such as they now endure, 

 while the greater portions of the northern and southern 

 hemispheres would suffer from climates of extreme 

 severity. In such a sketch as this it is needless to argue 

 the question, at all events as regards this special glacial 

 epoch, for the obvious reason that it is an established 

 fact that during most of that epoch, the continents of the 

 world, mountain chains and all, were distributed much 

 as they are at present, with occasional minor variations 

 in detail due to short local submergences. 



Neither is it worth while to discuss the facile 

 explanation of variation of climate, being due to the sun 

 with all its planets travelling through alternate hot and 

 cold regions of space. Such an idea crops out now and 

 then in conversation, but I do not remember to have 

 met any educated physicist who seriously entertained 

 it. 



I believe that the day may come, when both astro- 

 nomers and geologists will be forced to allow that, in 

 great cycles of geological time, changes have taken 

 place in the position of the earth's axis of rotation, in a 

 slowly cumulative manner, by gradual disturbances of 



