14 MARINE BIOLOGY OF THE H1iL)ASEYE RED SEA. on the MAMIKEBIOLOGY of the 81-D-4NESE R E D S E A . - l V , The RECEKTHISTOHY of the C O R A L KEEFS of -the Mid-West Shores of HEPORTS the RED SEA. By CYRILCROSSLAXD, M.A., ELSO.,F.Z.S. cated by Prof. K. A. HERDRIAX, F.R.S., P.L.S.) (Cloinmuiii- (PLATW1 & 4.) [Read 2nd May, 1907.1 CONTENTS. General Description of the Coast .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evidences of' Elovation .......................................... Its regularity-Corals and material of reefs remaining i n position. Raised Atoll of 'I'elh Telln Sorap. The Living Reefs-Fringing, Scattered, Barrier .................... The Forination of Coral-Rar ................................... Conclusionu. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PRp 14 17 19 24 26 GENERAL DESCRIIWON OF THE COAST. d e - of the Eccl Sea " Rift V:iliey '' is varied on the west by tlirce proniiiient point., Kas Beiias, Ras Elba, and Kar Etawaya (bee Map, fig. 1, p. 6). Betvecii R:i+ TZawaga and Suakiiii the coast-line ib, in the iliain, very regiilar, but it bends eastward again at Trinliitat, aiid is broken by soiric large bays about Agig. As is well liiiowii, the hides of this trough are composed of r2tnges of inountainv of granite and other crptalline rocks remarkable for their wild and jagged shapes and utter b a r r c ~ i e > s ,wliile the actual coast is :I low plain. Except for it* scnwarcl hortlcr, which ih invariably of elevated coral, tlie surface of this plain is coinpoarcl of :illmial gravel aiid sand froiii the hills *. Ti1 certain localities the plain i- di\ided by ranges of low hills, horntimes capped with coraI, which riw in tlic midst of the plain and run psrallel, iiiorc or less, to both hea-eoa-t and inountains t. This is the structure of the whole Red Sea coast at least a s far south as latitude 18" 10' N., with the exception of the northern part of the Gulf of Suez, which ic totally different $. A very >tril;iiig feature of this coad is the large number of canal-like bays or '' Khor5 " wliich rim into the coa-t-plain. Reference to the i m p 011 p. ti shows nine large exniiiplcs in tlic slxicie of only 40 miles just north of RawaFi, THE remarkablc parallelism o l the * Artesian borings made by the Public Works Department of the Sudan Government two miles inland from l'ort Sndsn show that tlie same materials extend downwards as far as tlie borings went, i. e. to a depth of at least 1000 metres. i. As these hills are of special interest 1 gite an enumeration of them and description of those I have climbed in a postscript (p. 27). 1 The crystalline hills are here absent, the eoa9t being formed of high, flat-topped, walllike hills of Ct.etitceoos litnestone which rise alniost directly from the ma. t.t.O!C I 1\IAP O F DONGOLA AND ITS T71CINITY. It; I MARINE BIOLOQY OF THE HUDANERE RED S E d . and the Chart of Suakini Harbour gives tlie featnres of a typical specimen. The characteristics more or less prominently developed in all are :(I) ( ‘anal-like outline. (2) Ckossed branches riinniiig N. & S. and E. L% W. ( 3 ) A nearly uniforni depth gently bhoaling as one passes landwards. (4) A flat bottom of coral-mud. ( 5 ) Precipitous sides of growing reef below sea-level and cliffs above. The latter are 01 course the height of the neighbohring plain, generally 2 feet or so in Suakini Harbour, or up to 12 feet in Khor ShiIYdb. The late Mr. Barron, who was Geologist to the Sudan Government, explains thebe formations as resulting from two systems of faults at right angles to one another. The almost rainless climate, the absence of very strong tidal Currents, and the protection afforded by growing coral, have allowed tlie above-mentioned characteristic and almost bizarre features to remain, whereas in the corresponding structures on the equatorial coasts of East Africa the erosion of powerful tidal currents and of freshwater streams has, in most cases, widened creelcs into bays and broken down their vertical sides into shelving shores. The Red Sea has an evil reputation with navigators on account of the number and complexity of its coral-reefs and the presence, in certain areas, of that especial horror, tlie coral pinnacle which rises out of deep blue water to?just under the surface. The reefs may be divided into :(u) Fringing rcefs along thc. coast and round islands. ( b ) A Barrier system, which is especially typically developtd between 19’ N. and 24‘ N. (c) Scattwetl reefs, including sonic of Atoll form. The climate is noted for an extreme aridity, which is however not so complete in the southern part of the coast as in the northern. Among the hills furious rain-storms are frequent in August and winter, which flood the valleys for a few hours a t a time in tlie way described by travellers in :he Sinai Peninsula, and by Egyptian Survey Departtnent officials for the desert bordering the Gulf of Suez *. These conditions are of coursi’ the best possible for the conveyance of thc large quantities of detritus which liave formed the great alluvial maritime plain. The tides have been referred to in the Narrative (p. 3), where it is stated that the middle part of the Red Sea is pract,ically tideless. There arc>,however, variations of level of two feet or thereabouts which occur almost daily, but their occurrence is subject to no fixed rule. The rise or fall inay be fairly regular for several days, high water being at nearly the same time each day, then it may remain continuously low or continuously high for :L short period. Occasionally the water niay sink to a foot below its lowest normal * Uarron &L Hume : (‘Topography and Geology of the Eastern Desert of Egypt, Central portion,” issued by the Survey Department, I’nblic n’orks 31 inistrg, p. 282. HISTORY O F THE CORAL REEF‘S. 0. CHOSYLAND-RECENT 17 level and remain down for oiie or inore days. Further, the mean I c ~ e li b about three feet lower in winnier than in winter. It is found that there i- a true tide of a few inches, but th,it this iswamped by the greater changes of level indnced Lp ch:mges of barometric pressure. The ‘‘tides ” are thereforc qiiite irregular and cannot, be l’redicted. My own obieriations h a i e betin innde daily for about six imnths a t Dongola, where, owing to itq po-ition ncar the head of a very long bay, the effect of the qtrength and direction of the ivinda is very marked, \but even there the wind is not the onlj- factor in producing c h a n g ~ of i Irvel. The difference in the level of the whole of the Red Sea in thc winter and summer is a result of the i n o n + o o ~in~ ~the Indiaii Ocean. The t o t d difference between high mntrr i n winter a i d lowed in suinnier is about six feet. C‘AVSES OP THE FORMS O K T H E YRESRXT HEEE’ SYSTEM. The tlieory advanced is tliat-(i.) A- the region i- one where conderablc upward niovenient (at least 500 feet) ha+ occurred since modern reefs began in the Sea, and w h t w elevation i- still in progrestG, Darwin’s ~nl)sicJencc~ theory cminot be calletl into nccoiint for the existence of the barrier reefs at a distance from l m d aiitl wpratcvl froin it by d e q ) water, or of ihe atoll forms of: certain reefs. (ii.) The rchtious between elevated and <till living reefs show that the foundations of hoth are ranges of sandstone hills partly below wa-level. (iii.) The forinu of the reef- are due to the balance of aggrading over degrading agmtq, the fornier being the growth of coral, nullipores, kc., and tlie latter the corro-ive action of the -pa and tlir rotting cau.etl hy boring orginisin<, &c. A not,c on the probable cause of tlic change- effected after tlie rrpliea\.al of a coral-reef, whereby it takes structure characteristic of *- 011 the Iiardness, homogeneity, and crvstalliiie C‘ord-R~g,’’is appended. EVIDENCES OF ELEVATION. The cxistence of‘ the coast-plain :inti its I)readtli, which on tht. Sudan coast averages five miles, czw again-t a n y conzidcrahle inovenien t of depreshon during its forination. The r a i d reef- inclicate upward inoveiiients of 1500 feet or more, ant1 more recent cEianges are rccordetl as raihed beacheand oltl erosion-lines 01- cliffs, of wliich esainpl~sare foniid througl~outthe length of the Sudan coast. A t Agig. a mile inland, h i t only :I few feet ; L ~ J O W tlie SKI, i h n raiscd beach in the form of :L long ridge of rollccl pe1)liles of crj-stalline rock correq~onding to tlyat of coral-fraginent.. which hortlers the present shore. Both are surmoniited by :I line of graws, the seashore having been, apparently, :I favourite place of hepnlture through d l ages. Even the cle-ign of tho. grave5 LISN. J0URN.-ZOOLOGY VOL. S X S I . 3 15: XQHINE BIOLOGY OF THE SUDANESE RED SEA. remains much the same, except that the modern stones are but a fraction of the size of the monoliths of the old days. Some particularly conspicuous cases of raised erosion-lines on cliffs are to be seen a t Tella Tella Suraya Island in the south, and on Haysoit Island in Khor Dongonab in the north. These are treble lines, the modern one at sealevel, and two others at points 3 and 5 feet above this. A single raised line may be seen in the harbours of Port Sudan and Suakim. The larger of the islets of the landlocked North Bay of Khor Dongonab is of limestone, and has a line of undermined cliffs along its northern, or exposed, side, the base of which is now sonie feet above sea-level and separated from the shore by a stretch of a n d about a hundred yards wide, and covered with the xerophytic plants usually found on these dry sandy islets. One should note that t,he higher cliffs, seen from a distance, appear to show lines of underinining in some cases, but that these, on a nearer view, turn out to be the results of subaerial denudation. The hills are formed of horizontal strata whicli are in some cases alternately hard and soft, and the latter, standing out from the general level, siinulate the overhanging part of a marine cliff. But in these selected cases, as in that described below, the smaller details of wave-action are preserved so that the former, lower, position of the cliff' is evident beyond doubt. The horizontal position of the strata shows that the elevation of the limestone from the sea has been effected in a rcgiilar manner ; but evidence of the steadiness of this movement is given by the occurrence everywhere of elevated coral colonies, which are in the same position exactly, relatively to the surrounding rock, as that in which they once grew in the sea. Such colonies may be of quite delicate branched species, and good examples of such are exposed in the trench which surrounds one of the blockhouses which once protected Suakim from the Dervishes, but generally the fact is more easily made out in the case of massive species. Vary conspicuous examples were seen on Tella Tella Suraya, an island of which the Admiralty ' Pilot Book ' notes that it is dist'inguished by t,he presence of many cairns of stones upon its highest parts. The island is about 40 feet high and surrounded by cliffs, mostly of rather loose coral-fragments and abundant shells &c. of present-day species. The " cairns on the level summit of these cliffs are simply large hemispherical coral colonies which survive t,he weathering that has removed the surrounding softer material. Corals similarly situated .are exposed in inany places on these coasts, the sunmiit of Jebel Tgtiwib in Khor Dongonab bearing a part,icularly fine series. There would be no difficulty in identifying them as species at present living in the adjacent sea. The island is a miniature elevated atoll, the present bottom of the lagoon being just above the sea-level, and consisting of a flat sheet of mud, which was drying and slightly crusted with salt at the time of our visit, a t the beginning of March. This depression is surrounded by high ground, its present coininunication with the sea being probably subterranean. The lagoon is hordered by I d i c s , of the species which live in " C . (‘ROWLAND-RECENT HISTORY O F THE CORAL lilCEl~-. 1!I salt s w t n i p , ant1 about 4s f w t a h thein :tr(l sniall cliff5 n1iic.h 41ow very distinct inarks of having been untlerniined hy waves at ;L tinw ~ v h e nthe lagoon was a t a lower level a i d contailid water. Above these the ground dopes steeply to the level of the summit of thr cliff;. For other elevations see the postscript on the randstone coast-hills (11. 27). Two sets of clcvations have been shown to have occurred since reefs of modern corals began to be formed in tlic Hctl Sea : (i.) A niajor b:eries up t o a t least 500 feet (Rawnyn, Mal~nw:r,dic.). (ii.) Minor rlevatioss which are very recent and are prohnblp still going on. (Recent raised heac.hes and erosion-lines on the cliff..) (1 THE LI\ I,CG fiEEF\. Pri/t~/i//g Rc<titx. Tlicse reefs O C C I I ~ practically w crywherc on the ol)en coast an<l in the bays ant1 “ khors ” except at tlirir extreme l a n d w r d end.;. Tlicir absence from pasts of Khor Dongonab has hoen noted in the NarriLtive (p. 5). They are absent also froin wi:i11 lengths of the coast-linc at Has Bnridi (noted by Darwin, 011 the east 4de of the sra) and from the ,shore!, of ‘‘ 1)okhana Bay ” in Khor Dongona\). As in onc or two &dar cases in Briti-11 East Africa, I can give no reasons for tllesc oniis,cions cscept in one case-the Iinrl~ourof Dabaclib, on the Sudan toad betwcen Salak and Port Sudan. Here the coral-border to the coaht-plain is almost entirely covrred by a deposit of the very fine hand, tlerivetl froin crystalline rocks, which a t times Bsf high winds causes thick “fogb” in tlic R d Sea. This fouls the water and prew n t s coral-growth in the immediate Iicinity. Probably the case of “ Dokhana Ray ” is the same ; the water certainly i.: middy there, but I have not determined the origin of the mud, ant1 in sonie cascs, e. g . Suakim Harbour and wpcially the North B:iy of Khor Dongonab, coral can flourish in very cloudy uater *. The shore-reef is broad where tlie coast is low : e. y., at the entrance to Sualiini Harbour, where it i b but a foot or two above sea-level, the reef is 1; sca-miles broad, while at Port Sudan (Mersa Sh6kh Barixd) the coast is 6 feet high and the reef only $ mile wide. This difference is obviously due to the lesscr rate of erosion where fallen rock protects the base of the <~liffsfor tt time against the sea. It is obvious that the formation of reef-flats by erosion is going on here just as on the Zanzibar coast, but that here ii is in1poq4i)le to tlrterniine wliat proportion of tlie total breadth attained is due to the cutting-tlmvii of the land and what to tlie addition due to recent coral-growth. At a point just north of Mersn Abu Hainhiiia, at Dabadib, Mersa Fijab, and sonic‘ other points, the coral-liinestone has been nearly completely renioved, the shore being forined of the rolled grey pebbles of tlle alluvial * Note added Sept. 1907.--Fre.li nnd ‘<Dokhana Bay” nearlj water, flowing underground, enters the all the J e l l round. Z ~ a Rt 2* Llabadlb 20 MARINE BIOLOGY OF T H E SITDAKF:SE RED SEA. jiI;iin. That the greater part of tlic reef-surface was formed by the removal of elevated coral is conspicuously demonstrated, in the first caqe mentioned, by the presence of numerous isolated pillars and grotezque rocks of this material bcattered over its landward portion, in tlw third hy the presenct' of coral-rock islands ; but at Dabadil), as already mentioned, only a narrow band of rock surface is visible at water-level. Only narrow reefs are found below the comparatively high cliffs (3040 feet) of the east side of the North Bay of Khor Dongonab. I a t t r i h t e this to the reefs being sheltered from the prevailing minds, and consider it merely as :L further indication of the inq)ortance of erosion as a factor in reef-formation on coral comts. Thc surface of the shore-reefs is gcnerallp sandy, but 1m-e rock-patches occur, ehpecially a* a border to the growing edge. Stones :%rerare or quite absent except at the slightly raieetl seaward margin, wliere the remains of dead coral colonies are soinetiines watteretl. Such large and solid stones as those so plentifully scattered on the reef-edge in Zanzibar are, froin the nature of the case, altogether ab:,ent from thi:, area, which h a i been formed directly by growth of coral and nu1liI)ore *. The raised edge comibtz of a gravel of broken coral well covered with nullipore. Outside this is a gentle slope of corals, generally stunted and accompanied by nullipore and a1cjonari:ms (Xeniid=), which extends to a depth of n fathom or so, after which i:, a precipitous slope of luxuriant corals reaching as far as we can see with the water-telescope, say to 10 fathoms or beyond. There is generally a more or less regulw boat-channel or series cif pools of water a fathom or SO deep, with niinicrou9 outlet:, to the open sea, some of which provide anchorage- for +a nl li d<~ (or " tlhows " a s they are callrtl in Zanzibar). I n shallower water than that found generally off the shore-reefs the lnaxiiiiuln depth at which coral grow- lu\uri:iiitlj- decreases, for instance on the Shubuk boundary-reef, to he dtwril,ed later. A slope of inud with scattered corals is visible below the precipice of luxuriant growihs. Siniilarlp inside the harbours, wliere also the species of corals change, the delicately brancliecl and fail-like form., which are in the majority outside, giye place to Inassive P o i d e s , Meantdrizia, and -o on. On this coast it, i, the ma&ve gener:i which are c1iar:icteriztic of slielteretl water?, not vier wmG. The fringing-reef on the east &le of Jebel Jfayiti is also wortliy of note. The island is a pyramidal hill, thti enbt side heing precipitous. From the su~nniitone looliz down upon :I brown fringing-reef covered hy a foot or two * The stones foond on the raised reef-edge of the Zanzibx coasta are the harder portions of the rock removed diirimg the forinntion or the reef, which lave been preserl ed in the w6y described in the p n p r (Zoc. czt.). The " nepro-hands" r,f Pacific atolls probabl: have x siniihr origin. of' water, pwlia1": one Iiundi-cd y i r d s wide, antl apparently of iiniforrn structure throughout. I th clear-cut, ~ t l g emeets tlie hliie-l~laclr tleep sea (lirwtly, without t l i ~ intervention of :I 1):itcIiof lighter colour anywhere, giving the inost forcihlc inipr(b-ion of tlic c d p of~ ;L f:itlionilcss ahysq. Scutteretl lierjs trrctl the Sl~itlitdArea. It will be conveiiicnt in considering the rernaining reefs to divide the coast into two section+- tlic first from Has Bcnns to Su:Lkini, the wcond from Siiakitn to tht. bounclary with the 1t:ili:ui territory, Ras Kas:ir. T11e foriiier is characterized I)y tho p r w w c e of well-deiclopetl harrier systems antl deep water close to all reef<; the latter by inniinier:il)lc>small scattered reefs and sand arid coral ihlaiiils (tlic Su:tkini :irehipelago) in comparatively shallow water and ii ,ihelving Imttom, and, i n thc hoiith, :I coniparatiiely plight tlevrlopment of recent coral-growth. The contrast is thus n very strongly in:irkecl one ant1 very .ignificant al\o *. One of thehe wattered islaiids, Tella Tella Kuraya, lias :ilre:icly bccn tlrtcribctl. The only other feattire of interest in thi+ neiglibourhood is thci Shuhuk area, a hundred scliiarc miles of tlie moht intricate p i w ~ g wamong reefb, surrounded on tlie N.E. and E. sitlcs by an uiibrolreii reef +iniil:ir to the ortlinarp fringing-rwf of the shore, ant1 on the S. :inti W.hy the ~n:iiiilantl. Most of the rcefs euclosetl arc iiirwly ~and1)aiiksc:ip~wlwith a fatliom or so of growing coral, but on tht. in or(^ c i l ) o w l N.and W. sides thc coral goes d e q m xiitl, whcrc not growing c w x l , the hottorn i+ 1:irgely composed of cleat1 fragiiir.iits of coral, 41clls, k c . The land hercabout.; is $0 lo\\ :UHI *o cut into by great shalloil lagoons, that it i h clifficult to clcfinc a coast-line :it all. It is easy to rc~niarknhlcarea Iia. bec.11 formed, the diorr-reef spreading srawartls rapidly in the hallow water, where its t1t:hri.: would be :it once :~~il:1l)lt3 as n substratum on which the growing zone of coral roultl aclvance seawards. Within the g r o i!ig ~ edge thc accumulated mass of corals &c. wonld tindergo decay ant1 bolution into tlic niazc of reefs now found there; while others wrre being carled out of the low coast, :mtl tlicw, when their siirfaces were covtvxl with coral, living ant1 dcatl, wonlcl liccoine indistiiiguiP1ia~)l~ from thow which hat1 grown up in sittr. T h e llurriei. System. But little is to be added to the account given by Darwin in his 'Coral Heefs,' and for a description I refer to his work p. 102 & pp. 143--146 (the latter refrrencc including x special section on the reefs brtwecii Siiakirn ant1 lialaib), :ind to tlic niap on 1). 6 (scipiu). But the facts I h a w given abolc show that it is impossible to suppose ;L secondary subsitlenor of the * Note added Sept. 1907.-On land tlie contiast consists in the absence of sandstonr coast-hille, the significance of which will be ~ e e nafter readirip the following paps and the postscript. 22 MARINE BIOLOGY OF THE SUDANESE BED SEA. middle part of the Red Sea in orclcr to explain the occurrence of barrier-reefs on this coast. The absence of the regular barriers in the southern section is not it consequence of elevation ; for tlie evidence is distinct that the recent movements have been the sitme here as in the north, but is rather correlated with the different slope of the sea-bottoni, and probably, as will be explained later, with the structure of the side. of this part of the rift-valley. That the latest moveinelit ha. been one of elevation is shown by the character of the erosion-marlis left on the cliffs, which include details which ~vouldbe coniparatively soon weathered away froin so soft a rock, especially when one c o n d e r s the important part played by mid-erosion 011 this desert coast. The very recent c1iar:icter of the fossils contained in tlie cliff:. iy evidence in the sanie direction. The present form of most of tlic3c reef- is the result of the saiiie procesres as have foriiied those of Zanzibar :ind East Africa *, where the forceb of erosion, with tlie aid of organic growth insufficient to add to the bulk of the material, ha\ e c a n ed ont a11 the cliaractcristic forms of coral reef,. I n the Red Sea, coral-growth is still adding greatly to the buIk of the reefs, but that the foundations of the present reef3 : i i ~due to erosion the exainination of tile physical geography of this cw;lst 1)ro\es conclusively. Ehrenberg, over seventy years ago, stated that the foundation of the Iled Sea reefs originated in this way, but here the problem i b more complicated than on the Equatorial coasts by the iinposddity of distiiiguisliing between that area of a reef wliich i:. due to the erosion of eleaated coral-rocb and that which has been added by recent growth, except in the ease of some of the Khor Dongola (Dongonab) reefq, u here :.ncli additions have not completely concealed the original found a t’ionq. A t first sight it seems m0.t prol)nl)le that these last-mentioned reefs are examples of thc mode of formation mggested by Darwint in the caws of the Farsail and Dahlali archipelagoes, iiz. by the collection of hediineiit on an uneven bottom froin which coral-growth took its ri,se. The Dongola Barrier (map, fig. 3, p. 15), indeed, seem:. an obvious ca*e of this occurrence, the bank continning awash for a long tli*tance to the west of the sand islands, and the bottom in the neighbourhootl being sandy with n o rery consideralile masses of coral or iiullipore growth. But a closer examination shows that this explanation, wemingly ,SO plau:.ible, i b not correct, and that, in fact, the bank has a rock foundation merely overlaid Iiy the sandbank and such coral- * C. Crossland, ‘< The Coral Reefs of Zanzibar,” Proc. Camb. Phil. SOC.xi. p. 493 ; and The Beefs of Pemba tmd the East African mainlaad,” 1. c. xii. 1904,p. 35. f I confine my quotation to I)arwin, as he is the only author who has described the reefs of this part of the Red Sea in general terms. The wider problem has been attacked by numerous worliers. For an epitome of recent views see J. Stanley Gardiner’s paper “ The Formation of (!oral Reefs,” iu ‘ Katiii-e,’veb. 18th, 1904. C. CHOSSLANU-KECEXT HISTORY OF THE CORAL REEFS. 2 ;3 growths as occur *. I n places, a strip of hard coral-rock showing. sections of the contained corals and shells is left k J W P about water-level, below the sand of which the islets are composed, aiid this is true of most of the other sand islets in Kltor Dongola. I have seen no reefs of any importance, on the Sudan coast at least, which could have had banks of beditmiit, tr~nsportedand deposited by currents, for their foundations. On a smaller scale, however, p o d d e examples are those bounding the fine harbour of Mohamed Ghul, several square miles of reefs in the neighbourhood of the village of Dongola, and many cd those described in the Sliuhulr area in the south. Such banlis are only to t e found in comparatively sheltered situations, never in the open sea around the Barrier spstern, Below water-level n careful examination of the bottoin show<, in inany places, bare rock-flats usually thinly coated with tnud. It is thus shown th:tt the Barrier aiid other reefs of Khor L)ongola h a r e been niercl~cai 1 ed ont of the rock by the action of the sea. Tlie ma1 Barrier system is even more easily qeen to he formed lty c rosion. Tlie fact that the reefs Itetween Maltawa I+land and the pniiisula of &a\\ aya are the rernains of a former land-connection between the two i5 ahnost obvious, especially a G a rrinnant of this lo-t latic3 remains as an i+olatcrl pillar of coral-limestone, 8 feet high, on “ St. Fillan’s Reef,” nearly lnidwaj between the two. Although Maliawa 1.lantl i+ high, about 500 feet mill not be far from tlir mark, it* southern end i+ little above bea-level. Si iiilarly, Rawaya, thong11 hill. up to 200 fcct in height are present, has cons derablt. :ireas which mu-t 111 course of time IN> reduced to reefs, S O low are they. The iiccli jorning it to the mainland i+ Itarticu1:irly low and narrow, SO that Raw:t):i inuzt soon 1)ecornea chaiu of limestolie irland+ connected by a coniplicated reef zy-teni iiidi,stingiiiihnl,Ir from that now connecting it to Idakan :t Island. Similarly the Barrier bystern to the ,iouth of Maliawa, p e r h a p a s far as the Tiflah Idands, iz a continuation of the Hawaya-Makawa Itill+, either as eroded reiriiiants of the range o r a3 hubmarine hills not yet elernteti above sea-level. The latter must be true of the southern reefs, in whicli case the most likely poe.tnlate to account hoth for tlieir growth and for tlie tAiistcnce of other rangee. of coral-capped hill5 tt+/iic/t are jm-allel to t h e sides of ~ I L?.jftP valley, is that the faulting which produced it rebultcd in the side.; bein,; in the form of a series of s t e p or parallel ridges, upon which the reinaine. of or,ganisms ~ o u l daccumulate f ar more rapidl?- than on the intervening trough- or flats, SO raising the ridge. until corals, fin:tllp of reef-building ipecieh, coil d carry the ridge up to the surface coinparati\ ely rapidly t. ‘ * From. Rlr. Stanley Gardiner’s last iepoit (in Nature ’) on the Percy Sladen Expedition in H.M.S. Sealarlr ’ (January 1906) it, appears that the fringing-reefs ot the Seyr idles art: another Ciwe where recent coral merelv coats over older rock. t J . S’tnnlrp G‘aidiner, “ The I3uilding of Atolls,” Proc. Camb. Phil. Soc. 1902. 24 MARINE BIOLOGP OF T H E SUDANESE RED YEA. Further north is another 11rominent point, Ras Benas, which shelters a large bay and has a large island to the south, exactly in the same way as Ras Rawaya. So like, indeed, are the appearances of the two points that the same name, Makawa *, has been given to the islands in both cases. Tlie Jezira Ridge on Ras Benas corresponds to the high ground on Rawaya, and the Makawa of latitude 23’ 50‘ N. is obviously the continuation of this range. Similarly the southern continuation of tlie range is represented by the “Horseshoe Reefs,” which still retains a fragment of the ancient island in the forin of ‘‘ a white rock” which “looks like a boat,” below which this range ends in another set of reefs. The third prominent point of this coast is at the mouth of the Straits of Jubal (leading into the Gulf of Suez), where the Zet Hills run southwards into the sea, where they are broken into a chain of islands ending in that of Shadwan. The process of denudation has here made comparatively little progress. Few reefs exist except those fringing islands-and the islands are high on all their areus, e.g. Shadwall 990 feet, Jubal 410. But imagine denudation to have progressed to a far greater extent and to have cut down the smaller islands to sea-level, their foundations being at tlle same time preserved or added to by the growth of coral, and we see that the conversion of a range of hills to a peninsula, island, and c h i n of barrier-reefs becomes complete, as in the case of Rawaya, Maltawa, and Tiflah. The repetition of this arrangement on this coast is striking, but just as some ranges are entirely inland (e. 9.the Abu Hamhma range), so otliers arc completely transformed to barrier-reefs, or perhaps were never elevated so high a s to be continuous with the land, or even to project above the sea. For instance, the long reef, SIiab Snadi, seems to be a continuation of the promontory of Salalra, but there are no hills in the vicinity that could be part of its range. Tlie Barrier system inside Foul Bay, south of Ras Benas, is not connected with that point or its “ Jexira ridge.” To the south this reef system bears the Mirear (lat. 24’ 40‘) and S i p 1 (lat. 23’) Islands and some small rocks, the islands heing low and flat, only ti to 12 feet above sea-level, and composed of elevated coral. THE FORMATION OF CORAL-RAG. An instructive comparison can be made between the elevated coral of the Red Sea coast and that of Zanzibar. Both are low, and contain fossilized specimens of the species now living in the adjacent sea, and are consequently among the latest of geological formations. I n both cases the alteration of the relative level of land and water has been effected in so regular a manner that the corals of the land retain relatively the position in which they grew when submerged. But in spite of these fundamental resemblances the physical properties of the rocks are as widely different as possible. * This Arabic name, meaning than its donors supposed, that which is resistant,” has therefore a deeper significance C. CHOSSLtW1)-RECENT HISTORY O F THE CORAL REEFY. 25 In one’s mental photograph+ of tlic t v o coast<, thr most prominent feature in either case is the black colour and fantastic forim of the Zanzibar coast rocks and the light yellow and usiially forinlebb c1i:iracters of thobe of the Sudan. These are the outward Iisiblr signs of cqnnlly striking tlifferencrs in their physical constitutions. In Zanzibar and B r i t i h East Africa the low cliffh arc of an exceedingly hard crystalline rock, which is so homogeneous and tenaciou.; that the overhanging shelf left by the iinderniining of the 5ea may, under favourable circumstances, project for more than four feet witliont its breaking down. The surface of the rock, both on the coa<t and inland, i., as already mentioned, alniost black, though when broken the rock itself is seen to bc white or nearly so. The raised coral of the Red Sen coast, on the other hand, is genernlly soft, often indeed like loose sandy gravel of h e l l s and coral fragineiits with the larger coral colonies a h embedded stones. The bases of‘ the cliffs are somewhat harder ; and though their underniining results in form5 which distinctly recall tlie cliffs of East Africa, the lack of homogeneity and cohesion of their material rrsults either in a monotonous formlessncss or, wlierr somewhat harder rock occurh, the substitution of picturc~squeinasses of fallen rock for the bizarre pillars, brackets and arches of the Equatorial coasts. Although fossilr a1-e T cry abuntl:int in the Zanzibar rock antl :ire visible in section in the cliffs, yet the greater part of the rock near the -urface is SO homogeneous and shows so little trace of its origin from a loose conglomerate, that there would be great tlifflculty in hepirating wcli fossils a s o c m r from the surrounding matrix. It is important to notice, honmer, that in places at n little depth below thr surface, the rock becomes much IPSS crystalline and its fossil5 niore easily sep:il-able ; in short, it :ipproachcs the condition of the Red Sea rock From quarries made for road nietalling, MY.T. J. Last, Slavery ~‘omniissionerin Zanzibar, made large collectious of fo+il* and their casts, nnd a wholr serirs of stages in the crptallizing process could he easily obtained. On tlir Sudan toad again the rocli hah the characters of that of :!anaibar in certilin localitieb, hut only i ~ l i e w the corditions to icl~ic~hi t is e.cposetE appmcirnute to t l m e rtormu1 011 t h e F 5 p t o r ; that is, to being “ ’twist u ind antl water ” at the sea-level in places where it is under wa\ e-action *. The whole surface of Zanzibar i b nndrr h i i c l i conditions owing to ii s heavy rainfall, but rainfall heing practically nil on the Sudan coast we iind the hardened crystalline rock only I)? thr sea at \vater-level. Further, owing to the illappreciable tide, the recfs giw constant protection to the cliff 5’ bases, so that only in exceptional places does one find coral-rock freely exposed to wahei and spray. For Some reason the rerf i b abhent froin the eastern cliffb of the islet of Tella Tella Kebira, which are thereforr alniost constantly bathed in spray, and h a w taken on a11 tlic characteribtics of thr Zanziknr rocks, in * See also the preceding paper 011 the coast 31ear hlexmiclris. inarked contrast to the other islet> in the yicinity, which have only the usual partial hardening of tlie undermined portionb of their cliff?. I n Khor Dongonab, where protection of the coast by coral is 1 ~ complete, s the foundations of the sand islets is, as already mentioned, of coral-rock, and :it yen-level, where it is alternately wetted and dried, it has talreii on the typical yualitie3 of " Coral-Rag." In the same way, beach sandstone has in seFcra1 cabes been formed by the cementation of the sand of which the islets are coniposed, and this is as hard as the coral-rag where it overlies it, becoming softer and softer a3 it rises above the water-level till, at a height of 3 fect or so, it becomes of a crumbling consistence and passes into tlie loose sand from which it is derired. (See PI. 1.fig. 3 and it, explanation.) The factq cited certainly warrant the conclusion that the alteration of tlie Zanzibar rock from the ma53 of more or 1e.s loosc material, in which conditioii it was elerated from the sea, to it3 present cryatalline state, and that tlic roclr of quite similar origin in the Hed Sea has rcniainecl nearly in it.; pritnitive condition, is due to the absence of' rain and generally of wave-action and spray in the latter region. Thc~details of this action of water have not bern worked out, but probably a recrystallization of the calcimn carbonate anti a partial substitution of magnesium, or dolomitixation, occurs, both of thehe proce$ses tending to harden the reni:iiiiin,a product. C'OKCLrSIONS. The conclusions to follows :- lie clrawii froin the :ibo\-e III:IJ- be suminarized as I. That thc coast of: the Sudiiii, bebides its major elevations amounting to more than 1100 feet, ha- recently undergone cereral sinall elelations, the inoveiiients having been uniform in their action and not recently reversed. 2. That the diff'erences between this elerated coral and roclr of piinilar origin elsewhere, e . y. on the coast of Equatorial East Africa, is due inainly to the abseiicc of tick and rain in the Red Sea. 3. That the present forin of the reefb is due as much to the eroding action of the bea upon t1ii.j elevated rock as to the growth of corals. In the case of F&igiiy-Keyfs (including the Shubuli area) the laid is cut down to sea-level behind the rim of' growing coral. Burrie~f h f s are formed: (1) By the direct growth of coral upon subinarine hill ranges ; tlie northern ends of these have been elevated, and are now ranges of coral-capped hills, the middle parts remain as pcninbulas and islands, the southern as Barrier-reefs. (2) By the ciitting down hy marine crosion of proniontories and islands, a i d of Cora 1- reefs previously rlcvated. 27 HISTORY UF T H E CORAL 1tEEEh. C . CKOYYI,ANI)--REC'EK'I' POSTSCRIPT. As the theory of the Barrier Borniations lierc advanced turns on the rtrnctnrc :md topography of the coast hill*, I give the inforinntion I have beea able to collect. The hills are iiot distinguishable on any map, indeed part of the country i,, a < yet, unsurveyed, and I have bud few opportunities of travelling inland. My account is therefore incomplete, but enough has been done to justify the CollclUsiOlls clrawn. Froin beawards these hills are easily distinguidhed from the jagged hills of igneous origin by their cliffs being of R light yellow colour and by their flat tops ; generally also by their position being nearer the hea than the lowest of the igneous hills, and e l e n tliau the mounds of gravel wbioh occur in places a t the foot of the latter. PasAig froin South to North, the first range i h met with a few miles nortli of Mersa Dumr (see nial) p. 6) a h a chain of low butts rising from the alluvial plain four or five mile- inland. These become higher and inor(' continuous as one passes northwards till they end in two considerahle hills, of about the saiiie height and area of h e , tlic northern of wliicli i, niarketl on the charts, whew it is called Table Mountain and given a Iic ight of 1100 feet. At about five miles inland from L)ongonab are one or two wiall hill5 standing alone, but further north is a range of higher hills lying inland from the middle of the North Bay and extending towards the Hamiima range, from which it ih only separated hy a n interval of a fcw inileb. The A h Haiulmn * range (which T +o named from its most 1m11iiinent though iiot highe*t ~ ~ e a lai ,famous lantliriark for sailors) extends from around the inner branches of Rhor Sliimil) to come distance beyond Khct Abu Hamiima, lying much nearer the sea than tlo tlir above. Its height is clstiniated by the Government S u r ~ e y o rat from 500 to 700 feet. These range- are wliolly inlantl, ri-iiig : i h p t I y froill the maritimc~plain, which they divide Ioiigitntlinally. J hat c iiot Iieen able to bee whether they are coral-capped, but IIOJ seen that their h:ises, at least, have the cti.uctur(1 of the range next described. The hill+ of the Kawaya Peninsula tind cJebel Makawa form an outer range parallel to the above, and are capped with a stratum of modern coral3 They were once barrier reefs, growing upon sandstone foundation-. The highest poilit* of the Rawiya. peninsula are near its north end, and eastwards froin the *alt-worlih in the south, the northcrii Iieing mi extensive platean of a height of about 100 f w t . the 1:itter a hill of about twiw this (1 . * This may \be translated its sniumit. '' Pigeon IIiIl,'' and is so called from the curiouL shape of 28 MARISE BIOLOaY OF THE YIrDANESE RED SEA. height. The rest of the surfacc. i+ low or untlulating, generally about twelve feet above wa-level but in inaiiy place- w r y little above the sea. The outline of the> peninsula as given on the chart is only approximately correct ; i t is broken u p by many intricate “ Kliors ” or inlets of the sea, of which “ Khor Hawaya ” is but on(’. A section of the Northern plateau is given by a cliff about 60 feet high which borders a fault, now full of sea-water, known as Khor AtGf. The thawing given (Pl. 4) shows its structure, which is typical of tlie whole of this peninsula. Above is a mass about six feet thick of recent corals (A), all in the position of growth. The large numbers of‘ shells which accompany these arc still more easily identified as being of living species. Next is a thicker stratum apparently of con+olidated coral-mud (B & U), containing two characteristic f o d s , nullipore nodules and a clypeastrid echinoderin. The latter occurs in large nmnbers and is easily recognized as a species still living in Khor AtGf. This is all the reef formation present *. Below it is a bed of gypsum (D) of varying thicknesses up to 18 feet, this resting on n finely laminated and very soft sandstone (E), genemll? yellow in colour but here often dark green or red. The coral foriiiationi :irp in horizontal strata, and w e uiicor!forntcthZe to the strafu O I L icliic*li they w s t , which are c onderably folded and contorted. I n the lower parts of the peiiin+iila, tlie coral stratuin ic thicker, e . y. at Haysoit Island up to 15 feet are exposed, the coral-mud and other strata not appearing above the sea-level. The hill in the south (Jebel Abu Shagara) is similar, forming a great cliff hounding the fault in which lie two inlets of the sea, “Khor Rawaya ” opening northwards, and Khor Abu Shagara which r u m in froiii the open sea, i. P . froin the south. The T-iew eastwards from tlie summit is striking, thp peninbula liere consisting of a series of parallel ridges and troughs, the result of einallrr fault*, some of which contain seawater inlets. These, like all the Khors above named, rim approximately north great north and south and south. Jebel Abu Shagara is itself cleft by gash i n its southern part, which may he taken as one of the coast inlets elevated above the sea for oiir examination. It shows the same structure as the cliff 011 the east sitle, viz., a shnllow cap of coral underlaid by gypsum and sand.;tone as in the north, but here the gypsuni band is narrow and may be absent, the sandatone in any ca3e foriiiiiig the great bulk of the hill. This rock forins also the bulk of the islaiicla of Makawa and Mayiti, which are about 500 and 300 feet high respectively. 1have not clinibed Makawa, but Mayiti is coral-capped. The east side of all these hills is precipitous, the west sloping. I have found no elevated reefs thicker than 16 feet on the Sudan coast. This agrees with the accounts given of those of both shores of the Gulf of Suer and the Sinai Peninsula. 'l'liib chain of bill-, it i a e\ i h t , H wc mice islands ant1 s i i l ~ i i i , i r i i i (b~inh~ lying parallel to the cotid, tlieir formation aiicl po.ition being a mechaiiic:il result of the opening of the rift alley. By iiiariiie erosion and the g r o tli ~ of Coral+, thew became levelled dowii ant1 built ul) to a line of barrier reef-, ])art of which ha- n o w been elevntetl :ibo\ c the -ea-lc\ el as the co:ist-r:iiigt+ ;ind a- i+land., other part> being oltw tetl to +ncli height$ t h t t rapid coralgrowth has heeii p d l e 011 their suiiiinitr. The deeljer portion- have 1)robntd \r bccii raised to within tlie limits of growth of reef corals hy the accuniulation upon theiii of the reinailis of niarine org:inimi+, which flourish e+pecidly on any elevation of the sra-bottom in the ~ r n ydeacaribeil by J. Stanley Gardiner". Thus, by degradation of the part- above sea-level and aggradatioii be ow', arr formed the lwei lines of rcrfs of tlio h r r i c r \ysteni of the present daj Thimode of forin:itioii is probably the origin of the level h:ind of coral-rock which lies between thc alluvial p v e l of tlie ni:iritiinc plain and the deep mi, and to the presence of l\liieli tlii. plain, con-i-tiiig ah it docs of large grakc.1 and -:iiicl to great dclkhs, i. due. ( S e e 1). 14, note at hottoni of page.) . CjTKIT, ('Rl IbbLASI). Sept. 20, 190i. EXlJLA4XATI0KOF THE PLATES. PLATIC 1. Fig. 1. A pearling canoe and its occupants. 2. Diver returning to the canoe with a captured pearl-oyster. 3. Bewh sandstone 011 the island of Sararat, in Ilhor Dongonab. ( a ) . Sea horizon. ( 6 ) . IIard coral-rock, eroded domn to sea-level, whici forins the foundatic,n oi +he sand island and of the ad,jacent living reefs. (c) to ( e ) . Band of beach sandstone, smooth and hard a t ( c ) ,rough and nioilerately hard in the dark band, very soft at ((J). (f)is the loose sand of which tlie island was origiually composed entirely. 4. Nouth of one of the shallow wlleys wliicli drain Rawaya and open into the, North Bay. The opening lies between two low digs of coral-rock, one of which it shown in the background of t h s photograph. During the winter, fresh water l'ercolates tlirongh the sand of the valley, which it consolidates into bench sandstone vdiere it meets the sea-water. At I the sandstoue has a smooth surface, a t B Irolten and apparently undergoing reduction by the sen. This sandstone is found in all tlie valley moofhs in this bay. Figs. 6 & 6. Part of a large heap of olil pearl-shells which forms a considerable part of the island of Uiiiin es Shelih, the Holy Island of Iihor Dongouab. Presuinably these are the reninins of fisheries made in the days when this species was fished only for its pearls, not as now maiiily for the mother-of-pearl shell. Keediess t o say their d u e , which would be very p e a t if the hells were fresh, is now nil. * Lb The Building of ;ItolL,'' Proc. Canib. Phil. S O ~190?. . I’IATIS2. Fig. 1. General view of the clitEs eastwards of Alexandria. The rocks in the foreground are made of rubble conso1id;tted by the sea, the similarly situated reef in the niiddle distance is natural sandstone. 2 . Hill of rubble at Chatby, Alexandria, remaining after the surrounding land has been excavated to its original level. 3. Portion of cliff face situated between 2 and 1 abnve. The upper part of looserubble containing pottery, &c. : the lower of rery soft sandstone. ]’LATE 3. Fig. 1. Catacomb passages exposed by the sea, in the eanie locality as above. The cliff is coherent sand below, rubble &c. above, m in P1. 2. fig. 3. Eight niches for bodies can be made out i n the foreground, to the left of the main passage. 2 . The ‘‘ Spouting Roclr ” beyond Pnlais station, showing the regular shelf surrounding d laid bare during the retreat of a the rock just below water-level ~ n partially waye. 1’LATI< 4. Jebel T&tlwib,in Khor Dongonab, a butt near its southern extremity, seen from its south side. The letter B is a t a level about 25 feet above the foreground. A. Coral colonies, in position of growth, bedded in a mass of loose coral ddbris, shells, Szc. 13. Corals bedded in consolidated coral-mud, forming n harder layer than that formed by either constituent done and 80 projecting shelfwise a t B. This layer passes gradually into C. C. Hardened coral-mud. The weathered surface forms rounded masses in low relief. D. Gypsum strata, here steeply tilted, and upturned a t their ends in the piece shown in the foreground. They are much folded in other parts of the cliff. 12. Green and red shaly rock underlying and sometimes interstratified with the gypsum. It is here broken down into sand. This rock contains sheets of glasslike recrystallized gypsum. F. A fallen piece of B. Contrast its roughness mith the smooth rounded surfaces of C upon which it lies. (+.Scree material from A 8 B, consisting of coral and shells. II. A dark line of reddish broken shell-material, containing many Cidaris-spines. This is more prominent in some other parts of the hill. ~'RCl~.l.\\l! JOURN.LINN. SOC.,Z O O L . VoL. XXXl. (rn,iit, S C E N E S I N THE RED SEA. PL.1 l<nzr. CE0ssl.aXI ) . JOURN. LINN. S o c . , ZOOL.VOL. >(xXI. P L . 2 1 I I 2 i I I I i 3 I 1 <I. Grout, E n s c'. Photo. S H O R E CLIFFS NEARtTALEXANDRIA. C’ROS21.4Sll. - JOURN. LINN.SOC.,ZOOL.VOL. XXXI. PL. 3. I FIG. I . C’. C’. Pllottl. CLIFFS N E A R ALEXANDRIA. CROSSLAXD. JOURN. LINN. Soc.. ZOOL.VOL. XXXI. PL. 4. /
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