The Student's Elements of Geology Part 47

(FIGURE 412. Mytilus septifer, King. Syn. Modiola ac.u.minata, Sowerby. Permian crystalline limestone.)

This formation is seen upon the coast of Durham and Yorkshire, between the Wear and the Tees. Among its characteristic fossils are Schizodus Schlotheimi (Figure 410) and Mytilus septifer (Figure 412). These sh.e.l.ls occur at Hartlepool and Sunderland, where the rock a.s.sumes an oolitic and botryoidal character. Some of the beds in this division are ripple-marked. In some parts of the coast of Durham, where the rock is not crystalline, it contains as much as 44 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 ma.s.ses, varying from the size of a marble to that of a cannon-ball, and radiating from the centre. Occasionally earthy and pulverulent beds pa.s.s into compact limestone or hard granular dolomite. Sometimes the limestone appears in a brecciated form, the fragments which are united together not consisting of foreign rocks but seemingly composed of the breaking-up of the Permian limestone itself, about the time of its consolidation. Some of the angular ma.s.ses in Tynemouth cliff are two feet in diameter.

(FIGURE 413. Magnesian Limestone, Humbleton Hill, near Sunderland. (King's Monograph Plate 2.) a. Fenestella retiformis, Schlot, sp. Syn. Gorgonia infundibuliformis, Goldf.; Retepora fl.u.s.tracea, Phillips.

b. Part of the same highly magnified.)

The magnesian limestone sometimes becomes very fossiliferous and includes in it delicate bryozoa, one of which, Fenestella retiformis (Figure 413), is a very variable species, and has received many different names. It sometimes attains a large size, single specimens measuring eight inches in width. The same bryozoan, with several other British species, is also found abundantly in the Permian of Germany.

(FIGURE 414. Productus horridus, Sowerby. (P. calvus, Sowerby) Sunderland and Durham, in Magnesian Limestone; Zechstein and Kupferschiefer, Germany.)

(FIGURE 415. Lingula Crednerii. (Geinitz.) Magnesian Limestone, and Carboniferous Marl-slate, Durham; Zechstein, Thuringia.)

The total known fauna of the Permian series of Great Britain at present numbers 147 species, of which 77, or more than half, are mollusca. Not one of these is common to rocks newer than the Palaeozoic, and the brachiopods are the only group which have furnished species common to the more ancient or Carboniferous rocks. Of these Lingula Crednerii (Figure 415) is an example. There are 25 Gasteropods and only one cephalopod, Nautilus Freieslebeni, which is also found in the German Zechstein.

(FIGURE 416. Spirifera alata, Schloth. Syn. Trigonotreta undulata, Sowerby., King's Monograph. Magnesian Limestone.)

Sh.e.l.ls of the genera Productus (Figure 414) and Strophalosia (the latter of allied form with hinge teeth), which do not occur in strata newer than the Permian, are abundant in the ordinary yellow magnesian limestone, as will be seen in the valuable memoirs of Messrs. King and Howse. They are accompanied by certain species of Spirifera (Figure 416), Lingula Crednerii (Figure 415), and other brachiopoda of the true primary or palaeozoic type. Some of this same tribe of sh.e.l.ls, such as Camarophoria, allied to Rhynchonella, Spiriferina, and two species of Lingula, are specifically the same as fossils of the carboniferous rocks. Avicula, Arca, and Schizodus (Figure 410), and other lamellibranchiate bivalves, are abundant, but spiral univalves are very rare.

(FIGURE 417. Restored outline of a fish of the genus Palaeoniscus, Aga.s.siz.

Palaeothrissum, Blainville.)

Beneath the limestone lies a formation termed the marl-stone, 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 ten species, Pygopterus two species, Coelacanthus two species, and Platysomus two species, which as genera are common to the older Carboniferous formation, but the Permian species are peculiar, and, for the most part, identical with those found in the marl-slate or copper-slate of Thuringia.

(FIGURE 418. Shark. Heterocercal.)

(FIGURE 419. Shad. (Clupea. Herring tribe. h.o.m.ocercal.)

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 Figure 418.) The "h.o.m.ocercal" fish, which comprise almost all the 9000 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 Figure 419.) 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 characterises the earlier periods of the earth's history, whereas in the secondary strata, or those newer than the Permian, 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 Professor 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 may be useful to a geologist in determining the age of the rock.

(FIGURES 420-425. SCALES OF FISH. MAGNESIAN LIMESTONE.

(FIGURE 420. Palaeoniscus comptus, Aga.s.siz. Scale, magnified. Marl-slate.)

(FIGURE 421. Palaeoniscus elegans, Sedgwick. Under surface of scale, magnified.

Marl-slate.)

(FIGURE 422. Palaeoniscus glaphyrus, Aga.s.siz. Under surface of scale, magnified.

Marl-slate.)

(FIGURE 423. Coelacanthus granulatus, Aga.s.siz. Granulated surface of scale, magnified. Marl-slate.)

(FIGURE 424. Pygopterus mandibularis. Aga.s.siz. Marl-slate.

a. Outside of scale, magnified.

b. Under surface of same.)

(FIGURE 425. Acrolepis Sedgwickii, Aga.s.siz. Outside of scale, magnified. Marl- slate.))

We are indebted to Messrs. Hanc.o.c.k and Howse for the discovery in this marl- slate at Midderidge, Durham, of two species of Protosaurus, a genus of reptiles, one representative of which, P. Speneri, has been celebrated ever since the year 1810 as characteristic of the Kupfer-schiefer or Permian of Thuringia. Professor Huxley informs us that the agreement of the Durham fossil with Hermann von Meyer's figure of the German specimen is most striking. Although the head is wanting in all the examples yet found, they clearly belong to the Lacertian order, and are therefore of a higher grade than any other vertebrate animal hitherto found fossil in a Palaeozoic rock. Remains of Labyrinthodont reptiles have also been met with in the same slate near Durham.

LOWER PERMIAN.

The inferior sandstones 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 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. But the princ.i.p.al development of Lower Permian is, as we have seen by Mr. Hull's Table 22.1, in the northwest, where the Penrith sandstone, as it has been called, and the a.s.sociated breccias and purple shales are estimated by Professor Harkness to attain a thickness of 3000 feet.

Organic remains are generally wanting, but the leaves and wood of coniferous plants, and in one case a cone, have been found. Also in the purple marls of Cornc.o.c.kle Muir near Dumfries, very distinct footprints of reptiles occur, originally referred to the Trias, but shown by Mr. Binney in 1856 to be Permian.

No bones of the animals which they represent have yet been discovered.

ANGULAR BRECCIAS IN LOWER PERMIAN.

A striking feature in these beds is the occasional occurrence, especially at the base of the formation, of angular and sometimes rounded fragments of Carboniferous and older rocks of the adjoining districts being included in a paste of red marl. Some of the angular ma.s.ses are of huge size.

In the central and southern counties, where the Middle Permian or Magnesian Limestone is wanting, it is difficult to separate the upper and lower sandstones, and Mr. Hull is of opinion that the patches of this formation found here and there in Worcestershire, Shropshire, and other counties may have been deposited in a sea separated from the northern basin by a barrier of Carboniferous rocks running east and west, and now concealed under the Tria.s.sic strata of Cheshire. Similar breccias to those before described are found in the more southern counties last mentioned, where their appearance is rendered more striking by the marked contrast they present to the beds of well-rolled and rounded pebbles of the Trias occupying a large area in the same region.

Professor Ramsay refers the angular form and large size of the fragments composing these breccias to the action of floating ice in the sea. These ma.s.ses of angular rock, some of them weighing more than half a ton, and lying confusedly in a red, unstratified marl, like stones in boulder-drift, are in some cases polished, striated, and furrowed like erratic blocks in the moraine of a glacier. They can be shown in some cases to have travelled from the parent rocks, thirty or more miles distant, and yet not to have lost their angular shape. (Ramsay Quarterly Geological Journal 1855; and Lyell Principles of Geology volume 1 page 223 10th edition.)

PERMIAN ROCKS OF THE CONTINENT.

Germany is the cla.s.sic ground of the Magnesian Limestone now called Permian. The formation was well studied by the miners of that country a century ago as containing a thin band of dark-coloured cupriferous shale, characterised at Mansfield in Thuringia by numerous fossil fish. Beneath some variegated sandstones (not belonging to the Trias, though often confounded with it) they came down first upon a dolomitic limestone corresponding to the upper part of our Middle Permian, and then upon a marl-slate richly impregnated with copper pyrites, and containing fish and reptiles (Protosaurus) identical in species with those of the corresponding marl-slate of Durham. To the limestone they gave the name of Zechstein, and to the marl-slate that of Mergel-schiefer or Kupfer- schiefer. Beneath the fossiliferous group lies the Rothliegendes or Rothtodt- liegendes, meaning the red-lyer or red-dead-lyer, so-called by the German miners from its colour, and because the copper had DIED OUT when they reached this underlying non-metalliferous member of the series. This red under-lyer is, in fact, a great deposit of red sandstone, breccia, and conglomerate with a.s.sociated porphyry, basalt, and amygdaloid.

According to Sir R. Murchison, the Permian rocks are composed, in Russia, of white limestone, with gypsum and white salt; and of red and green grits, occasionally with copper ore; also magnesian limestones, marl-stones, and conglomerates.

PERMIAN FLORA.

(FIGURE 426. Walchia piniformis, Schloth. Permian, Saxony. (Gutbier, Die Versteinerungen des Permischen Systemes in Sachsen volume 2 plate 10.) a. Branch.

b. Twig of the same.

c. Leaf magnified.)

About 18 or 20 species of plants are known in the Permian rocks of England. None of them pa.s.s down into the Carboniferous series, but several genera, such as Alethopteris, Neuropteris, Walchia, and Ullmania, are common to the two groups.

The Permian flora on the Continent appears, from the researches of MM. Murchison and de Verneuil in Russia, and of MM. Geinitz and von Gutbier in Saxony, to be, with a few exceptions, distinct from that of the coal.

In the Permian rocks of Saxony no less than 60 species of fossil plants have been met with. 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, and several species of Walchia (see Figure 426), a genus of Conifers, called Lycopodites by some authors, are said by Geinitz to be common to the coal-measures.

(FIGURE 427. Cardiocarpon Ottonis. Gutbier, Permian, Saxony. 1/2 diameter.)

(FIGURE 428. Neoggerathia cuneifolia. Brongniart. (Murchison's Russia volume 2 Plate A figure 3.)

Among the genera also enumerated by Colonel Gutbier are the fruit called Cardiocarpon (see Figure 427), 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. Neoggerathia (see Figure 428), the leaves of which have parallel veins without a midrib, and to which various generic synonyms, such as Cordaites, Flabellaria, and Poacites, have been given, 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 in the true Permian.

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.

Upon the whole, it is evident that the Permian plants approach much nearer to the Carboniferous flora than to the Tria.s.sic; and the same may be said of the Permian fauna.

CHAPTER XXIII.

THE COAL OR CARBONIFEROUS GROUP.