A Study in the Natural History of Ice 287 



the ice age to have been at least 500 metres thicker than the 

 present glaciers in the Chamonix district, they must have 

 travelled many times faster than the present ones do. 

 Applying the formula derived from his work on the Mer de 

 Glace to the case of the great glacier which in former times 

 filled the valley of the Arve, he shows that, allowing it to 

 have a total thickness of 1000 metres at its exit from the 

 valley at Geneva, it may have had a rate of travel of 4/5 metres 

 per day ; that of the Mer de Glace at the present day is only 

 o - 35 metre per day. 



If the velocity of the ice radiating from the centre of 

 snow and neve in the Alps at that spot was anything like 

 4 metres per day, and the thickness of it in the valleys was 

 anything like iooo metres, it is evident that when the decrease 

 began, and the supply of snow and neve ceased to be able to 

 feed a drain of ice from the mountains to the plains on the 

 colossal scale which these figures indicate, the flow of ice 

 would continue for a long time with very little abatement 

 before the length of the glaciers would be much reduced. 

 But as the supply at the fountain-head has been reduced, the 

 persistence of the rate of flow of the glaciers must have the 

 effect of stretching the ice in their upper reaches, and this 

 would cause a diminution of their thickness without the 

 necessity for melting to intervene. The melting, however, 

 would proceed all the same, in proportion to the energy of 

 insolation in the locality and to the superficial area of ice 

 exposed to it. 



The total effect of melting and stretching combined would 

 be to accelerate the disappearance of the ice in the earlier part 

 of the period of decrease, while the final rate of disappearance 

 could not fall below that which it is at present. 



Consequently, it may with certitude be affirmed that, 

 when the ice of the ice age really began to disappear, the 

 first half would do so at a greater rate than the second half, 

 and that the time required for its reduction to a state such as 

 that which it exhibited in the middle of last century may 

 have been less than four hundred years. 



