ion about this thunder sound?" Bob persisted in saying.

"Oh! he thinks the same as several gentlemen did who came out here a
few years ago on some business. They declared that once, hundreds of
years ago, perhaps, old Thunder Mountain must have been a volcano; and
that it still grumbles now and then, as the fires away down in the
earth begin to kick up some of their old monkeyshines."

"Yes, I heard one man say that," laughed Bob. "He declared that
there's going to be the biggest rumpus some fine day, when the fires
inside get to going out of bounds. Then the whole cap of the mountain
will go flying into a million pieces; and good-bye to any unlucky
cow-puncher caught napping near this place."

"Well," remarked Frank, as he prepared to settle down again into his
snug blanket, "I reckon we're not going to be scared away by a little
thing like that growl. Unless we hit a snag, or Peg Grant and his
guides break up our game, a few days ought to see us heading back to
Circle Ranch with a story calculated to make the boys sit up and take
notice; or else----"

"Just pull up right there, Frank," interrupted his chum, with a laugh.
"There's nothing going to happen to knock us out. If that same Peg
comes around, making a nuisance of himself, why, he's due for a nice
little surprise, mark me. Besides that; what could there be to make
trouble?"

"Oh, I'm not bothering my head over it, Bob," declared the other, as he
dropped into the nest he had made in his blanket. "But say, did you
take notice of the way our horses acted while that thing was going on?"

"Just what I did," the other replied. "They must have been trembling
all over. I could hear your Buckskin snorting to beat the band, and
pawing just like he does when he's worried. Reckon they didn't know
what to make of it, either, seeing that there's nary a sign of a storm
cloud around. But both horses have quieted down again. They think all
danger of a howler has passed away."

Frank made no reply. He was already getting ready to resume his
interrupted nap; and Bob lost no time in following his example, both
confident that in the alert Buckskin they had a sentry capable of
giving ample warning should peril threaten.

Once more Frank composed himself for sleep. The many noises of the
night, which had seemed to cease while that mysterious rumbling was
going on in the heart of the lofty mountain, had again resumed sway.
The hum of insects; the melancholy hoo