I will give only a few
instances. In barrier-reefs it has long been remarked with surprise
that the passages through the reef exactly face valleys in the
included land, even in cases where the reef is separated from the
land by a lagoon-channel so wide and so much deeper than the actual
passage itself, that it seems hardly possible that the very small
quantity of water or sediment brought down could injure the corals
on the reef. Now, every reef of the fringing class is breached by a
narrow gateway in front of the smallest rivulet, even if dry during
the greater part of the year, for the mud, sand, or gravel
occasionally washed down kills the corals on which it is deposited.
Consequently, when an island thus fringed subsides, though most of
the narrow gateways will probably become closed by the outward and
upward growth of the corals, yet any that are not closed (and some
must always be kept open by the sediment and impure water flowing
out of the lagoon-channel) will still continue to front exactly the
upper parts of those valleys at the mouths of which the original
basal fringing-reef was breached.
We can easily see how an island fronted only on one side, or on one
side with one end or both ends encircled by barrier-reefs, might
after long-continued subsidence be converted either into a single
wall-like reef, or into an atoll with a great straight spur
projecting from it, or into two or three atolls tied together by
straight reefs--all of which exceptional cases actually occur.
Pages:
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919