In the meantime they were steadily descending. The trees hemmed them closer. Thickets of willows and alders had to be crossed. Dimly through the tree-tops they seemed to see the sky darkening by degrees as they worked their way down. At first Bob thought it the lateness of the afternoon; then he concluded it must be the smoke of the fire; finally, through a clear opening, he saw this apparent darkening of the horizon was in reality the blue of the cañon wall opposite, rising as they descended. But, too, as they drew nearer, the heavy smoke of the conflagration began to spread over them. In time it usurped the heavens, and Bob had difficulty in believing that it could appear to any one anywhere as so simple a mushroom-head over a slender smoke column. By the time the horses stepped from the slope to the bed of the cañon, it was quite dark. Jack turned down stream. "We'll cut the trail to Burro Rock pretty quick," said he. Within five minutes of travel they did cut it; a narrow brown trough, trodden by the hoofs of many generations of cattlemen bound for the back country. Almost immediately it began to mount the slope. Now ahead, through the gathering twilight, lights began to show, sometimes scattered, sometimes grouped, like the camp-fires of an immense army. These were the stubs, stumps, down logs and the like left still blazing after all the more readily inflammable material had been burned away. As the little cavalcade laboured upward, stopping every few minutes to breathe the horses, these flickering lights defined themselves. In particular one tall dead yellow pine standing boldly prominent, afire to the top, alternately glowed and paled as the wind breathed or died. A smell of stale burning drifted down the damp night air. Pretty soon Jack Pollock halted for a moment to call back: "Here's their fire line!" Bob spurred forward. Just beyond Jack's horse the country lay blackened. The pine needles had burned down to the soil; the seedlings and younger trees had been withered away; the larger trees scorched; the fuel with which every forest is littered consumed in the fierceness of the conflagration. Here and there some stub or trunk still blazed and crackled, outposts of the army whose camp-fires seemed to dot the hills. The line of demarcation between the burned and the unburned areas seemed extraordinarily well defined. Bob looked closer and saw that this definition was due to a peculiar path, perhaps two yards wide. It looked as though some one had gone along there with a huge broom, sweeping as one would sweep a path in deep dust. Only in this case the broom must have been a powerful implement as well as one of wide reach. The brushed marks went not only through the carpet of pine needles, but through the tarweed, the snow brush, the manzañita. This was technically the fire line. At the sight of the positiveness with which it had checked the spread of the flames, Bob's spirits rose.