Ancient Extehi of the Glticiers df Chamonix. ^Z 



Lake of Geneva. The mean temperature of that toWn is 9-°66. 

 On the surrounding mountains, the limit of perpetual snow, 

 as we have seen, is found to be 2700 metres above the sea. 

 The great glaciers of the valley of Chamonix descend to 1550 

 metres below that line. This settled, suppose that the mean 

 temperature of Geneva was lowered 4 degrees only, and be- 

 came 5°*56 ; the decrease of the temperature with the height 

 being one degree for 188 metres, the limit of perpetual snow 

 would descend to 750 metres, and would be more than 1955 

 metres above the sea. It will be readily allowed that the 

 glaciers of Chamonix would descend below this new limit in 

 a proportion at least equal to that which exists between the 

 actual limit and their lower extremity. Now, at present the 

 foot of these glaciers is at 1150 metres above the ocean ; with 

 a climate 4 degrees colder, it would be 750 metres lower, that 

 is to say, it would come to the level of the Swiss plain. Thus, 

 then, the lowering of the line of eternal snow would suffice 

 to make the glacier of the Arve descend to the neighbour- 

 hood of Geneva. But we must not forget that a glacier de- 

 scends so much lower in proportion to the extent of the amphi- 

 theatre from which it proceeds ; now, the glaciers, having, as 

 a source of supply all the valleys and all the gorges elevated 

 above 1950 metres, would descend, from that cause alone, 

 much lower than before. Accordingly, the united action of 

 these two causes, the lowering of the line of eternal snow, 

 and the enlargement of the amphitheatres, causes, each of 

 which, taken by itself, would be sufficient to explain the an- 

 cient extension of glaciers, enable us readily to comprehend 

 how that of the Arve could formerly advance to the environs 

 of Geneva. Let it not be forgotten, that this extension has 

 been the work of a long series of ages, the number of which 

 is, so to speak, declared to us by those millions of blocks 

 which the glacier has slowly and successively carried from 

 the foot of Mont Blanc to the margin of the Leman lake. 



The climate which has favoured this prodigious develop- 

 ment of glaciers has nothing of which we cannot form a very 

 exact idea ; it is the climate of Upsal, Stockholm, Christiania, 

 and the northern part of America in the state of New York. 

 Geologists who do not hesitate to raise the mean tempera- 



