A Manual of Elementary Geology Part 46

Magnesian limestone, Humbleton Hill, near Sunderland.[303-A]]

Sh.e.l.ls 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.)

[Ill.u.s.tration: Fig. 337. _Productus calvus_, Sow. Min. Con. Syn. _Productus horridus_, Bronn's Index, &c., King's Monogr., &c.; _Leptaena_, Dalman.

Magnesian Limestone.]

[Ill.u.s.tration: 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 sh.o.r.e. But the Permian species are peculiar, and, for the most part, identical with those found in the marl-slate or copper-slate of Thuringia.

[Ill.u.s.tration: Fig. 339. Restored outline of a fish of the genus _Palaeoniscus_, Aga.s.s. _Palaeothrissum_, Blainville.]

The _Palaeoniscus_ above mentioned belongs to that division of fishes which M. Aga.s.siz 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 "h.o.m.ocercal" 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.)

[Ill.u.s.tration: Fig. 340. Shark.

_Heterocercal._]

[Ill.u.s.tration: Fig. 341. Shad. (_Clupea_, Herring tribe.)

_h.o.m.ocercal._]

Now it is a singular fact, first pointed out by Aga.s.siz, 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 h.o.m.ocercal 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.

[Ill.u.s.tration: Fig. 342. _Palaeoniscus comtus_, Aga.s.siz. Scale magnified. Marl-slate.]

[Ill.u.s.tration: Fig. 343. _Palaeoniscus elegans_, Sedg. Under surface of scale magnified. Marl-slate.]

[Ill.u.s.tration: Fig. 344. _Palaeoniscus glaphyrus_, Ag. Under surface of scale magnified. Marl-slate.]

[Ill.u.s.tration: Fig. 345. _Coelacanthus caudalis_, Egerton. Scale showing granulated surface magnified. Marl-slate.]

[2 Ill.u.s.trations: 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 Yorks.h.i.+re and Durham. In some instances, red marl and gypsum have been found a.s.sociated with these beds. They have been cla.s.sed 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 Somersets.h.i.+re, 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 princ.i.p.ally 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 sh.e.l.ls 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.).

[Ill.u.s.tration: Fig. 348. Tooth of _Palaeosaurus_ platyodon, nat. size.]

[Ill.u.s.tration: 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 cla.s.sic ground of the Lower New Red, or Magnesian Limestone, or Permian formation, on the Continent. It consists there princ.i.p.ally 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 a.s.sociated 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 Los.h.i.+_, _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.

[Ill.u.s.tration: 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 ma.s.s of air-roots, which often const.i.tuted 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 tria.s.sic; 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 a.s.soc., Eleventh Meeting, 1841, p. 197.

[307-A] Murchison's Russia, vol. ii. pl. A. fig. 3.

CHAPTER XXIV.

THE COAL, OR CARBONIFEROUS GROUP.

Carboniferous strata in the south-west of England--Superposition of Coal-measures to Mountain limestone--Departure from this type in North of England and Scotland--Section in South Wales--Underclays with Stigmaria--Carboniferous Flora--Ferns, Lepidodendra, Calamites, Asterophyllites, Sigillariae, Stigmariae--Coniferae--Endogens--Absence of Exogens--Coal, how formed--Erect fossil trees--Parkfield Colliery--St.

Etienne, Coal-field--Oblique trees or snags--Fossil forests in Nova Scotia--Brackish water and marine strata--Origin of Clay-iron-stone.