back. I think we can go on to the city of gold, after all."
"How do you mean?" asked Tom quickly. "Do you think we can bring the balloon down here and float across?"
"Bless my watch chain!" exclaimed Mr. Damon, "but that would be a way. I wonder"
"No, I don't mean that way at all," went on Ned. "But it seems to me as if this river isn't a natural one—I mean that it flows along banks of smooth stone, just as if they were cut for it, a canal you know."
"That's right," said Tom, as he looked at the edge of the channel of the underground stream "These stones are cut as cleanly as the rest of the tunnel. Whoever built that must have made a regular channel for this river to flow in. And it's square on the other side, too," he added, flashing his lamp across.
"Then don't you see," continued Ned, "that this river hasn't always been here."
"Bless my gaiters!" gasped Mr. Damon, "what does he mean? The river not always been here?"
"No," proceeded Tom's chum. "For the ancients couldn't have cut the channel out of stone, or made it by cementing separate stones together while the water was here. The channel must
