Perhaps the eye of a scrutinizing observer might have discovered a barely perceptible fissure, which, extending from the roof of the building in front, made its way down the wall in a zigzag direction, until it became lost in the sullen waters of the tarn.
This quotation appears in the narrator’s observations of the exterior of the mansion, as he describes its gloomy architecture. Although before describing the crack the narrator insists that the house looks stable, the crack casts doubt on his assessment. This crack foreshadows the house’s impending fall. Although it may be hard to see at first, the roots of the house’s downfall have been present from the start. Because the house and the Usher family are metaphorically connected, this crack also signals the decaying state of the Usher twins.
It was no wonder that his condition terrified — that it infected me. I felt creeping upon me, by slow yet certain degrees, the wild influences of his own fantastic yet impressive superstitions.
The narrator makes this comment not long after Madeline’s death, as Roderick’s mental state decays even further. Caught up in his friend’s complete breakdown, even the staunchly rational narrator begins to notice his sanity fraying. He likens Roderick’s mood to an infection capable of spreading to others, just as rot spreads. Notably, Roderick’s decline and its effect on the narrator occur just after Madeline’s death. Everything frightening about the house, its occupants, and its circumstances stems from its proximity to death, as signaled by decay.