284 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
though growing healthily above and partly coated with Lithothamnia &e. below, 
is full of minute holes round the base, which are the openings of sponge and 
worm borings, while another fragment showed the openings of 18 Lithodomus 
borings. This borer is very common ; one meets corals, still living, in which 
its burrows are much more abundant; being nearly as close-packed as those 
of Teredo in waterlogged wood. In short the active growth of Coral and 
Lithothamna may delay, but certainly does not prevent, the extension of the 
sea all along the western shore of the Bay. 
SUMMARY. 
1. There have been three successive lines of barrier reef along the Red Sea 
Coast, which, by continual uplift, have become : 
(1) A range of sandstone hills rising from the alluvial maritime plain ; 
(2) A fringe of limestone along the present coast-line ; 
(3) The present barrier system. 
2. These three ridges were formed by folding and 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. 
3. The northern ends of several sections of the present barrier reefs are 
elevated above sea-level, and examination of these, especially the one forming 
Rawaya Peninsula, and of the hills forming the maritime plain, enables us to 
reach the above conclusions. 
At the same time Rawaya gives evidence of a seaward movement as well 
as uplift, Khor Dongonab and some at least of the channels within the Barrier 
Reefs being recent fault-depressions, not merely the original anticlinal fold 
formed by the opening of the Rift Valley. 
The harbours and other fissures in the coast are due to the same secondary 
faulting. 
4. Tne maritime plain has had two maxima of seaward extension, the first 
being before, the second 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 coast by secondary faulting. 
5. 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, an extensive plain near Dongonab showing perfect 
uniformity in its formation. The effect of climate and vegetation on the 
formation of the plain is described (Plate 84). 
6. Of the possible mode of formation of the widely distributed gypsum 
deposits the theory favoured is by the evaporation of a shallow sea which is 
presumed to have existed before the deep Rift Valley was made. The recent 
