118 PEOCEEDINGS OP THE GEOLOGICAL SOCIETY. 



Society's Quarterly Journal*, is exposed in the side of a dry water- 

 course. And this is what we might expect to find ; for a considerable 

 deposit would take place over this portion of the valley on the 

 river finding an outlet more to westward. 



The watercourse above mentioned must at times form the bed of 

 a considerable torrent issuing from a narrow gorge running nearly 

 N.E. and S.W., the bottom of which is strewed with large rounded 

 boulders of greenstone and syenite. The gorge is bounded on both 

 sides by perpendicular cliffs traversed by dykes ; and at the upper 

 end, after the deluges of rain which occasionally, though rarely, 

 occur, there would be a considerable fall of water, as a hollow of 

 some depth shows the successive heights at which the water has 

 stood until dried up by evaporation. 



The author walked about fourteen miles to the eastward of this 

 gorge, and ascended what appeared to be the highest hill in the neigh- 

 bourhood. The base of this hill consists of crystalline rocks, the 

 upper 200 feet being coarse sandstone of a dark-purple colour 

 (specimen 115), the height of the summit above the Nile at Assouan 

 being about 1000 feet. 



Of the crystalline rocks at the Eirst Cataract, the coarse-grained 

 pink syenite, described by Professor Delesse in vol. vii., of the So- 

 ciety's Journal, is perhaps the most abundant variety. The rocks 

 forming the eastern bank of the Cataract consist of this, as also do 

 the higher portions of the Islands of Sehayl and Hesseh : in the 

 latter place they are associated with hornblende-schists (specimen 

 75) much contorted. In places they are vertical and horizontal 

 within a few yards. 



In a quarry to the east of Assouan (D on map) this pink syenite 

 may be seen and obtained in homogeneous masses of almost any size. 

 A block said to be (for it is partially covered with rubbish) 95 feet 

 X 12 feet X 12 feet, squared on three sides, and still attached to 

 the rock on the fourth, may still be seen in this ancient quarry. 

 The material of this block is the same as specimen 19 throughout 

 the exposed part, with the exception of a few angular patches simi- 

 lar to specimens 20 and 25. Wear this spot (D on map) a mass of 

 greenstone is quarried ; it occurs with veins of quartz and felspar 

 containing garnets (specimens 27 and 29). This coarse pink syenite 

 varies much in durability. Among the ancient monuments of 

 Egypt masses of it weighing many tons (as much as 800 tons in one 

 case) may be seen unaffected by the action of the weather after an 

 exposure of from 2000 to 3000 years. On the other hand, at the 

 Eirst Cataract we see on all sides signs of its perishable nature, in the 

 rugged hills formed of rectangular blocks piled one on another not 

 unlike rude masonry, or more often covered with huge boulders 

 heaped one on another in the greatest confusion. These spherical 

 masses weather by scaling off in thin concentric shells, as may be 

 seen in a curious block on the Island of Hesseh. 



Associated with the coarse-grained syenite occur finer-grained 

 varieties, sometimes in narrow veins, sometimes in large compact 

 * Vol. 3a. p. 15. 



