248 ]\jk. a. wade on the [Hay 191 1, 



they consisted of almost every type of local rock, quartz-grains 

 slightly predominating. They were all polished and worn, and very 

 frequently facetted so as to form tiny dreikanter (fig. 2, p. 246, & 

 PI. XY, tig. 1). Dreikanter on so small a scale do not appear to 

 have been previously noted. These large grains are not often carried 

 into the air by the wind ; but, when the wind is sufficiently strong, 

 the effect of such flying grains is painful in the extreme, and it is 

 impossible to face them for any length of time. This type of sand 

 owes its character to the sifting action of the wind : the fine sand 

 has all been removed, and is often found filling gullies high up 

 in the hills. It leaves behind these bigger and heavier grains, 

 which are just able to withstand the ordinary force of the wind. 



On the islands no siliceous sand is to be found, as a rule. The 

 sands are calcareous, and are chiefly formed from comminuted frag- 

 ments of corals and shells, as on Jubal Seria; of calcareous algae, as 

 on the northern side of Gaysum Island ; or of foraminifera, as on 

 the southern side of Gaysum Island (fig. 3, p. 247), where Orbitolites 

 coniplanata is the predominant form. 



VIII. Effects op Wind-blown Sand. 



The action of the wind upon the various rocks was very striking. 

 The softer limestones are sometimes worn into fantastic shapes 

 (PL XIV, fig. 2) ; and sometimes they are regularly grooved in such 

 a manner as often to simulate bedding-planes. The effect of the 

 sand-blast upon soft limestone is nowhere more magnificently 

 shown than on Jubal Island, although the sand there is soft and 

 calcareous. In the limestone-cliffs occur cirques or semicircular 

 bay -like openings, which go far inland, leaving hard outliers, carved 

 into curious shapes, standing near the shore. They vary greatly in 

 width, but are sometimes over 100 yards across. On the sides of 

 these cliffs the fossils stand out in relief, and sometimes present 

 the only means of determining the angle and direction of dip in the 

 massive beds. Similar cirques are described by Barron & Hume l on 

 the western side of the limestone outlier of Abu Had : they explain 

 the origin of such cirques as due to the widening of joint- and fault- 

 planes by the sand (PI. XV, fig. 2). Afterwards the sand-eddies 

 scoop out the hollows thus formed, until wide semicircular recesses 

 are finally formed in the cliffs. On Jubal Island, however, these 

 cirques are connected with raised-beach phenomena, and are cer- 

 tainly of marine origin in their early stages. 



The effect of the wind-blown sand upon the harder rocks is some- 

 what different. The chief action of the sand-blast upon the granite 

 is to assist in the removal of the felspar-crystals from the surface 

 of the rock : these crystals become weakened along the planes of 

 cleavage by the constant expansion and contraction due to the heat 

 of the sun. The outer layers are more affected than the deeper-lying 

 layers, and hence the tendency to flake off at the surface. The 

 quartz stands out in relief all over the surface of the granite, but 

 it soon becomes dislodged in the general break-up of the felspathic 



1 Op. cit. Egypt. Geol. Surv. Rep. 1902, p. 289. 



