Two generations of rock critics and fans have enshrined Astral Weeks as a sui generis work of wonder and borderline madness. It has a reputation as something of a sacred text of Rock and Roll Mystery, a journey into the mystic that has scores of devoted fans, but virtually no artistic heirs. IF ROCK HAS a canon, Van Morrison’s 1968 LP Astral Weeks contributes its gnostic gospels - a marginal, weird, eruptive text, full of leads for songs never sung, sermons never preached.