Chapter IX: Geography of Red Sea and Foundations of the Reef Systems (2)

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_

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.