Thoreau wants nature to speak for itself, yet it also wants it to have human meaning. I love a broad margin to my life. This is one of his most loved lines. I have been as sincere a worshipper of as the Greeks. I can understand that.
Morning brings back the heroic ages.
In season, corn grows extremely quickly. A tantivy is a rush. Probably these were passenger pigeons, once very prevalent but now extinct. Note how he manages to weave the railroad into his description of natural events around him. The sound of the railroad was at least as constant as any natural sounds at Walden, as it passed frequently on its way between Concord and Boston.