26 ; MARINE BIOLOGY OF THE SUDANESE RED SEA. 
marked contrast to the other islets in the vicinity, which have only the usual 
partial hardening of the undermined portions of their cliffs. 
In Khor Dongonab, where protection of the coast by coral is less complete, 
the foundations of the sand islets is, as already mentioned, of coral-rock, and 
at sea-level, where it is alternately wetted and dried, it has taken on the 
typical qualities of “ Coral-Rag.” In the same way, beach sandstone has in 
several cases been formed by the cementation of the sand of which the islets 
are composed, and this is as hard as the coral-rag where it overlies it, becoming 
softer and softer as it rises above the water-level till, at a height of 3 feet 
or so, it becomes of a crumbling consistence and passes into the loose sand 
trom which it is derived. (See Pl. 1. fig. 3 and its explanation.) 
The facts cited certainly warrant the conclusion that the alteration of the 
Zanzibar rock from the mass of more or less loose material, in which condition 
it was elevated from the sea, to its present crystalline state, and that the 
rock of quite similar origin in the Red Sea has remained nearly in its 
primitive condition, is due to the absence of rain and generally of wave-action 
and spray in the latter region. The details of this action of water have not 
been worked out, but probably a recrystallization of the calcium carbonate and 
a partial substitution of magnesium, or dolomitization, occurs, both of these 
processes tending to harden the remaining product. 
CONCLUSIONS. 
The conclusions to he drawn from the above may be summarized as 
follows : 
1. That the coast of the Sudan, besides its major elevations amounting 
to more than 1100 feet, has recently undergone several small eleva- 
tions, the movements having been uniform in their action and not 
recently reversed. 
2. That the differences between this elevated coral and rock of similar 
origin elsewhere, e. g. on the coast of Equatorial East Africa, is due 
mainly to the absence of tide and rain in the Red Sea. 
3. That the present form of the reefs is due as much to the eroding 
action of the sea upon this elevated rock as to the growth of corals. 
In the case of Fringing-Reefs (including the Shubuk area) the land 
is cut down to sea-level behind the rim of growing coral. Barrier- 
Reefs are formed: (1) By the direct growth of coral upon submarine 
hill ranges; the northern ends of these have been elevated, 
and are now ranges of coral-capped hills, the middle parts remain as 
peninsulas and islands, the southern as Barrier-reefs. (2) By the 
cutting down by marine erosion of promontories and islands, and of 
coral-reefs previously elevated. 
