PEOFESSOE TY^fDALL ON THE VEINED STEIJCTUEE OF OLACIEES. 303 
§ 11. Bemarks on Glacier Motion. 
It is only by slow degrees that we master from actual observation, a problem so large 
as that presented by the glaciers ; the muscular labour alone being such as to render 
the expenditure of a considerable amount of time unavoidable. The examination of the 
various questions connected with glaciers, has been therefore, in my case, distributed 
over some years, and not until last summer was I able to devote the requisite attention 
to the subject of the present section, which, however, is essential to a right compre- 
hension of the physics of glacier motion. 
It would be a problem eminently worthy of any geologist, to lay down upon a trust- 
worthy map of Switzerland the directions of the stri® oh the rocks over which ancient 
glacieis have moved; and to one who sees its importance and desires exact information 
upon this subject, it must be a matter of surprise that nothing of the kind, in a 
systematic way, has yet been attempted. A suitable map furnished with such lines of 
direction, carefully and conscientiously drawn, would impart more satisfactory informa- 
tion than aU the volumes that ever have been, or ever will be written upon the subject. 
Here is a piece of work loudly calling for accomplishment, and one on which any young 
geologist may base an honom-able reputation. 
Mr. Hopkins, I believe, was the first to urge the existence of roclies polies at the ends 
of existing glaciers and along the continuations of existing glacier valleys as an evidence 
in support of the sliding theory. That such facts exist is known to every body, and that 
the rocks are thus polished and rounded by the glaciers sliding over them is incontro- 
vertible. Let a traveller, if he wish to obtain a wealth of information upon this subject, 
transport himself to the terminus of the Unteraar glacier, and walk thence down the 
valley through which the river Aar now fiows. On all sides he will obtain the most 
striking evidence that the base of the valley was once the bed of the glacier. The rocks 
are polished and striated, and present at some places the appearance of huge rounded 
mounds, which, at first sight, would appear to offer an insuperable barrier to the motion 
of the glacier, but which show by their aspect that the ice actually moved over them, 
grinding off their angles and furrowing their summits and sides. All along the valley 
towards Meyringen, similar evidences exist. In fact, the phenomenon is very common, 
and admitted on all hands. 
The conclusion which Mr. Hopkins has drawn from these facts is unavoidable ; the 
glaciers must have slidden over the rocks on which such traces are left. To an eye a 
little practised in those matters, the precise limits reached by the ancient glaciers are 
perfectly visible. The junction of the rounded and abraded portions of the mountains, 
with those portions which in ancient times rose over the then existing ice, is per- 
fectly distinct ; and I should say in the valley of the Aar reaches to a height of more 
than a thousand feet above the present bed of the river. The valley of Saas, in the 
Canton de V alais, furnishes magnificent examples of the same kind. 
At all places, from the base of the ancient glacier to its surface, sliding must have 
occurred; the evidence of it is perfectly irresistible. The summit of the Grimsel pass 
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