Arkell.1956.Jurassic..

CLASSIFICATION AND CORRELATION 5
freedom from 'reform'. But they grade into formations and like them are
strictly limited geographically. In the other direction, formations can be
subdivided to any extent required.
In Britain and other European countries the formation names in
geological literature have historical associations and antiquity and a consequent
individuality which makes them useful and easy to remember.
It is not so with many formations recently introduced in some other parts
of the world. A superabundance of formation names (which by themselves
convey nothing) can choke the literature and build a formidable
barrier between writer and reader.
ZONES. Palaeontologically the old formations are often too comprehensive
and also inconsistent in content from place to place. The need
for time-planes independent of lithology and geography, so that rocks
may be correlated more satisfactorily, led to the concept of the Zone.
The hallmark of a zone is the assemblage of guide fossils, of which one
is selected as index species and gives its name to the zone. These are
supposed to have lived, for practical purposes, contemporaneously,
wherever they occur.
Like so many fundamental concepts, the zone is a subject of unending
controversy. There are many kinds of zone: faunizones based on assemblages
of fossils, biozones based on the evolutionary duration of a species,
teilzones based on the local presence of a species, and so on. (For a
full exposition see Arkell, 1933.) But over and above these considerations
there is uncertainty as to the basic concept of the zone: whether as originally
conceived by Oppel it was a stratum or bed, or a time-interval, or an
abstract combination of the two,—a hypothetical column of sediment
(probably nowhere actually existing) representing the time of duration of
the index species on the assumption of continuous sedimentation at some
unknown average rate.
British geologists have always envisaged a zone as a bed or stratum,
a tangible object accessible to the hammer, though differing lithologically
from place to place. Consequently some have thought it necessary to
construct a parallel terminology to express the time units to which the
various kinds of zones correspond. The need to keep time and rock distinct
in our thoughts is obvious, and to the extent that this elaborate terminology
has led to clarification of thought it has served a useful purpose. But
beyond that it is unnecessary. No one uses it, nor ever will.
That Oppel himself fully appreciated the time element cannot be
doubted (Schindewolf, 1950). The fact that he nowhere defined a zone,
nor made it clear whether his zones were strata or time intervals, may be
taken to mean that he visualized zones from both aspects at once. The
argument that a zone must be one or the other is sterile. A zone is
much more than a mere bed or stratum, or a formation, because it is an
abstraction and a generalization: it is in theory any bed, stratum, or
formation deposited in any part of the world during the period in which
the index fossils lived. To mention a concrete example: the Mariae Zone
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