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 behinl 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 Ls 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 debacle 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 

 llth 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 



