The fall of the house of usher

The narrator sees a "fissure" or crack that compromises the structural integrity of the house.

Perhaps the eye of a scrutinising 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.

The crack is significant in that it mirrors the mental stability of the home's inhabitants. The problems between Roderick and Madeline tears the family (the house of Usher) into peices.

The fissure extends from. To.