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NUMBER 57.‘
Geological Agency of the Sea.
mar 11.
As the Author ofNature has not given laws to the
universe, which, like human institutions, carry with
them the elements of their own destruction; as he has
not permitted in his works any symptom of infancy, or
of old age, we must endeavour to ascertain the means
by which reprarlltction is allowed to take place. And
these have been found by geologists to consist prin-
cipally-
1. In the etfortsof the coral insect, in forming islands
and reefs.
2. In tlle transportation by currents of the materials
which the sea obtains from the wasting clitis, rivers,
and floods; and
S. In the elevation of land from the action of sub-
terranean a
1. In Mr. Stutchbury’s excellent dissertation on tile
formation of coral reefs and islands, the construction of
the principal part is ascribed to the genera caryopbyllili,
I n a
niandrina, astrma, parites, and madre ora.
branch of living coral is drawn up from the sea, it pre-
sents a kind of bark spotted with little round tubercles,
and covered with a thick adhesive humour, which ap-
pears to flow particularly from the summit of the
branches, where may be seen large mill:-like drops.
This humour is supposed to be the material from which
the bark or skin of the branches is formed; and when
this becomes solidified by age, it hardens into 3 central
core, which then forms the stone-like substance which
we call coral. Whatever he the age or the size of ii
living coral branch, it is always found that there is an
inner substance, hard, compact, and capable of receiv-
ing a polis ; an n outer substance, soft, spongy,
thin, and which soon dries when exposed to the air. It
is in this spongy bark that are found- numbers of little
soft whitish polypi, inhabiting smail ce ls. no ex- 6
tremities of the branch are small and tender, and con-
tain very little of the interior substance, all of which
is consistent with the formation of the central coral
from successive layers of the viscous humour. '
animals possess minute glands, in w ich is secreted
this viscous humour, a humour composed of animal
gluten, calcareous earth, and some othensubstaiices. b
Poiret, (Voyage en Barbirie). who paid great at-
tention to the growth of coral off the coast of Barbary,
offers a conjecture that this humour may contain the
eggs of polypi, and that these eggs either attach them-
selves to foreign substances. and then form new gene-
rations, or they remain fixed u on the paternal bralich,
row up into olypi, and live and die there. When
these zoophytic architects cease to exist, their struc-
v s the
being gradually filled up by sand and shells, a mass o
rock is at length formed. In process of time salt plants
take root upon it, and I soil begins to be formed. Be-
fore long the new bank is visited by the seli-bird, after-
wards the nut ol' the cocoa or the pandanus is thrown
ashore. and the wearied land-bird, resting its wings on
the soil, deposits on it the seeds of herbs and trees.
Every high tide and every gale, adds something to the
bnnl.-, and it gradually spreads into an island of luxu-
rious vegetation.
In 1857, Mr. Darwin was copducted, by a considera-
tion ofthe structure of coral islands, to I remarkable
speculation, viz., that in the southern ocean the distri-
bution of the coral masses on a great sca e, and their pe-
culior forms in detail, are explicable on the supposition
of certain lines,‘or rather long narrow SP3?" 07 039“,
in which the land has undergone and is 81!" Humering
great depressions, and alternating with these other long
spaces in which the land is rising. Where depression
.;e-‘e%.
' Mr. Darwin states that the flora of coral island: has 0'1"‘
the character of c for Ihe destitute. Professor llenaluw
lnfunnrll him, um of the twenty -pt-can forming almost tie per-
illllllll, ninclern bulonsul Io dllirrenl
genera, and these again to no less than Illleen onlerll
No. 25.-VoL II.
r -
SATURDAY, JUNE 25, 1342.
has taken place, coral is supposed to have grown on the
submerged points; and, as the depression proceeded,
to have coiltinued to grow and keep the surface as high
as the sea .
Coral islands and reefs form the basis of, or surround
the shores of, most of the islands in the warm palts of
the Pacific, and stretch for a thousand miles parallel to
the north-cast coast of Australia, in a narrow reef of
several hundred miles in length. In like manner,
about the Indian and West Indian islands, in the Red
Sea, Persian Gulf, and Mediterranean. coral abounds,
so as to constitute a considerable portion of the products
of the sea.
2. It is only along the shares, at the mouths of
rivers, and in shallow gulfs and seas, that we have any
positive knowledge concerning the for-iiiation of marine
sedimentary strata, altliough, from the enormous quantity
of solid matter sent into the sea, we feel persuaded that
deposits to ti very considerable extent are taking place.
On the shores of the Ilerlnudus a most interesting de-
position oflimestone is in progress of formation. The
ocean which surrounds these islands abounds in corals
and shells, alid from the action of tho wavt-s on the coral
reefs, and on the dead shells, the waters become lorldetl
with calcareous matter. Much of this detritus is carried
down to the profound depths of the ocean, but a great
proportion is walled h the waves towards the shores,
and is deposited in the state of fine sand. This sand
is drifted inland by the winds, and becomes more or less
consolidated by the percolation of water, and the lI'llll-
tration ofcrystallized carbonate of lime, and a fine white
calcareous stone is thus formed. Similar formations are
frequent along the shores of the “lest Indian Archipe-
lago. On the nortli-east coast of the niainlanll of Gua-
daloupe, a bed of recent linivstone forms li sloping bank
from the steep cliffs of the island to the sea. It is com-
posed of consolidated sand, and coiulniuutcd shells and
orals, f species now inhabiting the nlljacent seas.
Sand-shells, frliglnents of pottery, stone arrow-licalls
carvcll stone and wooden ornaments, and human skele-
tons, are found [l.l0l't‘lH clivclopt:d.t Dr. Paris gives an
account of the recent forniritiou of S.'lll(l>‘t0nt! on the
northern coast of Cornwall, in the Transactions of the
Royal Geological Society of Cornwall. This sandstone
ears li great una ogy to that ofGuall.-lloupc.
The Elbe, “loser, Rhine, llnll Schuldt, pour out vast
quantities ofsrlnd into the German oceun. This sand,
as it subsides through the water, is swept by the alter-
nate flood and ebb into long sinuous and shallow banks,
which, rising in narrow ridges toward.-) the sllrface,
place themselves in directions pnrlillel to the contiguous
shores, or rather in lines radiating from the strait of
Dover, through which the North sea tides enter and
turn. There are other banks which do not seem to be
3
I attached to the shores, or to assume the same ridge-like
form, but which have been equally the result of the
same causes. Such me the Long North Bank, the
Dagger Bank, the Well Bank, the Broad Fourteens, and
others which need not be cnuvnc-ratetl. To give an idea
of the extent of these banks, the dimensions ofmlz will
sutlice. The Dogger bankis, in some places, sixty miles
broad, but it is contracted towards the east, and termi-
nates nearl in ii point. In length it extends from
about twelve leagues from Flamborougli Head, in York-
shire, to within twenty leagues of Jutland. .
A river, charged with solid matter, is always most
loaded in the middle of the stream, bl-cause its rapidity
is then greatest; but when that rapidity is diminished.
it is of necessity on reaching the elnbouchure or
mouth, a central deposit is formed, which in time rises
above the surface, and becomes nli island, dividing; the
ream into two branches. Every succeeding flood
brings a fresh tribute to the island, which increases sea-
ward; for while the upper end is abraded by the im-
12
&
pinging ofthe main-stream, tlmre is li dead water ininic- .
I now talmn during
Geological Prncvctl g
urological Soclvly.
Prom -, ls:n;
Ille mlulng of Mr. Darwin‘: hp.-r Iu the
9 Dr. MantcIl'i ivlmllm of Geology.
- PRICE THREE HALFPENCE.
dilitelyin front of the lower part where fresh detritus
continually subsides. The island increases not only in
length but breadth, the two branches of the stream di-
verging more as they advance. The alluvial laud, acquir-
ing something of a triangular form, has been denominated
a delta, because it somewhat resembles the fourth letter
of the Greek alphabet, A. If the river be large, the
lateral branches are disposed each to form a minor delta,
upon the same principle as the main stream did. a
formation of deltas is not, however, effected upon coasts
subject to strong tidal currents. The numerous blinks
at the mouth ofthe Thames, would soon rise into islands,
and n delta would be formed, were it not for the effect
of the tides. In some cases, deltas owe their formation
almost wholly to the nature and direction of the cur-
rents of the ocean, the form and nature of its bed in the
neigbhourliood of the mouths of rivers, the prevalent
winds and tides, &c. Ill almost all cases, these circum-
stances greatly modify their form, extent, and nature.
' he space, then, which is now occupied by the delta of
a river was once a part of the sea, which was hllcd up
by the debris and earth matter brought down by the
river from the mountains and hilly country through
which its upper and middle course lie.
The principal deltas are, in Europe, the Danube, Po.
and Rhine; ill Asia, the Ganges, Indus, Cavery,
Euphrates, Oural, Lena, null Kolimzi; in Africa, the
Nile and Niger; and in America, the Mississippi and
H080.
The magnitude of a delta, generally, although not
always, corresponds, to the volume o the waters by
which it has been created. The head of that of the
lthine is about ninety milus distant from the general line
of sea-coast of Holland; and although the name of the
main river be almost lost by the subdivision of its waters,
0
S
and the junction of other rivers, we include within the.
Itliine dulta the whole ofthe low land from the neighbour-
llood of Calais to the nortli-eastern shores of the Zuyder
Zee,wliich makes the blise of tlle triangle nearly ?00 miles.
The head of the delta of the Ganges is 2‘!!! miles from
the sea, its base is 200 miles long, including the space
occupied by the two great arms of the Ganges which
hound it on either side.
3. When u volcano bursts beneath the waters of the
sea, the substances thrown up ohen rise above the sur-
face. forming rocks and islands. The first on record is
in 1538; another in 1720; and in Feb. 1811. a vol-
cano burst forth otf the west-end of St. Michael, in the
zores, tllroiring up from the depth of forty fathoriis a
very dangerous shoal. On the 18th June, after raging
several (lays, and ejecting ashes, cinders, and stones, it
increased in size, so that it measured ti mile in circum-
ereuce, and 500 feet in height. Subsequently, how-
ever, it sank gradually into the sea, and by the middle
of October, in the same car, no part of it was visi-
ble above water, but ti diangcrous shoal still remains.
In 1783, two new islands were thrown u of the
coast of Iceland, but they also have been since swal-
0 up.
Many islands which have long been inhabited by man,
bear lill the appearance of having risen, in like manner,
from the bosom of the deep. The islands of St. Helena
.nnd Ascension, the Azores, the West India islands, and
many of the islands in the Pacific, are evidently the pro-
duce of volcanic action. “0ivhyhee," says M. De la
lleclio, “is a magnilicent example of such an island;
the whole ninss, estimated as exposing a surface of 4,000
square miles, is composed oflava, or other volcanic matter,
which rises in the peliksof Jlouna Rea, and Mouna Kaa,
to the height ofbetvreen 15,000 and 16,000 feet above
the level of the sea."
Other phenomena are producnd in the sea by the sub.
terraneau movements. The most remarkable effect of the
cartliquake which happened in Chili, on the 20;], Fe.
brtiary, H135. was the permanent tleralilvn of the limd.
Captain Fitzroy having twice visited tlieisland of Santa
Maria, for the purpose of exaininingevery circumstance
with extreme accuracy.‘ has brought a mass of evidence
in proof of such elevation. This part of the co“; of
my .