In material, the alluvial accumulations are of the same varied nature as the mountains from whose degradation they have originated, ranging from very basic mineral substances, such as serpentine, to the most acid, such as quartz. The relative proportions of the different materials in the alluvial deposits do not, however, correspond as a rule with the proportions in which they enter into the mountain masses. Not only are heterogeneous rocks, such as granites and diorites, far more rapidly disintegrated by temperature-changes than are the more homogeneous rocks such as felsites and serpentines, but the fragments produced by the disintegration of granitic rocks are rounder and of much smaller size than the fragments resulting from the breaking up of felsites, schists, and serpentines. The result is that granitic detritus is transported to greater distances and distributed over larger areas than the detritus of the closer-grained and more homogeneous rocks. In approaching a mountain composed of about equal parts of, say, granite and serpentine, we accordingly find the lower portions of the wadi consist chiefly of granitic sand, while the coarser and more angular detritus of serpentine is only seen in abundance in the higher parts of the drainage channel. The nature of the alluvium of the wadi floor occasionally gives its name to the wadi; thus, of the two wadis which by their junction form Wadi Khoda, one is called Wadi Salib el Azrak, because its alluvium consists of dark rock fragments, while the other is called Wadi Salib el Abiad, because its floor is formed of white granitic sand. (_Azrak_ = dark; _Abiad_ = white). In size, the fragments composing alluvial detritus vary from huge blocks weighing tons down to the finest sand whose grains are only a fraction of a millimetre in diameter. As a rule, of course, the larger fragments are found near the heads of wadis, and the finer sands in their lower reaches and on the plains. But often we find great boulders mixed with the sand at immense distances from the parent mountain mass; the heavier rain storms which occasionally break over the mountains produce for short periods such immense rushes of water down the wadis, that great boulders are swept along and incorporated in the finer material which is deposited in times of more normal rainfall. Thus it is no uncommon thing in sinking a well in a wadi fifty kilometres or more from the mountains to encounter great boulders in the sandy alluvium.