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6 On they went, around a bend of the trail and over some rough rocks, where the horses had to step with care, for fear of slipping into a gully on the left. Then they reached a patch of timber and plunged beneath the low-drooping trees. Here it was both dark and cool, and Darry breathed a long sigh of relief.

"How delicious!" he murmured. "It's almost like going into a cave. Benson, there must be lots of caves in these mountains," he went on reflectively.

"There are," answered the old scout. "I've been in a score or more."

"I should like to explore a big cave," came from Joe. "It would be a novelty to me."

"You may get the chance, lad," said Benson; "and get it soon."

"What do you mean? Are we going to ride by a cave?"

"There are a dozen or more ahead, and we may have to seek one of 'em for shelter. Do you hear that?"

Benson threw back his head to listen, and the two boys did likewise. From a great distance came the rumble of thunder, echoing and re-echoing throughout the mountains. To the westward the sun was hidden by a dense mass of black clouds which grew more ominous each instant.