Vol. 5 9. J SEMNA CATARACT OF THE NILE. 71 



The Syenite-Porphyry. 



Dykes of syenite-porphyry are well defined in the gneiss, both 

 on the left bank south of the ISemna temple, and on the right bank 

 in the hill behind Kumna. In the latter place the dykes are far 

 from vertical, dipping steeply northward ; on the eastern bank they 

 are on gently sloping ground and the dip is not seen, but the 

 strike is about east-south-east. On a fresh fracture, the rock is of 

 an ashen-grey colour with a brownish tinge ; it contains in places 

 pink porphyritic crystals of felspar up to 3 millimetres in diameter,, 

 in a fine-grained grey groundmass. The exposed surfaces are of a 

 rusty-brown ; and where below the high-Nile level they are polished, 

 so as to give the rock the appearance of eisenkiesel. The brown 

 skin penetrates to a depth of about 1 centimetre, the limit between 

 it and the normal grey rock being very sharply defined. The specific 

 gravity of the porphyritic rock is 2-45. Examined microscopically 

 (see PI. IV, fig. 2), it is seen to consist mainly of felspar (orthoclase), 

 of which there are two generations : the first in large porphyritic 

 crystals, and the second, forming the bulk of the groundmass, in 

 rod-shaped ones. Both classes of felspar are much kaolinized, but 

 they are fresh enough to render their orthoclastic nature evident. 

 There is a considerable amount of haematite in large irregular 

 grains, often partly surrounding the felspars of the groundmass ; 

 and quartz is sparingly present. 



The sections having been purposely cut to show the transition 

 from the grey interior to the brown skin, the explanation of the 

 formation of the latter is readily seen from them. In the internal 

 portions, the haematite is in perfectly opaque grains, mostly of 

 considerable size, and there is only a moderate sprinkling of 

 smaller grains, so that the felspars, except for kaolinization, are 

 clear. In the brown skin, the larger iron-oxide grains are trans- 

 lucent, of a red-brown colour ; and strings of the same mineral 

 extend through the groundmass between the felspar-crystals in all 

 directions, while at the same time the felspars are themselves full 

 of minute granules and strings of haematite and limonite (see PI. IV, 

 fig. 3). There has thus been brought about by hydrous action a 

 migration of iron-oxides, from the separate clearly defined grains 

 of the normal rock, into the substance of the felspathic groundmass. 

 I am inclined to think that the large opaque haematite-grains of 

 the normal rock are a product of oxidation of a ferromagnesian 

 silicate, perhaps biotite. The well-known alteration of biotite 

 with separation of large flakes of iron-oxide is strongly marked in 

 rocks which I have examined from the First Cataract, where it 

 can be seen in every stage. Here, however, not a vestige of 

 biotite or hornblende remains. 



The variety of- the rock which is free from the pink porphyritic 

 crystals is otherwise exactly the same as the one described, and 

 shows the same process of migration of iron-oxides by water-action. 

 Its specific gravity is, however, slightly higher (2*51). 



