mark, half way down the Mesopotamian lowland, by a difference of a geological structure, which is very conspicuous. Near Hit on the Euphrates, and a little below Samarah on the Tigris,[7] 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 new 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 neighbour."[8] Whether the two States had independent and separate life, or whether, as in after years, one of the two had, by its political and military superiority reduced the other to the condition of a vassal, the line of demarcation was constant, a line traced in the first instance by nature and rendered more rigid and ineffaceable by historical developments. Even when Chaldaea became nominally a mere province of Assyria, the two nationalities remained distinct. Chaldaea was older than Assyria. The centres of her civil life were the cities built upon the alluvial lands between the thirty-first and thirty-third degree of latitude. The most famous of these cities was Babylon. Those whom we call Assyrians, a people who rose to power and glory at a much more recent date, drew the seeds of their civilization from their more precocious neighbour. These expressions, Assyria and Chaldaea, are now employed in a sense far more precise than they ever had in antiquity. For Herodotus Babylonia was a mere district of Assyria;[9] in his time both States were comprised in the Persian Empire, and had no distinct existence of their own. Pliny calls the whole of Mesopotamia Assyria.[10] Strabo carries the western frontier of Assyria as far as Syria.[11] To us these variations are of small importance. The geographical and historical nomenclature of the ancients was never clearly defined. It was always more or less of a floating quantity, especially for those countries which to Herodotus or Diodorus, to Pliny or to Tacitus, were dimly perceptible on the extreme limits of their horizon.