GEOLOGY. 261 
and as the arch of this bridge, dug in summer by the 
torrent, closes up in winter, it happened in 1597, and in 
our times, in 1818, that the early months of spring 
sufficed not for the Drause to open for itself a passage, 
and a lake was formed behind the ice. 
‘When this became known (wrote M. Simond, some 
months after the event), alarm spread at once, not only 
throughout the whole valley but in Le Valais, and on so 
far as “Italy. Travellers feared to take the route of the 
Simplon ; it was felt that when this dyke should come to 
break up there would be there a sudden débacle which 
would sweep over the country to a great distance. The 
‘preceding winter had been severe; the ice had even 
then cast a dam across the valley, but without stopping 
the water, which had eaten out a passage for itself; but a 
second severe winter had produced such a fall of ice that 
the obstacle had become insurmountable and impervious. 
‘The Government sent an engineer (M. Venetz) ; he 
found that the dyke was 110 toises (nearly 700 feet) in 
length from the one mountain to the other, 66 toises (or 
about 400 feet) in height, and 500 toises (or 3000 feet) in 
thickness at its base. The lake was 1200 toises (or up- 
wards of 7000 feet in length, and had already risen to half 
the height of the dyke, that is to say, was from 30 to 40 
toises (from 180 to 240 feet) in depth. The engineer 
determined to cut a gallery or tunnel through the thick- 
ness of the ice, beginning 54 feet above the actual level of 
the lake, to give time to complete the work before that 
height should be reached by the accumulating waters, 
which were rising at the rate of from 1 to 5 feet per day, 
according to the temperature ; and he began the work on the 
11th of May at both ends of the tunnel. Fifty men in 
relays, relieving one another alternately, wrought there 
night and day at the peril of their lives,—one and another 
of the avalanches, which were falling every moment, 
threatening to bury them alive in the tunnel; many were 
wounded by lumps of ice, or had their feet frozen, and the 
ice was so hard that it frequently broke the pick-axes 
