less they would soon lie down.
Bob was already breathing heavily, which would indicate that he had
passed beyond the open door to slumber-land.

The minutes passed, and several hours must have gone.

Frank was dreaming of the excitement attending some of the many dashing
gallops he had lately enjoyed in company with his chum, looking up
stray cattle, helping to brand mavericks, watching the cowmen mill
stampeding herds, or chasing fleet-footed antelopes just to give the
horses a run.

He was suddenly aroused by a strange sound that seemed to cause the
very earth under him to tremble. The trample of a thousand hoofs would
make such a noise; if one of those old-time mighty herds of bison could
have come back to earth again; or a stampede of an immense herd of
long-horns might cause a similar vibration.

But Frank Haywood knew that neither of these explanations could be the
true one, even as he thus sat upright on his blanket to listen. The
ominous, growling, grumbling noise was more in the nature of
approaching thunder, just as though one of those furious summer storms,
tropical in their nature, and often encountered in this country where
plains and mountains sharply meet, had crept upon them as they calmly
slept.

And yet, strange to say, neither of the two boys jumped quickly to
their feet in wild dismay, seeking to prepare for the rain that might
soon burst upon them. On the contrary they continued to sit there,
straining their ears to catch the rumbling reverberations that kept
coming, with little respites between.

"Say, now, what d'ye think of that, Bob?" asked Frank, when silence
again held sway for a brief period. "Nary a cloud as big as your hand
in the sky; and yet all that grumbling oozing out of old Thunder
Mountain! Looks like we might have the biggest job of our lives
finding out the secret of that pile of rocks. There she starts in
again, harder than ever. Listen, Bob, for all you're worth!"

CHAPTER VI

A SECOND ALARM

"It's stopped again!" remarked Bob, after possibly five minutes had
passed, during which time the ominous rumbling, accompanied by earth
tremors, had kept up, now rising to a furious stage, and then almost
dying away.

Frank gave a big sigh.

"It sure has," he admitted; "and I don't wonder now, after I've heard
the racket with my own ears, that the reds for a hundred years back
have always declared the Great Manitou lived in Thunder Mountain, and
every little whi