all the streamlets and rivers, like many of those in Tuscany and the
south of Italy, hold abundance of carbonate of lime in solution, and
precipitate travertin, or sometimes bind together the sand and gravel
into solid sandstones and conglomerates; every delta and sand-bar thus
acquires solidity, which often prevents streams from forcing their way
through them, so that their mouths are constantly changing their
position.[350]

_Delta of the Nile._--That Egypt was "the gift of the Nile," was the
opinion of her priests before the time of Herodotus; and Rennell
observes, that the "configuration and composition of the low lands leave
no room for doubt that the sea once washed the base of the rocks on
which the pyramids of Memphis stand, the _present_ base of which is
washed by the inundation of the Nile, at an elevation of 70 or 80 feet
above the Mediterranean. But when we attempt to carry back our ideas to
the remote period when the foundation of the delta was first laid, we
are lost in the contemplation of so vast an interval of time."[351]
Herodotus observes, "that the country round Memphis seemed formerly to
have been an arm of the sea gradually filled by the Nile, in the same
manner as the Meander, Achelous, and other streams, had formed deltas.
Egypt, therefore, he says, like the Red Sea, was once a long narrow bay,
and both gulfs were separated by a small neck of land. If the Nile, he
adds, should by any means have an issue into the Arabian Gulf, it might
choke it up with earth in 20,000 or even, perhaps, in 10,000 years; and
why may not the Nile have filled a still greater gulf with mud in the
space of time which has passed before our age?"[352]

The distance between Memphis and the most prominent part of the delta in
a straight line north and south, is about 100 geographical miles; the
length of the base of the delta is more than 200 miles if we follow the
coast between the ancient extreme eastern and western arms; but as these
are now blocked up, that part only of Lower Egypt which intervenes
between the Rosetta and Damietta branches, is usually called the delta,
the coast line of which is about 90 miles in length. The bed of the
river itself, says Sir J. G. Wilkinson, undergoes a gradual increase of
elevation varying in different places, and always lessening in
proportion as the river approaches the sea. "This increase of elevation
in perpendicular height is much smaller in Lower than in Upper Egypt,
and in th