502 Mr Grossland, The Coral Reefs of Zanzibar. 



The height of the raised edge is that of the original shore 

 platform, the boat channel being a secondary formation. Darwin 1 

 gives instances of sandstone formations which simulate coral reefs, 

 e.g. a bar off Pernambuco, explaining the formation of their boat 

 channels by the removal of the inner parts of reefs and sandbanks 

 by the currents which return the water thrown upon them by 

 the surf. The fewness of the depressions in so long a reef would 

 make both these and the tidal current here exceptionally powerful. 

 Near low tide while the edge of the reef is out of water the tidal 

 current in the boat channel is running most strongly. Several 

 times when travelling at spring tides where the boat channel is 

 shallow or obstructed I have had to wait six hours for water to 

 float the boat. This means that water (often muddy) is pouring 

 over the reef flat for at least six hours longer than over the 

 raised edge, and these hours are stated by Dana to be the period 

 during which wave action is least powerful. 



This explains the absence of stones similar to those on the reef 

 edge from the channel and flat. These stones must have originally 

 been evenly distributed over the platform, as they are remnants 

 of the hardest portions of the rock which has been removed. It 

 is impossible to believe that such waves as occur on this coast 

 could break into this rock, which can be quarried for building 

 purposes only with difficulty. Their absence from certain areas 

 of reef edge is only explicable on the former theory of their 

 origin. It is hardly possible that numerous stones should be 

 broken from one part of the reef while none at all are from 

 others which are exposed to the same surf. But it is conceivable 

 that some parts of the original reef should contain specially 

 hardened portions of rock, which might be absent from other 

 areas. 



It is remarkable that the reef edge should not be growing. 

 All the physical conditions seem favourable to coral growths. 

 Their absence from the Somali coast (north of the equator) is 

 explained by the presence there of an off-shore current and a conse- 

 quent rising of cold water from the depths to take its place. But here 

 we have an on-shore current from the surface of the ocean under 

 the equator. Mud is fatal to corals, unless kept in motion by 

 a current. The bottom is visible here only to the depth of 

 5 fathoms, so that the water is dirty compared to that around 

 the Pacific Islands 2 . But the good effect of the current one 

 would expect to neutralise the evil of the mud, as it does on the 

 sand-banks at the south entrance to Kokotoni harbour. The 

 water here is positively thick, yet corals flourish, since strong- 

 tidal currents are continually flowing through these narrows. 



1 Corals and Coral Islands, pp. 72 and 73. 

 - Gardiner, on Funafuti, etc. 



