ACTION OF RIVERS. 
205 
steeper and steeper. In some cases, and especially 
in calcareous districts, the valleys end in a precipitous, 
more or less semicircular “Cirque.” Springs rising 
at the foot of such escarpments are known as Vau- 
clusian, from the celebrated and typical instance at 
Vaucluse. 
Another interesting point brought out by the 
study of Swiss rivers, is that just as in Geology, 
though there have no doubt been tremendous cata- 
clysms, still the main changes have been due to the 
continuous action of existing causes; so also in the 
case of rivers, however important the effects produced 
due to floods, still the configuration of river valleys 
is greatly due to the steady and regular flow of the 
water. 
Floods may be divided into two classes, (1) those 
due to the bursting of some upper reservoir, such, 
for instance, as the great flood of the Dranse de 
Bagnes in 1818, due to the outburst of the lake, 
which had been dammed back by the glacier of 
Gietroz, or the more recent flood of St. Gervais 
owing to the bursting of a subglacial reservoir in the 
little Glacier de T6te Rousse which rushed down the 
valley in the dead of the night, in a few minutes 
swept away the Baths, and drowned most of the 
visitors; and (2) those due to heavy rains. No one 
