thou wert

Along with the contracted form 'twere, the expression "thou wert" offers a striking instance of poetic language in "Scenes Beyond the Western Border" that is not in the style of Philip St George Cooke anywhere else in recorded history, but does conform to a poetic way of talking that Herman Melville adopted at need, repeatedly.



 The grammar explained:

Wert wẽrt The second person singular, indicative and subjunctive moods, imperfect tense, of the verb be. It is formed from were, with the ending -t, after the analogy of wast. Now used only in solemn or poetic style. --fine dictionary

Compare the apostrophe in Mardi to "Orienda," Melville's mythologized Near East:

Oh Orienda! thou wert our East, where first dawned song and science, with Mardi's primal mornings!  --Mardi, vol. 2, chapter 64

with the Captain's manner of apostrophizing the Sweetwater as the mythical river of forgetfulness:

Ah! not long, bright Sweet Water, did we refrain thy tempting embrace: Thou wert a Lethe to the desert behind; all illusion faded from the delightful realities of thy bath. 

--August 1852 Scenes Beyond the Western Border