So, one of the interesting things I discovered in working on MR. BAGGINS was that "The Lonely Mountain" was not Tolkien's original name for the place that had held the Kingdom Under the Mountain. In fact, as the original manuscript page makes clear (Marq. 1/1/22:4), Tolkien originally wrote "the Black mountain", then at some point cancelled the word "Black" and also at some point (not necessarily the same time) capitalized "Mountain".
Now, in itself this is simply another example of Tolkien changing his mind about the proper name of a person or place within the tale, alongside Medwed > Beorn, Gandalf > Thorin, and Bladorthin > Gandalf. But just this week while looking something up in THE SHAPING OF MIDDLE-EARTH, I was reminded that this is not the only place where Tolkien used the name:
"Morgoth flees from Valinor . . . and returns to the Northern World and rebuilds his fortress of Angband beneath the Black Mountain, Thangorodrim." (Annals of Beleriand, first sentence; HME.IV.295).
This is made all the more interesting by the fact that these two references are more or less contemporary with each other. Tolkien began work on THE HOBBIT in the summer of 1930, according to our best evidence, and the page naming The Black Mountain as the goal of their quest belongs to that very first layer, one of the three surviving sheets of what I have named 'The Pryftan Fragment'. And while we don't know the exact date of the "(Earliest) Annals of Beleriand", they were written to accompany the 1930 Quenta, so they must date from 1930 or very shortly thereafter.
While I don't think this means that the dwarves and wizard were inviting Bilbo along for a raid on the ruins of Thangorodrim, it is interesting to see Tolkien try out a name or idea in two entirely different contexts at about the same time (and a warning that not all places which share the same name are necessarily the same place). More interesting still, even after the lost dwarvenhome became 'the Lonely Mountain' some connection between the two remained in Tolkien's mind, some mental image: compare his depictions of The Lonely Mountain in the painting "The Death of Smaug" with his drawing of Thangorodrim in "Tol Sirion" (PICTURES BY TOLKIEN, plate 36; see especially the original pencil sketch on the left-hand page of this spread).
Was there ever a writer so adept are adapting and re-using odds and ends from earlier works into later parts of his ongoing mythology?