I recently came across a particular essay by the author of The Last Ringbearer, which states:
The Middle Earth has several built-in physical defects, and there’s no getting away from that. In his well-known essay Must Fantasy Be Stupid? Pereslegin provides a detailed classification of errors commonly committed by fantasy authors. He uses Tolkien’s work as an example of one of them, an “irreversible professional error”: “It occurs in a geologically unstable world....”
(To explain: if a planet has a single continent – Middle Earth.... 
.... It turns out that all the seeming contradictions of Middle Earth’s natural history can be resolved with a single assumption: that Tolkien is describing only the northwestern part of the local landmass, rather than the whole thing.
Now, this assumption had always been fact to me, so much that I assumed that I definitely did read it somewhere in the letters / HoME etc. Can someone point me to a definite source of this info from Tolkien's own pen? And if possible, a date on when that info became publicly available?
ETA: The info in specific referring to anything Tolkien himself mentioned about
whether or not the lands that we see in the maps of ME drawn by Tolkien are all that exist in that world
any correlations to real world geography (that it is / is not supposed to be referring to Western Europe as per Tolkien's conceit of 'Middle Earth is our world in the mythic past')