THE LEVELLING POWER OF RAIN. 369 



remarkable than those which are caused by the unaided 

 action of heavy rainfalls. 



; The most energetic action of aqueous destructive forces 

 is seen when water which has accumulated in the higher 

 regions of some mountain district breaks its way through 

 barriers which have long restrained it, and rushes through 

 such channels as it can find or make for itself into valleys 

 and plains at lower levels. Such catastrophes are fortunately 

 not often witnessed in this country, nor when seen do they 

 attain the same magnitude as in more mountainous countries. 

 It would seem, indeed, as though they could attain very 

 great proportions only in regions where a large extent of 

 mountain surface lies above the snow-line. The reason why 

 in such regions floods are much more destructive than else- 

 where will readily be perceived if we consider the phenomena 

 of one of these terrible catastrophes. 



Take, for instance, the floods which inundated the plains 

 of Martigny in 1818. Early in that year it was found that 

 the entire valley of the Bagnes, one of the largest side-valleys 

 of the great valley of the Rhone, above Geneva, had been 

 converted into a lake through the damming up of a narrow 

 outlet by avalanches of snow and ice from a loftier glacier 

 overhanging the bed of the river Dranse. The temporary 

 lake thus formed was no less than half a league in length, 

 and more than 200 yards wide, its greatest depth exceeding 

 200 feet The inhabitants perceived the terrible effects which 

 must follow when the barrier burst, which it could not fail to 

 do in the spring. They, therefore, cut a gallery 700 feet long 

 through the ice, while as yet the water was at a moderate 

 height. When the waters began to flow through this channel, 

 their action widened and deepened it considerably. At length 

 nearly half the contents of the lake were poured off. Un- 

 fortunately, as the heat of the weather increased, the middle 

 of the barrier slowly melted away, until it became too weak to 

 withstand the pressure of the vast mass of water. Suddenly 

 it gave way ; and so completely that all the water in the lake 

 rushed out in half an hour. The effects of this tremendous 



