Page 18 - A Hand book of Arabia Vol 1 (iii) Ch 1,2

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10 PHYSICAL SURVEY

preserved either by harder composition, or by caps of erupted
matter, which overlie the soft material of the mass of '-the peninsula.,
Since many summits on the upper edge of the shelf m both the
north and the south of the Red Sea region—Midian and Yemen—
exceed 8,000 feet, it is only to be expected that the central mass ot
the peninsula should lie, for the most part, at <\considerable eleva­
tion. As a matter of fact, Nejd—as the central districts are collec­
a mean
tively but incorrectly called by modern geographers has
elevation of fully 2,500 feet, and here and there, as in Jebel Aja in
*. •
the north centre, harder masses stand up to over 5,000 feeE. ^he
only notable exceptions to the generally uniform^sstward decline
occur in the extreme south-east, where hard rocks in Oman have
resisted denudation and break the slope wit0h summits rising in ^
Jebel Akhdhar as nigh as any in Midian or Y emeu ; * and in tl\e
east centre, where a long curving escarpment, facing west, defines
a broad plateau uplifted about 600 feet-*-Jebei Toweiq.

Wadis.

There are no rivers in Arabia which How perennially from source *
to mouth ; but there are incipient perennial streams in’Asir, Yemen,
the Aden district, Hasa, Oman and Nejd, and countless fiumaras ot
river-valleys {wadis), which carry floods (seils) after rainstorms.
Those which originate east of the western watershed are mostly
long and shallow, their bottoms being very little depressed below
the general level. • The longest of these, the Rummah, whose course
from the neighbourhood of Medina, through Qasim, to the Shatt
el-‘Arab, falls 0,000 feet in about 1,000 miles, is one good example,
and the Wadi Hanlfah, which runs from the westward flank of Jebel
Toweiq through a gap in the ridge towards the Persian Gulf (but,
perhaps, nevtV reaches it), is another. Both these great wadis might
be crossed at many points in their lower and middle courses alnxist
without the traveller being aware of their existence. They are not,
however, without importance ; for at all times the}' carry water
beneath th&ir beds, which can be reached by wells at viiryino-
depths; they, provide lines of possible communication ; and
wherever, as in the Rummah in Qasim, or the Hanlfah in Nejd,
the ground moisture rises near or on to their surface, they create
chains of oases.

The wadis, on the other hand, which fall to the Red Sea have
as a
whole, deeply eroded beds very steeply inclined. In their
upper courses they are of little service to communicationsTrom wesd
to east, and an obstacle to passage from north to south. In them

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