A man walks into a river and the water makes way for him. That is the whole event of these lines, but Shelley turns it into something close to a vision. The title tells us the passage comes from Virgil's Fourth Georgic, where the visitor is Aristaeus, descending to his mother, the water-nymph Cyrene. Shelley's version never names him. He is simply "him", and the "great Mother" rules a "humid reign" that no human has entered before.
The opening image does something odd with physics. The "cloven waters" become "a chasm of mountains" and then a "mighty portal". Liquid has turned into rock and architecture, as though the sea were building a gateway for its guest. Once inside, the world is defined mostly by what it lacks. The path is "untrampled", the groves are "profaned not by the step of mortal", and the lakes are ones "which rain / Replenished not". Even the rain is unnecessary here, because everything is fed from below. The place feels sacred because it has been left untouched.
The form suits this journey. The poem is in terza rima, the interlocking pattern aba bcb cdc, in which the middle line of each three-line stanza supplies the rhyme for the outer lines of the next. Nothing ever quite closes, so the verse keeps pulling forward like a current. Shelley's rhymes also carry meaning. In the second stanza, "immortal" and "mortal" rhyme with "portal" from the first, so the human and the divine are literally bound together by sound, just as the traveller is crossing between them. The sentences refuse to stop at the stanza breaks as well. "Lakes which rain" ends one stanza's line and "Replenished not" begins the next, so the syntax pours over the gap. The rhythm is loosely iambic pentameter, and many lines end on an extra unstressed syllable ("portal", "mortal", "fountains"), which gives them a trailing, liquid softness.
Then comes the traveller's reaction: "'Wildered", and a few lines later, "Half 'wildered". The clipped, apostrophe-marked word is repeated, and the reader starts to feel dizzy too. What bewilders him is a catalogue of rivers. Phasis, Lycus, Enipeus, Tyber, Caicus, Hypanis and Eridanus come from far-apart places, yet here they are gathered under one roof of earth. The poem is imagining the hidden sources of all the world's rivers, and its language is of family and birth: "old Enipeus has its birth", "father Tyber", Caicus "comes forth". The cavern is a nursery of water.
It is also a court. Words like "portal", "reign", "empire's sign" and "power" gather into a vocabulary of kingship. Eridanus, with "two golden horns upon thy taurine brow", is pictured as a bull-headed monarch wearing his emblem of rule. The final lines turn from admiring him to measuring his force: no stream "with fiercer power" bursts "on the purple brine". The verb echoes the earlier "bursting waves", so the passage is framed by the same violent word. All that pent-up energy, hidden under the earth, is released outward into the open sea.
The text as we have it is clearly unfinished. There is a missing word in "the ... sand paves" and a query beside "Anienas[?]". Judging by those gaps, it reads like a working draft rather than a polished piece. That is part of its interest, because we can see a translator still choosing. The Latinate diction ("humid", "profaned", "taurine") keeps something of Virgil's weight, while terza rima, a form associated with Italian poetry, gives the English its sense of continuous flow.
The poem matters less as a finished statement than as a demonstration. It shows how a Romantic poet could take an ancient scene of descent and wonder and make the sound of the verse do what the story describes. The reader is carried through the rhymes much as the man is carried through the water.