Arkell.1956.Jurassic..

286 THE MIDDLE EAST
date (Seward, 1935). In many places the sandstone contains abundant
silicified wood and tree trunks, which may have been drifted long distances,
but leaves as perfect as those from the Red Sea Hills figured by Seward
can hardly have been drifted out of one climatic zone into another; and
their growth must have taken place in a humid tropical climate (Seward,
1935, p. 5). On the other hand, near El Fasher, in eastern Darfur, other
plants were found which turned out to be xerophytic, probably strand
dune dwellers. They are of world-wide distribution and of either Lower
Cretaceous or early Upper Cretaceous date (Edwards, 1926).
The bedding is agreed by all recent observers to indicate deposition in
water, and the beds might best be described as fluvio-marine, though perhaps
in large part lacustrine in the south. The grains are usually subangular,
but Sandford (1935) points out that the shales, mudstones and clays
sometimes contain rounded and polished sand grains, or lenticles of white
sandstone composed of such grains, indicating wind transport and drift
sand not far away. He also figures (1935, pp. 345-6) from SW. of Kharga
lenticles up to 16 ft. thick of 'clayey sandstone and white and purple
sandstone' breccia, which he interprets as 'scree from a buried rock-face',
but which it might be suggested could be desiccation breccias due to
temporary drying and flooding of braided distributaries on deltas or
sandflats.
A remarkable feature is the absence of polygenetic basal conglomerate
except in a very few places, although the sandstones everywhere rest on an
almost level surface of granite and other crystalline rocks (Andrew, 1937).
Shukri and Said have shown that the mineral composition of the sandstone
is equally unexpected: it consists overwhelmingly of quartz, but the other
minerals present are not such as could have been derived direct from the
underlying basement complex. Rather it appears that the crystalline
complex was peneplaned before deposition of the Nubian Sandstone,
perhaps in Palaeozoic times, and that the materials of the Cretaceous
sandstones were derived from the destruction of their Palaeozoic predecessors.
This idea receives support from the intimate mixture of fresh
and decayed grains. Locally, however, quantities of pebbles of assorted
igneous and metamorphic rocks are found in isolated pebble beds (Arkell,
i95i)-
Shukri & Said, like Hume, infer deposition in a shallow sea on the fringe
of a sinking peneplain. That the sea at least occasionally invaded the
area is shown by the single Inoceramus far in the interior of the massif,
at Aswan, and by the ironshot oolite bands there and round Wadi Haifa.
But as Beadnell and others have pointed out, a marine origin for the whole
series is unconvincing. At the present time, as through all previous
geological periods down to the Cambrian, marine life teems in any sea in
these latitudes, however shallow, and there seems no reason why it should
not have left its remains in the Nubian Sandstone.
Andrew (1937) has rightly concluded that all Nubian Sandstone south
of 28° 40' N., i.e. south of St Paul Monastery, is Upper (and Middle)
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