The Democratic advocate. [volume] (Westminster, Md.) 1865-1972, 1884-01-12
Cur @lio.
CORAL ISLANDS AND REEFS.
Causes OF Their Observation-Disclosures of
Darwin and Damn.

: Darwin and damn.
Coral reefs, fringing and barrier, and
D coralislands, have, since their first discovery,
, been an attractive theme for traveller and
, poet, and a riddle for the students of
nature. Throughout the tropical portions
: of both the great oceans extensive stretches
of the coasts of continents and large
r islands are fringed with a belt of coral
} limestone, or surrounded by barrier reef
separated from the land by a canal of
greater or less width. Besides these con-
tinuous reefs there are rings of coral islets
enclosing broad lagoon of still water, with
small, rocky islet in the centre; atolls or
, rings of coral islets outside the central is
land; completed atolls of more or less cir
cular form, with central lagoon entirely
cut off from the sea; and, finally, islands of
coral, with an obsolescent lagoon.
There is, in fact, almost every possible
gradation between mere fringing reef and
solid island, the visible portion of which
is composed of the debris Of species that
still inhabit its ocean-washed coasts.
= If reef-building corals were inhabitants
of either deep or shallow water, as are, for
example, certain serpent stars which range
from near the coast line to depths of 1000
fathoms, there would be no difficulty in ac-
counting for the existence of reefs and is
lands over so large portion of the globe,
but it is well known that the species which,
through their gregarious habits, make up
the bulk of these masses of recent lime
stone, cannot exist in deep water. The
evidence accumulated by Ehrenberg, Dana
and Quoy and Gaimard goes to prove that
the limit of growth of these corals is found
at about twenty fathoms. Prof. Alex.
Agassiz, Judging from his examination of
the Tortugas, limits the depth to six or
seven fathoms.
How has it then been possible for islands
of coral to exist in mid-ocean-to rise out
of depths of thousands of fathoms. The
low beach with its line of palms Gaping
a pyramid, the base of which is set in depths
that, until the late improvements in sound
ing apparatus, were literally unfathomable ?
This is the riddle that has exercised
many able minds, and has received various
answers, which probably contain the truth
among them The oldest theory ascribed
the origin of coral islands and barrier reefs
to volcanic action. The first might be
built upon the summit of volcano, the
second upon an area raised from the action
of the earth's internal forces.
Darwin, during that memorable voyage
that gave him the opportunity of fame
studied the various coral superstructures in
connection with each other, and gave to the
world his celebrated theory of subsidence.
In one sense, elevation and subsidence are
the results of volcanic action, but Darwin's
theory did not suppose volcanic peak for
every coral island, not comparatively re-
cent great disturbance in regions where no
volcanoes now exist, to account for the
foundation of barrier reef. In shallow
waters around the coast line corals grew
upwards till they reached the highest level
perpetually bathed by the waves, thus
forming fringing reef. If, now, the land
surrounded by the fringing reef were slowly
to sink beneath the sea level, the corals
would continue to build upwards, keeping
the summit of the reef level with the tide,
and leaving a canal of comparatively still
water between the reef and the shore
Thus a barrier reef would be produced.
This often forms continuous ring around
an island, the further sinking of which will,
by degrees, leave only the reef, thus con-
verting it into an atoll.
As corals are extremely sensitive to at
mospheric action, and are killed off by low
tides or by exposure to the action of the
sun, the theory propounded by Darwin and
elaborated by Dana recognized other agents
as the causes by which portion of the
summit of the reef became raised above the
water. These causes were the growth of
nullipores or encrusting sea weeds upon the
summit of the reef, and still more the de-
position, by the action of storm waves, of
fragments and blocks of broken coral, to-
gether with the finer calcareous silt ground
from the reef by the waves, upon portion
of the reef, usually some distance from the
slope of the living portion of the reef.
THE ToRTUGAs AND FLORIDA REEFS.
However applicable to the Indies and
Australasia the explanation of Darwin and
Dana may be, it has long been recognized
that it will not apply in the West Indies.
Most barrier reefs and atolls are formed in
an area where the land is slowly sinking,
whereas those of the West Inlies and sur
rounding seas are formed in an area of ele-
walton. In areas of subsistence, the action
of the waves and the growth of sea-weeds
are sufficient to account for any slight ele-
vation the coral islands may acquire, but in
the West Indies we have to explain the
existence of all kinds of reefs without SUBJ
sidence. The elder Agassiz, LIE Conte, E.
B. Hunt and Alex. Agassiz have all at
tempted to explain the mode of formation
of the Florida Reefs, and the latter has re-
Gently put together the facts relating to
them IN a manner which not only seems to
offer a satisfactory explanation in their
case. bat also to shed light upon the causes
of the formation of other reefs in the Ca
ribbean Sea and Gulf of Mexico.
The entire southern portion of Florida
is formed of concentric barrier reefs, which
have been gradually cemented into con-
tinuous sheet of land by the accumulation
and consolidation of mud fats between
them. These reefs and fats are founded
upon fold or folds formed in the earth's
crust in an earlier geological period. Smith
and Hilgard have shown that this axis
sinks gradually to the south, while on either
side of it are deposited the more recent
Emestones that make up the width of Flo
rida. Beyond the coast line of Florida, as
result of this folding, lies an immense
submarine plateau, rich, as has been proved
BY the dredgings of Pourtales and others,
almost beyond conception in animal life.
Here live and die countless millions of mol
lusks, echinoderms, corals, worms, crusta-
ceans, ETC., all robbing the sea-water of the
carbonate of lime contained within it, and
; depositing it upon the plateau to ultimate
ly form modern limestone Westward of
the shore line Florida stretches out as an
immense submarine plateau coated over with
veneer of coral limestone at its very top.
Easfvard of tho ranins,,la this nlafaau is