This passage is quoted from William Wordsworth's poem Lines Written a Few Miles Above Tintern Abbey, published in the collection Lyrical Ballads in 1798. The poem was inspired by a ruined abbey located on the Welsh bank of the River Wye. Shelley gives another nod to the early-Romantics' importance for her own writing several pages later when, visiting the Cumberland Lake District, Victor suggestively describes meeting "some acquaintances, who almost contrived to cheat me into happiness" (p.134).
Online text of the poem
Online text of 'Recollections of the Lake Poets' by Thomas de Quincey, (1834-1840)