300 THE BEAtrxlES 01? NATURE chai'. 



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 

 ti-ansverse '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. 



