In spite of their apparent variety, all the buildings we shall describe in the present chapter may be referred to a single fundamental type. They are each formed of several cubic masses superimposed one upon another and diminishing in volume in proportion to their height in the monument. We have already explained how such a system came to be adopted.[452] It was determined by the limitations of the only material at the architect's disposal, and it had at least this advantage, that it enabled him to relieve the monotony of the Chaldaean plains with artificial mountains whose vast size and boldness of line were calculated to impress the minds of the people, and to give them a great idea of their master's power and of the majesty of the deities in whose honour they were raised. [Illustration: FIG. 168.--View of the Birs Nimroud; after Felix Thomas.] Mesopotamia was covered, then, by buildings resembling a stepped pyramid in their general outlines. We find them in the reliefs (Fig. 10), and in the oldest cities we can frequently recognize the confused ruins of their two or three lower stories. Our only doubt is connected with the possible use of these buildings, the _zigguratts_ of the Assyrian texts. We shall not here stop to recapitulate the evidence in favour of their religious character; it will suffice to quote the description given by Herodotus of the temple of Bel or Belus at Babylon. As to whether the ruins of that building are to be identified with _Babil_ (Fig. 37) or the _Birs-Nimroud_ (Fig. 168) we shall inquire presently. This is the description of Herodotus:-- "In the other (fortress) was the sacred precinct of Jupiter Belus, a square inclosure two furlongs each way with gates of solid brass; which was also remaining in my time. In the middle of the precinct there was a tower of solid masonry, a furlong in length and breadth, upon which was raised a second tower, and on that a third, and so on up to eight. The ascent to the top is on the outside, by a path which winds round all the towers. When one is about half way up one finds a resting-place and seats, where persons are wont to sit some time on their way to the summit. On the topmost tower there is a spacious temple, and inside the temple stands a couch of unusual size, richly adorned with a golden table by its side. There is no statue of any kind set up in the place nor is the chamber occupied of nights by any one but a single native woman.... Below in the same precinct there is a second temple, in which is a sitting figure of Jupiter all of gold ... outside the temple are two altars."[453]