Half a day's journey beyond, at a point where two great wadis enter the Euphrates, on the Syrian side, stands Jabriya, an unidentified ruined town of Babylonian type, with walls of unbaked brick, instead of the stone heretofore encountered.
The range from the same point of view presents a singularly uniform outline, having the appearance of an unbroken wall; in reality, however, it is traversed by a number of deep ravines (wadis), of which the most important are the Yabis, the Ajlun, the Rajib, the Zerka (Jabbok), the Hesban, and the Zerka Ma`in.
The central zone includes Hejaz (or Hijaz), Nejd and El Hasa; much of it is a dry, stony or sandy steppe, with few wells or watering-places, and only occupied by nomad tribes; but the great wadis which intersect it contain many fertile stretches of alluvial soil, where cultivation is possible and which support a considerable settled population, with several large towns and numerous villages.
Under Turkish protection, he visited the territory of the Hashid and Bakil tribes north-east of Sana, and though their hostile attitude compelled him to return after reaching their first important town, Khamr, he had time to reconnoitre the plateau lying between the two great wadis Kharid and Hirran, formerly covered with Himyaritic towns and villages; and to trace the course of these wadis to their junction at El Ish in the Dhu Husen country, and thence onward to the Jauf.
Hadramut; the two others run for a long distance through fertile valleys and, like many of the wadis on the seaward side of the range, have perennial streams down to within a few miles of the sea.
The Arhab district, through which these two great wadis run, was formerly the centre of the Himyar kingdom; cultivation is now only to be found in the lower parts on the borders of the watercourses, all above being naked rock from which every particle of soil has been denuded.
The wadis (or valleys) of Oman (like the wadis of Arabia generally) are merely torrential channels, dry for the greater part of the year.
Though there are no perennial rivers, there are watercourses (khors or wadis) in the rainy season; the chief being the Khor Abu Habl, which traverses the southcentral region.
The weathering of this desert area is probably faimly rapid, and the agents at work are principally the rapid heating and cooling of the rocks by day and night, and the erOsive action of sand-laden wind on the softer lnyers; these, aided by the occasional rain, are ceaselessly at work, and produce the successive plateaus, dotted with small isolated hills and cut up by valleys (wadis) which occasionally become deep ravines, thus foiming the principal type of scenery of these deserts.
It is probable that certain rudely chipped flints, so-called eoliths, in the alluvial gravels (formed generally at the mouth of wadis opening on to the Nile) at Thebes and elsewhere, are the work of primitive man; but it has been shown that such are produced also by natural forces in the rush of torrents.