The tunnel had been built as a chute for the disposition of the rock crushed in the mine.
There was no knowing how many years the natives had worked in that underground mine, crushing out the gold with rude appliances and disposing of the refuse by means of the tunnel cut through the fault in the rock.

The canyon into which the crushed rock had been cast was a wild and almost inaccessible break almost at the top of the mountain range, and might have been used for years--perhaps for centuries--without the truth of its gradual filling up becoming known to hostile peoples.
Looking down into the canyon, Nestor wondered if an easy route to the bottom might not be found there.

He was already more than 200 feet below the shelf of rock from which the mine opened.

The floor of the canyon was at least 400 feet below him, and at the south another cut, running east and west, seemed to connect with the first.

He heard the trinkle of water below, and was satisfied that there was a succession of canyons leading to the plain below, in which case descent would be comparatively easy.
This piece of good fortune, Nestor congratulated himself, would enable the boys to reach the camping place of the renegade and his men shortly after dark, as the approach to the sandy plain would be comparatively free of obstruction.