Note 4 — INTRODUCTORY CHAPTER.

Gibel Assoud and the hills on this side have the same name. The valley is bounded on both sides by hills, from 400 to 600 feet high—tops in general tabular; but a few are irregular, and two or three end in conical peaks; the sides of all are covered with much debris. The colour of the hills gives a very peculiar character to the valley; the tops of a shining black, as if covered over with black lead, that often extends some way down the sides, which are of a light brown, mixed with a dirty yellow: this is often observable in patches in the black, which gives to the whole a very striking appearance. The lower strata are limestone, of a yellowish colour, almost entirely formed of marine remains: this, although hard, is easily acted on by the air, and the exposed surface mouldering away leaves cavities in the rock, which, undermining the superincumbent ones, gives rise to the quantity of detached fragments. There are several thin strata of earthy gypsum: above that, limestone, with a fine fibrous-looking external surface, something like wood: this has the jingling sound of burnt lime; above is the shining basalt, of a fine texture, mixed with amygdaloid.

About six miles from where we halted, are a range of low white hills, running about west by north, of the same name as the plains. The top is a fine shining white, from thick beds of a milk-white marble, the base of porphyritic limestone. W. O.