et of
basaltic lava has been poured, and has descended the deep gorge of the
Zerka Main, which enters the Dead Sea some 2000 feet below. This gorge
had been eroded before the basaltic eruption, so that the stream of
molten lava took its course down the bed of this stream to the water's
edge, and grand sections have been laid bare by subsequent erosion along
the banks. Pentagonal columns of black basalt form perpendicular walls,
first on one side, then on the other; while considerable masses of
scoriae, peperino, and breccia appear at the head of the glen, probably
marking the orifice of eruption. Other eruptions of basalt occur, one at
Mountar ez Zara, to the south of Zerka Main, and another at Wady
Ghuweir, near the north-eastern end of the Dead Sea. There are no
lava-streams on the western side of the Ghor, or of the Dead Sea.[9]

The outburst of the celebrated thermal springs of Callirrhoe, together
with nine or ten others, along the channel of the Zerka Main, is a
circumstance which cannot be dissociated from the occurrence of basaltic
lava at this spot. In a reach of three miles, according to Tristram,
there are ten principal springs, of which the fifth in descent is the
largest; but the seventh and eighth, about half a mile lower down, are
the most remarkable, giving forth large supplies of sulphurous water.
The tenth and last is the hottest of all, indicating a temperature of
143 deg. Fahr. Thus it would appear that the heat increases with the depth
from the upper surface of the table-land; a result which might be
expected, supposing the heated volcanic rocks to be themselves the
source of the high temperature. To a similar cause may be attributed the
hot-springs of Hammath, near Tiberias, and those of the Yarmuk near its
confluence with the Jordan. Some of these and other springs break out
along, or near, the line of the great Jordan-Arabah fault which ranges
throughout the whole extent of this depression, from the base of Hermon
to the Gulf of Akabah, generally keeping close to the eastern margin of
the valley.

(_e._) _The Arabian Desert._--The basaltic lava-floods occupy a very
large extent of the Arabian Desert, from El Hisma (lat. 27 deg. 35' N.) to
the neighbourhood of Mecca on the south, a distance of about 440 miles,
with occasional intervals. The lava-sheets are called "Harras" (or
"Harrat"), one of which, Harrat Sfeina, terminates about ten miles north
of Mecca. The lava-sheets rest sometimes on the r