The ruins of which I have here given a brief account are of the first importance for the elucidation of the early history of the arts of Islâm. They can all be dated within a period of forty years falling in the middle of the ninth century, and are therefore among the earliest existing examples of Mohammadan architecture. They bear witness to the Mesopotamian influences under which it arose. The spiral towers of Sâmarrâ and Abu Dulâf[141] are an adaptation of the temple pyramids of Assyria and Babylonia which had a spiral path leading to the summit; the technique of arch and vault was invented by the ancient East and transmitted through Sassanian builders to the Arab invaders; the decoration is Persian or Mesopotamian and almost untouched by the genius of the West.[142] In the palaces and mosques of Sâmarrâ, we can see the conquerors themselves conquered by a culture which had been developing during thousands of years on Mesopotamian soil, a culture which had received indeed new elements into its composition, which had learnt from the Greek and from the Persian, but had maintained in spite of all modifications its distinctive character. Side by side with Sâmarrâ stand the ruins at Raḳḳah, where the mosque repaired by Nûr ed Dîn probably preserves a plan which can be dated even earlier than the two mosques on the Tigris; and finally the scheme and decoration of the Mesopotamian mosque is reproduced with certain variations in the latter half of the ninth century by Ibn Ṭûlûn, and the last descendant of the Babylonian zigurrat is the minaret of his mosque at Cairo.[143] CHAPTER VII MÔṢUL TO ZÂKHÔ _April 28--May 10_