CHAP. IX.] 



ANCIENT GLACIAL EPOCHS. 



169 



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 another 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 import- 

 ance to our present inquiry, because the wide extent of marine 

 Tertiary deposits in the northern hemisphere and their occur- 

 rence 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 submergences and emergences of the land com- 

 bined 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 in arms of the sea, or 

 of the terminal face of the ice-sheet which passes beyond the 

 land into the ocean. In both these cases abundance of rocks 

 and debris, such as form the terminal moraines of glaciers on land, 

 are carried out to sea and deposited over the sea-bottom of the 

 area occupied by icebergs. In the case of an ice- sheet it is 

 almost certain that much of the ground-moraine, consisting of 

 mud and imbedded stones, similar to that which forms the till" 

 when deposited on land, will be carried out to sea with the ice 

 and form a deposit of marine till " near the shore. 



It has indeed been objected that when an ice-sheet covered 



