The Call of the Canyon by Zane Grey

Carley, clutching her support, with abated breath and prickling skin, gazed in fascinated suspense over the rim of the gorge. Sometimes the wheels on that side of the vehicle passed within a few inches of the edge. The brakes squeaked, the wheels slid; and she could hear the scrape of the iron-shod hoofs of the horses as they held back stiff legged, obedient to the wary call of the driver.

The first hundred yards of that steep road cut out of the cliff appeared to be the worst. It began to widen, with descents less precipitous. Tips of trees rose level with her gaze, obstructing sight of the blue depths. Then brush appeared on each side of the road. Gradually Carley's strain relaxed, and also the muscular contraction by which she had braced herself in the seat. The horses began to trot again. The wheels rattled. The road wound around abrupt corners, and soon the green and red wall of the opposite side of the canyon loomed close. Low roar of running water rose to Carley's ears. When at length she looked out instead of down she could see nothing but a mass of green foliage crossed by tree trunks and branches of brown and gray. Then the vehicle bowled under dark cool shade, into a tunnel with mossy wet cliff on one side, and close-standing trees on the other.