~ C, CROSSLAND—PHYSICAL DESCRIPTION OF KHOR DONGONAB. 273 
20 feet above sea-level. These are now separated from the rest of the same 
formation by a broad flat valley, very slightly raised above the sea and 
opening into a bay on the west side of Khor Dongonab (see Map 2, PI. 29). 
It is exactly comparable to the valley at Salak, or to Khor Dongonab itself, 
in origin and topography, the gravel being substituted for raised coral. 
The old character of the hill is shown not only by its present isolation from 
the rest of the gravel plain, but by the existence of the strata in which the 
gravel is bound together by gypsum, these more coherent strata protecting 
the mass of loose gravel below them and forming steep-sided butts. 
The wells which supply Dongonab village and the nomads’ flocks are sunk 
in this flat valley bottom, about a mile from the sea. As might be expected, 
the water obtained is too salt to be used by any but natives ; the wells, 
though only ten feet deep, reaching sea-level. for the first few feet they 
pass through yellow blown sand, which suddenly becomes blue in colour and 
of a clayey consistence, the change indicating a former sea-level 6 feet or 
so above the present one, as will be shown later. Near the level at which the 
influx of water stopped our digging, some decayed wood was brought up 
with a species of Ostrea attached and other marine shells with them. 
We have thus additional proof, if that were needed, of the origin of these 
valley-flats by the filling in of bays, principally by blown sand. 
The Gypsum Deposits—Gypsum is abundant in many places along the 
coast, among the sandstone hills, which are generally a few miles inland. I 
figured its contorted strata underlying the horizontal coral of Jebel Tétawib 
in 1907, and Pl. 82. fig. 3 shows it, less clearly, in the ravine of Jebel Abu 
Shagara. Its unconformity with the coral in Jebel Tétawib proves its 
formation prior to the present sides of the Red Sea trough. 
Theories of Formation.—Gypsum, calcium sulphate, might be formed either 
by conversion of limestone, calcium carbonate, by the action of sulphurous 
geyser springs, or the life activities of sulphur bacteria, the product being 
afterwards oxidised. These origins would seem feasible in view of its juxta- 
position to coral-reef limestones, but, as we have seen, this implies no other 
relation, the strata being unconformable. Further, the action of volcanic 
gases or springs, or of sulphur bacteria, would probably not be perfect 
throughout the mass of rock, but in the one case would result in local meta-- 
morphoses, gypsum passing into unaltered limestone on either hand, or, in 
the second case, of alteration of the upper strata, the lower being mixed 
carbonate and sulphate, or of carbonate alone. There is no visible trace of 
such local action anywhere, and samples of every part of the Jebel Tétawib 
deposit have been analysed * and found to be very pure sulphate. 
* By Dr. Beam, of the Wellcome Research Laboratories, Khartim. See Wellcome 
Laboratories Annual Report, 1908, 
