Selim Capitan made three observations upon the Island of Tshànker, and the northern latitude was confirmed as 4° 30″. The stream, having a direction to S.S.W., is found to be three hundred mètres[3] broad, from the island to the right shore. The two arms of the river may amount to something over one hundred mètres from the island to the left shore, before which another little island lies. A rocky bar of gneiss extends here from E. to W. right across the stream, and continues beyond the islands, the highest points of which it forms, to the left shore, where we may follow the traces of this rocky bank to a still greater distance, for it projects in a slight breadth over the ascending grounds. This reef, running from E. to W. which may give the direction of the succeeding chasms in the valley broken through by the stream, rises in the middle of the river to a larger rock, and other blocks peep out of the water towards the right shore, whilst the other rocky part towards the Island of Tshànker is only superficially covered by water. The current gushes by the before-named rocks as at the Bingen Loch,[4] and it is only there where we may expect to find a passage. Our Arabs are glad at this tschellàl, as they call a waterfall, or even a current, and want to take it immediately for an insurmountable wall, even before sounding the passage. The conchylia, similar to oysters, clinging to these gneiss rocks, are the thorn-muscles, which are found also at the cataracts in Nubia (_Etheria tubifera_). By mistake, I previously called these muscles _ampulla tubulosa_.[5] Besides the snails mentioned on a former occasion, I found on the shores of the White Stream the large water-snails (_ampullaria ovata var._), as well as the muscles (_Iridina rubens_, and _Anodonta acuta_). The specimens of sand I brought back with me, and which were subjected to an examination in Berlin, with my other geognostical collections, have been already defined by Dr. Girard, in C. Ritters’ “Glance at the Source-Territory of the Nile.” He says:—“A sand from the shore of the Bahr-el-Abiad, in the kingdom of Bari, is similar to this sand (from the Sobàt river), only a little coarser in the grain, which, however, is only at the most of the size of millet-grain: it contains principally quartz, but more mica than the former, and has considerably more of those black grains, from which we see that it is horn-blende. This is either derived from syenite and diorite masses, as they often appear in gneiss and mica-slate mountains, or they might be also of volcanic origin, for the lava of the Gebl Defafaungh lying to the north boundary of these plains, contains the same in great abundance.” With respect to the constituent part of the rocks of this cataract, Dr. Girard observes:—“It is gneiss, consisting of white feldspar, and a good deal of white mica, and mica-slate, containing much granular, brittle, white and yellow quartz, no feld-spar, and small scaly black mica.”