In the Heart of the Rockies by G. A. Henty

Historical novel, set in the Wild West. The Preface begins: Until comparatively lately that portion of the United States in which I have laid this story was wholly unexplored. The marvelous canons of the Colorado River extend through a country absolutely bare and waterless, and save the tales told by a few hunters or gold-seekers who, pressed by Indians, made the descent of some of them, but little was known regarding this region. It was not until 1869 that a thorough exploration of the canons was made by a government expedition under the command of Major Powell. This expedition passed through the whole of the canons, from those high up on the Green River to the point where the Colorado issues out on to the plains. According to Wikipedia: George Alfred Henty (8 December 1832 - 16 November 1902), referred to as G. A. Henty, was a prolific English novelist, special correspondent, and Imperialist born in Trumpington, Cambridgeshire, England. He is best known for his historical adventure stories that were popular in the late 19th century. His works include Out on the Pampas (1871), The Young Buglers (1880), With Clive in India (1884) and Wulf the Saxon (1895).