224 Lieut. Newbold on the Geology of Egypt. 
of Nubia, and on the southern limit of Egypt, it contains more silex 
and less calcareous or argillaceous matter than at Cairo, which stands 
on the great limestone deposit, and in the Delta, which rests on that 
formation. It varies also in texture and composition, according to its 
position relative to the main channel of the river and the force of 
the current. The finest mud, as that of Ghennah, is generally dark 
brown passing to lighter shades; it. is also highly tenacious, reten- 
tive of moisture, effervesces, and fuses per se, with extrication into 
a greenish glass. The annual deposit or layer varies in thickness in 
the same situation from an inch to a few lines, the upper part being 
generally lighter than the lower; and each layer is separable from 
that above or beneath it ; but the deposition of one year is frequently 
removed by the flood of the next. 
Mr. Newbold does not knowif the thickness of the mud in the centre 
of the river’s bed has been ascertained; the greatest accumulation in 
a transverse section being near the stream’s channel ; but in Upper 
Egypt ke has measured cliffs composed of it forty feet in height ; 
ard the average thickness in Middle Egypt is thirty feet, while at 
the apex of the Delta it is eighteen feet. According to Sir G. Wil- 
kinson, the deposit has increased during the last 1700 years at Ele- 
phantine in Upper Egypt nine feet, at Thebes seven feet, and at He- 
liopolis five feet ten inches; but the amount of accumulation dimi- 
nishes in general more rapidly towards the Delta and Mediterranean. 
All calculations, however, on the progressive rate of increase through- 
out Egypt, deduced from the actual addition around the bases of 
nilometers, statues or buildings, in particular localities, are liable, 
Mr. Newbold says, to uncertainty, on account of the shifting of the 
river’s bed, and the intermingling of the drift sand of the desert. 
Moreover, the alluvium at the foot of these monuments has been 
disturbed by the plough and spade of cultivators ; and in most cases 
it has not been proved at what period the Nile reached these bases ; 
but judging from the thickness of the annual layers, of which the 
author has counted upwards of 900 im the clifis of the Nile, he 
concludes that the yearly deposition has not varied in the aggregate 
for the last thousand years. It is equally difficult, he adds, to calcu- 
late the progressive superficial extension of the mud. 
Few pebbles or detritus of any size are found in Lower Egypt and 
in the Delta, and only the finest ingredients escape into the Mediter- 
ranean, but Mr. Newbold has observed the sea coloured by this 
drifted matter to the distance of forty miles from the shore. The 
northern or Etesian winds, which commence about May, or the period 
of the inundation, retard, he says, the downward freshes, and contri- 
bute materially to the accumulation of the mud upon the land, as well 
as to the silting up of the embouchures of the river, by raising the 
level of the Mediterranean along the coast, and checking the currents 
in the estuaries. Near the mouths of the Nile the mud is inter- 
mingled with marine sand, and contains Mediterranean species of 
Mollusca, associated with terrestrial and fluviatile remains. Ac- 
cording to Ehrenberg, the river-mud contains an immense number 
of infusoria, 
