It is not necessary to suppose that the Medes ever apostatized altogether from the worship of Ormazd, or formally surrendered their Dualistic faith.

But, practically, the Magian doctrines and the Magian usages--elemental worship, divination with the sacred rods, dream expounding, incantations at the fire-altars, sacrifices whereat a Magus officiated--seem to have prevailed; the new predominated over the old; backed by the power of an organized hierarchy, Magism over-laid the primitive Arian creed, and, as time went on, tended more and more to become the real religion of the nation.
Among the religious customs introduced by the Magi into Media there are one or two which seem to require especial notice.

The attribution of a sacred character to the four so-called elements--earth, air, fire and water--renders it extremely difficult to know what is to be done with the dead.

They cannot be burnt, for that is a pollution of fire; or buried, for that is a pollution of earth; or thrown into a river, for that is a defilement of water.

If they are deposited in sarcophagi, or exposed, they really pollute the air; but in this case the guilt of the pollution, it may be argued, does not rest on man, since the dead body is merely left in the element in which nature placed it.