THE JURASSIC SYSTEM.
The Jurassic rocks were formerly separated on account of differences
of lithological character into Oolites and Lias, but it was apparent
that the Oolites were more important than the Lias, and a fourfold
division was made into:--
Upper or Portland Oolites }
Middle or Oxford Oolites } = Malm
Lower or Bath Oolites = Dogger
Lias.
The Lias strata have also been spoken of as the Black Jura, the Lower
Oolites and part of the Oxford Oolites as Brown Jura, and the rest of
the Oxford Oolites with the Portland Oolites as White Jura.
As the outcome of a detailed study of the faunas of the Jurassic
rocks, a further subdivision has been made, partly based upon the
original British series, but the divisions are defined with greater
accuracy, so that they are applicable over wider areas. They are as
follows:--
{ Purbeckian
Upper Oolites { Portlandian
{ Kimmeridgian
{ Corallian
Middle Oolites { Oxfordian
{ Callovian
Lower Oolites { Bathonian
{ Bajocian
{ Toarcian
Lias { Liassian
{ Sinemurian.
Many of these series have been still farther subdivided into smaller
stages, and the whole differentiated into a number of zones
characterised by different forms of Ammonites. Dr E. von Mojsisovics
gives thirty-two Ammonite zones, of which fourteen occur in the Lias,
eight in the Lower Oolites, six in the Middle Oolites, and four in the
Upper Oolites.
_Characters of the strata._ The whole of the Jurassic rocks and also
those of Lower Cretaceous age may be regarded as having been deposited
during the first shallow water phase of the third marine period, but
this shallow water phase is represented by strata which are varied
owing to numerous marine changes resulting in the production of land
at times, and estuarine conditions, shallow water, marine conditions,
and somewhat deeper sea conditions respectively at other times, and
accordingly the strata of the British Isles vary greatly when traced
laterally. That the uplifts of the Permo-Triassic periods produced
some effect on the nature and distribution of the Jurassic rocks is
certain, but it is not quite clear how far the ridges produced by
these uplifts were submerged and denuded during the deposition of the
main portion of the Jurassic strata.
Viewed broadly, the Jurassic rocks of Britain may be regarded as
consisting of three great clay deposits, the Lias, Oxford and
Kimmeridge Clays, alternating with the deposits of variable
lithological characters, which compose the Bajocian, Bathonian,
Corallian, Portlandian and Purbeckian subdivisions. This essentially
argillaceous character of a large part of the deposits of Jurassic age
is often overlooked, as, owing to their sameness and the comparative
paucity of organisms constituting the faunas in the clays, their
description in text-books can be given at much shorter length than
that of the more variable and highly fossiliferous deposits which
separate the clays. The following figure (Fig. 23) roughly represents
the nature of the different divisions of the rocks of this system when
traced across England from south-west to north-east.
[Illustration: Fig. 23.
Vertical scale: 1 in. = about 1000 feet.]
It will be seen that the greatest variations in lithological character
occur in the Bathonian and Bajocian beds, and it will be of interest
to give some account of the principal variations and to attempt to
account for them. In so doing it will be convenient to consider the
four major divisions of the Jurassic rocks separately, and to enter
into particulars concerning the local classification applied to the
rocks of these divisions.
_The Lias._ The British Lias deposits are divided into the Lower Lias,
the Marlstone, and the Upper Lias corresponding in general terms only
with the Sinemurian, Liassian, and Toarcian. The Marlstone is
separated from the Upper and Lower Lias on account of the greater
percentage of carbonate of lime which it contains, so that the bands
of argillaceous limestone are much more marked in the Marlstone than
in the upper and lower divisions, which consist chiefly of clay. The
three divisions possess very much the same characters throughout the
country, though the presence of the Mendip ridge and its continuation
beneath London is marked by the attenuation of this and succeeding
strata, and by the conglomeratic character of some of the Liassic
strata where they abut against it. The British Lias, as a whole, seems
to have been deposited in a fairly shallow sea at no great distance
from the land. It passes down conformably into the Rhætic beds, indeed
the zone of Ammonites (_Aegoceras_) _planorbis_, referred by British
geologists to the Lower Lias is included by some continental writers
with the Rhætic beds, and the plane of demarcation here as in other
cases is conventional.
_The Lower Oolites._ Of all the British strata, these perhaps cause
most trouble to the learner, on account of the different nomenclature
applied to the rocks in different parts of England, and the rapid
variations in lithological character, when the beds are traced
laterally. The following divisions are usually adopted for the beds of
the south-western counties where the most marked marine development
occurs:--
Cornbrash,
Forest Marble,
Great Oolite (with Bradford Clay),
Fuller's Earth,
Inferior Oolite.
Of these divisions, the uppermost one, the Cornbrash, though thin,
retains its characters with great constancy across the island. Of the
others the Forest Marble may be looked upon as a local development of
the upper portion of the Great Oolite, and the Fuller's Earth is a
local deposit, so that the Inferior Oolite and Great Oolite constitute
the important divisions of the Lower Oolites. The variations in the
characters of the rocks may be best shown in tabular form.
-----------------+------------------+-------------------+-----------------
Gloucestershire, | South | N. |
&c. | Northamptonshire | Northamptonshire | Yorkshire
| | and Lincoln |
-----------------+------------------+-------------------+-----------------
Cornbrash | Cornbrash | Cornbrash | Cornbrash
-----------------+------------------+-------------------+-----------------
Great Oolite | Great Oolite | Great Oolite Clay |
| (Upper part) | Great Oolite | Upper
| | Limestone |
| | Upper | Estuarine
| Northamptonshire | Estuarine |
............... | ................ | ................. | ................
| | Series | Series
| | Lincolnshire | Scarbro'
| | Limestone | Limestone
| Sands | | Middle Estuarine
| | | Series
Inferior Oolite | | Lower Estuarine | Millepore Oolite
| | Series |
| | | Lower Estuarine
| | | Series
-----------------+------------------+-------------------+-----------------
Upper Lias | Upper Lias | Upper Lias | Upper Lias
-----------------+------------------+-------------------+-----------------
The dotted line shows roughly the division between Bathonian
and Bajocian.
The changes may be explained very simply if we leave out of account
for the moment the development of Lincolnshire Limestone, with its
equivalent the Scarbro' Limestone, and the Millepore series. The beds
in Gloucestershire and other south-western counties are essentially
marine; whilst in Northamptonshire and Lincolnshire estuarine
conditions set in after the deposition of the Upper Lias, and
continued throughout the deposition of the Bajocian and Lower
Bathonian beds, being replaced by marine conditions during the
formation of the Upper Bathonian strata, and still further north in
Yorkshire the estuarine conditions generally prevailed throughout
Bajocian and Bathonian times. These changes point to the existence of
land towards the north. The general simplicity is modified by
temporary prevalence of marine conditions twice over (during the
deposition of the Millepore Oolite and the Scarbro' Limestone) in
Yorkshire, and once (during the deposition of the Lincolnshire
Limestone) in Lincolnshire.
Certain local deposits have not been noticed, but two of them merit
brief reference. At the base of the Great Oolite of Oxfordshire is an
estuarine deposit of finely laminated mechanical sediment mixed with
calcareous matter known as the Stonesfield Slate, especially
interesting on account of its fossils, while a bed with similar
lithological characters but with a different fauna occurring at the
base of the Lincolnshire Limestone (of Bajocian age) is termed the
Collyweston Slate. Neither of these deposits is a slate in the true
sense of the word, as they have not been affected by cleavage
subsequently to their accumulation, but each has been somewhat
extensively used for roofing purposes.
The Middle Oolites are much less complicated though considerable
variations arise with respect to the Corallian Rocks. The Oxfordian
with Callovian consist chiefly of clay, though the Callovian of the
south of England is represented by calcareous sandstone, with a
peculiar fauna which seems to be represented in the lower part of the
Oxford Clay further north, though this Callovian fauna has not been
everywhere recognised.
The Corallian of the southern counties consists of limestones with
calcareous grits, the limestones being often largely composed of the
remains of reef-building corals, and a similar development of the
rocks of this series is found in Yorkshire, while a local development
of the same character is found at Upware in Cambridgeshire, though in
the other parts of the Fenland counties the Corallian is represented
by an argillaceous deposit with Corallian fossils known as the
Ampthill Clay.
The Upper Oolites have a tolerably constant base, the Kimmeridge Clay,
usually consisting of laminated bituminous argillaceous material, but
the Portlandian and Purbeckian divisions vary greatly, and are only
locally developed, though their absence in some parts of central
England is no doubt due to unconformity.
The Portlandian rocks of the south of England consist of limestones
and sandstones which pass further northward into shallower water
mechanical deposits often charged with iron hydrate, and the beds
disappear in Oxfordshire. The Purbeckian rocks of the south are also
limited as regards area of exposure: they consist of estuarine
deposits with some terrestrial accumulations of the nature of old
surface soils. Representations of the Portlandian and Purbeckian beds
are found in Lincolnshire and Yorkshire, as arenaceous deposits in the
former county and argillaceous ones in the latter. Both are marine
deposits of a northern type, developed elsewhere in northern European
and circumpolar regions, and in these counties we find a complete
passage from the Jurassic rocks through the Cretaceous rocks, but the
exact lines of demarcation between the different series of the passage
beds are difficult to define.
The foreign Jurassic rocks of Europe and of some parts of Asia
strongly resemble in general characters those which have been
described above as occurring in Britain. One of the most remarkable
features of the Jurassic rocks as a whole, is the absence of the Lias
over wide areas, the continental period which in Britain existed in
Permo-Triassic times is elsewhere frequently replaced by one of
Liassic age.
The Jurassic and Cretaceous rocks are of interest on account of the
evidence which they supply as to the existence of climatic zones in
these periods, which run fairly parallel with those at present
existing. The late Dr Neumayr in a paper already cited divides the
world during later Mesozoic times into four distinct climatic zones,
equatorial, north and south temperate and boreal zones (the
corresponding austral zone is not known owing no doubt to the
extensive sea of South Polar regions and our general ignorance of its
lands). In Europe the Mediterranean Province belongs to the equatorial
zone, the Middle European to the North temperate zone, and the Russian
or Boreal to the Boreal zone. The last-named is marked partly by
negative characters, the absence of certain Ammonite-genera and of
coral reefs being noticeable, whilst the lamellibranch _Aucella_ is
very frequent. In the North temperate zone, certain Ammonite genera as
_Aspidoceras_ and _Oppelia_ are abundant and there are also extensive
coral-reefs. The Equatorial zone is marked by the Ammonite-genera
_Phylloceras_ and _Lytoceras_ and by the _Diphya_ group of
_Terebratulæ_. It is of special interest to note that the fauna of the
South temperate bears closer relationship to that of the North
temperate than to that of the intermediate Equatorial zone.
_Jurassic floras and faunas._ The Jurassic flora is very similar in
its characters to that of the Lower Cretaceous rocks, and the two
taken together afford a decided contrast with that of later Palæozoic
times, and also with that which succeeds them in the Upper Cretaceous
rocks, which bears a marked resemblance to the existing flora. Cycads
predominate, accompanied by conifers, and a fair number of ferns and
Equisetaceæ.
The Jurassic fauna is specially noteworthy on account of the character
of the vertebrata, but some notice of the invertebrates must also be
taken. The abundance of corals in the Temperate zones has already been
pointed out, but the mollusca form the bulk of the invertebrate fauna,
lamellibranchs, gastropods and cephalopods being all abundant; of the
last-named the ammonites and belemnites contribute most largely. The
vertebrates include remains of fishes, amphibia, reptiles, birds and
mammals. The Jurassic reptilia furnish representatives of some modern
orders as the Chelonia and Crocodilia, but the most important orders
are essentially characteristic of later Mesozoic times and their
representatives abound in the Jurassic strata. These are the
Sauropterygia (including the Plesiosaurs), the Ichthyopterygia
(including the Ichthyosaurs), the Dinosauria, and the Pterosauria
commonly known as Pterodactyls. No birds have hitherto been discovered
in the British Jurassic rocks, but the Solenhofen Slate of Bavaria (of
Kimmeridgian age) has furnished the celebrated _Archæopteryx
macrura_, which is not only placed in a family but also in an order by
itself, the order Saururæ. Many remains of mammals have been extracted
from the estuarine deposits of Stonesfield, and the old surface soils
of the Purbeckian beds; representatives of the Monotremata are
furnished by the _Plagiaulacidæ_ and _Tritylodontidæ_, the former
family containing the genus _Plagiaulax_ of the Purbeck Beds and the
latter, _Stereognathus_ of the Stonesfield slate. The Marsupialia are
represented by the _Amphitheridæ_, _Spalacotheridæ_ and
_Triconodontidæ_. Some forms have been referred to the Insectivora,
but there is still disagreement concerning the correctness of this
reference.
Before dismissing the subject of the Jurassic fossils, attention may
be called to a feature which has been frequently commented upon,
namely, the general resemblance of the flora and fauna of Jurassic
times to the modern Australian fauna and flora. The explanation which
has been offered to account for this resemblance has been given in a
preceding chapter, where it was stated that Mr A. R. Wallace
considers, after review of the geological and biological evidence,
that Australia was severed from the adjoining continental lands in
Mesozoic times, and that the higher forms of life which on the larger
continents have replaced the earlier and lower forms have not
succeeded in obtaining a footing in Australia, which therefore
furnishes us with a local survival of a once widespread fauna. In
connection with this matter the actual existence of the genus
_Trigonia_ (a form peculiarly abundant in Jurassic strata and
characteristic of Mesozoic strata in Britain) in the Australian sea is
of considerable interest.[100]
[Footnote 100: A good account of the British Jurassic rocks will be
found in Mr H. B. Woodward's Memoir on "The Jurassic Rocks of
Britain." _Mem. Geol. Survey_, 1893--.]