You who celebrate bygones,
Who have explored the outward, the surfaces of the races, the life
that has exhibited itself,
Who have treated of man as the creature of politics, aggregates,
rulers and priests,
I, habitan of the Alleghanies, treating of him as he is in himself
 in his own rights,
Pressing the pulse of the life that has seldom exhibited itself,
(the great pride of man in himself,)
Chanter of Personality, outlining what is yet to be,
I project the history of the future.
The speaker is saying that, unlike historians, he’s treating the humankind in it’s own complexity, skipping time and space and any historical coordinate that consists in the “surfaces of races”. 
 He wants to freely explore the human being and discover the powerful energy that moves all about our life, “pressing” it. Whitman’s use of the verb “press” adds an element of sensuality, a physical touch that appeals to readers.
To a Historian