256 Dr. A. P. Young—Glaciation of Navis Valley. 
If the views here set forth be correct, the lip of the corrie marks 
the lower end of the ice-tongue under which the basin was excavated ; 
the boundary of the glacier is defined by the lip and the watershed 
of the catchment area above it. By means of these data some precision 
could be introduced into the computation of the corresponding snow- 
line. Briickner’ has found that about one-third of the whole surface 
covered by a glacier belongs to the tongue; thus the contour-line 
which marks off the lower one-third portion from the upper two-thirds 
of the glaciated area is the snow-line. Where contoured maps on 
a large scale are available, a rule such as this, though empirical, 
should give good results in the case of corries, the areas dealt with 
being small. 
Erosion UNDER THE ICcE-TONGUE. 
The direct action of glacier-ice in wearing and scouring the standing 
rock is here accepted as an established fact. It has not been thought 
worth while to qualify the terms in which this postulate is implied. 
For the purpose of this paper, however, it would suffice to assume 
merely that the bed of a glacier tongue differs distinctly in form or 
dimensions” from that of a channel eroded by running water alone. 
The truth is that all conclusions hitherto drawn regarding the relative 
rates of erosion by ice and by water are of doubtful validity. The 
two processes cannot be observed apart. 
The facts to be taken into account are— 
The glacier tongue being below the snow-line, where all the snow 
that falls within the year melts within the year, the whole of the 
local precipitation must reach the land surface under the ice as 
running water by the numerous clefts which form at the firn-line 
and below it. Generally, however, the channels in the ice deflect 
the water, which thus reaches the bed not at the point directly under 
the spot at which it falls, but lower down the valley. 
Some snow above the firn-line melts and reaches the bed to form 
part of the stream. The snow-line is only the limit at which the 
falling snow just balances the melting snow. 
The whole of the glacier-ice is contributed by the firn, that is, by 
a part of the precipitation above the snow-line. By ablation this all 
joins the stream at various points above the lower end of the 
ice-tongue, so that the water running out at the ice-portal during the 
year is exactly what with the same amount of precipitation would 
flow over the bed at this place if no glacier existed. 
The glacier-bed is a water-channel as well as an ice-channel; the 
form taken by the bed is due to the combined action of water and ice. 
GRADIENT OF GLACTIER-ICE. 
The gradient of the ice surface during the last maximum of cold 
has been inferred from a great number of records by various observers 
1 Briickner, Meteorologische Zeitschrift, 1887, p. 31. Hess, Die Gletscher, p. 74. 
The ratio actually observed diverges widely in extreme cases from Briickner’s mean. 
Richter holds that the proportion of glacier-ice to firn is, as Briickner himself hints, 
more nearly 1:4 than 1:3 (E. Richter, Gletscher der Ost Alpen, 1888, p. 41). 
Hess adopts the ratio 1 : 31 (loc. cit., p. 88). 
2 See A. Penck, ‘‘ Uebertiefung der Alpentiiler”” : Internat. geogr. Congress, 1899, 
p. 289. - 
