we shall learn presently, the early stages of the Glacial period were
accompanied by a marine transgression over Northern Russia and
Germany--an overflow, as it were, of the waters of the Arctic Ocean
covering a great part of Northern Europe, with the exception of Norway.
One continuous ocean ultimately extended from the east coast of England
across Holland, Northern Germany, and Russia to the White Sea (Fig. 12,
p. 156). The south of England being at that time joined to France, and
Scotland to Scandinavia, there was no direct communication between this
large North European Sea and the Atlantic. The glaciers which took their
origin in the Scandinavian Mountains discharged icebergs into this sea,
and many of them no doubt were stranded on the east coast of England.
The boulders of Scandinavian origin which have been discovered in recent
geological deposits on that coast have generally been traced to the
action of land-ice, but the supposition that they have been carried by
icebergs--the older theory--appears to me the more probable one. Such
boulders begin to make their first appearance in the Red Crag, a deposit
which is now looked upon as belonging to the newer pliocene series. But
whether we call it pliocene or pleistocene really matters little. The
important fact is, that glacial phenomena, consisting of the appearance
of boulders foreign to the country together with an invasion of Arctic
shells, are now ushered in upon a coast which shortly before teemed with
the southern life of a Mediterranean character. Among the new arrivals
in these English crags there are no less than eighteen species of North
American marine mollusca. Since the German Ocean had then no direct
communication with the Atlantic, these mollusca could only have come
from the White Sea, and Forbes's _Arctic current_ would offer an
explanation of the manner in which they were enabled to migrate there
from their original home.

It might be urged that we have no grounds for the supposition that the
German Ocean was practically a closed basin; and that these American
species probably inhabited at that time the whole of the North Atlantic
Ocean. But if such had been the case, we ought to have evidence of the
occurrence of some of these species in the newer Tertiary deposits along
the west coasts of the British Islands. Such beds exist; there is,
however, not a trace in any of them of any American mollusca. In
examining the marine deposits of St. Erth, on