Arkell.1956.Jurassic..

8 CLASSIFICATION AND CORRELATION
From the Hettangian up to the Middle Kimeridgian these stages can
be recognized all over the world, but after that the scheme breaks down
owing to regional differentiation of faunas. The terms Portlandian and
Purbeckian are applicable only to NW. Europe, except that Purbeckian
faunas extend to Savoy and Portlandian to Greenland and perhaps central
Russia. In Russia and the Arctic there is otherwise a completely separate
ammonite fauna, for which the stage-name Volgian has to be used. Over
all the rest of the world the presumed equivalents of these late Jurassic
stages have nothing in common with either NW. Europe or Russia and
are known as the Tithonian.
Vast numbers of other stages have been proposed by different authors
but their very numbers proclaim their futility.* Some synonyms die
harder than others, because locally they may seem more reasonable and
more practical than the universal terms. Examples are the Aalenian and
Domerian (on which I have conducted a lengthy correspondence with
colleagues in my own and other countries before deciding not to use
them); but on a world view these are no more necessary than the hundred
or more other stage names available.
The principles, or rules, on which the names now used have been
chosen have been explained at length elsewhere (Arkell, 1946). They
are a compromise between priority, suitability and usage. On the whole
these self-imposed rules have been adhered to, but a few decisions then
made have had to be reconsidered in the light of seven years' further
experience. Purbeckian has been reinstated as a separate stage instead
of merging it in the Portlandian as advocated by Haug, a procedure likely
to cause confusion; and Berriasian has been adopted for the lowest stage
of the Cretaceous, in conformity with almost universal modern usage.
Although Oppel's Tithonian is not named after a place, it is too late to
abolish it after a hundred years of continuous use.
In the above table the Lower, Middle and Upper Jurassic are constituted
as nearly as practicable in accordance with von Buch's original
arrangement in his classic paper (1839). Von Buch's scheme has priority
and produces a better balance than the more usual French and English
custom of regarding the Callovian as Upper Jurassic. Even this arrangement
is not quite the same as von Buch's, for he included in the Middle
Jurassic the condensed Lower Oxfordian of the Swabian Jura, which is
often a mere nodule bed (Oppel's Biarmatum Zone) at the top of the Brown
Jura (Zeta); and he was followed by Quenstedt. However, it is undesirable
to split the Oxfordian stage between the Middle and Upper Jurassic, and
since it is always regarded as Upper Jurassic in regions where it is more
fully developed, a small departure from von Buch's pioneer scheme seems
necessary.
* To the 120 listed by me in 1933 (p. 617) can be added : Alpinian (de Gregorio,
1885), Ardescian (Toucas, 1890), Dubisian (Gardet, 1942), Nevisian (Rozycki, 1948),
Suebian (Hennig, 1943), Wetlianian (Ilovaisky & Florensky, 1941). We have even
reached the stage of unconscious homonyms : for Dubisian Gardet, 1942, is applied
to quite a different part of the stratigraphical column from Dubisian Desor, 1859.
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