Chap10

[139]

CHAPTER X.

PERMIAN STRATA.

IN England there are certain red strata, known as PERMIAN, which occupy a sort of debatable ground, lying between the Carboniferous and New Red or Triassic series. Sometimes they have been classed with the former, sometimes with the latter, by those who like to insist on hard and fast lines of division between each formation. These strata, lying not quite conformably either with the underlying or the overlying formation, I prefer to consider as in some sense transition beds, making one of the steps in that change of the physical geography of our area which put an end to the development of Coal-measures, and made it possible under new conditions for the Permian strata to be deposited.

They are usually divided (as in Germany) into two subformations, viz. :—

Magnesian Limestone and Marl Slate,
Rothe-todteliegende.

The higher English beds in certain areas consist chiefly of Magnesian Limestone or Dolomite, interstratified with certain mans, and the lower of red mans, sandstones, and conglomerates. But if we take England as a whole this division does not hold good, for in the eastern part of England the Magnesian Limestone often lies directly on the Coal-measures, and in Lancashire and

[140 Permian Strata.]

the Vale of Eden, in the north, only a few thin beds of Magnesian Limestone lie in the middle of red sandstones and marls. Hard and fast lines of division by no means hold good in this case.

The Permian strata were for long considered as forming a lower part of the New Red Sandstone, till separated from it by Professor Sedgwick, in his celebrated Memoir on the Magnesian Limestone. They were afterwards called Permian by Sir Roderick Murchison, from the ancient Government of Perm in European Russia, where they are extensively developed.

Between the neighbourhood of Nottingham and Tynemouth in Northumberland, they have been subdivided by Professor King, into—

Crystalline and other limestones.
Brecciated limestone.
Fossiliferous limestone.
Compact limestone.
Marl slate.

The Marl slate lies at the base, but these subdivisions are by no means constant, and the lines between them are not always definite. In many places the rock consists of round masses of all sizes, often as large as good-sized cannon balls, all cemented together. The section is finely exposed on the sea-cliffs between Hartlepool and South Shields, with great outlying masses of rock rising out of the sands like ruined castles, pierced by caverns with lofty ragged pillars and arches, worn out by the restless sea, and through which the daily tide flows. In their range from Nottingham to this district the Magnesian Limestone is interstratified with three minor beds of red marl.

In Nottinghamshire the position of these Permian strata to the underlying Coal-measures, and the overlying Trias, or New Red series, is shown in the following diagram:—

[Permian Stratz. 141]

FIG. 30.
Section acros Coal-measures, Permian, and New Red strata, Nottinghamshire.

Looked on as a whole, the Magnesian Limestone of this district lies quite unconformably on the Carboniferous

[142 Permian Strata.]

series, for while between Nottingham and the neighbourhood of Leeds they lie upon Coalmeasures, between Leeds and the vicinity of Darlington they overlap the north edge of the Yorkshire coal-field, and rest directly on the Millstone Grit and associated shales as far as the south end of the Durham coal-field, north of which they again lie on Coal-measures.

The limestone and marl slate are often fossiliferous.

In Lancashire, Cheshire, and North Staffordshire, the Permian strata chiefly consist of red marls and sandstones, interstratified near Manchester with a few thin bands of Magnesian Limestone, where both limestones and marls are fossiliferous, containing bivalve shells of the genera Pleurophorus, Bakevillia, and Schizodus, Turbo, Natica, &c. Similar marls and sandstones, bordered by New Red Sandstone, stretch at intervals from the border of the North Staffordshire coal-field to that of Shrewsbury, and skirt the Denbighshire coal-field on the east. In the more central parts of England the same kinds of rock border the Coalbrookdale, Forest of Wyre, South Staffordshire, and Warwickshire coal-fields. In the Permian strata of Warwickshire there are beds of conglomerate, the waterworn pebbles of which largely consist of fragments of Carboniferous Limestone. A few stems of trees have been found in them, together with Calamites, and two or three casts of shells of the genus Strophalosia (fig. 31), together with a Labyrinthodont Amphibian, Dasyceps Bucklandi.

A large extent of Permian red sandstones and marls occupy the beautiful Vale of Eden in Westmoreland and Cumberland (see fig. 104, p. 521), from whence Permian strata extend into the valleys of the Nith and

[Permian Boulder Beds. 143]

the Annan in Scotland, brecciateci like those of the Clent and Abberly Hills.

In the South Staffordshire district, and in the Clent and Bromsgrove Lickey Hills, the Permian marls and sandstones are capped by a remarkable brecciated conglomerate, consisting of pebbles and large blocks of stone, generally angular, imbedded in a marly paste, once soft clay. These conglomerate beds are about 400 feet thick. South of Colebrookclale, near Enville, and between that country and the Abberly and Malvern Hills, the same rocks occur, largely associated with coarse brecciated conglomerates, similar to those of the Clent Hills. The fragments have mostly travelled from a distance, apparently from the borders of Wales, and some of them are three feet in diameter. In some cases the smooth surfaces of the stones still retain striations, identical in character with those found in ordinary boulder-clay, or made by modern glaciers. Many of the stones are of greenstone and feistone, apparently derived from the Silurian traps of Montgomeryshire and North Wales, and at the south end of the South Staffordshire coal-field, near Northfield, I found in these strata large slabs of Pentamerus limestone, such as are only known in the Longmynd country, on the borders of the Cambrian rocks in Shropshire. So completely, indeed, does the whole deposit resemble the Post-pliocene boulder-clay, that I have no doubt that there was a glacial episode during part of the Permian epoch. In Thuringia the conglomerates of the Rothliegende have the same lithological character as the brecciated conglomerates of the Abberly Hills and Clent Hills, and they may be considered equivalents both in position and origin.

The chief part of the Permian fossils have been

[144 Permian Fossils and]

found in the Magnesian Limestone, and they are, generically and specifically, few in number, but, as a whole, their affinities and grouping are decidedly Palæozoic. Some of the genera of plants have a Coal-measure aspect, including Calamites, Lepidodendron, Neuropteris, Sphenopteris, and Alethopteris, besides Walchia, Ullmannia, Cardicocarpon, and fragments of silicified coniferous wood. Only 9 genera and 21 species of Brachiopoda are found in these strata, viz. Camarophoria 3, Crania 2, Discina 1, Lingula 2, Producta 2, Spirifera 3, Spiriferina 2, Strophalosia 4, and Terebratula 2. These partly belong to genera which also occur in the Carboniferous rocks. The same strata contain 16 genera and 31 species of Lamellibranchiate molluscs, the most common of which are of the genera Schizodus, Gervillia, Solemya, &c.; 26 species of Gasteropoda, 2 Nautili, and many ganoid fishes, the most common belonging to the very characteristic genus Palœoniscus, of which there are 6 species (fig. 31, p. 148). All the Permian fish have heterocercal tails, like the majority of the Palaeozoic genera, in which the vertebral column is prolonged into the upper lobe of the tail, whereas in most of the modern fishes the vertebral column is not prolonged into either lobe. The reptilian remains, both of the red rocks and of the Magnesian Limestone, are partly Amphibian, as shown by the Labyrinthodont Dasyceps Bucklandi of Kenilworth, the footprints in the red Permian sandstones of the Vale of Eden, and Corncockle Moor, in Dumfriesshire, and Lepidotosaurus Duffii of the lower part of the Magnesian Limestone; while others from the marl slate, Proterosaurus Speneri and P. Huxleyi, were true land Lacertilian reptiles.

Excepting the Magnesian Limestone, all the

[Physical Geography. 145]

Permian rocks are red. As with the thin pellicle of peroxide of iron that incrusts the grains of sand and mud of the Old Red Sandstone, so the colour of the red Permian sandstones and marls is due to a thin incrusting pellicle of peroxide of iron, such as I have elsewhere attempted to show is often characteristic of deposits in inland waters.

I now come to the main point:—What were the peculiarities of the Physical Geography of the British area in Permian times? To explain this I shall partly use the matter published in 1871, in the 'Journal of the Geological Society,' in my paper 'On the Red Rocks of England of older date than the Trias.'

First, the plants found in our Permian strata are chiefly of genera, but not of species, common to the Coal-measures, viz., Calamites, Lepidodendron, Walchia, Chondrites, Ullmannia, Cardiocarpon, Alethopteris, Sphenopteris, Neuropteris, and many fragments of coniferous wood of undetermined genera. Inland waters would be likely to receive land plants borne into them by rivers, but this yields no certainly conclusive evidence, since land plants are not very uncommon in marine strata of the Lias and Oolites.

The evidence derived from the remains of Labyrinthodont Amphibia and of land reptiles, clearly points to the close proximity of land. First, there is the Labyrinthodont Dasyceps Bucklandi from the red Permian strata near Kenilworth, and next, Lepidotosaurus Duffii, found near the base of the Magnesian Limestone, where it gradually passes into the underlying marl slate, and from the marl slate itself were obtained Proterosaurus Speneri and P. Huxleyi, both, according to Huxley, true land Lacertilian reptiles. Further north, in the red sandstones of the Vale of Eden, Professor Harkness

[146 Physical Geography.]

found footprints, apparently of Labyrinthodonts, at Brownrigg, in Plumpton, and near Penrith; and many years ago numerous footprints were described by the late Sir William Jardine, which were found on the surfaces of beds of sandstone in Corncockle Moor and in other parts of Dumfriesshire. All of these footprints clearly indicate that the animals were occasionally accustomed to walk on bare damp surfaces, which were afterwards dried by the heat of the sun, before the flooded waters overspread them with new layers of sediment in a manner such as now takes place during variations of the seasons in many modern salt lakes. Pseudomorphs of crystals of salt in the Permian beds of the Vale of Eden, and deposits of gypsum and peroxide of iron, help to this conclusion, together with the occurrence of sun-cracks or rain-pittings impressed on the beds. The Pseudomorphous crystals of salt tell of the evaporation of pools by solar heat, for neither crystals of chloride of sodium (salt), nor deposits of sulphate of lime (gypsum), could have been formed amid common mechanical sediments at the bottom of an open ocean. Only concentration of salts, by solar evaporation of inland waters, could have produced this result.

Eight genera and 21 species of fishes have been found, chiefly in the marl slate. They are Acrolepis 1, Cœlacanthus 2, Dorypterus 1, Gyracanthus 1, Gyropristis 1, Palæoniscus 11, Platysomus 2, and Pygopterus 2. Generically they have strong affinities with those of the Carboniferous age, some of which were undoubtedly truly marine, while others certainly penetrated shallow lagoons bordered by peaty flats. There is nothing extraordinary in the occurrence of seafish in an inland salt lake.

[Physical Geography. 147]

If we now turn to the assemblage of shells we shall find it to be very poor in number. In the red marls and bands of Magnesian Limestone at and near Manchester, the very few species found in the marls and thin limestones are poor and dwarfed in aspect, and in this respect, and the small number of genera they somewiiat resemble the living molluscan fauna of the Caspian Sea.

In the true Magnesian Limestone district of Nottinghamshire, Yorkshire, and Durham, the case is somewhat different. There we find a more numerous molliiscan fauna, but wonderfully restricted when compared with that of Carboniferous Limestone times. I give it in some detail, that the reader may judge for himself, a the facts have an important bearing on my argument. rIlle numbers are taken from Mr. Etheridge's forthcoming work.

BRACHIOPODA.—Camarophoria 3, Crania 2, Discina 1, Lingula 2, Producta 2, Spirifera 3, Spiriferina 2, Strophalosia 4, Terebratula 2: in all, 9 genera and 21 species.

LAMELLIBRANCHIATA. —Aucella 1, Mytilus 2, Avicula 2, Gervillia 5, Arca 2, Cardiomorpha 1, Ctenodonta 1, Leda 1, Myalina 1, Myochoncha 1, Pleurophorus I, Edmondia 1, Astarte 2, Schizodus 5, Solemya 4, Tellina 1: in all, 16 genera and 31 species.

UNIVALVES.—Calyptrœa 1, Chemnitzia 1, Chiton 3, Chitonellus 4, Dentalium 1, Natica 2, Pleurotomaria 3, Rissoa 1, Straparolus 1, Turbo 5, Turbonilla 4 in all, 11 genera and 26 species.

PTEROPODA.—Theca 1.

CE PHAL0PODA.—Nautilus 1.

The whole comprises only 38 genera and 80 species, a very poor representative of the teeming life in the

[148 Physical Geography.]

Carboniferous Limestone sea, from which more than 1,500 species have been named. It is to be remarked also,

FIG. 31.

Group of Permian Fossils.

that all of the Permian shells are dwarfed in aspect, when compared with their Carboniferous congeners.

In this poverty in number, and dwarfing of the forms, these Permian fossils may be compared with the

[Physical Geography. 149]

still less numerous fauna of the Caspian Sea, as far as that fauna is known, which sea, or brackish lake, was, it is believed, once connected with the northern ocean, as the fauna seems to testify. My belief is, that these Permian waters were also of an inland, unhealthy nature, and, like those of the Caspian, had previously been connected with the open ocean.

Besides the poverty in number and small size of the mollusca, the chemical composition and lithological structure of the Magnesian Limestone, seem to me to afford strong hints that it was originally deposited in a large inland salt lake, and not that it was entirely derived from calcareous organisms, and subsequently altered into dolomite by chemical changes. I am well aware that there are such masses, occasionally, for example, in the Carboniferous Limestone which was formed in an open sea. Some modern atolls are knowii to become dolomitiseci, as described by Dana, but in the Magnesian Limestone corals are chiefly conspicuous by their absence. I repeat that the Permian Magnesian Limestone was not, as used to be supposed, formed in the sea, but in an inland salt lake, under such circumstances that carbonates of lime and magnesia. were deposited simultaneously, probably, by concentration of solutions due to evaporation. In an open sea, lime and magnesia only exist in solution in very small quantities, and limestone rocks there are formed, as in coral reefs, by organic agency.

In some of the lower strata of the Magnesian Limestone, when weathered, it is observable that they consist of many curious thin layers, bent into a number of very small convolutions, approximately fitting into each other, like sheets of paper crumpled together. These dolomitic layers convey the impression that they are

[150 Physical Geography.]

somewhat tufaceous in character, as if the layers, which are unfossiliferous, had been deposited from solutions.

In other parts of the district, along the coast of Durham, large tracts of the limestone consist of vast numbers of ball-shaped agglutinated masses, large and small, and I have observed in limestone caverns, in pools of water surcharged with bicarbonate of lime, that sometimes precipitation takes place on a small scale producing similar nodular bodies. It is notable also that when broken in two, many of the balls are seen to have a radiated acicular structure, that is to say, from the centre rudely crystalline-looking bodies all united, radiate to the circumference. In other places we find numerous bodies radiating in a series of rays that gradually widen from the centre, and are unconnected at their outer ends, which remind the spectator of radiating corals. There is, however, nothing organic about them, and I do not doubt that they owe their growth to some kind of crystalline action going on at the time that the limestone was being formed.

The occurrence of gypsum in the many strata of the Permian series, helps to the conclusion that they were all deposited in inland waters, for it is impossible to conceive of pure sulphate of lime having been thrown down from solution in the ocean.

In these views I do not stand alone, for similar conclusions are held by Dr. Sterry Hunt, as shown in Sir William Logan's 'Geology of Canada,' and Professor Dana in his 'Manual of Geology.'

The chemical argument is not, however, what first led me to suspect that the Permian Magnesian Limestone was deposited chiefly from solution, in an inland salt sea, but rather the poverty and dwarfed character of the fauna alone, while I soon saw that the chemical

[Physical Geography. 151]

deposition of the limestone may account for the total absence of fossils in the larger part of the formation. Whether or not the water was too salt for the healthy production of numerous shells and corals, is a question I have not yet attempted to solve, being in the meanwhile content to prove (as I think) that the waters formed inland lakes, that lay in a large continent which began in Old Red Sandstone times, but had undergone many modifications in its physical geography before the Permian lake-basins came into existence.