But by and by this green and colorful and verdant beauty, the almost level floor of the cañon, the banks of soft earth, the thickets and the clumps of cotton-woods, the shelving caverns and the bulging walls--these features gradually were lost, and Nonnezoshe Boco began to deepen in bare red and white stone steps, the walls sheered away from one another, breaking into sections and ledges, and rising higher and higher, and there began to be manifested a dark and solemn concordance with the nature that had created this rent in the earth. There was a stretch of miles where steep steps in hard red rock alternated with long levels of round boulders. Here one by one the mustangs went lame. And the fugitives, dismounting to spare the faithful beasts, slipped and stumbled over these loose and treacherous stones. Fay was the only one who did not show distress. She was glad to be on foot again and the rolling boulders were as stable as solid rock for her. The hours passed; the toil increased; the progress diminished; one of the mustangs failed entirely and was left; and all the while the dimensions of Nonnezoshe Boco magnified and its character changed. It became a thousand-foot walled cañon, leaning, broken, threatening, with great yellow slides blocking passage, with huge sections split off from the main wall, with immense dark and gloomy caverns. Strangely, it had no intersecting cañon. It jealously guarded its secret. Its unusual formations of cavern and pillar and half-arch led the mind to expect any monstrous stone-shape left by an avalanche or cataclysm. Down and down the fugitives toiled. And now the stream-bed was bare of boulders, and the banks of earth. The floods that had rolled down that cañon had here borne away every loose thing. All the floor was bare red and white stone, polished, glistening, slippery, affording treacherous foothold. And the time came when Nas Ta Bega abandoned the stream-bed to take to the rock-strewn and cactus-covered ledges above. Jane gave out and had to be assisted upon the weary mustang. Fay was persuaded to mount Nack-yal again. Lassiter plodded along. The Indian bent tired steps far in front. And Shefford traveled on after him, footsore and hot. The cañon widened ahead into a great, ragged, iron-hued amphitheater, and from there apparently turned abruptly at right angles. Sunset rimmed the walls. Shefford wondered dully when the Indian would halt to camp. And he dragged himself onward with eyes down on the rough ground. When he raised them again the Indian stood on a point of slope with folded arms, gazing down where the cañon veered. Something in Nas Ta Bega's pose quickened Shefford's pulse and then his steps. He reached the Indian and the point where he, too, could see beyond that vast jutting wall that had obstructed his view. A mile beyond all was bright with the colors of sunset, and spanning the cañon in the graceful shape arid beautiful hues of a rainbow was a magnificent stone bridge.