APPENDICES 377

with black mottlings, black and dark purple rhyolites, quartz
porphyries, epidosites and epidotized volcanic tuffs.

The only types frequently collected by Philby, which are very
rare in Cheesman's samples, are fine-grained red and white
granites.1

In the discussion of the problem of the origin of the gravels
encountered by Major Cheesman (loc. cit., p. 423) it was stated
thatc we have no knowledge of the original source of the pebbles.'
That was the position in 1926 but Mr. PMlby has succeeded in
clearing up this question very satisfactorily.

He finds west of a line running roughly from Bisha and Tathlith
(south-east of Mecca), to about Duwadami (west of Riyadh), a
mass of old igneous and metamorphic rocks forming the back-
bone of the country. From the eastern part of Jizl to Turaba he
found a variety of red and white granites, dark porphyries, brown,
banded, and dark red rhyolites, and vein-quartz (often with
epidote). These are types abundantly represented in the gravels,
and there is no reason to look elsewhere for their source. One
may definitely ascribe the gravels of the southern area (Sahma to
Hidba Farsha) to these older igneous rocks outcropping farther
west along the course of the Wadi Dawasir, and one may reason-
ably infer a western source for the extensive gravels of Jafura
and Summan.

Evidence of the age of the gravels is very scanty. The sugges-
tion made in the note on Major Chessman's collection that they
were Pliocene corresponding to those in the Eaktiari series of
'Iraq was little more than a guess, but may be near the truth.
From their position with relation to rocks of known age all we
can say is that they are definitely post-Miocene. Their relation
to the white chalky sandstone of Jabrin and Hufuf is not clear,
but some of the evidence points to their being older than these
rocks, which, as pointed out, may be a recent or sub-recent surface
formation.

The vast extent of the gravels shows them to be something
much more extensive than the deposits of such Wadis as Sahba
and Dawasir, and they are clearly of earlier date than the erosion
of the present Wadi beds.

The yellow and buff limestone pebbles in these gravel plains
show to perfection the remarkable effects of etching by solutions,
which takes place when the limestone pebbles are embedded in
the moist sand below the surface. These * Billensteine5 or etched

1A piece of red granite, such as could have been derived from a large
pebble, was found at Qasr Dahbash (Jabrin), and the fragment found b;f
Cheesman in the well spoil-heap near Jabal Jawamir may be a chip of
such a stone.