of the Baltic, including
the Skagerrak and Kattegat, is 166,397 sq. m., and its volume 6907 cub. m.,
giving a mean depth of 36 fathoms, which is markedly less than that of any
other arm of the sea of similar area.

In the deeper hollows in the south part of the Baltic the bottom consists
almost invariably of either soft brown or grey mud or hard clay, while on
the shallow banks and near the low coasts fine sand, of white, yellow or
brown colour with small pebbles, is usually found.

[v.03 p.0286]

[Sidenote: Coasts--changes and character.]

At the time of the last great subsidence, in glacial times, an arm of the
sea extended across Sweden, submerging a great part of the littoral up to
the Gulf of Bothnia, and including the present lakes Vener, Hjelmar and
Malar. During this period the waters of the northern Baltic were
sufficiently salt for oysters to flourish. The subsequent upheaval
restricted direct communication with the open sea to the Danish channels,
and the Baltic waters became fresher: the oyster disappeared, but a number
of cold salt-water fishes and crustaceans, and even seals, became
acclimatized. It has been suggested that the presence of the remains of
these animals indicates a communication to the north with the Arctic Ocean;
but in view of the severe climatic conditions still prevailing at the time,
this seems an unnecessary assumption. In the next stage of its history the
Baltic is transformed by further elevation into a vast freshwater lake, the
_Ancylus_ lake of G. de Geer (named from the remains of the mollusc
_Ancylus fluviatilis_), which is supposed to have covered an area of about
220,000 sq. m., including the whole of the present Baltic area and a large
part of Finland, with Lake Ladoga. Then followed a subsidence, which not
only re-established communication through the Danish channels, but allowed
the Baltic to become sufficiently salt for such forms as _Cardium edule_
and _Littorina littorea_. At this time the Gulf of Bothnia must have
suffered greater depression than the Baltic proper, for the deposits of
that epoch show a thickness of 100 metres (328 ft.) near Hernosand, but of
only 25 metres (82 ft.) in the neighbourhood of Gotland. After this period
of subsidence the process of elevation set in which gave the Baltic its
present form and physical condition, and appears to be still in progress.
Dr R. Sieger has traced a series of isobasic lines, or lines of equal rate
of elevation, for po