Arkell.1956.Jurassic..

FRENCH SOMALILAND 3°7
complex overlain by Adigrat Sandstone (missing in places) and Antalo
Limestone, which in turn are largely covered by Tertiary lavas. The
outcrops have been worked in considerable detail, but the result suffers
from scarcity of ammonites and of overall measurements. (Vinassa de
Regny, 1924, 1931; Diaz-Romero, 1931.)
As in Shoa and at the eastern extremity of British Somaliland (see
p. 312), the lowest post-Archaean strata consist of unfossiliferous sandstone
followed by thick beds of unfossiliferous dark limestone. The earliest
fossiliferous limestone is again Bathonian, yielding among other fossils
Burmirhynchia and Gryphaea costellata. The next recognizable horizon
is one with Somalirhynchia africana, which in British Somaliland is
Upper and perhaps also Lower Oxfordian (Muir-Wood, 1935, facing
p. 81; and see p. 309), and in Syria is high Upper Oxfordian (bed 56,
p. 295). The only identifiable ammonite recorded, from the topmost
division of the limestone, is referred to Idoceras rufanum Dacque
(1914, p. 4; figd. 1905, p. 147, pi. xiv, fig. 15), a species from the Lower
Kimeridgian of Harro Rufa in the Harrar Province. A second doubtful
ammonite from the same level is compared to Anavirgatites subambiguus
Spath, which would correlate with the Gawan Limestone of British
Somaliland (see p. 308).
FRENCH SOMALILAND
The only Jurassic outcrop in French Somaliland is in the centre of a
dome at Ali Sabieh, 45 miles SW. of Jibouti, where the upper part only
of the limestones rises through the surrounding lavas. The lowest formation
exposed is 15 m. of thin-bedded limestones with crushed ammonites;
next come 200 m. or more of hard, compact, poorly-fossiliferous limestones;
then a varied shelly series (thickness not stated) with many pelecypods
and gastropods. The limestones finally pass up gradually and with
perfect conformity into sandstones, at least 300 m. thick and unfossiliferous.
(Dreyfuss, 1931.)
Owing to poor preservation, the three figured fragments of Perisphinctids
from the bottom of the series are inadequate for dating, and
judgment must be reserved. Plate xiii, figs. 1 and 2 of Dreyfuss' memoir
are indeterminable and could be many other forms besides those to which
he assigns them, while fig. 3 is certainly not Katroliceras pottingeri
(Sowerby), though it may be of much the same date. A more suitable
identification for both figs. 2 and 3 might be Aulacosphinctoides cf.
kachhensis Spath (1931, Cutch, p. 528, pi. lxxxiii, fig. 2), a species of the
Middle and Upper Katrol Beds (Middle or Upper Kimeridgian) of Cutch.
This would make the base of the Ali Sabieh section equivalent to some
part of the Daghani Shales of British Somaliland (see p. 308).
From the gradual upward passage of the top of the limestone, the very
highest layers containing Exogyra bruntrutana, into sandstones, Dreyfuss
insists that at least the lower part of the sandstones must be Jurassic
(he suggests Portlandian); but the Jurassic Purbeck Beds pass up by
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