The speaker of "Splendour Falls" is alone in the mountains, looking across the valley at sunset and describing the scene. It's gorgeous, but there is something about this particular scene of nature that is especially evocative. It reminds the speaker of fairy tales, of the past, of ancient history and mythology, and eventually of our lives' legacies after death. Woah. That's a lot of power, even for a beautiful valley!
Put your thinking caps on. In "The Splendour Falls," the grandeur of nature makes the speaker think about major, capital-letter ideas like Philosophy, History, Mythology, and the Future.
The incredible shrinking speaker—the distance across the valley in "The Splendour Falls" makes the speaker feel suddenly tiny and insignificant in the scope of history.