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 rendered more clearly intelligible.

Every river produces at the present day beds of fine mud and loam, and accumulations of gravel, which it deposits at various parts of its course—­the gravel generally occupying the lowest position, and the finer sands and mud coming above.  Numerous deposits of a similar nature are found in most countries in various localities, and at various heights above the present channels of our rivers.  Many of these fluviatile (Lat. fluvius, a river) deposits consist of fine loam, worked for brick-making, and known as “Brick-earths;” and they have yielded the remains of numerous extinct Mammals, of which the Mammoth (Elephas primigenius) is the most abundant.  In the valley of the Rhine