78 FRINGING REEFS. Ch. III. 



are less favourable in several respects on the inner 

 side of these patches, the growth of the coral is more 

 vigorous on the outside ; thus causing the reefs to be 

 generally higher and more perfect in their marginal 

 than in their central parts. Hence these reefs some- 

 times assume (and this circumstance ought not to be 

 overlooked) the appearance of atolls ; but as they are 

 based on a shallow foundation, and as their central 

 expanse is much less deep and their form less defined, 

 this resemblance is easily seen to be merely superficial. 

 On the other hand, when, in a deep sea, banks of sedi- 

 ment have accumulated round islands or submerged 

 rocks, and they become fringed with reefs, they are dis- 

 tinguished with difficulty from encircling barrier-reefs 

 or atolls. In the West Indies there are reefs, w T hich I 

 should probably have arranged under these two classes, 

 if the existence of large and level banks, lying a little 

 beneath the surface and ready to serve as the basis for 

 the attachment of coral, had not been present; the 

 formation of, such banks through the accumulation of 

 sediment being sufficiently evident. Fringing-reefs 

 sometimes coat, and thus protect the foundations of 

 islands, which have been worn down by the surf to the 

 level of the sea. According to Ehrenberg, this has been 

 extensively the case with the islands in the Eed Sea, 

 which formerly ranged parallel to the shores of the 

 mainland, with deep water within them : hence the 

 reefs now coating their bases, are situated relatively 

 to the land like barrier-reefs, although not belonging 

 to that class ; — but there are, as I believe, in the Eed 



