n comparatively arid mountains with scanty
snow-fields, or in a low forest-clad plateau. The Po, on the other hand, is
wholly in a district of abundant rainfall, while its sources are spread
over a great amphitheatre of snowy Alps nearly 400 miles in extent, where
the denuding forces are at a maximum. As Scotland is a mountain region of
rather abundant rainfall, the denuding power of its rains and rivers is
probably rather above than under the average, but to avoid any possible
exaggeration we will take it at a foot in 4,000 years.

Now if the end of the glacial epoch be taken to coincide with the
termination of the last period of high excentricity, which occurred about
80,000 years ago (and no geologist will consider this too long for the
changes which have since taken place), it follows that the entire surface
of Scotland must have been since lowered an average amount of twenty feet.
But over large areas of alluvial plains, and wherever the rivers have
spread during floods, the ground will have been raised instead of lowered;
and on all nearly level ground and gentle slopes there will have been
comparatively little denudation; so that proportionally much more must have
been taken away from mountain sides and from the bottoms of valleys having
a considerable downward slope. One of the very highest authorities on the
subject of denudation, Mr. Archibald Geikie, estimates the area of these
more rapidly denuded portions as only one-tenth of the comparatively level
grounds, and he further estimates that the former will be denuded about ten
times as fast as the latter. It follows that the valleys will be deepened
and widened on the average about five feet in the 4,000 years instead of
one foot; and thus many valleys must have been deepened and widened 100
feet, and some even more, since the glacial epoch, while the more level
portions of the country will have been lowered on the average only about
two feet. {63}

Now Dr. Croll gives us the following account of the present aspect of the
surface of a large part of the country:--

"Go where one will in the lowlands of Scotland and he shall hardly find a
single acre whose upper surface bears the marks of being formed by the
denuding agents now in operation. He will observe everywhere mounds and
hollows which cannot be accounted for by the present agencies at work....
In regard to the general surface of the country the present agencies may be
said to be just beginning to carve