Throughout our morning’s ride, as well as on the journey between Sheshaoua and Misra ben Kara, we noticed the apparently unaccountable way in which certain social species prevail over a considerable tract, and then suddenly give place to others, without any apparent reference to the composition of the soil. Where Chenopodiaceæ, such as _Suæda_ and _Caroxylon_ prevail, it is reasonable to conjecture the presence of nitre, gypsum, or other salts in the superficial layer; but such plants as _Artemisia Herba alba_, _Genista monosperma_, and a local form of _Helianthemum virgatum_ will sometimes take almost exclusive possession of the surface, though this in some places is mainly composed of siliceous sand, in others of disintegrated calcareous tufa, and in others of decomposed volcanic rock, nowhere seen by us _in situ_, but derived from scattered blocks of various sizes. In the plain near the city siliceous sand predominates, and, as a consequence, the vegetation is more meagre than elsewhere. We hereabouts first saw the only works of public utility which we encountered during our journey. What first struck the eye were long lines of irregular earthen mounds traversing the plain in a north and south direction, and we soon ascertained that these were watercourses rudely arched over. The streams from the mountains south of the city are distributed through irrigation canals over a large part of the plain, and thus render it fit for cultivation. Early experience must have taught the people that by protecting these canals from evaporation, they could be made available to a much greater extent; and it is probable that the construction of these covered waterways, some of which were in a ruinous condition, goes back to a remote period. In point of fact, the whole drainage of three considerable valleys, whose torrents we afterwards crossed, appears to be intercepted by this irrigation process, and absorbed by the vegetation of the plain. It is probable that by the skilful extension of the same system wide tracts, now barren, might be made productive. FOOTNOTES: [Footnote 1: For fuller particulars as to the Argan tree and its economic uses, see Appendix D.] [Footnote 2: It may be hoped that the plant will now become well known to botanists, as our friend M. Cosson has obtained a good supply of seed, which he has liberally distributed among many of the chief botanic gardens of Europe. See Appendix D.] [Footnote 3: This was apparently the _Pistacia atlantica_. The true Pistachio tree (_P. vera_ of Linnæus), so extensively cultivated in the warmer parts of the Mediterranean region, was not seen by us in Marocco.]