There is no reason to think that the terns Chaldaea had at anytime the extensive signification of Mesopotamia, much less that it applied to the entire flat country between the desert and the mountains.

Chaldaea was not the whole, but a part of, the great Mesopotamian plain; which was ample enough to contain within it three or four considerable monarchies.
According to the combined testimony of geographers and historians, Chaldaea lay towards the south, for it bordered upon the Persian Gulf; and towards the west, for it adjoined Arabia.

If we are called upon to fix more accurately its boundaries, which, like those of most countries without strong natural frontiers, suffered many fluctuations, we are perhaps entitled to say that the Persian Gulf 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.