374 THE EMPTY QUARTER

Mutaiwi, Qaliba, Maqainama, Bir Fadhil, Raqqat al Shalfa,
Bir Mukassar ; also from Bani Jafnan, Mamura, Abu Muhairat,
and Shuqqat al Hawaya, west of Abu Muhairat. At the last
locality the ' varnish ' is not grey, but buff and brick red.

A rock of more than ordinary interest outcrops among the
sands at Zuwaira and west of Abu Muhairat. At first sight it
looks like an oolitic limestone consisting of spherical grains be-
tween | and 1 millimetre in diameter. Closer inspection shews
that quite 50 per cent, of the grains are of colourless quartz
and the rest are rounded grains of limestone coated with a thin
film of recently deposited calcium carbonate. The rock appears
to be a consolidated quartz-limestone-sand like those found by
Mr. Bertram Thomas1 about Shanna. It is lightly cemented
with a calcareous cement, which has also coated the detrital
grains, giving them a dull surface.

In the same region very fine-grained friable quartz-sandstones
with a very small amount of calcareous cement were-found at
Abu Muhairat, Abal Khadim, and Umm Tina. These are pale
olive-buff or light buff in colour and must have a very low content
of the iron oxides which give colour to the more recent sands,
Slightly more coarse-grained sandstones of the same type, some
free from any calcareous cement, occux at 'Ain Sala, They con-
sist almost entirely of quartz, the grains of which are often as
much as 1 millimetre in diameter and well rounded. It is notice-
able that the smallest grains are angular or only slightly rounded.
At a depth of 22 fathoms in a well at Bir Fadhil the san4stone
Contains small pebbles up to 2 or 3 millimetres in diameter.

Pure sandstones of this kind may have i>een the original
rocks in which the meteorite craters of Wabar were found. The
rocks surrounding the craters, described by Dr. Spencer in his
note on the meteorites, are cream-coloured sandstones with a
peculiar fritted appearance. Mr. Hey's analyses showed them to
contain 92 per cent, silica. Under the microscope the quartz
grains, about half a millimetre in average diameter, are seen to
be shattered and traversed by unusually numerous fracture
planes. With them, often forming a matrix, is a quantity of pure
white opaque powdery mineral which has not at present been
fully determined, but which is in all probability amorphous
silica.

White chalky sandstones similar to those found by Major R. E.
Cheesman at Jabal Ghanima, near Hufuf, are found to have a
wide distribution. They consist of abundant small rounded
quartz grains set in a hard opaque white calcareous cement.

1 Bertram Thomas, Arabia FeUx, 19S2, p. 367.