A Snake
swamp with its secrets
Dickinson lived in Amherst, Massachusetts—no swamps. This is imaginative landscape, not observation. The swamp represents something psychologically murky.
enthralling gallop
"Enthralling" means captivating but also enslaving. The child's terror is both frightening and exhilarating—she can't choose to stop running.
summer's treason
Treason requires betrayal by one of your own. The snake belongs to summer—warm-blooded season, life season—but breaks summer's promise of safety.