"Nothing immediately alarming; merely an indication of what might be. Where the river emerges from your cut-off tunnel below the dam, it has worn out a deep pit in the old bed, as you know. The bottom of this pit must, in the nature of things, be far below the foundations of the masonry. Had you thought of that?"
"I have—more than once or twice," Ballard admitted.
"Very well," continued the Master of the Rocks; "that circumstance suggests three interrogation points. Query one: How has the diverted torrent managed to dig such a deep cavity if the true primitives—your workman's 'bed-rock'—under-lie its channel cutting? Query two: What causes the curious reverberatory sound like distant thunder made by the stream as it plunges into this pit—a sound suggesting subterranean caverns? Query three—and this may be set down as the most important of the trio: Why is the detritus washed up out of this singular pot-hole a friable brown shale, quite unlike anything found higher up in the bed of the stream?"
The two young men exchanged swift glances of apprehension. "Your deductions, Professor?" asked Bromley, anxiously.
"Now you are going too fast. True science doesn't deduce: it waits until it can prove. But I might hazard a purely speculative guess. Mr. Braithwaite's foundation stratum—your contractor's 'bed-rock'—may not be the true primitive; it may in its turn be underbedded by this brown shale that the stream is washing up out of its pot-hole."
"Which brings on more talk," said Ballard, grappling thoughtfully with the new perplexities forming themselves upon Gardiner's guess.
"Decidedly, one would say. Granting my speculative answer to Query Number Three, the Arcadia Company's dam may stand for a thousand years—or it may not. Its life may possibly be determined in a single night, if by any means the water impounded above it should find its way through Fitzpatrick's 'bed-rock' to an underlying softer stratum."
Ballard's eyes were fixed upon a blue-print profile of Elbow Canyon pinned upon the wall, when he said: "If that pot-hole, or some rift similar to it, were above the dam instead of below it, for example?"
"Precisely," said the geologist. "In five minutes after the opening of such an underground channel your dam might be transformed into a makeshift bridge spanning an erosive torrent comparable in fierce and destructive energy, to nothing milder than a suddenly released Niagara."
Silence ensued, and afterward the talk drifted to other fields; was chiefly reminiscent of the younger men's university years. It was while Bromley and Gardiner were carrying the brunt of it that Ballard got up and went out. A few minutes later the out-door stillness of the night was shattered by the sharp crack of a rifle, and other shots followed in quick succession.