G86 REPORT— 1903. 



continuous growtli of corals and nullipores. AVhen examined in detail tbeir structure 

 negatives this conclusion, and shows them to have been carved out by the sea 

 from the dead mass of crystalline coral limestone whose upheaval formed the 

 island of Zanzibar and the adjacent coasts of the mainland. Growing corals are 

 almost totally absent from the reef edge, and nullipores do no more than thinly 

 cover at most half its surface. Further, stones of exactly the properties of the 

 rock of the cliffs occur on the reef edge, and their presence in this situation is 

 inexplicable, except by the supposition that they are the hardest remnants of the 

 mass of rock removed by the sea during the formation of the reefs. 



In a few places corals and nullipores flourish in the boat channel either as 

 cylinders of Porites or as irregular blocks formed by the co-operation of several 

 coral genera with a vigorous growth of nullipores, the appearance of the whole 

 block being ■wonderfully rich and beautiful. In one place the growth of these 

 has continued until their coalescence has produced a continuous surface in appear- 

 ance like that of the old reef rock, but distinguishable from that by its softness 

 and appearance when broken. 



The island of Pemba is very similar in structure to that of Zanzibar, and it has 

 along its eastern coast a fringing reef, but very much naiTower, and its boat 

 channel is rudimentary. The reef edge is similar to that of Zanzibar, except in its 

 flora and fauna, which are strikingly difierent. Instead of the areas being 

 covered by brown filamentous seaweeds, there is on the Pemba reef edge a 

 vigorous growth of nullipores which cover every available space. Stunted corals, 

 Alcyonaria of various genera are also abundant. Even if nothing is being added to 

 the mass of the reef by these organisms, their presence affords a perfect protection 

 from wave action to the underlying rock. 



The theory of the origin of these reefs by the erosion and solution of elevated 

 coralline limestone is corroborated by the state of the shore where it is sheltered 

 from the surf by the outlying Mnemba reef. The definite reef form is lost, the 

 shore not only becoming narrow, but consisting of irregular patches of sand and 

 rock as in the shores of temperate seas. 



The Mnemba reef itself bears a proportionately tiny sand islet, interesting as 

 having been in the past a rendezvous for pirates, as evidenced by a most carefully 

 built well, every stone of which must have been imported from the reef edge three 

 miles away or from Zanzibar. It is quite certain that no natives of this district, 

 in which a stone house is a rarity, would ever dream of such exertions as this 

 structure must have required, especially for the water supply of an island now 

 inhabited but temporarily by octopus fishers. 



The pear-shaped reef has a raised edge like that of the reefs of Zanzibar along 

 its exposed eastern and southern sides, within which is a succession of pools and 

 channels just passable afoot at low spring tides. 



If this reef had been situated in the open ocean, so that with the change of the 

 monsoons every side would be exposed to the surf, it would have a raised 

 edge all round, the centre becoming a shallow lagoon. Thus would be produced a 

 typical atoll formed, not by growth of organisms in situ, but by the solvent and 

 eroding action of the sea upon a mass of upheaved long-dead coral limestone. 



Other reefs which have assumed the atoll form by the action of the .same 

 forces are found in every stage of formation in the immediate neighbourhood 

 of Zanzibar town and elsewliere. The first stage, that of an island of rock 

 standing on a flat much larger than itself, is exemplified by Prison Island. 

 The original purpose of the building upon this is explained by the name, but the 

 prisoners petitioned so earnestly against their removal to the island, lest they 

 should die of cold, that it was never used for its original purpose, and has become 

 one of the segregation houses of the quarantine station. Obviously a continuation 

 of this process of erosion will result in the total removal of the island, leaving a 

 rock flat level with the surface of the water at low spring tides. The edges of 

 this are protected from further destruction by the growth upon them of organisms, 

 including in many of these cases vigorously-growing corals. The central parts 

 are, however, removed by solution, forming a miniature lagoon as shown in the 

 charts exhibited. 



