12 MARINE BIOLOGY OF THE SUDANESE RED SHA. 
tock found below high-tide Jevel is apparently a cementation of the sand by 
deposition of mineral matter from the sea-water, in the same way probably as. 
in the case of the beach sandstone of the Khor Dongola Islands (p. 24 et seq.) 
(Pl. 1. fig. 3 and its explanation). 
Any calcareous matter may be hardened in this way. In places, masses of 
the rubble of ancient Alexandrine suburbs come to lie below sea-level *, and 
have been cemented into a breccia containing broken pottery. Such is the 
origin of the rocks in the foreground of fig. 1 (PI. 2), while those in the near 
distance are of homogeneous sandstone. 
Between tide-marks the shore forms a rock-flat, cut up into pools, and with 
areas of sand bound together by luxuriant patches of phanerogamous weeds. 
About Ibrahimieh this flat is artificially regular through the operations of the 
ancient Alexandrians, who filled the ground with graves and catacombs the 
bottoms of which now remain exposed in the shore-flat, the sea having invaded 
a large part of the great eastern necropolis. Plate 3, fig. 1 shows a catacomb 
passage just above high-tide level. There are side branches to right and left 
in which the niches for the dead are plainly seen. Further down the shore 
the graves remain as shallow troughs cut in the rock. The graves spared by 
the sea are now being carefully despoiled prior to the division of the cemetery 
into desirable building lots. 
Further west the levelling of the shore seems to be the result of quarrying 
operations rather than of grave-digging. 
Along the seaward Bice of this rock-flat is a series of masses of harder 
rock, which, having undergone further hardening under the action of the 
waves, are being eroded but slowly. Below water-level the edge of the flat 
is protected by an incrustation very similar to that I described as occupying 
the same position in the Cape Verde Islands t. The same species of Vermetus 
is here, but 
(1) It never forms masses alone. 
(2) There is no gradation between the proportions of nullipore and Ver- 
metus such as is characteristic of sheltered and exposed situations in. 
the Cape Verdes. 
(3) The incrustation consists largely of Serpulid, Sabellaria, and Terebellid 
tubes, which was never the case in the Cape Verdes. The latter two 
being sandy are a source of weakness. 
(4) The incrustation never contains a high proportion of nullipore. Large 
masses of nullipore are not found ; and the species which covers the 
* There seems to have been a sinking of the coast, cf. the ruins found out at sea near 
ancient Canopus, and the now submarine catacombs and excavations of the shore near 
Ibrahimieh. But of course masses of débris might reach their present position by local 
subsidence of portions of the cliffs. 
+ Proc. Zool. Soc. 1905, vol. i. p. 178. 
