52 SPARKS i'HOM A GEOLOGIST'S HAMMER. 



kind have occurred, besides several severe contusions. 

 Some years ago three tourists tried the experiment of 

 exploding a bomb beneath the vault of ice. The arch 

 came tumbling about their heads ; one was crushed to 

 death, and the two others were very severely wounded. 



The movements of the glacier are among its greatest 

 marvels. It marches and it retreats. The Lower Glacier 

 of the Aar, which was the scene of Agassiz' observations, 

 moves downward at an average rate of 250 feet per 

 annum (Dolfuss-Ausset). HugiYhut, according to Agassiz, 

 had been carried 5,900 feet in 13 years. A record bot- 

 tled up by Hugi, stated that it had traveled 197 feet in 

 three years and 2,345 feet in 9 years. The Mer de Glace 

 travels better. Forbes demonstrated by observations that 

 at Montanvert it moves 822 feet per annum, and at the 

 source of the Arveiron 209 feet per annum. The stream 

 of ice, pressed into a narrower channel, moves with in- 

 creased velocity. 



This march of the glacier, unless counteracted, would 

 result in a rapid encroachment upon the cultivated lands 

 of the valley. The compensation is found in the sum- 

 mer's warmth. The ice undergoes a rapid dissolution, 

 as the river testifies which issues from its base. This 

 causes a diminution of the glacier in a longitudinal as 

 well as a vertical sense. The advance of the lower ex- 

 tremity is melted off during the summer. If the season 

 prove unusually warm, the dissolution exceeds the ad- 

 vance, and a retreat is the net result. If the season 

 prove cooler than usual, the advance exceeds the amount 

 of dissolution, and the resultant is a net advance. 



Now, it is apparent that this glacier before which we 

 stand, has formerly occupied a more advanced position. 



