even from the smallest particle to large fragments of rock,
bears the stamp of having been subjected to the power of
organic arrangement. Captain FitzRoy, at the distance of
but little more than a mile from the shore, sounded with a
line, 7 2 0 0 feet long, and found no bottom. This island is,
therefore, a lofty submarine mountain, which has a greater
inclination than even those of volcanic origin on the land.
I will now give a sketch* of the general results at which I
have arrived, respecting the origin of the various classes of
reefs, which occur scattered over such large spaces of the
intertropical seas.
The first consideration to attend to, is, that every observation
leads to the conclusion that those lamelliform corals,
which are the efficient agents in forming a reef, cannot live at
any considerable depth. As far as I have personally seen, I
judge of this from carefully examining the impressions on
the soundings, which were taken by Captain FitzRoy at
Keeling Island, close outside the breakers, and from some
others which I obtained at the Mauritius. At a depth under
ten fathoms, the arming came up as clean as if it had been
dropped on a carpet of thick turf; but as the depth increased,
the particles of sand brought up became more and more
numerous, until, at last, it was evident the bottom consisted
of a smooth layer of calcareous sand, interrupted only at
intervals by shelves, composed probably of dead coral rock.
To carry on the analogy, the blades of grass grew thinner and
thinner, till, at last, the soil was so sterile, that nothing sprung
from it.
As long as no facts, beyond those relating to the structure
of lagoon islands were known, so as to establish some
more comprehensive theory, the belief that corals constructed
their habitations, or, speaking more correctly, their
skeletons, on the circular crests of submarine craters, was
from M a la c c a a n d J a v a , a n d th e sm a ll fra gm en ts o f p um ic e , d r ifte d h e re ,
to g e th e r w ith th e se ed s o f E a s t I n d ia n p la n ts . T h e o n e b lo ck o f g re e n s
to n e , m o reo v e r, on th e N o r th e rn L a g o o n m u s t b e e x c e p te d .
* T h is sk e tc h was re ad b e fo re th e G eo lo g ic a l S o c iety , M ay , 1937.
both ingenious and very plausible. Yet the sinuous margin
of some, as in the Radack Islands of Kotzebue, one of which
is fifty-two miles long, by twenty broad, and the narrowness
of others, as in Bow Island (of which there is a chart on a
large scale, forming part of the admirable labours of Captain
Beechey), must have startled every one who considered this
subject.
The very general surprise of aU those who have beheld
lagoon islands, has perhaps been one chief cause why other
reefs, of an equally curious structure have been almost overlooked
:* I allude to the encircling reefs. We will take, as an
instance, Vanikoro, celebrated on account of the shipwreck
of La Peyrouse. The reef there runs at the distance of
nearly two, and in some parts three miles from the shore,
and is separated from it hy a channel having a general depth
between thirty and forty fathoms, and, in one part, no less
than fifty, or three hundred feet. Externally, the reef rises
from an ocean profoundly deep. Can any thing be more singular
than this structure ? It is analogous to that of a lagoon,
but with an island standing, like a picture in its frame, in the
middle. A fringe of low alluvial land in these cases generally
surrounds the base of the mountains; this, covered by the
most beautiful productions of a tropical land, backed by the
abrupt mountains and fronted by a lake of smooth water, only
separated from the dark waves of the ocean by a line of
breakers, form the elements of the beautiful scenery of Tahiti—
so weU called the Queen of Islands. We cannot
suppose these encircling reefs are based on an external
crater, for the central mass sometimes consists of primary
rock, or on any accumulation of sedimentary deposits, for the
reefs follow indifferently the island itself, or its submarine
prolongation. Of this latter case there is a grand instance
M r. D e la B e ch e , h ow ev e r, seems to h a v e b e e n fu lly aw a re o f th e
d ifficu lty . H e says, “ th e r e a rc c e rta in s itu a tio n s , w h e re c o ra l ree fs ru n ,
as i t w e re , in a lin e w ith th e co a st, b u t s e p a r a te d from i t by d e e p w a te r,
w h ich w o u ld seem to r e q u ir e a d iffe re n t e x p la n a tio n .” — Geological
Manual, p . 142.
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