RECENT FORMATIONS. 195 



evidently the remains of a plain, the greater portion of 

 which has been cut away by streams and surface-waters. 

 Further north the plain still exists unbroken by ravines, 

 and forms the desert tract known as Shob. From the 

 quantity of boulders and pebbles, and the scarcity of 

 corals and shells, there can be but little doubt that this 

 plain has not been formed by the sea, but is a deposit 

 from the wash of rain and floods from the hills. As 

 already remarked, the sedimentary beds interstratified 

 with the volcanic rocks of the neighbourhood are pro- 

 bably, in great part at least, of similar origin. 



Coral Formation of the Red Sea Coast. — The island 

 of Massowa and the coast in the neighbourhood are 

 composed of fragments of corals, with shells and sand, 

 agglutinated together into an impure limestone rock. A 

 large number of the islands fringing the coast in this 

 2')art of the Red Sea are of the same formation,^ and from 

 the descriptions given it is highly probable that the 

 islands of the Dahalac Archipelago are all similar. 



AU these islands are perfectly flat, and stand about 

 twenty or thirty feet above high-water mark,^ being sur- 

 rounded by a low cliff of that height. A reef extends 

 for a varying distance outside of them, and terminates 

 abruptly. Doubtless the island was formerly co-exten- 

 sive with the reef, but so much of the rock as is exposed 

 to the influence of the breakers has been swept away by 

 the sea. The reef is naturally broadest in the direction 

 from which the heaviest breakers come, the open sea. 



' In the island of Massowa, the eastern or seaward side is much higher 

 than the other. 



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