animal had passed, but, whatever it was, it did not approach the clump of
trees and work was resumed at once. When dusk came the moist black soil
found in the pit had all been carried away and the boys had reached, to
their intense disgust, a stratum of hard packed gravel. That meant
infinitely more difficult work for them and the use of some new utensil.

There was nothing daunting in the new problem. When it came to the mere
matter of securing a tool for digging the hard gravel, both Ab and Oak
were easily at home. The cave dwellers, haunting the river side for
centuries, had learned how to deal with gravel, and when Ab returned to
the scene the next day he brought with him a sturdy oaken stave some six
feet in length, sharpened to a point and hardened in the fire until it
was almost iron-like in its quality. Plunged into the gravel as far as
the force of a blow could drive it, and pulled backward with the leverage
obtained, the gravel was loosened and pried upward either in masses which
could be lifted out entire, or so crumbled that it could be easily dished
out with the clamshell. The work went on more slowly, but not less
steadily nor hopefully than on the days preceding, and, for some time,
was uninterrupted by any striking incident. The boys were becoming
buoyant. They decided that the grassy valley was almost uninfested by
things dangerous. They became reckless sometimes, and would work in the
pit together. As a rule, though, they were cautious--this was an inherent
and necessary quality of a cave being--and it was well for them that it
was so, for when an emergency came only one of them was in the pit, while
the other was aloft in the lookout and alert.

It was about three o'clock one afternoon when Ab, whose turn it chanced
to be, was working valiantly in the pit, while Oak, all eyes, was perched
aloft. Suddenly there came from the treetop a yell which was no boyish
expression of exuberance of spirits. It was something which made Ab leap