The World Before the Deluge Part 16

KEUPER SUB-PERIOD.

The formation which characterises the Keuper, or saliferous period, is of moderate extent, and derives the latter name from the salt deposits it contains.

These rocks consist of a vast number of argillaceous and marly beds, variously coloured, but chiefly red, with tints of yellow and green.

These are the colours which gave the name of _variegatea_ (Poikilitic) to the series. The beds of red marl often alternate with sandstones, which are also variegated in colour. As subordinate rocks, we find in this formation some deposits of a poor pyritic coal and of gypsum. But what especially characterises the formation are the important deposits of rock-salt which are included in it. The saliferous beds, often twenty-five to forty feet thick, alternate with beds of clay, the whole attaining a thickness of 160 yards. In Germany in Wurtemberg, in France at Vic, at Dieuze, and at Chateau-Salins, the rock-salt of the saliferous formation has become an important branch of industry. In the Jura, salt is extracted from the water charged with chlorides, which issues from this formation.

Some of these deposits are situated at great depths, and cannot be reached without very considerable labour. The salt-mines of Wieliczka, in Poland, for example, can be procured on the surface, or by galleries of little depth, because the deposit belongs to the Tertiary period; but the deposits of salt, in the Tria.s.sic age, lie so much deeper, as to be only approachable by a regular process of mining by galleries, and the ordinary mode of reaching the salt is by digging pits, which are afterwards filled with water. This water, charged with the salt, is then pumped up into troughs, where it is evaporated, and the crystallised mineral obtained.

What is the origin of the great deposits of marine salt which occur in this formation, and which always alternate with thin beds of clay or marl? We can only attribute them to the evaporation of vast quant.i.ties of sea-water introduced into depressions, cavities, or gulfs, which the sandy dunes afterwards separated from the great open sea. In PLATE XIV.

an attempt is made to represent the natural fact, which must have been of frequent recurrence during the saliferous period, to form the considerable ma.s.ses of rock-salt which are now found in the rocks of the period. On the right is the sea, with a dune of considerable extent, separating it from a tranquil basin of smooth water. At intervals, and from various causes, the sea, clearing the dune, enters and fills the basin. We may even suppose that a gulf exists here which, at one time, communicated with the sea; the winds having raised this sandy dune, the gulf becomes transformed, by degrees, into a basin or back-water, closed on all sides. However that may be, it is pretty certain that if the waters of the sea were once shut up in this basin, with an argillaceous bottom and without any opening, evaporation from the effects of solar heat would take place, and a bed of salt would be the result of this evaporation, mixed with other mineral salts which accompany chloride of sodium in sea-water, such as sulphate of magnesia, chloride of pota.s.sium, &c. This bed of salt, left by the evaporation of the water, would soon receive an argillaceous covering from the clay and silt suspended in the muddy water of the basin, thus forming a first alternation of salt and of clay or marl. The sea making fresh breaches across the barriers, the same process took place with a similar result, until the basin was filled up. By the regular and tranquil repet.i.tion of this phenomenon, continued during a long succession of ages, this abundant deposit of rock-salt has been formed, which occupies so important a position in the Secondary rocks.

There is in the delta of the Indus a singular region, called the Runn of Cutch, which extends over an area of 7,000 square miles, which is neither land nor sea, but is under water during the monsoons, and in the dry season is incrusted, here and there, with salt about an inch thick, the result of evaporation. Dry land has been largely increased here, during the present century, by subsidence of the waters and upheavals by earthquakes. "That successive layers of salt may have been thrown down one upon the other on many thousand square miles, in such a region, is undeniable," says Lyell. "The supply of brine from the ocean is as inexhaustible as the supply of heat from the sun. The only a.s.sumption required to enable us to explain the great thickness of salt in such an area, is the continuance for an indefinite period of a subsidence, the country preserving all the time a general approach to horizontally." The observations of Mr. Darwin on the atolls of the Pacific, prove that such a continuous subsidence is probable. Hugh Miller, after ably discussing various spots of earth where, as in the Runn of Cutch, evaporation and deposit take place, adds: "If we suppose that, instead of a barrier of lava, sand-bars were raised by the surf on a flat arenaceous coast, during a slow and equable sinking of the surface, the waters of the outer gulf might occasionally topple over the bar and supply a fresh brine when the first stock had been exhausted by evaporation."

Professor Ramsay has pointed out that both the sandstones and marls of the Tria.s.sic epoch were formed in lakes. In the latter part of this epoch, he is of opinion, that the Keuper marls of the British Isles were deposited in a large lake, or lakes, which were fresh or brackish at first, but afterwards salt and without outlets to the sea; and that the same was occasionally the case with regard to other portions of northern Europe and its adjoining seas.

By the silting up of such lakes with sediment, and the gradual evaporation of their waters under favourable conditions, such as increased heat and diminished rainfall--where the lakes might cease to have an outflow into the sea and the loss of water by evaporation would exceed the amount flowing into them--the salt or salts contained in solution would, by degrees, become concentrated and finally precipitated. In this way the great deposits of rock-salt and gypsum, common in the Keuper formation, may be accounted for.

Subsequently, by increase of rainfall or decrease of heat, and sinking of the district, the waters became comparatively less salt again; and a recurrence of such conditions lasted until the close of the Keuper period, when a partial influx of the sea took place, and the Rhaetic beds of England were deposited.

The red colour of the New Red Sandstones and marls is caused by peroxide of iron, which may also have been carried into the lakes in solution, as a carbonate, and afterwards converted into peroxide by contact with air, and precipitated as a thin pellicle upon the sedimentary grains of sandy mud, of which the Tria.s.sic beds more or less consist. Professor Ramsay further considers that all the red-coloured strata of England, including the Permian, Old Red Sandstone, and even the Old Cambrian formation, were deposited in lakes or inland waters.[55]

[55] A. C. Ramsay, _Quart. Jour. Geol. Soc._, vol. 27, p. 191.

There is little to be said of the animals which belong to the Saliferous period. They are nearly the same as those of the Muschelkalk, &c.

Among the most abundant of the sh.e.l.ls belonging to the upper Trias, in all the countries where it has been examined, are the _Avicula, Cardium_, and _Pecten_, one of which is given in Fig. 85. Foraminifera are numerous in the Keuper marls. The remains of land-plants, and the peculiarities of some of the reptiles of the Keuper period, tend to confirm the opinion of Professor Ramsay, that the strata were deposited in inland salt-lakes.

In the Keuper period the islands and continents presented few mountains; they were intersected here and there by large lakes, with flat and uniform banks. The vegetation on their sh.o.r.es was very abundant, and we possess its remains in great numbers. The Keuper Flora was very a.n.a.logous to those of the Lias and Oolite, and consisted of Ferns, Equisetaceae, Cycads, Conifers, and a few plants, which M. Ad.

Brongniart cla.s.ses among the dubious monocotyledons. Among the Ferns may be quoted many species of _Sphenopteris_ or _Pecopteris_. Among them, _Pecopteris Stuttgartiensis_, a tree with channelled trunk, which rises to a considerable height without throwing out branches, and terminates in a crown of leaves finely cut and with long petioles; the _Equiset.i.tes columnaris_, a great Equisetum a.n.a.logous to the horse-tails of our age, but of infinitely larger dimensions, its long fluted trunk, surmounted by an elongated fructification, towering over all the other trees of the marshy soil.

[Ill.u.s.tration: Fig. 85.--Pecten orbicularis.]

The _Pterophyllum Jageri_ and _P. Munsteri_ represented the Cycads, the _Taxodites Munsteria.n.u.s_ represented the Conifers, and, finally, the trunk of the Calamites was covered with a creeping plant, having elliptical leaves, with a re-curving nervature borne upon its long petioles, and the fruit disposed in bunches; this is the _Preissleria antiqua_, a doubtful monocotyledon, according to Brongniart, but M.

Unger places it in the family of _Smilax_, of which it will thus be the earliest representative. The same botanist cla.s.ses with the canes a marsh-plant very common in this period, the _Palaeoxyris Munsteri_, which Brongniart cla.s.ses with the _Preissleria_ among his doubtful Monocotyledons.

The vegetation of the latter part of the Tria.s.sic period is thus characterised by Lecoq, in his "Botanical Geography": "The cellular _Cryptogameae_ predominate in this as they do in the Carboniferous epoch, but the species have changed, and many of the genera also are different; the _Cladephlebis_, the _Sphenopteris_, the _Coniopteris_, and _Pecopteris_ predominate over the others in the number of species. The Equisetaceae are more developed than in any other formation. One of the finest species, the _Calamites arenaceus_ of Brongniart, must have formed great forests. The fluted trunks resemble immense columns, terminating at the summit in leafy branches, disposed in graceful verticillated tufts, foreshadowing the elegant forms of _Equisetum sylvatic.u.m_. Growing alongside of these were a curious Equisetum and singular Equiset.i.tes, a species of which last, _E. columnaris_, raised its herbaceous stem, with its sterile articulations, to a great height.

"What a singular aspect these ancient rocks would present, if we add to them the forest-trees _Pterophyllum_ and the _Zamites_ of the fine family of Cycadeaceae, and the Conifers, which seem to have made their appearance in the humid soil at the same time!

"It is during this epoch, while yet under the reign of the dicotyledonous angiosperms, that we discover the first true monocotyledons. The _Preissleria antiqua_, with its long petals, drooping and creeping round the old trunks, its bunches of bright-coloured berries like the _Smilax_ of our own age, to which family it appears to have belonged. Besides, the Tria.s.sic marshes gave birth to tufts of _Palaeoxyris Munsteri_, a cane-like species of the Gramineae, which, in all probability, cheered the otherwise gloomy sh.o.r.e.

"During this long period the earth preserved its primitive vegetation; new forms are slowly introduced, and they multiply slowly. But if our present types of vegetation are deficient in these distant epochs, we ought to recognise also that the plants which in our days represent the vegetation of the primitive world are often shorn of their grandeur. Our Equisetaceae and Lycopodiaceae are but poor representatives of the Lepidodendrons; the Calamites and Asterophyllites had already run their race before the epoch of which we write."

The princ.i.p.al features of Tria.s.sic vegetation are represented in PLATE XIV., page 198. On the cliff, on the left of the ideal landscape, the graceful stems and lofty trees are groups of _Calamites arenaceus_; below are the great "horse-tails" of the epoch, _Equisetum columnare_, a slender tapering species, of soft and pulpy consistence, which, rising erect, would give a peculiar physiognomy to the solitary sh.o.r.e.

The Keuper formation presents itself in Europe at many points, and it is not difficult to trace its course. In France it appears in the department of the Indre, of the Cher, of the Allier, of the Nievre, of the Saone-et-Loire; upon the western slopes of the Jura its outliers crop out near Poligny and Salins, upon the western slopes of the Vosges; in the Doubs it shows itself; then it skirts the Muschelkalk area in the Haute-Marne; in the Vosges it a.s.sumes large proportions in the Meurthe at Luneville and Dieuze; in the Moselle it extends northward to Bouzonville; and on the Rhine to the east of Luxembourg as far as Dockendorf. Some traces of it show themselves upon the eastern slopes of the Vosges, on the lower Rhine.

It appears again in Switzerland and in Germany, in the canton of Basle, in Argovia, in the Grand Duchy of Wurtemberg, in the Tyrol, and in Austria, where it gives its name to the city of Salzburg.

In the British Islands the Keuper formation commences in the eastern parts of Devonshire, and a band, more or less regular, extends into Somersetshire, through Gloucestershire, Worcestershire, Warwick, Leicestershire, Nottinghamshire, to the banks of the Tees, in Yorkshire, with a bed, independent of all the others in Cheshire, which extends into Lancashire. "At Nantwich, in the upper Trias of Cheshire," Sir Charles Lyell states, "two beds of salt, in great part unmixed with earthy matter, attain the thickness of 90 or 100 feet. The upper surface of the highest bed is very uneven, forming cones and irregular figures.

Between the two ma.s.ses there intervenes a bed of indurated clay traversed by veins of salt. The highest bed thins off towards the south-west, losing fifteen feet of its thickness in the course of a mile, according to Mr. Ormerod. The horizontal extent of these beds is not exactly known, but the area containing saliferous clay and sandstones is supposed to exceed 150 miles in diameter, while the total thickness of the Trias in the same region is estimated by Mr. Ormerod at 1,700 feet. Ripple-marked sandstones and the footprints of animals are observed at so many levels, that we may safely a.s.sume the whole area to have undergone a slow and gradual depression during the formation of the New Red Sandstone."

Not to mention the importance of salt as a source of health, it is in Great Britain, and, indeed, all over the world where the saliferous rocks exist, a most important branch of industry. The quant.i.ty of the mineral produced in England, from all sources, is between 5,000 and 6,000 tons annually, and the population engaged in producing the mineral, from sources supposed to be inexhaustible, is upwards of 12,000.

The lower Keuper sandstones, which lie at the base of the series of red marls, frequently give rise to springs, and are in consequence called "water-stones," in Lancashire and Cheshire.

[Ill.u.s.tration: Fig. 86.--Productus Martini.]

[Ill.u.s.tration: Fig. 87.--Patella vulgata.

(Living.)]

If the Keuper formation is poor in organic remains in France, it is by no means so on the other side of the Alps. In the Tyrol, and in the remarkable beds of Saint Ca.s.sian, Aussec, and Hallstadt, the rocks are made up of an immense number of marine fossils, among them Cephalopods, Cerat.i.tes, and Ammonites of peculiar form. The Orthoceras, which we have seen abounding in the Silurian period, and continued during the deposit of the Devonian and Carboniferous periods, appears here for the last time. We still find here a great number of Gasteropods and of Lamellibranchs of the most varied form. Sea Urchins--corals of elegant form--appear to have occupied, on the other side of the Alps, the same seas which in France and Germany seem to have been nearly dest.i.tute of animals. Some beds are literally formed of acc.u.mulated sh.e.l.ls belonging to the genus _Avicula_; but these last-mentioned deposits are to be considered as more properly belonging to the Rhaetic or Penarth strata, into which the New Red or Keuper Marl gradually pa.s.ses upwards, and which are more fully described at page 207.

In following the grand mountainous slopes of the Alps and Carpathians we discover the saliferous rocks by this remarkable acc.u.mulation of Aviculae. The same facies presents itself under identical conditions in Syria, in India, in New Caledonia, in New Zealand, and in Australia. It is not the least curious part of this period, that it presents, on one side of the site of the Alps, which were not yet raised, an immense acc.u.mulation of sediment, charged with gypsum, rock-salt, &c., without organic remains; while beyond, a region presents itself equally remarkable for the extraordinary acc.u.mulation of the remains of marine Mollusca. Among these were _Myophoria lineata_, which is often confounded with Trigonia, and _Stellispongia variabilis_.

France at this period was still the skeleton of what it has since become. A map of that country represents the metamorphic rocks occupying the site of the Alps, the Cevennes, and the Puy-de-Dome, the country round Nantes, and the Islands of Brittany. The Primary rocks reach the foot of the Pyrenees, the Cotentin, the Vosges, and the Eifel Mountains.

Some bands of coal stretch away from Valenciennes to the Rhine, and on the north of the Vosges, these mountains themselves being chiefly composed of Tria.s.sic rocks.

RHaeTIC, OR PENARTH SUB-PERIOD.

The attention of geologists has been directed within the last few years, more especially, to a series of deposits which intervene between the New Red Marl of the Trias, and the blue argillaceous limestones and shales of the Lower Lias. The first-mentioned beds, although they attain no great thickness in this country, nevertheless form a well-defined and persistent zone of strata between the unfossiliferous Tria.s.sic marls and the lower Lia.s.sic limestone with _Ostrea Lia.s.sica_ and _Ammonites planorbis_, _A. angulatus_ and _A. Bucklandi_; being everywhere characterised by the presence of the same groups of organic remains, and the same general lithological character of the beds. These last may be described as consisting of three sub-divisions, the lowermost composed of alternations of marls, clays, and marly limestones in the lower part, forming a gradual pa.s.sage downwards into the New Red Marls upon which they repose. 2. A middle group of black, thinly laminated or paper-like shales, with thin layers of indurated limestone, and crowded in places with _Pecten Valoniensis_, _Cardium Rhaetic.u.m_, _Avicula contorta_, and other characteristic sh.e.l.ls, as well as by the presence, nearly always, of a remarkable bed, which is commonly known as the "Bone-bed." This thin band of stone, which is so well known at Aust, Axmouth, Westbury-on-Severn, and elsewhere, is a brecciated or conglomerated band of variable thickness which, sometimes a sandstone and sometimes a limestone, is always more or less composed of the teeth, scales, and bones of numerous genera of Fishes and Saurians, together with their fossilised excrement, which will be more fully and subsequently described under the name of Coprolites, under the Lia.s.sic period.

The molar tooth of a small predaceous fossil mammal of the Microlestes family (?????, _little_; ??st??, _beast_), whose nearest living representative appears to be some of the Hypsiprymnidae or Kangaroo Rats, has been found by Mr. Dawkins in some grey marls underlying the bone-bed on the sea-sh.o.r.e at Watchett, in Somersetshire; affording the earliest known trace of a fossil mammal in the Secondary rocks. Several small teeth belonging to the genus Microlestes have also been discovered by Mr. Charles Moore in a breccia of Rhaetic age, filling a fissure traversing Carboniferous Limestone near Frome; and in addition to the discovery of the remains of Microlestes, those of a mammal more closely allied to the Marsupials than any other order, have been met with at Diegerloch, south-east of Stuttgart, in a remarkable bone-breccia, which also yielded coprolites and numerous traces of fishes and reptiles.

The uppermost sub-division includes certain beds of white and cream-coloured limestone, resembling in appearance the smooth fracture and closeness of texture of the lithographic limestone of Solenhofen, and which, known to geologists and quarrymen under the name "white lias," given to it by Dr. William Smith, was formerly always considered to belong to, and was included in, the Lias proper. The most remarkable bed in this zone is one of only a few inches in thickness, but it has long been known to collectors, and sought after under the name of Cotham Marble or Landscape Stone, the latter name having reference to the curious dendritic markings which make their appearance on breaking the stone at right angles to its bedding, bearing a singular resemblance to a landscape with trees, water, &c.; while the first name is that derived from its occurrence abundantly at Cotham, in the suburbs of Bristol, where the stone was originally found and noticed.

This band of stone is interesting in another respect, because it sometimes shows by its uneven, eroded, and water-worn upper surface, that an interval took place soon after it had been deposited, when the newly-formed stone became partially dissolved, eroded, or worn away by water, before the stratum next in succession was deposited upon it. The same phenomenon is displayed, in a more marked degree, in the uppermost limestone or "white lias" bed of the series, which not only shows an eroded surface, but the holes made by boring Molluscs, exactly as is produced at the present day by the same cla.s.s of animals, which excavate holes in the rocks between high and low-water marks, to serve for their dwelling-places, and as a protection from the waves to their somewhat delicate sh.e.l.ls.

The "White Lias" of Smith is the equivalent of the Koessen beds which immediately underlie the Lower Lias of the Swabian Jura, and have been traced for a hundred miles, from Geneva to the environs of Vienna; and, also, of the Upper St. Ca.s.sian beds, which are so called from their occurrence at St. Ca.s.sian in the Austrian Alps.

The general character of the series of strata just described, is that of a deposit formed in tolerably shallow water. In the Alps of Lombardy and the Tyrol, in Luxembourg, in France, and, in fact, throughout nearly the whole of Europe, they form a sort of fringe in the margin of the Tria.s.sic sea; and, although of comparatively inconsiderable thickness in England, they become highly developed in Lombardy, &c., to an enormous thickness, and const.i.tute the great ma.s.s of the Rhaetian Alps and a considerable part of the well-known beds of St. Ca.s.sian, and Hallstadt in the Austrian Alps. (See page 205.)

The Rhaetic beds of Europe were, as a whole, formed under very different conditions in different areas. The thickness of the strata and the large and well-developed fauna (chiefly Mollusca) indicate that the Rhaetic strata of Lombardy, and other parts of the south and east of Europe, were deposited in a broad open ocean. On the other hand, the comparatively thin beds of this age in England and north-western Europe, the fauna of which, besides being poor in genera and species, consists of small and dwarfed forms, point to the conclusion that they were in great part deposited in shallow seas and in estuaries, or in lagoons, or in occasional salt lakes, under conditions which lasted for a long period.[56]

[56] See A. C. Ramsay, "On the Physical Relations of the New Red Marl, Rhaetic Beds, and Lower Lias," _Quart. Jour. Geol. Soc._, vol. 27, p. 189.

In consequence of the importance they a.s.sume in Lombardy (the ancient Rhaetia), the name "Rhaetic Beds" has been given to these strata by Mr.

Charles Moore; Dr. Thomas Wright has proposed the designation "Avicula Contorta Zone," from the plentiful occurrence of that sh.e.l.l in the black shales forming the well-marked middle zone, and which is everywhere present where this group of beds is found; Jules Martin and others have proposed the term "Infra-lias," or "Infra-lia.s.sic strata;" while the name "Penarth Beds" has been a.s.signed to these deposits in this country by Mr. H. W. Bristow, at the suggestion of Sir Roderick Murchison, in consequence of their conspicuous appearance and well-exposed sections in the bold headlands and cliffs of that locality, in the British Channel, west of Cardiff.

A fuller description of these beds will be found in the Reports of the Bath Meeting of the British a.s.sociation (1864), by Mr. Bristow; also in communications to the _Geological Magazine_, for 1864, by MM. Bristow and Dawkins;[57] in papers read before the Geological Society by Dr.

Thomas Wright,[58] Mr. Charles Moore,[59] and Mr. Ralph Tate,[60] as printed in their _Quarterly Journal_; and by Mr. Etheridge, in the Transactions of the Cotteswold Natural History Club for 1865-66. The limits of the Penarth Beds have also been lately accurately laid down by Mr. Bristow in the map of the Geological Survey over the district comprised between Bath, Bristol, and the Severn; and elaborately detailed typical sections of most of the localities in England, where these beds occur, have been constructed by MM. Bristow, Etheridge, and Woodward, of the Geological Survey of Great Britain, which, when published, will greatly add to our knowledge of this remarkable and interesting series of deposits.

[57] _Quart. Jour. Geol. Soc._, vol. xx., p. 396.