erhaps a
rockfall, deflected the Rhine, just as we see in Fig. 30 that the Rhone
was pushed on one side by the Borgne. The Rhone, however, had no choice,
it was obliged to force, and has forced its way over the cone deposited
by the Borgne. The Rhine, on the contrary, had the option of running
down by Vaduz to Rheinach, and has adopted this course. The watershed
between it and the Weisstannen is, however, only about 20 feet in
height, and the people of Zurich watch it carefully, lest any slight
change should enable the river to return to its old bed. The result of
all these changes is that the rivers have changed their courses from
those shown in Fig. 43 to their present beds as shown in Fig. 44.

Another interesting case is that of the Upper Engadine (Fig. 45), to
which attention has been called by Bonney and Heim. The fall of the Val
Bregaglia is much steeper than that of the Inn, and the Maira has
carried off the head-waters of that river away into Italy. The Col was
formerly perhaps as far south as Stampa: the Albegna, the Upper Maira,
and the stream from the Forgno Glacier, originally belonged to the Inn,
but have been captured by the Lower Maira. Their direction still
indicates this; they seem as if they regretted the unwelcome change, and
yearned to rejoin their old companions.

[Illustration: Fig. 45.--River system of the Maloya.]

Moreover, as rivers are continually cutting back their valleys they must
of course sometimes meet. In these cases when the valleys are at
different levels the lower rivers have drained the upper ones, and left
dry, deserted valleys. In other cases, especially in flatter districts,
we have bifurcations, as, for instance, at Sargans, and several of the
Italian lakes. Every one must have been struck by the peculiar
bifurcation of the Lakes of Como and Lugano, while a very slight
depression would connect the Lake Varese with the Maggiore, and give it
also a double southern end.

ON LAKES

The problem of the origin of Lakes is by no means identical with that of
Valleys. The latter are due, primarily as a rule to geological causes,
but so far as their present condition is concerned, mainly to the action
of rain and rivers. Flowing water, however, cannot give rise to lakes.

It is of course possible to have valleys without lakes, and in fact the
latter are, now at least, exceptional. There can be no lakes if the
slope of the valley is uniform. To what then are lakes due?

Professor