APPENDICES	373
The Miocene rocks are best represented in Jaub Anbak and
at Nasla near Anbak. The fossififerous beds are white clayey
limestones or marls, the calcium carbonate being hi a very fine
state of division and probably chemically precipitated. Some of
it is cemented with salt into a hard rock.
Above the fossiliferous stratum the beds consist of :
No. 1.  Pink oolitic limestone with small shells and f oraminif era.
No. 2.   Brick-red very friable calcareous sandstone.
No. 3.   Pink and white calcareous sandstone.
Beds very similar to Nos. 2 and 3 also occur at Jaub Ba'aij
and at Al Kharza, associated in the last place with a pebbly
conglomerate. At Qarn Abu Wail near Sikak the rocks associated
with the Miocene fossils are pale green sandy clay impregnated
with salt, and a pink calcareous clay with the carbonate in an
extremely fine state of division. A similar red clay comes from
Judairat wells.
The white marls are similar to the material quarried for the
manufacture of earthenware between Hufuf and Mubarraz.
Quarries near Hufuf also show 10 or 12 feet of pale green and pink
clays with fibrous gypsum and plates of selenite. No fossils were
found in these beds. A similar greenish clay occurs on a ridge
near *Uyun.
Compact limestones found at various localities give good
examples of the polish or desert varnish so often recorded in dry
desert regions. The colour of this polished surface varies from buff
or drab to brick red, but the most common colour is grey in various
shades. In some of the grey fragments the polished surface is
an actual skin—the ^c Schutzrinde ' described by J. Walther.^1
One of these limestones with a pale grey polish from Mamura
contains foraminifera (Alveolina) and seems to be a pure lime-
stone. Another, dull brown in colour, contains abundant minute
angular quartz grains in a cryptocrystalline calcite cement.
The grey' Schutzrinde'^2 seems to form on limestones of more than
one kind, but was not found on the limestone pebbles collected
from the gravel plains. It occurs both on projecting parts of
outcrops and on loose pieces. Some calcareous concretions are
entirely coated with it.
Localities from which Mr,  Philby brought examples are :
^1	Walther (Johannes).—Das Gesete der Wustenbildvng in Gegenwart und
Vorzeti.   Leipzig, 1912, pp. 144-153.
^2	That the grey colour and polish are a surface effect is indicated by a
specimen of a quite different yellow crystalline shelly limestone from
Mulaiha near Khuthaiqan which lithologically resembles some of the
Cretaceous limestones.