498 Mr Crossland, 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 Pliaeophyceae, 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 slopes 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. 
