Meaning and Nothingness

So I started rereading my old posts, going back to the beginning. And then I reread "Out of the Spent and Unconsidered Earth" and came to the end. The part where the narrator realizes that nothing was real except the City.

Does it cause hallucinations? Are the shadows -- the Nightlanders, as I call them now, thank you, William Hope Hodgson -- part of it? Are they a symptom of the City? Or does the City just target people who are already mentally unstable? Fuck it, I don't know.

But the second-to-last post, before that R Kipling realized he was crazy, was this:

Reality is that which, when you stop believing in it, doesn't go away. 

Majoresque cadunt altis de montibus umbræ.

What does that even mean? And the Latin...I recognize "umbrae." That means shadow. What does the rest of it mean?