dency is to cut through any projections so that finally their course
assumes some such curve as that below, from the source (_a_) to its
entrance into the sea (_b_).

[Illustration: Fig. 46.--Final Slope of a River.]

Glaciers, however, have in addition a scooping power, so that if
similarly _a d b_ in Fig. 47 represent the course of a glacier, starting
at _a_ and gradually thinning out to _e_, it may scoop out the rock to
a certain extent at _d_; in that case if it subsequently retires say to
_c_, there would be a lake lying in the basin thus formed between _c_
and _e_.

[Illustration: Fig. 47.]

On the other hand I am not disposed to attribute the Swiss lakes
altogether to the action of glaciers. In the first place it does not
seem clear that they occupy true rock basins. On this point more
evidence is required. That some lakes are due to unequal changes of
level will hardly be denied. No one, for instance, as Bonney justly
observes,[55] would attribute the Dead Sea to glacial erosion.

The Alps, as we have seen, are a succession of great folds, and there is
reason to regard the central one as the oldest. If then the same process
continued, and the outer fold was still further raised, or a new one
formed, more quickly than the rivers could cut it back, they would be
dammed up, and lakes would result.

Moreover, if the formation of a mountain region be due to subsidence,
and consequent crumpling, as indicated on p. 217, so that the strata
which originally occupied the area A B C D are compressed into A' B' C'
D', it is evident, as already mentioned, that while the line of least
resistance, and, consequently, the principal folds might be in the
direction A' B', there must also be a tendency to the formation of
similar folds at right angles, or in the direction A' C'. Thus, in the
case of Switzerland, while the main folds run south-west by north-east
there would also be others at right angles, though the amount of folding
might be much greater in the one direction than in the other. To this
cause the bosses, for instance--at Martigny, the Furca, and the Ober
Alp,--which intersect the great longitudinal valley of Switzerland, are
perhaps due.

The great American lakes also are probably due to differences of
elevation. Round Lake Ontario, for instance, there is a raised beach
which at the western end of the lake is 363 feet above the sea level,
but rises towards the East and North until near Fine it reaches an
ele