“Ny 
926 PHYSIOGRAPHICAL GEOLOGY: [Book VII. 
_ watercourses that diverge from them. Where a watershed lies 
symmetrically along the centre of a country or continent with on 
either side an equal declivity and rainfall, and an identity of geo- 
logical structure, it will be permanent, because the erosion on each 
slope proceeds at the same rate. But such a combination of cireum- 
stances can happen rarely, save on a small and local scale. As a 
rule watersheds lie on one side of the centre of a country or con- 
tinent, and the declivity is steeper on the side nearest the sea. Hence, 
apart from any influence from difference of geological structure, the © 
tendency of erosion, by wearing the steep slope more than the gentle 
one, is to carry the watershed backward nearer to the true centre of 
the region, especially at the heads of valleys. Of course this is an 
extremely slow process; but it must be admitted to be one of real 
efficacy in the vast periods during which denudation has continued. 
Excellent illustration of its progress, as well as of many other features 
of land-sculpture, may often be instructively studied on clay bank 
exposed to the influence of rain. 7 
~The crests of mountains are watersheds of the sharpest type 
where erosion has worked backward upon a steep slope on either 
side. Their forms are mainly dependent upon structure, and espe- 
cially upon systems of joints. It will often be observed that the © 
general trend of a crest coincides with that of one set of joints, and 
that the bastions, recesses, and peaks have been determined by the 
intersection of another set. If the rock is uniform in structure and 
the declivity equal in angle on either side, a crest may retain its 
position, but as one side is usually considerably steeper than the 
other, the crest advances at the expense of the top of the gentler 
declivity. But under any circumstances it is continually lowered in 
level, for it may be regarded as the part of a mountain where the 
rate of subaerial denudation reaches a maximum. An ordinary cliff 
is attacked only in front, but a crest has two fronts and is further 
splintered along its summit. Nowhere can the guiding influence of 
geological structure be more conspicuously seen than in the array of 
spires, buttresses, gullies, and other striking outlines which a 
mountain crest assumes. 
Valleys are mainly due to erosion, guided either by original de- 
pressions of the ground, or by geological structure, or by both. Their 
contours depend partly on the structure and composition of the rocks, 
and partly on the relative potency of the different denuding agents. 
Where the influence of air, rain, frost, and general subaerial weather- 
ing has been slight, and the streams, supplied from distant sources, 
have had sufficient declivity, deep, narrow, precipitous ravines or 
gorges have been excavated. The canons of the Colorado are a 
magnificent example of this result (Fig. 485). Where, on the other 
hand, ordinary atmospheric action has been more rapid, the sides of 
the river channels have been attacked, and open sloping glens and 
1 See on this subject Mr. Gilbert's suggestive remarks in the essay on “ Land- 
Sculpture”’ already cited (p. 920), 
