REPRODUCTIVE EFFECTS OF RIVERS.
Lake deltas--Growth of the delta of the Upper Rhine in the Lake of
Geneva--Computation of the age of deltas--Recent deposits in Lake
Superior--Deltas of inland seas--Course of the Po--Artificial
embankments of the Po and Adige--Delta of the Po, and other rivers
entering the Adriatic--Rapid conversion of that gulf into
land--Mineral characters of the new deposits--Marine delta of the
Rhone--Various proofs of its increase--Stony nature of its
deposits--Coast of Asia Minor--Delta of the Nile.
DELTAS IN LAKES.
I have already spoken in the 14th chapter of the action of running
water, and of the denuding power of rivers, but we can only form a just
conception of the excavating and removing force exerted by such bodies
of water, when we have the advantage of examining the reproductive
effects of the same agents: in other words, of beholding in a palpable
form the aggregate amount of matter, which they have thrown down at
certain points in their alluvial plains, or in the basins of lakes and
seas. Yet it will appear, when we consider the action of currents, that
the growth of deltas affords a very inadequate standard by which to
measure the entire carrying power of running water, since a considerable
portion of fluviatile sediment is swept far out to sea.
Deltas may be divided into, first, those which are formed in lakes;
secondly, those in island seas, where the tides are almost
imperceptible; and, thirdly, those on the borders of the ocean. The most
characteristic distinction between the lacustrine and marine deltas
consists in the nature of the organic remains which become imbedded in
their deposits; for, in the case of a lake, it is obvious that these
must consist exclusively of such genera of animals as inhabit the land
or the waters of a river or a lake; whereas, in the other case, there
will be an admixture, and most frequently a predominance, of animals
which inhabit salt water. In regard, however, to the distribution of
inorganic matter, the deposits of lakes and seas are formed under very
analogous circumstances.
_Lake of Geneva._--Lakes exemplify the first reproductive operations in
which rivers are engaged when they convey the detritus of rocks and the
ingredients of mineral springs from mountainous regions. The accession
of new land at the mouth of the Rhone, at the upper end of the Lake of
Geneva, or the Leman Lake, presents us with an example of a considerable
thickness of strata which have accumulated since the historical era.
This sheet of water is about thirty-seven miles long, and its breadth is
from two to eight miles. The shape of the bottom is very irregular, the
depth having been found by late measurements to vary from 20 to 160
fathoms.[335] The Rhone, where it enters at the upper end, is turbid and
discolored; but its waters, where it issues at the town of Geneva, are
beautifully clear and transparent. An ancient town, called Port Vallais
(Portus Valesiae of the Romans), once situated at the water's edge, at
the upper end, is now more than a mile and a half inland--this
intervening alluvial tract having been acquired in about eight
centuries. The remainder of the delta consists of a flat alluvial plain,
about five or six miles in length, composed of sand and mud, a little
raised above the level of the river, and full of marshes.
Sir Henry De la Beche found, after numerous soundings in all parts of
the lake, that there was a pretty uniform depth of from 120 to 160
fathoms throughout the central region, and on approaching the delta, the
shallowing of the bottom began to be very sensible at a distance of
about a mile and three quarters from the mouth of the Rhone; for a line
drawn from St. Gingoulph to Vevey gives a mean depth of somewhat less
than 600 feet, and from that part of the Rhone, the fluviatile mud is
always found along the bottom.[336] We may state, therefore, that the
new strata annually produced are thrown down upon a slope about two
miles in length; so that, notwithstanding the great depth of the lake,
the new deposits are inclined at so slight an angle, that the dip of the
beds would be termed, in ordinary geological language, horizontal.
The strata probably consist of alternations of finer and coarser
particles; for, during the hotter months from April to August, when the
snows melt, the volume and velocity of the river are greatest, and large
quantities of sand, mud, vegetable matter, and drift-wood are
introduced; but during the rest of the year, the influx is comparatively
feeble, so much so, that the whole lake, according to Saussure, stands
six feet lower. If, then, we could obtain a section of the accumulation
formed in the last eight centuries, we should see a great series of
strata, probably from 600 to 900 feet thick (the supposed original depth
of the head of the lake), and nearly two miles in length, inclined at a
very slight angle. In the mean time, a great number of smaller deltas
are growing around the borders of the lake, at the mouths of rapid
torrents, which pour in large masses of sand and pebbles. The body of
water in these torrents is too small to enable them to spread out the
transported matter over so extensive an area as the Rhone does. Thus,
for example, there is a depth of eighty fathoms within half a mile of
the shore, immediately opposite the great torrent which enters east of
Ripaille, so that the dip of the strata in that minor delta must be
about four times as great as those deposited by the main river at the
upper extremity of the lake.[337]
_Chronological computations of the age of deltas._--The capacity of this
basin being now ascertained, it would be an interesting subject of
inquiry, to determine in what number of years the Leman Lake will be
converted into dry land. It would not be very difficult to obtain the
elements for such a calculation, so as to approximate at least to the
quantity of time required for the accomplishment of the result. The
number of cubic feet of water annually discharged by the river into the
lake being estimated, experiments might be made in the winter and summer
months, to determine the proportion of matter held in suspension or in
chemical solution by the Rhone. It would be also necessary to allow for
the heavier matter drifted along at the bottom, which might be estimated
on hydrostatical principles, when the average size of the gravel and the
volume and velocity of the stream at different seasons were known.
Supposing all these observations to have been made, it would be more
easy to calculate the future than the former progress of the delta,
because it would be a laborious task to ascertain, with any degree of
precision, the original depth and extent of that part of the lake which
is already filled up. Even if this information were actually obtained by
borings, it would only enable us to approximate within a certain number
of centuries to the time when the Rhone began to form its present delta;
but this would not give us the date of the origin of the Leman Lake in
its present form, because the river may have flowed into it for
thousands of years, without importing any sediment whatever. Such would
have been the case, if the waters had first passed through a chain of
upper lakes; and that this was actually the fact, seems indicated by the
course of the Rhone between Martigny and the Lake of Geneva, and, still
more decidedly, by the channels of many of its principal feeders.
If we ascend, for example, the valley through which the Dranse flows, we
find that it consists of a succession of basins, one above the other, in
each of which there is a wide expanse of flat alluvial lands, separated
from the next basin by a rocky gorge, once perhaps the barrier of a
lake. The river seems to have filled these lakes, one after the other,
and to have partially cut through the barriers, some of which it is
still gradually eroding to a greater depth. Before, therefore, we can
pretend even to hazard a conjecture as to the era at which the principal
delta of Lake Leman or any other delta commenced, we must be thoroughly
acquainted with the geographical features and geological history of the
whole system of higher valleys which communicate with the main stream,
and all the changes which they have undergone since the last series of
convulsions which agitated and altered the face of the country.
_Lake Superior._--Lake Superior is the largest body of freshwater in the
world, being above 1700 geographical miles in circumference when we
follow the sinuosities of its coasts, and its length, on a curved line
drawn through its centre, being more than 400, and its extreme breadth
above 150 geographical miles. Its surface is nearly as large as the
whole of England. Its average depth varies from 80 to 150 fathoms; but,
according to Captain Bayfield, there is reason to think that its
greatest depth would not be overrated at 200 fathoms, so that its bottom
is, in some parts, nearly 600 feet below the level of the Atlantic, its
surface being about as much above it. There are appearances in different
parts of this, as of the other Canadian lakes, leading us to infer that
its waters formerly occupied a higher level than they reach at present;
for at a considerable distance from the present shores, parallel lines
of rolled stones and shells are seen rising one above the other, like
the seats of an amphitheatre. These ancient lines of shingle are exactly
similar to the present beaches in most bays, and they often attain an
elevation of 40 or 50 feet above the present level. As the heaviest
gales of wind do not raise the waters more than three or four feet, the
elevated beaches have by some been referred to the subsidence of the
lake at former periods, in consequence of the wearing down of its
barrier; by others to the upraising of the shores by earthquakes, like
those which have produced similar phenomena on the coast of Chili.
The streams which discharge their waters into Lake Superior are several
hundred in number, without reckoning those of smaller size; and the
quantity of water supplied by them is many times greater than that
discharged at the Falls of St. Mary, the only outlet. The evaporation,
therefore, is very great, and such as might be expected from so vast an
extent of surface. On the northern side, which is encircled by primary
mountains, the rivers sweep in many large boulders with smaller gravel
and sand, chiefly composed of granitic and trap rocks. There are also
currents in the lake in various directions, caused by the continued
prevalence of strong winds, and to their influence we may attribute the
diffusion of finer mud far and wide over great areas; for by numerous
soundings made during Captain Bayfield's survey, it was ascertained that
the bottom consists generally of a very adhesive clay, containing shells
of the species at present existing in the lake. When exposed to the air,
this clay immediately becomes indurated in so great a degree, as to
require a smart blow to break it. It effervesces slightly with diluted
nitric acid, and is of different colors in different parts of the lake;
in one district blue, in another red, and in a third white, hardening
into a substance resembling pipeclay.[338] From these statements, the
geologist will not fail to remark how closely these recent lacustrine
formations in America resemble the tertiary argillaceous and calcareous
marls of lacustrine origin in Central France. In both cases many of the
genera of shells most abundant, as Limnea and Planorbis, are the same;
and in regard to other classes of organic remains there must be the
closest analogy, as I shall endeavor more fully to explain when speaking
of the imbedding of plants and animals in recent deposits.
DELTAS OF INLAND SEAS.
Having thus briefly considered some of the lacustrine deltas now in
progress, we may next turn our attention to those of inland seas.
_Course of the Po._--The Po affords an instructive example of the manner
in which a great river bears down to the sea the matter poured into it
by a multitude of tributaries descending from lofty chains of mountains.
The changes gradually effected in the great plain of Northern Italy,
since the time of the Roman republic, are considerable. Extensive lakes
and marshes have been gradually filled up, as those near Placentia,
Parma, and Cremona, and many have been drained naturally by the
deepening of the beds of rivers. Deserted river-courses are not
unfrequent, as that of the Serio Morto, which formerly fell into the
Adda, in Lombardy. The Po also itself has often deviated from its
course, having after the year 1390 deserted part of the territory of
Cremona, and invaded that of Parma; its old channel being still
recognizable, and bearing the name of Po Morto. There is also an old
channel of the Po in the territory of Parma, called Po Vecchio, which
was abandoned in the twelfth century, when a great number of towns were
destroyed.
_Artificial embankments of Italian rivers._--To check these and similar
aberrations, a general system of embankment has been adopted; and the
Po, Adige, and almost all their tributaries, are now confined between
high artificial banks. The increased velocity acquired by streams thus
closed in, enables them to convey a much larger portion of foreign
matter to the sea; and, consequently, the deltas of the Po and Adige
have gained far more rapidly on the Adriatic since the practice of
embankment became almost universal. But, although more sediment is borne
to the sea, part of the sand and mud, which in the natural state of
things would be spread out by annual inundations over the plain, now
subsides in the bottom of the river-channels; and their capacity being
thereby diminished, it is necessary, in order to prevent inundations in
the following spring, to extract matter from the bed, and to add it to
the banks of the river. Hence it happens that these streams now traverse
the plain on the top of high mounds, like the waters of aqueducts, and
at Ferrara the surface of the Po has become more elevated than the roofs
of the houses.[339] The magnitude of these barriers is a subject of
increasing expense and anxiety, it having been sometimes found necessary
to give an additional height of nearly one foot to the banks of the
Adige and Po in a single season.
The practice of embankment was adopted on some of the Italian rivers as
early as the thirteenth century; and Dante, writing in the beginning of
the fourteenth, describes, in the seventh circle of hell, a rivulet of
tears separated from a burning sandy desert by embankments "like those
which, between Ghent and Bruges, were raised against the ocean, or those
which the Paduans had erected along the Brenta to defend their villas on
the melting of the Alpine snows."
Quale i Fiamminghi tra Guzzante e Bruggia,
Temendo il fiotto che in ver lor s'avventa,
Fanno lo schermo, perche il mar si fuggia,
E quale i Padovan lungo la Brenta,
Per difender lor ville e lor castelli,
Anzi che Chiarentana il caldo senta.--
_Inferno_, Canto xv.
In the Adriatic, from the northern part of the Gulf of Trieste, where
the Isonzo enters, down to the south of Ravenna, there is an
uninterrupted series of recent accessions of land, more than 100 miles
in length, which, within the last 2000 years, have increased from _two
to twenty miles in breadth_. A line of sand-bars of great length has
been formed nearly all along the western coast of this gulf, inside of
which are lagunes, such as those of Venice, and the large lagune of
Comacchio, 20 miles in diameter. Newly deposited mud brought down by the
streams is continually lessening the depth of the lagunes, and
converting part of them into meadows.[340] The Isonzo, Tagliamento,
Piave, Brenta, Adige, and Po, besides many other inferior rivers,
contribute to this advance of the coast-line and to the shallowing of
the lagunes and the gulf.
_Delta of the Po._--The Po and the Adige may now be considered as
entering by one common delta, for two branches of the Adige are
connected with arms of the Po, and thus the principal delta has been
pushed out beyond those bars which separate the lagunes from the sea.
The rate of the advance of this new land has been accelerated, as before
stated, since the system of embanking the rivers became general,
especially at that point where the Po and Adige enter. The waters are no
longer permitted to spread themselves far and wide over the plains, and
to leave behind them the larger portion of their sediment. Mountain
torrents also have become more turbid since the clearing away of
forests, which once clothed the southern flanks of the Alps. It is
calculated that the mean rate of advance of the delta of the Po on the
Adriatic between the years 1200 and 1600 was 25 yards or metres a year,
whereas the mean annual gain from 1600 to 1804 was 70 metres.[341]
Adria was a seaport in the time of Augustus, and had, in ancient times,
given its name to the gulf; it is now about twenty Italian miles inland.
Ravenna was also a seaport, and is now about four miles from the main
sea. Yet even before the practice of embankment was introduced, the
alluvium of the Po advanced with rapidity on the Adriatic; for Spina, a
very ancient city, originally built in the district of Ravenna, at the
mouth of a great arm of the Po, was, so early as the commencement of our
era, eleven miles distant from the sea.[342]
But although so many rivers are rapidly converting the Adriatic into
land, it appears, by the observations of M. Morlot, that since the time
of the Romans, there has been a general subsidence of the coast and bed
of this sea in the same region to the amount of five feet, so that the
advance of the new-made land has not been so fast as it would have been
had the level of the coast remained unaltered. The signs of a much
greater depression anterior to the historical period have also been
brought to light by an Artesian well, bored in 1847, to the depth of
more than 400 feet, which still failed to penetrate through the modern
fluviatile deposit. The auger passed chiefly through beds of sand and
clay, but at four several depths, one of them very near the bottom of
the excavation, it pierced beds of turf, or accumulations of vegetable
matter, precisely similar to those now formed superficially on the
extreme borders of the Adriatic. Hence we learn that a considerable area
of what was once land has sunk down 400 feet in the course of ages.[343]
The greatest depth of the Adriatic, between Dalmatia and the mouths of
the Po, is twenty-two fathoms; but a large part of the Gulf of Trieste
and the Adriatic, opposite Venice, is less than twelve fathoms deep.
Farther to the south, where it is less affected by the influx of great
rivers, the gulf deepens considerably. Donati, after dredging the
bottom, discovered the new deposits to consist partly of mud and partly
of rock, the rock being formed of calcareous matter, incrusting shells.
He also ascertained, that particular species of testacea were grouped
together in certain places, and were becoming slowly incorporated with
the mud or calcareous precipitates.[344] Olivi, also, found some
deposits of sand, and others of mud, extending half way across the gulf;
and he states that their distribution along the bottom was evidently
determined by the prevailing current.[345] It is probable, therefore,
that the finer sediment of all the rivers at the head of the Adriatic
may be intermingled by the influence of the current; and all the central
parts of the gulf may be considered as slowly filling up with horizontal
deposits, similar to those of the Subapennine hills, and containing many
of the same species of shells. The Po merely introduces at present fine
sand and mud, for it carries no pebbles farther than the spot where it
joins the Trebia, west of Piacenza. Near the northern borders of the
basin, the Isonzo, Tagliamento, and many other streams, are forming
immense beds of sand and some conglomerate; for here some high mountains
of Alpine limestone approach within a few miles of the sea.
In the time of the Romans, the hot-baths of Monfalcone were on one of
several islands of Alpine limestone, between which and the mainland, on
the north, was a channel of the sea, about a mile broad. This channel is
now converted into a grassy plain, which surrounds the islands on all
sides. Among the numerous changes on this coast, we find that the
present channel of the Isonzo is several miles to the west of its
ancient bed, in part of which, at Ronchi, the old Roman bridge which
crossed the Via Appia was lately found buried in fluviatile silt.
_Marine delta of the Rhone._--The lacustrine delta of the Rhone in
Switzerland has already been considered (p. 251), its contemporaneous
marine delta may now be described. Scarcely has the river passed out of
the Lake of Geneva before its pure waters are again filled with sand and
sediment by the impetuous Arve, descending from the highest Alps, and
bearing along in its current the granitic detritus annually brought down
by the glaciers of Mont Blanc. The Rhone afterwards receives vast
contributions of transported matter from the Alps of Dauphiny, and the
primary and volcanic mountains of Central France; and when at length it
enters the Mediterranean, it discolors the blue waters of that sea with
a whitish sediment, for the distance of between six and seven miles,
throughout which space the current of fresh water is perceptible.
Strabo's description of the delta is so inapplicable to its present
configuration, as to attest a complete alteration in the physical
features of the country since the Augustan age. It appears, however,
that the head of the delta, or the point at which it begins to ramify,
has remained unaltered since the time of Pliny, for he states that the
Rhone divided itself at Arles into two arms. This is the case at
present; one of the branches, the western, being now called Le Petit
Rhone, which is again subdivided before entering the Mediterranean. The
advance of the base of the delta, in the last eighteen centuries, is
demonstrated by many curious antiquarian monuments. The most striking of
these is the great and unnatural datour of the old Roman road from
Ugernum to Beziers (_Boeterrae_) which went round by Nismes
(_Nemausus_). It is clear that, when this was first constructed, it was
impossible to pass in a direct line, as now, across the delta, and that
either the sea or marshes intervened in a tract now consisting of terra
firma.[346] Astruc also remarks, that all the places on low lands, lying
to the north of the old Roman road between Nismes and Beziers, have
names of Celtic origin, evidently given to them by the first inhabitants
of the country; whereas, the places lying south of that road, towards
the sea, have names of Latin derivation, and were clearly founded after
the Roman language had been introduced.
Another proof, also, of the great extent of land which has come into
existence since the Romans conquered and colonized Gaul, is derived from
the fact, that the Roman writers never mention the thermal waters of
Balaruc in the delta, although they were well acquainted with those of
Aix, and others still more distant, and attached great importance to
them, as they invariably did to all hot springs. The waters of Balaruc,
therefore, must have formerly issued under the sea--a common phenomenon
on the borders of the Mediterranean; and on the advance of the delta
they continued to flow out through the new deposits.
Among the more direct proofs of the increase of land, we find that Mese,
described under the appellation of Mesua Collis by Pomponius Mela,[347]
and stated by him to be nearly an island, is now far inland. Notre Dame
des Ports, also, was a harbor in 898, but is now a league from the
shore. Psalmodi was an island in 815, and is now two leagues from the
sea. Several old lines of towers and sea-marks occur at different
distances from the present coast, all indicating the successive retreat
of the sea, for each line has in its turn become useless to mariners;
which may well be conceived, when we state that the Tower of Tignaux,
erected on the shore so late as the year 1737, is already a mile remote
from it.[348]
By the confluence of the Rhone and the currents of the Mediterranean,
driven by winds from the south, sand-bars are often formed across the
mouths of the river; by these means considerable spaces become divided
off from the sea, and subsequently from the river also, when it shifts
its channels of efflux. As some of these lagoons are subject to the
occasional ingress of the river when flooded, and of the sea during
storms, they are alternately salt and fresh. Others, after being filled
with salt water, are often lowered by evaporation till they become more
salt than the sea; and it has happened, occasionally, that a
considerable precipitate of muriate of soda has taken place in these
natural salterns. During the latter part of Napoleon's career, when the
excise laws were enforced with extreme rigor, the police was employed to
prevent such salt from being used. The fluviatile and marine shells
inclosed in these small lakes often live together in brackish water; but
the uncongenial nature of the fluid usually produces a dwarfish size,
and sometimes gives rise to strange varieties in form and color.
Captain Smyth in his survey of the coast of the Mediterranean, found the
sea opposite the mouth of the Rhone, to deepen gradually from four to
forty fathoms, within a distance of six or seven miles, over which the
discolored fresh water extends; so that the inclination of the new
deposits must be too slight to be appreciable in such an extent of
section as a geologist usually obtains in examining ancient formations.
When the wind blew from the southwest, the ships employed in the survey
were obliged to quit their moorings; and when they returned, the new
sand-banks in the delta were found covered over with a great abundance
of marine shells. By this means, we learn how occasional beds of drifted
marine shells may become interstratified with freshwater strata at a
river's mouth.
_Stony nature of its deposits._--That a great proportion, at least, of
the new deposit in the delta of the Rhone consists of _rock_, and not of
loose incoherent matter, is perfectly ascertained. In the Museum at
Montpelier is a cannon taken up from the sea near the mouth of the
river, imbedded in a crystalline calcareous rock. Large masses, also,
are continually taken up of an arenaceous rock, cemented by calcareous
matter, including multitudes of broken shells of recent species. The
observations lately made on this subject corroborate the former
statement of Marsilli, that the earthy deposits of the coast of
Languedoc form a stony substance, for which reason he ascribes a certain
bituminous, saline, and glutinous nature to the substances brought down
with sand by the Rhone.[349] If the number of mineral springs charged
with carbonate of lime which fall into the Rhone and its feeders in
different parts of France be considered, we shall feel no surprise at
the lapidification of the newly deposited sediment in this delta. It
should be remembered, that the fresh water introduced by rivers being
lighter than the water of the sea, floats over the latter, and remains
upon the surface for a considerable distance. Consequently it is exposed
to as much evaporation as the waters of a lake; and the area over which
the river-water is spread, at the junction of great rivers and the sea,
may well be compared, in point of extent, to that of considerable lakes.
Now, it is well known, that so great is the quantity of water carried
off by evaporation in some lakes, that it is nearly equal to the water
flowing in; and in some inland seas, as the Caspian, it is quite equal.
We may, therefore, well suppose that, in cases where a strong current
does not interfere, the greater portion not only of the matter held
mechanically in suspension, but of that also which is in chemical
solution, may be precipitated at no great distance from the shore. When
these finer ingredients are extremely small in quantity, they may only
suffice to supply crustaceous animals, corals, and marine plants, with
the earthy particles necessary for their secretions; but whenever it is
in excess (as generally happens if the basin of a river lie partly in a
district of active or extinct volcanoes), then will solid deposits be
formed, and the shells will at once be included in a rocky mass.
_Coast of Asia Minor._--Examples of the advance of the land upon the sea
are afforded by the southern coast of Asia Minor. Admiral Sir F.
Beaufort has pointed out in his survey the great alterations effected
since the time of Strabo, where havens are filled up, islands joined to
the mainland, and where the whole continent has increased many miles in
extent. Strabo himself, on comparing the outline of the coast in his
time with its ancient state, was convinced, like our countryman, that
it had gained very considerably upon the sea. The new-formed strata of
Asia Minor consist _of stone_, not of loose incoherent materials. Almost
all the streamlets and rivers, like many of those in Tuscany and the
south of Italy, hold abundance of carbonate of lime in solution, and
precipitate travertin, or sometimes bind together the sand and gravel
into solid sandstones and conglomerates; every delta and sand-bar thus
acquires solidity, which often prevents streams from forcing their way
through them, so that their mouths are constantly changing their
position.[350]
_Delta of the Nile._--That Egypt was "the gift of the Nile," was the
opinion of her priests before the time of Herodotus; and Rennell
observes, that the "configuration and composition of the low lands leave
no room for doubt that the sea once washed the base of the rocks on
which the pyramids of Memphis stand, the _present_ base of which is
washed by the inundation of the Nile, at an elevation of 70 or 80 feet
above the Mediterranean. But when we attempt to carry back our ideas to
the remote period when the foundation of the delta was first laid, we
are lost in the contemplation of so vast an interval of time."[351]
Herodotus observes, "that the country round Memphis seemed formerly to
have been an arm of the sea gradually filled by the Nile, in the same
manner as the Meander, Achelous, and other streams, had formed deltas.
Egypt, therefore, he says, like the Red Sea, was once a long narrow bay,
and both gulfs were separated by a small neck of land. If the Nile, he
adds, should by any means have an issue into the Arabian Gulf, it might
choke it up with earth in 20,000 or even, perhaps, in 10,000 years; and
why may not the Nile have filled a still greater gulf with mud in the
space of time which has passed before our age?"[352]
The distance between Memphis and the most prominent part of the delta in
a straight line north and south, is about 100 geographical miles; the
length of the base of the delta is more than 200 miles if we follow the
coast between the ancient extreme eastern and western arms; but as these
are now blocked up, that part only of Lower Egypt which intervenes
between the Rosetta and Damietta branches, is usually called the delta,
the coast line of which is about 90 miles in length. The bed of the
river itself, says Sir J. G. Wilkinson, undergoes a gradual increase of
elevation varying in different places, and always lessening in
proportion as the river approaches the sea. "This increase of elevation
in perpendicular height is much smaller in Lower than in Upper Egypt,
and in the delta it diminishes still more; so that, according to an
approximate calculation, the land about Elephantine, or the first
cataract, lat. 24 degrees 5 minutes, has been raised nine feet in 1700
years; at Thebes, lat. 25 degrees 43 minutes, about seven feet; and at
Heliopolis and Cairo, lat. 30 degrees, about five feet ten inches. At
Rosetta and the mouths of the Nile, lat. 31 degrees 30 minutes, the
diminution in the perpendicular thickness of the deposit is lessened in
a much greater decreasing ratio than in the straitened valley of Central
and Upper Egypt, owing to the great extent, east and west, over which
the inundation spreads."[353]
For this reason the alluvial deposit does not cause the delta to
protrude rapidly into the sea, although some ancient cities are now a
mile or more inland, and the mouths of the Nile, mentioned by the
earlier geographers, have been many of them silted up, and the outline
of the coast entirely changed.
The bed of the Nile always keeps pace with the general elevation of the
soil, and the banks of this river, like those of the Mississippi and its
tributaries (see p. 265), are much higher than the flat land at a
distance, so that they are seldom covered during the highest
inundations. In consequence of the gradual rise of the river's bed, the
annual flood is constantly spreading over a wider area, and the alluvial
soil encroaches on the desert, covering, to the depth of six or seven
feet, the base of statues and temples which the waters never reached
3000 years ago. Although the sands of the Libyan deserts have in some
places been drifted into the valley of the Nile, yet these aggressions,
says Wilkinson, are far more than counterbalanced by the fertilizing
effect of the water which now reaches farther inland towards the desert,
so that the number of square miles of arable soil is greater at present
than at any previous period.
_Mud of the Nile._--On comparing the different analyses which have been
published of this mud, it will be found that it contains a large
quantity of argillaceous matter, with much peroxide of iron, some
carbonate of lime, and a small proportion of carbonate of magnesia. The
latest and most careful analysis by M. Lassaigne shows a singularly
close resemblance in the proportions of the ingredients of silica,
alumina, iron, carbon, lime, and magnesia, and those observed in
ordinary mica;[354] but a much larger quantity of calcareous matter is
sometimes present.
In many places, as at Cairo, where artificial excavations have been
made, or where the river has undermined its banks, the mud is seen to be
thinly stratified, the upper part of each annual layer consisting of
earth of a lighter color than the lower, and the whole separating easily
from the deposit of the succeeding year. These annual layers are
variable in thickness; but, according to the calculations of Girard and
Wilkinson, the mean annual thickness of a layer at Cairo cannot exceed
that of a sheet of thin pasteboard, and a stratum of two or three feet
must represent the accumulation of a thousand years.
The depth of the Mediterranean is about twelve fathoms at a small
distance from the shore of the delta; it afterwards increases gradually
to 50, and then suddenly descends to 380 fathoms, which is, perhaps, the
original depth of the sea where it has not been rendered shallower by
fluviatile matter. We learn from Lieut. Newbold that nothing but the
finest and lightest ingredients reach the Mediterranean, where he has
observed the sea discolored by them to the distance of 40 miles from the
shore.[355] The small progress of the delta in the last 2000 years
affords, perhaps, no measure for estimating its rate of growth when it
was an inland bay, and had not yet protruded itself beyond the
coast-line of the Mediterranean. A powerful current now sweeps along the
shores of Africa, from the Straits of Gibraltar to the prominent
convexity of Egypt, the western side of which is continually the prey of
the waves; so that not only are fresh accessions of land checked, but
ancient parts of the delta are carried away. By this cause, Canopus and
some other towns have been overwhelmed; but to this subject I shall
again refer when speaking of tides and currents.