500 Mr Grossland, The Coral Reefs of Zanzibar. 



About Muyuni the boat channel tends to be filled up by 

 blocks of stone formed by coral and nullipore growths. These 

 occur mostly on the outer third of the reef, where they form as 

 it were a giant pavement level with the surface of the water, the 

 interstices of which may be 1 or more feet wide by 2 to 4 

 deep, containing clear water and floored with clean coarse sand 

 which often but thinly covers the underlying rock. Scattered 

 blocks may occur anywhere. Some are dead, but most are living 

 round the edges, which overhang considerably. Nullipores flourish 

 exceedingly, both on the surface and under the edges, encrusting 

 and foliose forms covering every particle of non-living surface, 

 down to the dead "leaflets" of Halimeda. Various genera of coral, 

 e.g. Pocillopora, Madrepora, Pavonia and Oalaxea are found on 

 each block, though their interior appears homogeneous soft white 

 rock bored by mollusca and worms. Caulerpas and Fucaceae also 

 grow on it, giving this part of the reef a very rich appearance, 

 in marked contrast to its usual barrenness. 



Obviously if the growth of these blocks continues far enough, 

 so that the spaces between them become obliterated, a bare flat 

 surface of dead rock is again formed. A month or two later while 

 travelling near Pongwe Bay I found that this obliteration had 

 taken place. The inner third of the reef consists of old limestone 

 and slimy mud banks. When the boat channel attains a depth 

 of about three feet, scattered growing blocks occur similar to 

 those near Muyuni, except that less coral and more nullipore 

 in proportion grow on them. The smaller blocks at least, such 

 as I was able to overturn, are not attached to the substratum. 

 Further out these become more numerous and fewer are living, 

 until a continuous surface is formed, having all the appearance 

 of the ordinary reef flat of old rock, but for its softness and the 

 presence of many deep holes and channels with overhanging 

 edges. Close to the reef edge is an area of shingle — rounded by 

 the growth of nullipores — partly loose, partly cemented. All this 

 recently formed rock surface is level with that of the water in the 

 boat channel, i.e. the same as that of the reef edge. 



The Mnemba Reef has a continuous raised stony edge along 

 its east and south sides. Its surface gives instances of every- 

 thing shewn by the reefs of the main island, though no definite 

 channels occur. The west side is of sand, similar to that at 

 Jembiani but cleaner, descending gradually into several feet of 

 water, after which is a steep slope down to about 50 fathoms. 

 On the southern parts are growing blocks like those near 

 Muyuni and Pongwe, and on the north rock-pools like those off 

 Ras Michamve. 



This reef shelters a few miles of the Zanzibar shore from the 

 action of the surf, and consequently this shore is neither so broad 



