He saw, before he reached the height, where Miss Allen had come down again--and he saw where she had, to avoid a clump of boulders and a broken ledge, gone too far to one side.

He followed that way.

She had descended at an angle, after that, which took her away from the canyon.
In Montana there is more of daylight after the sun has gone than there is in some other places.

Andy, by hurrying, managed to trail Miss Allen to the bottom of the peak before it grew really dusky.

He knew that she had been completely lost when she reached the bottom, and had probably wandered about at random since then.