The Birket-el-Keroun, or Lake Moaris of the classical writers, is a natural basin—­not, as Herodotus imagined, an artificial one—­situated on the western side of the Nile valley, in a curious depression which nature has made among the Libyan hills.  This depression—­the modern district of the Faioom—­is a circular plain, which sinks gradually towards the north-west, descending till it is more than 100 feet below the surface of the Nile at low water.  The Northern and northwestern portion of the depression is occupied by the lake, a sheet of brackish water shaped like a horn (whence the modern name) measuring about thirty-five or thirty-six miles from end to end, and attaining in the middle a width of between five and six miles.  The area of the lake is estimated roughly at 150 square miles, its circumference at about ninety miles.  It has a depth varying from twelve to twenty-four feet.  Though the water is somewhat brackish, yet the Birket contains several species of fresh-water fish; and in ancient times its fisheries are said to have been exceedingly productive.

The principal cities of the Empire were, besides Pesargadae and Persepolis, Susa—­the chief city of Susiana—­which became the capital; Babylon, Ecbatana, Rhages, Zadracarta, Bactra (now Balkh), Maracanda (now Samarcand), Aria, or Artacoana (Herat), Caspatyrus on the Upper Indus,Taxila (Attock?), Pura (perhaps Bunpoor), Carmana (Kerman), Arbela, Nisibis, Amida (now Diarbekr); Mazaca in Cappadocia; Trapezus (Trebizond), Sinope, Dascyleium, Sardis, Ephesus, Miletus, Gordium, Perga, and Tarsus in Asia Minor:  Damascus, Jerusalem, Sidon, Tyre, Azotus or Ashdod, and Gaza in Syria; Memphis and Thebes in Egypt; Cyrene and Barca in the Cyrenaica.  Of these, while Susa had from the time of Darius Hystaspis a decided pre-eminence as the main residence of the court, and consequently as the usual seat of government, there were three others which could boast the distinction of being royal abodes from time to time, either regularly at certain seasons, or occasionally at the caprice of the monarch.  These were Babylon, Ecbatana, and Persepolis, the capitals respectively of Chaldaea, Media, and Persia Proper, all great and ancient cities, accustomed to the presence of Courts, and all occupying positions sufficiently central to render them not ill-suited for the business of administration.  Next to these in order of dignity may be classed the satrapial residences, often the chief cities of old monarchies, such as Sardis, the capital city of Lydia, Dascyleium of Bithynia, Memphis of Egypt, Bactra of Bactria, and the like; while the third rank was held by the towns, where there was no Court, either royal or satrapial.