Those walls have long ago disappeared. For many centuries their ruins afforded building materials for the inhabitants of the cities that have succeeded each other upon and around the site of ancient Babylon, and now their lines are only to be faintly traced in slight undulations of the ground, which are here and there hardly distinguishable from the banks that bordered the canals. But in those deserts of Lower Chaldaea, where the nomad tent is now almost the only dwelling, structures have been found but little damaged, in which layers of reeds placed at certain intervals among the bricks may be easily distinguished. As a rule three or four layers are strewn one upon the other, the rushes in one being at right angles to those above and below it. Here and there the stalks may still be seen standing out from the wall.[182] Fragments of bitumen are everywhere to be picked up among the _debris_ about these buildings, upon which it must have been used for mortar. It never seems to have been employed, however, over the whole of a building, but only in those parts where more than the ordinary cohesive power was required. Thus, at Warka, in the ruin called _Bouvariia_, the buttresses that stand out from the main building are of large burnt bricks set in thick beds of bitumen, the whole forming such a solid body that a pickaxe has great difficulty in making any impression upon it.[183] Travellers have also found traces of the same use of bitumen in the ruins of Babylon. It seems to have been in less frequent employment in Assyria. It has there been found only under the two layers of bricks that constitute the ordinary pavement of roofs, courts, and chambers. The architect no doubt introduced this coat of asphalte for two purposes--partly to give solidity to the pavement, partly to keep down the wet and to force the water in the soil to flow off through its appointed channels. A layer of the same kind was also spread under the drains.[184]