Mr Grossland, The Coral Reefs of Zanzibar. 501 



as usual, nor is it in the form of a regular reef. Its breadth 

 is in fact only one-fifth of a mile and the reef edge ceases just 

 south of Muyuni, commencing again at the point marked B. 

 Between these points is an irregular shore of hummocky old 

 rock and sand. No coral grows at low tide, though close to deep, 

 clear water. 



Zanzibar Channel. 



The numerous islands and rocks of the channel are all formed 

 of coral rock, the former being deeply undermined, standing on 

 flats of the same rock whose extent shews the original size of the 

 islands. In the course of time the islands will disappear, as in 

 the case of the majority of the reefs and sandbanks of the 

 channel, which have been formed in this way, thus the sandbank 

 Pange still shews a nucleus of limestone. Coral flourishes round 

 all these flats, e.g. on the sandbanks at the S.W. entrance to 

 Kokotoni Harbour, Mungopwani, Prison Island, Pange, Murogoand 

 Kisiki. Round the latter four its growth has formed a low preci- 

 pice surrounding the sandbanks, whose top is a foot or two below 

 low tide level, but little fresh surface has been formed. As nulli- 

 pores are not abundant in any of these places it is doubtful 

 whether real rock is being formed. 



Conclusions. 



The island of Zanzibar was formed as part of a great barrier 

 reef on the coast of East Africa, which is here exceptionally 

 distant from land in correspondence with the outward bend of 

 the 100 fathom line. Coral exists at present at a level of 250 ft. 

 above the sea, but the original elevation must have been much 

 more, the commonness of the red earth and underground caves, 

 being an indication of considerable rain erosion. 



In addition to upheaval is the lowering of the level of the 

 ocean, which Gardiner estimates at about 14 feet and to which 

 he attributes the formation of many atoll islands'. It is interest- 

 ing that if the ocean returned to this level large areas of the 

 eastern parts of Zanzibar and Pemba islands and of the mainland 

 would be submerged at high tide. 



The fringing reefs of the east coast have not been formed by 

 recent growth, but are due to the erosion of the raised rock by 

 the waves. 



1 Gardiner, on Funafuti, Bbtuma, and Fiji, Proc. Camb. Phil. Soc, vol. ix. 

 Pt. in. ; Formation of the Maldives, Geog. Journ. , March 1902. 



