— 22 — 
near Luxor, the Nubian sandstone which overlies the crystalline rocks 
dips under the Nile and its place along the Nile Valley is taken by 
green and grey clays containing nitrate and phosphate deposits. The 
former are inexhaustible and have constituted the manure of this part 
of the valley for thousands of years. With these deposits are thick 
banks of soft white limestone. 
From Luxor northwards the clays dip under the Nile and the Nile 
Valley is bounded by the superposed white eocene limestone up to 
Cairo. 
The Nubian sandstone is always soft and porous. The limestone is 
generally soft, though hard siliceous beds are sometimes met with. 
North of Cairo there is no building stone of any value except the 
siliceous sandstone of Gebel Ahrnar near Cairo and the basalt of Abu 
Zabel, a recent outcrop furnishing a black rock of great durability. The 
area covered by this rock is small. 
Thick deposits of sand and gravel underlie the Nile mud deposits of 
the Nile Valley. All along the Nile, but especially south of Luxor, 
river deposits of dark sandy mud exist on either side of the Nile 
Valley considerably above the level of the deposit of to-day. The best 
known of these is the plain of Kom Ombos. The thickness of the 
layer of Nile mud in the valley is as much as 18 metres in places, but 
the average depth is, I should say, 10 metres. 
8. The discharges of the Nile and its tributaries. — Refer- 
ence should be made to tables 24 and 25 which embody the results of an 
exhaustive examination of the observed discharges, the cross sections, 
the gauges of the Nile Valley, and the calculated discharge tables 
made for these gauges. Many of these tables are founded on only two 
or three discharges and some on only one, but they have been prepared 
with the greatest care and referred to all the existing gauge obser- 
vations, and are good working tables, which can be modified and 
improved as time places more information at our disposal. Until 
then they may be used as about the best approximations available to-day. 
In 1902 the Albert Nile discharged 600 cubic metres per second as 
against 520 discharged by the Victoria Nile. In 1903 the Victoria 
Nile discharged 730 cubic metres per second and the Albert Nile 800. 
Leaving a poor year like 1902 which was much below the average, and 
taking 1903 which was all round a good average year and only 
slightly below the mean, we have the following results : — 
The Victoria Nile was at its highest in July with 840 cubic metres 
