268 FRINGING-REEFS. 



been straightly deposited in front of a lofty island, 

 and so far beyond its termination in the open sea. 

 Finally, if we look to other oceanic islands of 

 about the same height and of similar geological 

 constitution, but not encircled by coral-reefs, we 

 may in vain search for so trifling a circumambient 

 depth as 30 fathoms, except quite near to their 

 shores ; for usually land that rises abruptly out of 

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

 cled oceanic islands, plunges abruptly under it. 

 On what, then, I repeat, are these barrier-reefs 

 based 1 Why, with their wide and deep moat- 

 like channels, do they stand so far from the in- 

 cluded land 1 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 

 riband or fringe round the shores : where the land 

 elopes 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 prolonga- 

 tion of the land is gently inclined. In fact, the 

 reefs extend only to that distance from the shore 

 at which a foundation within the requisite depth 

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

 actual reef is concerned, there is uo essential dif- 

 ference 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 sedi- 

 ment 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 



