Volume 4 is a survey of every aspect of the civilisations which flourished in the Iranian region from the Arab conquests to the Saljuq expansion: in particular, it studies the gradual transition of Iran from Zoroastrianism to Islam, the uniting of all Iranians under one rule, the flowering into full magnificence of the Persian language, and the establishment of those other acts which were to flourish so brilliantly after the Mongol conquest. The volume as a whole provides a comprehensive record of the formative centuries of Islam in Iran.