stalline floor is overlaid by the great
sandstone series which covers nearly the whole of the country north of
Hail. Upon the sandstone rest a few scattered outliers of limestone,
probably of Cretaceous age, the largest of which occur near Jauf and
east of Bureda. Over both sandstone and granite great sheets of lava
have been poured, and these, protecting the softer beds beneath from
further denudation, now stand up as the high plateaus and hills called
_harra_. Volcanic cones still exist in large numbers, and the sheets
of lava appear as fresh as any recent flows of Etna or Vesuvius.
Arabian manuscripts describe an eruption on the harra near Medina in
A.D. 1256. In the south of Arabia the crystalline floor appears at
intervals along the southern coast and on the shores of the Gulf of
Oman. At Marbat the granite is overlaid by sandstone, presumably the
Nubian sandstone: this is followed by marls containing Cenomanian
fossils; and these are overlaid by Upper Cretaceous limestones, upon
which rest isolated patches of _Alveolina_ limestone. Generally,
however, the Cretaceous beds do not appear, and the greater part of
southern Arabia seems to be formed of _Alveolina_ and nummulite
limestones of Tertiary age. An extinct volcano occurs at Aden, and
volcanic rocks are found at other places near the Straits of
Bab-el-Mandeb. Throughout the whole of Arabia, so far as is known, the
sedimentary beds show no signs of any but the most gentle folding.
Faulting, however, is by no means absent, and some of the faults are
of considerable magnitude. The Gulf of Akaba is a strip of country
which has been let down between two parallel faults, and several
similar faulted troughs occur in the Sinai peninsula. The Red Sea
itself is a great trough bounded by faults along each side.]

_Climate._--Owing to its low latitude and generally arid surface,
Arabia is on the whole one of the hottest regions of the earth; this
is especially the case along the coasts of the Persian Gulf and the
southern half of the Red Sea, where the moist heat throughout the year
is almost intolerable to Europeans. In the interior of northern and
central Arabia, however, where the average level of the country
exceeds 3000 ft., the fiery heat of the summer days is followed by
cool nights, and the winter climate is fresh and invigorating; while
in the highlands of Asir and Yemen in the south-west, and of