Letter to Charles K. Newcomb
p77“from the brain of the purple mountain" flowed forth cheer to my somewhat weary mind”
Fuller, Margaret. The Letters of Margaret Fuller. Edited by Robert N. Hudspeth. Vol. III. Ithaca: Cornell University Press, 1986.
Inferred Hudspeth identified the source as Tennyson's "The Poet's Mind" from Poems (1:51), and Fuller quotes the exact phrase "brain of the purple mountain" from the poem's second stanza.