“Rocks.”

“But round the water⁠—where the reeds were?”

“It was a bluish soil. It looked like clay.”

“Exactly. A volcanic tube full of blue clay.”

“What of that?” I asked.

“Oh, nothing, nothing,” said he, and strolled back to where the voices of the contending men of science rose in a prolonged duet, the high, strident note of Summerlee rising and falling to the sonorous bass of Challenger. I should have thought no more of Lord John’s remark were it not that once again that night I heard him mutter to himself: “Blue clay⁠—clay in a volcanic tube!” They were the last words I heard before I dropped into an exhausted sleep.