C. CROSSLAND—PHYSICAL DESCRIPTION OF KHOR DONGONAB. 281 
Ecology of Coral.—In the Red Sea from the entrance to the Gulf of Suez 
southwards to Shubuk*, 7. ¢. from latitude 28°N. to 18°40'N., coral 
flourishes exceedingly on every reef; even within the canal-like harbours 
certain species survive in the muddy water near their heads. But there is 
a curious restriction of coral growth within Khor Dongonab; between 
Sararat Island in the south and Umm Tarda in the north the reef flats ending 
in walls of luxuriant coral (such as are universal everywhere) give place to 
sloping banks covered with stones and weed or sponges of the genus 
Phyllospongia. It is only after some residence and travel on this coast that 
the phenomenal nature of these conditions, normal elsewhere, is fully felt. 
There is nothing like it elsewhere on the Sudan Coast, and I should not be 
surprised to find that Khor Dongonab is altogether unique in this respect. 
This reduction of coral growth becomes all the more puzzling when one 
finds that in certain places many species flourish perfectly f, and above all 
that in the northern basin of the Khor, above Umm Tarda, the normal Red 
Sea conditions are met with again. The shoals of this basin have been 
converted into level surface-reefs with precipitous sides, and the islands are 
surrounded by coral in the same way. But in the main basin of the Khor 
coral growth, in any quantity, is restricted to water two or three feet deep, 
and then is far from forming a complete reef-flat. 
Fig. 3. 
under cut 
Cliff 
coral Cole Sian lOfath 
| 
a ree ee ol) ne 
As a ms) Sea Level 
= 3 fa 
The zigzag lines indicate masses of coral. 
Diagram of the results of marine erosion on a bank of elevated coral according as the 
growth of coral does or does not preponderate over forces of degradation. The shaded 
area, bounded by the line marked 2, isa section of a bank such as is characteristic of the 
Main Basin of Khor Dongonab; 1 is the outline of the original bank, and 3 that of 
the resulting reef when coral growth and denudation work together, 
The slope of the sea-bottom round any of the islets or along the limestone 
shore of the west side follows one rule which is illustrated by the diagram of 
fig. 8. Ata distance of two or three hundred yards from shore the gradual 
* The southernmost point of which I have personal knowledge. 
+ Professor J. Stanley Gardiner is describing a collection mainly from Dongonab Harbour 
with his own collections from the Indian Ocean. 
