Mr.. LyeI|'s Fourth Lecture en- Geology.
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Concluded


Bu, wha, causes the singular. ring-lik'. shaPe oi
these islands and why 50 many Jusl on icSci
with the sea and neither higher nor lower : lhesc
questions have been as ked ever since the peculiar
formation and mineral character OF these islands
were ascertaiued 1t was q, first imagined, and
once maintained the opinion, that they we're the
cratcrs Of submarine volcanoes. " hen II wa
discovered that Finder. wrong in supPosins
that they could bs built at great depth ~ ~
cold and dark regions Of the ocean, up IRON ~,
bottom il was natural 20 suppose that 'f! ''''s Ut
haze grown upon the tops OF some -'o'''a'i"
mountains But what mountains "ouio give t',s
sloDc IN The corals7 Or was 1t lndl7pensaoic
that they should be built in this Pecuiiar ioI'h ;!
Now when EHREHBERG examined the coFais o! 'lie
R,d Son he found thar some banks were square
while o,hers were risbon-likc strips with hat tops
and WITHOUT lagoons and ret they were OF the Same
genera as those which hoi lasoons, the same as
,he atoN,. as they were named from those observed
in the Maldive and Laccadve Islands, the term
expressing an island with rim of coral o,lt-
side and a LAGOON within It thus found
that this annuar shape is not essential; but that
the corals raay form in different shape. 1t was
therefore natural to suppose that tins sorndcPend
e;i on the outline OF the submarine bottom, and
that they were based volcanic craters, thousands
of which were known 10 exist III that part OF the
sea These claters would give the cup-shaped
center and the rirn might be covered with corals
by these zoophvles. In the South Shetland stands
and in Barren Island in the Bay OF Bengal, are
submarine volcanoes with craiers in the center
and breach in the side SO that YOU may sail with
in and see ali around the walls Of the crater. Now
when submerged the corals ight easily have been
built upon them. In farther support of this thco-
rv it was observed that in Gamblers group OF
islands there were volcanic rocks with lagoon
10 the center just as medcrn volcanoes have sprung
up in the Gulf of Santorn


Notwithstanding all these arguments in favor
Of the theory of volcanic origis, ii found
necessary TO abandon it entirely: because, thOucli
ii would accountfor Of the facts, there were
many others which il could not explain and which
could be explained by another theory 1t was
perfectly satisfactory far the rim and lagoon.
were concerned and it also explained why the
Ocean near by should be unfathomablc, which al
first seemed an argument in favor Of the volcanic
theory. Mr.. DARWIN, after considering all the
different facts have related, observed that there
were in some cases islands which had this same
corallinc ring and vet were not Uolconic but er0-
mite; and vet the ring Of coral surrounded 4
precisely sirailur !agoon.-How this to BE-
explained Again FLISDgRs discovered nag
nitccnt rccl-on the North East Coast Of New
Holland more than thousand mics in length :;
aud he sailed for more than three hundred and
fifty miles and yet found passage through tha
narrow reef Instead of being circular or oval
or in any such shape ii was parallel to the Coast
of New Holland. In the island Of New Calcdo
nia, which is grnnitic, there also long ridg,
of coral 400 miles long and prolonged at each
end under the water beyond these limits between
the coral reel and the shore what might E,
called lagoon. Now must have theory
which will explain these facts OF these cncir-
cled islands, us DARWIN called them-island>
with encircling reefs around tlcm-Vancouvcr',
is one and Otahcite is another. Here we have "a
coral reef with lagoon inside. In the figure W..
have given a section Of one of these cncircled
islands