The chief natural division of the country is that made by the Tigris, which, having a course nearly from north to south, between Til and Samarah, separates Assyria into a western and an eastern district. Of these two, the eastern or that upon the left bank of the Tigris, although considerably the smaller, has always been the more important region. Comparatively narrow at first, it broadens as the course of the river is descended, till it attains about the thirty-fifth parallel a width of 130 or 140 miles. It consists chiefly of a series of rich and productive plains, lying along the courses of the various tributaries which flow from Mount Zagros into the Tigris, and often of a semi-alluvial character. These plains are not, however, continuous. Detached ranges of hills, with a general direction parallel to the Zagros chain, intersect the flat rich country, separating the plains from one another, and supplying small streams and brooks in addition to the various rivers, which, rising within or beyond the great mountain barriers, traverse the plains on their way to the Tigris. The hills themselves--known now as the Jebel Maklub, the Ain-es-sufra, the Karachok, etc.--are for the most part bare and sterile. In form they are hogbacked, and viewed from a distance have a smooth and even outline but on a nearer approach they are found to be rocky and rugged. Their limestone sides are furrowed by innumerable ravines, and have a dry and parched appearance, being even in spring generally naked and without vegetation. The sterility is most marked on the western flank, which faces the hot rays of the afternoon sun; the eastern slope is occasionally robed with a scanty covering of dwarf oak or stunted brushwood. In the fat soil of the plains the rivers commonly run deep and concealed from view, unless in the spring and the early summer, when through the rains and the melting of the snows in the mountains they are greatly swollen, and run bank full, or even overflow the level country.