22 MARINE BIOLOGY OF THE SUDANESE BED SEA. 
middle part of the Red Sea in order to explain the occurrence of barrier-reefs 
on this coast. The absence of the regular barriers in the southern section is 
not a consequence of elevation; for the evidence is distinct that the recent 
movements have been the same here as in the north, but is rather correlated 
with the different slope of the sea-bottom, and probably, as will be explained 
later, with the structure of the sides of this part of the rift-valley. 
That the latest movement has been one of elevation is shown by the 
character of the erosion-marks left on the cliffs, which include details which 
would be comparatively soon weathered away from so soft a rock, especially 
when one considers the important part played by sand-erosion on this desert 
coast. The very recent character of the fossils contained in the cliffs is 
evidence in the same direction. 
The present form of most of these reefs is the result of the same processes 
as have formed those of Zanzibar and Hast Africa*, where the forces of 
erosion, with the aid of organic growth insufficient to add to the bulk of the 
material, have carved out all the characteristic forms of coral reefs. In the 
Red Sea, coral-growth is still adding greatly to the bulk of the reefs, but that 
the foundations of the present reefs are due to erosion the examination of the 
physical geography of this coast proves conclusively. 
Ehrenberg, over seventy years ago, stated that the foundation of the Red 
Sea reefs originated in this way, but here the problem is more complicated 
than on the Equatorial coasts by the impossibility of distinguishing between 
that area of a reef which is due to the erosion of elevated coral-rock and that 
which has been added by recent growth, except in the case of some of the 
Khor Dongola (Dongonab) reefs, where such additions have not completely 
concealed the original foundations. 
At first sight it seems most probable that these last-mentioned reefs are 
examples of the mode of formation suggested by Darwin in the cases of the 
Farsan and Dahlak archipelagoes, viz. by the collection of sediment on an 
uneven bottom from which coral-growth took its rise. The Dongola Barrier 
(map, fig. 2, p. 15), indeed, seems an obvious case of this occurrence, the 
bank continuing awash for a long distance to the west of the sand islands, 
and the bottom in the neighbourhood being sandy with no very considerable 
masses of coral or nullipore growth. But a closer examination shows that 
this explanation, seemingly so plausible, is not correct, and that, in fact, the 
bank has a rock foundation merely overlaid by the sandbank and such coral- 
* C. Crossland, ‘The Coral Reefs of Zanzibar,” Proc. Camb. Phil. Soc. xi. p. 493; and 
“The Reefs of Pemba and the East African mainland,” /. c. xii, 1902, p. 35. 
+ I confine my quotation to Darwin, as he is the only author who has described the reefs 
of this part of the Red Sea in general terms. The wider problem has been attacked by 
numerous workers. 
For an epitome of recent views see J. Stanley Gardinev’s paper “ The Formation of Coral 
Reefs,” in ‘ Nature,’ Feb. 18th, 1904. 
