1836.] FIUNGING-BEEFS. 453 



of the island, in the same straight line with which it fronts the 

 west coast, it is hardly possible to believe that a bank of sediment 

 could thus have 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 

 80 fathoms, except 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 tinder 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 requisite 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 sediments 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 



