What kind of poem is "To Marguerite—Continued"?

Matthew Arnold's "To Marguerite--Continued" is a romantic
poem (both literally and figuratively), filled with passionate despair regarding the
sea's isolation, a metaphor for his possible relationship with the woman to whom it is
dedicated.

The poem title looks like a letter, or a
post-script of a former, longer letter.  Indeed, another poem "Isolation: To Marguerite"
precedes it:

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[Arnold] travelled to Switzerland in the course
of his duties. There, in 1848-49, he met the "Marguerite" who inspired "Isolation: To
Marguerite" and "To Marguerite--Continued." No one knows Marguerite's real identity,
although there is of course a great deal of speculation; some critics even insist that
Marguerite is an imaginary
figure.

Later, in 1851,
Arnold married someone else, so he did lose Marguerite, if she ever
existed.

The poem is heavy in personification, imagery, and
metaphor, as it likens humans to the seas:

Oh!
then a longing like despair

Is to their
farthest caverns sent;

For surely once,
they feel, we were

Parts of a single
continent!

Arnold says that modern man is like the
current watery world, isolated and disconnected.