22 



than in winter, and faster by day than by night. A ladder, lost by 

 De Saussure, travelled 4757 yards between 1788 and 1832 ; a knap- 

 sack lost on another occasion, moved 140 yards each year. In 1861, 

 '63 and '65, the bodies of three guides were disgorged by the 

 Glacier des Bossons ; they had been advancing with the glacier 

 since 1820, at the rate of 160 yards per annum. 



The cause of the motion of a glacier is still a disputed point. 

 Forbes was of opinion that it moves from its plasticity almost as if 

 it were made of treacle ; Tyndall teaches that it is owing to fracture 

 of the ice in multitudinous points, and subsequent refreezing ; while 

 CroU thinks that heat variously derived passes through the glacier. 

 This heat is partly carried by water, which permeates everywhere ; 

 and each particle of ice, as it is individually melted, utilises its 

 momentarily liquid form to move forward, impelled by the laws of 

 gravity. 



It is found that when ice has passed over rough and jagged 

 rocks, the latter present a rounded outline, so that they look 

 almost like sheeps' backs. Such rocks are hence teraied "rockes 

 nioutonnees." Several Highland valleys, also, have a rounded appear- 

 ance when the rocks are viewed from the head of the valley, while 

 they preserve much of their primitive roughness when seen from 

 below. This condition of things is characteristic of certain districts, 

 and shows^that at some time or other glaciers have been in action 

 there. An ice-sheet, in passing over a country, will often, on 

 melting, deposit blocks at a vast distance from the rock which 

 originally gave rise to them, and these are known as •'•'erratics," or 

 " blocs perches." Moraines and erratics abound in the British Isles, 

 and are proof positive that at one time glaciers existed as far south 

 as the Thames. Deposits, one above the other, of glacier- formed 

 material or " Till," are evidence in our islands that several glacial 

 periods followed successively. 



The Lecturer then went into the subject of Lake Basins, and 

 showed that there is much probability that many of these owed 

 their origin to glaciers. 



Glacial periods occurred in very remote geological times ; their 

 is proof of glaciation in the Permian, and it is believed, even in the 

 Silurian, epoch. The causes of frequently recurring ice periods are 



