The breadth of the Mesopotamian Valley affected also the building materials and the character of the art. Stone was much more difficult to obtain than in Egypt. Clay only was abundant. All buildings were consequently of brick. These structures were far less enduring than those in Egypt; their upper parts have disintegrated and buried the lower portions. Babylonian ruins are accordingly all under ground. The abundant clay was also used by the Babylonians as writing material. When baked, it proved far more enduring than the Egyptian papyrus. Thus, notwithstanding the general similarities which the Mesopotamian Valley presents to Egypt, its differences profoundly affected Babylonian history and Babylonian art. =2. The Preservation of Antiquities.=--Babylonian cities were usually built on terraces of brick. The walls of the cities and their buildings were constructed of the same material. Refuse from the houses in these towns was always thrown out into the streets, so that, as the centuries passed, the streets were gradually elevated. The walls of the brick houses gradually became unstable in the lapse of time, and as the houses were repaired they were brought up to the level of the street. Consequently even in peaceful times the mounds on which the cities were built gradually grew higher. Most of these cities were at various times destroyed in warfare. Sometimes all the houses would be partially demolished and the site would be for a time practically uninhabited. When at length the place was repeopled, the top of the mound would be smoothed off and taken as the base of a new city. In this way through the many centuries of Babylonian history the sites of her cities have become great mounds. When these cities finally fell into ruin, the clay of the upper part of the walls gradually disintegrated in the weather and formed a coating of earth over the whole, which preserved the foundations of the walls both of cities and houses, as well as the inscribed clay, stone tablets, and the works of art buried underneath. Connected with each Babylonian and Assyrian temple was a kind of staged tower, shaped in a general way like the stepped pyramid of Zoser at Sakkarah in Egypt. The Babylonians called these towers _Ziggurats_. As the bricks of these towers decayed, they formed in connection with the city mound a kind of hillock or peak, which varied in accordance with the height of the tower. The ruin of the _Ziggurat_ at Birs Nimrûd, the ancient Borsippa, is one of the most imposing to be seen in ancient Babylonia; it was long thought to be the original of the Tower of Babel (Gen. 11:9). It thus came about that no ancient temple of Babylonia, like some of those in Egypt, has remained above ground. Explorers have had to dig to discover antiquities; (see Fig. 22).