495 
Mr Crossland, The Coral Reefs of Zanzibar. 
Zanzibar is composed of (a) hard coral limestone, ( b ) white or 
yellow chalky deposits, (c) sand, (d) red earth. 
A very soft sandstone is found occasionally, and harder beach 
sandstone occurs at a few places on the coast, and in particular 
near the town 1 . On the east this formation is insignificant. 
The red earth is undoubtedly formed in the same way as in 
all other cases of its occurrence, viz. by the disintegration of coral 
rock, and the chalky deposits probably have a similar origin. 
The bulk of these rocks is confined to the western and central 
areas. Coral limestone forms the mass of the island, including 
the whole of the eastern side. It also forms the nucleus of the 
sandbanks and reefs of the channel, e.g. Mwamba Bawe and 
Pange, which were doubtless formed by the removal of islands 
of this rock. The present small islands standing on large rock 
flats, such as Prison Island, Kebandiko and Chapani represent 
stages in island formation. 
The distribution of cliffs of this rock on the coast is shewn 
by a black line in the appended map, the thinner shore line re- 
presenting a sandy beach. In the latter regions remains of coral- 
rock cliffs occur frequently as a line of pinnacles between the 
beach and the reef flat, or as a line of undermined cliff more or 
less buried in the sand of the beach. At Muyuni the little 
village with its grove of coconut palms is situated on such an 
accumulation, and the top of the cliff is traceable for half a mile 
behind the village, emerging from the sand at either end. No 
such definite remains occur on the Paje and Jembiani beaches, 
though directly one goes inland the ground rises slightly and its 
surface is of coral rock. 
I conclude therefore that when first the island was upheaved 
the east coast presented a uniform line of coral rock, the present 
variations being due to the accumulation of sand by the sea. 
The cliffs are never high, 20 feet being the maximum in 
exposed situations, their height remaining very uniform. They 
are deeply undermined always, but falls of rock are rare. The 
hardness and density of the rock cause the whole overhanging 
mass to break off in one piece, and the fallen mass is then itself 
undermined. I have observed a casuarina tree, with a trunk four 
inches in diameter, which had grown on a fallen block the com- 
mencement of whose undermining was just visible. 
This undermining is due to wave erosion, since the bare rock, 
continuous with that of the cliffs, is left as a level platform at 
the base. 
The coral rock which rings to the hammer, is hard, crystalline, 
yellowish or white when broken, but on the surface dark, 
1 Bounded pebbles of quartz occur in these beds. 
