ara," finally announced the Professor.
"The rapids of the Niagara River would be lost in this turbid stream."

Great knife-like rocks projected from the flood. When the water struck
these sharp edges it was cleanly cut, spurting up into the air like
geysers, sending a rainbow spray for many yards on either side.

What puzzled the lads more than all else were the great leaping waves
that rose without apparent cause from spaces of comparatively calm
water. These upturning waves, the guide explained, were the terror of
explorers who sought to get through the Canyon in boats.

"Has any one ever accomplished it?" asked Tad.

"Yes; that intrepid explorer, Major J.W. Powell, made the trip in the
year 1869, one of the most thrilling voyages that man ever took. Several
of his men were lost; two who managed to escape below here were killed
by the Indians."

"I think I should like to try it," said Tad thoughtfully.

"You won't, if I have anything to say about the matter," replied Dad
shortly.

"No one would imagine, to gaze down on this stream from the rim, that
it was such a lively stretch of water," remarked the boy. "It doesn't
seem possible."

"Yes, if they had some of this water up on the plateau it would be
worth almost its weight in gold," declared Nance. "Water is what
Arizona needs and what it has precious little of. Speaking of the
danger of the river," continued Nance, "it isn't wholly the water, but
the traveling boulders."

"Traveling boulders!" exclaimed the boys.

"Yes. Boulders weighing perhaps a score or more of tons are rolled
over and over down the river by the tremendous power of the water,
almost with the force and speed of projectiles. Now and again they
will run against snags. The water dashing along behind them is
suddenly checked under the surface. The result is a great up-wave,
such as you have already observed. They are just as likely to go
downward or sideways as upward. You never know."

"Then that is the explanation of the cause of those up-waves?" asked
the Professor.

"That's the way we figure it out. But we may be wrong. Take an old
man's advice and don't monkey with the river."

"I thought you said Dad's beloved Canyon would not hurt him," said Tad
teasingly.

"Dad's Canyon won't. The river isn't Dad's The river is a demon. The
river would scream with delight were it to get Dad in its cruel
clutches," answered the old man thoughtfully, his bristling whiskers
dro