Arkell.1956.Jurassic..

312 EAST AFRICA
prohexagonum Spath not in situ but probably from here and comparable
with a Bathonian (Stonesfield Slate) species. Eligmus rollandi Douville
and E. weiri Cox abundant, Gryphaea costellata Douville abundant,
also the brachiopod genera Burmirhynchia, Cererithyris, Charltonithyris,
etc. (Muir-Wood, 1935, facing p. 79.)
UNDATED
Adigrat Sandstone, 152 m. at Bihendula. Yellow and grey gritty
current-bedded sandstones. No fossils.
The brachiopods of the Lower Bihen Limestone are said to be of
Callovian appearance. Unless the Eligmus rollandi-Gryphaea costellata
fauna passes up from the Upper Bathonian to the Callovian southwards,
however, the age of these beds is Upper Bathonian by analogy with Sinai,
Palestine, Syria and Arabia. The evidence from Jebel Tuwaiq strongly
suggests that they are not Callovian. There is no reliable evidence for
the presence of anything earlier than Upper Bathonian.
It is possible that the boundary between the Bathonian and Middle
Callovian or Oxfordian does not coincide with the line of division between
Lower and Upper Bihen Limestone drawn by Macfadyen, but falls lower
than 50 m. above the base.
Correlation of the ammonite fauna of the Gawan Limestone with the
standard Jurassic sequence still remains an open problem. The Anavirgatites
and Sublithacoceras spp. identified by Spath with species of the
Neuburg beds figured by Schneid were first referred by Spath (1925)
to the Upper Kimeridgian. Later, however, (1935, p. 208) he regarded
this fauna as post-Portlandian. The evidence for this is questionable,
and since in the Daghani section the Anavirgatites and Sublithacoceras
are only 80-100 m. above the Middle Kimeridgian and 144 metres below
the top of the Jurassic limestones, there is more room for Portlandian and
later faunas above than below them. Moreover, Sublithacoceras bears an
extremely close resemblance to Pectinatites. We do not, however, commit
ourselves either way by classing the Gawan Limestone as Tithonian, using
that term in the historically correct sense, to mean the southern European
equivalents of Upper Kimeridgian, Portlandian and Purbeckian, or any
additional stages there may be between or above those and below the
Cretaceous. (See Arkell, 1946, p. 21.)
Various other isolated outcrops occur farther east (Macfadyen, 1933,
pp. 30-31) but have not been studied in detail, though they contributed
material used in the palaeontological symposium of 1935. The most
easterly is at the extremity of British Somaliland, near the boundary with
Italian Somaliland, in the precipitous scarp of the Al Hills, south of Alayu
and Bunda Ziada (Barrington Brown, 1931, p. 263 and pi. xxiii). Resting
on the crystalline complex at the base are 90 m. of sandstones and grits
with a shale band, followed by 180 m. of unfossiliferous limestone stringed
with quartz grains and including two thick bands of sandstone; these
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