78 
FRINGING REEFS. 
Ch. III. 
and appear as separate and irregularly scattered 
patches often of considerable area. As the conditions 
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 he merely superficial. 
On the other hand, when, in a deep sea, hanks 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, which I 
should probably have arranged under these two classes, 
if the existence of large and level hanks, 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 foundation? 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 Red 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 
