344 fRINGING-REEFS. [CHAP, xx 



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 thirty 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 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 founda- 

 tion within the requisite depth from twenty to thirty 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 satis- 

 factory 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 con- 

 structed 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 

 subside. Now as the island sinks down, either a few feet at a time or 

 quite insensibly, we may safely infer, from what is known of the condi- 

 tions favourable to the growth of coral, that the living masses, bathed 

 by the surf on the margin of the reef, will soon regain the surface. 

 The water, however, will encroach little by little on the shore, the 

 island becoming lower and smaller, and the space between the inner 

 edge of the reef and the beach proportionally broader. A section of the 

 reef and island in this state, after a subsidence of several hundred feet, 

 is given by the dotted lines. Coral islets are supposed to have been 



