[Footnote 24: A short list of the more important works relating to
the Eocene rocks and fossils will be given after all the Tertiary
deposits have been treated of.]

CHAPTER XIX.

THE MIOCENE PERIOD.

The Miocene rocks comprise those Tertiary deposits which contain
less than about 35 per cent of existing species of shells
(_Mollusca_), and more than 5 per cent--or those deposits in
which the proportion of living shells is less than of extinct
species. They are divisible into a _Lower Miocene_ (_Oligocene_)
and an _Upper Miocene_ series.

In _Britain_, the Miocene rocks are very poorly developed, one
of their leading developments being at Bovey Tracy in Devonshire,
where there occur sands, clays, and beds of lignite or imperfect
coal. These strata contain numerous plants, amongst which are
Vines, Figs, the Cinnamon-tree, Palms, and many Conifers, especially
those belonging to the genus Sequoia (the "Red-Foods"). These
Bovey Tracy lignites are of Lower Miocene age, and they are
lacustrine in origin. Also of Lower Miocene age are the so-called
"Hempstead Beds" of Yarmouth in the Isle of Wight. These attain a
thickness of less than 200 feet, and are shown by their numerous
fossils to be principally a true marine formation. Lastly, the
Duke of Argyll, in 1851, showed that there existed at Ardtun, in
the island of Mull, certain Tertiary strata containing numerous
remains of plants; and these also are now regarded as belonging
to the Lower Miocene.

In _France_, the Lower Miocene is represented in Auvergne, Cantal,
and Velay, by a great thickness of nearly horizontal strata of
sands, sandstone, clays, marls, and limestones, the whole of
fresh-water origin. The principal fossils of these lacustrine
deposits are _Mammalia_, of which the remains occur in great
abundance. In the valley of the Loire occur the typical European
deposits of Upper Miocene age. These are known as the "Faluns,"
from a provincial term applied to shelly sands, employed to spread
upon soils which are deficient in lime; and the Upper Miocene
is hence sometimes spoken of as the "Falunian" formation. The
Faluns occur in scattered patches, which are rarely more than 50
feet in thickness, and consist of sands and marls. The fossils
are chiefly marine; but there occur also land and fresh-water
shells, together with the remains of numerous Mammals. About 25
per cent of the shells of the Faluns are identical with existing
species. The sands, limes