y 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.

Bromley sprang afoot at the first discharge, but before he could reach
the door of the adobe, Ballard came in, carrying a hatful of roughly
crumbled brown earth. He was a little short of breath, and his eyes were
flashing with excitement. Nevertheless, he was cool enough to stop
Bromley's question before it could be set in words.

"It was only one of the colonel's Mexican mine guards trying a little
rifle practice in the dark," he explained