1866.] DAWSON — ICEBERGS AND GLACIERS. 39 



table-land or series of wide valleys are thus emptied ' into one 

 narrow ravine, and pour their whole accumulations through the 

 Mer de Glace. Leaving however the many interesting phenomena 

 connected with the motion of glaciers, and which have been so 

 well interpreted by Saussure, Agassiz, Forbes, Hopkins, Tyndall 

 and others, we may consider their effects on the mountain valleys 

 in which they operate — 



1. — They carry quantities of debris from the hill-tops and 

 mountain valleys downward into the plains. From every peak, 

 cliff and ridge, the frost and thaw ar& constantly loosening stones 

 and other matters which are swept by avalanches to the surface of 

 the glacier, and constitute lateral moraines. When two or more 

 glaciers unite into one, these become medial moraines, and at 

 length are spread over and through the whole mass of the ice ; 

 eventually all this material, including stones of immense gize, as 

 well as fine sand and mud, is deposited in the terminal moraine or 

 carried off by the streams. 



2. — They are mills for grinding and triturating rock. The 

 pieces of rock in the moraine are, in the course of their movement, 

 crushed against one another and the sides of the valley, and 

 are cracked and ground as if in a crushing-mill. Farther the 

 stones on the surface of the glacier are ever falling into crevasses, 

 and thus reach the bottom of the ice, where they are further 

 ground against one another and the floor of rock. In the 

 movement of the glacier these stones seem in some cases to come 

 again to the surface, and their remains are finally discharged in 

 the terminal moraine, which is the waste-heap of this great mill. 

 The fine material which has been produced, the flour of the mill, 

 so to speak, becomes diffused in the water which is constantly 

 flowing from beneath the glacier, and for this reason all the 

 streams flowing from glaciers are .turbid with whitish sand and 

 mud. 



The Arve which drains the glaciers of the north side of Mont 

 Blanc, carries its burden of mud into the Rhone, which sweeps it 

 with the similar material of many other Alpine streams into the 

 Mediterranean, to aid in filling up the bottom of that sea, whose 

 blue waters it discolours for miles from the shore, and to increase 

 its own ever enlarging delta which encroaches on the sea at the 

 rate of about half a mile per century. The upper waters of the 

 Rhone, laden with similar material, are filling up the Lake of 

 Geneva ; and the great deposit of ' loess ' in the alluvial plain of 



