162 SCIENCE IN SHORT CHAPl'ERS. 



ated for glacial accumulations were partially or -wholly filled 

 with ice. There may have been many intermediate fluctua- 

 tions of climate and glaciation, and probably were such, but as 

 these do not affect my present argument they need not be here 

 considered. 



So far I agree with the general conclusions of Mr. Geilde as 

 I understand them, and with the generally received hypothe- 

 ses, but in what follows I have ventured to diverge materially. 



It appears to me that the existing Antarctic glaciers and 

 some of the glaciers of Greenland are essentially different in 

 their conformation from the present glaciers of the Alps, and 

 from those now occupying some of the fields and valleys of 

 Norway ; and that the glaciers of the earlier or greater glacial 

 epoch were similar to those now forming the Antarctic barrier, 

 while the glaciers of the later or minor glacial epoch resembled 

 those now existing in temperate climates, or were intermediate 

 between these and the Antarctic glaciers. The nature of the 

 difference which I suppose to exist between the two classes of 

 glaciers is this : The glaciers (properly so called) of temperate 

 climates are the overflow of the neve (the great reservoir of ice 

 and snow above the snow line). They are composed of ice 

 which is protruded below the snow-line into the region where 

 the summer thaw exceeds the winter snowfall. This ice is 

 necessarily subject to continual thinning or wasting from its 

 upper or exposed surface, and thus finally becomes liquefied, 

 and is terminated by direct solar action. 



Many of the characteristic phenomena of Alpine glaciers 

 depend upon this ; among the more prominent of which are 

 the superficial extrusion of boulders or rock fragments that 

 have been buried in the neve, or have fallen into the crevasses 

 of the upper part of the true glacier, and the final deposit of 

 these same boulders or fragments at the foot of the glaciers 

 forming ordinary moraines. 



But this is not all. The thawing which extrudes, and finally 

 deposits the larger fragments of rock, sifts from them the 

 smaller particles, the aggregate bulk of which usually exceeds 

 very largely that of the larger fragments. This fine silt or sand 

 thus washed away is carried by the turbid glacier torrent to 

 considerable distances, and deposited as an alluvium wherever 

 the agitated waters find a resting-place. 



Thus the debris of the ordinary modern glacier is effectively 

 separated into two or more very distinct deposits ; the moraine 

 at the glacier foot consisting of rock fragments of considerable 



