Egypt, historically the oldest of countries, is geologically the youngest. It consists entirely of the soil deposited in comparatively recent times by the Nile. The triangle of the Delta marks the site of the ancient mouth of the river; and though the land has encroached upon the sea but slightly since the age of the Pharaohs, its height has year by year been slowly increasing. Some of the mouths of the river which were navigable streams in classical times have now ceased to be so; the Serbonian lake has in part become dry land, while desolate marshes are now cultivated fields. To the south of the Delta,—with the exception of the Fayûm, which owes its fertility to the canal called Bahr Yusûf, the former feeder of Lake Mœris,—Egypt is confined to the narrow strip of mud which lines both sides of the river, and is bounded by low hills of limestone, or the shifting sands of the desert. The Nile now flows for 1600 miles without receiving a single tributary ; the heated deserts on either bank absorb all the moisture of the air, and almost wholly prevent a rainfall, and it is consequently only where the waters of the river extend during the annual inundation, or where they can be dispersed by artificial irrigation, that cultivation and settled life are possible.