It is also the opinion of high authorities that a
considerable number of the so-called "cave-deposits," with the
bones of extinct Mammals, truly belong to the Glacial period,
being formed during warm intervals when the severity of the Arctic
cold had become relaxed. It is further believed that some, at
any rate, of the so-called "high-level" river-gravels and
"brick-earths" have likewise been deposited during mild or warm
intervals in the great age of ice; and in two or three instances
this has apparently been demonstrated--deposits of this nature,
with the bones of extinct animals and the implements of man,
having been shown to be overlaid by true Boulder-clay.
The fossils of the undoubted Glacial deposits are principally
shells, which are found in great numbers in certain localities,
sometimes with _Foraminifera_, the bivalved cases of Ostracode
Crustaceans, &c. Whilst some of the shells of the "Drift" are such
as now live in the seas of temperate regions, others, as previously
remarked, are such as are now only known to live in the seas of
high latitudes; and these therefore afford unquestionable evidence
of cold conditions. Amongst these Arctic forms of shells which
characterise the Glacial beds may be mentioned _Pecten Islandicus_
(fig. 254), _Pecten Groenlandicus, Scalaria Groenlandica, Leda
truncata, Astarte borealis, Tellina proxima, Nattra clausa_,
&c.
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