Many a load of dirt was carried on the second day from the pit to the marsh's edge, and only once did the lookout have occasion to suggest to his working companion that he had better climb the tree. A movement in the high grass some hundred yards away had aroused suspicion; some wild 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 from the excavation as he heard it and reach the side of Oak as the latter came literally tumbling down the bole of the tree of watching. "Run!" Oak said, and the two darted across the valley and reached the forest and clambered into safe hiding among the clustering branches. Then, in the intervals between his gasping breath, Oak managed to again articulate a word: "Look!" he said.