There was no doubt but that Beaver Bill had hit the nail on the head.
All accepted this hypothesis as a correct one. But darkness was coming on rapidly now.
CHAPTER XIII.
THE END.
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.