particularly, are perpetually varying the course of the Tigris, the shore
of the Persian Gulf, and the line of demarcation between the sands of
Arabia and the verdure of the Euphrates valley. But nature has set a
permanent mark, half way down the Mesopotamian lowland, by a difference
of geological structure, which is very conspicuous. Near Hit on the
Euphrates, and a little below Samarah on the Tigris, the traveller who
descends the streams, bids adieu to a somewhat waving and slightly
elevated plain of secondary formation, and enters on the dead flat and
low level of the mere alluvium. The line thus formed is marked and
invariable; it constitutes the only natural division between the upper
and lower portions of the valley; and both probability and history point
to it as the actual boundary between Chaldaea and her northern neighbor.

The extent of ancient Chaldaea is, even after we have fixed its
boundaries, a question of some difficulty. From the edge of the alluvium
a little below Hit, to the present coast of the Persian Gulf at the mouth
of the Shat-el-Arab, is a distance of above 430 miles; while from the
western shore of the Bahr-i-Nedjif to the Tigris at Serut is a direct
distance of 185 miles. The present area of the alluvium west of the
Tigris and the Shat-el-Arab maybe estimated at about 30,000 square miles.
But the extent of ancient Chaldaea can scarcely have been so great. It
is certain that the alluvium at the head of the Persian Gulf now grows
with extraordinary rapidity, and not improbable that the growth may in
ancient times have been even more rapid than it is at present. Accurate
observations have shown that the present rate of increase amounts to as
much as a mile each seventy years, while it is the opinion of those best
qualified to judge that the average progress during the historic period
has been as much as a mile in every thirty years! Traces of
post-tertiary deposits have been found as far up the country as Tel
Ede and Hammam, 10 or more than 200 miles from the embouchure of the