sation, like fine spray, didn't you?"
demanded Bob, as soon as he could speak with comfort. "Why, touch your
face right now; and you'll find it moist. Whatever can it mean?"

"I think I know," Frank said, slowly. "I suspected it before, and this
seems to make it look more than ever that way."

"Do you mean that you've guessed what makes all that frightful noise?"
asked Bob, astonished.

"I believe I have," came the reply.

"And it has to do with this misty feeling in the air; has it?"
continued the Kentucky boy.

"If my idea proves the right one, and I'm bound to find out before I go
away from this place, it's got everything to do with it, Bob."

"Where there's smoke you'll find fire; and where there's mist I reckon
water can be looked for," remarked Bob, quickly.

"Just so. Now Bob, have you ever been up in the Yellowstone Park
region?"

"I can't say that I have, Frank."

"Then you see I've got the advantage over you; and that's what gave me
a point in the game. Because I've stood and watched Old Faithful and
the other great geysers play every half hour or so," Frank went on, as
they slowly advanced into the passage which seemed possibly to act as
one of many funnels through which the tremendous roaring sound was
carried to the outside world.

"Geysers!" cried Bob. "Oh! now I get onto what you mean. You think,
then, that in the heart of Thunder Mountain a giant geyser spouts every
once in a while; and that as the water is dashed against the rocky
walls it makes the ground shake. Is that it, Frank?"

"Yes," replied the other, "and the noise is so like thunder that when
it is forced out through several queer, funnel-shaped openings like
this one, it has puzzled the Indians for hundreds of years. Bob, more
than that, I believe that every once in so many years, when an extra
convulsion shakes things up here, the water bursts out through some
passage, and rushes down that _barranca_ in a wave perhaps twenty feet
high."

"But they call it a cloud burst, Frank," suggested Bob.

"I know they do, but still I stick to my idea," Frank went on.

"And this promises to be an extra strong outburst. Nick said so
anyhow; didn't he, Frank?" Bob queried, a new anxiety in his tone.

"Just what he did. You're wondering now, that if what I said is true,
whether this passage right here is one of those through which all that
water dashes, on its way to the rocky _barranca_?"

"Yes, that's the truth. How about i