The Desert forms a natural boundary to Egypt, on the west extending from the _Natron Valley_ to the mountains of _Ummesogier_; to the north, the dreary and barren plain is bounded by a chain of lofty hills, in view during the whole course of the caravan; and to the south, extends a journey, probably, of several days, by the ordinary mode of computation in these countries; but in this direction its limits are not defined, or are not known. In this vast tract of sands, petrified wood is found, of various forms and size: sometimes are seen whole trunks of trees, of twelve feet circumference or more; sometimes only branches and twigs, scarcely of a quarter of an inch diameter; and sometimes merely pieces of bark of various kinds, and in particular of the oak, are to be found. Many of the great stems yet retain their side branches, and in many the natural timber has undergone so little change, that the circular ranges of the wood are discernible, and especially in those trunks which apparently were of oak. The interior of other bodies of timber was become a petrifaction, shewing no distinctions of grain or fibre, but bearing the appearance of mere stone; though the outward coat and form of the substance clearly denoted the tree. Several Arabs informed me, that in travelling over this Desert, petrified trees were often found upright, and as if growing in the soil; but I presume, respecting those I did not see, from those I inspected, that they were merely trunks raised by hand, round the base of which the sand had quickly gathered before the winds, and formed a mound, as if heaved up by a root. The colour of the petrified wood is in general black, or nearly so; but in some instances it is of a light gray, and then so much resembling the wood in its natural state, that our slaves would often collect, and bring it in, for the purposes of firing. These petrifactions are sometimes scattered in single pieces, but are oftener found in irregular layers, or strata, covering together a considerable space of ground.