We are told that there are certain expressions in the Assyrian language which lead to the belief that the earliest writing was on the bark of trees, that it offered the first surface to the scribe in those distant northern regions from which the early inhabitants of Chaldaea were emigrants. It is certain that the dwellers in that vast alluvial plain were compelled by the very nature of the soil to use clay for many purposes to which no other civilization has put it. In Mesopotamia, as in the valley of the Nile, the inhabitants had but to stoop to pick up an excellent modelling clay, fine in texture and close grained--a clay which had been detached from the mountain sides by the two great rivers, and deposited in inexhaustible quantities over the whole width of the double valley. We shall see hereafter what an important part bricks, crude, fired, and enamelled, played in the construction and decoration of Chaldaean buildings. It was the same material that received most of their writing. Clay offered a combination of facility with durability which no other material could equal. While soft and wet it readily took the shape of any figure impressed upon it. The deftly-handled tool could engrave characters upon its yielding surface almost as fast as the reed could trace them upon papyrus, and much more rapidly than the chisel could cut them in wood. Again, in its final condition as solid terra-cotta, it offered a chance of duration far beyond that of either wood or papyrus. Once safely through the kiln it had nothing to fear short of deliberate destruction. The message intrusted to a terra-cotta slab or cylinder could only be finally lost by the reduction of the latter to powder. At _Hillah_, the town which now occupies a corner of the vast space once covered by the streets of Babylon, bricks are found built into the walls to this day, upon which the Assyrian scholar may read as he runs the royal style and titles of Nebuchadnezzar.[48] As civilization progressed, the dwellers upon the Persian Gulf felt an ever-increasing attraction towards the art of writing. It afforded a medium of communication with distant points, and a bond of connection between one generation and another; by its means the son could profit by the accumulated experience of the father. The slab of terra-cotta was the most obvious material for its reception. It cost almost nothing, while such an elaborate substance as the papyrus of Egypt can never have been very cheap. It lent itself kindly to the service demanded of it, and the writer who had confided his thoughts to its surface had only to fire it for an hour or two to secure them a kind of eternity. This latter precaution did not require any very lengthy journey; brick kilns must have blazed day and night from one end of Chaldaea to another.