In Assyria, on the other hand, stone was easily procurable and therefore readily used, though not to the extent one would expect, the reason being that the Assyrian was not an inventor but an imitator of his predecessor, the Babylonian, who afforded him little or no example in the working of stone. Accordingly even in Assyria, stone was for the most part used only for pavements, plinths and the lining of walls: at times however it was also used for the retaining walls which enclosed an artificial mound. The blocks of stone used for this latter purpose were sometimes of colossal size, measuring even as much as 6 × 6 × 9 feet and weighing some tons. The principal kinds of stone employed by the Assyrian architects were limestone, of varying degrees of hardness, and alabaster, which latter is often found in Assyria itself a little below the surface of the soil. Alabaster is a sulphate of chalk, it is grey in colour, soft, and admits of a high polish, but it is brittle and deteriorates in course of time. At Nimrûd (Calah) some of the drainage channels were covered with large slabs of limestone, and the ziggurat of Nimrûd, of which only one storey remains, was faced with a massive stone revetment wall, while occasionally stone columns appear to have been used, and one part of a column composed of carved limestone, some forty inches high and including both the capital and the upper part of the shaft in one piece has been actually discovered. Layard further found four bases of columns made of limestone, on the northern side of Sennacherib’s palace at Nineveh (cf. Fig. 14). Sometimes the lintels of doors were made of stone; one such stone lintel was found by George Smith at the entrance to the hall in Sennacherib’s palace, while the sill or threshold generally, or at all events very frequently, consisted of alabaster or limestone. Similarly the floors of the more important rooms were formed of limestone-slabs.