Note 53 — 11. The Sand-Dunes.

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.