corals flourish in one and not in the other, and as so many
conditions before enumerated must affect their existence, it would
be an inexplicable fact if, during the changes to which earth,
air, and water are subjected, the reef-building corals were to
keep alive for perpetuity on any one spot or area. And as by our
theory the areas including atolls and barrier-reefs are subsiding,
we ought occasionally to find reefs both dead and submerged. In
all reefs, owing to the sediment being washed out of the lagoon
or lagoon-channel to leeward, that side is least favorable to the
long-continued vigorous growth of the corals; hence dead portions of
reef not infrequently occur on the leeward side; and these, though
still retaining their proper wall-like form, are now in several
instances sunk several fathoms beneath the surface. The Chagos group
appears from some cause, possibly from the subsidence having been
too rapid, at present to be much less favorably circumstanced for
the growth of reefs than formerly: one atoll has a portion of its
marginal reef, nine miles in length, dead and submerged; a second
has only a few quite small living points which rise to the surface;
a third and fourth are entirely dead and submerged; a fifth is a
mere wreck, with its structure almost obliterated. It is remarkable
that in all these cases the dead reefs and portions of reefs lie at
nearly the same depth; namely, from six to eight fathoms beneath the
surface, as if they had been carried down by one uniform movement.
One of these “half-drowned atolls,” so called by Captain Scoresby (to
whom I am indebted for much invaluable information), is of vast size;
namely, ninety nautical miles across in one direction and seventy
miles in another line; and is in many respects eminently curious. As
by our theory it follows that new atolls will generally be formed
in each new area of subsidence, two weighty objections might have
been raised; namely, that atolls must be increasing indefinitely in
number; and, secondly, that in old areas of subsidence each separate
atoll must be increasing indefinitely in thickness, if proofs of
their occasional destruction could not have been adduced. Thus have
we traced the history of these great rings of coral-rock, from their
first origin through their normal changes, and through the occasional
accidents of their existence, to their death and final obliteration.
Authors have noticed with surprise that, although atolls are the
commonest coral structures throughout some enormous oceanic tracts,
they are entirely absent in other seas, as in the West Indies: we
can now at once perceive the cause, for where there has not been
subsidence, atolls can not have been formed; and in the case of the
West Indies and parts of the East Indies, these tracts are known
to have been rising within the recent period. The larger areas are
all elongated; and there is a degree of rude alternation, as if
the rising of one had balanced the sinking of the other. Taking
into consideration the proofs of recent elevation both on the
fringed coasts and on some others (for instance, in South America)
where there are no reefs, we are led to conclude that the great
continents are for the most part rising areas; and from the nature
of the coral-reefs, that the central parts of the great oceans are
sinking areas. The East Indian archipelago, the most broken land in
the world, is in most parts an area of elevation, but surrounded
and penetrated, probably in more lines than one, by narrow areas of
subsidence.
Bearing in mind the statements made with respect to the upraised
organic remains, we must feel astonished at the vastness of the areas
which have suffered changes in level either downward or upward,
within a period not geologically remote. It would appear, also,
that the elevatory and subsiding movements follow nearly the same
laws. Throughout the spaces interspersed with atolls, where not a
single peak of high land has been left above the level of the sea,
the sinking must have been immense in amount. The sinking, moreover,
whether continuous, or recurrent with intervals sufficiently long for
the corals again to bring up their living edifices to the surface,
must necessarily have been extremely slow. This conclusion is
probably the most important one which can be deduced from the study
of coral formations; and it is one which it is difficult to imagine
how otherwise could ever have been arrived at. Nor can I quite pass
over the probability of the former existence of large archipelagoes
of lofty islands, where now only rings of coral-rock scarcely break
the open expanse of the sea, throwing some light on the distribution
of the inhabitants of the other high islands, now left standing so
immensely remote from each other in the midst of the great oceans.
The reef-constructing corals have indeed reared and preserved
wonderful memorials of the subterranean oscillations of level; we see
in each barrier-reef a proof that the land has there subsided, and in
each atoll a monument over an island now lost. We may thus, like unto
a geologist who had lived his ten thousand years and kept a record of
the passing changes, gain some insight into the great system by which
the surface of this globe has been broken up, and land and water
interchanged.
MAGNITUDE AND COLOR OF THE SEA
--G. HARTWIG
Of all the gods that divide the empire of the earth, Neptune rules
over the widest realms. If a giant hand were to uproot the Andes and
cast them into the sea, they would be engulfed in the abyss, and
scarcely raise the general level of the waters. The South American
Pampas, bounded on the north by tropical palm-trees, and on the south
by wintry firs, are no doubt of magnificent dimensions, yet these
vast deserts seem insignificant when compared with the boundless
plains of earth-encircling ocean. Nay! a whole continent, even
America or Asia, appears small against the immensity of the sea,
which covers with its rolling waves nearly three-fourths of the
entire surface of the globe.
The length of all the coasts which form the boundary between sea and
land can only be roughly estimated, for who has accurately measured
the numberless windings of so many shores? The entire coast-line of
deeply indented Europe and her larger isles measures about 21,600
miles, equal to the circumference of the earth; while the shores of
compact Africa extend to a length of only 14,000 miles. The coasts of
America measure about 45,000 miles, those of Asia 40,000, while those
of Australia and Polynesia may safely be estimated at 16,000. Thus
the entire coast-line of the globe amounts to about 136,000 miles,
which it would take the best pedestrian to traverse from end to end.
How different is the aspect of these shores, along which the
ever-restless sea continually rises or falls! Here steep rock-walls
tower up from the deep, while there a low sandy beach extends its
flat profile as far as the eye can reach. While some coasts are
scorched by the vertical sunbeam, others are perpetually blocked up
with ice. Here the safe harbor bids welcome to the weather-beaten
sailor, the lighthouse greets him from afar with friendly ray; the
experienced pilot hastens to guide him to the port, and all along the
smiling margin of the land rise the peaceful dwellings of civilized
man. There, on the contrary, the roaring breakers burst upon the
shore of some dreary wilderness, the domain of the savage or the
brute. What a wonderful variety of scenes unrolls itself before
our fancy as it roams along the coasts of ocean from zone to zone!
What changes, as it wanders from the palm-girt coral island of the
tropical seas to the melancholy strands where, verging toward the
poles, all vegetable life expires! And how magnificently grand does
the idea of ocean swell out in our imagination, when we consider that
its various shores witness at one and the same time the rising and
setting of the sun, the darkness of night and the full blaze of day,
the rigor of winter and the smiling cheerfulness of spring!
The sea is not colorless; its crystal mirror not only reflects the
bright sky or the passing cloud, but naturally possesses a pure
bluish tint, which is only rendered visible to the eye when the light
penetrates through a stratum of water of considerable depth. In the
Gulf of Naples, we find the inherent color of the water exhibited to
us by Nature on a most magnificent scale. The splendid “Azure Cave,”
at Capri, might almost be said to have been created for the purpose.
All profound and clear seas are more or less of a deep blue color,
while, according to seamen, a green color indicates soundings. The
bright blue of the Mediterranean, so often vaunted by poets, is found
over all the deep pure ocean, not only in tropical and temperate
zones, but also in the regions of eternal frost. Scoresby speaks with
enthusiasm of the splendid blue of the Greenland seas, and all along
the great ice-barrier which under 77° S. lat. obstructed the progress
of Sir James Ross toward the pole, that illustrious navigator found
the waters of as deep a blue as in the classical Mediterranean. The
North Sea is green, partly from its water not being so clear, and
partly from the reflection of its sandy bottom mixing with the
essentially blue tint of the water. In the Bay of Loanga the sea has
the color of blood, and Captain Tuckey discovered that this results
from the reflection of the red ground-soil.
But the essential color of the sea undergoes much more frequent
changes over large spaces, from enormous masses of minute _algæ_,
and countless hosts of small sea-worms, floating or swimming on its
surface.
“A few days after leaving Bahia,” says Mr. Darwin, “not far from
the Abrolhos islets, the whole surface of the water, as it appeared
under a weak lens, seemed as if covered by bits of hay with their
ends jagged. Each bundle consisted of from twenty to sixty filaments,
divided at regular intervals by transverse septa, containing a
brownish-green flocculent matter. The ship passed several bands
of them, one of which was about ten yards wide, and, judging from
the mud-like color of the water, at least two and a half miles
long. Similar masses of floating vegetable matter are a very common
appearance near Australia. During two days preceding our arrival at
the Keeling Islands, I saw in many parts masses of flocculent matter
of a brownish-green color floating in the ocean. They were from half
to three inches square, and consisted of two kinds of microscopical
confervæ. Minute cylindrical bodies, conical at each extremity, were
involved in large numbers in a mass of fine threads.”
“On the coast of Chili,” says the same author, “a few leagues north
of Concepcion, the _Beagle_ one day passed through great bands of
muddy water; and again a degree south of Valparaiso, the same
appearance was still more extensive. Mr. Sullivan, having drawn up
some water in a glass, distinguished by the aid of a lens moving
points. The water was slightly stained, as if by red dust, and after
leaving it for some time quiet a cloud collected at the bottom.
With a slightly magnifying lens, small hyaline points could be
seen darting about with great rapidity and frequently exploding.
Examined with a much higher power, their shape was found to be oval,
and contracted by a ring round the middle, from which line curved
little setæ proceeded on all sides, and these were the organs of
motion. Their minuteness was such that they were individually quite
invisible to the naked eye, each covering a space equal only to the
one-thousandth of an inch, and their number was infinite, for the
smallest drop of water contained very many. In one day we passed
through two spaces of water thus stained, one of which alone must
have extended over several square miles. The color of the water was
like that of a river which has flowed through a red clay district,
and a strictly defined line separated the red stream from the blue
water.”
In the neighborhood of Callao, the Pacific has an olive-green color,
owing to a greenish matter which is also found at the bottom of the
sea in a depth of 800 feet. In its natural state it has no smell, but
when cast on the fire it emits the odor of burned animal substances.
Near Cape Palmas, on the coast of Guinea, Captain Tuckey’s ship
seemed to sail through milk, a phenomenon which was owing to an
immense number of little white animals swimming on the surface and
concealing the natural tint of the water.
The peculiar coloring of the Red Sea, from which it has derived its
name, is owing to the presence of a microscopic alga, _sui generis_,
floating at the surface of the sea and even less remarkable for its
beautiful red color than for its prodigious fecundity.
I could add many more examples, where, either from minute _algæ_, or
from small animals, the deep blue sea suddenly appeared in stripes
of white, yellow, blue, brown, orange, or red. For fear, however,
of tiring the reader’s patience, I shall merely mention the _olive
green_ water which covers a considerable part of the Greenland seas.
It is found between 74° and 80° N. lat., but its position varies with
the currents, often forming isolated stripes, and sometimes spreading
over two or three degrees of latitude. Small yellowish Medusæ, of
from one-thirtieth to one-twentieth of an inch in diameter, are the
principal agents that change the pure ultramarine of the Arctic Ocean
into a muddy green.
When the sea is perfectly clear and transparent, it allows the
eye to distinguish objects at a very great depth. Near Mindora,
in the Indian Ocean, the spotted corals are plainly visible under
twenty-five fathoms of water.
The crystalline clearness of the Caribbean Sea excited the admiration
of Columbus. “In passing over these splendidly adorned grounds,” says
Schöpf, “where marine life shows itself in an endless variety of
forms, the boat, suspended over the purest crystal, seems to float in
the air, so that a person unaccustomed to the scene easily becomes
giddy. On the clear sandy bottom appear thousands of sea-stars,
sea-urchins, mollusks, and fishes of a brilliancy of color unknown
in our temperate seas. Fiery red, intense blue, lively green, and
golden yellow perpetually vary; the spectator floats over groves of
sea-plants, gorgonias, corals, alcyoniums, flabellums, and sponges
that afford no less delight to the eye, and are no less gently
agitated by the heaving waters, than the most beautiful garden on
earth when a gentle breeze passes through the waving boughs.”
TIDAL ACTION
--SIR ROBERT S. BALL
Every one is familiar with the fact that the moon raises tides on the
earth; these tides ebb and flow along our coasts, and in virtue of
them the satellite exercises a certain control on the movements of
our globe. If the moon had liquid oceans on its surface there can not
be a doubt that the attraction of the earth would generate tides in
the oceans on the moon just as the attraction of the moon generates
tides in the oceans of the earth. But there would be a fundamental
difference between the two cases; the shores of the lunar seas would
be periodically inundated by tides far vaster than any tides which
the moon can create on the earth. But it may be said that as the
moon contains no water it seems idle to talk of the tides that might
have been produced in oceans if they had existed. It is no doubt
true that the moon contains no visible liquid water on its surface
at the present time; it is, however, by no means certain that our
satellite was always void of water; it is not at all impossible
that spreading oceans may have once occupied a large part of that
surface now an arid wilderness. The waters from those oceans have
vanished, but the basins they presumably filled are still left as
characteristic features on our satellite. For our present argument,
however, it is really not material that the moon should ever have had
oceans as we understand them. The water at those remote periods must
have been suspended in the form of vapor around the more solid parts
of the glowing globe. But tides can be manifested in other liquids
besides that which forms our seas. In fact if the basins of our great
oceans were filled with oil or with mercury, or even with molten iron
instead of water, the moon would still cause tides to ebb and flow,
no matter what the material might be, so long as it possessed to some
extent the properties of a liquid. It need not be a perfect liquid,
for any material which is in some degree viscous, like honey or
treacle, would still respond to tidal influence, though not, it may
be well believed, with the same alacrity and freedom of movement as
would a fluid of a more perfect character. In the molten moon itself,
throughout the very body of our satellite, the tidal influence of the
earth must have been experienced in these primitive ages.
There can not be a doubt that in ancient days when the moon was
sufficiently fluid, the action of the tides tended without ceasing to
the establishment of such an adjustment between the rotation of the
moon around its axis and the revolution of the moon around the earth,
that the two should be brought to have equal periods. Friction would
incessantly operate until this adjustment had been effected, and
owing to the preponderating mass of the earth such strenuous tides
must have been evoked in the moon that our satellite was brought
under tidal control with comparative facility. Hence it arose that
in those early days the habit of bending the same face incessantly
toward the earth around which it revolved was established on our
satellite.
Time passed on, the moon gradually dispensed its excessive heat by
radiation into space, and it gradually became transformed from a
molten globe to a globe with a solid crust. It may be that the water
was condensed from vapor and then collected together into oceans on
the newly formed surface; if so, these oceans would not have any
ebbing tide or flowing tide, for it would be constant high tide at
some places and constant low tide at others. Such a state of things
would at all events endure so long as the adjustment of equality
between the moon’s rotation and its revolution continued. In fact,
should any departure from this adjustment have manifested itself,
corresponding tides would have begun to throb in the lunar oceans,
and their tendency would be to restore the adjustment which was
disturbed. This arrangement between the two movements was necessarily
stable when tidal control was always at hand to check any tendency to
depart from it.
It may be that the moon has now cooled so thoroughly that not only
is it hard and congealed on the exterior as we see, but it seems
highly probable that the heat may have so entirely forsaken even
the interior that there is no longer any fluid in the globe of our
satellite to respond to tidal impulse. There is, therefore, in all
probability, no longer any actual tidal control. On the other hand,
however, there is nothing to disturb the adjustment. It was, as
we have seen, caused by the tides which have done their work; the
consequences of that work are still exhibited in the constant face of
the moon, which, now that it has been brought about, seems likely to
exist permanently as a stable adaptation of the movement.
The tendency of the tides on a tide-disturbed globe is to adjust the
movements of that globe in such a way that the tides shall no longer
ebb or flow, but that permanent high tide shall be established in
some places and permanent low tide in others. If the rotation of the
body be not fast enough the tide will pull the body round in order to
effect this object. If the rotation of the body be too rapid, then
the influence of the tide will tend to check the movement and bring
down the speed of rotation until the desired adjustment is obtained.
At present the earth is spinning too fast to permit the high tides to
remain at permanent localities, and consequently tides are applied
with the effect of checking the rotation. The earth is, however, so
vast, and the tides generated by so small a body as the moon are
relatively so impotent, that their effects in reducing the speed of
the earth’s rotation are insignificant. Nevertheless, small though
they are, they unquestionably exist, and there can not be a doubt
that to some extent the earth is affected by the unremitting action
of the tides; the consequence is that the rapidity with which the
earth rotates upon its axis is gradually declining.
One result of this can be stated in a very simple manner. The
length of the day must be increasing. It is true that this gradual
stretching of the day is very slow; it is indeed quite inappreciable
in so far as our ordinary use of the day as a measure of time is
concerned. The alteration almost eludes any means of measurement
at our disposal. Even in a thousand years the change is so small
that the increase in the length of the day is only a fraction of a
second. We can doubtless afford to disregard so trifling a variation
in our standard of time so far as the period contemplated in mere
human affairs is concerned. In fact the change is absolutely devoid
of significance within such periods as are contemplated since the
erection of the Pyramids, or indeed since any other human monument
has been reared. We must not, however, conclude that the change in
the length of the day has no significance in earth history.
The significance of the gradual elongation of the day by the tides
arises from the circumstance that the change always takes place in
one direction. In this form of effect the tide differs from other
more familiar astronomical phenomena which sometimes advance in one
direction and then after the lapse of suitable periods return in
the opposite direction, and thus restore again the initial state of
things. But the alteration of the length of the day is not of this
character, it is not periodic, its motion is never reversed, is never
even arrested. Only one condition is therefore necessary to enable
it to obtain tremendous dimensions, and that is sufficient time in
which it can operate.
There are many lines of reasoning which show the extreme antiquity
of our globe: the disclosures of geology are specially instructive
on this head. Think, for instance, of that mighty reptile the
Atlantosaurus, which once roamed over the regions now known as
Colorado. The bones of this vast creature indicate an animal
surpassing in proportions the greatest elephant ever known. No one
can count the æons of years that have elapsed since the Atlantosaurus
whose bones are now to be seen in the museum at Yale University
breathed its last. A still more striking conception of time than
even the antiquity of this creature affords is derived from the
consideration that his mighty form was itself the product of a long
and immeasurable line of ancestry, extending to a depth in the remote
past far beyond the limits of computation. I have mentioned this
illustration of the antiquity of the earth for the purpose of showing
the ample allowance of time that is available for tides to accomplish
great work in earlier stages of our globe’s history.
As the evidence of the earth’s crust proves that our globe has
lasted for incalculable ages, it becomes of interest to think how
far the gradual elongation of the day may have attained significant
proportions since very early time. It may be that even in a thousand
years the effect of the tides is not sufficient to alter the length
of the day by so much as a single second. But the effect may be
very appreciable or even large in a million years, or ten million
years. We have the best reasons for knowing that in intervals of time
comparable with those I have mentioned, the change in the length of
the day may have amounted not merely to seconds or minutes, but even
to hours. Looking into the remote past, there was a time at which
this globe spun round in twenty-three hours instead of twenty-four;
at a still earlier period the rate must have been twenty hours, and
the further we look back the more and more rapidly does the earth
appear to be spinning. At last, as we strain our gaze to some epoch
so excessively remote that it must have been long anterior to those
changes which geology recognizes, we see that our globe was spinning
round in a period of six hours or five hours, or possibly even less.
Here then is a lesson which the tides have taught us: they have shown
that if the causes at present in operation have subsisted without
interruption for a sufficiently long period in the past, the day must
have gradually grown to its present length from an initial condition
in which the earth seems to have spun round about four times as
quickly as it does at present.
We should, however, receive a very inadequate impression of what
tides are able to accomplish if we merely contemplated this change in
the length of the day, striking and significant though it doubtless
is. The student of natural philosophy is well aware that there is
no action without a corresponding reaction, and it is instructive
to examine in this case the form which the reaction assumes. Our
reasoning has been founded on the supposition that it is the
attraction of the moon on the waters of our globe that gives rise to
the tides. It is, therefore, the influence of the moon which checks
the speed of the earth’s rotation and adds to the length of the day.