Boox VIL.] INFLUENCE OF DENUDATION. - 927 
valleys have been hollowed out. A gorge or defile is usually due to 
the action of a waterfall, which, beginning with some abrupt declivity 
or precipice in the course of the river when it first commenced to 
flow, or caused by some hard rock crossing the channel, has eaten its 
way backward, as already explained (p. 375). 
A pass is a portion of a watershed which has been cut down by 
the erosion of two valleys, the heads of which adjoin on opposite 
sides of a ridge. Tach valley is cut backward until the intervening 
ridge is demolished. Most passes no doubt lie in original but subse- 
quently deepened depressions between adjoining mountains. The 
continued degradation of a crest may obviously give rise to a pass. 
- Lakes may have been formed in several ways. 1. By subter- 
ranean movements, as, for example, in mountain-making and in 
voleanic explosions. ‘The subsidence of the central part of a mountain 
system might conceivably depress the heads of the valleys below the 
level of portions further from the sources of the streams. Or the 
elevation of the lower parts of the valleys might cause an accumula- 
tion of water in their upper parts. Or each lake-basin might be 
supposed to be due to a special subsidence. But these hollows, 
unless continually deepened by subsequent movements of a similar 
nature, would be filled up by the sediment continually washed into 
them from the adjoining slopes. The numerous lakes in such a 
mountain system as the Alps cannot be due merely to subterranean 
movements, unless we suppose the upheaval of the mountains to have 
been quite recent, or that subsidence must take place continuously 
or periodically below each independent basin. But there is evidence 
that the Alpine uplift is not of such recent date, while the idea of 
perpetuating lakes by continued local subsidence would demand, not 
in the Alps merely, but all over the northern hemisphere, where 
lakes are so abundant, an amount of subterranean movement of 
which, if it really existed, there would assuredly be plenty of other 
evidence. 2. By irregularities in the deposition of superficial 
accumulations prior to the elevation of the land, or, in the northern 
parts of Hurope and America, during the disappearance of the ice- 
sheet. The numerous tarns and lakes enclosed within mounds and 
ridges of drift-clay and gravel are examples. 3. By the accumula- 
tion of a barrier across the channel of a stream and the consequent 
ponding back of the water. ‘This may be done, for instance, by a 
Jandslip, by a lava stream, by the advance of a glacier across a 
valley, or by the throwing up of a bank by the sea across the mouth 
of a river. 4. By erosion. The only agent capable of excavating 
hollows out of the solid rock such as might form lake-basins is 
eglacier-ice (p. 416). It is a remarkable fact, of which the signiti- 
cance may now be seen, that the innumerable lake basins of the 
northern hemisphere lie on surfaces of intensely ice-worn rock. The 
strie can be seen on the smoother rock-surfaces slipping into the 
water on all sides. ‘hese striae were produced by ice moving over 
the rock. If the ice could, as the strie prove, descend into the 
