CHAP. IX ANCIENT GLACIAL EPOCHS 175 



period itself, the unequal amount of ice at the two poles 

 causing the centre of gravity of the earth to be displaced 

 when, of course, the surface of the ocean will conform to it 

 and appear to rise in the one hemisphere and sink in the 

 other. If this is the case, subsidences of the land are 

 natural concomitants of a glacial period, and will power- 

 fully aid in removing all evidence of its occurrence. We 

 have seen reason to believe, however, that during the 

 height of the glacial epoch the extreme cold persisted 

 through the successive phases of precession, and if so, both 

 polar areas would probably be glaciated at once. This 

 would cause the abstraction of a large quantity of water 

 from the ocean, and a proportionate elevation of the land, 

 which would react on the accumulation of snow and ice, 

 and thus add avother to that wonderful series of physical 

 agents which act and react on each other so as to intensify 

 glacial epochs. 



But whether or not these causes would produce any 

 important fluctuations of the sea-level is of comparatively 

 little importance to our present inquiry, because the wide 

 extent of marine Tertiary deposits in the northern hemi- 

 sphere and their occurrence at considerable elevations above 

 the present sea-level, afford the most conclusive proofs that 

 great changes of sea and land have occurred throughout 

 the entire Tertiary period ; and these repeated sub- 

 mergences and emergences of the land combined with 

 sub-aerial and marine denudation, would undoubtedly 

 destroy all those superficial evidences of ice-action on 

 which we mainly depend for proofs of the occurrence of the 

 last glacial epoch. 



What Evidence of Early Glacial Epochs may he Expected, — 

 Although we may admit the force of the preceding argument 

 as to the extreme improbability of our finding any clear 

 evidence of the superficial action of ice during remote 

 glacial epochs, there is nevertheless one kind of evidence 

 that we ought to find, because it is both wide-spread and 

 practically indestructible. 



One of the most constant of all the phenomena of a 

 glaciated country is the abundance of icebergs produced by 

 the breaking off of the ends of glaciers which terminate 



