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Professor Sedgwick in 1832 described what is now recognised as the central member of this group, the Magnesian limestone, showing that it attained a thickness of 600 feet along the north-east of England, in the counties of Durham, Yorkshire, and Nottinghamshire, its lower part often passing into a fossiliferous marl-slate and resting on an inferior Red Sandstone, the equivalent of the Rothliegendes of Germany.
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. To the limestone they gave the name of Zechstein, and to the marl-slate that of Mergel-schiefer or Kupfer- schiefer.
The mountains on the left are composed of slate-marl, and not sandstone, as before stated by myself and Dr. Oudney. Overweg considers them of a very peculiar character and is delighted with their castle-like and battlemented shapes. But we shall have much to say of these marl-slate mountains, coloured so beautifully, and looking nobly to the eye.
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.
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.
Thickness of calcareous and sedimentary Rocks in North of England. Upper, Middle, and Lower Permian. Marine Shells and Corals of the English Magnesian Limestone. Reptiles and Fish of Permian Marl-slate. Foot-prints of Reptiles. Angular Breccias in Lower Permian. Permian Rocks of the Continent. Zechstein and Rothliegendes of Thuringia. Permian Flora. Its generic Affinity to the Carboniferous.
Pygopterus mandibularis. Agassiz. Marl-slate. a. Outside of scale, magnified. b. Acrolepis Sedgwickii, Agassiz. Outside of scale, magnified. We are indebted to Messrs.
Some of this same tribe of shells, such as Camarophoria, allied to Rhynchonella, Spiriferina, and two species of Lingula, are specifically the same as fossils of the carboniferous rocks. Restored outline of a fish of the genus Palaeoniscus, Agassiz. Beneath the limestone lies a formation termed the marl-stone, which consists of hard calcareous shales, marl-slate, and thin-bedded limestones.
Remains of Labyrinthodont reptiles have also been met with in the same slate near Durham. 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 associated with these beds.
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