204 Dr. A. P. Young—Glaciation of Navis Valley. 
the surface under the ice-tongue is not considered.) This zone does 
not necessarily coincide with the snow-line; it is to be sought 
near the isotherm of 0°C. of mean annual temperature, which in the 
Alps is probably above the snow-line.’ It may be that the standing 
rock under the firn is subject to frequent alternations of the kind in 
question; the changes may then be determined by variations of 
pressure and other causes only indirectly dependent on the temperature 
of the air; the freezing-point under a heavy cover of firn may be other 
than zero Centigrade. 
Above the zone indicated the oscillations of temperature about 
freezing-point diminish in frequency with the height, for not only is 
the mean annual temperature reduced, but the range of temperature- 
variation is narrowed in ascending,” and if denudation still goes on it 
must be due to other causes. 
It might be expected that a study of the volcanic cones which 
project above the snow-line would throw light on the question 
whether a cover of firn serves to protect the land surface. From the 
observations of E. Richter* in the Alps, I. Russell* on Mount Rainier, 
W. Reiss*® in Ecuador and Colombia, it seems clear that the glaciers 
at the snow-line are the most active agents of destruction. According 
to Russell the work of demolition goes on with the greatest activity 
along the crevasses which form at the firn-line. The beds of the 
glaciers are continually receding, with the result that, to use Richter’s 
phrase, the mountain is ultimately beheaded at the snow-limit. 
The outlines of extinct volcanoes which have been long subject 
to this levelling action are well shown in Hans Meyer's photo- 
graphs of Sincholagua and Quilindana.® In the latter figure the 
plain of nival denudation is especially conspicuous. The sketches 
given by A. Stiibel,’ preserving, as they do, the correct relation 
between vertical and horizontal dimensions, are also instructive. 
Russell* maintains that above this zone of rapid disintegration the 
ground is protected by the snow and firn, especially at higher levels, 
where the snow retains the powdery form until it is removed by the 
wind or falls to lower levels. 
Admitting the facts, the proof that a cover of firn or snow retards 
erosion is still incomplete. In estimating the comparative rates of 
erosion all the variables must be taken into account, for example— 
(1) The amount of precipitation varies with the height above sea. 
It reaches a maximum in the Alps at 1800 to 2400 metres, and above 
this level diminishes with the height. In other regions, with average 
rainfall, this maximum is accordingly to be looked for at levels below 
the snow-line. 
1 See J. Hann, Handbuch der Klimatologie, p.268. Inthe Eastern Alps near Innsbruck 
the temporary isotherm 0° C. is below the snow-line only during the winter months. 
2 H. Hess, loc. cit., p. 41. 
3 E. Richter, Geomonphologische Studien in den Hoch Alpen. 
4 I. C. Russell, ‘‘ The Glaciers of Mount Rainier ’’: U.S. Geol. Surv. Reports 
for 1896-7 (1898), vol. xviii, p. 382. 
5 W. Reiss & A. Stiibel, Das Hochgebirge der Republik Eeuador, 1902, ii, 
pp. 165, 173. 
6 Hans Meyer, In den Hoch-Anden von Ecuador, Berlin, 1907. 
7 Die Vulcanberge von Ecuador, p. 407. 8 J. Russell, loc. cit., p. 382. 
