more than
half the width of those of Dover, but are very shallow, the greatest
depth not exceeding two or three fathoms. The new bay is of a somewhat
circular form, and between _thirty_ and _forty_ miles in diameter. How
much of this space may formerly have been occupied by Lake Flevo is
unknown. (See map, fig. 38.)

_Destruction of islands._--A series of islands stretching from the Texel
to the mouths of the Weser and Elbe are probably the last relics of a
tract once continuous. They have greatly diminished in size, and have
lost about a third of their number, since the time of Pliny; for that
naturalist counted twenty-three islands between the Texel and Eider,
whereas there are now only sixteen, including Heligoland and
Neuwerk.[445] The island of Heligoland, at the mouth of the Elbe,
consists of a rock of red marl of the Keuper formation (of the Germans),
and is bounded by perpendicular red cliffs, above 200 feet high.
Although, according to some accounts, it has been greatly reduced in
size since the year 800, M. Wiebel assures us, that the ancient map by
Meyer cannot be depended upon, and that the island, according to the
description still extant by Adam of Bremen, was not much larger than
now, in the time of Charlemagne. On comparing the map made in the year
1793 by the Danish engineer Wessel, the average encroachment of the sea
on the cliffs, between that period and the year 1848 (or about half a
century), did not amount to more than three feet.[446] On the other
hand, some few islands have extended their bounds in one direction, or
become connected with others, by the sanding-up of channels; but even
these, like Juist, have generally given way as much on the north towards
the sea as they have gained on the south, or land side.

_The Dollart formed._--While the delta of the Rhine has suffered so
materially from the movements of the ocean, it can hardly be supposed
that minor rivers on the same coast should have been permitted to extend
their deltas. It appears that in the time of the Romans there was an
alluvial plain of great fertility, where the Ems entered the sea by
three arms. This low country stretched between Groningen and Friesland,
and sent out a peninsula to the northeast towards Emden. A flood in 1277
first destroyed part of the peninsula. Other inundations followed at
different periods throughout the fifteenth century. In 1507, a part only
of Torum, a considerable town, remained standing; and in spit