Be this as it may, one thing is clear, namely, that the Wady was and is the great central receptacle of the plain, and corresponds pretty closely with its neighbour, the still existing Dead Sea, while the Wady-er-Rajel entering it from the north, holds towards it the position of the Jordan. Water in the Wady Sirhan is found at a nearly uniform level of 1850 feet; and this rule applies to that part of it which is known as the Jôf as well as to the rest. The abundance of water obtainable from its wells along a line extending 300 miles from the frontier of Syria, to within 200 of the frontier of Nejd, points out Wady Sirhan as the natural high road of Northern Arabia, and such it must from the earliest times have been. It is probable that in the days when Arabia was more populous than now, villages existed in it at intervals from Ezrak to Jôf. At present, the wells of these only remain, if we except the twin oases of Kâf and Ithery, still preserved in life by the salt lakes which supply them with an article of trade. These are but poor places, and their population can hardly exceed two hundred souls.
Jôf and Meskakeh are still flourishing towns, but I have reason to think their population has been over estimated by Mr. Palgrave. I cannot put the total number of houses in Jôf at more than 500, nor in Meskakeh at more than 600, while 100 houses are an ample allowance for Kara and the other hamlets of the Jôf oasis. This would give us a census of hardly 8,000 souls, whereas Mr. Palgrave puts it at 40,000. I do not, however, pretend to accuracy on this point.
With regard to the geology of the Hamád and the adjacent districts north of the Nefûd, I believe that sandstone is throughout the principal element. In the extreme north, indeed, limestone takes its place or conglomerate; but, with the exception of a single district about 100 miles south of Meshhed Ali on the Haj road, I do not think our route crossed any true calcareous rock. The cliffs which form the eastern boundary of the Wady Sirhan are, I think, all of sandstone, south at any rate of Jebel Mizmeh, as are certainly the hills of Jôf and Meskakeh, the rocks of Aalem and Jobba, and all the outlying peaks and ridges north-west of Haïl. These have been described as basaltic or of dark granite, the mistake arising from their colour which, though very varied, is in many instances black. The particular form of sandstone in which iron occurs, seems indeed to acquire a dark weathering with exposure, and unless closely examined, has a volcanic look. I do not, however, believe that south of latitude 31° the volcanic stones of the Haura are really met with; unless indeed it be west of the Wady Sirhan. Jebel Mizmeh, the highest point east of it, is alone perhaps basaltic. The whole of the Jôf district reminded me geologically of the sandstone formation of Sinaï, both in the excentric outline of its rocks, which are often mushroom shaped, and in their colour, where purple, violet, dark red, orange, white and even blue and green are found, the harder rock assuming generally an upper weathering of black. I can state positively that nothing basaltic occurs on the road between Jôf and Jebel Aja. Of Jebel Mizmeh I am less certain, as I did not actually touch the stone, but, if volcanic it be, it is the extreme limit of the Harra southwards. The tells of Kâf I certainly took to be of basalt when I passed them. But I did not then consider how easily I might be mistaken. On the whole I am inclined to place latitude 31° as the boundary of the volcanic district east of the Wady.