472 FRINGING-REEFS. [chap. xx. 



quite near to their shores ; for usually land that rises abruptly out 

 of water, as do most of the encircled and non- encircled oceanic 

 islands, plunges abruptly under it. On what then, I repeat, are 

 these barrier-reefs based ? Why, with their wide and deep moat- 

 like channels, do they stand so far from the included land ? We 

 shall soon see how easily these difficulties disappear. 



We come now to our third class of Fringing Reefs, which will 

 require a very short notice. Where the land slopes abruptly 

 under water, these reefs are only a few yards in width, forming 

 a mere ribbon or fringe round the shores : where the land slopes 

 gently under the water the reef extends further, sometimes even 

 as much as a mile from the land ; but in such cases the soundings 

 outside the reef, always show that the submarine prolongation of 

 the land is gently inclined. In fact the reefs extend only to that 

 distance from the shore, at which a foundation within the requi- 

 site depth from 20 to 30 fathoms is found. As far as the actual 

 reef is concerned, there is no essential difference between it and 

 that forming a barrier or an atoll : it is, however, generally of 

 less width, and consequently few islets have been formed on it. 

 From the corals growing more vigorously on the outside, and 

 from the noxious effect of the sediment washed inwards, the outer 

 edge of the reef is the highest part, and between it and the land 

 there is generally a shallow sandy channel a few feet in depth. 

 Where banks of sediment have accumulated near to the surface, 

 as in parts of the West Indies, they sometimes become fringed 

 with corals, and hence in some degree resemble lagoon-islands or 

 atolls ; in the same manner as fringing -reefs, surrounding gently- 

 sloping islands, in some degree resemble barrier-reefs. 



No theory on the formation of coral-reefs can be considered 

 satisfactory which does not include the three great classes. We 

 have seen that we are driven to believe in the subsidence of 

 those vast areas, interspersed with low islands, of which not one 

 rises above the height to which the wind and waves can throw 

 up matter, and yet are constructed by animals requiring a 

 foundation, and that foundation to lie at no great depth. Let 

 us then take an island surrounded by fringing-reefs, which offer 

 no difficulty in their structure ; and let this island with its 

 reef, represented by the unbroken lines in the woodcut, slowly 



