تغيرات مستوى سطح البحر خلال البلايستوسين وآثارها الجيمورفولوجية على طول الساحل الشرقي

ميراد · #9 09-08-2019, 02:54 PM

هذه بعض الصفحات الذي ذكرت فيها أٍرخبيل فرسان مع التفريغ النصي لها:

262 APPENDIX.

western coast, is siuTonncled by an intricate archipelago of

islets and shoals ; the main island is irregular in outline,

and includes a bay seven miles loftg, by four across, in

which no bottom was found with 252 feet ; there is only

one entrance into it, half a mile wide, and with an island

in front. The submerged banks on the eastern coast,

within the same latitudes, round Farsan Island, are, like-

wise, penetrated by many narrow creeks of deep water ;

one is twelve miles long, in the form of a hatchet, and close

to its broad upper end, soundings were not struck with

360 feet ; its entrance is only half a mile wide. In

another creek of the same nature, but even with a more

irregular outline, there was no bottom with 480 feet.^ The

island of Farsan itself, has as singular a form as any of its

surrounding banks. The bottom of the sea round the

Dhalac and Farsan Islands consists chiefly of sand and

agglutinated fragments of coral, but, in the deep and narrow

creeks, it consists of mud ; the islands consist of thin,

horizontally stratified, modern tertiary beds, containing

but little broken coral ; - their shores are fringed by living

coral-reefs.

From the account given by Eiippell ^ of the manner in

which Dhalac is rent by fissures, the opposite sides of

which have been unequally elevated (in one instance to

the amount of 50 feet), it seems probable that this irregular

form, as well as that of Farsan, may have been partly

caused by unequal elevation ; but, considering the general

form of the banks, and of the deep-water creeks, together

with the composition of the land, I think their configura-

* [The islands of this group are of upraised coral, as is the fore-

shore of the opposite coast of Abyssinia. In many parts of the Ked

Sea coast the low coral cliffs give evidence of upheaval. There are,

nevertheless, reefs which would be classed as barrier-reefs on both

sides of the central part of the Ked Sea.— Capt. Wharton.]

- Rlippell, Eeise in Abyssinie, Band. i. s. 247.

3 Ibid. s. 2io

264 ArrENDix.

twenty miles long, less than a mile in width (bat expanding at the northern end into a disk), slightly sinuous, and parallel to the main land at the distance of five miles from it, with very deep water inside, so that in one place sound- ings were not obtained with 205 fathoms. Some leagues further south, there is another very narrow reef, ten miles long, with other small portions of reef, north and south, almost connected with it ; and within this line of reefs (as w^ell as outside) the water is profoundly deep. There are also some small linear and sickle-formed reefs, lying a little way out at sea. All these reefs are covered, as I am iiiformed by Captain Moresby, by living corals. Here, then, we have all the characters of reefs of the barrier class, and some of the outlying reefs partially resemble atolls. My only source of doubt arises from the narrowness and straightness of the spits of sand and rock in the Dhalac and Farsan groups ; one of these spits in the former group is nearly fifteen miles long, only two broad, and is bordered on each side with deep water ; so that, if worn down by the surf, and coated with living corals, it would form a reef nearly similar to those within the space under considera- tion. Nevertheless I cannot believe that the many small, isolated, and sickle-formed reefs, as well as others long, nearly straight, and very narrow, with the water unfathom- ably deep close round them, could have been formed by corals merely coating banks of sediment or the abraded surfaces of irregularly shaped islands. It seems more pro- bable that the foundations of these reefs have subsided, and that the corals, during their upward growth, have given to them their present forms. I have, therefore, with much hesitation coloured this part blue. The West Coast, from Lat. 22° to 24°.— This part of the coast (north of the space coloured blue on the map) is fronted by an irregularly shelving bank, from 10 to 30 fathoms deep ; numerous little reefs, some of which have

RED SEA. 263

tion is more probably due in great part to currents having

drifted sediment over an uneven bottom. It is almost

certain that their form cannot be attributed to the growth

of coral. The greater number of banks on the eastern

side of the Red Sea seems to have originated in nearly the

same manner, whatever this may have been, as the Dhalac

and Farsau archipelagoes. I judge of this from their

similar configuration (in proof of which I may instance a

bank on the east coast in lat. 22"") and from their' similar

composition. The depth, however, within the banks north-

ward of lat. 17° is usually greater, and their outer sides

shelve more abruptly (circumstances which seem to go

together) than in the Dhalac and Farsan archipelagoes ;

but this may have been caused by a stronger action of the

currents during their formation : moreover, the greater

abundance of hving coral on the northern banks, tends to

give them steeper margins.

From this account, brief and imperfect as it is, we can

see that the great chain of banks on the eastern side of the

Eed Sea, and on the western side of the southern portion,

differ greatly from true barrier-reefs, which are wholly

formed by the gi'owth of coral. Ehrenberg also concludes

(Ueber die, &c. pp. 45 and 51) that these banks owe their

origin in a quite secondary manner to the growth of coral.

He remarks that the islands off the coast of Norway, if

worn down level with the sea, and merely coated with

living coral, would present a nearly similar appearance.

It seems, however, from information given me by Dr. Mal-

colmson and Captain Moresby, that Ehrenberg has rather

under-rated the influence of corals on the formation of the

tertiary deposits of the Red Sea.

The West Coast of the Bed Sea heticeen Lat.l9° and 22°.

— Eeefs exist here, which, if I had known nothing of the

others in the Red Sea, I should unhesitatingly have con-

sidered as barrier-reefs. One of these reefs, in 20° 15', is

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