ough the Alleghanies. The case of the Dranse will be
alluded to further on (p. 292). In these cases the rivers preceded the
mountains. Indeed as soon as the land rose above the waters, rivers
would begin their work, and having done so, unless the rate of elevation
of the mountain exceeded the power of erosion of the river, the two
would proceed simultaneously, so that the river would not alter its
course, but would cut deeper and deeper as the mountain range gradually
rose.

Rivers then are in many cases older than mountains. Moreover, the
mountains are passive, the rivers active. Since it seems to be well
established that in Switzerland a mass, more than equal to what remains,
has been removed; and that many of the present mountains are not sites
which were originally raised highest, but those which have suffered
least, it follows that if in some cases the course of the river is due
to the direction of the mountain ridges, on the other hand the direction
of some of the present ridges is due to that of the rivers. At any rate
it is certain that of the original surface not a trace or a fragment
remains _in situ_. Many of our own English mountains were once valleys,
and many of our present valleys occupy the sites of former mountain
ridges.

Heim and Ruetimeyer point out that of the two factors which have produced
the relief of mountain regions, the one, elevation, is temporary and
transitory; the other, denudation, is constant, and gains therefore
finally the upper hand.

We must not, however, expect too great regularity. The degree of
hardness, the texture, and the composition of the rocks cause great
differences.

On the other hand, if the alteration of level was too rapid, the result
might be greatly to alter the river courses. Mr. Darwin mentions such a
case, which, moreover, is perhaps the more interesting as being
evidently very recent.

"Mr. Gill," he says, "mentioned to me a most interesting, and as far as
I am aware, quite unparalleled case, of a subterranean disturbance
having changed the drainage of a country. Travelling from Casma to
Huaraz (not very far distant from Lima) he found a plain covered with
ruins and marks of ancient cultivation, but now quite barren. Near it
was the dry course of a considerable river, whence the water for
irrigation had formerly been conducted. There was nothing in the
appearance of the water-course to indicate that the river had not flowed
there a few years previously; in