76 CORAL-REEFS. 



as the breakers are prevented acting on the shores of the 

 island, and as they do not ordinarily tear up many frag- 

 ments from the outside, and as every streamlet has its bed 

 prolonged in a straight line through the reef, this channel 

 could be filled up only very slowly with sediment. But a 

 beach of sand and of fragments of the smaller kinds of 

 coral seems, in the case of Mauritius, to be slowly 

 encroaching on the shallow channel. On many shelving 

 and sandy coasts, the breakers tend to form a bar of sand a 

 little way from the beach, with a slight increase of depth 

 within it; for instance, Capt. Grey 1 states that the west 

 coast of Australia, in lat. 24 , is fronted by a sand-bar 

 about 200 yards in width, on which there is only two feet 

 of water ; but within it the depth increases to two fathoms. 

 Similar bars, more or less perfect, occur on other coasts. 

 In these cases I suspect that the shallow channel (which no 

 doubt during storms is occasionally obliterated) is scooped 

 out by the flowing away of the water thrown beyond the 

 line, on which the waves break with the greatest force. At 

 Pernambuco a bar of hard sandstone, 2 which has the same 

 external form and height as a coral-reef, extends nearly 

 parallel to the coast ; within this bar currents, apparently 

 caused by the water thrown over it during the greater part 

 of each tide, run strongly, and are wearing away its inner 

 wall. From these facts it can hardly be doubted, that 

 within most fringing-reefs, especially within those lying 

 some distance from the land, a return stream must carry 

 away the water thrown over the outer edge; and the 

 current thus produced, would tend to prevent the channel 

 being filled up with sediment, and might even deepen it 



1 Capt. Grey's Journal of Two Expeditions, vol. i. p. 369. 



2 I have described this singular structure in the Lond. and Edin. 

 Phil. Mag., October 1S41. 



