500 
Mr Crossland, The Coral Reefs of Zanzibar. 
About Muyuni the boat channel tends to be filled up b} T 
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 Galaxea 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 
