The captain clutched his arm.
“My God!” he cried, “there is the end of half of them!”
A startling incident had occurred in the valley, which was so vivid and horrible as to leave its impress upon the minds of the spectators forever.
Suddenly half a dozen of the counterfeiters were seen to become enveloped in what seemed like a momentary flash of blue flame.
Then their bodies were seen lying upon the sand, and the ghostly balls of light were seen dancing above them.
“Heavens! How do you explain that?” gasped the captain.
“They must have unluckily stepped upon an undeveloped mine of the gas,” replied Frank. “It has come up suddenly and overpowered them.”
“Ah, might not that same fate overcome us if we should try to cross the valley?”
“It is not impossible,” agreed Frank. “But——”
He never finished the sentence.