"They would see us go out into the plains," hazarded Bob, calmly, "but the smoke of these very convenient boilers has kept them from noticing our return."
Mackay nodded. "They've got about a solid ton to shift before the door will swing," he said musingly. "Now I wonder if we should go an' help them wi' the job or no'?"
"I reckon we has had enough for one day, Mac," answered Emu Bill, wearily. "Let the skunks work their own passage."
Another rattle, louder than the first, reached their ears.
"'Pears to me they is in a mighty hurry," grinned the Shadow.
Bob rose to his feet. "I'm going to have a look," he said. "Come on, Jack;" and they tip-toed over to the origin of the disturbances, leaving their companions apparently deeply and solely intent on bringing the billy on the fire to a boil speedily.
Mackay had examined the barricade once or twice earlier in the day, and noticed no change in its appearance, and was convinced that nothing short of gelignite cartridges could shift their obstruction from the inside. Bob held the same opinion, but he was nevertheless curious to see what sort of efforts were being made. Making a short détour, they silently approached the entrance to the underground passage from the side farthest from the movable rocky slab. The interstice had been well-nigh filled with diorite boulders, leaving only the top of the solid panel showing; but when Bob looked now, he was alarmed to find a considerable shrinkage in the level of the barricade, and though the noisy echoes of falling rocks were still plainly heard, it was evident that nothing was rolling down from the top of the pile. Jack drew a quick breath of anxiety; Bob was perplexed beyond measure, but he made no sign, and as he looked, behold! the boulder stack was gradually, yet surely, sinking—sinking apparently into the earth beneath. Then his eye noticed some slight change in the position of the rocking wall; it was thrust up somewhat, and gaped widely. The solution of the mystery was now made clear: the great slab moved upwards as well as outwards, and the depletion of the pile was taking place from the bottom; the rock fragments were rolling inwards to the tunnel!
Hastily he beckoned on his companions, and they came forward at a run, just as the last stone was disappearing from view. But the natives had now taken alarm. There came a dull thud as the doorway relapsed into its accustomed place, and then their rapidly retreating footsteps were heard as they scurried back into the subterranean channel, and the peculiar tapping of the night before heralded the direction of their flight.
Mackay took in the position at a glance, and an expression of grave concern settled on his features.
"Their resources are positively marvellous," he groaned in despair. "It's a vera fortunate thing, Bob, that you werena influenced by my stupid over-confidence, and came to investigate. We might have been bowled over wi' their arrows before we had time to lift a rifle." He continued bitterly to abuse himself while he inspected the now bare cavity in the mountain.