In Emily Dickinson's "Because I Could Not Stop for Death," the extended metaphor used to express the process of dying is the unexpected ride in a horse-drawn carriage that leads to the grave.
Death itself is personified as a carriage driver, who "kindly" stops for the speaker. But, the speaker's ride is a solitary one as there are no other passengers in this carriage. Nor is there any haste in this ride as the speaker has finished her worldly tasks of "labor" and "leisure."
As the ride continues, the carriage passes a school, fields of grain, and the sunset. Each of these symbolize a stage of life: Childhood, Maturity, and Old Age. Finally, they pause before a House that "seemed / A Swelling of the Ground--" whose "Roof was scarcely visible." This, of course, is the grave. But, it is only at this point that the speaker realizes--"surmises"-- that her carriage ride has been headed "toward Eternity."