Consist strictly of four hymn books: the Rig (prayers in verse), Sama (musical melodies), Yajur (prose prayers), and Atharva (spells and incantations). Each Veda, though, also contains a Brahmana (interpretation), and the Vedas also incorporate treatises on meditation (Aranyakas) as well as the Upanishads. Written in an archaic form of Sanskrit by early Aryan invaders, possibly between 1500 and 1200 BC, the Vedas concentrate on sacrifices to deities, such as Indra (god of thunder), Varuna (cosmic order), and Agni (fire). The major gods Vishnu and Shiva appear as minor deities in the Vedas; their elevation, as well as the concept of karma, does not develop until the Upanishads.