20 On the Arabian Frontier of Egypt. 



land gulf, subject to a greater evaporation than tlie current 

 that sets in from the Straits of Gibraltar can replace,* its 

 waters sank, at the eastern end, to a permanent level of equi- 

 librium between these two opposite agencies, — which is about 

 30 feet lower than the Red Sea at high water. 



It is evident that this central plateau must have conti- 

 nued to form the northern boundary of the Red Sea, until 

 another shoal, near Suez, was raised above the water's edge 

 by the simple process of accumulation, forming a second 

 natural dyke across the gulf, that stopped out the water 

 from the upper gulf-basin, and left a hollow nearly 60 feet 

 deep, which became a salt lake, and is now a dangerous 

 marsh. This second shoal begins about 20 geogr. miles south 

 of the head of the former gulf, and its length, to near Suez, 

 is about 11 J miles ;t but it is only a space about 2\ miles 

 of its southern end that is actually higher than the sea ; be- 

 yond this, it slopes downwards about 10 feet to the edge of 

 the upper gulf-basin. The process by which the surface of 

 this shoal was raised to its present height, may be seen still 

 going on in the Red Sea. The coral banks that grow up 

 rapidly in its shallows, rise to the low-water mark ; and then 

 the lithophytes cease their operations. The enormous quanti- 

 ties of drift-sand, blown into the shallow gulf, are washed up 

 by the waves, and deposited, with broken shells and gravel, 

 on the top of these shoals ; where, from the quantity of cal- 

 careous matter which the water of this sea contains, they form 

 hard banks up to the high-water level. It was at the foot of a 

 height on which Ptolemy Philadelphus built the marine sta- 

 tion of Arsinoc, that a line of sand-banks began to form on a 

 shoal of this description, at its southern end, the one most 

 exposed to the winter-tides, which are always 3 or 4 feet 

 higher at that season, because then the prevailing winds blow 

 from the south. It will be seen by the section that this end 

 of the bank is scarcely 3 feet above the ordinary winter high- 

 water mark of the Red Sea, and only a few lines above its 

 excessive tides in stormy seasons. 



Thus far, Mr Sharpe's positions appeared to be sustained 



* Lyell's Pr. of Geology, book ii., p. 1., chap. vii. 



t Its extent is indicated by a dotted line on the map, Plate IV. 



