The Nile flows over this sandstone for nearly 426 miles of the entire distance, but not continuously. At Abu Hammed, it flows over granitic rocks, and these continue from that place for about 120 miles. There is then about 215 miles of the sandstone, which is succeeded by igneous and metamorphic rocks, that continue for 195 miles without any interruption, except a narrow stripe of sandstone of about 15 miles near Amara. It is in this region of hard igneous rocks that nearly all the rapids occur, between that of Hannek and the great or second cataract at Wadi-Halfa. From the latter place there is sandstone throughout a distance of about 196 miles, and then commences the granitic region of the Cataract of Assuan, through which the Nile flows about 35 miles. Thus we have about 350 miles of igneous and metamorphic rocks, and about 426 of sandstone. The general hard nature of the igneous and metamorphic rocks, over which the Nile flows for about 155 miles above Semne, and for about 40 immediately below it, will be recognised by my naming some of the varieties described by Russegger, viz. granites of various kinds, often penetrated by greenstone dykes; sienite, diorite, and felspar porphyries; gneiss, and clay slate, penetrated by numerous quartz veins. The siliceous sandstone is very uniform in its character; and in Nubia, as in Egypt, the only organic bodies which it has as yet been found to contain, are silicified stems of wood. Occasionally, as in the neighbourhood of Korusko, interstratified beds of marly clay are met with.[71] Footnote 71: Russegger, Bd. ii., 1 Thl. 569 to 584.