Part 20

At the base of the system lies a band of lower sandstone (No. 6) of various colours, separating the Magnesian Limestone from the coal in Yorkshire and Durham; sometimes associated with red marl and gypsum, but with the same obscure relations in all these beds which usually attend the close of one series and the commencement of another; the imbedded plants being, in some cases, stated to be identical with those of the Carboniferous series. In Thuringia the Rothliegende, or red-lyer, a great deposit of red sandstone and conglomerate, associated with porphyry, basaltic trap, and amygdaloid, lies at the base of the system. Among the fossils of this age are the silicified trunks of Tree-ferns (Psaronius), the bark of which is surrounded by dense masses of air-roots, which often double or quadruple the diameter of the original stem; in this respect bearing a strong resemblance to the living arborescent ferns of New Zealand.

The marl-slate (No. 5) consists of hard calcareous shales, marl-slates, and thin-bedded limestone, the whole nearly thirty feet thick in Durham, and yielding many fine specimens of Ganoid and Placoid fishes--Palæoniscus, Pygopterus, Cœlacanthus, and Platysomus--genera which all belong to the Carboniferous system, and which Professor King thinks probably lived at no great distance from the shore; but the Permian species of the marl-slate of England are identical with those of the copper-slate of Thuringia. Agassiz was the first to point out a remarkable peculiarity in the forms of the fishes which lived before and after this period. In most living fishes the trunk seems to terminate in the middle of the root of the tail, whose free margin is “homocercal” (even-tail), that is, either rounded, or, if forked, divided into two equal lobes. In Palæoniscus, and most Palæozoic fishes, the axis of the body is continued into the upper lobe of the tail, which is thus rendered unsymmetrical, as in the living sharks and sturgeons. The latter form, which Agassiz termed “heterocercal” (unequal-tail) is only in a very general way distinctive of Palæozoic fishes, since this asymmetry exists, though in a minor degree, in many living genera besides those just mentioned. The compact limestone (No. 4) is rich in Polyzoa. The fossiliferous limestone (No. 3), Mr. King considers, is a deep-water formation, from the numerous Polyzoa which it contains. One of these, Fenestella retiformis, found in the Permian rocks of England and Germany, sometimes measures eight inches in width.

Many species of Mollusca, and especially Brachiopoda, appear in the Permian seas of this age, Spirifera and Producta being the most characteristic.

Other shells now occur, which have not been observed in strata newer than the Permian. Strophalosia (Fig. 73) is abundantly represented in the Permian rocks of Germany, Russia, and England, and much more sparingly in the yellow magnesian limestone, accompanied by Spirifera undulata, &c. S. Schlotheimii is widely disseminated both in England, Germany, and Russia, with Lingula Credneri, and other Palæozoic Brachiopoda. Here also we note the first appearance of the Oyster, but still in small numbers. Fenestella represents the Polyzoa. Schizodus has been found by Mr. Binney in the Upper Red Permian Marls of Manchester; but no shells of any kind have hitherto been met with in the Rothliegende of Lancashire, or in the Vale of Eden.

The brecciated limestone (No. 2) and the concretionary masses (No. 1) overlying it (although Professor King has attempted to separate them) are considered by Professor Sedgwick as different forms of the same rock. They contain no foreign elements, but seem to be composed of fragments of the underlying limestone, No. 3. Some of the angular masses at Tynemouth cliff are two feet in diameter, and none of them are water-worn.

The crystalline or concretionary limestone (No. 1) formation is seen upon the coast of Durham and Yorkshire, between the Wear and the Tees; and Mr. King thinks that the character of the shells and the absence of corals indicate a deposit formed in shallow water.

The plants also found in some of the Permian strata indicate the neighbourhood of land. These are land species, and chiefly of genera common in the Coal-measures. Fragments of supposed coniferous wood (generally silicified) are occasionally met with in the Permian red beds of many parts of England.

Among the Ferns characteristic of the period may be mentioned Sphenopteris dichotoma and S. Artemisiæfolia; Pecopteris lonchitica and Neuropteris gigantea, figured on pp. 143, 144. “If we are,” says Lyell, “to draw a line between the Secondary and Primary fossiliferous strata, it must be run through the middle of what was once called the ‘New Red.’ The inferior half of this group will rank as Primary or Palæozoic, while its upper member will form the base of the Secondary or Mesozoic series.” Among the Equiseta of the Permian formation of Saxony, Colonel Von Gutbier found Calamites gigas and sixty species of fossil plants, most of them Ferns, forty of which have not been found elsewhere. Among these are several species of Walchia, a genus of Conifers, of which an example is given in Fig. 75.

“Elements of Geology,” p. 456.

In their stems, leaves, and cones, they bear some resemblance to the Araucarias, which have been introduced from North America into our pleasure-grounds during the last half-century.

Among the genera enumerated by Colonel Von Gutbier are some fruits called Cardiocarpon, and Asterophyllites and Annularia, so characteristic of the Carboniferous age. The Lepidodendron is also common to the Permian rocks of Saxony, Russia, and Thuringia; also the Nöggerathia, a family of large trees, intermediate between Cycads (Fig. 72) and the Conifers. The fruit of one of these is represented in Fig 76.

PERMIAN ROCKS.--We now give a sketch of the physiognomy of the earth in Permian times. Of what do the beds consist? What is the extent, and what is the mineralogical constitution of the rocks deposited in the seas of the period? The Permian formation consists of three members, which are in descending order--

1. Upper Permian sandstone, or Grès des Vosges; 2. Magnesian Limestone, or Zechstein; 3. Lower Red Sandstone, Marl-slate or Kupferschiefer, and Rothliegende.

The grès des Vosges, usually of a red colour, and from 300 to 450 feet thick, composes all the southern part of the Vosges Mountains, where it forms frequent level summits, which are evidences of an ancient plain that has been acted on by running water. It only contains a few vegetable remains.

The Magnesian Limestone, Pierre de mine, or Zechstein, so called in consequence of the numerous metalliferous deposits met with in its diverse beds, presents in France only a few insignificant fragments; but in Germany and England it attains the thickness of 450 feet. It is composed of a diversified mass of Magnesian Limestone, generally of a yellow colour, but sometimes red and brown, and bituminous clay, the last black and fetid. The subordinate rocks consist of marl, gypsum, and inflammable bituminous schists. The beds of marl slate are remarkable for the numbers of peculiar fossil fishes which they contain; and from the occurrence of small proportions of argentiferous grey copper-ore, met with in the bituminous shales which are worked in the district of Mansfeld, in Thuringia--the latter are called Kupferschiefer in Germany.

The Lower Red Sandstone, which attains a thickness of from 300 to 600 feet, is found over great part of Germany, in the Vosges, and in England. Its fossil remains are few and rare; they include silicified trunks of Conifers, some impressions of Ferns, and Calamites.

In England the Permian strata, to a great extent, consist of red sandstones and marls; and the Magnesian Limestone of the northern counties is also, though to a less degree, associated with red marls.

In Lancashire thin beds of Magnesian Limestone are interstratified with red marls in the upper Permian strata, beneath which there are soft Red Sandstones, estimated by Mr. Hull to be about 1,500 feet thick. These are supposed to represent the Rothliegende, and no shells of any kind have been found in them. The upper Permian beds, however, contain a few Magnesian Limestone species, such as Gervillia antiqua, Pleurophorus costatus, Schizodus obscurus, and some others, but all small and dwarfed.

The coal-fields of North and South Staffordshire, Tamworth, Coalbrook Dale, and of the Forest of Wyre, are partly bordered by Permian rocks, which lie unconformably on the Coal-measures; as is the case, also, in the immediate neighbourhood of Manchester, where they skirt the borders of the main coal-field, and consist of the Lower Red Sandstone, resting unconformably on different parts of the Coal-measures, and overlaid by the pebble-beds of the Trias.

At Stockport the Permian strata are stated by Mr. Hull to be more than 1,500 feet thick.

In Yorkshire, Nottinghamshire, and Derbyshire, the Permian strata are stated by Mr. Aveline to be divided into two chief groups: the Roth-liegende, of no great thickness, and the Magnesian Limestone series; the latter being the largest and most important member of the Permian series in the northern counties of England. The Magnesian Limestone consists there of two great bands, separated by marls and sandstone, and quarried for building and for lime. In Derbyshire and Yorkshire the magnesian limestone, under the name of Dolomite, forms an excellent building-stone, which has been used in the construction of the Houses of Parliament.

In the midland counties and on the borders of Wales, the Permian section is different from that of Nottinghamshire and the North of England. The Magnesian Limestones are absent, and the rocks consist principally of dark-red marl, brown and red sandstones, and calcareous conglomerates and breccias, which are almost entirely unfossiliferous. In Warwickshire, where they rest conformably on the Coal-measures, they occupy a very considerable tract of country, and are of very great thickness, being estimated by Mr. Howell to be 2,000 feet thick.

In the east of England the Magnesian Limestone contains a numerous marine fauna, but much restricted when compared with that of the Carboniferous period. The shells of the former are all small and dwarfed in size when compared with their congeners of Carboniferous times, when such there are, and in this respect, and the small number of genera, they resemble the living mollusca of the still less numerous fauna of the Caspian Sea.

Besides the poverty and small size of the mollusca, the later strata of the true Magnesian Limestone seem to afford strong indications that they may have been deposited in a great inland salt-lake subject to evaporation.

The absence of fossils in much of the formation may be partly accounted for by its deposition in great measure from solution, and the uncongenial nature of the waters of a salt-lake may account for the poverty-stricken character of the whole molluscan fauna.

The red colouring-matter of the Permian sandstones and marls is considered, by Professor Ramsay, to be due to carbonate of iron introduced into the waters, and afterwards precipitated as peroxide through the oxidising action of the air and the escape of the carbonic acid which held it in solution. This circumstance of the red colour of the Permian beds affords an indication that the red Permian strata were deposited in inland waters unconnected with the main ocean, which waters may have been salt or fresh as the case may be.

“The Magnesian Limestone series of the east of England may, possibly, have been connected directly with an open sea at the commencement of the deposition of these strata, whatever its subsequent history may have been; for the fish of the marl strata have generically strong affinities with those of Carboniferous age, some of which were truly marine, while others certainly penetrated shallow lagoons bordered by peaty flats.”

“On the Red Rocks of England,” by A. C. Ramsay. Quart. Jour. Geol. Soc., vol. xxvii., p. 246.