e delta it diminishes still more; so that, according to an
approximate calculation, the land about Elephantine, or the first
cataract, lat. 24 degrees 5 minutes, has been raised nine feet in 1700
years; at Thebes, lat. 25 degrees 43 minutes, about seven feet; and at
Heliopolis and Cairo, lat. 30 degrees, about five feet ten inches. At
Rosetta and the mouths of the Nile, lat. 31 degrees 30 minutes, the
diminution in the perpendicular thickness of the deposit is lessened in
a much greater decreasing ratio than in the straitened valley of Central
and Upper Egypt, owing to the great extent, east and west, over which
the inundation spreads."[353]

For this reason the alluvial deposit does not cause the delta to
protrude rapidly into the sea, although some ancient cities are now a
mile or more inland, and the mouths of the Nile, mentioned by the
earlier geographers, have been many of them silted up, and the outline
of the coast entirely changed.

The bed of the Nile always keeps pace with the general elevation of the
soil, and the banks of this river, like those of the Mississippi and its
tributaries (see p. 265), are much higher than the flat land at a
distance, so that they are seldom covered during the highest
inundations. In consequence of the gradual rise of the river's bed, the
annual flood is constantly spreading over a wider area, and the alluvial
soil encroaches on the desert, covering, to the depth of six or seven
feet, the base of statues and temples which the waters never reached
3000 years ago. Although the sands of the Libyan deserts have in some
places been drifted into the valley of the Nile, yet these aggressions,
says Wilkinson, are far more than counterbalanced by the fertilizing
effect of the water which now reaches farther inland towards the desert,
so that the number of square miles of arable soil is greater at present
than at any previous period.

_Mud of the Nile._--On comparing the different analyses which have been
published of this mud, it will be found that it contains a large
quantity of argillaceous matter, with much peroxide of iron, some
carbonate of lime, and a small proportion of carbonate of magnesia. The
latest and most careful analysis by M. Lassaigne shows a singularly
close resemblance in the proportions of the ingredients of silica,
alumina, iron, carbon, lime, and magnesia, and those observed in
ordinary mica;[354] but a much larger quantity of calcareous matter is
sometimes present