Towards the foot of the first and most conspicuous of the hills above mentioned, which bears the inappropriate name Hank el Gemmel (Camel’s back), the plain rises gently rather more than one hundred feet in all; above this the slope of the hill becomes steep, and finally exhibits an almost vertical face at the top. At the foot of the steeper slope, about four hours’ ride from Aïn Oumast, our track passed by an ancient well, now almost dry, and often completely so; and here, under the imperfect shade of a lotus tree, we made a short halt. The direct rays of the sun being very powerful, we were somewhat surprised to find the temperature of the air to be only 77° Fahr. Leaving our escort, we ascended the low but steep hill above the well. The scarped face exhibited a section of the yellowish-white limestone that appears to underlie nearly the whole of the low country between the coast and the base of the Atlas. No fossils were found; and in the present state of our knowledge, or rather ignorance, of the whole region, it seems impossible to fix its position in the geological series. The level summit is capped by a thin layer of coarse chalcedony, in which we recognised the origin of the siliceous fragments scattered over the plain below. This layer would offer resistance to superficial denudation, and account for the tabular forms of the hills, but where these were attacked from below by marine or river action the covering would necessarily be broken up and the fragments scattered over the plain below. With reference to the opinion expressed by Maw in his paper in the Proceedings of the Geological Society, and in the Appendix to this volume, as to the origin of the tufaceous coating of the plain between Aïn Oumast and Marocco, the only difficulty that presents itself arises from the presence of these siliceous fragments on the surface along with the disintegrated tufa. If, as he and other geologists believe, such a superficial coating is due to evaporation from the underlying mass of water charged with carbonate of lime, it seems hard to account for the diffusion of the chalcedony fragments, unless we suppose a submergence of the plain subsequent to the formation of the tufa layer, and a renewed supply of such fragments by further erosion of the hills that formed the sea or river coast line. To confirm this conjecture, we may note the fact that the fragments of chalcedony became progressively rarer as we advanced from the lower portion of the plain over which we this day travelled to the upper level surrounding the city of Marocco.