PERMIAN OR MAGNESIAN LIMESTONE GROUP.
Fossils of Magnesian Limestone and Lower New Red distinct from the
Triassic--Term Permian--English and German equivalents--Marine shells
and corals of English Magnesian limestone--Palaeoniscus and other fish
of the marl slate--Thecodont Saurians of dolomitic conglomerate of
Bristol--Zechstein and Rothliegendes of Thuringia--Permian Flora--Its
generic affinity to the carboniferous--Psaronites or tree-ferns.
When the use of the term "Poikilitic" was explained in the last chapter, I
stated, that in some parts of England it is scarcely possible to separate
the red marls and sandstones so called (originally named "the New Red"),
into two distinct geological systems. Nevertheless, the progress of
investigation, and a careful comparison of English rocks between the lias
and the coal with those occupying a similar geological position in Germany
and Russia, has enabled geologists to divide the Poikilitic formation; and
has even shown that the lowermost of the two divisions is more closely
connected, by its fossil remains, with the carboniferous group than with
the trias. If, therefore, we are to draw a line between the secondary and
primary fossiliferous strata, as between the tertiary and secondary, it
must run through the middle of what was once called the "New Red," or
Poikilitic group. The inferior half of this group will rank as Primary or
Paleozoic, while its upper member will form the base of the Secondary
series. For the lower, or Magnesian Limestone division of English
geologists, Sir R. Murchison has proposed the name of Permian, from Perm, a
Russian government where these strata are more extensively developed than
elsewhere, occupying an area twice the size of France, and containing an
abundant and varied suite of fossils.
Mr. King, in his valuable monograph, recently published, of the Permian
fossils of England, has given a table of the following six members of the
Permian system of the north of England, with what he conceives to be the
corresponding formations in Thuringia.[301-A]
North of England. Thuringia.
1. Crystalline or concretionary, |1. Stinkstein.
and non-crystalline limestone. |
2. Brecciated and pseudo-brecciated |2. Rauchwacke.
limestone. |
3. Fossiliferous limestone. |3. Dolomit, or Upper Zechstein.
4. Compact limestone. |4. Zechstein, or Lower Zechstein.
5. Marl-slate. |5. Mergel-schiefer, or Kupferschiefer.
6. Inferior sandstones of various |6. Rothliegendes.
colours. |
I shall proceed, therefore, to treat briefly of these subdivisions,
beginning with the highest, and referring the reader, for a fuller
description of the lithological character of the whole group, as it occurs
in the north of England, to a valuable memoir by Professor Sedgwick,
published in 1835.[302-A]
_Crystalline or concretionary limestone_ (No. 1.).--This formation is seen
upon the coast of Durham and Yorkshire, between the Wear and the Tees.
Among its characteristic fossils are _Schizodus Schlotheimi_ (fig. 333.)
and _Mytilus septifer_ (fig. 335.).
[Illustration: Fig. 333. _Schizodus Schlotheimi_, Geinitz. Syn. _Axinus
obscurus_, Sow. Crystalline limestone, Permian.]
[Illustration: Fig. 334. _Schizodus truncatus_, King; to show
hinge. Permian.]
[Illustration: Fig. 335. _Mytilus septifer_, King. Syn. _Modiola
acuminata_, James Sow. Permian crystalline limestone.]
These shells occur at Hartlepool and Sunderland, where the rock assumes an
oolitic and botryoidal character. Some of the beds in this division are
ripple-marked; and Mr. King imagines that the absence of corals and the
character of the shells indicate shallow water. In some parts of the coast
of Durham, where the rock is not crystalline, it contains as much as
forty-four per cent. of carbonate of magnesia, mixed with carbonate of
lime. In other places,--for it is extremely variable in structure,--it
consists chiefly of carbonate of lime, and has concreted into globular and
hemispherical masses, varying from the size of a marble to that of a
cannon-ball, and radiating from the centre. Occasionally earthy and
pulverulent beds pass into compact limestone or hard granular dolomite. The
stratification is very irregular, in some places well-defined, in others
obliterated by the concretionary action which has re-arranged the materials
of the rocks subsequently to their original deposition. Examples of this
are seen at Pontefract and Ripon in Yorkshire.
_The brecciated limestone_ (No. 2.) contains no fragments of foreign
rocks, but seems composed of the breaking-up of the Permian limestone
itself, about the time of its consolidation. Some of the angular masses
in Tynemouth Cliff are 2 feet in diameter. This breccia is considered by
Professor Sedgwick as one of the forms of the preceding limestone, No.
1., rather than as regularly underlying it. The fragments are angular
and never water-worn, and appear to have been re-cemented on the spot
where they were formed. It is, therefore, suggested that they may have
been due to those internal movements of the mass which produced the
concretionary structure; but the subject is very obscure, and after
studying the phenomenon in the Marston Rocks, on the coast of Durham, I
found it impossible to form any positive opinion on the subject. The
well-known brecciated limestones of the Pyrenees appeared to me to
present the nearest analogy, but on a much smaller scale.
_The fossiliferous limestone_ (No. 3.) is regarded by Mr. King as a
deep-water formation, from the numerous delicate corals which it includes.
One of these, _Fenestella retiformis_ (fig. 336.), is a very variable
species, and has received many different names. It sometimes attains a
large size, measuring 8 inches in width. The same zoophyte is also found
abundantly in the Permian of Germany.
[Illustration: Fig. 336. Fenestella.
_a._ _Fenestella retiformis_, Schlot.
Syn. _Gorgonia infundibuliformis_, Goldf.; _Retepora flustracea_,
Phillips.
_b._ Part of the same highly magnified.
Magnesian limestone, Humbleton Hill, near Sunderland.[303-A]]
Shells of the genera _Spirifer_ and _Productus_, which do not occur in
strata newer than the Permian, are abundant in this division of the series
in the ordinary yellow magnesian limestone. (See figs. 337, 338.)
[Illustration: Fig. 337. _Productus calvus_, Sow. Min. Con. Syn. _Productus
horridus_, Bronn's Index, &c., King's Monogr., &c.; _Leptaena_, Dalman.
Magnesian Limestone.]
[Illustration: Fig. 338. _Spirifer undulatus_, Sow. Min. Con. Syn.
_Triogonotreta undulata_, King's Monogr.
Magnesian Limestone.]
_The compact limestone_ (No. 4.) also contains organic remains,
especially corallines, and is intimately connected with the preceding.
Beneath it lies the _marl-slate_ (No. 5.), which consists of hard,
calcareous shales, marl-slate, and thin-bedded limestones. At East
Thickley, in Durham, where it is thirty feet thick, this slate has
yielded many fine specimens of fossil fish of the genera _Palaeoniscus_,
_Pygopterus_, _Coelacanthus_, and _Platysomus_, genera which are all
found in the coal-measures of the carboniferous epoch, and which
therefore, says Mr. King, probably lived at no great distance from
the shore. But the Permian species are peculiar, and, for the
most part, identical with those found in the marl-slate or
copper-slate of Thuringia.
[Illustration: Fig. 339. Restored outline of a fish of the genus
_Palaeoniscus_, Agass. _Palaeothrissum_, Blainville.]
The _Palaeoniscus_ above mentioned belongs to that division of fishes
which M. Agassiz has called "Heterocercal," which have their tails
unequally bilobate, like the recent shark and sturgeon, and the
vertebral column running along the upper caudal lobe. (See fig. 340.)
The "Homocercal" fish, which comprise almost all the 8000 species at
present known in the living creation, have the tail-fin either single or
equally divided; and the vertebral column stops short, and is not
prolonged into either lobe. (See fig. 341.)
[Illustration: Fig. 340. Shark.
_Heterocercal._]
[Illustration: Fig. 341. Shad. (_Clupea_, Herring tribe.)
_Homocercal._]
Now it is a singular fact, first pointed out by Agassiz, that the
heterocercal form, which is confined to a small number of genera in the
existing creation, is universal in the Magnesian limestone, and all the
more ancient formations. It characterizes the earlier periods of the
earth's history, when the organization of fishes made a greater approach to
that of saurian reptiles than at later epochs. In all the strata above the
Magnesian limestone the homocercal tail predominates.
A full description has been given by Sir Philip Egerton of the species of
fish characteristic of the marl-slate in Mr. King's monograph before
referred to, where figures of the ichthyolites which are very entire and
well preserved, will be found. Even a single scale is usually so
characteristically marked as to indicate the genus, and sometimes even the
particular species. They are often scattered through the beds singly, and
maybe useful to a geologist in determining the age of the rock.
[Illustration: Fig. 342. _Palaeoniscus comtus_, Agassiz. Scale
magnified. Marl-slate.]
[Illustration: Fig. 343. _Palaeoniscus elegans_, Sedg. Under surface of
scale magnified. Marl-slate.]
[Illustration: Fig. 344. _Palaeoniscus glaphyrus_, Ag. Under surface of
scale magnified. Marl-slate.]
[Illustration: Fig. 345. _Coelacanthus caudalis_, Egerton. Scale showing
granulated surface magnified. Marl-slate.]
[2 Illustrations: Scales of fish. Magnesian limestone.
Fig. 346. _Pygopterus mandibularis_, Ag. Marl-slate.
_a._ Outside of scale magnified.
_b._ Under surface of same.
Fig. 347. _Acrolepis Sedgwickii_, Ag. Marl-slate.]
The _inferior sandstones_ (No. 6. Tab. p. 301.), which lie beneath the
marl-slate, consist of sandstone and sand, separating the magnesian
limestone from the coal, in Yorkshire and Durham. In some instances, red
marl and gypsum have been found associated with these beds. They have been
classed with the magnesian limestone by Professor Sedgwick, as being nearly
co-extensive with it in geographical range, though their relations are very
obscure. In some regions we find it stated that the imbedded plants are all
specifically identical with those of the carboniferous series; and, if so,
they probably belong to that epoch; for the true Permian flora appears,
from the researches of MM. Murchison and de Verneuil in Russia, and of
Colonel von Gutbier in Saxony, to be, with few exceptions, distinct from
that of the coal (see p. 307.).
_Dolomitic conglomerate of Bristol._--Near Bristol, in Somersetshire,
and in other counties bordering the Severn, the unconformable beds of
the Lower New Red, resting immediately upon the Coal, consist of a
conglomerate called "dolomitic," because the pebbles of older rocks are
cemented together by a red or yellow base of dolomite or magnesian
limestone. This conglomerate or breccia, for the imbedded fragments are
sometimes angular, occurs in patches over the whole of the downs near
Bristol, filling up the hollows and irregularities in the mountain
limestone, and being principally composed at every spot of the debris of
those rocks on which it immediately rests. At one point we find pieces
of coal shale, in another of mountain limestone, recognizable by its
peculiar shells and zoophytes. Fractured bones, also, and teeth of
saurians, are dispersed through some parts of the breccia.
These saurians (which until the discovery of the _Archegosaurus_ in the
coal were the most ancient examples of fossil reptiles) are all
distinguished by having the teeth implanted deeply in the jaw-bone, and
in distinct sockets, instead of being soldered, as in frogs, to a simple
alveolar parapet. In the dolomitic conglomerate near Bristol the remains
of species of two distinct genera have been found, called
_Thecodontosaurus_ and _Palaeosaurus_ by Dr. Riley and Mr.
Stutchbury[306-A]; the teeth of which are conical, compressed, and with
finely serrated edges (figs. 348 and 349.).
[Illustration: Fig. 348. Tooth of _Palaeosaurus_ platyodon, nat. size.]
[Illustration: Fig. 349. Tooth of _Thecodontosaurus_, 3 times magnified.]
In Russia, also, Thecodont saurians occur, in beds of the Permian age, of
several genera, while others named _Protorosaurus_ are met with in the
Zechstein of Thuringia. This family of reptiles is allied to the living
monitor, and its appearance in a primary or paleozoic formation, observes
Mr. Owen, is opposed to the doctrine of the progressive development of
reptiles from fish, or from simpler to more complex forms; for, if they
existed at the present day, these monitors would take rank at the head of
the Lacertian order.[306-B]
In Russia the Permian rocks are composed of white limestone, with gypsum
and white salt; and of red and green grits, with occasionally copper ore;
also magnesian limestones, marlstones, and conglomerates.
The country of Mansfeld, in Thuringia, may be called the classic ground of
the Lower New Red, or Magnesian Limestone, or Permian formation, on the
Continent. It consists there principally of, first, the Zechstein,
corresponding to the upper portion of our English series; and, secondly,
the marl-slate, with fish of species identical with those of the bed so
called in Durham. This slaty marlstone is richly impregnated with copper
pyrites, for which it is extensively worked. Magnesian limestone, gypsum,
and rock-salt, occur among the superior strata of this group. At its base
lies the Rothliegendes, supposed to correspond with the Inferior or Lower
New Red Sandstone above mentioned, which occupies a similar place in
England between the marl-slate and coal. Its local name of Rothliegendes,
_red-lyer_, or "Roth-todt-liegendes," _red-dead-lyer_, was given by the
workmen in the German mines from its red colour, and because the copper has
_died out_ when they reach this rock, which is not metalliferous. It is, in
fact, a great deposit of red sandstone and conglomerate, with associated
porphyry, basaltic trap, and amygdaloid.
_Permian Flora._--We learn from the recent investigation of Colonel von
Gutbier, that in the Permian rocks of Saxony no less than sixty species of
fossil plants have been met with, forty of which have not yet been found
elsewhere. Two or three of these, as _Calamites gigas_, _Sphenopteris
erosa_, and _S. lobata_, are also met with in the government of Perm in
Russia. Seven others, and among them _Neuropteris Loshii_, _Pecopteris
arborescens_, and _P. similis_, with several species of _Walchia_
(Lycopodites), are common to the coal-measures.
Among the genera also enumerated by Colonel Gutbier are _Asterophyllites_
and _Annularia_, so characteristic of the carboniferous period; also
_Lepidodendron_, which is common to the Permian of Saxony, Thuringia, and
Russia, although not abundant. _Noeggerathia_ (see fig. 350.), supposed by
A. Brongniart to be allied to _Cycas_, is another link between the Permian
and carboniferous vegetation. Coniferae, of the Araucarian division, also
occur; but these are likewise met with both in older and newer rocks. The
plants called _Sigillaria_ and _Stigmaria_, so marked a feature in the
carboniferous period, are as yet wanting.
[Illustration: Fig. 350. _Noeggerathia cuneifolia._ Ad. Brongniart.[307-A]]
Among the remarkable fossils of the rothliegendes, or lowest part of the
Permian in Saxony and Bohemia, are the silicified trunks of tree-ferns
called generically _Psaronius_. Their bark was surrounded by a dense
mass of air-roots, which often constituted a great addition to the
original stem, so as to double or quadruple its diameter. The same
remark holds good in regard to certain living extra-tropical arborescent
ferns, particularly those of New Zealand.
Psaronites are also found in the uppermost coal of Autun in France, and in
the upper coal-measures of the State of Ohio in the United States, but
specifically different from those of the rothliegendes. They serve to
connect the Permian flora with the more modern portion of the preceding or
carboniferous group. Upon the whole, it is evident that the Permian plants
approach nearer to the carboniferous ones than to the triassic; and the
same may be said of the Permian fauna.
FOOTNOTES:
[301-A] Palaeontographical Society, 1848, London.
[302-A] Trans. Geol. Soc. Lond., Second Series, vol. iii. p. 37.
[303-A] King's Monograph, pl. 2.
[306-A] See paper by Messrs. Riley and Stutchbury, Geol. Trans., Second
Series, vol. v. p. 349., plate 29., figures 2. and 5.
[306-B] Owen, Report on Reptiles, British Assoc., Eleventh Meeting,
1841, p. 197.
[307-A] Murchison's Russia, vol. ii. pl. A. fig. 3.