Arkell.1956.Jurassic..

6 CLASSIFICATION AND CORRELATION
is part of the Oxford Clay in the south of England and part of the Lower
Calcareous Grit in the north of England and the marls of Mount Hermon
in Syria, but we do not know whether it exists in Japan, because no
comparable fauna has been found there. Wherever it is, the Mariae
Zone is a bed or part of a formation, but this does not express its whole
entity: it transcends all local occurrences, and the factor which enables
it to do this is the time element in the concept. Anything deposited
during the critical period of time is part of the zone.
In practice zones have their limitations because for various reasons
no fossils fulfil perfectly all the requirements of zonal indices, namely,
ease of identification, combined with short vertical and universally wide
horizontal range. The best zone fossils are often the most difficult to
identify, because their short vertical range depends on subtle characters
undergoing rapid evolution, and, owing to the existence of ecological
and palaeogeographical barriers and provinces, no fossils are evenly distributed
over the whole earth. It is therefore necessary to construct a
separate zonal column for each faunal province.
The degree of refinement of the zonal scale that may be possible depends
on a number of local factors, not the least of them the lithological. Given
ideal lithology and abundance of suitable fossils, a high degree of refinement
may be achieved in a restricted area, but the more the area is enlarged
the more the refinements break down. This has obvious explanations,
which have been discussed elsewhere (Arkell, 1933, pp. 30-35). For
correlations from one part of Europe to another the smallest units of
practical value are astonishingly near the first set of zones promulgated
by Oppel. His pioneer studies of western Europe led him to a remarkably
just appraisal of the possibilities. The few last attempts to apply Buckman's
'polyhemeraF chronology across Europe {e.g. Roche, 1939) have
been conspicuously unsuccessful, and the only justification put forward
for these attempts is a repetition of Buckman's claim that in relation to the
slow speed of evolution and deposition the time required for the universal
spread of a species is negligible—like the flight of an aeroplane in relation
to the process of bricklaying, as he put it. The argument is fallacious,
since many different lineages were evolving and migrating simultaneously
and so the succession is bound to vary in different places: all did not
wait their turn while each species completed its migrations (Arkell, 1933,
p. 33, fig. 1).*
Experience has now shown that minute subdivisions, such as those made
by Buckman in the English Lias, are not recognizable even in other
European countries, but that over wide areas only the general faunal
succession such as is expressed by Oppel's zones and a few subzones can
be recognized. This, moreover, is true even for the Lias, which is by
far the most favourable part of the Jurassic for zonal subdivision and
* Roche assumes that Buckman carefully chose as hemeral indices ammonite species
which were short-lived and spread rapidly. He did not. His polyhemeral tables
comprise a completely random selection of ammonites and only a very small fraction
was based on any field experience such as could enable such a choice to be made.
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