The measure of the excess of snow erosion over water erosion is therefore the difference between a northern or water-formed and a southern or snow-formed profile, [200] . This difference is also shown in [201] and from it and the restored initial profiles we conclude that the rate of water erosion is to that of nivation as 1:3. This ratio has been derived from numerous observations on cones so recently formed that the interfluves without question are still intact.
Fig. 202—Graphic representation of amount of glacial erosion during the glacial period. In the background are mature slopes surmounted by recessed asymmetrical peaks. The river entrenched itself below the mature slopes before it began to aggrade, and, when aggradation set in, had cut its valley floor to a′-b′-c. By aggradation the valley floor was raised to a-b while ice occupied the valley head. By degradation the river has again barely lowered its channel to a′-b′, the ice has disappeared, and the depression of the profile represents the amount of glacial erosion.
a′-b′-c = preglacial profile.
a-b-d-c = present profile.
b′-d-c-b = total ice erosion in the glacial period.
a-b = surface of an alluvial valley fill due to excessive erosion at valley head.
b-b′ = terminal moraine.
d-c = cirque wall.
e, e′ e″ = asymmetrical summits.
Thus far only those volcanoes have been considered which have been modified by nivation. There are, however, many volcanoes which have been eroded by ice as well as by snow and water. It will be seen at once that where a great area of snow is tributary to a single valley, the snow becomes compacted into névé and ice, and that it then erodes at a much faster rate. Also a new force—plucking—is called into action when ice is present, and this greatly accelerates the rate of erosion. While it lies outside the limits of my subject to determine quantitatively the ratio between water and ice action, it is worth pointing out that by this method a ratio much in excess of 1:3 is determined, which even in this rough form is of considerable interest in view of the arguments based on the protecting influence of both ice and snow. I have, indeed, avoided the question of ice erosion up to this point and limited myself to those volcanoes which have been modified by nivation only, since the result is more striking in view of the all but general absence of data relating to this form of erosion.