Arkell.1956.Jurassic..

LOWER JURASSIC—
CLASSIFICATION AND CORRELATION 11
STAGES ZONES
TOARCIAN . . -|
PLIENSBACHIAN . . -
SINEMURIAN . . •
HETTANGIAN . . |
TABLE I—continued
Lytoceras jurense
Hildoceras bifrons
Harpoceras falcifer
Dactylioceras tenuicostatum
Pleuroceras spinatum
Amaltheus margaritatus
Prodactylioceras davoei
Tragophylloceras ibex
Uptonia jamesoni
Echioceras raricostatum
Oxynoticeras oxynotum
Asteroceras obtusum
Euasteroceras turneri
Arnioceras semicostatum
Arietites bucklandi
Schlotheimia angulata
Psiloceras planorbis
The possibility of describing and analysing a geological system as a
whole, all over the world, depends primarily on availability of a single
universal language for use in classification. This language the stages
provide. Their great value for this purpose is impaired if different
countries introduce their own scheme of stages. Those recently proposed,
for instance, for New Zealand, are not true stages, since they are
not definable in terms of zones and are not applicable outside New Zealand.
They are in reality Series, or groups of formations. An independent scale
of classification for the Jurassic rocks of New Zealand was a necessity and
these names will no doubt be invaluable as a basis for further work in that
country; but in this book the terminations -ian, -an will be reserved for
stages in the old sense, which can be defined palaeontologically and used
virtually in any part of the world.
Those geologists who like to keep time terms separate from rock terms
consider a stage a rock term and an age its equivalent time-term. The
term age, however, has also been used (especially by Buckman) for the
time of dominance of a particular ammonite genus or family. These
latter ages are smaller subdivisions than stages and are in fact more nearly
equivalent to zones. The old classical species of the mid-nineteenth
century, which were selected by Oppel as index fossils for his zones, have
for the most part now become genera, with the result that the dominant
species of Oppel's day is much the same as the dominant genus of to-day.
It results that Buckman's ammonite 'ages' (tabulated in Arkell, 1933,
p. 24) are more or less reflections of Oppel's zones. They suffer from all
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