The Zend Avesta, Part I
Translated by James Darmesteter
The texts of the Avesta - which are all in the Avestan language - were collated over several hundred years. The most important portion, the Gathas, in Gathic Avestan, are the hymns thought to have been composed by Zarathushtra (Zoroaster) himself, and date linguistically to around 1000 [BC]. The liturgical texts of the Yasna, which includes the Gathas, is partially in Older and partially in Younger Avestan. The oldest portions may be older than the Gathas, later adapted to more closely follow the doctrine of Zoroaster. The various Yashts are in Younger Avestan and thought to date to the Achaemenid era (559-330 [BC]). The Visperad and Vendidad, which are also in Younger Avestan, were probably composed even later but this is not certain.