The fugitives were entering a desolate, burned-out world. It rose above them in limitless, gradual ascent and spread wide to east and west. Then the waste of sand began to yield to cinders. The horses sank to their fetlocks as they toiled on. A fine, choking dust blew back from the leaders, and men coughed and horses snorted. The huge, round hills rose smooth, symmetrical, colored as if the setting sun was shining on bare, blue-black surfaces. But the sun was now behind the hills. In between ran the streams of lava. The horsemen skirted the edge between slope of hill and perpendicular ragged wall. This red lava seemed to have flowed and hardened there only yesterday. It was broken sharp, dull rust color, full of cracks and caves and crevices, and everywhere upon its jagged surface grew the white-thorned choya. Again twilight encompassed the travelers. But there was still light enough for Gale to see the constricted passage open into a wide, deep space where the dull color was relieved by the gray of gnarled and dwarfed mesquite. Blanco Sol, keenest of scent, whistled his welcome herald of water. The other horses answered, quickened their gait. Gale smelled it, too, sweet, cool, damp on the dry air. Yaqui turned the corner of a pocket in the lava wall. The file of white horses rounded the corner after him. And Gale, coming last, saw the pale, glancing gleam of a pool of water beautiful in the twilight. Next day the Yaqui's relentless driving demand on the horses was no longer in evidence. He lost no time, but he did not hasten. His course wound between low cinder dunes which limited their view of the surrounding country. These dunes finally sank down to a black floor as hard as flint with tongues of lava to the left, and to the right the slow descent into the cactus plain. Yaqui was now traveling due west. It was Gale's idea that the Indian was skirting the first sharp-toothed slope of a vast volcanic plateau which formed the western half of the Sonora Desert and extended to the Gulf of California. Travel was slow, but not exhausting for rider or beast. A little sand and meager grass gave a grayish tinge to the strip of black ground between lava and plain.