Carter's pursuit of the yak became now a flight from an unseen thing, for though he dared not glance over his shoulder he felt that the presence behind him could be nothing wholesome or mentionable. His yak must have heard or felt it first, and he did not like to ask himself whether it had followed him from the haunts of men or had floundered up out of that black quarry pit. Meanwhile the cliffs had been left behind, so that the oncoming night fell over a great waste of sand and spectral rocks wherein all paths were lost. He could not see the hoofprints of his yak, but always from behind him there came that detestable clopping; mingled now and then with what he fancied were titanic flappings and whirrings. That he was losing ground seemed unhappily clear to him, and he knew he was hopelessly lost in this broken and blasted desert of meaningless rocks and untravelled sands. Only those remote and impassable peaks on the right gave him any sense of direction, and even they were less clear as the grey twilight waned and the sickly phosphorescence of the clouds took its place.