498 Mr Grossland, The Coral Reefs of Zanzibar. 



the hard nodule may break underfoot, soft gritty sand being found 

 underneath. They never cover more than half the area of the 

 upstanding stones, where they grow so thinly that all the mark- 

 ings and roughness of the stone are visible through them as 

 through a thin coat of paint. The rest of the stone is quite 

 bare, having not even a covering of brown or green alga. The 

 colour of the nullipores here is dull and unhealthy looking in 

 contrast with the bright, clean appearance they have in certain 

 places immediately under the cliffs and on the rocks of recent 

 growth found in the boat channel. In any case the stones are 

 partially exposed to erosion and must in course of time disappear. 

 The addition made to the reef by nullipores is thus negligible ; 

 the reef edge is not growing but is formed of the same dead rock 

 as forms the greater part of the island. 



The colour of the outer slope is generally brown, from the 

 presence of filamentous Phaeophyceae, or sometimes green pre- 

 dominates owing to the presence of scattered dwarf clumps of 

 Enteromorpha. These algae protect the surface of the rock from 

 further erosion by holding a layer of sand, matted together by 

 their basal parts, against the rocks. 



The Boat Channel. 



There are but few depressions in the reef edge, so that at low 

 tide water is held on the reef at a higher level than that of the 

 water outside, causing strong tidal currents to run over the reef- 

 flat to the openings. A channel has thus been hollowed out, 

 which may be over 6 feet deep near an opening, but at a dis- 

 tance of a few miles dwindles and finally disappears. One such 

 channel commences near Ras Mkunduchi, attains a depth of 6 

 feet at ordinary low tides opposite Jembiani, and empties at the 

 point marked by a small arrow. Another commences at A and 

 empties to the north of Ras Nungwe, and similarly along the 

 whole reef. 



Those openings between the channel and the sea which I saw 

 at the points D and E were due to a collapse of the surface 

 layers of rock, their surrounding slope's being cracked exactly as is 

 the ice on a pond from which the water has been run off. 



The rock flat has been eroded over the greater part of its 

 breadth, though to a depth of 6 feet or more only near its outer 

 side. But owing to deposition of fine and coarse sand, and to 

 coral growths, the actual channel is usually much narrower. 



Fine sand occurs in great quantity off Paje and Jembiani, 

 forming the beach and the surface of the inner two-thirds of the 

 reef. Through this winds the river-like channel of muddy water. 



