CHAP. XXIV SUMMARY AXD CONCLUSIOX 535 



shown, by a variety of evidence, that without higli land 

 there can be no permanent snow^ and ice. From these 

 facts and principles tlie very important conclusion is 

 reached, that the alternate phases of precession — causing 

 the Avinter of each hemisphere to be in aphelion and 

 IMrihelioii each 10,500 years — would produce a complete 

 change of climate only where a country was partially 

 snow-clad ; w^hile, whenever a large area became almost 

 ivholly buried in snow and ice — as was certainly the case 

 with Northern Europe and America during the glacial 

 epoch — then the glacial conditions would be continued 

 and perhaps even intensified when the sun approached 

 nearest to the earth in winter, instead of there being at 

 that time, as Mr. Croll maintains, an almost j^erpetual 

 spring. This important result is supported by reference 

 to the existing differences between the climates of the 

 northern and southern hemispheres, and by what is know^n 

 to have occurred during the last glacial epoch ; and it is 

 shown to be in complete harmony with the geological 

 evidence as to interglacial mild jDeriods. 



Discussing next the evidence for glacial epochs in 

 earlier times, it is showii that Mr. Croll's views are opposed 

 by a vast body of facts, and that the geological evidence 

 leads irresistibly to the conclusion that during a large 

 portion of the Secondary and Tertiary periods, uninter- 

 rupted warm climates prevailed in the north temj^erate 

 zone, and so far ameliorated the climate of the Arctic 

 regions as to admit of the growth of a luxuriant vegetation 

 in the highest latitudes yet explored. The geogTaphical 

 condition of the northern hemisphere at these periods is 

 then investigated, and it is shown to have been probably 

 such as to admit the warm troj^ical Avaters freely to 

 penetrate the land, and to reach the Arctic seas by 

 several channels ; and, adopting Mr. Croll's calculations 

 as to the enormous quantity of heat that would thus be 

 conveyed northwards, it is maintained that the mild 

 Arctic climates are amply accounted for. With such 

 favourable geographical conditions, it is shown, that 

 changes of excentricity and of the phases of precession 

 would have no other effect than to cause orpeater differences 



