.

In many places, as at Cairo, where artificial excavations have been
made, or where the river has undermined its banks, the mud is seen to be
thinly stratified, the upper part of each annual layer consisting of
earth of a lighter color than the lower, and the whole separating easily
from the deposit of the succeeding year. These annual layers are
variable in thickness; but, according to the calculations of Girard and
Wilkinson, the mean annual thickness of a layer at Cairo cannot exceed
that of a sheet of thin pasteboard, and a stratum of two or three feet
must represent the accumulation of a thousand years.

The depth of the Mediterranean is about twelve fathoms at a small
distance from the shore of the delta; it afterwards increases gradually
to 50, and then suddenly descends to 380 fathoms, which is, perhaps, the
original depth of the sea where it has not been rendered shallower by
fluviatile matter. We learn from Lieut. Newbold that nothing but the
finest and lightest ingredients reach the Mediterranean, where he has
observed the sea discolored by them to the distance of 40 miles from the
shore.[355] The small progress of the delta in the last 2000 years
affords, perhaps, no measure for estimating its rate of growth when it
was an inland bay, and had not yet protruded itself beyond the
coast-line of the Mediterranean. A powerful current now sweeps along the
shores of Africa, from the Straits of Gibraltar to the prominent
convexity of Egypt, the western side of which is continually the prey of
the waves; so that not only are fresh accessions of land checked, but
ancient parts of the delta are carried away. By this cause, Canopus and
some other towns have been overwhelmed; but to this subject I shall
again refer when speaking of tides and currents.

CHAPTER XVIII.

REPRODUCTIVE EFFECTS OF RIVERS--_continued_.

Deltas formed under the influence of tides--Basin and delta of the
Mississippi--Alluvial plain--River-banks and bluffs--Curves of the
river--Natural rafts and snags--New lakes, and effects of
earthquakes--Antiquity of the delta--Delta of the Ganges and
Brahmapootra--Head of the delta and Sunderbunds--Islands formed and
destroyed--Crocodiles--Amount of fluviatile sediment in the
water--Artesian boring at Calcutta--Proofs of subsidence--Age of the
delta--Convergence of deltas--Origin of existing deltas not
contemporaneous--Grouping of strata and stratification in