Throughout the valley we were struck by the proofs of native industry and skill given by the numerous irrigation channels, such as one sees in Piedmont, and in the tributaries of the Rhone valley in Switzerland, sometimes cut along steep faces of rock, sometimes maintained by high terraced banks. Where the ground is favourable, walnut trees are often planted along these watercourses, and must largely contribute to the dietary of the inhabitants. It thus appears that the drainage of the Great Atlas is, in great part, absorbed by irrigation, even before the streams enter the low country, while a further portion is there taken up for the same purpose, and but a small percentage reaches the sea in ordinary weather. This helps to account for much that at first sight appears so strange in the hydrography of Marocco. A vast mountain region, fully exposed to the currents of saturated warm air from the Atlantic, sends but four rivers to the ocean from its northern and western flanks, in a coast line of over 400 miles from El Araisch to Cape Guer; and these, at ordinary times, are all easily fordable. But when rain falls on the mountains, the irrigation channels are speedily filled to the brim, and the entire surplus reaches the rivers, which are then said to rise ten or twelve feet in the course of as many hours. As bridges are unknown, the Moors speak of travellers being detained for many days before a flooded river channel, as a common occurrence. Above the ford, the valley was again contracted to a mere gorge, and the narrow path mounts on its eastern flank, and winds along the extremely steep rugged slopes much after the fashion of some unfrequented valley of the Southern Alps. Although the sun was already high, the mountain rose so sheer upon our left that the shadow often gave welcome protection; and the track was so narrow in places that we were not free from anxiety for the baggage animals. The rock was now porphyry of a prevailing red colour, which, with occasional intrusive masses of diorite and dark green basalt, makes up the whole mass of the central ridge of the Great Atlas in this part of the chain. As compared with the rich and varied flora, insect life appeared, at least at this season, to be remarkably scarce, and the only butterfly noted was _Papilio podalyrius_.