464 Professor T. G. Bonney—Presidential Address— 
But to return. I have not selected petrology as my subject, partly 
because I think that the great attention which its more minute details 
have of late received has tended to limit rather than to broaden our 
views, while for a survey of our present position it is enough to refer 
to the suggestive and comprehensive volume published last year by 
Mr. A. Harker ;! partly, also, because the discussion of any branch of 
petrology would involve so many technicalities that I fear it would be 
found tedious by a large majority of my audience. . . . I purpose, 
then, to ask your attention this evening to some aspects of the glacial 
history of Western Europe. 
Much light will be thrown on this complex problem by 
endeavouring to ascertain what snow and ice have done in some 
region which, during the Glacial Epoch, was never submerged, and 
none better can be found for this purpose than the European Alps. 
In certain mountain regions, especially those where strong lime- 
stones, granites, and other massive rocks are dominant, the yalleys 
are often trench-like, with precipitous sides, having cirques or corries 
at their heads, and with rather wide and gently sloping floors, which 
occasionally descend in steps, the distance between these increasing 
with that from the watershed. Glaciers have unquestionably occupied 
many of these valleys, but of late years they have been supposed to 
have taken a large share in excavating them. In order to appreciate 
their action we must imagine the glens to be filled up and the district 
restored to. its former eared valor a a more or less undulating upland. 
As the mean temperature? declined snow would begin to accumulate 
in inequalities on the upper slopes. This, by melting and freezing 
would soften and corrode the underlying material, which would then 
be removed by rain and wind, gravitation, and avalanche. In course 
of time the hollow thus formed would assume more and more the 
outlines of a corrie or a cirque by eating into the hillside. With an 
increasing diameter it would be occupied, as the temperature fell, first 
by a permanent snow-field, then by the névé of a glacier. Another 
process now becomes important, that called ‘sapping’. While 
ordinary glacier-scour tends, as we are told, to produce ‘‘ sweeping 
curves and eventually a graded slope’’, ‘ sapping’ produces ‘‘ benches 
and cliffs, its action being horizontal and backwards”’, and often 
dominant over scour. The author of this hypothesis* convinced him- 
self of its truth in the Sierra Nevada. . . . Beneath the névé the 
temperature would be uniform, so its action would be protective, 
except where it set up another kind of erosion, presently to be noticed; 
but in the chasm, we are informed, there would be, at any rate for 
a considerable part of the year, a daily alternation of freezing and 
thawing. Thus the cliff would be rapidly undermined and be carried 
back into the mountain slope, so that before long the glacier would 
nestle in a shelter of its own making. Farther down the valley the 
1 The Natural History of Igneous Rocks, 1909. 
* In the remainder of this Address ‘ temperature’ is to be understood as mean 
temperature. The Fahrenheit scale is used. 
3 W. D. Johnson, Science, n.s., vol. ix, pp. 106, 112, 1899. 
