The term "marl" is very generally employed to designate the clays
of the Lower and Upper Trias; but the term is inappropriate, as
they may contain no lime, and are therefore not always genuine
marls. In Britain the Bunter Sandstein consists of red and mottled
sandstones, with unconsolidated conglomerates, or "pebble-beds,"
the whole having a thickness of 1000 to 2000 feet. The Bunter
Sandstein, as a rule, is very barren of fossils.
II. The Middle Trias is not developed in Britain, but it is largely
developed in Germany, where it constitutes what is known as the
_Muschelkalk_ (Germ. _Muschel_, mussel; _kalk_, limestone), from
the abundance of fossil shells which it contains. The Muschelkalk
(the _Calcaire coquillier_ of the French) consists of compact
grey or yellowish limestones, sometimes dolomitic, and including
occasional beds of gypsum and rock-salt.
III. The Upper Trias, or _Keuper_ (the _Marnes irisees_ of the
French), as it is generally called, occurs in England; but is
not so well developed as it is in Germany. In Britain, the Keuper
is 1000 feet or more in thickness, and consists of white and
brown sandstones, with red marls, the whole topped by red clays
with rock-salt and gypsum.
The Keuper in Britain is extremely unfossiliferous; but it passes
upwards with perfect conformity into a very remarkable group of
beds, at one time classed with the Lias, and now known under the
names of the Penarth beds (from Penarth, in Glamorganshire), the
Rhaetic beds (from the Rhaetic Alps), or the _Avicula contorta_ beds
(from the occurrence in them of great numbers of this peculiar
Bivalve).
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