jdworth is right. "The Uncharted Isle" is reminiscent of Poe's "Ms Found in a Bottle". I read the two stories within two months of each other a couple of years ago and literally noted that. And then my sieve-like memory forgot.
Scott's right about Burroughs and Flammarion. [
www.erbzine.com], a survey of ERB's library, shows he was pretty familiar with Flammarion.