In the tract of land lying between the lower course of the Great Zab River and the Tigris, in which was situated the important town of Calah (now Nimrud), a tract which is partly alluvial, but more generally of secondary formation, hard gravel, sandstone, or conglomerate, are the remains of a canal undoubtedly Assyrian, which was carried for a distance of more than five-and-twenty miles from a point on the Khazr or Ghazr Su, a tributary of the Zab, to the south-eastern corner of the Nimrud ruins. [PLATE CXXXIV., Fig. 1.] Originally the canal seems to have been derived from the Zab itself, the water of which was drawn off, on its northern bank, through a short tunnel--the modern Negoub--and then conducted along a cutting, first by the side of the Zab, and afterwards in a tortuous course across the undulating plain, into the ravine formed by the Shor-Derreh torrent. The Zab, when this part of the work was constructed, ran deep along its northern bank, and, sending a portion of its waters into the tunnel, maintained a constant stream in the canal. But after awhile the river abandoned its north bank for the opposite shore; and, water ceasing to flow through the Negoub tunnel, it became necessary to obtain it in some other way. Accordingly the canal was extended northwards, partly by cutting and partly by tunnelling to the Ghazr Su at about two miles above its mouth, and a permanent supply was thenceforth obtained from that stream. The work may have been intended in part to supply Calah with mountain water; but the remains of dams and sluices along its course sufficiently show that it was a canal for irrigation also. From it water was probably derived to fertilize the whole triangle lying south of Nimrud between the two streams, a tract containing nearly thirty square miles of territory, mostly very fertile, and with careful cultivation well capable of supporting the almost metropolitan city on which it abutted.