MOUNTAIN LAKES. 



347 



The quarrymen, when in search for good beds of slate, where the 

 side of a mountain is covered with stones and vegetation, form a lake 

 or pool near the top of the mountain, by damming up a rivulet where 

 it passes through a depression or small valley. When the water has 

 accumulated in sufficient quantity, they dig a trench near the dam., 

 to direct the current where they wish it to flow, and then break down 

 part of the dam. The water flows first through the trench, and rush- 

 ing with accelerated velocity down the mountain, carries with it the 

 stones near the surface, and in a very short time ploughs a deep chan- 

 nel in the rocks, exposing every bed to view. Thus, in a few hours 

 is effected, what the labour of many men, continued for months, could 

 not have accomplished. I have been informed, that in the upper 

 part of the valley of Long Sleddale, when the process of hushing 

 iakes place, the river Ken, (as it flows by Kendal, twelve miles dis- 

 tant,) is made turbid for some days, by the quantity of debris carried 

 into it. If such an effect can be produced by the small quantity of 

 water thus peqt up, it will not be difficult to believe, that the bursting 

 of extensive mountain lakes, may have scooped out passages for 

 mighty rivers. Even the bursting of a small mountain lake, in the 

 valley of Bagnes, in the year 1818, produced the most terrific effects. 

 The lake had been formed by a barrier of ice damming up the river 

 at a great elevation : this barrier suddenly gave way, and precipitated 

 the water into the great valley of the Rhone, near Martigny, tearing 

 down and overturning every obstacle it met in its passage. From 

 the quantity of mud and stones which it bore along, it resembled a 

 moving mass of stones and earth. An English gentleman who was 

 descending the valley at the time, observed his horse exhibit by its 

 motions, great trepidation, of which he could not discover the cause, 

 until a loud rushing noise occasioned him to look back, when he be- 

 held what appeared like a wall filling up the bottom of the valley, and 

 advancing rapidly towards him. He instantly alighted, and scram- 

 bled up the adjacent rocks, leaving his horse to its fate. Two years 

 afterwards, when I was at Martigny, the desolating ravages of this 

 catastrophe were apparent. 



Many of the valleys in the Alps have, evidently, once been lakes. 

 The upper valley of the Rhone, from its source to Martigny, formed 

 one lake : the whole valley of Geneva, between the Alps and the 

 Jura, formed a lower and more extensive lake, before a passage was 

 opened for the water at Porte I'Ecluse. When a fissure was once 

 made by earthquakes or by subsidence, the rushing of water charged 

 with stones, would enlarge and deepen the passage, and thus lay dry 

 and reduce the ancient lakes in a comparatively short period. In 

 the year 1819, part of a mountain immediately above the river Isere, 

 and opposite to the city of Moutiers, in the Tarentaise, suddenly fell 

 down into the river, and formed a dam across it, over which persons 

 might pass from one side to the other. When I was there in the 

 year 1821, all this mass of stone, had been carried away by the riv- 



