C. CROSSLAND—RECENT HISTORY OF THE CORAL REEFS. 23 
growths as occur *. In places, a strip of hard coral-rock showing sections of 
the contained corals and shells is left bare about water-level, below the sand 
of which the islets are composed, and this is true of most of the other sand 
islets in Khor Dongola. 
I have seen no reefs of any importance, on the Sudan coast at least, which 
could have had banks of sediment, transported and deposited by currents, 
for their foundations. On a smaller scale, however, possible examples are 
those bounding the fine harbour of Mohamed Ghul, several square miles of 
reefs in the neighbourhood of the village of Dongola, and many of those 
described in the Shubuk area in the south. Such banks are only to be found 
in comparatively sheltered situations, never in the open sea around the Barrier 
system. 
Below water-level a careful examination of the bottom shows, in many 
places, bare rock-flats usually thinly coated with mud. It is thus shown that 
the Barrier and other reefs of Khor Dongola have been merely carved out 
of the rock by the action of the sea. 
The real Barrier system is even more easily seen to be formed by erosion. 
The fact that the reefs between Makawa Island and the peninsula of 
Rawaya are the remains of a former land-connection between the two is 
almost obvious, especially as a remnant of this lost land remains as an isolated 
pillar of coral-limestone, 8 feet high, on “St. Fillan’s Reef,” nearly midway 
between the two. Although Makawa Island is high, about 500 feet will not 
be far from the mark, its southern end is little above sea-level. Similarly, 
Rawaya, though hills up to 200 feet in height are present, has considerable 
areas which must in course of time be reduced to reefs, so low are they. The 
neck joining it to the mainland is particularly low and narrow, so that 
Rawaya must soon become a chain of limestone islands connected by a com- 
plicated reef system indistinguishable from that now connecting it to Makawa 
Island. Similarly the Barrier system to the south of Makawa, perhaps as far 
as the Tiflah Islands, is a continuation of the Rawaya-Makawa hills, either as 
eroded remnants of the range or as submarine hills not yet elevated above 
sea-level. The latter must be true of the southern reets, in which case the 
most likely postulate to account both for their growth and for the existence of 
other ranges of coral-capped hills which are parallel to the sides of the rift- 
valley, is that the faulting which produced it resulted in the sides being in the 
form of a series of steps or parallel ridges, upon which the remains of organisms 
would accumulate far more rapidly than on the intervening troughs or flats, 
so raising the ridges until corals, finally of reef-building species, could carry 
the ridge up to the surface comparatively rapidly ft. 
* From Mr. Stanley Gardiner’s last report (in ‘ Nature’) on the Percy Sladen Expedition 
in H.M.S. ‘Sealark’ (January 1906) it appears that the fringing-reefs of the Seychelles are 
another case where recent coral merely coats over older rock. 
+ J. Stanley Gardiner, “‘The Building of Atolls,” Proc. Camb. Phil. Soc. 1902. 
