de opposite the Trap-Door City and
its mysterious menace. Day was swiftly dying, and the lower passes of
the mountains were already hazy with rapidly forming storm-clouds.

"Look!" cried Irma excitedly. "What are those things?"

Far in the distance a long line of wavering red lights snaked swiftly
through the dusky valley toward them. Penrun picked up his binoculars.

"Spiders," he announced. "Scores of them. Each is carrying a sort of
red torch. I have a feeling that those are what the monsters of the
Trap-Door City have been waiting for."

He urged the sphere to swifter flight along the range. Miles from the
Caves, he swept up over the peaks, and dropped down on the lowlands
side. Dusk was deepening rapidly as he raced back toward the White
River cataract under the pall of the gathering storm.

* * * * *

Among the boulders on the rough mountainside near the mouth of the
Caves he eased the craft down to a gentle landing.

"Wait here," he told Irma. "I'll investigate and see if it is safe to
enter the Caves."

They had seen the three men return to the ship, but others might have
gone to the Caves after that. Penrun made his way down the slope to
the lip of the cataract and the yawning blackness of the abysmal gorge
below it.

Overhead the storm was gathering swiftly, and the saffron light of the
dying day illuminated the plateau eerily. Half a mile away the
Trap-Door City shimmered fantastically in the uncertain light. Penrun
repressed a shudder. The Devil's own playground! Thank God, he and
Irma would be out of it soon!

He crept down the narrow path that led under the ledge of the
trickling cataract. Outside, a bolt of lightning stabbed down from the
darkened heavens. Its lurid flash revealed the huge figure of a man,
pistol in hand, beside the entrance to the Caves.

Too late to retreat now, even had he wished to. Penrun's weapon
flashed first. A scream of pain and fury answered the flash, and the
man's pistol clattered down on the rock. The next instant Penrun was
helpless in the clutch of a mighty pair of arms that tried to squeeze
the life out of him.

"Burn, me, will ye, ye dirty scum!" roared the giant of a man
tightening his grip. "I'll break your damned back for ye and heave ye
into the gorge!"

Penrun writhed frenziedly, trying to twist his pistol around against
his enemy's back, while they struggled desperately about the ledge
above the dizzy blackness of the