"We can't blame ourselves vera much for neglectin' to notice things," agreed Mackay, with a grim smile.
In the depths of the fissure which striated the mountain, only a number of loose boulders were to be seen.
"I reckon I'll try and shift some o' them," spoke Emu Bill. He stooped down and lifted one or two of the heaviest rock fragments, while Mackay held the light, and examined the markings on the walls of the cavern with keen interest.
"There ain't no opening here at all," cried Emu Bill, looking round fearfully, as if half expecting to find the savages still close in the vicinity. "I'm certain sure they couldn't get through here."
Mackay smiled. "I don't know what sort o' blacks we've struck, Bill," he said earnestly; "but if they constructed this trap-door arrangement they've got a wonderful amount o' intelligence."
He traced with his finger an irregularly shaped shelving crack in the southerly wall. It seemed a perfectly natural occurrence in every way, as indeed it was, but the edges gaped considerably towards the top, and Mackay, pressing lightly against the mossy front, caused the overlapping rim to close solid into the rock. By this time Bob, Jack, and the Shadow had scrambled in beside the two.
"But they couldn't get through that measly crack," protested Emu Bill, not quite understanding.
Mackay reached for the pick, and inserting the sharp point into the thin, almost unnoticeable crack now showing, pulled gently, and behold, the weighty rock swung back on end revealing a narrow, tunnel-like entrance penetrating into the mountain. At the same time a draught of damp and stifling air issued from the dark and gloomy passage way, extinguishing the lamp, and before they could look again, the rock fell softly back into position, and this time it closed with a snap. Again Mackay endeavoured to lever it open, but now the solid formation refused to be moved; try as he might, the doorway into the mountain seemed closed conclusively against him.
"Surely it cannot be a natural cave formation?" said Bob.
"Nary cave, Bob," returned Emu Bill, decisively.