ntinued Bob, anxious to know.

"What would cowmen be doing away off here, tell me that, Bob? And
lugging along a bunch of extra mounts, too, in the bargain? No, I
rather think, Bob, that those fellows must have some of Mendoza's
cattle rustlers. And they've been making a dandy raid on some ranch's
saddle herd; or I miss my guess."

"Perhaps the Circle outfit had gotten careless," suggested Bob.

"I sure hope not, for the boys have had plenty of warning; and I reckon
Bart Heminway is some too good an overseer to permit such a raid. I'd
rather believe it was the X-bar-X outfit that has gone and got nipped
this time. But stop and think Bob; what d'ye expect takes these
cattle-rustlers over this way right now, headed straight for the
canyons of Thunder Mountain?"

"Oh, I see what you mean!" exclaimed the taller lad, immediately.
"Perhaps the secret hiding place of Mendoza and his crowd of cattle
thieves may be somewhere around this same old rock pile. It'd be just
like the tricky rustler to have a hide-out where nobody else ever came!"

"Now, why didn't somebody ever think of that before?" ejaculated Frank,
in a tone of mingled surprise and disgust.

"Looks easy, doesn't it, after we've run across a clue?" admitted Bob,
laughing softly. "You remember what they said about discovering
America, after Columbus did it. But supposing this thing _does_ turn
out to be true; how's it going to affect our little business, Frank?
Oh! say, I wonder if that crowd can have anything to do with the
rumbling of the mountain?"

Frank laughed heartily at the suggestion.

"Well," he remarked, "they're a pretty tough lot, all right; but even
such a bad bunch could hardly get enough hot air together to make a
mountain shake and groan like that. Besides, don't you see, Bob, they
must have been out yonder, riding this way with their stolen horses,
when that little circus came off."

"But one thing is sure," the other went on, sturdily; "they don't seem
to take any stock in that notion about a volcano, because, as we saw,
they headed straight for Thunder Mountain. That gives it away; they're
so used to the row that they don't pay any attention to it any longer."

"Correct!" echoed Frank, as though his mind was made up.

"Do we need to hold the horses down any longer?" asked Bob, who could
feel that Domino was becoming very restless under his enforced silence.

"I reckon not," replied the other, at the same time taking the blan