C. CROSSLAND—RECENT HISTORY OF THE CORAL REEFS. iL) 
salt swamps, and about six feet above them are small cliffs which show very 
distinct marks of having been undermined by waves at a time when the 
lagoon was at a lower level and contained water. Above these the ground 
slopes steeply to the level of the summit of the cliffs. 
For other elevations see the postscript on the sandstone coast-hills (p. 27). 
Two sets of elevations have been shown to have occurred since reefs of 
modern corals began to be formed in the Red Sea: (i.) A major series up 
to at least 500 feet (Rawaya, Makawa, &c.). (ai.) Minor elevations which 
are very recent and are probably still going on. (Recent raised beaches and 
erosion-lines on the cliffs.) 
THe Livinc REEFs. 
Fringing Reefs. 
These reefs occur practically everywhere on the open coast and in the 
bays and “khors” except at their extreme landward ends. Their absence 
from parts of Khor Dongonab has been noted in the Narrative (p. 5). They 
are absent also from small lengths of the coast-line at Ras Baridi (noted by 
Darwin, on the east side of the sea) and from the shores of ‘“ Dokhana 
Bay” in Khor Dongonab. As in one or two similar cases in British Hast 
Africa, I can give no reasons for these omissions except in one case—the 
harbour of Dabadib, on the Sudan coast between Salak and Port Sudan. 
Here the coral-border to the coast-plain is almost entirely covered by a deposit 
of the very fine sand, derived from crystalline rocks, which at times of high 
winds causes thick “fogs” in the Red Sea. This fouls the water and pre- 
vents coral-growth in the immediate vicinity. Probably the case of ‘ Dokhana 
Bay” is the same ; the water certainly is muddy there, but I have not deter- 
mined the origin of the mud, and in some cases, e. g. Suakim Harbour and 
especially the North Bay of Khor Dongonab, coral can flourish in very cloudy 
water *. ; 
The shore-reef is broad where the coast is low: e. g., at the entrance to 
Suakim Harbour, where it is but a foot or two above sea-level, the reef is 
14 sea-miles broad, while at Port Sudan (Mersa Shékh Bariid) the coast 
is 6 feet high and the reef only ? mile wide. This difference is obviously 
due to the lesser rate of erosion where fallen rock protects the base of the 
cliffs for a time against the sea. Itis obvious that the formation of reef-flats 
by erosion is going on here justas on the Zanzibar coast, but that here it is im- 
possible to determine what proportion of the total breadth attained is due to the 
cutting-down of the land and what to the addition due to recent coral-growth. 
Ata point just north of Mersa Abu Hamama, at Dabadib, Mersa Fijab, 
and some other points, the coral-limestone has been nearly completely 
removed, the shore being formed of the rolled grey pebbles of the alluvial 
* Note added Sept. 1907.—Fresh water, flowing underground, enters the sea at Dabadib 
and “‘ Dokhana Bay” nearly all the year round, 
2% 
