Mr Crossland, The Coral Reefs of Zanzibar. 497 



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 beyond 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 Islajids, p. 136. 



3 Gardiner, loc. cit.; and The Formation of the Maldives, Geocj. Journal, March, 

 1902. 



4 The African pilot. 



5 Funafuti, Rotuma, and Fiji, loc. cit. 



