C. CROSSLAND—PHYSICAL DESCRIPTION OF KHOR DONGONAB. 285 
and still continuing deposition of gypsum beneath the Rawaya salt field is an 
interesting actual example of this method. 
7. The restriction of coral growth within Khor Dongonab, a very exceptional 
if not unique state of things in the Red Sea, is shown to be the result of 
abrasion by silt-carrying currents. Considerable effects follow from com- 
paratively feeble currents which carry very little sand, but that this is 
sufficient to prevent the fixation of coral larvee is borne out by the comparison 
of newly fixed spat of those mollusea which do flourish on these current-swept 
banks with those of the Aviculide, e.g., which flourish on the coral areas. 
8. That on its west shore Khor Dongonab is extending itself at the expense 
of the land in spite of the presence, in some abundance of coral and 
Lithothamnia. The presence of these must not be assumed to imply the 
addition of material to a reef unless their growth can more than counter- 
balance the forces of degradation. 
EXPLANATION OF THE PLATES. 
PLaTE 28. 
Map 1.—General view of the Red Sea, showing Jedda, Port Sudan, Xe. 
The outline of the coast is a double line, the outer representing the edge of 
the fringing reef. The shoal areas bearing the bariier reefs are shown outside 
these ; they are especially regular between Ras Salak and Sualin. 
The map includes the whole coast-line of the Anglo-Egyptian Sudan, between 
the Egyptian and Italian Boundaries. 
Sandstone hill-ranges are shaded. 
PLATE 29. 
Map 2.—ULarger scale, showing position of Khor Dengonab. 
(1) Small islands are black, thin lines enclose shoal areas bearing reefs. 
(2) Distribution of Coral. 
Growing coral is shown by shading. Such an area enclosed by a line represents 
a coral reef with definite edge, but the existence of many reefs can only be 
indicated in writing. Bare rock-slopes are indicated by lines of dots. 
(5) Depths :—10- to 20-fathom line om. —.— »— 
NOO=tathomeline: sya oy eee 
Tbe numbers refer to depths in fathoms. 
The chart made by Commander Walter in H.M.S. ‘Merlin’ in 1907 is extremely 
interesting (Admiralty Plan No. 3722). This is the only large-scale chart 
giving detailed soundings of a part of the outer edge of the Barrier System. The 
great submarine precipice found along the 100-fathom line and the practical 
coincidence of the 10- and 20-fathom lines (these cannot generally be shown 
separately even on the large chart) are striking examples of fault escarpments 
along the sides of the Rift Valley. The chart also demonstrates the connection of 
the Rawaya range with the Télat reefs by an area 9-15 fathoms deep Sumoler! 
with small cinnieaae: reefs rising precipitously from these depths. 
