276 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
creeping stems which root at the nodes, may be six feet long, and which must 
increase their effectiveness as sand collectors. Specially conspicuous accumu- 
lations are sometimes made by luropus, e. g. near Dabadib Harbour, where 
a considerable area is covered with conical hillocks of fine red dust resembling 
termites’ nests, four or more feet high, which have been accumulated in con- 
tinuously growing tufts of this grass. The 4lwropus grass 1s accompanied by 
another succulent Xerophyte, which is also found among the acacia bushes 
(Acacia tortilis with Lycium persicum) of the gravel, but the grasses are there 
absent. A very little of the ordinary woody grass of the maritime plain is 
present among the bushes, which, in contrast to the halophilous grasses above 
mentioned, is leafless all the summer, indeed it has now been practically so for 
four years. Beyond the gravel area the head of the well valley is blocked 
with sand-hills of considerable size, mostly quite bare, but in places covered 
with bushes up to six feet high, in shape and size strikingly recalling the 
British juniper, with dark green succulent and cylindrical leaves, and known 
” Of these plants the Acacia tortilis, 
having for all its meagre height a well-formed trunk, and the grass of the 
gravel ridge are not sand collectors *. The “asal,” one would think, should 
act in this way, but it certainly has done nothing towards forming the sand- 
to the natives as “asal’”’ or “‘ adlib. 
hills near and on which it is found, even if it has possibly raised the general 
level of the valley bottom. It is not found on the largest sand-hills, and one 
often meets with cases where a thicket has died in consequence of the removal 
of the sand, not the dune’s destruction, following the death of the bush. The 
xerophyte of the innermost section of the sand plain is, like the grasses, an 
efficient collector, but it occurs too sparsely on the gravel to have much effect. 
Indeed the action of the wind must be continually to reduce the height of the 
gravel while raising that of the inner side of the plain, and this latter area is 
extending over the bare part of the plain as fast as the reduction of the amount 
of salt in the sand will allow the grass to grow over it. 
Excavation emphasises the peculiar coherence of the sand noted above, and 
the striking difference in colour as well as other properties between it and the 
fresh sand-drifts. The latter appear by contrast almost white, after a sand- 
storm its accumulations remind one forcibly of snow-drifts. On the trench 
meeting one of the harder bands above mentioned, crowbars and hammers are 
called for and the sand is removed as lumps of soft stone, not by spadefuls. 
One notices that these bands are formed of a very coarse sand, the grains of 
which may be up to a millimetre in diameter. 
At a depth of two or three feet the sea-level is reached +, and the sand 
abruptly changes in colour from dull buff to a blue-grey. It ceases to be 
* Another species of Acacia (native name Sanganeb) is present, but rare, which forms 
large hillocks, nearly as big as the bush itself. 
+ Compare the depth in the well valley at which this change occurred. 
