C. CROSSLAND—-RECENT HISTORY OF THE CORAL REEFS. HD) 
In one’s mental photographs of the two coasts, the most prominent feature 
in either case is the black colour and fantastic forms of the Zanzibar coast 
rocks and the light yellow and usually formless characters of those of the 
Sudan. These are the outward visible signs of equally striking differences 
in their physical constitutions. 
In Zanzibar and British East Africa the low cliffs are of an exceedingly 
hard crystalline rock, which is so homogeneous and tenacious that the over- 
hanging shelf left by the undermining of the sea may, under favourable 
circumstances, project for more than four feet without its breaking down. 
The surface of the rock, both on the coast and inland, is, as already mentioned, 
almost black, though when broken the rock itself is seen to be white or nearly 
so. The raised coral of the Red Sea coast, on the other hand, is generally soft, 
often indeed like loose sandy gravel of shells and coral fragments with the 
larger coral colonies as embedded stones. The bases of the cliffs are somewhat 
harder ; and though their undermining results in forms which distinctly recall 
the cliffs of Hast Africa, the lack of homogeneity and cohesion of their material 
results either in a monotonous formlessness or, where somewhat harder rock 
occurs, the substitution of picturesque masses of fallen rock for the bizarre 
pillars, brackets and arches of the Hquatorial coasts. 
Although fossils are very abundant in the Zanzibar rock and are visible in 
section in the cliffs, yet the greater part of the rock near the surface is so 
homogeneous and shows so little trace of its origin from a loose conglomerate, 
that there would be great difficulty in separating such fossils as occur from 
the surrounding matrix. 
It is important to notice, however, that in places at a little depth below the 
surface, the rock becomes much less crystalline and its fossils more easily 
separable ; in short, it approaches the condition of the Red Sea rock. From 
quarries made for road metalling, Mr. T. J. Last, Slavery Commissioner in 
Zanzibar, made large collections of fossils and their casts, and a whole series 
of stages in the crystallizing process could be easily obtained. 
On the Sudan coast again the rock has the characters of that of Zanzibar 
in certain localities, but only where the conditions to which it is exposed 
approximate to those normal on the Equator; that is, to being “ *twixt wind and 
water ”’ at the sea-level in places where it is under wave-action *. 
The whole surface of Zanzibar is under such conditions owing to its heavy 
rainfall, but rainfall being practically mil on the Sudan coast we find the 
hardened crystalline rock only by the sea at water-level. Further, owing 
to the inappreciable tide, the reefs give constant protection to the cliffs’ bases, 
so that only in exceptional places does one find coral-rock freely exposed to 
waves and spray. For some reason the reef is absent from the eastern cliffs 
of the islet of Tella Tella Kebira, which are therefore almost constantly bathed 
in spray, and have taken on all the characteristics of the Zanzibar rocks, in 
* See also the preceding paper on the coast near Alexandria. 
