and Ouled bou Sba met at Sidi Moktar; but such places in Marocco are proverbially unsafe, because they are the frequent resort of robbers and outlaws. In case of a robbery or murder being committed, the people of each tribe throw the blame upon their neighbours, and the men of one province are very shy of attempting to pursue malefactors who take refuge within the boundaries of another. After the commission of many outrages at this place, it was found necessary to transfer a portion of territory to the Ouled bou Sba, at the same time making the Governor of that province and tribe responsible for the safety of those whom business or piety lead to the sanctuary of Sidi Moktar. As we rode onward the Great Atlas chain remained in view, but dimly seen through the haze that increased with the increasing heat of the day, and ahead of us rose some flat-topped hills of singular aspect which have attracted the attention of all travellers in this region. Some of these hills extend for a considerable distance, while others form small isolated masses; but they agree in two respects—all are flat-topped, and all show a steep escarpment especially on their westward faces. We afterwards saw reason to believe that they all rise about 450 feet above the portion of the plain near at hand, and reach nearly the same height as the plain surrounding the city of Marocco. The general appearance suggested the probability of a former wide extension westward of the latter plain, and subsequent erosion by marine or fluviatile action. As we approached the most conspicuous of these isolated hills, we were struck with the singular appearance of the stunted bushes of _Zizyphus Lotus_, which form the only arborescent vegetation of this region. From a little distance they looked as if covered by some white-flowered climbing plant, or else laden with white fruit. This appearance was due to the extraordinary number of two species of snails (_Helix lactea_ and _H. explanata_) that completely covered the branches. We frequently noticed the same appearance afterwards, but nowhere so markedly as here.