Arkell.1956.Jurassic..

420 JAPAN AND KOREA
radiolarites of deeper-water facies and devoid of recognizable fossils.
Except perhaps for this outer coastal fringe in the south-west and its
probable continuation in Hokkaido, the whole of the Japanese islands in
the Jurassic occupied a marginal belt oscillating between marine and
continental sedimentation. The region is of special importance for
dating the Mesozoic floras of continental Asia, since in Japan it is possible
to establish their relationships to marine horizons with ammonites.
Extensive heterochronous homotaxy is thus revealed. For instance,
the so-called Rhaetic-Lias flora of Asia and Europe, originally dated in
western Eurasia, occurs in Japan beneath marine beds with Entomonotis
ochotica and is of Norian age. The inference is that this flora migrated
westward during the Triassic epoch (Kobayashi, 1938a, 19386, 1939a,
1942).
Shales and sandstones like those in Japan and probably in part Jurassic
occur on both sides of the axial mountain range of Formosa (Kanehara,
1926, p. 33), while in the central range of Hokkaido there are radiolarites
of Lower Cretaceous and in part Jurassic age (Kobayashi, 1944, p. 237;
Hashimoto, 1952).
In the following description the Japanese Jurassic is illustrated by
successions first in the inner geosyncline (Nagato, northern Kyushu,
Korea, central Honshu), then in the outer (Torinosu Series of the Pacific
coast), and finally in the isolated outcrops of north-eastern Honshu.
NAGATO PROVINCE (YAMAGUCHI PREFECTURE)
In the peninsula of Nagato, forming the western tip of the main island
of Honshu, the Jurassic is represented by a group of sandstones and shales,
up to 900 m. thick, with a polygenetic basal conglomerate, resting unconformably
on Palaeozoic phyllites. It was called by Yabe the Toyora
Group and has been described by Kobayashi (1926) and carefully revised
by Matsumoto and Ono (1947). (See also Matsumoto, 1949.) From some
part of the group eight Toarcian ammonites—Dactylioceras, Peronoceras,
Harpoceras and Hildoceras—were figured and correctly dated by Yokoyama
(1904). The revisers have much reduced the original estimates of thickness
and have proved palaeontologically that almost the whole 900 m. are
Liassic and the highest 570 m. or so Toarcian with doubtful Lower
Bajocian. Both in facies and in thickness this development is comparable
with the Caucasus (p. 363).
I am much indebted to Professor T. Matsumoto for sending me on
loan the type specimens figured in his and Mr Ono's important paper,
and to Mr J. R. McEwan of King's College, Cambridge, for translating
the paper from the Japanese. The following is a tabulated summary,
substituting my own zonal attributions and incorporating some emendations
to nomenclature and revised determinations. The principal changes
are: (1) Their 'Echioceras Zone' is in my opinion based, not on Echioceras,
but on the homoeomorphous Fontanelliceras Fucini, a genus found in the
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