Bocks in the Nile Valley, in Nubia. 139 



from Abu Hammed to Phila3 ; viz. a silicious sandstone, similar to 

 that which occurs to a great extent on both sides of the Nile in 

 Upper Egypt, and which Russegger, after a very careful examination 

 of it there, considers to be an equivalent of the greensand of the cre- 

 taceous rocks of Europe. The tertiary nummulito limestone, so 

 abundant in Kgypt, has not hitherto been met with in Nubia. 



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 metmorphic 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 vari- 

 ous kinds, often penetrated by greenstone dykes ; sienite, diorite, 

 and felspar porphyries ; gneiss, and clay slate, penetrated by nu- 

 merous 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.* 



When, therefore, we take into account the hard nature of the si- 

 liceous sandstone, the durability of which is shewn by the very ancient 

 monuments of Egypt and Nubia, that are formed of it, and the still 

 greater hardness of the granites and other crystalline rocks, it is 

 manifest that the wearing action of a river flowing over so gentle a 

 fall, can scarcely be appreciable. If the occasional beds of marly 

 clay occur in the bank of the river, they may be washed out, and 

 blocks of the superincumbent sandstones may fall down ; but such an 

 operation would have a tendency to raise rather than deepen the bed 

 of the river at those places ; unless the transporting power of the 

 stream were far greater than can exist with so moderate a fall, 

 especially in that part of the river below Semne, where, for 96 miles, 

 it is not more than 5*3 inches, and for 115 miles below that, not 

 more than 12 inches in a mile. Even if we suppose the river to 



Russegger, Bd, ii., 1 Thl. 569 to 584. 



