THE PERMIAN SYSTEM.
_Classification._ It has already been observed that as the result of
the Pennine and Mendip systems of earth-movement, the Carboniferous
rocks of Britain are succeeded by a marked unconformity, and that the
rocks of the succeeding Permian and Triassic systems of Britain shew
an abnormal development. The principal areas where Permian rocks are
found are on either side of the Pennine Chain in the North of England,
but sporadic exposures of rocks of this age are found in some of the
Midland and Southern counties. The Permian rocks have been well
studied in Germany, and the German names are sometimes adopted in
Britain, and the following comparison will prove useful:--
Britain. Germany.
Magnesian Limestone Magnesian Limestone } Zechstein.
Marl Slate Kupferschiefer }
Lower Permian Sandstones Rothliegende.
The term Zechstein has been applied in a somewhat different sense by
different writers, but the one given in the table appears to find most
favour.
In a region which was essentially continental, considerable variations
in the lithological characters of the rocks may be expected, when the
strata are traced laterally, but we nevertheless find that the
differences are not so great as was formerly supposed to be the case
when certain red sandstones lying above recognised Permian strata in
the district on the west side of the Pennine Chain towards its
northern extremity were also referred to the Permian; these sandstones
(the St Bees Sandstones) are now generally admitted to be of Triassic
age, and comparison between the rocks on opposite sides of the Pennine
Chain is much simplified, as seen below.
West side. East side.
Thin Magnesian Limestones and Marls Magnesian Limestone
Hilton Shales Marl Slate
Penrith Sandstone and Brockrams Lower Permian Sandstones.
_Description of the Strata._ On the east side of the Pennine Chain,
the Lower Permian sandstone is an inconstant deposit often consisting
of yellow false-bedded arenaceous strata. The Marl Slate is an
argillaceous shale, often containing bituminous matter, and yielding
several fish-remains and some plants; it is usually only a few feet in
thickness. The Magnesian Limestone is typically developed in Durham as
a yellow or greyish limestone containing a variable percentage of
carbonate of magnesia; when traced southward, it alters its
characters, becoming mixed with mechanical deposits, and some chemical
precipitates in places, so that at Mansfield it appears as a red
sandstone with grains cemented by a mixture of carbonates of lime and
magnesia; and, like the rest of the Permian strata, it has disappeared
when we reach Nottingham. In addition to the southward thinning of the
Permian beds of this area, there is some evidence of their
disappearance in a westerly direction, though, as the present strike
of the beds is nearly north and south, the indications of this are
less convincing.
On the east side of the Pennine Chain, the main difference observable
is the relative thickness of the major divisions. The Lower Permian
sandstones have thickened out considerably, while the reputed
representatives of the Magnesian Limestone are thin. The Penrith
sandstone is of considerable interest. It contains in places, as near
Appleby, thick deposits of breccia consisting of angular fragments
chiefly composed of Carboniferous Limestone, which in many cases have
undergone subsequent dolomitisation, embedded in a matrix of red
sandstone. This breccia is known as brockram. Many beds of the Penrith
sandstone are composed of crystalline grains of sand, due to
deposition of silica in crystalline continuity with the quartz of the
original grain after the formation of the deposit; of more
significance, for our present purpose, is the presence of other
accumulations of the sand, in which the individual grains often
approach the form of spheres, thus resembling the 'millet-seed' sands
of modern desert regions. The Hilton shales are grey sandy shales,
with plant remains, and above them are variable deposits including
thin magnesian limestones which have yielded no fossils.
The isolated Permian deposits of the midland and southern counties of
England consist of red marls and sandstones with occasional breccias,
and in the absence of fossils, their exact position in the Permian
series is still unknown.
The German Permian rocks resemble those of Britain, especially as seen
in Durham, in many particulars, and give indications of formation
under physical and climatic conditions generally similar to those
which were then prevalent in the British area. At Stassfurt, in
Germany, the less soluble constituents of ocean water are accompanied
by a great variety of salts:--chlorides, sulphates and borates; and
the very soluble salts of potassium and magnesium known as the Abraum
salts are found in abundance as well as the less soluble salts of
sodium and calcium. The occurrence of these very soluble salts is so
infrequent on a large scale among the rocks of the Geological Column,
and the matter is one of so great theoretical import, that it is
necessary to take special note of their presence in the Permian
strata.
The frequent existence of chemical deposits in the Permian Rocks of
N.W. Europe, the formation of red sandstones, and the dolomitisation
of limestone beds and fragments of pre-existing limestones point to
inland seas of a Caspian character, while the evaporation necessary
for the formation of the precipitates also indicates a fairly warm
temperature. The presence of millet-seed sands, in very lenticular
patches, suggesting former sand-dunes, and the occurrence in places of
breccias (like some parts of the brockram) almost devoid of matrix,
piled up against pre-existing cliffs, recalling screes of modern
times, give almost certain evidence of the occurrence of land tracts
most probably of desert character, during part of the period of
accumulation of the materials of the Permian rocks. The fossil
evidence supports this view, and geologists are mostly agreed that the
Permian rocks of north-west Europe were accumulated in an area of
desert character, occupied in part by inland seas, though there is
much difference of opinion as to the extent of these seas, some
geologists holding that a number of isolated sheets of water were
necessary to produce the distribution and character of the
accumulations. It is still a vexed question with British geologists
how far the Pennine ridge stood up as land during the period, but
leaving this and other minor considerations out of account, it may be
noted that the similarity of deposits in the different areas, whether
we examine the order of succession, the lithological characters or the
included fossils, suggests communication between the water tracts of
different regions, though this communication need not have been more
than a series of straits, or comparatively narrow belts of water[96].
[Footnote 96: It should be mentioned that some writers have inferred
the evidence of glacial conditions over parts of the British area, on
account of the resemblance of some of the Permian breccias to recent
glacial deposits. The question is still _sub judice_. It is not
necessarily opposed to the existence of desert conditions, if the
mountains were sufficiently high, for the Wahsatch regions adjoining
the Basin Region of N. America have been glaciated.]
The extensive development of Permian and Triassic rocks with
terrestrial characters in the southern hemisphere also, and the
absence of newer deposits in many places, suggests that the land areas
of these times in that hemisphere have largely remained such ever
since, in which case, the Permo-Triassic series of movements produced
a marked direct effect upon our present continental areas, and at any
rate produced an indirect one upon the British land tracts.
The presence of anomalous deposits of Permian age over wide areas need
not be surprising, but it would be indeed remarkable if no ordinary
marine type of Permian rocks was known, and the researches of recent
years have proved that this type is extensively developed, in Eastern
Europe, Asia, and North America, where Permian rocks consisting of
limestones, with a greater or less admixture of mechanical deposits,
occur in some abundance. The studies of Waagen and others in India
have given us the farthest insight into the nature of these beds.
Below is a general classification taken from Waagen's work:--
Salt Range. Germany.
Base of Trias }
Unfossiliferous Shale and }
Sandstone } Passage Beds into Trias
Top Beds of Upper _Productus_ }
Limestone }
Cephalopoda Beds of Upper } Gypsum Beds
_Productus_ Limestone }
Middle Division of Upper }
_Productus_ Limestone } Zechstein (in restricted sense)
Lower Division of Upper }
_Productus_ Limestone }
Upper Division of Middle } Weissliegende and Kupferschiefer
_Productus_ Limestone }
Middle Division of Middle } Rothliegende.
_Productus_ Limestone }
It will be seen that in the Salt Range there is a complete passage
from the Permo-Carboniferous strata through the Permian into the
Trias, and the detailed work which has been carried out by Waagen and
others amongst the rocks of the Salt Range must make this, for the
present at all events, the type area for the marine development of the
strata of Permo-Carboniferous and Permian ages.
_The Permian flora and fauna._ The Permian flora presents some
difficulties. The flora of the Zechstein consists largely of ferns and
conifers, but that of the Rothliegende of Germany has been compared
with that of the Carboniferous, and if a true Permian flora of the
northern hemisphere has many forms of Carboniferous affinities, the
presence of the Glossopteris flora in Permo-Carboniferous rocks of
more southerly regions seems to imply its origin there and _slow_
migration northwards. It must be noted, however, that the Rothliegende
has been divided by some geologists into an upper and lower division,
of which the lower is actually referred to the Carboniferous system.
All that can be now said is, that our knowledge of the floras of
Permo-Carboniferous and Permian times is still incomplete, and that
the difficulties will no doubt be cleared up as the result of further
work.
The invertebrate fauna of the north-west European Permian deposits is
chiefly noticeable on account of the paucity of species, though
individuals are often abundant. The shells are also sometimes stunted
and occasionally distorted. These characters bear out the supposition
that the aqueous deposits were laid down in inland seas of Caspian
character and not in the open ocean. Polyzoa, brachiopods, and
lamellibranchs predominate, but other groups are found. The
vertebrates consist of forms of fish, amphibia and reptiles, and the
Permian rocks are the earliest strata in which the remains of true
Reptilia are known to occur with certainty. The Reptiles belong to the
orders Anomodontia (Theromora) and Rhynchocephalia, of which the
former is exclusively Permian and Triassic, while the latter is
abundant in the strata of those periods, but is represented at the
present day by the genus _Sphenodon_ of New Zealand. The Amphibia
belong to the order Labyrinthodontia which ranges from Carboniferous
to Lower Jurassic, but the members of the order are most abundant in
Permian and Triassic strata, and these periods may be spoken of as the
Periods of Labyrinthodonts.
A few words must be said of the fauna of the truly marine Permian
beds. It is much richer than that of the abnormal deposits of
north-western Europe, and its study is important as furnishing another
link between Palæozoic and Mesozoic life. Many Palæozoic genera pass
up into the Permian rocks, and, as will be ultimately seen, several
occur in those of the Triassic system, and one or two even in the
basal Jurassic strata, though Mesozoic forms predominate in the Lower
Jurassic Rocks, and there is a fairly equal admixture of forms usually
considered as Palæozoic and of those generally regarded as Mesozoic in
Triassic rocks, while the Palæozoic forms still predominate over the
Mesozoic in the Permian strata. Along with these characteristic
Palæozoic genera, it is interesting to find representatives of more
than one genus of the tribe of Ammonites, which is to take so
prominent a place in the fauna of the Mesozoic rocks, amongst the true
marine Permian sediments of India and other areas. The announcement of
the contemporaneity of ammonites with fossils regarded as exclusively
palæozoic was received with considerable doubt, but this
contemporaneity is now clearly established, and need not be regarded
as in any way anomalous.
With the deposition of the Permian rocks, Palæozoic time comes to an
end, but as already remarked there is no marked and sudden change to
characterise it. Had our classification been originally founded on
study of the Indian Rocks instead of those of Britain, and similar
terms adopted, the line of demarcation between Palæozoic and Mesozoic
rocks would probably have been drawn below the Permo-Carboniferous
deposits, and if it had been based on study of other areas, perhaps
elsewhere. The palæontological break is purely local, and it is of the
utmost importance that it should be recognised as such, and that it
should not be considered that division into Palæozoic and Mesozoic
implies some great and widespread change which occurred between the
times covered by the deposits of each of these great divisions[97].
[Footnote 97: The Permian Fossils of Britain are described by
Professor King in the Monographs of the Palæontographical Society (the
Brachiopods by Dr Davidson in the Monographs of the same Society). For
a general account of the marine type the student may consult the
second edition of Messrs Medlicott and Blanford's _Geology of India_.
For information concerning the Permian volcanic rocks see Sir A.
Geikie's _Ancient Volcanoes of Great Britain_.]