from it by deep water navigable for coasting vessels larger than
canoes.
III. _Atolls_, ring- or crescent-shaped reefs having no obvious
relation to any land and typically found far out in the ocean,
from the great depths of which they rise with steep slopes to,
at most, a few feet above high tide level.
Fringing reefs we have already dealt with; the two agents
described—growth and abrasion of coral—will account for all of
them. Barriers and atolls are more puzzling. Why should the barrier
form its line parallel to the coast, though at a distance from
it, and the very existence of atolls is one of the most striking
phenomena of Nature.
The problem is complicated by the fact that ordinary reef corals
die out at a depth of 50 fathoms or so. Now 50 fathoms is a mere
nothing compared to the depths from which the Pacific atolls rise,
and is only a quarter the depth often found within a few hundred
yards of the Red Sea reefs. How then to account for the building
of reefs in deep water?
One suggestion was that atoll rings were formed by the growth of a
mere cap of coral round the edge of the craters of huge submarine
volcanoes. But that postulates far too large a number of such
immense volcanoes[56], and the early stages of these formations have
not been found. Darwin’s hypothesis was hailed with joy as the
obvious solution, and held the field against all rivals for many
years. Briefly it is that corals formed a reef by direct growth in
shallow water on the coast of an island, forming a fringe thereto
in the way explained above. Now is postulated one of those great,
slow earth movements such as have very often occurred in the past
and are occurring at the present day. In this case the island is
to sink slowly, at such a rate that the reef grows upwards as fast
as it is submerged. The result is obviously a mass of corals of a
thickness equal to the total sinking movement of our island, though
every individual coral grew while in water under 50 fathoms deep.
When our island is half submerged the fringing reef has become a
barrier, when wholly gone the reef ring remains enclosing an empty
lagoon, and is the only mark of the grave of a drowned island. Thus
Darwin’s theory has the further merit of referring the two forms
of reef, barrier and atoll, to one common cause, the sinking of the
land. But we have no idea of how the original islands were formed
in such numbers, and many believe that no such vast sinking of the
ocean basins has occurred since they were formed. Also, if solution
be ignored, it is difficult to see why, as the island sank, coral
growth did not close in over the submerged land, and so form a vast
reef flat instead of leaving a lagoon up to 50 fathoms deep.
To settle the matter an expedition was sent to a typical atoll,
Funafuti, and a boring 1200 feet deep was made to find out what
the interior of the reef is made of. The material brought out of
the bore hole has been carefully examined by experts, and reported
to consist of the remains of exactly similar corals to those found
near the surface, and this result was taken by one or two geologists
as complete vindication of Darwin’s theory. But apart from the
extreme difficulty of the identification of all coral species,
especially those which have been subject to partial crystallisation
and so on, one remembers that a considerable part of the foundations
in deep water are formed of corals which have fallen down the steep
slope from the growing reef above, so that their presence buried a
thousand fathoms deep proves nothing, while the boring at Funafuti
only went to about 200 fathoms.
[Illustration: Diagram 6. Original elevation A of sea bottom shaded,
B₁-B₄ additions formed by growth, C₁-C₃ slopes of coral
&c. fallen from above. The thick line of C₃ is outline of section
of the atoll mass resulting.]
After all it is easier to imagine that the atoll grew up from the
bottom of the deep sea. The only postulate is a chance elevation on
the sea bottom. On such elevations it is found that the remains of
marine organisms, including deep sea corals (as distinct from reef
builders), tend to accumulate much more rapidly than on the floor
of the surrounding depths. The elevation is consequently slowly
but surely raised, and the higher it grows the more rapid the
accumulation, until at last reef corals obtain a footing forming
a cap or pinnacle reaching to the surface. From this masses of
coral, sand, stones or large boulders, are always falling on to the
foundation slopes, forming successive sloping layers indicated by
the dotted lines, upon which fresh growth takes its rise[57]. When
the coral reef has become of some breadth (and atoll rings may be
30 miles or more across) a boring at the edge might descend for a
thousand fathoms and never meet with the original foundations, but
would pass only through recent corals fallen from the shallow zone.
We should expect to find a continuous surface of coral at sea
level. As a matter of fact there is a broad lagoon, generally of
considerable depth, one or two gaps through the encircling reef
giving communication with the open ocean. This is the natural result
of the causes described when dealing with the boat channel of a
fringing reef; it is the same thing on a much larger scale. Seeing
that the rate of growth of coral masses is always only the excess
of growth over destruction and solution, the presence of growing
corals is no evidence against the fact that the lagoon shores may be
undergoing destruction, and that such coral growth as is present may
add nothing to the inner sides of the reef in the end. No more does
the accumulation of great quantities of mud prove that the lagoon
will in time be quite filled in. Mud and sand[58] are but stages in
the destruction of coral rock, and its presence where that process
is going on is to be expected. An abnormal tide, a shift of the
currents, and vast quantities are swept out through the gaps in the
reefs. My home on the Red Sea is beside a large landlocked lagoon in
which coral gardens of great luxuriance, whence collections of many
species can be procured, are frequent. Spite of this, the evidence
is as clear as possible that its shores and islands are undergoing
rapid denudation, and its reefs are being cut down by currents to
banks below water level. As in the Red Sea the level rarely alters
by more than a foot once in the twenty-four hours, and often the
rise or fall is much less, the action of tidal currents is at a
minimum, yet even so they produce well-marked effects.
Barrier reefs may be formed from fringing reefs by the enlargement
of the boat channel, while the reef is extending seawards.
The island of Zanzibar, 60 miles long by 20 wide, and 20 miles from
the mainland of Africa, seems to be a part of the East African
barrier system, and it certainly was separated from the mainland
by the destruction of the intervening land; the shallow dividing
channel being full of shoals and sandbanks formed by cutting down
of islands. The fauna of Zanzibar, including leopards, serval cats,
&c., can be accounted for in no other way. The Great Barrier of
Australia, a thousand miles long, is the same thing on a vastly
greater scale. But, as described in the next chapter, the Barrier
system of the Red Sea is quite another thing, and its mode of
formation may possibly be unique in the world.
CHAPTER IX
GEOGRAPHY OF RED SEA AND FOUNDATIONS OF THE REEF SYSTEMS
_The Climate_ has already been roughly described but it is
interesting enough to deal with in more detail.
One might suppose the extremes of dry heat and cold of the desert
climate to be moderated by the sea, and the resulting mean to be a
fairly mild and equable climate. Actually we get alternations of
desert and sea climates, extreme dry heat in summer and steaming
winds from the sea, both bringing great discomfort.
The winter from November to March is cool and pleasant so long
as the prevailing north-east wind blows, but there are spells
of very disagreeable weather even in winter. When the wind is
from the south-east the temperature rises and at the same time it
becomes very damp, saltish moisture being over everything, so that
even the natives become lazy and depressed and many suffer from
rheumatism, &c.
One has however the satisfaction of knowing that the south-east
wind usually lasts but three days or so, and never more than a week,
when the north wind comes back and we revive.
The south wind is generally preceded by a day’s calm and increases
in strength until the end, when a short calm ushers in a very strong
wind from the north. On several occasions I have actually seen the
approach of this sudden and welcome change as a line of low cloud,
formed by the condensation of vapour where the cold north wind
meets the damp from the south.
This sudden change was the cause of the wrecking of a _sambûk_ which
was beating down to Port Sudan in a south-east wind. Anchoring one
night in a long narrow harbour open to the north, they were caught by
the north wind next morning and, being unable to beat out against it,
were driven on to the reef. The crew had to walk in to Port Sudan,
distant about twenty-five miles, without food or water, one of them
having a badly crushed wrist. As I had cargo on the _sambûk_ I went
up immediately, and after only two or three days there was nothing
visible of the _sambûk_, but fragments scattered over miles of reef.
In winter the desert wind, due north or a little west of north,
is very much colder than the usual north-north-east The mornings
indeed may be quite chilly, and though this is very welcome to the
Englishman the natives suffer considerably. On the first day or
two of such a period the wind is strong, charged perhaps with sand,
and so dry that the backs of books curl as if they had been before
a fire.
In the summer the alternations of climate may be astonishingly
rapid, both may occur on one hot-weather day in July or August as
follows. The land breeze is very weak, and dies away about 6 a.m.,
when already the sun is blazing hot. By 8 a.m. it is intolerable,
but as it is still dead calm pearlers and fishermen are at sea
making use of their opportunity. If however they expect a day of
“hurûr” or hot wind they do not go far away, and when warned
by two or three preliminary puffs of wind off shore, they must make
all haste to return, or risk being swept out to sea. In half an
hour the wind may be furiously strong, heated as by a furnace and
bearing dense clouds of fine dust, of the colour and density of a
London fog, together with coarser sand that stings the face. Woe
to one who has to travel against such a storm! The dry heat soon
produces intolerable thirst, the eyes, nose and mouth are filled
with sand, while one’s face, eyelashes and even teeth are caked
with mud produced by it with the natural moisture.
These conditions continue until noon, when a change may be
expected, but may be deferred until 4 p.m., or rarely even 6
p.m. The wind suddenly ceases, the world becomes again visible,
and the temperature drops from say 105° F. to 95° F. But soon
there comes the reverse wind, almost equally strong, from the sea,
and the humidity increases so much that the fall of temperature is
not the relief that might be expected, being but the change from
oven to steam-kettle. The natives tell me that this wind, so hot
in the plains, among the mountains is cold, and is heated by its
passage over the sun-roasted plains. Apparently the great heat here
originates miniature local cyclones, cold air from the mountain tops,
or drawn over the mountains, rushing down to fill the low pressure
area on the plains, being heated there and rushing on a few miles
out to sea, whence the easterly return wind originates. At Dongonab
these “hurûr” winds are rarer than they are further south,
where they are of almost daily occurrence during the summer, while
at Halaib, 100 miles further north, the natives tell me they do not
occur at all. Consequently we are sometimes visited by the return
wind in the morning, caused by “hurûr” at a point further down
the coast. Such a cyclone is illustrated by the frontispiece, which
represents the combination of thunder clouds over the mountains
while a “hurûr” rages over the plain and for several miles
out to sea. But among the barrier reefs, though the wind is blowing
directly towards them, all is glassy calm.
The rainfall is extremely scanty and local, though markedly better
in the south, where the population is correspondingly greater and
the fauna richer.
There are two seasons when rain may be hoped for, viz. the
“kharîf” which centres round August, and which is referred to
in the frontispiece, and the winter months, but if rain fell for an
hour or two on three days it would be considered a liberal supply
for the whole year in most places. At Dongonab there has been no
rain (above a millimetre or two) since December, 1907, though one
or two showers have fallen on Rawaya and Makawar[59]. There is of
course much more rain on the hills than on the plains, but even so
grass grows only in scattered areas to which the people migrate.
_Tides._ The Red Sea undergoes considerable variations of level
at its extremities, up to seven feet at Suez, but in the middle
the variations are small, only a few centimetres at Port Sudan. At
Dongonab the difference between highest and lowest levels recorded
is 80 cm., but the maximum change in any 24 hours is rarely over
30 cm. Records shew a distinct tide, but this may be interfered
with by changes of level due to wind and changes of atmospheric
pressure, and in any case one of the usual two tides of the 24 hours
is practically suppressed, the water remaining near high tide level
until it falls for next day’s tide. In the summer the average level
is lower than in winter and the tidal effects are partially masked
by the results of the peculiar climatic conditions. The water may
remain low for days, so that all the coral which has grown above
that level since the last occasion of extreme low water, which may
have been one or even two years ago, dies off.
I suppose that every school-boy looking at an atlas, is struck
by the peculiar shape of the Red Sea, and is led to ponder on
the usefulness of this peculiar canal, the sole value of which
is that it gives communication between Europe and the East, a
value which needed but the trifling addition possible to human
effort to make it the great highway of the world. Its own shores
are desolate wastes, in itself it has no attraction for traffic,
and even its shape seems to indicate that it is but a passage
to other seas. (See map inside the cover.) For so narrow a sea,
only a little over a hundred miles wide, the depth is great,
two hundred to five hundred fathoms at the side and a thousand in
the middle. These peculiarities are also well marked in the deep
Gulf of Akaba which bounds Sinai on the east—the Gulf of Suez,
on the west, being a shallower branch valley. Both these gulfs,
like the Red Sea, are bounded on either side by high mountains,
and those of the southern part of Sinai are particularly grand in
the savage barrenness of their jagged peaks and vast precipices.
The Gulf of Akaba is directly in line with the Jordan Valley, a
similar depression on a smaller scale, only partially occupied by
water, the Dead Sea, while southwards we find another dry valley
running through British East Africa and adjoining territories,
a great trough bounded by plateaux, several thousand feet above
its bottom. We can thus trace
[Illustration: Diagram 7. Formation of a rift valley]
this trough-like valley from Palestine to some degrees south of
the Equator as a stupendous crack in the earth’s surface, well
named “The Great Rift Valley[60].” The Red Sea is its greatest
section, its total depth here being, say, 5000 feet from the summit
of the mountains[61] to sea level and 6000 feet to the sea bottom,
11,000 feet in all.
The formation of such a valley, by the dropping down of a series of
strips of country below the level of the remainder, is illustrated
by Diagram 7. To study the simplest possible case we draw a section
through the ground and imagine it formed of three kinds of rock,
of which two form horizontal sheets, AA and BB, over the third
CC. These were originally unbroken, and in the positions shewn by
the lines of dashes, but were broken by the dropping down of the
central part to form the valley shewn here in section. The floor
of the valley has the same structure as the original surface of
the ground, the same three beds, A, B and C, occurring in the same
positions, but at a lower level. They are found again in each of
the steps on the valley’s sides, their regular reappearance in
this way being conclusive proof of the earth movements postulated.
The vertical lines FFFF, between each step and the next drop, along
which the continuity of the beds is broken, are termed “faults,”
a geological term which should be remembered.
Rift valleys are found elsewhere in the world, but are exceptional,
ordinary valleys, with their winding courses and rounded outlines,
having been formed by the action of streams, which slowly wash away
the ground and hollow out their courses to the sea.
The actual structure of the middle portion of the Red Sea Valley
is shewn diagrammatically by the section on page 145. Five steps
are shewn, Nos. 2 and 3 being further separated by a minor fault
valley. The details are described later.
The southern part of the sea, below Masawa, has recently been
subjected to volcanic[62] action; many of the islands there are
quite well-preserved volcanic cones, but as regards the rest of the
sea, though earth movements have been frequent and considerable,
there are now no traces of volcanic action, and the movements that
have occurred have not necessarily involved cataclysms greater than
severe earthquakes.
There are however in the north two islands the existence of which
is most readily explained by volcanic action. I refer to the coral
formations known as “The Brothers” and “Daedalus Shoal,” the
former a pair of low islets, the latter a flat reef, rising out of
the centre of the sea and surrounded by water hundreds of fathoms
deep. They are extremely steep-sided cones, and what could form and
support such structures far out from land is puzzling. A certain view
of another section of the Rift Valley, that once seen can never be
forgotten, seems to offer an explanation. After passing through the
forests of the Kikuyu Plateau by the Uganda Railway one comes out
into the open on the brink of the great escarpment of the Rift Valley
and looks across a trough 3000 feet deep to the similar forest-clad
heights of the Mau on the other side. The continuity of the valley
is rudely broken by two volcanic cones rising abruptly in the middle
of the flat bottom of the trough. On consideration the strangeness
of their appearance in the middle of a valley passes away, one sees
that the bottom of such a rift must be a zone of weakness of the
earth’s crust where volcanoes might naturally be expected to arise.
[Illustration: Diagram 8. Formation of atoll as a cap of coral
growing on a mound of loose volcanic material. A original mound,
B as cut down by the sea, C the atoll.]
If the water were removed from the Red Sea Valley would not the
appearance of The Brothers and Daedalus be very much like that of
the two volcanoes of British East Africa, allowing for the steeper
angle at which their materials would lie under water? Given such
cones of loose volcanic ash, &c., wave action would quickly level
down their summits until coral growth afforded protection and formed
a cap of rock, part of which is now raised again above sea level as
the islands on one of which the lighthouse is built.
The ring-shaped reef of Sanganeb[63], opposite Port Sudan, which is
outside the Barrier system and separated from it by water 400 fathoms
deep, may be built on a similar foundation. Like the two coral reefs
above it rises with extremely steep slopes from this deep water,
and is the summit of a submarine pinnacle rather than hill.
On the other hand, the foundations of these strangely isolated
reefs may be like a certain island which, rising high above sea
level, shews its structure, a centre of olivine rock fringed with
coral. This island is variously known as Zeberjed, St Johns, and
Emerald Island, the latter name due to its possession of mines for
peridots, which are worked by the Khedive of Egypt. Its position
is 23° 30′ N., distant about 60 miles from the African coast,
a formation quite independent of the sides of the Rift Valley. It
is an example of the “Block Mountains” described by Professor
Gregory, portions of the original earth surface which have remained
standing when the surrounding country dropped down to form the
trough of the Rift Valley, not a mass of land thrust upwards and
subsequently carved into peaks and valleys by running water, which
is the way ordinary mountain ranges are formed.
One gets a good idea of the structure of the Red Sea coasts on
leaving the Gulf of Suez for the voyage south, before the ship’s
course passes far from land. On the western horizon is a range
of wild mountains, a grey plain ending in a yellow shore-line
separating them from the sea, and the off-lying islands are of the
same colour. The plain is formed of gravel from the high hills,
its yellow border seawards being coral limestone, and the islands
also. In the sea are numerous reefs, here of very intricate plan,
lines of white breakers separating the deep blue black water from
large areas of green and brown shoals in waveless lagoons. There are
deep channels between these reefs and the shore, which is itself
fringed by a shallow reef with its edge at low water level but
bearing perhaps one or two fathoms of water on its surface within.
This being the simple structure of both sides of the whole Red Sea
trough I may proceed to describe in detail one section of the coast,
that bounding the territory of the Anglo-Egyptian Sudan, between
18° and 22° N. This section includes two (Ras Rawaya and Ras
Salak) of the three promontories which break the straight line of
the west coast north of Masawa, the third being Ras Benas, further
north. The map opposite shews clearly the fringing reef which lies
along the whole coastline, the numerous harbours, of which Port
Sudan, Suakin, and Trinkitat are of commercial importance[64],
the deep channel separating the fringing from the barrier reefs,
and the atoll of Sanganeb on which the lighthouse is built.
[Illustration: Fig. 78. Coast of the Anglo-Egyptian Sudan
Sandstone hills shaded, _small_ islands black. Coastline double,
the outer line being the edge of the fringing reef. The thin lines
enclosing roughly oval or elongated areas at sea are the barrier
reefs. Figures on sea represent depths in fathoms.]
On land the bases of the high mountains are indicated, and certain
lower hills, of sandstone, which rise in the midst of the maritime
plain. A striking fact is visible on first inspection of this map,
viz. that not only is the Red Sea a nearly parallel-sided trough but
that the constituents of the sides are themselves placed in lines
parallel to the coast. The Archean hills[65], the lesser sandstone
ranges, the coral bounding the maritime plain, and the barrier reefs,
are all four roughly parallel to the main axis of the sea.
We will consider each feature in more detail. For the Archean
hills consult the extremely interesting memoirs of the Egyptian
Geological Survey[66]; for our purposes it is enough to note that
they are all of ancient igneous and metamorphic rocks, that they
rise to heights of from four to eight thousand feet, and the valley
bottoms are generally flat and filled in with gravel.
The maritime plain is from five to ten miles wide, sloping up
regularly from the sea towards the bases of the hills, where it
may attain a level of several hundred feet. Except at its seaward
edge, it is composed of black gravel, the product of the decay of
the hills carried down by the torrents resulting from the rare but
furious rain-storms, and spread out to form the plain. Sand-hills
occur, but not very commonly, though the gravel is mingled with
sand throughout, and in sections of the plain exposed by wells,
layers of gravel alternate with sand, fine or coarse, as far as
the deepest borings have been carried[67].
The pebbles, though black predominate, are of a most remarkable
variety of kinds and colours. Bright green and red, yellow and
clear white are abundant, and any square yard would yield a rich
collection in Petrology. As the torrents open out into the level
plain they lose themselves, continually taking to fresh channels,
so that the _débris_ from series of hills quite distant from one
another are mingled; in a given spot gravel from one valley is laid
down this year, from another and totally distinct one another. One
would expect gravel which had been carried by torrents a distance
of many miles to be rounded down by friction into smooth boulders
or pebbles, like those of our home streams. As a matter of fact it
is nearly always angular, the rounded surfaces we should expect
being rarely met with on the surface. The pebbles, as we now see
them, have been re-formed from larger stones since their transport
through the valleys and over the plain. Large stones, lying half
buried in smaller material, shew the usual rounded surfaces of
water-borne rock, but they are invariably split up by fissures,
which may be half an inch broad, so that the stone is as it were
built up of angular fragments fitted together after the style of a
puzzle picture. During the hundreds of years they have lain there,
apparently secure from all interference, they have been exposed to
innumerable fierce heats and cold nights, which, causing successive
minute expansions and contractions, have at last split the stones
into small pieces. This is the origin of the irregularly shaped
gravel; first indeed it was rounded by the grinding and pounding of
the torrents of hundreds of successive winters, then it was split
up again by the silent invisible stresses of heat and cold.
_Plate XXXVI_
[Illustration: Fig. 79. The maritime plain and Irba Mountains
Yemêna oasis in foreground and gravel-covered ridge across middle
distance]
_Plate XXXVII_
[Illustration]
[Illustration: Figs. 80 and 81. Corals, in position
of growth, on the summit of Jebel Têtâwib]
The accumulation of this vast mass of gravel and sand in the manner
described has taken a length of time compared to which a human
life is but a moment. Even from the geological point of view it has
been not inconsiderable. There is abundant evidence that the plain
was formed, much as it is now, at a time when the coast-line was
entirely different, and though there is no good evidence of the
country’s having been other than desert throughout historical
time, there probably was a greater rainfall when the formation of
the plain was in full swing.
The sandstone hills are particularly interesting in that one finds
a regular layer of coral on their summits, which shews that they
were once nearly level with the sea, and are in fact coral reefs
which have been elevated to heights of from 100 to 1000 feet. In
some of those hills to which I have had access the corals on the
summit are wonderfully well preserved, and by this fact, and that
the species are indistinguishable from those now living in the
sea, prove the elevation of the hills to have been geologically
recent. Further, the larger coral colonies are at once seen to be
still in the position in which they grew, not tilted or overthrown
in any way. This is not the case with the older rocks on which they
lie, the strata of which are frequently twisted and broken, and
this is particularly noticeable sometimes in the case of the layer
of gypsum which is often found between the coral and the sandstone
(Fig. 89 on page 144).
The hills are not marked on any map, indeed no survey has yet been
made hereabouts. My account is therefore incomplete, but this does
not invalidate the conclusions drawn. From seaward these hills are
very easily distinguished from the jagged hills of archean rock,
the true boundary of the Rift Valley, by their flat tops and the
light yellow colour of their cliffs, and also by their generally
being nearer the sea even than the great mounds of gravel which
sometimes form the foot-hills of the mountain range.
Passing from south to north the first range is met with a few miles
north of Mersa[68] Durûr, as a chain of low butts rising from the
alluvial plain a few miles inland. These become higher and more
continuous as one passes northwards, culminating in two considerable
hills, of about the same height and area of base, the northern of
which is marked on the charts, where it is called Table Mountain,
and given a height of 1000 feet.
At about five miles inland from Dongonab are a couple of small hills
standing alone, but a little farther north lying inland from the
middle of the North Basin of Dongonab Bay is a considerable range,
extending towards the hills about Khor Shinab, Hamama, &c., from
which it is separated by an interval of only a few miles.
The Abu Hamâma[69] range (which I so name from its most prominent
though not highest peak, a landmark for sailors) extends from about
the inner branches of Khor Shinab to some distance beyond Khor Abu
Hamâma, lying much nearer the sea than do the others. Its height
is estimated by a government surveyor at from 500 to 700 feet. (Map,
p. 126.)
These ranges are wholly inland, and rise from the maritime plain,
which they divide longitudinally. The scattered ranges of sandstone
are not the whole of this formation however. I give a view of a part
of the maritime plain in which it is seen to rise as a distinct fold
across the middle distance. This appears to consist of the usual
gravel, but where cut into by the Yemêna ravine a very different
state of things is displayed. It is practically all sandstone,
covered by a few feet of gravel and gypsum conglomerate. Of coral I
only saw one large boulder, having no time for a search. There is
yet another range rising from the sea, namely two small hills on
the peninsula of Rawaya, and the islands of Makawar and Mayitib,
of which Makawar is the only considerable elevation. This range is
of special interest and will be described in detail.
_Plate XXXVIII_
[Illustration: Fig. 82. Water-hole under a stratum of hard gravel
and gypsum conglomerate, in the lower part of the ravine]
[Illustration: Fig. 83. Exposure of _sandstone_ under the gravel
of the plain
Two views in Yemêna ravine, which cuts the maritime plain]
_Coral of the coast-line._
This band of elevated coral is never very wide, about a mile at
Suakin, exclusive of the reef, and rather less at Port Sudan. At
Suakin, and to the south it is very slightly raised above sea level,
but at Port Sudan and generally to the north it is from 10—20
feet higher, and is separated from the gravel plain by a depression
a few hundred yards wide. This depression is often very near sea
level and floored with mud in which grow the plants of salt marshes.
Although to the ordinary non-scientific person the idea that most
land was once beneath the sea, and nearly all rocks were formed
beneath the water, may be known, yet unfamiliar, no one can land
on these coral shores without being specially and personally
impressed by the fact that, as the ground is entirely formed of
corals and shells it has been raised up from the sea, beneath which
it was formed. One may walk about on a limestone hill in England
and, by patient collection of fossils, partially corroborate the
geologist’s assertion that the whole thing is a mass of ancient
shells, squeezed together and finally thrust up from the sea bottom,
but here, so fresh are the shells, so familiar their forms and so
abundant the coral, often complete in all its delicate detail[70]
(like those figured opposite pages 88 and 91) that every man may
be his own geologist and assert the origin of the rock as a matter
of personal knowledge. Further, he may assert that all the shells
and most of the corals[71] are exactly like those now living on
the Red Sea reefs, and so deduce the fact that the uplifting of
the original coral reef has been geologically recent, long ages
since the successive worlds of species of animal whose remains make
up the older, and to British minds, more usual, limestones had,
one after another, passed away and been finally replaced by the
inhabitants of our own world.
It is one of the most recent of rocks, and yet it gives some idea
of the meaning of the expression “Geologic time” to remember
that these very ordinary-looking shells lived thousands of years
before the builders of the pyramids.
We can better appreciate the raw newness of the Red Sea cliffs if
we digress a little to the comparison with the very different rocks
of Equatorial East Africa and elsewhere which have however much
the same origin. These latter are much more typical of elevated
coral the world over, the Red Sea, owing to its nearly rainless
climate, having peculiarly well preserved the corals of its raised
reefs. Plate XXXV of cliffs in Zanzibar should be compared with the
Red Sea rocks on Plates XXXII and XXXVII, and the comparison made
in Chapter VIII, page 111, referred to. Of course the differences
between the elevated coral of the Equatorial coast and that of the
Red Sea might be due to the former being of greater age; though, if
such a difference exists, as it is not considerable[72], we are led
to lay more stress on the different physical conditions under which
they are placed. These are that the equatorial rocks are exposed
to a considerable rainfall, and, owing to the tides, to far more
drenching by spray than are those of the Red Sea, resulting in the
solution of the surface layers of the rock and the crystallisation
of the dissolved limestone in all the cavities of the interior,
thus making the rock both crystalline and homogeneous within,
as before described.
Where the Red Sea rock is exposed to alternate wetting and drying
the beginnings of this change are evident. All the way along the
coast from sea level up to a few feet above that portion of the
cliff which is undermined by the waves, the rock is harder and more
homogeneous, but this is merely a local alteration due to the action
of spray. I had opportunities of examining the internal structure of
these cliffs both when the foundations of the quay walls were being
dredged out and when the slipway was excavated at Port Sudan. In
both cases I found that within the homogeneity of the outer crust
disappeared completely, giving place to exactly the structure of
a recent growing reef, the larger colonies of coral forming great
boulders bedded into a loose mass composed of smaller species
and the broken fragments of those more delicately branched. At a
depth of five metres I picked out shells retaining almost perfectly
the colours and appearance of their living relatives of the same
species. The general colour of the excavation was grey, the colour
of the mud which is formed by the disintegration of coral and shells
by boring worms, molluscs and sponges.
The finding of beds of coral on the tops of the sandstone hills at
heights of 500 feet and more above sea level, and the fact that
dead coral and shells form the ground along the coastline, are
explained, as we have seen, by a general uplifting of the whole
country whereby coral reefs have become dry land and even hill
tops. The breadth of the maritime plain is another evidence of the
same fact, for no such plain can be formed on a sinking coast-line;
in such places the successive deposits of sand and gravel from the
hills are submerged and the following form layers on the top of
the preceding and cannot be carried out beyond them to form a plain.
As an example of such a sinking coast-line compare Norway, where
the hills rise directly from the sea and the valleys have sunk
below the water forming the characteristic fjords.
Startling though the thought of such changes of the relative levels
of sea and land may be, they are of common occurrence, and always
have been, in fact they are the commonplace of geological history
everywhere. Our present case is a movement of very minor degree,
involving but a few hundred feet, a mere detail of the opening of
that stupendous fissure, the Rift Valley, of which the whole Red
Sea is but a portion.
It is interesting to note how very regular this elevation has been,
entirely without twisting or contortion of the strata, so that the
individual corals remain exactly in the same position, relative to
the surrounding rock, as they did when growing on the reef.
Hence also the almost perfect level of the coast-line, which,
spite of “faulting” by which a few small areas rise to a
higher level as hills, and the opening of fissures, preserves
the same level, within 20 feet, for several hundred miles. At
the same time the elevation has been effected in several stages,
as evidenced by the existence of level parallel lines of cliff
along the sides of hills, e.g. Jebel Zêt in the Gulf of Suez,
Jebel Makawar on the Sudan coast, which were cut out by the sea
when the hills were at lower levels, and by successive beds of
coral at different levels on hill sides in positions that could not
be due to tilting of the hill during elevation. Also, at various
sheltered points of the shores of Port Sudan and Suakin harbours,
and elsewhere, the latest stage of this elevation can be traced in
the form of a low cliff, standing a few yards back from the sea,
fronted by a reef flat now dry land, though only a foot or two above
the sea level. The cliff is undermined exactly as are those still
under the influence of the waves, and even the detailed marking of
the rock surface characteristic of this marine erosion remains,
not yet obliterated by the flaking away of the surface through
the action of the sun’s heat and the cold of the clear nights,
or by the filing action of the sand blasts of summer.
As explained in Chapter VIII, page 107, much of the fringing reef is
really a part of the coral limestone of the coast, and its formation
needs no further explanation, but it is interesting to note how
greatly it varies in width in correspondence with the height of the
land behind it. For instance, about Suakin it is up to 1½ miles
wide, at Port Sudan only one-third of this, in correspondence with
the fact that the shore about Suakin is raised scarcely two feet
above sea level, whereas at Port Sudan it rises six to ten feet.
This variation is exactly what we should expect on the theory of
reef formation by abrasion, the cutting down of the low-lying land
involving the removal of comparatively small masses of rock and so
proceeding quickly. Again, on the coast about Ankêfail (see Map,
p. 139), where the land is as high as at Port Sudan, the reef is
only a few yards wide, but this may be partially attributed to the
shelter from waves provided by the large island Makawar.
That the differences in breadth are so marked shews that abrasion
has had much more to do with the formation of the reef flats than
has growth of coral, for we see no reason why this latter factor
should not have operated equally well over the whole coast and
tended to equalise the reefs’ breadths.
But it was also explained in Chapter VII that ordinary corals
cannot grow in very deep water, and as we find depths of even 200
fathoms just over the edges of these reefs we are confronted with
a problem. We have explained the origin of only the surface of the
ground, what lies beneath and how it came about that there was a
foundation ready, within the narrow limits of depth in which corals
could build, continuous through so many hundreds of miles, are the
real problems. The barrier reefs, right away from land, from which
they are separated by deep water, still more conspicuously need an
explanation. These are the main questions of this chapter.
_Barrier Reefs._
The barrier system is not a single linear reef, or line of reefs,
but rather a line of areas of shallow water full of reefs of all
sizes, generally more or less crescentic or ring-shaped. The details
have not been surveyed, except very partially in some cases, the
charts from which the map on page 126 is copied merely giving the
outlines of the areas on which the reefs stand.
Some of these areas are very broad, the southernmost, Towartit,
being eight miles across, their size, intricacy, and their being
completely useless to all navigators but a few pearl fishers,
preventing their survey within the outer borders, except in the
case of that which bounds the passage to Port Sudan on the north,
shewn on the map opposite. This area is obviously a continuation
of the barrier system, spite of the fewness of its reefs, and the
fact that over the greater part of it an average depth of 10 fathoms
obtains. It is a young reef, mostly not yet grown to the surface.
_Origin of Barrier Reefs._
The origin of these reefs cannot be explained by any of the theories
discussed in Chapter VIII. Darwin’s theory is quite inapplicable
as the coast has risen continuously throughout recent geological
time, and no currents could have carved out such a channel as that
separating the reefs from the land, with such irregular great depths
as are shewn on the maps, where, within the barrier depths over 150
fathoms are seen in proximity to soundings of only 30 to 40 fathoms,
or even close alongside surface reefs.
Finally, coral growth alone, as already mentioned, could not
give rise to such sheer precipices as those in which these reefs
generally end.
The examination of two features of the land makes all clear at
once. These are the promontories and sandstone hills, of which
the Rawaya peninsula is the best example, and worth describing in
some detail.
[Illustration: Fig. 84. Approaches to Port Sudan, shewing Sanganeb
atoll, portions of the barrier reefs and fringing reef. The latter
dotted.]
The map opposite shews that Rawaya is a large area joined by a very
narrow neck to the mainland and enclosing a large bay, Khor Dongonab,
about 20 fathoms deep.
Directly south of its extremity are the islands of Makawar and
Mayitib, which, like Rawaya, enclose a deep basin of water (40
fathoms) on their west side, while on the east depths of 200 fathoms
are found only a mile and a half from Mayitib, and 300 fathoms only
three miles away, while half a mile from the islet of Shambaya the
same depth occurs. In comparison the elevation of the peninsula,
and even of the islands, is very trifling, and the difference
of level between them and the maze of reefs which separates them
absolutely negligible. Indeed
[Illustration: Diagram 9. Section through Rawaya and Makawar.]
Rawaya is extremely low, its average being about ten feet above
the sea, the areas of its two hills, Jebel Têtawib in the north,
and Jebel Abu Shagara in the south, being inconsiderable, and their
heights only about 40 and 127 feet. Further, an inspection of the
ground shews that these hills are merely parts of the peninsula which
have been thrust up to a higher level (see Diagram 9), and even on
Makawar, where much of the island attains a height of over 250 feet
with summits of 300 feet, the two ends and west side are low like
Rawaya. In short, Rawaya, Makawar and the reefs between and about
them are obviously one continuous ridge, the middle part of which
is slightly lower, and, by coral growth and wave erosion, has been
built up and cut down into the level area of reefs we now find there.
[Illustration: Fig. 85. Peninsula of Rawaya and reef systems
connected with it
The narrow fringing reef along shore of mainland shaded as are
coral beds in Khor Dongonab. Small Islands are black. Dotted areas
are reefs free from coral.
-·-·-·-·- 10 (and in south basin 20) fathom line.
--------- 100 fathom line.]
The conversion of such low-lying land as Rawaya into a reef maze
follows at once from the action of the sea, restrained by coral
growth, described in Chapter VIII, but the diagrams make the case
clearer.
A is the first stage, the thin line representing the outline of
a partly submarine hill range, the undulations of which are much
exaggerated in the diagram. The horizontal dotted line is the
sea-level, so the diagram represents one summit above the sea,
an island, another submerged, and a third emerging to the right.
[Illustration: Diagram 10. Conversion of a line of low hills,
partly submarine, into a maze of surface reefs
A First stage. B after elevation and second period of abrasion.
----- sea level.
[Legend: wavy lines] additions made by coral growth.
Coarse shading = remains of original hill after abrasion.
Fine shading = coral mud or sand.]
In A the first summit to the left appears above water as a rounded
island, which is cut down considerably and much of its area converted
into reef flat, and the deep lagoon to the right is narrowed by
coral growth on both this reef flat and over the next summit,
as indicated by zigzag shading in the diagram.
As this second summit is at about the right depth below the surface,
coral grows vigorously upon it forming a surface reef, slightly
hollowed out in the middle. The third summit is like the first.
_Plate XXXIX_
[Illustration]
[Illustration: Figs. 86 and 87. Two views on Rawaya
Upper figure in the north of the peninsula, lower near salt works
in the south. Both shew one of the canal-like inlets of the sea
which cut up the western side of the peninsula. In the lower one
the inlet is partially cut off from the sea and the great heat
has evaporated its water leaving a lake of salt. The low ground of
Rawaya is shewn behind as a mere line on the horizon in the upper,
but in the lower photograph the hill of Abu Shagara is included.]
In B the thin line represents the final stage of A, further elevation
and abrasion, with coral growth, resulting in the levelling down
of islands and reefs and partial filling of the deeper lagoons as
shewn by the shaded area of the diagram.
Summit No. 1 is not only cut away altogether but hollowed out into
a shallow lagoon, the deep lagoon has been narrowed considerably
while the ring-shaped reef on summit 2 is much as before, but has
spread out and encloses a larger lagoon, thus becoming a small
atoll. Summit 3 shares the fate of number one.
Now compare the outline of the original hill range in A, with the
shaded line in B, and the levelling action of the sea, both upwards
and downwards, is evident.
This explanation of the origin of the reefs between Makawar and
Rawaya can obviously be extended to those to the south as far as the
Têlat Islands, and to the whole barrier system in fact. The reefs
south of Salak are similarly related to a large area of raised coral
extending from the point northwards, and though there is no bay
here, corresponding to Khor Dongonab, there is a large salt marsh
separating this from other raised coral to the west, and formed by
the filling in of a bay by blown sand. The diagrammatic map overleaf
makes this clearer, and shews that on land we have continuations
of both kinds of reef, the barrier being continued as the eastern
coral ridge, the fringing reefs of Salak Seghir being one with the
limestone on the west side of the swamp. Similarly Ras Benas to the
north (lat. 24° N.) and the angle at the entrance to the Gulf of
Suez, have reefs and islands in continuation of them southwards,
the former being named Makawar, in this as in appearance recalling
the island off Ras Rawaya.
It is now evident that the origin of both barrier and fringing reefs
is identical with that of the whole coast-land, and is not to be
looked for in any laws of coral growth, or marine sedimentation
and abrasion, these factors having merely affected the summits of
submarine hills hundreds of miles long, nearly two thousand feet
high, often peculiarly narrow, and always more or less parallel to
the axis of the sea-filled Rift Valley.
[Illustration: Diagram 11. Fringing and barrier reefs in
neighbourhood of Ras Salak, shewing correspondence of both systems
with coral formations on land
Soundings with line and dot over mean that no bottom was found
after so much line was run out.]
The rocks of which these ranges are composed are laid bare in the
cliffs which have resulted from the upthrusting of the hills of
the Rawaya-Makawar range, and on the hills of the maritime plain.
_Plate XL_
[Illustration: Fig. 88. In a fault ravine of Abu Shagara. Cliff
coral, gypsum and sandstone, the latter containing sheets of
recrystallised gypsum, selenite, in every crack]
I illustrate overleaf part of Jebel Têtâwib in the north part of
Rawaya. It is about 40 feet high, and of this from one to six feet
are occupied by the basal sandstone, a soft laminated rock generally
yellow in colour, sometimes greenish or red. Next is a band, up
to 20 feet thick, of gypsum, the strata of which are considerably
contorted in contrast to the coral formations overlying them, which
are nearly horizontal, and as usual retain the relative positions
they occupied during the growth of the reef. In the south Jebel
Abu Shagara is higher, 127 feet, and its cliffs, being higher,
contain very much more sandstone, but are essentially the same, as
are those of Jebel Makawar and Mayitib, and those of the sandstone
hills of the mainland.
The sandstone ranges, coral coast-line, and barrier reefs are
then three parallel repetitions of the same structure extending
with great regularity along the sides of the Rift Valley from the
entrance to the Gulf of Suez to Suakin, a distance of about 700
miles. Southwards of this point, as we shall see, similar structures
occur, but without this extreme regularity.
Their formation is due to the opening of the Rift Valley which
resulted in these sandstones[73] being thrown into a series of
steps as it were along each side of the trough, as shewn on page
145. Of these we are acquainted with three, but more would probably
be discovered if detailed soundings were taken from outside the
barrier reefs to the narrow trough which runs down the centre of
the sea and is a thousand fathoms deep.
The further history of these three steps or ridges has been as
follows. We will distinguish them as numbers one, two and three,
the former being the highest, the present sandstone hills and ridges
of the maritime plain. The coral caps on these were formed when
the sea reached to the bases of the Archean hills, the sandstone
range No. 1 being a line of barrier reefs off the mountainous
coast-line. The mountains, then as now, were being broken down by
the action of the weather, and the resulting sand and gravel was
washed down into the sea as the beginning of the maritime plain.
[Illustration: Fig. 89. Jebel Têtâwib, in Khor Dongonab, a butt
near its southern extremity, seen from its south side.
The letter B is at a level about 25 feet above the foreground
A. Coral colonies, in position of growth, bedded in a mass of loose
coral débris, shells, &c.
C. Hardened coral-mud. The weathered surface forms rounded masses
in low relief.
D. Gypsum strata, here steeply tilted, and upturned at their ends
in the piece shown in the foreground. They are much folded in other
parts of the cliff.
E. Green and red shaly rock underlying and sometimes interstratified
with the gypsum. It is here broken down into sand. This rock
contains sheets of glass-like recrystallized gypsum.]
Meanwhile organic remains were accumulating on ridge No. 2, and as
elevation brought this within fifty fathoms or so of the surface,
reef corals took possession and covered the summits one after
another as elevation proceeded, so that
[Illustration: Diagram 12]
when ridge No. 1 emerged from the sea altogether, and its bases
were surrounded by the gravel from the hills, No. 2 was a second
barrier reef out at sea.
The same process has been repeated, so that coral growth and
levelling have made ridge No. 3 into the present barrier system
and the maritime plain has reached the one-time barrier No. 2 and
so made this the present coast-line.
During the last of these elevations a good deal of breaking and
cracking of ridges Nos. 2 and 3 took place. For instance, Rawaya was
originally connected with the mainland, the proof being the presence
upon it of scattered pieces of Archean rock which could not possibly
have reached it unless a continuous surface stretched from it to
the old hills. Dongonab Bay, and with it probably other parts of
the channel within the barrier system, have evidently been formed
or enlarged since the maximum extension of the maritime plain. The
harbours of the coast, which are so interesting in themselves as
to deserve separate consideration, were formed also at this time.
Natural harbours, almost completely surrounded by land or reef,
waveless in all weathers, more perfect than almost any made by
man, abound throughout the length of this coast. In one part,
just north of Rawaya, are ten of these strange inlets in a space of
only 40 miles (see map, page 126). That of Suakin has been already
described; on entering for the first time it is hard to believe
that this long parallel-sided, deep channel, bounded by reefs
covered only by a foot or two of water, and then by land only the
same amount above the sea, is not an artificial canal. It leads
nearly straight inland for two miles, but not _quite_ straight,
indeed there is a bend that large steamers frequently fail to clear,
and which led to the abandonment of Suakin as the Port of the Sudan.
Obviously this canal-like inlet is not the mouth of a river, past
or present, for present rivers there are none, and no river, flowing
over a wide plain, through loose and heterogeneous materials, could
cut out such a channel, but would end in a wide shallow estuary or
delta, if it formed a definite mouth at all.
The new harbour of Port Sudan is much wider both in the entrance
and within, but the origin of this deep landlocked basin is equally
puzzling.
The forms of all the harbours of the coast can be reduced to one
plan more or less easily, that of a cross with arms parallel and
at right angles to the coast-line, and are in fact formed by two
cracks in the earth’s surface nearly at right angles. The former
arm is generally the largest, in Port Sudan it is two miles long, the
other arm, which connects this with the sea and forms the shallower
branch harbour, being much the shorter. The same applies to for
instance Wiai, Fîjab, Salak Seghir and Ankêfail Kebir, whereas in
the case of the narrower harbours, like Suakin, Arûs, Shinab and
its neighbours, the arm at right angles to the sea is the longest,
and the plan of the inlet is more like the conventional cross.
[Illustration: Fig. 90. Two of the canal-like “Khor” which
run into the coral plain. Soundings in fathoms, note comparatively
great depth of the water
The arrow in Khor Shinab indicates the point at which the material
of the cliff changes from coral to gravel.]
In Wiai, Fîjab, Salak Seghir and other harbours most of the land
between the inner arm and the sea, corresponding to the East Town
in Port Sudan, has been cut down
[Illustration: Fig. 91]
and converted into reef, upon which strips of sand have accumulated
to form islands in places. In all three harbours currents flowing in
and out have buried the south end of this reef, next the entrance
passage, in a steep sandbank (the point is marked by an arrow on
the plans). As the water is too deep for convenient anchorage of
small vessels, the _sambûks_ run their noses on to these sandbanks,
a couple of sailors walk ashore with the anchor, and they are moored
for the night, as the prevailing wind is from the north.
Salak Seghir has a long narrow winding entrance, like a deep still
river between reefs. Having successfully but fearfully navigated
this in my launch, I found that my sailors’ design was to
run her on to the sandbank, _sambûk_ fashion. I declined this,
for my copper sheathing’s sake, and was all unprepared for the
fact that the passage there is about as wide as my launch is long
and that the inner branch is shallow and full of humps of coral,
giving me a choice of evils which I do not intend to make again. The
sand lying on the reef between the inner harbour and the sea has
become consolidated into sandstone in a narrow parallel-sided band,
perfectly level and almost as regular as an artificial breakwater. A
short length of such a formation would be striking, but this extends
to nearly two miles.
These curious and most useful splits of the land have been made since
the maritime plain was complete, as we saw was the case with Dongonab
Bay, and, consequently, part at least of the barrier system. In some
cases the innermost parts of the harbours are composed of gravel,
not elevated coral. At Fîjab this is due to erosion of the coral,
as shewn by rocks and islets of this material remaining on the
shallows which separate the gravel cliffs from the deeper water,
but in other cases the gravel bounds the actual fault.
This is well seen at Shinab, where almost the whole harbour is
bounded by raised coral cliffs, but near the innermost end this is
overlaid with gravel, and finally gravel replaces the coral in the
most regular manner, shewing that the two materials were in perfect
continuity when the split which made the harbour occurred. The north
and south limbs of the crosses have been largely filled in with water
and wind-carried sand; they were originally of much greater length.
The peculiarities of the coast impress unusual methods upon those
who travel along it by sea. The wave motion varies greatly; from
Port Sudan to Darûr for instance the waves are much the same
as on an open sea, from Darûr to Fîjab the barrier system gives
considerable shelter, the vessel passing into perfect calm for short
periods as she approaches near the reefs in tacking. From Shalak to
the Têlat Islands is a bad bit in stormy weather, quite open sea
and no possibility of anchoring anywhere in an emergency, so that
vessels are often windbound at Salak[74] anchorage, waiting a fall
in the wind in which to reach the next section of the barrier system.
To travel by night is obviously impossible, the navigation of a boat,
even among well-known reefs, when moonlight seems bright as day, is
an experience once tried never repeated, without urgent cause. Even
when the sun is low it is extremely difficult to see one’s way,
though a good native pilot sees indications of reefs where all is
a white glare to even an experienced Englishman. Consequently it
is the invariable custom to get into the nearest harbour about
four o’clock in the afternoon, and if the coast were not thus
liberally provided, the natives’ travel by sea would be nearly
impossible. The start is early next morning, between 2 a.m. and 4
a.m. according to the wind and the distance of the reefs. If the
wind is fairly off shore, so that the neighbourhood of reefs will
not be reached for some time, the start is early and the sail is
hoisted in a strange silence, the sleepy sailors, on these occasions
only, omitting their shouts and chants, and the vessel slips out
of harbour like a slowly-gliding ghost.
As already remarked the formation we have described is that of almost
the whole Red Sea, but south of Suakin it loses the regularity that
is so noticeable northwards. The area marked on the map (p. 126)
as “Suakin Archipelago” consists of innumerable small reefs,
shoals and islets, the water between being of very irregular
depth, three hundred fathoms being found close alongside surface
reefs. Opposite Trinkitat the water shoals gradually, only 30 to
40 fathoms being found as far as 45 miles from land. The shore
itself is extremely low and sandstone hills are absent. The area
marked Shubuk, south of Suakin, shewn on the map enclosed by a
thin semicircular line on the north and east sides and by the land
on the west and south, has a very remarkable structure. An area
of 100 square miles is enclosed by a regular and unbroken reef,
indicated by the above curved line, the space within being a most
intricate maze of reefs with comparatively deep canal-like passages
between them. In the south are broader passages and some islets of
elevated coral. The bounding reef is extremely regular and, on its
eastern side, unbroken. It consists of a steep slope and precipice
of growing coral up to near the surface, when the slope becomes very
gradual and forms a nearly smooth surface of stunted corals with grey
_Xenia_. This extends to among the breakers, above which is a band of
gravel formed of broken and wave-rounded pieces of coral. Within is
sand, and the coral capped sandbanks of the labyrinth. Landwards the
ground is for miles so low and so much broken into by salt lagoons
and marshes that a definite coast-line can scarcely be said to exist.
This is the remains of an old delta of the Khor Baraka, a river which
rises in the Abyssinian highlands, but which nowadays never reaches
the sea. Its floods come down on to the maritime plain at Tokar,
where, spreading over a considerable area, they render possible
the growth of cotton and other crops in the fine soil they leave
soaked with water. Tokar is thus the one fruitful spot of any size
on the whole Red Sea coast, but its character of fertile oasis is
but short-lived. When the crops are gathered even the dry cotton
stalks must be removed lest they should collect the sand, which,
every day of the summer, is carried over them by burning gales,
and so would convert the fertile ground into barren sandhills.
As it is, at any rate at present, impossible to predict these floods
the seed must be sown after each, even though it frequently happens
that another flood comes down and carries it all away. The seed must
then be patiently re-sown, and that left by the last flood will grow
and bear. In the old days, before man was there to make any use of
it, the Baraka formed a regular delta, subject to yearly floods,
a miniature Egypt. The growth of coral in this neighbourhood would
then be impossible, the shifting sand, muddy and freshened water
rendering its life impossible.
Now when the rainfall decreased so that for the greater part of the
year no freshwater stream entered the sea, and the materials of the
edges of the delta became stationary, coral growth arose here and
there, forming a fringing reef, the extension of which seawards must
have been exceptionally rapid in this gradually shelving water. There
was then an elevation of the sea-bottom, here comparatively slight,
and the sea began to cut into the raised coral, carving it out into
islets, surface reefs, tidal channels and lagoons.
At the seaward edge of the reefs, the coral in the purer water could
continue as it is doing now, a rapid and continuous growth, the
uniformity of the conditions producing the remarkable unbroken reef
already described. The disintegrating forces described in Chapter
VIII break the inner side of the reef into sandy flats and pools,
but the extension of these pools into wider spaces and continuous
channels makes coral growth again possible in the shallows on their
banks, so that all the inner sandbanks and rocks are capped with
live corals which prevents their further demolition and gives the
steep-sided canal-like form to the passages between them.
_Living Reefs._
In detail the living reefs of the Red Sea are characterised by (1)
their luxuriant growth of coral, (2) the absence of large stones or
“negro-heads” along their edges, (3) their frequently crescentic
or circular forms. There are remarkably few points on the coast
where coral is not growing in abundance, and at the edges of reefs
in the open sea its luxuriance is wonderful. Even at the inner ends
of the canal-like harbours, where the water is stagnant and dirty,
scattered colonies are found. The absence of rivers probably favours
these growths, but even so the floods bring fresh water into the
harbours occasionally, and colour their waters a deep red brown,
like the Nile in flood, for several days together once or twice a
year. The absence of strong currents carrying mud is another factor;
where such occur, in Khor Dongonab, the vertical sided coral reefs
give place to sloping bottoms of rock covered with stones formed
by the growth of lithothamnia—stony seaweeds—but I only know
this one place where these conditions prevail.
There are no stones of any size on the edges of the reefs, nothing
but a few pieces of coral, a foot or so in diameter at most,
project above lowest water level. It would decrease the danger
of navigation in dead calm weather, which at present, except in
frequented places, is considerable, if larger stones, such as the
“negro-heads” of some reefs, were present at intervals. As in
most cases these larger masses are the remains of former land and
are not, as sometimes stated, fragments of the living reef thrown
up by storms, one cannot expect their presence here, where the reef
edge has grown up _in situ_. But even large boulders such as form the
“hurricane beach” of Pacific atolls, and are cast up by storms
in profusion on the reefs of Queensland, are not found here. There
is plenty of strong wind in the Red Sea, but the strongest of all,
the hot winds of summer, blow off land and do not extend far to sea
and such vast breakers, up to 40 feet high, as are recorded of the
Pacific, never occur. The reef edge above the precipice consists
of a slope of stunted coral which above water-level changes to
a gravel of coral fragments coated with lithothamnia; within is
sand, generally with bands of seaweeds and marine flowering plants
(grass-like in appearance) on the shallows, with muddy pools and
channels a fathom or two deep. The edge is higher than the rest,
but does not appear continuously above lowest water-level. Reefs not
exposed to the waves have not this definite edge, the platform being
covered with two or three feet of water in which stand separate coral
colonies, numerous at the edge, rarer within towards the lagoon.
The crescentic or circular forms of isolated reefs is very far from
being a peculiarity of the Red Sea, but is worth while mentioning
as bearing on the formation of the lagoons of atolls on the larger
scale. From the smallest to the largest these reefs shew a hollowing
out in the centre, where the reef material, not being protected by
living matter, is exposed to the destructive influences detailed
in Chapter VIII.
There are some reefs in the Red Sea quite of the Atoll form, of which
the largest is Sanganeb, the plan of which, on the map on page 137,
is sufficiently explanatory. Another is Tella Tella Seghir, which
is elevated to 40 feet above sea-level, and consists of a ring of
high ground enclosing a depression, once a lagoon though its floor is
now a little above sea-level. The elevation of this ring-shaped reef
has been at least two stages, the lagoon having contained water to
about one-third of its depth comparatively recently, since a line
of undermined cliffs occurs at this level. The _Admiralty Pilot_
remarks that the edge of the ridge bears numerous cairns. Some of
these are artificial I am told, but those I examined were large coral
colonies, in the position in which they grew, left exposed by the
removal of the softer stuff in which they were embedded. One example
of the coral genus _Mussa_ was especially conspicuous, as much so as
the corals illustrated on Plate XXXVII on the top of Jebel Têtâwib.
SUMMARY OF GEOLOGICAL HISTORY OF THE RED SEA.
(1) There was originally a _shallow_ sea covering the space between
the high mountains of both sides of the present Red Sea. In this sea
were laid down sandy and gravelly sediments, and limestones were
formed which are now found in the sandstone hills of the maritime
plain, &c.
The gypsum beds found here are the result of the drying up of the
water of this shallow sea.
(2) The beds of rock thus formed were broken up by the sinking in
of a long strip of the earth’s crust forming the Rift Valley,
which extends from Jordan to Tanganyika. Part of this valley was
filled by sea water and became the Red Sea.
(3) There have been three successive systems of barrier reefs along
the Red Sea coast, which by continual uplift have become,
(_a_) A range of sandstone hills rising from the alluvial maritime
plain.
(_b_) A fringe of limestone along the present coast-line.
(_c_) The present barrier system.
(4) These three ridges were formed by the faulting of sedimentary
rocks which overlay the bases of the Archean hills at the time of
the great movement which opened the Red Sea section of the Great
Rift Valley.
(5) The northern ends of several sections of the present barrier
reefs are elevated above sea-level, and examination of these,
and of the hills of the maritime plain, enable us to reach the
above conclusions.
(6) At the same time Rawaya gives evidence of a seaward movement
as well as uplift, Khor Dongonab and some at least of the channel
within the barrier reefs being recent fault depressions, not merely
an anticlinal fold formed at the opening of the Rift Valley.
The harbours and other fissures in the coral limestones, &c. of
both coast-land and of the barrier reefs are due to the same
secondary faulting.
(7) The maritime plain had its maximum seaward extension after the
growth of coral on the second and third barriers. Owing to elevation
nothing has been added to its seaward slopes since the formation
of the features of the present coast-line by secondary faulting.
(8) The filling in of valleys and the completion of the connection
of the second barrier with the maritime plain has been largely due
to blown sand. The process is continuing, e.g. an extensive plain
near Dongonab shews perfect uniformity in its formation.
INDEX
Acacia bushes, beauty of 3
use 50
Administration by native Shêkhs 27
Akaba 122
Alexandria 111
Amulets 42
Animals resembling seaweed 8
Arabs 15
Archean rocks 127
Arms 23, 27
use of 28
Arûs harbour 148
Association of corals and higher animals 94
with microscopic plants 95
Atbara route to Nile ix
district 19
Axum, mediaeval trade route of 39
Baraka flood and delta 151
Benas, Ras 141
Biologist’s justification viii, x
Bisharia, tribe 15
Block mountains 125
Boring animals 101
Boundaries of Anglo-Egyptian coast vii
Brothers Islets 123
Cape Verde Islands 110
Cats 45
Challenger Society x
Clams (_Tridacna_) 69, 100
as food 59
pearls from 69
Climate, Red Sea coast 118
_Clione_, boring sponge 101
Corals, animal nature of 85
cups and polyps 85
colours 93
formation of colonies 89
genera _Caryophyllia_ (British coral) 87, 91
_Dendrophyllia_ 89
_Fungia_ 92
_Galaxea_ 94
_Madrepora_ 91
_Porites_ 89, 100
(see also plates 26 and 28 and explanation p. 88)
life-history 98
physiology 86, 95
place in evolution 96
plant-like nutrition 95
sizes of colonies 89, 91
variety of forms 90
Coral Reefs—see Reefs
“Coralline” seaweed 100
Crystallisation of coral limestone 111
_Cymodocea_, a marine plant 108
Daedalus shoal 124
Dance, religious 41
Darwin’s theory of atoll formation 114, 136
Delta of the Baraka 151
Depth of Red Sea 121
Destruction of corals 100
Devil fish or sting rays 70
Dhow, or _sambûk_ sailing vessels 59
Diving, as seamanship 63
for pearl oysters 66
Divorce 75
Dongonab Bay 104, 138, 141
reefs in 153
Dongonab plain 155
Dulawa harbour 147
Dunn, Sudan Government Geologist vii
Eclipse of the moon 47
Elevation of reefs above sea 131, 133
in Pemba and Zanzibar 109, 110
_Eunice_, marine worm 101
Evil eye 44
Evolution 96
Faulting, geologic term 122
Fijab harbour 148
Fishing 68
Fish spearing 70, 71
Funafuti atoll reef 114
Gardiner, Prof. J. Stanley ix
Geologic time 97, 132
Geological history of Red Sea, summary 154
Gordon 14
Gregory, Professor 122, 125
Hamitic natives 16
personality of 22
Harbours, natural 146
Hardihood of natives 30, 54, 78
Honesty 49
Ibrahim’s wife 76
Jebel Abu Shagara and J. Têtâwib 138, 143
Jedda 12, 21
Linnean Society x
_Lithodomus_, boring mollusc 101
Lithothamnia, stony seaweeds 110
Luxuries, native 55
Mabrûk, Swahili slave 20
Makawar Island 134, 135, 138
Manufactures 80
Maritime plain 3, 10, 127
Marital relations 32, 73
Medical ideas 47, 78
Medicine, ink as 43
Milking 44
Mohammedanism 33
Mûled dance and service 40
Murray, Messrs x
Nationalities on coast 15
Negroes 17
Newness (geologically) of Red Sea coast 132
Ornaments 24, 25
Osman Digna 20, 27
Pearling 65
Pearls, from _Tridacna_ clams 69
theory of origin of true pearls 44
_Pholas_, coral borer 103
Plant and animal interchange of oxygen 94
_Porites_—see Corals
Porpoise 45
Port Sudan 1, 9, 133, 146
approach by sea 1, 136
reefs at 83, 100, 103
Port Sudan-Atbara route to Khartum ix
Prayer 39
_Pseudoscarus_ 101
Rainfall 120
Rain squall 7
Raised shore-lines 134
Rawaya peninsula 127, 131, 138, 145
Reefs, coral Australian Barrier 117
classification of 113
constituents 99, 131
elevated above sea 83, 131
form of surface 104
fringing and barrier 1, 113, map 137
growth of 100, 105
living of Red Sea 152
origin by abrasion 106, 140
origin of atolls 114
pictorial 5, 8, 92
sandstone, Alexandria 111
sandstone, Cape Verde Islands 110
Zanzibar 89, 109
Religion 33, 35
Rift Valley, the Great 122
step formation of sides 122, 143
Rising and sinking lands 133
Sailors, Arabian 59
Hamitic 65
Salak 127, 141, 149
_Sambûk_ (or dhow) sailing vessel 59
Sand-eating animals 103
Sandstone hills 129
Sanganeb lighthouse 10
atoll reef 124
Sea gardens 8, 92
Seaweed, stony 100
Sexual morality 26, 32
Shêkh Barûd (Port Sudan), stories of 37
Shêkhs, living 27
as dead saints 36, 38
Shinab Harbour 147
Shubuk reefs 150
Slaves’ stories 19
Social relations 32
Songs of labour 31
Sting rays or devil fish 70
Suakin 10, 13, 146
archipelago 9, 150
Subsistence of natives 50
Sunset land 8
Superstitions, native 42
British 49
Tella Tella Island 154
Tents of nomads 53
Têtâwib hill 143
Tides of Red Sea 121
Tokar, dervish stronghold 20
oasis, delta of Baraka floods 151
Travel by sea 150
Trinkitat 150
_Tubipora_ 110
Uganda railway, crossing Rift Valley 124
Utensils 55
Vegetation 3, 52
_Vermetus_ 110
Volcanoes 123
Warning coloration of sting rays 71
Wedding and Arabian sword dances 57
Wiai harbour 148
Women, personality and influence 24, 73, 81
_Xenia_ 151
Yemêna ravine and oasis 130
Zanzibar reefs 89, 109, 117
CAMBRIDGE: PRINTED BY JOHN CLAY, M.A. AT THE UNIVERSITY PRESS.
_Plate II_
[Illustration: Fig. 2. Map of Red Sea]
FOOTNOTES:
[Footnote 1: For the story of Shêkh Barûd’s death see p. 37.]
[Footnote 2: _Acacia tortilis_.]
[Footnote 3: A species of _Carex_ I believe.]
[Footnote 4: I am writing of the neighbourhood of Lat. 22° N. About
Suakin conditions are better, at least for plant life.]
[Footnote 5: Actual temperature of the wind 100° to 115° Fahr.]
[Footnote 6: Besides the completest possible system of harbour
lights, the quays, etc., with every facility for handling cargo,
the needs of shipping are well provided for by a large stock of coal
electrically handled, tugs and water barges, and a complete dockyard
for repairs of any kind up to a considerable magnitude. Since
1910 a salvage tug has been stationed here and the slipway at the
dockyard has been completed and is in use. The town water supply,
though healthy, is slightly brackish (though much less so than most
desert wells), but the very large condensing plants produce and sell
fresh water very cheaply. The railway of course runs alongside the
shipping, the Customs godowns are liberally and conveniently planned,
and the railway bridge rises vertically upwards so that any vessel
may pass up to the dockyard without obstruction.
A first-class hotel has just been opened.
Spite of the harbour’s being practically tideless its water is
perfectly pure, all garbage being collected in barges and towed out
to sea, where they are emptied at a distance of about five miles
from land.
The whole town is a fine example of what can be done by scientific
forethought given free scope and a clear desert site, unhampered
by the presence of partially obsolete arrangements and conflicting
vested interests, and keeping ever in view the great extensions
of every department which the increasing trade of the Sudan will
soon need.]
[Footnote 7: Many of the smaller mosques in Egypt, particularly
those near the deserts (e.g. Suez and Belbês) are very similar.]
[Footnote 8: The population of my own tiny village well illustrates
the cosmopolitan nature of all commercial and administrative activity
in the Sudan; including Government _employés_ we have:
British: one, myself.
Syrians: two, my assistant and a carpenter.
Italian: one, engineer—we communicate in Arabic.
Egyptians: about six.
Arabians: also about six, including natives of Sinai in the north
and the Yemen and Hadramaut in the south.
Hamites: the natives of the country.
Negroes: these include several distinct nationalities, e.g. Nubas
and Nilotic tribes.
Swahili: one, from Zanzibar.
Elsewhere Greeks abound, as in so many countries.
We all speak Arabic, which was however the mother tongue of only
a dozen individuals, and even these speak three or four different
dialects!]
[Footnote 9: I am at a loss for a name for this particular Hamitic
nation. The names Hadendoa, Bisharia and so on, are those of large
tribal subdivisions of one nationality, which again is but one out
of several distinct Hamitic peoples. The name “Beja” is not
known to any native I have met.]
[Footnote 10: Apart from the use of henna, I have detected a distinct
red tinge in some men’s hair, but the wonderful gold and brown
mops of the Somalis are not seen here.]
[Footnote 11: Some Arabs grow bushy beards as dense as any
European’s, but the majority, at any rate before middle age have
only scanty beards, if any.]
[Footnote 12: Contrast Zanzibar and the tropical coast of East
Africa where nearly the whole population is supposed to be a cross
between Arab and negro.]
[Footnote 13: Presumably General Grenfell’s engagement with the
dervishes, Dec. 1888.]
[Footnote 14: Inverted commas indicate quotations from Mabrûk’s
statement. The probability is that the hardships of a seven months’
voyage were actually compressed into so many weeks.]
[Footnote 15: The camel kneels and his halter rope is bound round
one foreknee. Only a fractious beast will rise when so tied, and
straying or running away is impossible.]
[Footnote 16: I am told that the sexual freedom of women is
a peculiarity of one tribe only—the Aliab—but it certainly
applies to all the maritime people with whom I live.]
[Footnote 17: The word Shêkh, meaning literally an old man, is
used to designate alike a dead saint of the highest supernatural
powers or one whose sanctity is barely acknowledged by an occasional
decoration of his tomb by a piece of rag on a stick. Among the
living the Shêkh may be a native leader of any grade, from the
head of a great tribe, with whom the British Governor of a Province
may consult, and to whom is entrusted real power, to one of his
subordinate agents. In my village one of these combines the trade
of butcher with a dreadful dentistry.]
[Footnote 18: Anglicised into “hangar.”]
[Footnote 19: I give a few examples, but Dr and Mrs Seligmann,
being expert anthropologists, were able to extract more curious
information from my own men in half an hour’s talk than I had
done in five years. Their report will be of the highest interest.]
[Footnote 20: I need not say that I refer to national habit. To
say this of personal expression would be an obvious and gross libel.]
[Footnote 21: It is curious how all races of mankind, from Ireland
to India, Borneo, Japan and Zanzibar, should concur in decorating
their holy places in this way. Probably practical convenience is
a prosaic part of the explanation.]
[Footnote 22: In spite of its improbabilities the story illustrates
well-known and interesting phenomena. Such a sudden rise of the sea
as is demanded is quite possible, and the shore level about Jedda,
as elsewhere, is extremely low. Though practically tideless the Red
Sea is subject to variations of level which cannot be predicted,
and which may be as much as three feet vertically. Also objects from
the Arabian shore are very commonly carried across the whole sea and
stranded on the west side. A year or two ago we learnt that abnormal
floods had occurred in Arabia from the number of palm trunks found
stranded, and another time, the whole coast, for 100 miles at least,
was littered with the bases of palm leaves. These had been left on
the ground in some Arabian valley when the leaves were cut for use
or sale, and carried into the sea by a flood.]
[Footnote 23: In Egypt Mûled or Mowled means a fair held on a
saint’s birthday. On the Red Sea coast a man “makes a mûled”
to celebrate any private event, or simply by way of giving an
entertainment.]
[Footnote 24: Really dolphin, but this name is also used for a
certain fish. The true dolphins, like porpoises, are whales in
miniature, air-breathing animals which have become disguised
by a fish-like outer shape in accordance with their marine
life. Internally every structure is like that of the land mammals
from which they are descended, and totally different to that
of fishes.]
[Footnote 25: _Science of the Sea_, issued by the Challenger Society,
John Murray. Any reader interested in Part II of this volume should
obtain this book as a guide to work in Marine Biology.]
[Footnote 26: One would not _drown_ them simply because water
costs money.]
[Footnote 27: Suakin is populous in winter, the inhabitants going
into the hills to escape the summer heat. At my village the reverse
is the case.]
[Footnote 28: Flint and steel are sometimes used, but nowadays
matches are extremely cheap in all corners of the world.]
[Footnote 29: The leaves and midribs of the date palm are essentials
to the people of the desert, nowadays at least, as they use them
for all sorts of handicrafts. There are some date palms near Suakin,
but their matting is imported.]
[Footnote 30: Gourds are continually mentioned in books of travel,
and as the word remained mysterious to me from my eighth to my
twentieth year I explain in detail. Certain species of melon-like
plants produce a hard-skinned fruit, the bitter pulp of which dries
up to a powder, leaving the skins hollow. Cut a hole in this,
clear out the seeds and fibre, and you have a basin or a bottle
according to the shape of the fruit. Only the former shape is seen
on the Red Sea coast.]
[Footnote 31: Is “coffee bean” a corruption of the Arabic
_bûn_?]
[Footnote 32: This is of course the regular way of making Turkish
coffee and can be imitated at home by anyone. The coffee should be
finely ground or crushed like cocoa.]
[Footnote 33: The sheet is passed round a smooth thick thwart and
one or two of the crew take up the slack as the rest haul in. This
_to some extent_ takes the place of tackle.]
[Footnote 34: They do not _bale_ water out of a canoe, but throw
it with a paddle, shell or broken wooden bowl! The method is very
effective.]
[Footnote 35: Merely to see the bottom, without distinguishing
small objects, is often possible at greater depths still.]
[Footnote 36: The flesh of the pearl oysters and clams are only
eaten when all else fails and as fish is obtainable in times of
scarcity it is not much appreciated when anything else is to be had.]
[Footnote 37: Unlike the Ballistes, some species at least are able
to swallow the flesh of shellfish without the shells, so that the
only indications of the origin of their diet is by finding opercula
and radulae among the half-digested mass. They also break up pearl
oysters, leaving the ground strewn with the broken shells.]
[Footnote 38: The rather majestic term “Shêkh” often means
nothing more than this, though it also includes persons of real
importance and power in the country, and saints devoutly venerated.]
[Footnote 39: Donkeys’ dung provides a cheap substitute for
soap. The clothes are buried in the sand of the sea-shore with dung,
left overnight and washed out in the sea next day.]
[Footnote 40: Except certain rare forms obtainable only from very
deep water in the Atlantic.]
[Footnote 41: More correctly by decomposition of calcium sulphate,
thus,
CaSO₄ + H₂CO₃ = CaCO₃ + H₂SO₄,
since the sulphate forms 3·6% while the carbonate only forms 0·2%
of the salts dissolved in sea-water.]
[Footnote 42: Similar masses are common in the Red Sea, but I do
not know of a case quite so striking as this.]
[Footnote 43: The apes include a wide variety of type, e.g. the
baboon is on the whole lower than the ordinary monkey, which is lower
than the big man-like apes, chimpanzee, orang-outang and gorilla. The
lemurs link lower monkeys and ordinary quadrupeds. See Huxley’s
essay “On the relations of man to the lower animals,” 1863.]
[Footnote 44: Avoid the conception of descent as the highest fish
giving rise to the lowest amphibian, the highest amphibian giving
birth to the lowest reptile, and so on. The fishes did not cease
their evolution when the amphibia appeared, and ancestral types
must have been generalised, and so, in one sense, lowly forms.]
[Footnote 45: More accurately gypsum, CaSO₄, which is decomposed
to form limestone CaCO₃ as noted bottom of page 89 above.]
[Footnote 46: To obtain a specimen of coral of this beautiful
snow-white colour it is necessary to remove the flesh from any living
colony, which is usually coloured brown or yellow, by rotting the
same for a week in sea-water in a bucket. Cover the bucket to keep
out dust, and change the water a few times to reduce the smell. The
last traces of decayed flesh are removed by dashing buckets of
water on to the coral. Rinse in fresh water. Or the specimens may
be dried _thoroughly_, rotted in water after return to Europe,
and finally bleached with hydrogen peroxide.]
[Footnote 47: See for instance, among other literature by the same
author, “The Fauna and Geography of the Maldives and Laccadives,”
Cambridge University Press, and “The Percy Sladen Trust Expedition
to the Indian Ocean,” by J. Stanley Gardiner, M.A., F.R.S.,
etc., _Transactions Linnean Soc. Zoology_, Vol. XII., pp. 35, 51,
and illustrations on Plate IX, and on pages 128 and 135.]
[Footnote 48: The duller coloured and peculiarly shaped
“bladder” or “parrot” fish, _Tetraodon_ or “four tooth”
is distinguished by its peculiarly rounded shape and alteration of
scales into spines. Though its teeth are very similar to the coral
eater, _Pseudoscarus_, it eats softer animals, e.g. Ascidians and
Echinoderms, and sometimes shell-fish.]
[Footnote 49: Includes the whole oyster and bivalve shell-fish
family, and many other less generally known forms of life.]
[Footnote 50: Owing to the practical absence of tide in the Red Sea
the boat channels of the fringing reef are discontinuous, so that
at intervals the canoe men must wait for favourable weather and put
out to the open sea. In Zanzibar, e.g., one can travel the whole
60 miles of the east coast within the reef but for the crossing of
one bay, except at lowest spring tides. Some of the deeper parts
of the boat channel of Red Sea reefs are due to faulting (see next
chapter) as may be also a very peculiar reef channel just north of
Mombasa Harbour.]
[Footnote 51: For the qualities and formation of this rock see
page 111.]
[Footnote 52: The absence of similar stones from all other parts
of the reef is to be accounted for by their being subject to the
continual friction of strong sand-laden currents, while those on
the raised edge are exposed to waves of clean water for a portion
of each day only.]
[Footnote 53: These tubes form a mass by coiling loosely together
like a cluster of worms, though the animal which makes them is
exactly like those that have regular spiral shells and move about
freely, e.g. the whelks. The young _Vermetus_ has a shell like a
young whelk, but as it then fixes itself down, the shell degenerates,
and grows into the loosely coiled tube of the adult mollusc. Nothing
could be more different than the body of a _Vermetus_ and that of
a worm, the resemblances between the shell of the one and the tube
of the other being pure coincidence.]
[Footnote 54: More correctly a _compound_, not mixture of carbonates,
is formed, viz. dolomite, CaMg(CO₃)₂.]
[Footnote 55: Examples of all three are well shewn on the map
opposite page 136.]
[Footnote 56: I.e. in the great oceans. As shewn later, volcanic
mounds and islands are found in the Red Sea, and may have formed
the foundations of certain of its reefs.]
[Footnote 57: Gardiner, J. S., “The building of Atolls,”
_Proc. Camb. Phil. Soc._ or The Challenger Society’s _Science of
the Sea_, John Murray.]
[Footnote 58: In some lagoons quantities of sand are formed
of great numbers of the minute shells of lowly organisms, the
foraminifera. This is, of course, a very different thing to that
resulting from breaking up of coral and shells.]
[Footnote 59: 1912 was a rich year! 40 millimetres fell on Oct. 22nd
and 20 millimetres in December, about 2½ inches altogether.]
[Footnote 60: J. W. Gregory, _The Great Rift Valley_, John Murray,
1896.]
[Footnote 61: The ranges bordering the sea are from 4000 feet to
8000 feet high, but of course were much higher in the days when
the Rift opened.]
[Footnote 62: Recently, that is, geologically speaking.]
[Footnote 63: Such ring-shaped reefs, rising out of deep water,
are termed Atolls. Sanganeb is a small example compared to those
of the Indian and Pacific Oceans.]
[Footnote 64: I have marked other harbours as they are interesting
geologically, but there are only three miserable villages on this
whole coast in addition to the two towns. There is no village at
Trinkitat, but the cotton from Tokar is loaded on to _sambûks_
here.]
[Footnote 65: I.e. hills of Archean, ancient rock, as opposed to
the newer sandstones, gravels and limestones.]
[Footnote 66: The Eastern Desert of Egypt, southern section,
describes a part of the Red Sea coast range north of the Sudan
boundary, but essentially similar to the rest.]
[Footnote 67: Two borings, made in the hope of obtaining water two
miles inland, near Port Sudan, were carried down through 300 metres,
nearly 1000 feet, of this kind of material.]
[Footnote 68: “Mersa” in Arabic = anchorage.]
[Footnote 69: Arabic Hamâma = pigeon. The shape of this hill is
a cone with a cubical block on its apex, hence the appropriate
name Pigeon Hill. “Khor” is used for an inlet of the sea,
among other meanings.]
[Footnote 70: Actually many of these shells retain some of their
colour! e.g. the red of the common _Spondylus_, the bands of
_Strombus fasciatus_ and the mottlings of _Cyprina tigrina_ are
quite distinct on some specimens from 100 feet above, and on others
dug out 15 feet below sea level.]
[Footnote 71: _All_ the corals are of existing species, but the
identification is less easy than in the case of the shells.]
[Footnote 72: Such fossils as are found in the Equatorial rocks are
recent species, and the fault harbours of the coast have not had
time to lose altogether the peculiar characters of such structures
under the influence of the rivers which enter them and the strong
tidal currents.]
[Footnote 73: The gypsum often found between the sandstone and
the coral was most probably formed by the drying up of a _shallow_
sea which occupied the site before the Rift Valley appeared, and
probably the sandstones are the sediments deposited in the same sea;
also parts of the present maritime plain were formed as the shore
deposits of this ancient sea.]
[Footnote 74: The forms Salak and Shalak are both used by the
natives.]
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Transcriber's note:
The change indicated in the Errata has been done.
pg 101 (footnote 48) Changed: _Pseudocarus_ to: _Pseudoscarus_
Spelling inconsistencies have been left unchanged.
Pages 84, 88, 102, 126, 137, 144, 147, that contained full-page
captions or figures, have been placed between paragraphs
in the page after them.