Mr Grossland, The Coral Reefs of Zanzibar. 495 



Zanzibar is composed of (a) hard coral limestone, (6) 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 

 fiats, 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 Rounded pebbles of quartz occur in these beds. 



