116 PROCEEDrN'GS Oi" THE GEOLOGICAL SOCIETY. 



of the channels are of great depth, that to the west of Hesseh being 

 more than 50 feet deep in some places, even close to the detached 

 rocks which occur thronghont the channel. 



At the Pirst Cataract the Nile flows without exception over crys- 

 talline rocks, consisting principally of quartz, felspar, hornblende, 

 and mica combined in various proportions and then appearing under 

 the forms of syenite, greenstone, hornblende- and mica-schists, or 

 else occurring in separate masses. The whole district is traversed 

 by dykes and smaller veins of quartz, felspar, and pitchstone. The 

 more compact masses of these rocks, of which the surface is often 

 beautifully polished (specimens 1 and 5), divide the river into nu- 

 merous channels, the direction of many of them being determined 

 by the line of the dykes, which, as a rule, have caused the rocks 

 in immediate contact with them to be more destructible than 

 the same in their unaltered condition ; or else the material of the 

 dykes themselves has yielded readily to the action of the water, 

 sphting into small angular fragments along joints coated with brown 

 oxide of iron (specimens 31 to 38). As the direction of the dykes 

 is generally east and west, those channels already mentioned as con- 

 necting the two main divisions of the river, and which are at right 

 angles to the usual north and south course of the Wile, wiU generally 

 be found to be in the line of dykes. Fox instance, the channel to 

 the south of the Island of Sehayl is along the line of a dyke 30 feet 

 wide running east and west, and which may be traced for some 

 distance into the Arabian desert. Again, the Island of Sehayl is 

 nearly divided into two parts by a dyke parallel to the one above 

 mentioned. 



The crystalline rocks forming the bed of the river are overlain, 

 both on the eastern and western banks, by a sandstone of very 

 variable consistency (specimen 106), some beds being coarse and 

 gritty, others fine-grained and compact, containing but little lime, 

 occasional crystals of sulphate of barium (specimen 92), the whole 

 being generally strongly impregnated with iron, which gives it a red 

 or dark-purple, and sometimes almost black colour (specimens 93 to 

 106, 115). It seems to contain no vestige of organic remains, 

 unless some of the nodules and concretions of ironstone (specimen 

 112) can be regarded as indicative of such, as many of these 

 appear at first sight to be the actual casts of shells. This sand- 

 stone rests on the uneven surface of the syenite in slightly inchned 

 strata dipping to the N.N.E. It is nowhere penetrated by dykes 

 or altered by contact with the syenite, which is, however, often 

 much decomposed at its junction with the sandstone, so that at places 

 it is difficult to distinguish the one from the other. In the lower beds 

 of sandstone roUed pebbles of quartz and chalcedony abound, some- 

 times 6 or 8 inches in diameter. Opposite Assouan, on the western 

 bank of the river, the syenite is seen underlying the sandstone; and 

 as we ascend this bank it attains a gradually higher elevation until, 

 opposite the southern extremity of the Island of Hesseh, it forms a 

 plateau about 200 feet above the Mle, extending about a mile to the 

 west of the river, when we again meet with the overlying sandstone 



