EROSION. 259 



streams themselves have for thousands of years been at work to 

 deepen their own beds. It could occur only to the boldest 

 Neptunist that the Rhine may have hollowed out the deep 

 ravines of the Roflen and the Via Mala, or that the Linth alone 

 formed the rocky cleft running from the Pantenbriick to the 

 Thierwehd, the depth of which may render a person giddy who 

 looks down it; but undoubtedly these mountain -streams, fol- 

 lowing natural fissures and gorges, have gradually widened and 

 deepened them. 



A different result of aqueous action is seen in the region of 

 the Miocene. In Central and Eastern Switzerland the Miocene 

 formation is traversed by numerous brooks and rivers, the beds 

 of which have a direction nearly at right angles with the line of 

 the Alpine chain. Here we may observe that the Miocene lies 

 horizontally, and is only upraised at its edges, so that, at least 

 in the horizontal portion, neither hills nor valleys have been 

 produced by foldings. In this district hills and valleys could 

 only originate by erosions, which may have commenced in the 

 period of the Miocene, although their chief activity belongs to 

 the Pliocene period ; for we see in many places that the strata 

 of the newest Miocene rocks on the two sides of a valley exactly 

 correspond, and therefore, no doubt, were once connected, whilst 

 they are now separated by a deep valley. We have already 

 (vol. i. pp. 301, 302, and vol. ii. p. 116) attempted to describe 

 the appearance of the land in the later period of the Miocene ; 

 and the localities there described should be considered with 

 reference to the subsequent changes of the Pliocene epoch. It 

 would appear that these changes were connected with the large 

 freshwater lake spreading over the Cantons of Zurich and Thur- 

 govia in Eastern Switzerland, along the Alps (p. 116). Pre- 

 cisely in this region the Miocene was much upheaved, and in 

 the place of the bed of the lake high mountains (such as the 

 Speer and the Righi) rose aloft. Great masses of water were 

 poured over the horizontal Miocene, and flowed off in a direction 

 almost perpendicular to the line of upheaval, causing, with the 

 waters coming from the ascending region of the Alps, those 

 enormous erosions which so greatly exceed any thing that we 

 now witness. All the gorges and valleys which traverse the 

 region of the Miocene have been eroded by rivers and brooks ; 



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