After that he watched with a thrilling eagerness.

Straight at the waterfall the rustlers drove the burros, and straight through the middle, where the water spread into a fleecy, thin film like dissolving smoke.

Following closely, the rustlers rode into this white mist, showing in bold black relief for an instant, and then they vanished.
Venters drew a full breath that rushed out in brief and sudden utterance.
"Good Heaven! Of all the holes for a rustler!...

There's a cavern under that waterfall, and a passageway leading out to a canyon beyond.

Oldring hides in there.