118 SCIENCE IN SHORT CHAPTERS. 



glacial epoch resembled those now existing in temperate 

 climates, or were intermediate between these and the Ant- 

 arctic glaciers. The nature of the difference which I sup- 

 pose 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 reservoirof 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 snow-fall. 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 of fragments at the foot of the gla- 

 ciers 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 de- 

 posited as an alluvium wherever the agitated waters find a 

 resting-place. 



Thus the debris of the ordinary modern glacier is effec- 

 tively separated into two or more very distinct deposits ; the 

 moraine at the glacier foot consisting of rock fragments of 

 considerable size with very little sand or clay or other tine 

 deposit between them, and a distant deposit of totally dif- 

 ferent character, consisting of gravel, sand, clay, or mud, 

 according to the length and conditions of its journey. The 

 "chips," as they have been well called, are thus separated 

 from what I may designate the filings or sawdust of the 

 glacier. 



The filings from the existing glaciers of the Bernese Alps 

 are gradually filling up the lake-basins of Geneva and Con- 

 stance, repairing the breaches made by the erosive action of 



