The attention of geologists has been directed within the last few years,
more especially, to a series of deposits which intervene between the New
Red Marl of the Trias, and the blue argillaceous limestones and shales
of the Lower Lias. The first-mentioned beds, although they attain no
great thickness in this country, nevertheless form a well-defined and
persistent zone of strata between the unfossiliferous Triassic marls and
the lower Liassic limestone with _Ostrea Liassica_ and _Ammonites
planorbis_, _A. angulatus_ and _A. Bucklandi_; being everywhere
characterised by the presence of the same groups of organic remains, and
the same general lithological character of the beds. These last may be
described as consisting of three sub-divisions, the lowermost composed
of alternations of marls, clays, and marly limestones in the lower part,
forming a gradual passage downwards into the New Red Marls upon which
they repose. 2. A middle group of black, thinly laminated or paper-like
shales, with thin layers of indurated limestone, and crowded in places
with _Pecten Valoniensis_, _Cardium Rhæticum_, _Avicula contorta_, and
other characteristic shells, as well as by the presence, nearly always,
of a remarkable bed, which is commonly known as the “Bone-bed.” This
thin band of stone, which is so well known at Aust, Axmouth,
Westbury-on-Severn, and elsewhere, is a brecciated or conglomerated band
of variable thickness which, sometimes a sandstone and sometimes a
limestone, is always more or less composed of the teeth, scales, and
bones of numerous genera of Fishes and Saurians, together with their
fossilised excrement, which will be more fully and subsequently
described under the name of Coprolites, under the Liassic period.
The molar tooth of a small predaceous fossil mammal of the Microlestes
family (μικρος, _little_; ληστης, _beast_), whose nearest living
representative appears to be some of the Hypsiprymnidæ or Kangaroo Rats,
has been found by Mr. Dawkins in some grey marls underlying the bone-bed
on the sea-shore at Watchett, in Somersetshire; affording the earliest
known trace of a fossil mammal in the Secondary rocks. Several small
teeth belonging to the genus Microlestes have also been discovered by
Mr. Charles Moore in a breccia of Rhætic age, filling a fissure
traversing Carboniferous Limestone near Frome; and in addition to the
discovery of the remains of Microlestes, those of a mammal more closely
allied to the Marsupials than any other order, have been met with at
Diegerloch, south-east of Stuttgart, in a remarkable bone-breccia, which
also yielded coprolites and numerous traces of fishes and reptiles.
The uppermost sub-division includes certain beds of white and
cream-coloured limestone, resembling in appearance the smooth fracture
and closeness of texture of the lithographic limestone of Solenhofen,
and which, known to geologists and quarrymen under the name “white
lias,” given to it by Dr. William Smith, was formerly always considered
to belong to, and was included in, the Lias proper. The most remarkable
bed in this zone is one of only a few inches in thickness, but it has
long been known to collectors, and sought after under the name of Cotham
Marble or Landscape Stone, the latter name having reference to the
curious dendritic markings which make their appearance on breaking the
stone at right angles to its bedding, bearing a singular resemblance to
a landscape with trees, water, &c.; while the first name is that derived
from its occurrence abundantly at Cotham, in the suburbs of Bristol,
where the stone was originally found and noticed.
This band of stone is interesting in another respect, because it
sometimes shows by its uneven, eroded, and water-worn upper surface,
that an interval took place soon after it had been deposited, when the
newly-formed stone became partially dissolved, eroded, or worn away by
water, before the stratum next in succession was deposited upon it. The
same phenomenon is displayed, in a more marked degree, in the uppermost
limestone or “white lias” bed of the series, which not only shows an
eroded surface, but the holes made by boring Molluscs, exactly as is
produced at the present day by the same class of animals, which excavate
holes in the rocks between high and low-water marks, to serve for their
dwelling-places, and as a protection from the waves to their somewhat
delicate shells.
The “White Lias” of Smith is the equivalent of the Koessen beds which
immediately underlie the Lower Lias of the Swabian Jura, and have been
traced for a hundred miles, from Geneva to the environs of Vienna; and,
also, of the Upper St. Cassian beds, which are so called from their
occurrence at St. Cassian in the Austrian Alps.
The general character of the series of strata just described, is that
of a deposit formed in tolerably shallow water. In the Alps of Lombardy
and the Tyrol, in Luxembourg, in France, and, in fact, throughout nearly
the whole of Europe, they form a sort of fringe in the margin of the
Triassic sea; and, although of comparatively inconsiderable thickness in
England, they become highly developed in Lombardy, &c., to an enormous
thickness, and constitute the great mass of the Rhætian Alps and a
considerable part of the well-known beds of St. Cassian, and Hallstadt
in the Austrian Alps. (See page 205.)
The Rhætic beds of Europe were, as a whole, formed under very different
conditions in different areas. The thickness of the strata and the large
and well-developed fauna (chiefly Mollusca) indicate that the Rhætic
strata of Lombardy, and other parts of the south and east of Europe,
were deposited in a broad open ocean. On the other hand, the
comparatively thin beds of this age in England and north-western Europe,
the fauna of which, besides being poor in genera and species, consists
of small and dwarfed forms, point to the conclusion that they were in
great part deposited in shallow seas and in estuaries, or in lagoons, or
in occasional salt lakes, under conditions which lasted for a long
period.[56]
[56] See A. C. Ramsay, “On the Physical Relations of the New Red Marl,
Rhætic Beds, and Lower Lias,” _Quart. Jour. Geol. Soc._, vol. 27,
p. 189.
In consequence of the importance they assume in Lombardy (the ancient
Rhætia), the name “Rhætic Beds” has been given to these strata by Mr.
Charles Moore; Dr. Thomas Wright has proposed the designation “Avicula
Contorta Zone,” from the plentiful occurrence of that shell in the black
shales forming the well-marked middle zone, and which is everywhere
present where this group of beds is found; Jules Martin and others have
proposed the term “Infra-lias,” or “Infra-liassic strata;” while the
name “Penarth Beds” has been assigned to these deposits in this country
by Mr. H. W. Bristow, at the suggestion of Sir Roderick Murchison, in
consequence of their conspicuous appearance and well-exposed sections in
the bold headlands and cliffs of that locality, in the British Channel,
west of Cardiff.
A fuller description of these beds will be found in the Reports of the
Bath Meeting of the British Association (1864), by Mr. Bristow; also in
communications to the _Geological Magazine_, for 1864, by MM. Bristow
and Dawkins;[57] in papers read before the Geological Society by Dr.
Thomas Wright,[58] Mr. Charles Moore,[59] and Mr. Ralph Tate,[60] as
printed in their _Quarterly Journal_; and by Mr. Etheridge, in the
Transactions of the Cotteswold Natural History Club for 1865-66. The
limits of the Penarth Beds have also been lately accurately laid down by
Mr. Bristow in the map of the Geological Survey over the district
comprised between Bath, Bristol, and the Severn; and elaborately
detailed typical sections of most of the localities in England, where
these beds occur, have been constructed by MM. Bristow, Etheridge, and
Woodward, of the Geological Survey of Great Britain, which, when
published, will greatly add to our knowledge of this remarkable and
interesting series of deposits.
[57] _Quart. Jour. Geol. Soc._, vol. xx., p. 396.
[58] Ibid, vol. xvii., p. 483.
[59] Ibid, vol. xvi., p. 374.
[60] Ibid, vol. xx., p. 103.