As for the Hamadah, we know that near Sokna the plateau breaks up and
forms what are called the Jebel-es-Soudy, or Black Mountains, a most
picturesque group of cliffs; and again on the route to Egypt from
Mourzuk, six days' journey south-east from Sokna, it also breaks into
huge cliffs, and bears the name of El-Harouj. These mountain buttresses
are either the bounds of the Hamadah, or masses of rock where it breaks
into hills, forming ravines or valleys. But, in fact, how far the
Hamadah extends between Ghadamez on the west and Augila on the east is
not yet properly ascertained. It seems to be like a broad belt
intercepting the progress of commerce, civilisation, and conquest, from
the shores of the Mediterranean to Central Africa. The kingdom of
Fezzan, however, advances like a promontory beyond it; and then on every
side stretches the desert ocean with its innumerable oases or islands,
which, from being once mere fluctuating names, as it were, on a guess
map, are now by degrees dropping one by one into their right places.

On the breaking-up of the plateau we observed its geological structure
to consist of three principal strata: first, a covering or upper crust,
limestone with flints and red earth; then masses of marl; and then
sandstone, lumps and masses of which were blackened by the contact of
the air with the iron they contain. Under the sandstone was likewise a
bed of yellow clay, with a mixture of gypsum.

The face of the cliffs of the plateau was blackened as with the smoke of
a huge furnace, which gave a majestic and yet gloomy appearance to the
scene as we descended the pass towards the valley of El-Hasee. We found
the plain strewed with great masses of dark sandstone, seeming to have
been detached by some convulsion from the rocky walls, which now rose in
apparently interminable grandeur behind us. We glanced back in awe, and
yet in some triumph, towards the iron-bound desert we had thus safely