Pioneer Boys of the Gold Fields by Edward Stratemeyer

The day after they abandoned the wagon, and divided the outfit into packs, the young pioneers found themselves on something of a plateau among the Rockies. Each carried a stiff load on his shoulders, and what was left of the outfit was strapped to the back of the mule, -- enough to make Darling move along slower than ever.

They had come over a broken trail, where they had often to leap from one rock to the next. More than once one or another of them had fallen, and Si had received a wrench of his left ankle that hurt him considerably.