to
intensify glacial epochs.

But whether or not these causes would produce any important fluctuations of
the sea-level is of comparatively little importance to our present inquiry,
because the wide extent of marine Tertiary deposits in the northern
hemisphere and their occurrence at considerable elevations above the
present sea-level, afford the most conclusive proofs that great changes of
sea and land have occurred throughout the entire Tertiary period; and these
repeated submergences and emergences of the land combined with sub-aerial
and marine denudation, would undoubtedly destroy all those superficial
evidences of ice-action on which we mainly depend for proofs of the
occurrence of the last glacial epoch.

_What Evidence of Early Glacial Epochs may be Expected._--Although we may
admit the force of the preceding argument as to the extreme improbability
of our finding any clear evidence of the superficial action of ice during
remote glacial epochs, there is nevertheless one kind of evidence that we
ought to find, because it is both wide-spread and practically
indestructible.

One of the most constant of all the phenomena of a glaciated country is the
abundance of icebergs produced by the breaking off of the ends of glaciers
which terminate {65} in arms of the sea, or of the terminal face of the
ice-sheet which passes beyond the land into the ocean. In both these cases
abundance of rocks and _debris_, such as form the terminal moraines of
glaciers on land, are carried out to sea and deposited over the sea-bottom
of the area occupied by icebergs. In the case of an ice-sheet it is almost
certain that much of the ground-moraine, consisting of mud and imbedded
stones, similar to that which forms the "till" when deposited on land, will
be carried out to sea with the ice and form a deposit of marine "till" near
the shore.

It has indeed been objected that when an ice-sheet covered an entire
country there would be no moraines, and that rocks or _debris_ are very
rarely seen on icebergs. But during every glacial epoch there will be a
southern limit to the glaciated area, and everywhere near this limit the
mountain-tops will rise far above the ice and deposit on it great masses of
_debris_; and as the ice-sheet spreads, and again as it passes away, this
moraine-forming area will successively occupy the whole country. But even
such an ice-clad country as Greenland is now known to have protruding peaks
and rocky masses whi