CHAr. XXIV.] 



SUMMAllY AND CONCLUSION. 



503 



case with Northern Europe during the giacial epoch — then the 

 glacial conditions would be continued and perhaps even inten- 

 sified when the sun approached nearest to the earth in winter, 

 instead of there being at that time, as Mr. Croll maintains, 

 an almost perpetual 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 

 known 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 periods. 



Discussing next the evidence for giacial epochs in earlier 

 times, it is shown 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, uninterrupted warm climates prevailed in 

 the north temperate 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 geogra- 

 phical condition of the northern hemisphere at these periods is 

 then investigated, and it is shown to have been such as to 

 admit the warm tropical waters freely to penetrate the land, 

 and to reach the Arctic seas by several channels ; and, adopt- 

 ing Mr. Croll's views 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 show^i, that changes of 

 excentricity and of the phases of precession would have no other 

 effect than to cause greater differences of temperature between 

 summer and winter ; but, wherever there was a considerable 

 extent of very lofty mountains the snow-line would be lowered, 

 and the snow-collecting area being thus largely increased a con- 

 siderable amount of local glaciation might result. Thus may be 

 explained the presence of enormous ice-borne rocks in Eocene 

 and Miocene times in Central Europe, while at the very same 

 period all the surrounding country enjoyed a tropical or sub- 

 tropical climate. 



The general conclusion is thus reached, that geographical 

 conditions are the primary causes of great changes of climate, 



