Dizzy Ravine! and when I gaze on thee
I seem as in a trance sublime and strange
To muse on my own separate fantasy,
My own, my human mind, which passively
Now renders and receives fast influencings,
Holding an unremitting interchange
With the clear universe of things around;
One legion of wild thoughts, whose wandering wings
Now float above thy darkness, and now rest
Where that or thou art no unbidden guest.

Percy Bysshe Shelley's poem is about the highest mountain in the Alps, Mont Blanc, and its surrounding scenery. The landscape is pretty stunning, and in these lines, Shelley is describing the effect it has on him.

Looking down on a ravine, the speaker of this poem is taken into that exalted state: the sublime. Shelley's description shows us that actually the sublime is about an interaction between nature and the mind. It isn't just one thing and the other; it's both: "My own, my human mind…/ Holding an unremitting interchange/ With the clear universe of things around." The sublime is both outside and inside the speaker.

And as usual, the sublime takes the speaker to a crazy place. He describes his thoughts as "wild thoughts" with "wandering wings." The sublime is intense, people. And we can see that in the speaker's description of his state here.

The sublime is about this interaction between the mind and the external environment. The phrase "[d]izzy ravine!" sums up that interaction exactly. It's not the ravine that's dizzy of course; it's the speaker who gets dizzy as he looks at the ravine. But he projects his own perception or emotion onto the ravine. In the phrase, "[d]izzy ravine," we see this combination of the mind working on the external environment, and vice versa.