560 HISTORY OF GEOLOGY. 



changes of climate. 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 extreme vicissitudes of a 

 burning summer and a rigorous winter." 1 



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 supposed various distribution of 

 land and water at different periods of the earth's history. If the land 

 were all gathered into the neighborhood of the poles, it would become 

 the seat of constant ice and snow, and would thus very greatly reduce 

 the temperature 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 couM fasten a firm hold, while the torrid zone 

 would act like a furnace to heat the whole. And, supposing a cycle 

 of terrestrial changes in which these conditions should succeed each 



O 



other, the winter and summer of this "great year" might differ 

 much more than the elevated temperature which we are led to ascribe 

 to former periods of the globe, can be judged to have differed from the 

 present state of things. 



The ingenuity and plausibility of this theory cannot be doubted : and 

 perhaps its results may hereafter be found not quite out of the reach 

 of calculation. Some progress has already been made in calculating 

 the movement of heat into, through, and out of the earth ; but when 

 we add to this the effects of the currents of the ocean and the atmo- 

 sphere, the problem, thus involving so many thermotical and atmologi- 

 cal laws, operating under complex conditions, is undoubtedly one of 

 extreme difficulty. Still, it is something, in this as in all cases, to 

 have the problem even stated ; and none of the elements of the solu- 

 tion appears to be of such a nature that we need allow ourselves to 

 yield to despair, respecting the possibility of dealing with it in a use- 

 ful manner, as our knowledge becomes more complete and definite. 



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



