It is a decasyllabic line, with a trochee substituted for an iambus in the third foot-Around : me gleamed : many a : bright se : pulchre.
"The Complete Poetical Works of Percy Bysshe Shelley Volume III"
Percy Bysshe Shelley Edited by Thomas Hutchinson, M. A.
If so, he must have been one of the first of English poets to adopt the very loose enjambed decasyllabic couplet in which his work, like that of Marmion and still more Chamberlayne, is written.
"A History of English Literature Elizabethan Literature"
George Saintsbury
The Chanson contains 3500 lines, dates probably from the twelfth century, and is written, like Roland, in decasyllabic verse, but, unlike Roland, has a shorter line of six syllables and not assonanced at the end of each stanza.
"A Short History of French Literature"
George Saintsbury