42 Mr Crossland, The Coral Reefs of Pemba Island 



the deep water and the current everything is different. There 

 is no trace of coral, nothing but weed and dead rock being seen 

 through the clouded water. 



The same fact is better illustrated at Pungutiayu, where 

 I carefully examined and compared the depths to which corals 

 extend on the north and south sides of the island. The latter is 

 exposed to the open ocean, the north is separated from the islands 

 of Pungutiachi by a channel two-thirds of a mile wide and nine 

 fathoms deep. On this side the corals end quite definitely at 

 a depth of two fathoms, below which the dredge travelled quite 

 smoothly, bringing up sponges, soft alcyonarians (e.g. Xeniidae) 

 and a little weed. The bottom is apparently of smooth rock 

 without loose stones. On the S. and S.E. sides, on the other 

 hand, coral growths extend to depths of from five to seven fathoms. 

 Below this is again smooth rock. Still further out, in twenty 

 fathoms, occur patches of gravel formed of much corroded coral 

 fragments. 



From this one concludes that around atoll reefs of the ocean 

 coral growth would not only be much more luxuriant than 

 near a continental area, but would extend to a greater depth. 



General Conclusions concerning the whole coast of 

 East Africa. 



1. There are no reefs due to growth in situ. 



This is shewn to be true not only by those places specially 

 examined, but of this region generally, by the state of things seen 

 where coral is growing most vigorously. 



2. There have been great coral reefs in geologically recent times, 

 consisting of a fringing reef and a barrier in deep water, whose 

 upheaval and consequent crystallization forms the rock of the whole 

 coast and the outlying islands and reefs. 



3. The islands of Pemba and Mnemba are formations inde- 

 pendent of the above system. 



4. That all the forms typical of growing coral reefs are formed 

 by physical agencies operating upon this dead rock aided by the 

 protection against erosion afforded by organisms (principally algae, 

 calcareous or not) which grow especially where the rock is exposed 

 to the open sea. 



Examples. Fringing reefs, east coasts of Zanzibar and Pemba 



Islands. 

 Barriers (a) off the mainland, broken as in Pemba channel, 



regular as at Chale and Mombasa ; (b) off the west coast 



of Pemba. 



