THE LAKE OF GENEVA. 
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layers. The boulders with which the river is laden, 
by degrees broke through the harder rock in various 
places, and then began to act more effectively on 
the softer stratum below, which they gradually ate 
into more and more, finally meeting, and thus form- 
ing a sort of tunnel sufficient to carry all the water 
of the river when it is low. The lower Calcareous 
layer has in some places been itself worn through, 
so that the same process is beginning a second time. 
Neither the Rhone nor the Arve would by itself 
have been able to effect this. It requires gravel and 
boulders, animated by a sufficient force of water. 
The Rhone is large enough, but does not carry down 
enough stones. The Arve has plenty of gravel and 
stones, but not enough flow of water. Thus the 
Rhone supplies the force and the Arve the tools. 
From the Fort de I’Ecluse to below Malpertuis 
the Rhone is not a river, but a torrent. It has not 
had time to approach its “regimen.” 
Below Bellegarde the slope of the river is the 
reverse of that of the valley. The river slopes from 
Bellegarde to Malpertuis, the valley from Malpertuis 
to Bellegarde. The river falls 20 to 25 metres, the 
ground rises over 200. At Bellegarde the gorge has 
a depth of 200 metres, at Malpertuis of 450. 
Thus while the river falls the valley rises. If 
