ACTION OF TIDES AND CURRENTS--_continued_.
Inroads of the sea at the mouths of the Rhine in Holland--Changes in
the arms of the Rhine--Proofs of subsidence of land--Estuary of the
Bies Bosoh, formed in 1421--Zuyder Zee, in the 13th century--Islands
destroyed--Delta of the Ems converted into a bay--Estuary of the
Dollart formed--Encroachment of the sea on the coast of Sleswick--On
shores of North America--Tidal wave, called the Bore--Influence of
tides and currents on the mean level of seas--Action of currents in
inland lakes and seas--Baltic--Cimbrian deluge--Straits of
Gibraltar--No under-current there--Whether salt is precipitated in
the Mediterranean--Waste of shores of Mediterranean.
_Inroads of the sea at the mouths of the Rhine._--The line of British
coast considered in the preceding chapter offered no example of the
conflict of two great antagonist forces; the influx, on the one hand, of
a river draining a large continent, and, on the other, the action of the
waves, tides, and currents of the ocean. But when we pass over by the
Straits of Dover to the Continent, and proceed northeastwards, we find
an admirable illustration of such a contest, where the ocean and the
Rhine are opposed to each other, each disputing the ground now occupied
by Holland; the one striving to shape out an estuary, the other to form
a delta. There was evidently a period when the river obtained the
ascendancy, when the shape and perhaps the relative level of the coast
and set of the tides were very different; but for the last two thousand
years, during which man has witnessed and actively participated in the
struggle, the result has been in favor of the ocean; the area of the
whole territory having become more and more circumscribed; natural and
artificial barriers having given away, one after another; and many
hundred thousand human beings having perished in the waves.
_Changes in the arms of the Rhine._--The Rhine, after flowing from the
Grison Alps, copiously charged with sediment, first purifies itself in
the Lake of Constance, where a large delta is formed; then swelled by
the Aar and numerous other tributaries, it flows for more than six
hundred miles towards the north; when, entering a low tract, it divides
into two arms, about ten miles northeast of Cleves,--a point which must
therefore be considered the head of its delta. (See[A], map, fig. 8.)
In speaking of the delta, I do not mean to assume that all that part of
Holland which is comprised within the several arms of the Rhine can be
called a delta in the strictest sense of the term; because some portion
of the country thus circumscribed, as, for example, a part of Gelderland
and Utrecht, consists of strata which may have been deposited in the sea
before the Rhine existed. These older tracts may either have been raised
like the Ullah Bund in Cutch, during the period when the sediment of
the Rhine was converting a part of the sea into land, or they may have
constituted islands previously.
[Illustration: Fig. 38.
The dark tint between Antwerp and Nieuport, represents part of the
Netherlands which was land in the time of the Romans, then overflowed by
the sea before and during the 5th century, and afterwards reconverted
into land.]
When the river divides north of Cleves, the left arm takes the name of
the Waal; and the right, retaining that of the Rhine, is connected, a
little farther to the north, by an artificial canal with the river
Yssel. The Rhine then flowing westward divides again southeast of
Utrecht, and from this point it takes the name of the Leck, a name which
was given to distinguish it from the northern arm called the old Rhine,
which was sanded up until the year 1825, when a channel was cut for it,
by which it now enters the sea at Catwyck. It is common, in all great
deltas, that the principal channels of discharge should shift from time
to time, but in Holland so many magnificent canals have been
constructed, and have so diverted, from time to time, the course of the
waters, that the geographical changes in this delta are endless, and
their history, since the Roman era, forms a complicated topic of
antiquarian research. The present head of the delta is about forty
geographical miles from the nearest part of the gulf called the Zuyder
Zee, and more than twice that distance from the general coast-line. The
present head of the delta of the Nile is about 80 or 90 geographical
miles from the sea; that of the Ganges, as before stated, 220; and that
of the Mississippi about 180, reckoning from the point where the
Atchafalaya branches off to the extremity of the new tongue of land in
the Gulf of Mexico. But the comparative distance between the heads of
deltas and the sea affords no positive data for estimating the relative
magnitude of the alluvial tracts formed by their respective rivers, for
the ramifications depend on many varying and temporary circumstances,
and the area over which they extend does not hold any constant
proportion to the volume of water in the river.
The Rhine therefore has at present three mouths. About two-thirds of its
waters flow to the sea by the Waal, and the remainder is carried partly
to the Zuyder Zee by the Yssel, and partly to the ocean by the Leck. As
the whole coast to the south as far as Ostend, and on the north to the
entrance of the Baltic, has, with few exceptions, from time immemorial,
yielded to the force of the waves, it is evident that the common delta
of the Rhine, Meuse, and Scheldt, for these three rivers may all be
considered as discharging their waters into the same part of the sea,
would, if its advance had not been checked, have become extremely
prominent; and even if it had remained stationary, would long ere this
have projected far beyond the rounded outline of the coast, like that
strip of land already described at the mouth of the Mississippi. But we
find, on the contrary, that the islands which skirt the coast have not
only lessened in size, but in number also, while great bays have been
formed in the interior by incursions of the sea.
In order to explain the incessant advance of the ocean on the shores and
inland country of Holland, M. E. de Beaumont has suggested that there
has in all probability been a general depression or sinking of the land
below its former level over a wide area. Such a change of level would
enable the sea to break through the ancient line of sand-banks and
islands which protected the coast,--would lead to the enlargement of
bays, the formation of new estuaries, and ultimately to the entire
submergence of land. These views appear to be supported by the fact that
several peat-mosses of fresh-water origin now occur under the level of
the sea, especially on the site of the Zuyder Zee and Lake Flevo,
presently to be mentioned. Several excavations also made for wells at
Utrecht, Amsterdam, and Rotterdam have proved, that below the level of
the ocean, the soil near the coast consists of alternations of sand with
marine shells, and beds of peat and clay, which have been traced to the
depth of fifty feet and upwards.[443]
I have said that the coast to the south as far as Ostend has given way.
This statement may at first seem opposed to the fact, that the tract
between Antwerp and Nieuport, shaded black in the annexed map (fig. 38),
although now dry land, and supporting a large population, has, within
the historical period, been covered with the sea. This region, however,
consisted, in the time of the Romans, of woods, marshes, and
peat-mosses, protected from the ocean by a chain of sandy dunes, which
were afterwards broken through during storms, especially in the fifth
century. The waters of the sea during these irruptions threw down upon
the barren peat a horizontal bed of fertile clay, which is in some
places three yards thick, full of recent shells and works of art. The
inhabitants, by the aid of embankments and the sand dunes of the coast,
have succeeded, although not without frequent disasters, in defending
the soil thus raised by the marine deposit.[444]
_Inroads of the Sea in Holland._--If we pass to the northward of the
territory just alluded to, and cross the Scheldt, we find that between
the fourteenth and eighteenth centuries parts of the islands Walcheren
and Beveland were swept away, and several populous districts of Kadzand,
losses which far more than counterbalance the gain of land caused by the
sanding up of some pre-existing creeks. In 1658 the Island Orisant was
annihilated. One of the most memorable inroads of the sea occurred in
1421, when the tide, pouring into the mouth of the united Meuse and
Waal, burst through a dam in the district between Dort and
Gertrudenberg, and overflowed seventy-two villages, forming a large
sheet of water called the Bies Bosch. (See map, fig. 38.) Thirty-five of
the villages were irretrievably lost, and no vestige, even of their
ruins, was afterwards seen. The rest were redeemed, and the site of the
others, though still very generally represented on maps as an estuary,
has in fact been gradually filled up by alluvial deposits, and had
become in 1835, as I was informed by Professor Moll, an immense plain,
yielding abundant crops of hay, though still uninhabited. To the north
of the Meuse is a long line of shore covered with sand dunes, where
great encroachments have taken place from time to time, in consequence
chiefly of the prevalence of southeasterly winds, which blow down the
sands towards the sea. The church of Scheveningen, not far from the
Hague, was once in the middle of the village, and now stands on the
shore, half the place having been overwhelmed by the waves in 1570.
Catwyck, once far from the sea, is now upon the shore; two of its
streets having been overflowed, and land torn away to the extent of 200
yards, in 1719. It is only by the aid of embankments that Petten, and
several other places farther north, have been defended against the sea.
_Formation of the Zuyder Zee and Straits of Staveren._--Still more
important are the changes which have taken place on the coast opposite
the right arm of the Rhine, or the Yssel, where the ocean has burst
through a large isthmus, and entered the inland lake Flevo, which, in
ancient times, was, according to Pomponius Mela, formed by the
overflowing of the Rhine over certain lowlands. It appears that, in the
time of Tacitus, there were several lakes on the present site of the
Zuyder Zee, between Friesland and Holland. The successive inroads by
which these and a great part of the adjoining territory, were
transformed into a great gulf, began about the commencement, and were
completed towards the close, of the thirteenth century. Alting gives the
following relation of the occurrence, drawn from manuscript documents of
contemporary inhabitants of the neighboring provinces. In the year
1205, the island now called Wieringen, to the south of the Texel, was
still a part of the mainland, but during several high floods, of which
the dates are given, ending in December, 1251, it was separated from the
continent. By subsequent incursions the sea consumed great parts of the
rich and populous isthmus, a low tract which stretched on the north of
Lake Flevo, between Staveren in Friesland and Medemblick in Holland,
till at length a breach was completed about the year 1282, and
afterwards widened. Great destruction of land took place when the sea
first broke in, and many towns were swept away; but there was afterwards
a reaction to a certain extent, large tracts, at first submerged, having
been gradually redeemed. The new straits south of Staveren are more than
half the width of those of Dover, but are very shallow, the greatest
depth not exceeding two or three fathoms. The new bay is of a somewhat
circular form, and between _thirty_ and _forty_ miles in diameter. How
much of this space may formerly have been occupied by Lake Flevo is
unknown. (See map, fig. 38.)
_Destruction of islands._--A series of islands stretching from the Texel
to the mouths of the Weser and Elbe are probably the last relics of a
tract once continuous. They have greatly diminished in size, and have
lost about a third of their number, since the time of Pliny; for that
naturalist counted twenty-three islands between the Texel and Eider,
whereas there are now only sixteen, including Heligoland and
Neuwerk.[445] The island of Heligoland, at the mouth of the Elbe,
consists of a rock of red marl of the Keuper formation (of the Germans),
and is bounded by perpendicular red cliffs, above 200 feet high.
Although, according to some accounts, it has been greatly reduced in
size since the year 800, M. Wiebel assures us, that the ancient map by
Meyer cannot be depended upon, and that the island, according to the
description still extant by Adam of Bremen, was not much larger than
now, in the time of Charlemagne. On comparing the map made in the year
1793 by the Danish engineer Wessel, the average encroachment of the sea
on the cliffs, between that period and the year 1848 (or about half a
century), did not amount to more than three feet.[446] On the other
hand, some few islands have extended their bounds in one direction, or
become connected with others, by the sanding-up of channels; but even
these, like Juist, have generally given way as much on the north towards
the sea as they have gained on the south, or land side.
_The Dollart formed._--While the delta of the Rhine has suffered so
materially from the movements of the ocean, it can hardly be supposed
that minor rivers on the same coast should have been permitted to extend
their deltas. It appears that in the time of the Romans there was an
alluvial plain of great fertility, where the Ems entered the sea by
three arms. This low country stretched between Groningen and Friesland,
and sent out a peninsula to the northeast towards Emden. A flood in 1277
first destroyed part of the peninsula. Other inundations followed at
different periods throughout the fifteenth century. In 1507, a part only
of Torum, a considerable town, remained standing; and in spite of the
erection of dams, the remainder of that place, together with
market-towns, villages, and monasteries, to the number of fifty, were
finally overwhelmed. The new gulf, which was called the Dollart,
although small in comparison to the Zuyder Zee, occupied no less than
six square miles at first; but part of this space was, in the course of
the two following centuries, again redeemed from the sea. The small bay
of Leybucht, farther north, was formed in a similar manner in the
thirteenth century; and the bay of Harlbucht in the middle of the
sixteenth. Both of these have since been partially reconverted into dry
land. Another new estuary, called the Gulf of Jahde, near the mouth of
the Weser, scarcely inferior in size to the Dollart, has been gradually
hollowed out since the year 1016, between which era and 1651 a space of
about four square miles has been added to the sea. The rivulet which now
enters this inlet is very small; but Arens conjectures that an arm of
the Weser had once an outlet in that direction.
_Coast of Sleswick._--Farther north we find so many records of waste on
the western coast of Sleswick, as to lead us to anticipate that, at no
distant period in the history of the physical geography of Europe,
Jutland may become an island, and the ocean may obtain a more direct
entrance into the Baltic. Indeed, the temporary insulation of the
northern extremity of Jutland has been affected no less than four times
within the records of history, the ocean having as often made a breach
through the bar of sand, which usually excludes it from the Lym Fiord.
This long frith is 120 miles in length including its windings, and
communicates at its eastern end with the Baltic. The last irruption of
salt water happened in 1824, and the fiord was still open in 1837, when
some vessels of thirty tons' burden passed through.
The Marsh islands between the rivers Elbe and Eider are mere banks, like
the lands formed of the "warp" in the Humber, protected by dikes. Some
of them, after having been inhabited with security for more than ten
centuries, have been suddenly overwhelmed. In this manner, in 1216, no
less than ten thousand of the inhabitants of Eiderstede and Ditmarsch
perished; and on the 11th of October, 1634, the islands and the whole
coast, as far as Jutland, suffered by a dreadful deluge.
_Destruction of Northstrand by the sea._--Northstrand, up to the year
1240, was, with the islands Sylt and Fohr, so nearly connected with the
mainland as to appear a peninsula, and was called North Friesland, a
highly cultivated and populous district. It measured from nine to eleven
geographical miles from north to south, and six to eight from east to
west. In the above-mentioned year it was torn asunder from the
continent, and in part overwhelmed. The Isle of Northstrand, thus
formed, was, towards the end of the sixteenth century, only four
geographical miles in circumference, and was still celebrated for its
cultivation and numerous population. After many losses, it still
contained nine thousand inhabitants. At last, in the year 1634, on the
evening of the 11th of October, a flood passed over the whole island,
whereby 1300 houses, with many churches, were lost; fifty thousand head
of cattle perished, and above six thousand men. Three small islets, one
of them still called Northstrand, alone remained, which are now
continually wasting.
The redundancy of river water in the Baltic, especially during the
melting of ice and snow in spring, causes in general an outward current
through the channel called the Cattegat. But after a continuance of
northwesterly gales, especially during the height of the spring-tides,
the Atlantic rises, and pouring a flood of water into the Baltic,
commits dreadful devastations on the isles of the Danish Archipelago.
This current even acts, though with diminished force, as far eastward as
the vicinity of Dantzic.[447] Accounts written during the last ten
centuries attest the wearing down of promontories on the Danish coast,
the deepening of gulfs, the severing of peninsulas from the mainland,
and the waste of islands, while in several cases marsh land, defended
for centuries by dikes, has at last been overflowed, and thousands of
the inhabitants whelmed in the waves. Thus the island Barsoe, on the
coast of Sleswick, has lost, year after year, an acre at a time, and the
island Alsen suffers in like manner.
_Cimbrian deluge._--As we have already seen that during the flood before
mentioned, 6000 men and 50,000 head of cattle perished on Northstrand on
the western coast of Jutland, we are all well prepared to find that this
peninsula, the Cimbrica Chersonesus of the ancients, has from a remote
period been the theatre of like catastrophes. Accordingly, Strabo
records a story, although he treats it as an incredible fiction, that,
during a high tide, the ocean rose upon this coast so rapidly, that men
on horseback were scarcely able to escape.[448] Florus, alluding to the
same tradition, says, "Cimbri, Teutoni, atque Tigurini, ab extremis
Galliae profugi, cum terras eorum inundasset Oceanus, novas sedes toto
orbe quaerebant."[449] This event, commonly called the "Cimbrian Deluge,"
is supposed to have happened about three centuries before the Christian
era; but it is not improbable that the principal catastrophe was
preceded and followed by many devastations like those experienced in
modern times on the islands and shores of Jutland, and such calamities
may well be conceived to have forced on the migration of some maritime
tribes.
_Inroads of the sea on the eastern shores of North America._--After so
many authentic details respecting the destruction of the coast in parts
of Europe best known, it will be unnecessary to multiply examples of
analogous changes in more distant regions of the world. It must not,
however, be imagined that our own seas form any exception to the general
rule. Thus, for example, if we pass over to the eastern coast of North
America, where the tides rise, in the Bay of Fundy, to a great
elevation, we find many facts attesting the incessant demolition of
land. Cliffs, often several hundred feet high, composed of sandstone,
red marl, and other rocks, which border that bay and its numerous
estuaries, are perpetually undermined. The ruins of these cliffs are
gradually carried, in the form of mud, sand, and large boulders, into
the Atlantic by powerful currents, aided at certain seasons by drift
ice, which forms along the coast, and freezes round large stones.
At Cape May, on the north side of Delaware Bay, in the United States,
the encroachment of the sea was shown by observations made consecutively
for sixteen years, from 1804 to 1820, to average about nine feet a
year;[450] and at Sullivan's Island, which lies on the north side of the
entrance of the harbor of Charleston, in South Carolina, the sea carried
away a quarter of a mile of land in three years, ending in 1786.[451]
_Tidal wave called "the Bore."_--Before concluding my remarks on the
action of the tides, I must not omit to mention the wave called "the
Bore," which is sometimes produced in a river where a large body of
water is made to rise suddenly, in consequence of the contraction of the
channel. This wave terminates abruptly on the inland side; because the
quantity of water contained in it is so great, and its motion so rapid,
that time is not allowed for the surface of the river to be immediately
raised by means of transmitted pressure. A tide wave thus rendered
abrupt has a close analogy, observes Mr. Whewell, to the waves which
curl over and break on a shelving shore.[452]
The Bore which enters the Severn, where the phenomenon is of almost
daily occurrence, is sometimes nine feet high, and at spring-tides
rushes up the estuary with extraordinary rapidity. The finest example
which I have seen of this wave was at Nova Scotia,[453] where the tide
is said to rise in some places seventy feet perpendicular, and to be the
highest in the world. In the large estuary of the Shubenacadie, which
connects with another estuary called the Basin of Mines, itself an
embranchment of the Bay of Fundy, a vast body of water comes rushing up,
with a roaring noise, into a long narrow channel, and while it is
ascending, has all the appearance of pouring down a slope as steep as
that of the celebrated rapids of the St. Lawrence. In picturesque
effect, however, it bears no comparison, for instead of the transparent
green water and snow-white foam of the St. Lawrence, the whole current
of the Shubenacadie is turbid and densely charged with red mud. The same
phenomenon is frequently witnessed in the principal branches of the
Ganges and in the Megna as before mentioned (p. 279). "In the Hoogly,"
says Rennell, "the Bore commences at Hoogly Point, the place where the
river first contracts itself, and is perceptible above Hoogly Town; and
so quick is its motion, that it hardly employs four hours in travelling
from one to the other, though the distance is nearly seventy miles. At
Calcutta it sometimes occasions an instantaneous rise of five feet; and
both here, and in every other part of its track, the boats, on its
approach, immediately quit the shore, and make for safety to the middle
of the river. In the channels, between the islands in the mouth of the
Megna, the height of the Bore is said to exceed twelve feet; and is so
terrific in its appearance, and dangerous in its consequences, that no
boat will venture to pass at spring-tide."[454] These waves may
sometimes cause inundations, undermine cliffs, and still more frequently
sweep away trees and land animals from low shores, so that they may be
carried down, and ultimately imbedded in fluviatile or submarine
deposits.
CURRENTS IN INLAND LAKES AND SEAS.
In such large bodies of water as the North American lakes, the
continuance of a strong wind in one direction often causes the elevation
of the water, and its accumulation on the leeward side; and while the
equilibrium is restoring itself, powerful currents are occasioned. In
October, 1833, a strong current in Lake Erie, caused partly by the set
of the waters towards the outlet of the lake, and partly by the
prevailing wind, burst a passage through the extensive peninsula called
Long Point, and soon excavated a channel more than nine feet deep and
nine hundred feet wide. Its width and depth have since increased, and a
new and costly pier has been erected; for it is hoped that this event
will permanently improve the navigation of Lake Erie for
steamboats.[455] On the opposite, or southern coast of this lake, in
front of the town of Cleveland, the degradation of the cliffs had been
so rapid for several years preceding a survey made in 1837, as to
threaten many towns with demolition.[456] In the Black Sea, also,
although free from tides, we learn from Pallas that there is a
sufficiently strong current to undermine the cliffs in many parts, and
particularly in the Crimea.
_Straits of Gibraltar._--It is well known that a powerful current sets
constantly from the Atlantic into the Mediterranean, and its influence
extends along the whole southern borders of that sea, and even to the
shores of Asia Minor. Captain Smyth found, during his survey, that the
central current ran constantly at the rate of from three to six miles an
hour eastward into the Mediterranean, the body of water being three
miles and a half wide. But there are also two lateral currents--one on
the European, and one on the African side; each of them about two miles
and a half broad, and flowing at about the same rate as the central
stream. These lateral currents ebb and flow with the tide, setting
alternately into the Mediterranean and into the Atlantic. The excess of
water constantly flowing in is very great, and there is only one cause
to which this can be attributed, the loss of water in the Mediterranean
by evaporation. That the level of this sea should be considerably
depressed by this cause is quite conceivable, since we know that the
winds blowing from the shores of Africa are hot and dry; and
hygrometrical experiments recently made in Malta and other places, show
that the mean quantity of moisture in the air investing the
Mediterranean is equal only to one half of that in the atmosphere of
England. The temperature also of the great inland sea is upon an average
higher, by 3-1/2 degrees of Fahrenheit, than the eastern part of the
Atlantic Ocean in the same latitude, which must greatly promote its
evaporation. The Black Sea being situated in a higher latitude, and
being the receptacle of rivers flowing from the north, is much colder,
and its expenditure far less; accordingly it does not draw any supply
from the Mediterranean, but, on the contrary, contributes to it by a
current flowing outwards, for the most part of the year, through the
Dardanelles. The discharge, however, at the Bosphorus is so small, when
compared to the volume of water carried in by rivers, as to imply a
great amount of evaporation in the Black Sea.
_Whether salt be precipitated in the Mediterranean._--It is, however,
objected, that evaporation carries away only fresh water, and that the
current from the Atlantic is continually bringing in salt water: why,
then, do not the component parts of the waters of the Mediterranean
vary? or how can they remain so nearly the same as those of the ocean?
Some have imagined that the excess of salt might be carried away by an
under-current running in a contrary direction to the superior; and this
hypothesis appeared to receive confirmation from a late discovery, that
the water taken up about fifty miles within the Straits, from a depth of
670 fathoms, contained a quantity of salt _four times greater_ than the
water of the surface. Dr. Wollaston,[457] who analyzed this water
obtained by Captain Smyth, truly inferred that an under-current of such
denser water flowing outward, if of equal breadth and depth with the
current near the surface, would carry out as much salt below as is
brought in above, although it moved with less than one-fourth part of
the velocity, and would thus prevent a perpetual increase of saltness in
the Mediterranean beyond that existing in the Atlantic. It was also
remarked by others, that the result would be the same, if the swiftness
being equal, the inferior current had only one-fourth of the volume of
the superior. At the same time there appeared reason to conclude that
this great specific gravity was only acquired by water at immense
depths; for two specimens of the water, taken within the Mediterranean,
at the distance of some hundred miles from the Straits, and at depths of
400 and even 450 fathoms, were found by Dr. Wollaston not to exceed in
density that of many ordinary samples of sea-water. Such being the case,
we can now prove that the vast amount of salt brought into the
Mediterranean _does not_ pass out again by the Straits; for it appears
by Captain Smyth's soundings, which Dr. Wallaston had not seen, that
between the capes of Trafalgar and Spartel, which are twenty-two miles
apart, and where the Straits are shallowest, the deepest part, which is
on the side of Cape Spartel, is only 220 fathoms. It is therefore
evident, that if water sinks in certain parts of the Mediterranean, in
consequence of the increase of its specific gravity, to greater depths
than 220 fathoms, it can never flow out again into the Atlantic, since
it must be stopped by the submarine barrier which crosses the shallowest
part of the Straits of Gibraltar.
The idea of the existence of a counter-current, at a certain depth,
first originated in the following circumstances:--M. De l'Aigle,
commander of a privateer called the Phoenix of Marseilles, gave chase
to a Dutch merchant-ship, near Ceuta Point, and coming up with her in
the middle of the gut, between Tariffa and Tangier, gave her one
broadside, which directly sunk her. A few days after, the sunken ship,
with her cargo of brandy and oil, was cast ashore near Tangier, which is
at least four leagues to the westward of the place where she went down,
and to which she must have floated in a direction contrary to the course
of the _central_ current.[458] This fact, however, affords no evidence
of an under-current, because the ship, when it approached the coast,
would necessarily be within the influence of a lateral current, which
running westward twice every twenty-four hours, might have brought back
the vessel to Tangier.
What, then, becomes of the excess of salt?--for this is an inquiry of
the highest geological interest. The Rhone, the Po, the Nile, and many
hundred minor streams and springs, pour annually into the Mediterranean
large quantities of carbonate of lime, together with iron, magnesia,
silica, alumina, sulphur, and other mineral ingredients in a state of
chemical solution. To explain why the influx of this matter does not
alter the composition of this sea has never been regarded as a
difficulty; for it is known that calcareous rocks are forming in the
delta of the Rhone, in the Adriatic, on the coast of Asia Minor, and in
other localities. Precipitation is acknowledged to be the means whereby
the surplus mineral matter is disposed of, after the consumption of a
certain portion in the secretions of testacea, zoophytes, and other
marine animals. But before muriate of soda can, in like manner, be
precipitated, the whole Mediterranean ought, according to the received
principles of chemistry, to become as much saturated with salt as Lake
Aral, the Dead Sea, or the brine-springs of Cheshire.
It is undoubtedly true, in regard to small bodies of water, that every
particle must be fully saturated with muriate of soda before a single
crystal of salt can be formed; such is probably the case in all natural
salterns: such, for example, as those described by travellers as
occurring on the western borders of the Black Sea, where extensive
marshes are said to be covered by thin films of salt after a rapid
evaporation of sea-water. The salt _atangs_ of the Rhone, where salt has
sometimes been precipitated in considerable abundance, have been already
mentioned. In regard to the depth of the Mediterranean, it appears that
between Gibraltar and Ceuta, Captain Smyth sounded to the enormous depth
of 950 fathoms, and found there a gravelly bottom, with fragments of
broken shells. Saussure sounded to the depth of two thousand feet,
within a few yards of the shore, at Nice; and M. Barard has lately
fathomed to the depth of more than six thousand feet in several places
without reaching the bottom.[459]
The central abysses, therefore, of this sea are, in all likelihood, at
least as deep as the Alps are high; and, as at the depth of seven
hundred fathoms only, water has been found to contain a proportion of
salt four times greater than at the surface, we may presume that the
excess of salt may be much greater at the depth of two or three miles.
After evaporation, the surface water becomes impregnated with a slight
excess of salt, and its specific gravity being thus increased, it
instantly falls to the bottom, while lighter water rises to the top, or
flows in laterally, being always supplied by rivers and the current from
the Atlantic. The heavier fluid, when it arrives at the bottom, cannot
stop if it can gain access to any lower part of the bed of the sea, not
previously occupied by water of the same density.
How far this accumulation of brine can extend before the inferior strata
of water will part with any of their salt, and what difference in such a
chemical process the immense pressure of the incumbent ocean, or the
escape of heated vapors, thermal springs, or submarine volcanic
eruptions, might occasion, are questions which cannot be answered in the
present state of science.
The Straits of Gibraltar are said to become gradually wider by the
wearing down of the cliffs on each side at many points; and the current
sets along the coast of Africa, so as to cause considerable inroads in
various parts, particularly near Carthage. Near the Canopic mouth of the
Nile, at Aboukir, the coast was greatly devastated in the year 1784,
when a small island was nearly consumed. By a series of similar
operations, the old site of the cities of Nicropolis, Taposiris, Parva
and Canopus, have become a sand-bank.[460]