Before long they were in a wilderness of bare, stern peaks whose snow-covered summits towered high against the horizon. At times the road zigzagged along narrow shelves cut in the faces of precipices and guarded here and there by low retaining-walls built of cut stones laid without mortar, but so perfectly that the blade of a knife could not be thrust between them. The air became colder, and the scientist told them that often the temperature in these mountain-valleys would vary as much as one hundred degrees within twenty-four hours.
As they approached the crest of a great ridge which towered above them, Jud began to find great difficulty in breathing and complained of nausea and a feeling of suffocation.
"It's the soroche, the mountain-sickness," explained Professor Ditson. "It will pass soon."
"I'm the one that's goin' to pass—pass out," panted Jud.
Soon he became so exhausted that, like Will, he had to be half-carried along the trail.
"You an' me are a fine pair to fight Injuns," he whispered to the boy, who smiled wanly in reply.
Beyond the ridge the road ran downward toward a vast gorge. From its dark depths rose and fell at intervals the hoarse, roaring bellow of a river rushing among the rocks a thousand feet below.
"It is Apurinac, the Great Speaker," said Pinto.
As the trail led downward again, Jud began to feel better, and before long he was able to walk without any help.
At length, far below them, looking like a white thread against the threatening blackness of the cañon, they saw swinging in the wind a rude suspension bridge of the kind which travelers had used in these mountains ever since the days of the Incas. When Pinto, who knew the bridge well, learned that Professor Ditson intended to cross it at once, he was much disturbed.