20) MARINE BIOLOGY OF THE SUDANESE RED SEA. 
plain. That the greater part of the reef-surface was formed by the removal 
of elevated coral is conspicuously demonstrated, in the first case mentioned, 
by the presence of numerous isolated pillars and grotesque rocks of this 
material scattered over its landward portion, in the third by the presence of 
coral-rock islands; but at Dabadib, as already mentioned, only a narrow 
band of rock surface is visible at water-level. 
Only narrow reefs are found below the comparatively high cliffs (30— 
40 feet) of the east side of the North Bay of Khor Dongonab. I attribute 
this to the reefs being sheltered from the prevailing winds, and consider it 
merely as a further indication of the importance of erosion as a factor in 
reef-formation on coral coasts. 
The surface of the shore-reefs is generally sandy, but bare rock-patches 
occur, especially as a border to the growing edge. Stones are rare or quite 
absent except at the slightly raised seaward margin, where the remains of 
dead coral colonies are sometimes scattered. Such large and solid stones as. 
those so plentifully scattered on the reef-edge in Zanzibar are, from the 
nature of the case, altogether absent from this area, which has been formed 
directly by growth of coral and nullipore *. 
The raised edge consists of a gravel of broken coral well covered with 
nullipore. Outside this is a gentle slope of corals, generally stunted and 
accompanied by nullipore and aleyonarians (Xeniide), which extends to a 
depth of a fathom or so, after which is a precipitous slope of luxuriant 
corals reaching as far as we can see with the water-telescope, say to 
10 fathoms or beyond. 
There is generally a more or less regular boat-channel or series of pools of 
water a fathom or so deep, with numerous outlets to the open sea, some of 
which provide anchorages for sambtks (or “ dhows” as they are called in 
Zanzibar ). 
In shallower water than that found generally off the shore-reefs the 
maximum depth at which coral grows luxuriantly decreases, for instance on 
the Shubuk boundary-reef, to be described later. A slope of mud with seat- 
tered corals is visible below the precipice of luxuriant growths. Similarly 
inside the harbours, where also the species of corals change, the delicately 
branched and fan-like forms, which are in the majority outside, give place to 
massive Porites, Meandrina, and so on. On this coast it is the massive 
genera which are characteristic of sheltered waters, not vice versd. 
The fringing-reef on the east side of Jebel Mayiti is also worthy of note. 
The island is a pyramidal hill, the east side being precipitous. From the 
summit one looks down upon a brown fringing-reef covered by a foot or two 
* The stones found on the raised reef-edge of the Zanzibar coasts are the harder portions 
of the rock removed during the formation of the reef, which have been preserved in the way 
described in the paper (/oc. czt.). The “negro-heads” of Pacific atolls probably have a. 
similar origin. 
