on the south, the Tigris on the east, the Arabian desert on the west, and the limit between Upper and Lower Mesopotamia on the north, formed the natural bounds, which were never greatly exceeded and never much infringed upon.  These boundaries are for the most part tolerably clear, though the northern only is invariable.  Natural causes, hereafter to be mentioned more 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 Shat-el-Arab; and there is ample reason for believing that at the time when the first Chaldaean monarchy was established, the Persian Gulf reached inland, 120 or 130 miles further than at present.  We must deduct therefore from the estimate of extent grounded upon the existing state of things, a tract of land 130 miles long and some 60 or 70 broad, which has been gained from the sea in the course of about forty centuries.  This deduction will reduce Chaldaea to a kingdom of somewhat narrow limits; for it will contain no more than about 23,000 square miles.