ed at this time. This would have
effectually prevented an overflow of the fauna from Siberia. Only in
deposits later than the lower continental boulder-clay do we find traces
of a Siberian migration. The time of maximum glaciation had then passed
away; the great glacier which was believed to have invaded the lowlands
of Northern Europe had again retreated, before the Siberian mammals made
their appearance in Germany.

It has been stated above (p. 226) that while the Russian boulder-clay
was being laid down, the Aralo-Caspian probably had some communication
with the White Sea.

But how can this view be reconciled with the existence of a huge _mer de
glace_ in the northern plains of Russia? The existence of the ice-sheet
has been conjured up in order to explain the presence of the
boulder-clay. But not long ago a very different interpretation of the
origin of this clay was given; and one, I may say, which explains the
history of the Siberian and the European fauna in a more satisfactory
manner than is done by the ice-sheet hypothesis. It is that the
boulder-clay is not the product of land-ice, but has been deposited by a
sea with floating icebergs. Thus the latter hypothesis does not deny the
existence of glaciers, but allows the mud to be deposited on the floor
of a turbid sea, instead of beneath an immense _mer de glace_. I need
hardly mention that this view, which was formerly universally accepted
by geologists, is now scouted by almost every authority, both British
and Continental. I should scarcely venture the attempt to revive old
memories and stir up again long forgotten controversies, were it not for
the fact that many new points have arisen in the course of the above
inquiries, which appear to me so very difficult to explain by the
land-ice hypothesis, while they are comparatively easy to understand
when we adopt the old theory of the marine origin of the boulder-clay.
But a few geologists even at the present day, while believing in the
land-ice theory, recognise that the marine hypothesis should have some
consideration shown to it. I need only remind glacialists of the work
recently published by Professor Bonney. "The singular mixture," he
remarks (p. 280), "and apparent crossing of the paths of boulders, as
already stated, are less difficult to explain on the hypothesis of
distribution by floating ice than on that of transport by land-ice,
because, in the former case, though the drift of winds and currents
wo