r
a large extent of European territory. In Switzerland it was long ago
pointed out that after the ancient glaciers had for a long time occupied
the low grounds of that country they, for some cause, retreated to
the mountain valleys, and allowed streams and rivers to work over the
_debris_ left behind them. At Wetzikon most interesting conclusions have
been drawn. We there learn that, after the retreat of the glaciers,
a lake occupied the place, which in course of time became filled with
peat, and that subsequently the peat was transformed into lignite. To
judge from the remains of animals and plants, the climate must have been
at least as warm as that at present; and this condition of things must
have prevailed over a period of some thousands of years to explain the
thick deposits of peat, from which originated the lignites.<27>

But we also know that this period came to an end, and that once more
the ice descended. This is shown by the fact that directly overlying the
lignite beds are alternating layers of sand and gravel, and, resting
on these, glacier-born bowlders. The same conclusion follows from the
discoveries made at many other places.

In Scotland it is well known that the bowlder clay contains every now
and then scattered patches of peat and beds of soil either deposited
in lakes or rivers. The only explanation that can be given for their
presence is that they represent old land surfaces; that is, when the
land was freed from ice, and vegetation had again clothed it in a mantle
of green. In this cut is shown one of these beds. Both above and below
are the beds of bowlder clay. The peat in the centre varies from an inch
to a foot and a half in thickness, and contains many fragments of wood,
sticks, roots, etc.; and of animals, numerous beetles were found, one
kind of which frequents only places where deer and ruminant animals
abound.

Diagram of Interglacial Bed------------

From a large number of such discoveries it is conclusively shown that,
after all, Scotland was smothered under one enormous glacier, a change
of climate occurred, and the ice melted away. Then Scotland enjoyed a
climate capable of nourishing sufficient vegetation to induce mammoths,
Irish deer, horses, and great oxen to occupy the land. But the upper
bowlder clay no less conclusively shows that once more the climate
became cold, and ice overflowed all the lowlands and buried under a new
accumulation of bowlder clay such parts of the o