The road continued through rather monotonous country, the altitude varying from 6,300 to 7,700 feet.  Grass began to be scarce, and the animals suffered accordingly.  It is the custom with Mexican muleteers to select from among themselves a few, whose business throughout the journey it is to guard the animals at night.  These men, immediately after having had their supper, drive the animals to a place where suitable pasture is found, never very far from the camp, and bring them back in the morning.  They constitute what is called la sabana.  Comparatively few men suffice for this duty, even with a large herd, as long as they have with them a leader of the mules, a mare, preferably a white one.  She may be taken along solely for this purpose, as she is often too old for any other work.  The mules not infrequently show something like a fanatic attachment for their yegua, and follow blindly where they hear the tinkling of the bell, which is invariably attached to her neck.  She leads the pack-train, and where she stops the mules gather around her while waiting for the men to come and relieve them of their burdens.  Sometimes a horse may serve as a leader, but a mare is surer of gaining the affection of all the mules in the train.  This is an important fact for travellers to bear in mind if they use mules at all.  In daytime the train will move smoothly, all the mules, of their own accord, following their leader, and at night keeping close to her.  In this way she prevents them from scattering and becomes indispensable to the train.

But in spite of the vigilance of the sabana and the advantage of a good yegua, it may happen, under favourable topographical and weather conditions, that robbers succeed in driving animals away.  While giving the pack-train a much-needed rest of a day in a grassy spot, we woke next morning to find five of our animals missing.  As three of the lot were the property of my men, they were most eagerly looked for.  The track led up a steep ridge, over very rough country, which the Mexicans followed, however, until it suddenly ran up against a mountain wall; and there the mules were found in something like a natural corral.

Not until then did our guide inform me that there lived at Calaveras (skulls), only three miles from where we were stopping, a band of seven robbers and their chief, Pedro Chaparro, who was at that time well-known throughout this part of the Tarahumare country.  I had no further experience with him, but later heard much of this man, who was one of a type now rapidly disappearing in Mexico.  He did not confine his exploits to the Mexicans, but victimised also the Indians whenever he got an opportunity, and there are many stories in circulation about him.