300 THE BEAUTIES OF NATURE CHAP. 
of the Danube from its source to Vienna, and 
as just mentioned, of the Aar from Bern to 
Waldshut. Hence also, whenever the Swiss 
rivers running east and west break into a 
transverse valley, as the larger ones all do, 
and some more than once, they invariably, 
whether originally running east or westwards, 
turn towards the north. 
But although we thus get a clue to 
the general structure of Switzerland, the 
whole question is extremely complex, and 
the strata have been crumpled and folded 
in the most complicated manner, sometimes 
completely reversed, so that older rocks have 
been folded back on younger strata, and even 
in some cases these folds again refolded. 
Moreover, the denudation by aerial action, by 
glaciers, frosts, and rivers has removed hun- 
dreds, or rather thousands, of feet of strata. 
In fact, the mountain tops are not by any 
means the spots which have been most 
elevated, but those which have been least 
denuded; and hence it is that so many of the 
peaks stand at about the same altitude. 
