C. CROSSLAND—-NARRATIVE OF THE EXPEDITION. 
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3. SupDAN Coast. 
Between latitudes 21° 30’ N. and 18° N. I made a particular examination 
of the coast and found but one exception to the above described uniformity. 
This is the large enclosed bay of Dongola (or Dongonab as it is more properly 
called), the peculiar structure of which is probably connected with its 
biological uniqueness. In the map on p. 15 the thin lines represent reefs 
and shoals, and it is seen that the area may be described as three deep basins, 
of which the first is open to the south. The boundary between this and 
the second is a reef covered by 1 fathom on the west and 24 fathoms on the 
east side of the long narrow island which occupies its middle part. 
The partition between the second and third basins is formed by a large 
sand-island and the shoals lying round it. Now on the south side of the 
former barrier all shoals lying below the level at which coral grows with 
luxuriance are covered with sand and sparsely scattered coral colonies, of 
which many are dead, just as elsewhere in those places where there is any 
step between the coral cliff and the floor of coral-mud. North of the barrier 
on the other hand, the corresponding position is generally occupied by 
nullipores, either in the form of nodules composed of Lithothamnion alone or 
larger masses of a combination of coral and nullipores, of which the latter 
is predominant and practically hides the former. There are exceptions, 
some small nullipore patches being found just south of the barrier and 
coral occurring in places north of it; but the description holds good that south 
is a coral area, north is nullipore. But it must be remembered that this 
applies only to water of 2 or 3 fathoms and over. In the shallows down 
to 1 fathom corals flourish on both sides of the barrier alike. 
In a few places north of the barrier we find surface reefs of rather a 
peculiar constitution, viz., a combination of coral and nullipore in which 
the former is necessary as a nucleus for the latter but is almost hidden under 
it. The coral is not merely dead and overgrown, but still lives on at the 
ends of some of its branches, a balance being obtained like that between the 
conflicting interests of parasite and host. The combination has a wonderful 
rich appearance, being covered again with green and brown moss-like sea- 
weeds growing with such vigour that striking a “stone” with a hammer 
causes an effervescence of oxygen bubbles. 
If the reason for this change were known in the case of this restricted 
area, it might throw some light on the unexplained absence of coral, on the 
reef-forming scale, from the east coast of Zanzibar and British Hast Africa 
and other places, where there is no apparent reason for its inability to 
flourish. Unfortunately there is no solution as yet; I can only state the 
problem. 
