A Manual of Elementary Geology or, The Ancient Changes of the Earth and its Inhabitants as Illustrated by Geological Monuments

CHAPTER XXIII.

Chapter 642,271 wordsPublic domain

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.

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.).

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.

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.)

_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.

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.)

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.

[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.).

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.

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.