e Sound of Mull and the Minch, climbing up the sides of
opposing rocks and islands until even the Outer Hebrides and the
North-east of Ireland were covered by one vast mantle of ice and snow.
The movement of such a body of ice over the land must have been attended
with a large amount of abrasion of the rocky floor; nor have the
evidences of that abrasion entirely disappeared even at the present day.
We still detect the grooves and scorings on the rock-surfaces where they
have been protected by a coating of boulder clay; and we still find the
surface strewn with the blocks and _debris_ of that mighty ice-flood.

But whatever may have been the amount of erosion caused by the great
ice-sheet, it was chiefly confined to the more or less horizontal
surface-planes. Erosion of another kind was to succeed, and to produce
more lasting effects on the configuration of the surface. On the
disappearance of the ice-sheet, an epoch characterised by milder
conditions of climate set in. This was accompanied by subsidence and
submersion of large tracts of the land during the Interglacial stage; so
that the sea rose to heights of several hundred feet above the present
level, and has left behind stratified gravels with shells at these
elevations in protected places. During this period of depression and of
subsequent re-emergence the wave-action of the Atlantic waters must have
told severely on the coast and islands, wearing them into cliffs and
escarpments, furrowing out channels and levelling obstructions. Such
action has gone on down to the present day. The North-west of Scotland
and of Ireland has been subjected throughout a very lengthened period to
the wear and tear of the Atlantic billows. In the case of the former,
the remarkable breakwater which nature has thrown athwart the North-west
Highlands in the direction of the waves, forming the chain of islands
constituting the Outer Hebrides, and composed of very tough Archaean
gneiss and schist, has done much to retard the inroads which the waves
might otherwise have made on the Isle of Skye; while Coll and Tiree,
composed of similar materials, have acted with similar beneficent effect
for Mull and the adjoining coasts. But such is the tremendous power of
the Atlantic billows when impelled by westerly winds, that to their
agency must be mainly attributed the small size of the volcanic
land-surfaces as compared with their original extent, and the formation
of those grand headlands which a