Explaining to Captain Joe and the others that they were going only a short distance from the shore, the boys launched the canoe and were soon on the sloping shore of the peninsula. Once across they hid their canoe in a thicket which overhung the stream and disappeared in the interior.
“Now, look here,” Clay said as he stopped and sat deliberately down in the shade of a great tree, “I’ve got an idea.”
Alex stared hard in pretended wonder and amazement.
“Where did you get it?” he asked.
“Brain cell opened and gave it to me,” Clay answered.
“Well, come across with it,” Alex urged.
“Captain Joe wants to know where the water comes from to make the west river so large at its mouth,” Clay went on. “I started in to tell him that there might be a subterranean stream somewhere hereabouts, but I thought he would laugh at me and so kept my mouth shut.”
Alex sprang to his feet and swung round and round on his heels, chuckling and shaking hands with himself.
“That’s the idea!” he cried. “That’s just the idea! There is a subterranean stream here somewhere! Look at the way the rocks are piled up, and look at the long slope from the top of the ridges to the level of the river. There are catch basins here somewhere, and water pouring into the river that no one knows anything about.”
“Now go a little farther,” Clay suggested. “Figure that at some time, say two or three hundred years ago, this subterranean channel lay open to the sun. Now what do you make of it?”