as the sand will lie (about 50 deg.). On the summit of the falk there
is generally a mound known as _tas_ or _barkhus_ composed of white
sand which stands out conspicuously against the deep red of the
surrounding deserts; the exterior slopes are comparatively gentle. The
falks are singularly uniform in shape, but vary greatly in size; the
largest were estimated by Huber and Euting at 1-1/4 m. across and 330
ft. deep. They run in strings irregularly from east to west,
corresponding in this with their individual direction, the convex face
of the falk being towards the west, i.e. the direction of the
prevailing wind, and the cusps to leeward. In the south of the Nafud,
where Huber found the prevailing wind to be from the south, the falks
are turned in that direction. Though perhaps subject to slight changes
in the course of years, there is no doubt that these dunes are
practically permanent features; the more prominent ones serve as
landmarks and have well-known distinctive names. The character of the
vegetation which clothes their slopes shows that even superficial
changes must be slight. The general level of the Nafud was found by
Huber's observations to be about 3000 ft. above sea-level; the highest
point on the Jauf-Hail route is at Falk Alam, the rocky peaks of which
rise 200 or 300 ft. above the surface of the sand. Other peaks
cropping out of the Nafud are Jebel Tawil, near the wells of Shakik,
and J. Abrak Rada, a long black ridge in the middle of the desert.

The Harra.

The high plateau which from. J. Hauran southward forms the main
watershed of the peninsula is covered in places by deep beds of lava,
which from their hardness have preserved the underlying sandstones
from degradation, and now stand up considerably above the general
level. These tracts are known as _harra_; the most remarkable is the
Harrat El Awerid, west of the Haj route from Tebuk to El Ala, a
mountain mass 100 m. in length with an average height of over 5000
ft., and the highest summit of which, J. Anaz, exceeds 7000 ft. The
harra east of Khaibar is also of considerable extent, and the same
formation is found all along the Hejaz border from Medina to the Jebel
el Kura, east of Mecca. The surface of the harra is extremely broken,
forming a labyrinth of lava crags and blocks of every size; the whole
region is sterile and almost waterless, and compared with the Nafud it