shells which lie embedded in the subsoils
beneath the old coast-line are exactly those which still live in our
seas.

[Illustration: Fig. 87.

ASTARTE ARCTICA.]

[Illustration: Fig. 88.

TELLINA PROXIMA.]

Above this ancient line of coast we find, at various heights, beds of
shells of vastly older date than those of the low-lying terrace, and
many of which are no longer to be found living around our shores. I
spent some time last autumn in exploring one of these beds, once a sea
bottom, but now raised two hundred and thirty feet over the sea, in
which there occurred great numbers of shells now not British, though
found in many parts of Britain at heights varying from two hundred to
nearly fourteen hundred feet over the existing sea level. But though no
longer British shells, they are shells that still continue to live in
high northern latitudes, as on the shores of Iceland and Spitzbergen;
and the abundance in which they were developed on the submerged plains
and hill-sides of what are now England and Scotland, during what is
termed the Pleistocene period, shows of itself what a very protracted
period that was. The prevailing tellina of the bed which I last
explored,--a bed which occurs in some places six miles inland, in others
elevated on the top of dizzy crags,--is a sub-arctic shell (_Tellina
proxima_), of which only dead valves are now to be detected on our
coasts, but which may be found living at the North Cape and in
Greenland. The prevailing astarte, its contemporary, was _Astarte
arctica_, now so rare as a British species, that many of our most
sedulous collectors have never seen a native specimen, but which is
comparatively common on the northern shores of Iceland, and on the
eastern coasts of Norway, within the arctic circle. In this elevated
Scottish bed of the Pleistocene period I laid these boreal shells open
to the light by hundreds, on the spot evidently where the individuals
had lived and died. Under the severe climatal conditions to which
(probably from some change in the direction of the gulf stream) what is
now Northern Europe had been brought, this tellina and astarte had
increased and multiplied until they became prevailing shells of the
British area; and this increase must have been the slow work of ages,
during which the plains, and not a few of the table lands, of the
country, were submerged in a sub-arctic sea, and Great Britain existed
as but a scattered archipelago of wintry islands.