Could it be the voice of the cougar?
or, more fearful thought still, the snort of the grizzly bear?
The latter was not unlikely.

They were now in a region where these fierce animals are to be met with; and just in such a spot as one or more of them would choose for their abode.
It was a fearful apprehension, and it would have banished sleep from the eyes of the young hunters had they been certain that grizzly bears were in their neighbourhood.

They were not certain, however; still they resolved not to sleep all at one time, but to keep watch in turns.

The fire was replenished with fresh wood, so that the blaze would enable them to see for a good distance around; and then two of them lay down to sleep again, while the third watched, sitting up with his piece in readiness for any sudden attack.

Each took a two-hours' turn as sentinel until the morning broke, which put an end to their fears, as no enemy appeared to be near them.
They now bestirred themselves, let loose their horses upon the grass, performed their ablutions in the crystal water of the spring, and made ready their breakfast.