When afterwards he came to reason soberly over the adventure, the conclusion seemed obvious that the pitfall had been a consequent upon the breaking out of one of the ancient springs, so that the water, in endeavoring to find an outlet, had finally undermined the whole roadway. The chasm, as he looked back upon it, extended dear across the street. Its depth was only conjectural, but the mass presented the treacherous appearance of quaking sand, and Constans shuddered as he gazed. Yet he had escaped; the peril was past; let him forget what was behind and press forward. Half a block farther on and he found himself in a cul-de-sac. The street was filled from house-wall to house-wall with an immense mass of broken stone, brick, and other debris. The cause was not far to seek. Immediately upon the left rose one of the fabulously high buildings for which the ancient city had been famous. It could not compare in magnitude with the tremendous structures that he could discern still farther ahead, but its dozen and a half of stories loomed up imposingly when contrasted with the moderate sized houses adjoining it. Constans looked up in wonder at its towering facade, then started back with an exclamation of alarm. It appeared that the foundations of the structure had in some way become weakened, for the whole building had settled and was leaning over at a terrifying divergence from the perpendicular. Being constructed of iron truss-work similar to that of a bridge, the essential framework still held together, but the outside walls, mere shells of stone and brick, had cracked and given way under the strain, falling piece-meal into the street below. Even as he looked, a stone dropped from a window pediment and crashed into splinters on the pavement a few yards beyond where he stood. The angle of inclination seemed to grow larger as he gazed at it; the enormous mass poised itself above him, monstrous, informed, threatening to strike. With that uncomfortable contraction of the scalp-skin that attends upon the sudden presence of peril, Constans backed hastily away; not for worlds would he have ventured again under that overhang of artificial cliff. Yet behind him was the stretch of sunken pavement; he could not risk another passage of that. A single alternative remained--to enter one of the small houses that lined the street, ascend to its roof, and so escape to the nearest cross thoroughfare.