422 [Proc. B.N.F.C, 



the Lake of Geneva, has already reduced the area of that lake 

 thirty-four square miles. A town situated on the shore of the lake 

 300 years ago was now a mile inland. Mr. Harbison proceeded 

 to describe the origin and work of glaciers. On the Alps, the 

 height of the snow-line is 8,500 feet. Above this elevation, the 

 heat of Summer is unable to melt the snow that falls during the 

 previous Winter. If there were no means of getting rid of the 

 snow il would in the course of ages be piled up to an enormous 

 height. Some of it is removed by avalanches, which come tumbling 

 down the mountain slopes, but still more by glaciers. Mountains 

 that rise above the snow-line are not covered by a uniform cover- 

 ing of snow. Many of the peaks are so abrupt that the snow does 

 not lie on them. The snow is blown by the wind into hollows, 

 where it accumulates often to the depth of many hundreds of feet. 

 The snow, partly by pressure, which squeezes out the air, and 

 partly by melting and re-freezing, is transformed into ice, which 

 gradually moves down the slope at the rate of about two feet in the 

 day. De Saussure attributed the advance to the pressure exercised 

 on the upper portions by additional masses of snow. Forbes 

 believed that the ice was plastic, and behaved like a sluggish 

 stream. Agassiz maintained that the motion was due to the rains 

 and melted snow and ice, which, falling into the crevasses, get 

 frozen and increase in bulk, and thus force the glacier onward. 

 Tyndall is also of this opinion, which is now generally held. By 

 this theory the motion of a glacier over a ridge may be satisfactorily 

 accounted for. Blocks of stone derived from the cliffs on the 

 margin are scattered over the ice, then falling into the crevasses, 

 yawning gulfs, several hundred feet deep, get stuck in the bottom 

 of the ice, and assist it in grinding the rock beneath. Thus, in the 

 course of ages a deep groove is formed, and when, through change 

 of climate, the glacier retreats to the mountains, a valley is left 

 behind. It is only by studying the effects produced by existing 

 glaciers that one can understand the work done by that ancient 



