Gh. XIII.] 



DATE OF THE GLACIAL PERIOD 



291 



date 





which is subsequent to the beginning of the Glacial Epoch, 



be 



entire ocean would scarcely supply water enough to form an 



We 



ice-cap of the required thickness, 

 fore, to admit great modern changes in the relative level ol 

 sea and land, wholly independent of glacial conditions, and 

 due to such movements as are now going on in Sweden and 

 Greenland, or in the volcanic and coral areas of the Pacific .* 



How f( 



a date can be assigned to the Glacial Period by 



rference to former eras of 



From what I 



have already said in this and the preceding chapter, it will 



climate 



the quantity of ice now stored up in polar latitudes to have 

 been governed chiefly by geographical conditions. If, as is 





as 



r probable, a much larger excentncity ol tne eartn s orbit, 

 suggested by Mr. Croll, combined with that excess of 



nolar land, which I consider essential, would give rise to an 

 exaggeration of the cold of both hemispheres, and would espe- 

 cially augment the ice at that pole the winters of which oc- 

 Mirrprl in nrsnAlifm i+, would follow that we mi^ht obtain from 





f 





Glacial Period. We might 



modern 



whole of it, nearly all the animals and plants inhabiting the 

 northern hemisphere were the same as those now in exist- 

 ence. It is therefore a matter of no small interest to ascer- 

 tain the dates of those variations in the excentricity of the 

 orbit which may throw light on the times when the cold 

 first came on, when it reached its height, and when it was 



duced 





t limits. 



my applying to the Astronomer Eoyal, Mr 





Mr 



itory, to make so: 

 eminent mathem 











. 



* The reader is referred to important zine 1865 and 1866, in which the sup- 

 papers by Messrs. Heath, Croll, Moore, posed effect of polar ice-caps on the 

 and Pratt, in the Philosophical Maga- level of the ocean is ably discussed. 



u 2 



