, I always imagined that
there must have been great tracts of land, and that on such large tracts
there must have been mountains of immense altitudes. But not, it appears
to me, that one is only justified in supposing that groups of islands
stood there. There are (as I believe) many considerable islands and
groups of islands (Galapagos Islands, Great Britain, Falkland Islands,
Marianas, and, I believe, Viti groups), and likewise the majority of
single scattered islands, all of which a subsidence between 4,000
and 5,000 feet would entirely submerge or would leave only one or two
summits above water, and hence they would produce either groups of
nothing but atolls, or of atolls with one or two encircled islands. I
am far from wishing to say that the islands of the great oceans have not
subsided, or may not continue to subside, any number of feet, but if the
average duration (from all causes of destruction) of reefs on the same
spot is limited, then after this limit has elapsed the reefs would
perish, and if the subsidence continued they would be carried down; and
if the group consisted only of atolls, only open ocean would be left; if
it consisted partly or wholly of encircled islands, these would be left
naked and reefless, but should the area again become favourable for
growth of reefs, new barrier-reefs might be formed round them. As an
illustration of this notion of a certain average duration of reefs on
the same spot, compared with the average rate of subsidence, we may take
the case of Tahiti, an island of 7,000 feet high. Now here the present
barrier-reefs would never be continued upwards into an atoll, although,
should the subsidence continue at a period long after the death of the
present reefs, new ones might be formed high up round its sides and
ultimately over it. The case resolves itself into: what is the ordinary
height of groups of islands, of the size of existing groups of atolls
(excepting as many of the highest islands as there now ordinarily occur
encircling barrier-reefs in the existing groups of atolls)? and likewise
what is the height of the single scattered islands standing between such
groups of islands? Subsidence sufficient to bury all these islands (with
the exception of as many of the highest as there are encircled islands
in the present groups of atolls) my theory absolutely requires, but no
more. To say what amount of subsidence would be required for this end,
one ought to know the height of al