By the end of the decade a distinct genre of hard rock was emerging with bands like Led Zeppelin, who mixed the music of early rock bands with a more hard-edged form of blues rock and acid rock on their first two albums Led Zeppelin (1969) and Led Zeppelin II (1969), and Deep Purple, who began as a progressive rock group but achieved their commercial breakthrough with their fourth and distinctively heavier album, In Rock (1970). Who's lyrics contained elements that warned against the occult?