since Tolkien considered it a mythology, he approached it with a gravitas that other authors may not. And I would suggest that LOTR is not the mythology, but is a story built upon the mythology. He had already created a whole world of thousands of years of history before he ever started the Hobbit.

Remember that LOTR and the Hobbit are supposed to be the written works of Bilbo and Frodo as the "Red book of Westmarch." This make the two authors scholars of the ancient tales. Bilbo routinely translates tales he hears in Rivendell from Elvish to the common speech.

By the way, this summer a new book length version of "The Fall of Gondolin" will appear on bookshelves. I'm looking forward to it as it is one of my favorite tales of the First Age. (https://www.theguardian.com/books/2018/apr/10/the-fall-of-gondolin-new-jrr-tolkien-book-to-be-published-in-2018). A bit of trivia ties this story to later events: In the Hobbit, Elrond tells them that Bilbo's sword "Sting" was forged in the Realm of Gondolin (as was Gandalf's sword.)