Bh 2 MARINE BIOLOGY OF THE SUDANESE RED SHA.. 
reefs at rare intervals, their presence or absence being only inferred through 
the direction and size of the waves met with. 
Khor Dongonab again gives the clue to the formation of this channel. 
Although Rawaya is formed entirely of coral, gypsum and sandstone, in 
the way I described in 1905, a few fragments of black stone from the 
Archean hills of the mainland are to be found at intervals on the surface 
of the coral, and though these are so few as to occur singly they are too 
numerous to have been brought over by human agency, and are conclusive 
proof that Rawaya was once continuous with the maritime plain of the 
mainland. Khor Dongonab therefore is a very recent fault depression ; 
Rawaya, and with it the whole barrier system, having been torn away from 
the mainland, leaving a rift which the sea has filled. 
Age of the Maritime Plain.—The ages of these pebbles from the Archean 
hills to the coral formations are of great interest as indications of the relative 
ages of the maritime plain, the reef and the earth movements which have 
made the peculiar features of the present coastline. It must be remembered 
that the corals and shells which compose these limestones are all identical 
with living forms. Asa general rule the band of raised coral which now 
forms the coastline is at a somewhat higher level than the strip of finer 
alluvium which separates it from the gravel of the main mass of the plain. 
The arrangement is, indeed, like that described at Ras Shalak, but on a much 
smaller scale. Where, however, gravel and coral meet, the former overlies 
the latter. The most striking case is found in the cliffs of the innermost 
branch of Khor Shinab (see plan, fig. 2 on p. 268, and Map 1, PI. 28), 
where coral cliff passes into oravel without the least change in the outline 
of the Khor, and even without alteration of the shape of the cliff other than 
that due to the insolubility of the gravel. Nothing could more strikingly 
show that both alluvial plain and coral reefs were fully formed before the 
earth movements occurred which formed the present features of the coast, 
and that these affected both formations equally whenever both occurred 
within the sphere of their action. It proves also that the rest of the 
maritime plain was fully formed at that time, its later extension seawards 
having been insignificant, whatever may have been added to its landward 
slopes. Not only so, but the existence of beds of pebbles, a few feet or a 
few inches thick, below the coral in the ravine of Jebel Abu Shagara carries 
back the history of the maritime pla much further. They indicate the 
existence of a maritime plain and earth movements, which, compared with 
those which we are considering, are of a very old date, though in geological 
time still recent. 
A portion of this older maritime plain has become isolated, and so 
preserved distinct, as a low hill about 100 feet high two miles south of 
Dongonab village, and its continuation northwards as a gravel ridge 10 to 
