Evening star. [volume] (Washington, D.C.) 1854-1972, 1898-03-26
Professor Alexander Agassiz's Recent
Long Tour.
His STUDY OF MID-PACIFIC GEOLOGY
What He Says of the Formation
of Reefs.
ORIGIN OF THE ISLANDS



made on the YaraIla. twin-screw steamer
of about 800 tons, and there was a full
outfit of dredging, sounding and deep-sea
towing, as we'll as materials necessary to
preserve the collections. The day before
leaving Cambridge for the Pacific news
had arrived that the expedition of Prof.
David of the University of Sydney, had
bored into the atoll of F'unafuti to the
depth of nearly GO feet, the bottom being
still in coral But later experience would
seem to indicate that general theory of
the formation OF coral reefs is as far re-
moved as ever Prof Agassiz says:
Il came to FIJI under the impression that
We were TO visit characteristic area of
subsistence. Dana, in his last discussion of
the coral reef question, states that it is
impossible to find a better series of islands
than those of the Fiji to illustrate the
gradual changes brought about by subsi-
dence, which transforms volcanic island
with j fringlng reef to with barrier,
and next to one with circular reef ring,
and nnalIy to one in which the interior
island has disappeared, and has left only
more or less circular reefing. For these
reasons of the Fiji atols promised to
be an admirable location for boring and
settling the question of the thickness of
the coral reef of an atolI. My surprise was
great, therefore, to find within mile from
Suva an elevated reef about 30 feet thick
and I'D feet above the level Of the sea, the
base of the reef being underlaid by what is
locally called soapstone, probably a kind of
stratified volcanic mud.
Volcanic Rocks.
But the traces of extensive elevation
are not limited to the larger island of Viii
Levu- 1 found the islands on the rim of
the atoll of Ngele Levu TO consist entirely
of coral rock elevated to height of over
~ feet the larger island. 1t was found
that at Nanua Mbalavu the northern line
OF islands were parts of elevated reef
by u central volcanic mass of the main
island height of from 800 to GO feet.
On the south OF the main island they are
much lOwer. At Mango the vertical bluf1s
underiaid by volcanic rocks, which crop
out at the sea level. AT Tavutha the bluffs
are probably $40 feet high. and at Lakem-
ba about 80. On the island of Aiwa the
elevated reef is fully 2D feet thick. On
the three islands Of the Yangasa group it
attairs thickness of from 40 to 800 feet;
and hrally. on Ongea, II attalns Trick
ness of nearly 800 Get, and at Fulanga 800.
AT Natu Leile, the most westerly island
We examined. the elevated reef forming
the island is 10 feet thick at its northern
extremity.
.-AIl this plainly shows that the southern
part of Viii Lew and as far south as Vatu
Lc;le, and the whole length of the wind
ward islands of the Fiji group from Ngele
Levu on the north to Ongea on the south.
have been subject to an elevation OF at
least So feet; there is abundant proof
that great part of the thickness of the
elevated reef has been eroded to reduce 1t
in certain localities to the level of the sea
or TO leave at others bluffs and islands or
iSletS, the occurrence OF which have
traced at many points. But the evidence
of very considerable elevation is not linn
tied to that furnished by the remains of
the elevated reefs just mentioned; it is
natural TO assume that the elevation we
have just traced was but a part of a more
general elevation, which perhaps took place
in tertiary times and in which the whole
group was involved
Reefs
From this evidence / am inclined to
think that the corals of today have ac-
tualy played no part in the shaping of the
circular or irregular atolls scattered among
the Fiji Islands; Furthermore, that they
have had nothing to do in our time with
the building up of the barrier reefs sur
rounding either wholly or in part some OF
the islands. j also believe that their mod
flying influence has been entirely limited
In the present epoch to the formation OF
-frirging reefs. and that the recent corals
living upon the reefs either of the atolls
Or of the barriers form only a crust Of
very moderate thickness upon the under
lying base. This base may be either fiat
of an eroded, elevated reef or of a similar
substructure of volcanic rocks. the nature
of that base depending absolutely upon its
character when elevated in former period
to a greater height than II now occupies.
formulation and erosion act more rap
idly on the elevated reef rocks than upon
those of a volcanic character. 1t is. there
like kandavu. Tuvinin and Ovalau, are Of
volcanic origin, while the islands which
once occupied the area of. the lagoons of
Ngele Levu, OF the Nanuku reef, of Vanua
Mhalavu. of the Argo reefs. of the Oneata
Yangasa. Alwa Ongea and Vatu Leile clus-
tiers. being elevated coral reefs. they have
disappeared almost entirely. leaving only
here and there small island to attest to
the former existence of the more extensive
elevated reef. once covering the whole area
of what is atoll-
-Denu1atiOn hav haan still
mere active In the Ringgod Islands. in the
kimbobo cluster, and in Komo and 1t may
have gone so far as to leave no trace in an
atoll to indicate either its volcanic coral
elevated reef origin, the shape of the atoll
be-Lg entirely due to mechanical action,
and not being connected in any way with
the growth of the corals which have been
found footing upon reef flats formed by
atmospheric agencies by the action Of
the sea.
DenudatiOn and Erosion.
AS far as We can Judge from the Fiji
Islands, the shape of the atolls and of the
barrier reefs is due to causes during a pc
Rod preceding our own. The islands of the
whole group have been elevated and have
like the northern part of Queensland. re-
naired nearly stationary and exposed to
great and prolonged denudation and ero-
sion, which has reduced the islands to their
present height the platforms upon which
the barrier reef corals have grown being
merely the flats left by the denudation and
erosion OF central island OF greater size
than that now left. while the atolls are
similar flats from the interior of which the
Islands have been eroded, and the lagoons
OF which have been continually secured by
the action of the sea, the incessant rollers
pouring huge mass of water into the Ia
goon, which finds its way out through the
passages leading into it.
I'm the Fiji Islands the atolls and islands
or islets, surrounded in part or wholly by
barrier reefs, have not been formed by the
subsistence and disappearance of this CEN
trai island. as is claimed by Dana and OAR
win. The Fiji Islands are not situated as
was supposed, in an area of subsistence;
but. on the contrary, they are in an area
of elevation, so that the theory of Darwin
and of Dana is not applicable to the is
lands and atolls of the Fiji group.
w hat the age of the elevated reef of the
Fiji Is 1 am unable to state. Its aspect
and position show IL to be of considerable
age. probably antecedent to the present
period. In many ways it resembles some
OF the late tertiary elevated limestones
which L have seen on the northern and
southern coasts of Cuba The great thick
ness which the elevated coral reefs attain