that this might be the very worst thing they could
do. His quick ears had told him that the center of the slide was ahead
of them. That was his judgment, but he knew how easily it was to be
mistaken in a moment like this.

Glancing up the boy could see nothing but a great cloud of dust that
filled the air. His companions seemed powerless to stir, and it was
fortunate for them that such was the case, else they might have done
that which would have sent them to a quick death.

Tad unslung his rope with the intention of casting it over a sharp rock
that extended some six feet up above the level of the trail and on the
mountainside. In an emergency it would serve to anchor him. He motioned
to the others to do the same, but either they did not understand or they
were too frightened to act.

A sudden dust cloud obliterated the trail for fully five rods ahead of
Professor Zepplin, then went shooting out into the chasm beyond, and a
great mass of earth seemed to leap from the mountainside just above
them. It hovered right over the center of the line of ponies for an
agonizing second, then swept down on them.

The secondary slide, which this was, had but little width, perhaps a few
feet. Furthermore, it had fallen only a short distance, so that it had
not had time to gain great velocity. The mass smote the pack mule just
ahead of Tad Butler. Tad saw the pack mule's hind feet go out from under
him. For the smallest fraction of a moment the animal stood quivering,
then his hind hoofs slipped over the edge of the trail.

The little animal was making desperate efforts to cling to the trail
with its fore feet, at the same time trying to get its hind feet back on
solid ground. That effort was fatal. Little by little the frightened
beast slipped toward the great gulf. Evidently realizing the fate that
was in store for it, the mule brayed shrilly.

The Pony Rider Boys sat gazing on the scene with fascinated eyes. Even
Professor Zepplin was at a loss for words, and at a greater loss for a
remedy for the disaster that was upon them. Tad Butler's brain was
working, however.

Suddenly Tad raised his rope above his head and gave it three sharp
twirls. Then he let go. The big loop dropped over the head of the
unfortunate pack mule.

"Jump on him and hold him down," shouted Tad. "Be careful that you don't
go over."

The boys hesitated slightly. Perhaps they could not have accomplished
anything, but Butler did not wait to see. He ha