472 FRINGING-REEFS, [cmar. xx. - 
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 asa 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 requi- 
site depth from 20 to 80 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 Jagoon-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 vast areas, ititerspersed with low islands, of which not one 
rises above the height to which the wind and waves can throw 
up matter, and yet are constructed 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 woodent, slowly 
