[Footnote 43: _Geogr. Journ._, 43 (1924), p. 288.] [Footnote 44: ‘The Secret of the Sahara—Kufara’ (London, 1921), p. 235.] [Footnote 45: Beadnell, “The Sand-dunes of the Libyan Desert,” _Geogr. Journ._ 25 (1910), pp. 379-395; Harding King, “The Nature and Formation of Sand Ripples and Dunes,” _Geogr. Journ._, 47 (1916), pp. 189-209; Harding King, “Study of a Dune Belt,” _Geogr. Journ._, 51 (1918), pp. 16-33, and Discussion, pp. 250-258.] [Footnote 46: For particulars of the distribution and directions of the dune-lines near Gebel Owenat, and between that mountain and Sarra, I am indebted to Prince Kemal el Din Hussein, who devoted special attention to observing them on his expedition to Sarra in 1926.] [Footnote 47: It may be remarked that in the desert of Northern Sinai the dune-lines follow curved directions, instead of maintaining a single direction as in the Egyptian Libyan Desert, and this curvature of the Sinai dune-lines has generally been thought to be caused by the deflecting influence of the mountain masses of Moghara, Yelleg, and Hellal on the wind; but here also there may be regional influences at work.] [Footnote 48: See Jordan’s notes on pp. 204 and 208 of his “Physische Geographie und Meteorologie der libyschen Wüste,” Cassel, 1876.] [Footnote 49: _Geogr. Journ._, 49 (1917), p. 55.] [Footnote 50: The steep Miocene escarpment, some 200 metres high, which bounds the Qattara depression on the north, probably produces a great eddy in the wind sweeping southwards over it, causing a whirl round a horizontal axis south of the scarp; this may be very influential both in excavation and in lifting the sand to great heights. As a large portion of the floor of the depression at the foot of the escarpment is formed of a salty sludge (sabakha), it is likely that a considerable quantity of finely divided salt crystals are carried up with the sand in summer, and this may conceivably act to some extent as a binding material in the dunes.] [Footnote 51: _Geogr. Journ._, 47 (1916), p. 209.] [Footnote 52: _Geogr. Journ._, 51 (1918), p. 252.] [Footnote 53: Many years ago I discovered that this film can be removed by treating the sand with warm hydrochloric acid, leaving the grains of snowy whiteness (see my ‘Description of the First or Aswan Cataract’ (Cairo, 1907), p. 57). But I think traces of iron in the ferrous state must exist even within the white siliceous body of the grains, and be continually passing into the ferric state at the exposed surface; for otherwise it is difficult to account for the persistence of the yellow colour in spite of the mutual rubbing of the grains during their transport by wind. The film may possibly have an influence on the electrical behaviour of the grains, by reason of its possessing a higher conductivity than the silica of the interior.]