Plunge into a chasm
This could be a reference to Thomas Gray’s poem “The Bard”:
“… and headlong from the mountain’s height
Deep in the roaring tide he plung’d to endless night.”
There also are references to John Martin’s painting “The Bard” in Henry Holiday’s illustration to the last Snark chapter.
To help improve the meaning of these lyrics, visit "The Hunting of the Snark" by Lewis Carroll and leave a comment on the lyrics box