a new line of features out of the old
glacially-formed surface. But so little progress has yet been made, that
the kames, gravel-mounds, knolls of boulder clay, &c., still retain in most
cases their original form."[11]

The facts here seem a little inconsistent, and we must suppose that Dr.
Croll has somewhat exaggerated the universality and complete preservation
of the glaciated surface. The amount of average denudation, however, is not
a matter of opinion but of measurement; and its consequences can in no way
be evaded. They are, moreover, strictly proportionate to the time elapsed;
and if so much of the old surface of the country has certainly been
remodelled or carried into the sea since the last glacial epoch, it becomes
evident that any surface-phenomena produced by still earlier glacial epochs
_must_ have long since entirely disappeared.

_Rise of the Sea-level Connected with Glacial Epochs, a Cause of Further
Denudation._--There is also another powerful agent that must have assisted
in the destruction of any such surface deposits or markings. During the
last glacial epoch itself there were several minor oscillations of the
land, without counting the great submergence of over 1,300 feet, supposed
to be indicated by patches of shelly clays and gravels in Wales and
Ireland, and also in a few localities in England and Scotland, since these
are otherwise explained by many geologists. Other subsidences have no doubt
occurred in the same areas during the Tertiary epoch, and some writers
connect these subsidences with the glacial {64} period itself, the unequal
amount of ice at the two poles causing the centre of gravity of the earth
to be displaced when, of course, the surface of the ocean will conform to
it and appear to rise in the one hemisphere and sink in the other. If this
is the case, subsidences of the land are natural concomitants of a glacial
period, and will powerfully aid in removing all evidence of its occurrence.
We have seen reason to believe, however, that during the height of the
glacial epoch the extreme cold persisted through the successive phases of
precession, and if so, both polar areas would probably be glaciated at
once. This would cause the abstraction of a large quantity of water from
the ocean, and a proportionate elevation of the land, which would react on
the accumulation of snow and ice, and thus add another to that wonderful
series of physical agents which act and react on each other so as