miles and three-quarters per hour; but the mean velocity may be reckoned
at two miles per hour. In the year 1794, the late Mr. Rennie found the
velocity of the Thames at Windsor two miles and a half per hour[395].”
It will thus be seen that the velocity of the Nile is probably greatly
inferior to that of the Thames; for it appears that, except during the
inundation, for more than half the year the depth is inconsiderable.
The average fall when greatest, that is, including the province of
Batn-el-Hadjar, where the rapids chiefly occur, is considerably less than
that of any part of the above course of the Thames; so that there must
be long intervals between the rapids where the fall must be far less
than 13 inches in a mile. The breadth of the Nile is vastly greater; but
supposing the depth of the water to be the same as that of the Thames, on
account of the friction of the bed, the greater breadth would add very
little to the velocity. If we assume the average depth of the Thames
in the above distance to be 5 feet, and that it flows with an average
velocity of 2 miles in an hour, and if we assume the average depth of
the Nile in that part of its course where the fall is 13·12 inches to be
10 feet, when not swollen by the rise, the velocity would be 2⅘ miles
nearly in an hour[396], if the fall were equal to that of the Thames. We
shall probably come near the truth, by assuming the velocity of the Nile
on this part at 2 miles in an hour. That it must be considerably less in
the other divisions of the course I have named, and especially in that
part immediately below the second cataract, where the average fall is
only 5·30 inches for a distance of 96 miles, is quite evident.
The power of a river to abrade the soil over which it flows, so far as
water is by itself capable of doing so, must depend upon its volume and
velocity, and the degree of hardness of the material acted upon. The
power is increased when the water has force enough to transport hard
substances. But even transported gravel has little action on the rocks
with which it comes in contact, when it is free to move in running water,
unless the fall be considerable, and, consequently, the velocity and
force of the stream great. When stones are firmly set in moving ice, they
then acquire a great erosive power, cutting and wearing down the rocks
they are forcibly rubbed against; but this condition never obtains in
Lower Nubia, as ice is unknown there.
_Geological Structure of Lower Nubia._
One kind only of regularly stratified rock occurs in the 776 miles from
Abu Hammed to Philæ; viz., a silicious sandstone, similar to that which
occurs to a great extent on both sides of the Nile in Upper Egypt, and
which Russegger, after a very careful examination of it there, considers
to be an equivalent of the greensand of the cretaceous rocks of Europe.
The tertiary nummulite limestone, so abundant in Egypt, has not hitherto
been met with in Nubia.
The Nile flows over this sandstone for nearly 426 miles of the entire
distance, but not continuously. At Abu Hammed, it flows over granitic
rocks, and these continue from that place for about 120 miles. There is
then about 215 miles of the sandstone, which is succeeded by igneous and
metamorphic rocks, that continue for 195 miles without any interruption,
except a narrow stripe of sandstone of about 15 miles near Amara. It is
in this region of hard igneous rocks that nearly all the rapids occur,
between that of Hannek and the great or second cataract at Wadi Halfa.
From the latter place there is sandstone throughout a distance of about
196 miles, and then commences the granitic region of the Cataract of
Assuan, through which the Nile flows about 35 miles. Thus we have about
350 miles of igneous and metamorphic rocks, and about 426 of sandstone.
The general hard nature of the igneous and metamorphic rocks, over
which the Nile flows for about 155 miles above Semne, and for about
40 immediately below it, will be recognised by my naming some of the
varieties described by Russegger, viz., granites of various kinds, often
penetrated by greenstone dykes; sienite, diorite, and felspar porphyries;
gneiss, and clay slate, penetrated by numerous quartz veins.
The siliceous sandstone is very uniform in its character; and in Nubia,
as in Egypt, the only organic bodies which it has as yet been found
to contain, are silicified stems of wood. Occasionally, as in the
neighbourhood of Korusko, interstratified beds of marly clay are met
with[397].
When, therefore, we take into account the hard nature of the siliceous
sandstone, the durability of which is shown by the very ancient monuments
of Egypt and Nubia, that are formed of it, and the still greater hardness
of the granites and other crystalline rocks, it is manifest that the
wearing action of a river flowing over so gentle a fall, can scarcely
be appreciable. If the occasional beds of marly clay occur in the bank
of the river, they may be washed out, and blocks of the superincumbent
sandstones may fall down; but such an operation would have a tendency to
raise rather than deepen the bed of the river at those places; unless the
transporting power of the stream were far greater than can exist with so
moderate a fall, especially in that part of the river below Semne, where,
for 96 miles, it is not more than 5·3 inches, and for 115 miles below
that, not more than 12 inches in a mile. Even if we suppose the river to
have power to tear up its bed for some distance above Semne and below
it, as far as the rapid of Wadi Halfa, it is evident that the materials
brought down would be deposited, except the finest particles, in that
tranquil run of 96 miles, which may be almost compared to a canal. The
drains in Lincolnshire are inclined 5 inches to a mile[398]. When the
annual inundations commence, the water of the Nile comes down the rapid
at Assuan of a reddish colour loaded with sand and mud only; whatever
detrital matter of a larger and heavier kind the Nile may have brought
with it, is deposited before it reaches that point.
From all these considerations, therefore, I come to the conclusion, that
the bed of the Nile cannot have been excavated, as Professor Lepsius
supposes, since the date of the sculptured marks on the rock at Semne. He
says, “Es lässt sich kaum eine andere Ursache für das bedeutende Fallen
des Nils denken, als ein Auswaschen und Aushölen _der Katakomben_.” By
the word _Katakomben_[399] he can only mean natural caverns in the rock;
but such caverns are rarely, if ever, met with in sandstones, and only
occasionally in limestones. If the course of the Nile were over limestone
instead of sandstone, we could not for a moment entertain the idea of a
succession of caverns for 200 miles beneath its bed, sometimes two miles
in width, the roofs of which were to fall in; and where the igneous rocks
prevail, this explanation is wholly inapplicable.
But besides the objections arising from the nature of the rocks, and the
inconsiderable fall of the river, there is still another difficulty to
overcome. It is to be borne in mind, that this lowering of the bed of the
Nile, from Semne to Assuan, is supposed to have taken place within the
last 4000 years. Between the first cataract at Assuan and the second at
Wadi Halfa there are numerous remains of temples on both banks of the
Nile, some of very great antiquity. “From Wadi Halfa to Philæ,” says
Parthey, “there is a vast number of Egyptian monuments, almost all on the
left bank of the river, and so near the water that most of them are in
immediate contact with it[400].” We may rest assured that the builders of
these would place them out of the reach of the highest inundations then
known. Although we have many accurate descriptions of these monuments,
the heights of their foundations above the surface of the river are not
often given; they are, however, mentioned in some instances. I shall
describe the situations of some of these buildings relatively to the
present state of the river’s levels, and shall begin with those on the
island of Philæ.
This island, according to the measurements of General von Prokesch, is
1200 Paris feet (1278 English) in length, and 420 (447) in breadth, and
is composed of granite. Lancret informs us, that, “à l’époque des hautes
eaux, l’île de Philæ est peu élevée au-dessus de leur surface: mais,
lorsqu’elles sont abaissées, elle les surpasse de huit mètres.” It was
formerly surrounded by a quay of masonry, portions of which may be traced
at intervals, and in some places they are still in good preservation.
The south-west part of the island is occupied by temples. According
to Wilkinson, the principal building is a temple of Isis commenced by
Ptolemy Philadelphus, who reigned from 283 to 247 years before Christ;
and he adds, that it is evident an ancient building formerly stood
on the site of the present great temple. Lancret, in referring to
this more ancient building, says:—“Il y a des preuves certaines d’une
antiquité bien plus reculée encore, puisque des pierres qui entrent dans
la construction de ce même grand temple, sont des débris de quelque
construction antérieure.” Rosellini considers that it was built by
Nectanebus. The first king of Egypt, of the Sebennite Dynasty of that
name, ascended the throne 374 years B.C., the second and last ceased to
reign about 350 years B.C.[401]
Rosellini[402] informs us, that on the island of Bageh, opposite to
Philæ, there are the remains of a temple of the time of Amenophis II.,
and a sitting statue of granite representing him. He was a king in the
earlier years of the 18th Dynasty, which, according to the Chevalier
Bunsen[403], began in the year 1638, and ended in 1410 B.C.
GAU[404], in describing a temple at Debu, about 12 miles above Philæ,
which he visited in January, and consequently during the time of low
water, states that he discovered under the sand, at the edge of the
river, the remains of a terrace leading towards a temple.
A short distance north of Kalabsche, about 30 miles above Philæ, at
Beil-nalli, Rosellini[405] speaks of a small temple in the following
terms:—“Among the many memorials that still exist of Ramses II., the
most important, in a historical point of view, is a small temple or
grotto excavated in the rock!” and Wilkinson mentions it “as a small but
interesting temple excavated in the rock, of the time of Ramses II.,
whom Champollion supposes to be the father of Sesostris or Rameses the
Great[406].” He was the first king of the 19th Dynasty, which began in
the year 1409 B.C.[407]
Gau[408] thus describes a monument at Gerbé Dandour:—“La chaîne de
montagnes qui borde le Nil est, dans cet endroit, si approchée du lit de
ce fleuve, qu’il ne reste que très peu d’espace sur la rive. Cet espace
est presque entièrement occupé par le monument, et la rivière, dans ses
débordemens, arrive jusqu’au pied du mur de la terrasse.”
Parthey informs us that the temple of Sebûa is about 200 feet distant
from the river, in which distance there are two rows of sphinxes, and
that the road between them, from the temple, ends in wide steps at the
water’s edge; and he adds that Champollion refers this temple to the time
of Rameses the Great[409].
It thus appears that monuments exist close to the river, some of which
were constructed at least 1400 years before our era; so that taking the
time of Amenemha III. to be, as Professor Lepsius states, 2200 years
B.C., the excavation of the bed of the Nile which he supposes to have
taken place, must have been the work, not of 4000 years but of 800. If
the erosive power of the river was so active in that time, it cannot be
supposed that it then ceased; it would surely have continued to deepen
the bed during the following 3000 years.
At all events, the buildings on the island of Philæ demonstrate that the
bed of the Nile must have been very much the same as it is now, 2200
years ago; and even a thousand years earlier it must have been the same,
if the foundation of the temple on the island of Bageh, opposite to
Philæ, be near the limit of the highest rise of the Nile of the present
time; so that there could be no barrier at the Cataract of Assuan to dam
up the Nile when they were constructed; and thus the deafening sound of
the waterfall recorded by Cicero and Seneca must still be held to be an
exaggeration.
The existence of alluvial soil, apparently of the same kind as that
deposited by the Nile, in situations above the Cataract of Assuan, at
a level considerably above the highest point which the inundations of
the river have reached in modern times, to which allusion is made by
Professor Lepsius, has been noticed by other travellers, and even at
still higher levels than those he mentions. Whether that alluvial soil be
identical with, or only resembles the Nile deposit, would require to be
determined by a close examination, and especially with regard to organic
remains, if any can be found in it. There is no evidence to show that
it was deposited during the historical period, and it may be an evidence
of a depression and subsequent elevation of the land antecedent to that
period. It may not be of fresh-water origin, but the clay and sand, or
till, left by a drift while the land was under the sea. For remote as
is the antiquity of Nubia and Egypt, in relation to the existence of
the human race, it appears to be of very modern formation in geological
time. The greater part of Lower Egypt, probably all the Delta, is of
post-pliocene age, and even late in that age; and the very granite of
the Cataract of Assuan, that of which the oldest monuments in Egypt
are formed, and which, in the earlier days of geology, was looked upon
as the very type of the rock on which the oldest strata of the earth
were founded, is said to have burst forth during the later tertiary
period. We learn from Russegger, that the low land which lies between
the Mediterranean and the range of hills that extends from Cairo to the
Red Sea at Suez, and of which hills a nummulite limestone constitutes a
great part, is composed of a sandstone which he calls a “Meeresdiluvium,”
a marine diluvial formation, and considers to be of an age younger than
that of the sub-appennines[410]. This sandstone he found associated with
the granite above Assuan, and covering the cretaceous sandstone far into
Nubia. It appears, therefore, that, in the later ages of the tertiary
period, this north-eastern part of Africa must have been submerged, and
that very energetic plutonic action was going forward in the then bed of
the sea. The remarkable fact of the granite bursting through this modern
sandstone is thus described by Russegger:
“We arrived at a plateau of the Arabian Chain south-east of
Assuan. It is about 200 feet above the bed of the Nile, and
consists of the lower and upper sandstone, which are penetrated
by innumerable granite cones from 20 to 100 feet in height,
arranged over the plateau in parallel lines, very much resembling
volcanic cones rising from a great cleft. The sandstone is
totally altered in texture near the granite, and has all the
appearance as if it had been exposed to a great heat. ‘I cannot
refrain,’ he says, ‘from supposing that the granite must have
burst, like a volcanic product, through long wide rents in
the sandstone, and that, in this way, the conical hills were
formed[411].’”
An eruption of a true granite during the period of the sub-appennine
formations, one possessing the same mineral structure as that we know
to have been erupted during the period of the palæozoic rocks, would
be a fact of so extraordinary a kind, that its age would require to be
established on the clearest evidence, and especially by that of organic
remains in the sandstone.
Having thus ventured—I trust without any want of the respect due to so
eminent a person—to reject the hypothesis proposed by Professor Lepsius
for the high levels of the Nile at Semne, indicated by the sculptured
marks he discovered, it may perhaps be expected that I should offer
another more probable explanation. If in some narrow gorge of the river
below Semne, a place had been described by any traveller, where, from
the nature of the banks, a great landslip, or even an artificial dam,
could have raised the bed to an adequate height; that is, proportionate
to the fall of the river, as it was more distant from Semne, a bar that,
in the course of a few centuries, might have been gradually washed away,
I might have ventured to suggest such a solution of the problem. But
without any information of the existence of such a contraction of the
river’s channel, or any exact knowledge of the natural outlets and dams
to running water along the 250 miles of the Nile Valley, from Semne to
Assuan, it would be idle to offer even a conjecture. These marks are
unquestionably very difficult to account for, in the present imperfect
state of our knowledge of the structure of that portion of the Nile
Valley; and any competent geologist, well versed in the questions of
physical structure involved, who may hereafter visit Nubia, would have a
very interesting occupation in endeavouring to solve the difficulty.
_7th April, 1850._
_Translation of a Letter from Dr. Lepsius to Mr. Horner, dated Berlin
the 12th of April, 1853._
DEAR SIR,—I observe from a letter of your daughter, that she is
desirous of adding to her translation of my Letters a note upon
the height of the water of the Nile, with reference to your paper
in the “Edinburgh Philosophical Journal.” I wish that you would
get reprinted in that note the whole of the small memoir, as it
possesses great interest, and abounds in data not easily brought
together; for in that case the subject may probably be further
discussed.
I will, at all events, avail myself of this opportunity to
make some remarks, which you may, if you like, propose to have
introduced into the contemplated note.
I must first remark that the word _Katakomben_ was entirely a
typographical error for _Katarakten_, as was unfortunately the
case in many other instances in those things which were printed
during my absence.
But in respect of the explanation of the observed facts, my
views are perhaps less different from yours than you imagine.
You imagine a natural or artificial barrier which has broken
down, but this appears to me of insufficient magnitude; I too
imagine barriers to have existed, and natural ones, but that
there must have been several of them. I do not, moreover, regard
it as impossible, that at certain periods, when the country was
in its most flourishing condition, artificial dams may have been
constructed in order to obtain a higher rise of the water within
a particular space, such as was necessary for an overflowing. But
if we imagine an entire dam thrown across the river, this, if
I am not mistaken, could only hold back the current for a very
short way, namely, where there is a greater general fall. If,
for example, we imagine a barrier at Assuan, it would require to
be several hundred feet high to have any effect on the height
of the water at Semneh, and then the whole valley from Philæ to
Wadi Halfa would be a great lake, as it may indeed have been in
geological time.
If we imagine a succession of barriers which would be especially
formed where veins existed in the primitive rock, then the
present entire physiognomy of the Nile valley seems to be more
easily explained. The river-bed, amidst granitic or other
upheaved rock, is not level, like a chalk or sandstone channel,
but forms sometimes lakes, sometimes barriers. The force of
the swollen current at these last, of which there is one at
Semneh itself, does not act in the mean proportion of a space
of considerable extent, but with immense effects, exceeding all
calculation, especially when, in addition, there is a contraction
of the sides, as at Semneh. Immediately below this barrier the
bed again spreads out, and the rocks disappear in the current.
The colossal rock-fragments on that bank, whose inscriptions
sometimes show that above 4000 years ago they were still not
broken loose, display the Titan force of a current thus hemmed
in, and allow us to conceive how at that spot it gradually washed
out its bed, sometimes to a great depth, but sometimes also to
a greater breadth, which has the same effect, and how all that
is broken away, or that during the time of low water splits to
a considerable depth in the bed of the river from the summer
heat, rolls away, until arrested by falling into hollows. But
if these single barriers are only washed away in the course of
thousands of years, then the whole river must receive an equable
fall, and it will never rise in the very rocky districts, but can
only continue to be still more excavated, and will only again
deposit the heavier portions it bears along with it, below the
cataracts, where every obstruction disappears. The monuments can
hardly be cited in opposition to the view of a gradual sinking
of the bed of the river in the higher districts. All of them
lie tolerably far above the region of the rise of the Nile—for
example, the temple on the island of Bigeh, to which there is a
considerable ascent. Philæ has only been built upon since the
time of Nectanebus, and there is nothing to indicate buildings
of an earlier date. The sinking of the surface of the water even
at Philæ and Assuan must also have been far less than at Semneh.
Nevertheless, special researches with respect to the relative
condition of the ancient temple and rock-inscriptions to the
present surface of the water would certainly be of the greatest
utility.
Herr von Humboldt, after reading some observations on the same
subject by Wilkinson in the Nouv. Ann. des Voyages, i., without
recollecting my views, wrote to me as follows:
“Breaches in dams, I imagine, cause only temporary rises of
water, unless in earlier times (for which I see no reason)
there was a greater accumulation of water in the valley of the
river, from meteorological causes. Primeval conditions, where
broad valleys were filled with waters, are not applicable to
periods when there were inscriptions. Does it not seem to you
more probable, that the height of the water was at one time at a
greater elevation, on account of the bed of the river not having
been so much furrowed out, because at an earlier period the
bottom of the river was not at _c d_, but at _e f_.
[Illustration]
“There are rivers whose beds are elevated and rendered more
shallow by deposition, others which furrow out their bed _qui
creusent un lit plus profond_.”
With sincere respect, your faithful,
R. LEPSIUS.