"This way!" he shouted.
Frank heard his chum's feet swiftly crunching the sand and gravel, and followed the sound.

In a moment or two his vision cleared somewhat and he was able to see several rows of huge wooden tanks.

A plank incline led to the top of one row, and Ballard could be distinguished racing up the incline.

Beyond Ballard, traveling at speed over a plank gangway that spanned the tanktops, was a burly figure silhouetted against the lighter gloom of the night.

With a shout to Clancy, Merriwell hustled after Ballard.
Those tanks were part of the cyanide plant, wherein the refuse of the mill was treated with deadly cyanide of potassium for recovering what little gold was left after the refuse, or "tailings," had come from the stamp mill.
The cyanide plant, presumably, was familiar ground to Porter, whereas the boys had never seen it before.