HAWKSHAW riEST CATARACT. 119 



masses. Often the hornblende is nearly wanting. In a variety of 

 the coarse-grained syenite, which occurs in large quantities, the 

 hornblende is in such preponderance as to give a dark-green colour 

 to the rock, relieved here and there by large flesh-coloured crystals 

 of felspar. Near the northern end of the Island of Hesseh a great 

 variety of specimens may be obtained from the debris remaining 

 from the artificial cut before alluded to (specimens 41 to 53). The 

 prevaihng colour of the felspar in the neighbourhood of this cut is 

 deep red ; sometimes, however, it is much decomposed and chalky 

 white (specimen 50). The dykes that intersect the cataracts are 

 mostly of dark-green compact greenstone or felstone speckled with 

 crystals of pink felspar — or sometimes rotten and decomposed, and 

 traversed by detached veins of quartz and felspar, being then of 

 a brown colour in places (specimens 31 to 38). In the neighbour- 

 hood of the dykes the syenite is often separated into its component 

 parts (specimens 1 and 5). 



Hornblende-schist occurs at the western side of the river, oppo- 

 site the Island of Hesseh (point K on map), overlain by, and repo- 

 sing on, pink syenite. 



Hornblende -schist forms a large portion of the Island of Hesseh ; 

 a little occurs in the Island of Sehayl, and it is again met with in the 

 large eastern valley before described. At the northern extremity of 

 this valley the hornblende- schist or mica-schist is in places nearly 

 vertical, and is overlain by sandstone. At the junction of the two, 

 the sandstone contains pieces of schist and large angular fragments 

 of quartz (specimen 111). 



There is near this point a very conspicuous mass of white quartz, 

 about 150 yards long by 50 yards wide, rising 30 feet above the 

 schist, which is nearly vertical in its neighbourhood. 



In conclusion, I would call attention to the advantages offered 

 by the locality I have endeavoured to describe for observing the 

 different combinations of granitic and syenitic rocks and of their 

 elements, and more especially the gradual transition from one variety 

 of combination to another. Every variety of combination of these 

 minerals, sometimes weatherworn and in all stages of decay, at 

 other times having the surface polished as if by the hand of a lapi- 

 dary, may be seen at low Nile over a few square miles of country, 

 on the banks of the river and on the islands of the First Cataract. 



