SHAXNA	227
enemies or chance visitors. I sallied out to examine our sur-
roundings and to collect such specimens as Shanna might pro-
vide for the British Museum—rocks or insects or other things.
Climbing the cliff to eastward of our camp I found myself
on an extensive patch of gypsum, roughly circular in form
and sinking gently from its outer perimeter to a smaller cir-
cular depression lightly covered with sand and grit. Its some-
what crater-like appearance, together with the roughened
surface of the gypsum, churned up as if by the wind, sug-
gested that this might possibly be the desiccated site of an
ancient lake or pond; but I found nothing either to confirm
or invalidate such an impression. Near by, however, I came
upon a small circular shallow pit, only three feet in diameter
and four inches in depth, which I took to be an ancient and
buried well-mouth until some of my companions disillus-
ioned me with the information that such hollows are made
and used by the Badawin women for the extraction of dye
for their leather goods from the juicy shoots of the Abal bush,
pounded on sheets of leather laid over the pit, until they
yield their tannin.
A long line of scattered, discontinuous patches of exposed
gypseous rock extended south-westward from this point up
the valley between the dune-ranges on either hand lying
about a mile apart. The valley itself was fairly thickly
covered with the desert scrub of various kinds including the
charming yellow-flowered Zahr, apparently unknown or
but little known in the northern parts of the Rub' al Khali.
At Shanna itself the northern dune-range cuts straight across
the gypsum valley-bed which, however, continues beyond it
in a north-easterly direction for a mile or more. The whole
line of these exposed patches of the bed-rock suggested very
strongly to my mind the possibility of its being in fact the
dried-up bed of an ancient river. The very fact that the com-
paratively deep sweet-water well of Shanna lay in the line of
the supposed valley tended to confirm such an impression*
But it wets not till the following day that my search for
farther evidence was rewarded by the discovery of a consider-
able quantity of little spiral fresh-water shells^1 in the hollow
^1 Melanoides tuberculata, see Appendix.