INORGANIC GEOLOGICAL DYNAMICS. 617 



He finds, however, that though the effect of this 

 change on the mean temperature of the year may 

 be small, the effect on the extreme temperature of 

 the seasons will be much more considerable; "so 

 as to produce alternately, in the same latitude of 

 either hemisphere, a perpetual spring, or the ex- 

 treme vicissitudes of a burning summer and a 

 rigorous winter 13 ." 



Mr. Lyell has traced the consequences of another 

 hypothesis on this subject, which appears at first 

 sight to promise no very striking results, but which 

 yet is found, upon examination, to involve adequate 

 causes of very great changes : I refer to the sup- 

 posed various distribution of land and water at dif- 

 ferent periods of the earth's history. If the land 

 were all gathered into the neighbourhood of the 

 poles, it would become the seat of constant ice and 

 snow, and would thus very greatly reduce the tem- 

 perature of the whole surface of the globe. If, on 

 the other hand, the polar regions were principally 

 water, while the tropics were occupied with a belt 

 of land, there would be no part of the earth's sur- 

 face on which the frost could fasten a firm hold, 

 while the torrid zone would act like a furnace to 

 heat the whole. And, supposing a cycle of terres- 

 trial changes in which these conditions should suc- 

 ceed each other, the winter and summer of this 

 " great year," might differ much more than the ele- 

 vated temperature which we are led to ascribe to 



12 Geol. Trans, vol. iii. p. 298. 



