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 Würtemberg, in France
at Vic, at Dieuze, and at Château-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 Triassic 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 quantities
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 masses 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
potassium, &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 repetition 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 assumption
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 Triassic 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 Rhætic 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 Triassic 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 shells 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 shores was very
abundant, and we possess its remains in great numbers. The Keuper Flora
was very analogous to those of the Lias and Oolite, and consisted of
Ferns, Equisetaceæ, Cycads, Conifers, and a few plants, which M. Ad.
Brongniart classes 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 _Equisetites
columnaris_, a great Equisetum analogous 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.
[Illustration: Fig. 85.--Pecten orbicularis.]
The _Pterophyllum Jägeri_ and _P. Münsteri_ represented the Cycads, the
_Taxodites Münsterianus_ 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 classes with the canes a
marsh-plant very common in this period, the _Palæoxyris Münsteri_, which
Brongniart classes with the _Preissleria_ among his doubtful
Monocotyledons.
The vegetation of the latter part of the Triassic period is thus
characterised by Lecoq, in his “Botanical Geography”: “The cellular
_Cryptogameæ_ 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
Equisetaceæ 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
sylvaticum_. Growing alongside of these were a curious Equisetum and
singular Equisetites, 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 Cycadeaceæ, 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 Triassic marshes gave
birth to tufts of _Palæoxyris Münsteri_, a cane-like species of the
Gramineæ, which, in all probability, cheered the otherwise gloomy shore.
“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
Equisetaceæ and Lycopodiaceæ are but poor representatives of the
Lepidodendrons; the Calamites and Asterophyllites had already run their
race before the epoch of which we write.”
The principal features of Triassic 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 shore.
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 Nièvre, of
the Saône-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 assumes 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 Würtemberg, 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 masses 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 assume 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 quantity 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.
[Illustration: Fig. 86.--Productus Martini.]
[Illustration: 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 Cassian, Aussec, and Hallstadt, the rocks are
made up of an immense number of marine fossils, among them Cephalopods,
Ceratites, 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 destitute of
animals. Some beds are literally formed of accumulated shells belonging
to the genus _Avicula_; but these last-mentioned deposits are to be
considered as more properly belonging to the Rhætic or Penarth strata,
into which the New Red or Keuper Marl gradually passes 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 accumulation of
Aviculæ. 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
accumulation of sediment, charged with gypsum, rock-salt, &c., without
organic remains; while beyond, a region presents itself equally
remarkable for the extraordinary accumulation 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 Cévennes, and the Puy-de-Dôme, 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 Triassic rocks.