For a time the narrator comforts Roderick by reading and painting with him; one of Roderick's paintings is described as follows: 'A small picture presented the interior of an immensely long and rectangular vault or tunnel, with low walls, smooth, white, and without interruption or device. Certain accessory points of the design served well to convey the idea that this excavation lay at an exceeding depth below the surface of the earth.' What later event in the story does this picture foreshadow?