497 
Mr Crossland, The Coral Reefs of Zanzibar. 
tide level is about 10 yards, forming a regular slope, the corre- 
sponding slope to the boat channel being rather steeper. The 
outline of the reef edge is straight, in marked contrast to the 
irregularities and fissures characteristic of all growing reefs as 
described by Darwin 1 , Dana 2 , and Gardiner 3 , and of the small 
ones in Zanzibar channel and off certain islands near the African 
mainland. 
Beyond the breakers is a steep descent (Admiral Wharton 4 
speaks of it as “ wall-like”) into from 8 to 30 fathoms, the 
latter depth being generally found within three-quarters of a mile 
of the edge. Though these depths are comparatively small, the 
existence of blue water close to the reef edge forms a striking 
contrast to the west coast. 
Loose stones, up to 4 feet in diameter, formed of hard lime- 
stone absolutely similar to that which forms the cliffs may be 
present. The dotted line on the reef edge shews where they 
occur. If present they are numerous, especially on the highest 
point, though never covering the surface, and they occur on both 
slopes. They may be entirely absent over miles of the edge as 
indicated on the chart. They are never found in the boat channel 
or on the reef flat. 
The surface of the edge is soft, even crushing underfoot 
sometimes. When broken it gives the impression of a shallow 
layer of rounded stones held together by a soft, gritty cement. At 
a depth of a few inches is hard rock similar to that of the cliffs. 
A similar edge is found on the east and south sides of the 
Mnemba reef, but not on the sheltered western side, nor on the 
shore of the adjacent mainland. 
Coral is practically absent, entirely so from the outer slope 
even among the breakers, and bejmnd to a depth of at least 
5 fathoms are bare rock (on which grows a marine phanerogamous 
plant) and sand. I only saw clumps of a single species of coral 
at one point ( B ), two or three freshly dead fragments on the south 
end of the Mnemba reef, these being the only traces of coral 
living outside the reef edge. 
Gardiner 5 having shewn the great importance of nullipores 
(. Lithothamnion ) on reefs, I observed carefully the extent of their 
occurrence here. Though encrusting forms are frequent on the 
reef edge they are far from covering its surface, and never form 
a thickness of more than a quarter of an inch, and even then 
1 Darwin, the Keeling Atoll, Coral Reefs, p. 2. 
2 Dana, Corals and Coral Islands, p. 136. 
3 Gardiner, loc. cit.; and The Formation of the Maldives, Geog. Journal, March, 
1902. 
4 The African pilot. 
5 Funafuti, Rotuma, and Fiji, loc. cit. 
