is, the slight relief of the ground above high water of the Red Sea,
in the whole extent of the Isthmus. There are only two points somewhat
elevated. The first, proceeding from Suez, is met with before Lake
Timsah, and is that which we shall call the Serapeum bar; its greatest
elevation is 16 _met._, 5950, above low water in the Mediterranean. The
second point is at the outlet of the lake, and its greatest elevation is
fifteen _metres_, at the spot known as the bar of _El Guisr_; but the
line of the Canal may be carried in a direction where but ten _metres_
are met with for some _kilometres_ of length. Supposing therefore the bed
of the canal to be established at the depth of 6 _met._, 50, below low
water in the Mediterranean, the greatest excavation would be at the bar
of _El Guisr_, and would show a total depth of 16 _met._, 50, which is
nothing extraordinary; supposing it even twenty _metres_, the requisite
excavation would bear no comparison with what was executed in Mexico,
during the Spanish occupation. For, in their then difficult position,
and in the absence of tools and improved means, the Spaniards were able
to effect, near the town of Mexico, which was threatened with invasion
by the waters of the neighbouring lakes, the cutting of Huehuetoca, the
total length of which is 20,585 _metres_, and its depth from forty-five
to sixty _metres_, for a length of more than 800 _metres_, and from
thirty to fifty _metres_ for a length of 3500 _metres_. And yet the
expense of this work was only 31,000,000 francs.
The levelling also shows, that by adopting 6 _met._, 50, for the bed of
the Canal, there will be a length of 18 _kil._ in the Bitter Lakes, where
there will not be a shovelful to remove, and for another 18 _kil._ there
will be very little to do; and as these lakes are dry at a depth of 8
_met._, 39, below low water, all the earth-works for the whole length of
them could be performed in the dry, if found advantageous to do so.
The numerous transverse sections taken with the levelling of 1847, enable
us to ascertain approximately the superfice of the Bitter Lakes at the
water line. This superfice is about 330,000,000 _square metres_. If,
then, the action of the tide, which brings two _metres_ of moving water,
be admitted into these lakes, a disposable volume of 660,000,000 _cubic
metres_ of water would be accumulated, and which might be raised to
800,000,000 by adding Lake Timsah and the retaining basins at Suez and
Pelusium to these immense reservoirs.
Before pointing out the various directions of the adopted track, it
appears necessary to arrive at a fixed opinion as to the formation of
the Isthmus and of the downs by which it is partly covered, and also as
to the accumulations of sand which exist both on the coast of Pelusium
and at the bottom of the Gulf of Suez; for it is from the explanation
of these phenomena that we shall start in our justification of the
arrangements of the direct track in general and in detail.
By attentively observing what is passing before our eyes at the present
time, in respect of the destruction and recomposition of continuity, we
may come to an exact conclusion regarding the laws which operated towards
the first ages of the world in the formation of alluvial lands.
Let us first examine what is going on in the English Channel; for this
narrow sea having a large number of ports both on the French and English
coasts, has on that account been the object of numerous observations by
engineers.
The first well recognised fact is the destruction of the coast from the
point of Barfleur as far as the Somme, a distance of 338 _kilometres_;
and on the other side of the channel, from the Isle of Wight to Dover, a
distance of 250 _kilometres_. This action is produced by the alternation
of frost and thaw, by dry and moist winds, and by the saline evaporation
of the sea. The abrasion observed on the coast of Calvados is an average
of 0 _met._, 25, _per ann._ and on the coasts of Normandy and England
0 _met._, 30. The mean height of the cliffs on either side being sixty
_metres_, it follows that the channel swallows up an amount of 10,000,000
_cubic metres_ of earth and stones every year, which must find a place
somewhere.
The second fact, equally well established, and which, though opposed
to the opinion of the ancients, can no longer leave any doubt on the
mind, is, that rivers, with a few rare exceptions,—such as the Loire
for instance,—only carry to the sea an extremely thin mud, destined to
be lost in the mass of matter held in suspension by the latter; that
the sands of rivers do not in general reach the sea, and that the muddy
or sandy deposits observed in tidal rivers, are entirely owing to the
matters brought by the tide. This discovery has been arrived at as
follows.
In making the analysis of the alluvial lands forming the Bay of St.
Michael, it was found that the principal substances of their formation
are silex and the carbonate of lime; that the nearer the sea is
approached, the more the proportion of silex increases; the more it is
receded from, the more considerable the proportion of carbonate of lime
becomes. Now if the basins of the three rivers which discharge themselves
into this bay, the Sée, the Selime and the Couësnon, be examined, they
will be found entirely destitute of calcareous substances. It is the same
with the coasts of the channel and of Brittany. It cannot, therefore, be
either from these rivers or from the coasts that the enormous proportion
of silex proceeds which has just been described. If samples are examined
with a magnifying glass, commencing with those nearest the sea, and
afterwards proceeding farther into the bay, in the first, fragments of
shells are perceived quite distinguishable, then these fragments are
reduced and become so impalpable, that the best glass will no longer
enable us to distinguish the form in the most calcareous portions.
It is, therefore, certain that the calcareous part comes exclusively
from the sea, and even from the bottom of the roadstead of Cancale. As
for the silex and clay, a part in their deposit may be attributed to the
rivers; but it should first be understood how unimportant these three
small rivers are, each discharging not more than an average of eight to
ten _cubic metres_ of water per second. Farther, if the contributions
of the rivers reckoned for anything in the deposits which are made in
this locality, clayey or gravelly stratifications would be seen on their
banks at the parts where the tide is least felt. Nothing of the kind
occurs. The mixture of the calcareous matter, the grains of silex, and
the argillaceous atoms is so intimate, that it is evident it could only
be made at the very centre of the production of the calcareous matter;
that is to say, at the bottom of the sea. If the fluviatile deposit
was appreciable, it would counterbalance entirely, or in part, the
calcareous overplus in the drift taken from the top of the roadstead, as
compared with that taken at the bottom. Far from this being the case, the
progression of the calcareous element, which can only come from the sea,
is seen in proportion to the elevation of the shores. Finally, if the
fluviatile deposit ought to be reckoned for anything, a larger proportion
of clay would be seen upon the brink of the Sée, which traverses fissile
lands, than in the neighbouring channel of the Couësnon, which traverses
lands of a much harder character, furnishing less clay than the fissile
ground of the Sée and the Selime. Now, the contrary is the case; the
drifts of the neighbouring channel of the Couësnon are more clayey than
the others, solely because this channel being more sheltered than the
beds of the other two rivers, the muddy matter which the sea always holds
so abundantly in suspension, and which it deposits in the basins of
ports, can be carried there concurrently with the drifts.
On making the same investigations for the Seine, it was found that the
sands transported by this river do not pass Rouen, and that all the
accretions that are seen lower down, as far as the flats which are met
with at its mouth, are deposits by the sea.
The same results were arrived at for the Scheld.
As to the Meuse and the Rhine, the following deductions have been made.
The abrasion of the coasts of the channel supplies the sea with fragments
of chalk and siliceous rocks, which being rolled about by the sea become
shingle. This shingle forms banks along the English and French coasts,
and forced by the double action of flood and wind towards the straits
it approaches them; but the shingle on the coast of France continually
decreasing in size, reaches the mouth of the Somme, where it finds
the point of Cayeux formed by its accumulation. Stopped at this point
by the waters of the Somme, and by the change in the direction of the
current of the sea which turns towards the Pas de Calais, this shingle
increases the point of Cayeux, so long as its continual collision has not
sufficiently reduced the size of the stones for them to be carried away
by the sea; but when they are small enough, the flood bears them away and
distributes them on the numerous banks which are found between the Somme
and the Pas de Calais. From the inspection of Marine Charts, it is seen
that the fineness of the deposit increases in proportion as these banks
are nearer to the straits, and if the banks disappear in the Straits,
it is because the force of the current does not allow the sands, which
from being sifted for a long time have become finer and finer, to stop
in that passage. They pass it therefore and some go to form the downs
between Dunkirk and the Scheld, others in like manner to form downs on
the English coast, others remaining in the strongest currents are carried
as far as the mouths of the Humber in England, and of the Meuse and Rhine
on the Continent.
If the shape of the English and French coasts to the north and south
of the Straits is observed attentively, it will strike every one that
those to the south are cut out into concave indentations, while those to
the north all affect the convex form. It is because the coasts to the
south of the Straits are abraded by the tide, and those to the north,
on the contrary, are fed by the accretions. As for the muddy matters in
this long course, they can only be deposited in a few perfectly tranquil
creeks, or in the basins of open ports on either coast. Wherever the tide
penetrates they are carried with it, and, when finally it has entered the
northern sea, and made a course sufficiently long to abate its swiftness,
it finds itself in an excellent condition for depositing these muddy
matters, which it holds in suspension. This is what it does at the mouth
of the Humber, where it completely chokes up the port of Hull.
In like manner, the muddy matters form at the mouth of the Rhine, of the
Meuse, and of the Scheld, those immense polders, which constitute such an
essential part of the territory of Holland, and the numerous banks at the
mouth of these rivers are only composed of sand and carbonate of lime.
Now the rapidity of the current, long before reaching the mouth, is not
sufficiently great to carry down the sands; in fact, no trace of them is
perceived; these banks are therefore the production of the sea.
Finally, in order to appreciate at the _maximum_ the power of the
fluviatile deposit in the formation of the coasts, observations have been
made upon the Yssel, that branch of the Rhine which discharges itself
into the Zuyderzee. This sea has but very feeble tides, 0 _met._, 40,
at ordinary high water, and very much resembles the Mediterranean, the
Black Sea, and the Adriatic Gulf in this respect. A muddy Delta has also
been formed at the mouth of the Yssel, of the same shape as those of
the Rhone, the Po, the Nile, &c. &c. This Delta cannot be exclusively
owing to the Yssel, because, although it is true that the tides of the
Zuyderzee are very feeble, on the other hand the shores which surround
it are of an exceedingly friable nature; now, however feeble the tides
may be, they yet attack the banks, and what proves it, is, that the
Hollanders are obliged to defend them. By considering the Delta of the
Yssel as a fluviatile deposit solely, we shall therefore have an extreme
case. Now, this Delta has a superfice of only 1500 _hectares_, while the
superfice of the land in Holland, which is evidently of modern deposit,
is at least 1,000,000 _hectares_. If it is observed that the Yssel only
emits a fifteenth of the whole volume of the Rhine and the Meuse united,
it will give 22,500 _hectares_ for the deposit of the river, against
1,000,000 deposited by the sea; which is scarcely two per cent. of what
the sea has furnished in the formation of the polders of Holland.
From the examination of all these facts, it evidently results, as we have
said, that in seas with tides, the rivers not only do not form banks,
alluvium, or deltas at their mouths, but farther, that the alluvium found
in the regions of these rivers submitted to the action of the tide, is
deposited by the sea.
We shall now prove that these conclusions are equally true for the
rivers of the Mediterranean, notwithstanding the opinion of the Italian
engineers, who have considered the fluviatile origin of their deltas as
demonstrated.
To give an idea of the propagation of the waves or billows of the sea
agitated by the wind, they have been compared to a field of corn under
the action of the air. It seems as if the ears of corn had an impulsive
swiftness, which however does not exist, since they do not quit their
places. Farther, if the wind is feeble, it is only the ears which waver
without the stalks being shaken; but, in proportion as the wind rises,
the stalks take part in the movement to a greater and greater depth down
to the root.
The waves have been again compared to the movements of a cord, which
is made to undulate by shaking one of its extremities in the hand. It
seems as if the cord was going at a rapid rate, while in reality it does
not quit the hand that shakes it, only each point of it rises and falls
alternately, and this movement is the greater according as the impulse is
stronger; if the extremity of the cord opposite to that which receives
this impulse encounters an obstacle, as the surface of a wall for
instance, it will strike it at each movement of the hand.
It is exactly the same with the waves of the sea; every fluid molecule
placed at the surface of the billow experiences an oscillatory movement
nearly vertical, so that if a body floating on the surface of a wave is
watched it will be seen to remain in the same place, sometimes in the
hollow of the wave, sometimes on its summit, and if at length it changes
its place, that depends upon other circumstances, such as the force of
the wind or the direction of the currents.
This oscillatory movement which is perceived on the surface of the sea,
is necessarily developed to a certain depth, which will be greater in
proportion as the undulations are stronger at the surface. This fact
has been confirmed by experiment; it has been ascertained in effect
that the agitation of the sea caused by the wind, is communicated to a
certain depth, variable according to the wind, according to the sea, and
the places where the observations were made, and that beyond that depth
the sea is perfectly calm. Thus it may be admitted as an observed and
well proved fact, that the waves require a certain depth for their free
developement; if an obstacle is presented to this developement, there
will be a forcible re-action of the wave against this obstacle which will
be carried off, if it is moveable, and will enter into the system of the
wave. This action of the waves against the deeps is what is called the
_ground swell_.
This established, it has already been seen that the coast of the
sea, as well as the projecting capes, resign to the sea every year a
certain amount of earthy and rocky matters. These matters are removed
by the waves which break upon the shore, the soft portions are quickly
disintegrated by this powerful action, and form muddy sand and mud,
and the hard portions are rounded into pebbles the size of which is
diminished more and more by the prolonged action of the force which set
them in motion and which reduces them to sand; but in proportion as these
matters arrive at a sufficient degree of tenuity, they become susceptible
of submitting to the transporting force of the waves and currents, and
quit the place where they were formed.
This transporting force depends both upon the height of the tides and the
direction of the winds, as well as their intensity, combined with that of
the currents which are observed in all seas. So that while considerable
masses of matter are set in motion along the shores, the rivers,
especially those which traverse a great extent of country, transport as
far as their mouths only muddy matters, so light that they are carried
to a distance, and afterwards deposited in the depths of the sea. This
is remarkably the case with the Nile, whose waters at the time of the
inundation are distinguished by their colour for more than ten leagues
into the sea. All the deposits and accretions of the river up to 20
_kilom._ above its mouth are muddy, while all the banks which are at its
mouth are composed of sand alone.
Thus, all the impediments of the mouth of the Nile evidently emanate from
the sea. To demonstrate it by still farther evidence, we will repeat the
reasoning of the engineer M. Bonniceau relative to the alluvium of the
river Mersey, in his excellent work upon the navigation of tidal rivers:
“If the deposits emanated from the elevated lands in a sensible degree,
the quantities deposited from time to time ought to be proportional to
the quantity of rain that falls at the same epochs, because the same
amount of matters descending from the elevated lands and transported by
the river, ought to be partly regulated by the quantity of water that
carries them; but it is a fact well ascertained, that the accumulations
of sand which exist in the vicinity of the mouth are greater in
proportion as the waters of the river are less abundant, while on the
contrary at the time of the increase, when the Nile contains nearly 0
_met._ 008 of matter in suspension, the sand banks are removed and thrown
back far off into the sea.”
It is said that Alexander the Great was determined in his choice of the
situation for the port of Alexandria by the consideration of the winds
and littoral currents which carry eastward the matters held in suspension
by the Nile, and thus cover the coast with sand. If this theory were
true, no alluvium ought to be perceived westward of the mouth of the
river. Now all the coast from Tripoli as far as El Aritch is covered with
sand banks, which frequently form downs, and these downs are found at the
present time transported several leagues into the interior of the lands
westward of the Rosetta mouth.
The port of Alexandria itself has not escaped the action of the ground
swell, for a sand bank has been formed which occupies a good third of the
total superfice of the roadstead. Happily for the port the accumulation
of sand appears to have been arrested long ago, or rather its increase
has become imperceptible.
The roadstead of Alexandria owes its depth to the disposition of its
sides in respect to the winds and currents. It is like the roadstead of
Algiers, which is everywhere very deep, while the neighbouring ports have
sand banks. It cannot be said that these sand banks are owing to the
presence of rivers, which do not exist in the whole extent of the coast
of Barbary, for the few land streams that are scattered along the shore
cannot be called by that name.
We have seen that the winds and the currents carry the detritus of
the coasts reduced to sand to great distances. The currents however do
not arrest the motion of the waves and the ground swell; they bend to
their forms, and as their direction necessarily tends to the shore, the
sands clear the currents with the ground swell which contains them, and
which thus conducts them to the shore. When the direction of the waves
is oblique to the coast, the sands are borne to a distance, but when it
is perpendicular to the coast, the waves raise the sands brought by the
ground swell into dykes and banks, which protect the low shores. The most
minute and lightest portions are accumulated at the more elevated points
of the flats, where, being dried by the sun, they are soon carried away
by the wind, which leaves them, in its turn, in the shape of downs. The
ground swell, therefore, furnishes the materials of those downs, which
usually border flat shores, and it is that which has drawn from the
depths of the sea the sands of those immense deserts of Africa, and of so
many other plains which are found in various parts of the globe.
“Often,” says M. Jomard, “have I remained for whole hours, contemplating
the origin and progress of the phenomenon of the formation of sands. I
saw the waves break and deposit a small line, scarcely discernible, of
very fine sand. Another wave came, burdened like the first, and this new
line of sand pushed the first slightly on. This, once beyond the reach of
the water, and exposed to the rays of a burning sun, was quickly dried,
and became the prey of the wind, which immediately seized and carried it
off into the air; the less light particles of gravel did not reach so
far, but, subjected to the alternate motion, diminished more and more,
and were converted by degrees into sand.”
We may also say with Colonel Emy, that
“All river bars are deposits, brought or arrested by the ground swell,
and without it these deposits would be repelled into the main as far
as the rivers extend their course. The Delta of the Nile, those of the
Mississippi, of the Ganges, of the Scheld, of the Meuse, of the Rhine,
and the Camargue of the Rhone, were originally bars formed by this same
ground swell.”
The tongues of sand which separate the lake of Thau from the gulf of the
Lion, the tongue of earth upon which Alexandria is built, those which
separate the lakes Bourlos and Menzaleh from the Mediterranean, are bars
of sand formed by the ground swell. The sand bank which separates from
the Red Sea the vast basin of the Bitter Lakes, was, without any doubt, a
ford elevated by the ground swell, which, in tempestuous weather, ascends
this sea with the current of the tides charged with sand. The ford, which
answers at the present time at Suez, was certainly formed in this manner
by the ground swell.
We may say also, that the whole Isthmus of Suez was formed by the
maritime deposits of the Mediterranean and of the Red Sea. We believe
that, previous to historic times, the two Seas were in communication
with each other, that the detritus of the chains of mountains situated
to the right and left, carried down by rain, filled up the space which
separates them, and that when that space was elevated to such a height
that the ground swell could reach it, its action was applied in such a
way that by the meeting of the swell of the two Seas, a bank was formed,
which is no other than the bar of El Guisr. After the formation of this
bank, the combined action of the ground swell, both on one side and the
other, and the accretions from the neighbouring mountains continued until
the Isthmus was dry. Then the soil thus constituted was covered by the
downs, which advanced upon it from the direction of Pelusium, driven by
the north winds, and from the direction of Suez, driven by the winds and
currents from the south.
In this state the Isthmus is at present, and the numerous soundings which
we have asked for from His Highness the Viceroy, will prove whether our
hypothesis is well founded or not.
The same theory may, as Colonel Emy observes in his remarkable work,
throw a new light on important geological facts:—
“For instance, those ancient and elevated plains, composed of sand and
pebbles, the formation of which, it has been attempted to explain, by
the revolutions of the globe and violent convulsions of nature, or which
have been regarded as deposits left by rivers, appear to be maritime
accretions. If is, indeed, easy to conceive rivers capable of bringing
down the fragments detached from mountains, by shocks, and by the
decomposition of the rocks; but how could they extend those fragments
uniformly, and over spaces so extensive as the plains in question?
Moreover, was not the course from the summits of the mountains generally
too short for the fragments of the excessively hard rock found in some
of those plains, to have time to acquire their roundness? The rivers
have prolonged their courses through these accumulations of pebbles;
they may, in overflowing, have covered them with sand and earth, but it
is more probable that they contributed in nowise to the formation of
these accretions, unless it were by transporting the rough materials to
the sea. Nothing but the ground swell could spread these fragments of
mountains so uniformly as they are, convert them into shingle and sand by
a long trituration on the shores, where it had driven them; gather them
either into banks or plains, and thus fill up spaces over which the sea
formerly extended.
“The ancient collections of shingle, pebbles and sand are owing, like
those at present forming in a similar manner, to maritime accretions,
and must henceforth be regarded as an incontestable proof that the
ocean formerly reached and was long stationary at different heights far
exceeding its present level.”
It is not surprising, then, to find on divers points of the Isthmus
pieces of hard stone broken into small fragments, and half rounded,
covering the sand-banks at variable heights above the level of the
Mediterranean.