"a ceaseless Spectacle of Transition"

Endings get interesting in late Pynchon, starting with the bizarre funding-cut deus ex in Vineland.  M&D has a few different endings, including the sentimental final pages.  But the one I like most is the fantasy that, for project #2, the Ley-borne boys will "inscribe a Visto on the Atlantick Sea" (712).  First it solves longitude & navigational problems, then it gets co-opted by real estate developers and other disreputable capitalists: "Too soon, word will reach the Land-Speculation Industry, and its Bureaus seek Purchase, like some horrible Seaweed, the length of the Beacon Line" (712).  Love that image of seaweed as industrial predation...

But the close of this great mini-scene is M&D on this new salty Line --

'Tis here Mason and Dixon will retire, being after all Plank-Holders of the very Scheme, having written a number of foresighted Stipulations into their Contract with the Line's Proprieter, the transnoctially charter'd 'Atlantick Company.'  Betwixt themselves, neither feels British enough anymore, nor quite American, for either Side of the Ocean.  They are content to reside like Ferrymen or Bridge-keepers, ever in Ubiquity of Flow, before a ceaseless Spectacle of Transition.  (713)

Great stuff there -- political / national refusal, an ability to play capitalism's structures against itself, a commitment to "flow" and "transition."

Is St Brenden's Isle an underground place, in the terms you're putting together?

I used some quotations from this section in an article I wrote last year on dolphins and humans in Shakespeare & other 16-17c writers (out next year, perhaps, in a collection called The Indistinct Human).  But really it's part of a larger project, about fantasies of human-ocean mixing and collaboration.  There might be a chapter on Pynchon's ocean someday...