s 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.

[Illustration: Fig. 254.--Left valve of _Pecten Islandicus_, Glacial
and Recent.]

III. POST-GLACIAL DEPOSITS.--As the intense cold of the Glacial
period became gradually mitigated, and temperate conditions of
climate were once more re-established, various deposits were
formed in the northern hemisphere, which are found to contain
the remains of extinct Mammals, and which, therefore, are clearly
of Post-Pliocene age. To these deposits the general name of
_Post-Glacial_ formations is given; but it is obvious that, from
the nature of the case, and with our present limited knowledge,
we cannot draw a rigid line of demarcation between the deposits
formed towards the close of the Glacial period, or during warm
"interglacial" periods, and those laid down after the ice had
fairly disappeared. Indeed it is extremely improbable that any
such rigid line of demarcation should ever have existed; and it
is far more likely that the Glacial and Post-Glacial periods,
and their corresponding deposits, shade into one another by an
imperceptible gradation. Accepting this reservation, we may group
together, under the general head of "Post-Glacial Deposits,"
most of the so-called "Valley-gravels," "Brick-earths," and
"Cave-deposits," together with some "raised beaches" and various
deposits of peat. Though not strictly within the compass of this
work, a few words may be said here as to the origin and mode of
formation of the Brick-earths, Valley-gravels, and Cave-deposits,
as the subject will thus be r