With the coming of darkness the curious phenomena of the valley again manifested itself.

It was a wonderful scene.

But Frank took Captain Elmo to the verge of the plateau and said:

“I told you that I would explain my theory to you to-night. If you will look closely you will see that those curious balls of light dancing up from the ground exist only in sections.”

“I do observe that fact,” agreed Captain Elmo.

“Very good. Now the noxious gas which overcomes one in crossing the valley comes only from those spots. Wherever those balls of light are not to be seen there is no gas, and it is consequently safe to travel.”

The captain was deeply impressed with this explanation.

“Upon my word, Mr. Reade,” he cried, “that is strong logic!”

“I thought you would agree to it. Now those cunning rascals of counterfeiters are aware of the fact and cross the valley with impunity at any time of night.”

“But—but what of the nuggets of gold which are said to tempt the unwary into the place? If they were able to enter and close the valley, why should Mason and his gang need to manufacture counterfeit money?”

Frank snapped his fingers.

“That for the nuggets of gold!” he declared. “Ten to one they are worthless iron pyrites. But we will investigate that later. Ah! what did I tell you?”

With this excited cry Frank pointed down into the valley.

Dark forms were seen to flit into the shadows between the dancing sections of gas fire. That they were the counterfeiters bent on escape there was no doubt.

“Heavens! there they are!” gasped the captain. “What shall we do?”

“Explain the secrets of the valley to your men. Close in upon them and——”

But Frank Reade, Jr., did not finish the sentence.

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.

A thrilling cry went up from the lips of all the spectators at that moment.

The surviving counterfeiters were to be seen running wildly across the valley.

Suddenly there was a low, sullen roar, and the whole valley seemed alive with a blue flame.

Even those on the cliffs were hurled back, and cries of horror went up.

When, the next moment, the exhibition subsided, the bodies of all of the counterfeiters were seen far out in the deadly sands.

Once too often they had dared the deadly perils of Satan’s Hole. It was a horrible, awful thought to the awestruck spectators.

Frank Reade, Jr., stood like one dumbfounded.

Bert Mason was among the doomed ones. In a moment, as it were, the entire gang was wiped out. But Frank was only thinking of poor Astley.

“Come,” said Capt. Elmo huskily, “I believe that valley is a part of hell. Let us get out of here as quickly as possible.”

None had any desire to remain longer in the vicinity of the Sandy Trail of Death. Silently, and with a keen sense of horror they picked up their belongings and left the spot behind them forever.

But Benjamin Astley did not suffer death upon the gallows for a crime of which he was wholly innocent.

Enough evidence had been found to substantiate the fact that he was not identical with Bert Mason who had perished in Satan’s Hole.

So he was released from prison a happy man and restored to his faithful friends and his happy bride.

All this was certainly due to the disinterested kindness of Frank Reade, Jr., and Astley expressed his gratitude in warmest terms.

The story spread all over the country and Frank Reade, Jr., was eulogized as the prince of benefactors and philanthropists.

The Steam Horse, however, was not destined to long remain idle. Almost as soon as he returned to Readestown Frank became interested in another thrilling case, the full details of which may be found in exciting form in No. 10 of the Frank Reade Library, entitled:

“FRANK READE, JR., WITH HIS NEW STEAM HORSE AND THE MYSTERY OF THE UNDERGROUND RANCH.”