C. CROSSLAND—NARRATIVE OF THE EXPEDITION. 7 
The possible causes of the restriction of coral-growth in general are :-— 
(1) Cold currents. 
(2) Very strong currents. 
(3) Dirtiness of the water. 
(4) Chemical composition of the water including its dilution 
by fresh water. 
(1) and (4) are dismissed at once as inapplicable to either this case or to that 
of British East Africa. It has been shown too that the effect of an ordinary 
river upon reefs a comparative short distance from its mouth is generally 
inappreciable. But there are no permanent rivers in this part of the world. 
(2) Strong currents are present in this neighbourhood, but as a matter of 
fact currents stronger than occur here are generally favourable to coral-growth. 
(3) The water in the north basin, where coral grows luxuriantly, forming 
reefs rising precipitously from deep water, is very dirty, as cloudy as that of 
Suakim Harbour, for example. That of the middle and southern basins 
is clear ; even when the streng north wind of winter is blowing, the bottom 
is clearly visible to a depth of 5 fathoms, and in the summer, when winds 
of a more normal strength prevail, objects are distinguished clearly at 
10-13 fathoms. 
Mernops OF COLLECTING. 
Collecting by dredge and trawl is difficult in a sea where coral is so 
ubiquitous. Hven on the nullipore beds of Dongola “ stones” large 
enough to stop the working of small apparatus abound nearly everywhere. 
Consequently the majority of my specimens from depths of more than a 
few feet were obtained by divers or by trawling on coral-mud. Compared 
with the harvests reaped from other grounds, the results from the latter 
seem poor. Large quantities of the slender branches of an Antipatharian, 
certain Polyzoa, several species of Lamellibranchs, &., were often obtained, 
but their repeated appearance became monotonous. 
Coral colonies brought up by divers, and either broken up at once or 
left standing in fresh water until the inhabitants came out of their burrows 
and tubes, yielded many and varied specimens. On one occasion I employed 
three men for several days in taking coral from a shoal and breaking it up on 
shore. One result of this was to alter my ideas of the relative abundance 
of certain forms of life. 
One hears little of the use of divers in accounts of collecting expeditions. 
In most places it is easy to find skilful “swimming divers,” and by learning 
a few words of their language one can easily get them to bring up the 
particular coral, sponge, aleyonarian, or weed which one has picked out by 
the water-teleseope. In a tideless sea like this their help is invaluable, 
and indeed in most seas the desirable specimens seem to be just outside the 
reach of the collector confined to between tide-marks and dredging. 
Trawling in the Bay of Agig Suraya was exceptional, resulting in the- 
