The Desert Valley

Helen and her father were a content lot—traveling and supporting each other through their various travels. But Mexico is a different case altogether. While pitching their tent for the night, they come face to face with a mysterious stranger and things alter dramatically from that point onwards. Who is this stranger? What terrible secrets are waiting to be unfolded?
Excerpt:
"In the dusk a pack-horse crested a low-lying sand-ridge, put up its head and sniffed, pushed forward eagerly, its nostrils twitching as it turned a little more toward the north, going straight toward the water-hole. The pack was slipping as far to one side as it had listed to the other half an hour ago; in the restraining rope there were a dozen intricate knots where one would have amply sufficed. The horse broke into a trot, blazing its own trail through the mesquite; a parcel slipped; the slack rope grew slacker because of the subsequent readjustment; half a dozen bundles dropped after the first. A voice, thin and irritable, shouted 'Whoa!' and the man in turn was briefly outlined against the pale sky as he scrambled up the ridge. He was a little man and plainly weary; he walked as though his boots hurt him; he carried a wide, new hat in one hand; the skin was peeling from his blistered face. From his other hand trailed a big handkerchief. He was perhaps fifty or sixty. He called 'Whoa!' again, and made what haste he could after his horse."