SUBSIDENCE MORAINESSLOPE OF GLACIERS. 243 



several effects may usually be distinguished by observation.* 

 During the height of summer, near the Montanvert, I found the 

 daily average ablation to be 3'62 inches, the daily subsidence to be 

 1*63 inches. Seven-tenths of the geometrical depression are due, 

 therefore, to the former cause, and three-tenths to the latter. This 

 is a very large amount, and it is certain that during the colder 

 period of the year, and whilst the glacier is covered with snow, 

 the subsidence is not only suspended, but that the glacier recruits 

 in thickness a portion of its waste during the season of summer 

 and autumn. To this subject we shall again return. 



One point about moraines we have not yet mentioned. As 

 we ascend any considerable glacier we almost invariably observe 

 several parallel trails of debris extending throughout its length, 

 and not mixing with one another. These medial moraines may 

 in all cases be traced to a rocky promontory where two tributary 

 glaciers have united. The rocky masses detached by frost and 

 rain which have rolled upon the margin of the confluent glaciers 

 are borne along by the progress of each to the point of union. 

 But where the icy streams unite the trails of rock do so also ; 

 and being continually retained on the surface by the causes we 

 have mentioned, float, as it were, down the middle of the com- 

 mon glacier, preserving throughout the distinctive character of 

 their origin. Four such medial moraines may readily be traced 

 to their sources on the great Glacier of Chamouni ; but the 

 grandest specimen of a medial moraine is that on the Glacier of 

 the Lower Aar, effectively represented in one of the plates in 

 M. Agassiz' work (Etudes sur les Glaciers). 



The middle region of the great glaciers of the Alps extends 

 from the level of about 6000 to 8000 feet above the sea. The 

 inclination is usually there most moderate say from 2J to 6 6 . 

 But this is not invariably the case. Beyond 8000 feet we reach 

 the snow line. The snow line is a fact as definite on the surface 

 of a glacier as on that of a mountain, only in the former case it 



* Eleventh Letter on Glaciers. Edinburgh PkilosophicalJournal, 1846. [Re- 

 printed in the present volume.] Observations of the ablation of the ice have also been 

 made by MM. Martins and Agassiz. The amount, of course, depends materially 

 on the elevation and exposure of the glacier as well as on the weather and season. 



