the _kanat_ or _karez_ of Persia
and Afghanistan, are also largely used. The principal villages on the
eastern slopes are Rustak, Nakhl and Semail in the well-watered valley
of the same name; on the western slopes are Tanuf and Nizwa, lying
immediately below the highest summit of the range; Semed, Ibra and
Bidiya in the W. Betha are all well-built villages with palm-groves
and irrigated fields. In the north-west the Dhahira district sloping
towards the Jewasimi coast is more steppe-like in character; but there
two oases of great fertility are found, of which Birema, visited by
both Miles and Zwemer, supports a population of 15,000. West of Abu
Dhabi a low flat steppe with no settled inhabitants extends up to the
Katr peninsula, merging on the north into the saline marshes which
border the Persian Gulf, and on the south into the desert.

The southern desert.

The great desert known as the Dahna or the Rub'a el Khali ("the empty
quarter") is believed to cover all the interior of southern Arabia
from the borders of Yemen in the west to those of Oman in the east.
Halevy in Nejran, Von Wrede in Hadramut, and Wellsted in Oman reached
its edge, though none of them actually entered it, and the guides
accompanying them all concurred in describing it as uninhabitable and
uncrossed by any track. Its northern fringe is no doubt frequented by
the Bedouin tribes of southern Nejd after the rains, when its sands,
like those of the northern desert, produce herbage; but towards the
east, according to Burckhardt's information, it is quite without
vegetation even in the winter and spring. The farthest habitable spot
to the south of Nejd is the Wadi Yabrin, which L. Pelly heard of from
the Ahl Murra Bedouins as once a fertile district, and which still
produces dates, though, owing to malaria, it is now deserted; thence
southward to the Hadramut valley no communication is known to exist.

[_Geology._--The geological structure of Arabia is very similar to
that of Egypt. The oldest rocks consist of granite and schist,
penetrated by intrusive dykes, and upon this foundation rest the
flat-lying sedimentary deposits, beginning with a sandstone like the
Nubian sandstone of Egypt. In the northern part of Arabia the
crystalline rocks form a broad area extending from the peninsula of
Sinai eastwards to Hail and southwards at least as far as Mecca.
Towards the north the cry