A early idea. In Tolkien's "latter" works, Tolkien held that Arda had always been round, rather then it being made round after the fall of Númenór.
Many of Tolkien's BoLT ideas were not incorporated into his works. They served (in the end) as a beginning. BoLT is a lot more fantastical and archaic then his later works. It could be that that the BoLT mythology was more influenced by other mythologies then his latter works.
Curiosity is insubordination in its purest form-Vladimir Nabokov
Do not read as children do to enjoy themselves, or, as the ambitious do to educate themselves. No, read to live. -Gustave Flaubert
We are not provided with wisdom, we must discover it for ourselves, after a journey through the wilderness which no one else can take for us, an effort which no one can spare us.-Marcel Proust