GEOLOGY. 645 



domains of Pluto, though ample of entrance, was yet 

 shrouded in impenetrable darkness toAvards the bottom. 

 The ancient Hercynian Forest, which we had just traversed, 

 was buried under half a yard of rime ; the cold was 25° 

 below freezing-point (Fahr.); and our vehicle, in spite of 

 the skids, Avhich made large showers of ice tly on all sides, 

 dragged us with frightful rapidity towards the precipice. 

 It was altogether superb, and vividly recalled the icy 

 forests of the north. 



Sometimes, on the contrary, the mountain tops in splin- 

 tering off have left upright here and there long narrow 

 segments of rock, which, seen from afar in the dim mists of 

 night, seem like so many fantastic shades hovering in the 

 clouds. These are the Avitch-dances of the superstitious 

 inhabitants of the Harz forests. 



When the rending asunder of mountains takes place on 

 a grand scale, and their flanks are deeply cut into, advan- 

 tage is taken of the natural openings thus formed in order 

 to trace out roads or passes, distinguished by the name of 

 gates, because they offer easy means of communication 

 between nations. The Iron Gates of Algeria have acquired 

 a certain degTee of celebrity. 



It should also be remarked, that certain gorges are due 

 to the erosion of the waters alone, which, rolling over their 

 walls, incessantly wear them away, and in time form large 

 valleys. These gorges of erosion are less rugged than those 

 caused by rents; the waters, by the friction of the frag- 

 ments carried down by them and by their own movement, 

 having smoothed their sides. Often also rivers roar at the 

 bottom of these ravines, leaping over the pebbles, or preci- 

 pitating themselves from fall to fall amid the rocks. 



Many of the cascades which we meet with among moun- 

 tains issue from gorges of erosion. There are some which, 



82 



