Campbell (p. 52) continues:
What Cosmas signifies by placing Paradise across the Ocean Stream, other writers and theorists would signify by surrounding it with walls of flame and/ or impenetrable rock, or with the "Vale Perilous," or by situating it on an unscalable height to which even the waters of the Flood had not reached. A surprising number of itineraria and cosmographiae end with the description of one or another "burning hill" (and "burning hills" becomes thus representative of the contents of such books in one Renaissance cosmographer's remarks).