Arkell.1956.Jurassic..

BRITISH SOMALILAND 313
together presumably represent the Adigrat Sandstone and Abai Beds
of Shoa (p. 306). At Haurartiro a little to the west (longitude 48 0
43' E.) geologists of the Shell Company collected Bouleiceras arabicum
Arkell and fragments of other species from near the base of the Jurassic;
these were sent me in 1953 through the kindness of Dr P. E. Kent and
the late Mr E. J. White.
The base of the Bihen Limestone in the Al Hills is marked by the
Eligmus rollandi-Gryphaea costellata fauna, including these and many
other pelecypods and brachiopods, in a 7 m. band of grey limestone.
The total thickness of Jurassic limestones from the base of the Eligmus
bed upwards is 290 m.—not much more than a quarter of the thickness at
Bihendula. At the top, with perfect conformity, follows a thin grey marl
with Barremian ammonites. The Nubian Sandstone of more westerly
outcrops has here passed into limestones, and from the basal marl band
have been identified the Barremian ammonites Holcodiscus cf. gastaldianus
(d'Orb.), H. cf. caillaudianus (d'Orb.), Pseudothurmannia ? sp., Procheloniceras
sp. (Spath, 1935, p. 214). In view of this some of the underlying
limestones classed as Jurassic may belong to the earlier stages of the
Neocomian.
Farther east along the south coast of the Gulf of Aden, Stefanini's
map (1933) shows five more inliers of Jurassic limestone and Adigrat
Sandstone extending almost to Cape Guardafui. One of the last, at Ras
Hantara, shows 300 m. of Jurassic limestones, with Somalirhynchia
africana near the base, resting on sandstones and they in turn on granite
(Stefanini, 1932, p. 48). On the islands of Abd el Kuri and Socotra, on
the other hand, the Cretaceous rests directly on the crystalline basement
complex (Kossmat, 1907), as at Dhofar on the opposite coast of Arabia
(Lees, 1928, pi. Ii). The thinning of the Jurassic from Bihendula to the
Al Hills, the overlap of the Lower Kimeridgian on to the pre-Cambrian
near Makalla, and this final overlap by the Cretaceous, indicates that we
have crossed the eastern margin of the Trans-Erythraean trough on to a
separate massif (Arabo-Somali massif of Picard). Similar overlap of
Nubian Sandstones on to pre-Cambrian takes place southward throughout
British Somaliland, where no Jurassic has been found between these formations
on the southern plateau east of longitude 43 0
E. (Macfadyen, 1933,
p. 29).
THE SOUTH ABYSSINIAN PLATEAU (HARRAR)
About latitude 43° E., in western British Somaliland, the buried coast
of the massif turns south and then SE., and a broad sheet of Jurassic
limestones spreads over the plateau south of the funnel of the Rift valley,
across central Ethiopia to the valley of the Juba River. In the central
region, around and south of Harrar, outcrops cover an extensive area in
the basin of the upper Webi (or Ouabi) Shebeli River, of which numerous
tributaries have cleared away the Cretaceous rocks and the overspreading
lavas. Travellers of several nations have brought collections from different
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