40 H. F. lieid — Studies of Muir Q lacier. 



haps the tenth to the sixteenth centuries, the glaciers of the Alps 

 were much less extensive than at present, and that horses were 

 able to cross passes now considered difficult by mountaineers. 

 During the seventeenth and eighteenth centuries the glaciers 

 increased, attaining their greatest extent in the beginning'of this 

 century.* At present they are in general retreating. This shows 

 a variation almost as great and almost as rapid as that mentioned 

 for the glaciers of Glacier bay. 



A possible Cause of the recent Retreat. 



When the tide in Muir inlet is very low one can see on its 

 eastern shore the stumps of large trees, which Professor Muir 

 assures me are in place. The trees must have grown, of course, 

 above high tide; they are now twenty feet below that level. 

 Although I cannot say so with certainty, it is not unreasonable 

 to suppose that these trees, like those of the buried forest, are 

 spruce, and of the same species as those now growing in Alaska ; 

 but we must remember that any results deduced from this sup- 

 position have no more weight than the supposition itself. If, 

 therefore, these trees were growing at t;he same time as those of 

 the buried forest, there has been a subsidence of the land of at 

 least 20 feet since the last advance of the glacier ; it may have 

 been much more ; if so, it would have produced an increase in 

 the mean annual temperature, which Avould have increased the 

 rate of melting and would also have decreased the proportion 

 of the solid to the liquid precipitation; and on account of the 

 general lowering of the mountains, more of the moisture from the 

 ocean may have been carried over them and precipitated further 

 inland. All of these results would tend to diminish the extent 

 of the glacier. Not only that, but the diminution itself would 

 increase the rate of diminution, for the presence, of extensive 

 snow-fields must lower the mean annual temperature (see page 

 52) and thus increase the proportion of snow to the total pre- 

 cipitation. If for any cause these snow-fields become smaller, 

 their influence on the mean temperature becomes less, the snow- 

 fall is diminished, and the snow-fields become smaller still. So 

 we see that anything causing a slight change in the mean tem- 

 perature may result finally in quite a large variation in the ex- 

 tension of the glacier, although this large variation may reach 

 its limit only long after the cause which started it has ceased to 



*Agassiz, Etudes sur les Glaciers, 1840, chap. xvi. 



