fact, until his first case had been filled.

"Can you stop that thing for a moment, Rob?" he asked at this point. "I don't want to lose track of these, and will have to hold up while I open a new case."

"All right. I thought you'd want to stop for thought soon anyway."

"Why?"

"Because the mole is nearly four meters down, well below the depth at which we hit bedrock before, and is still in soil."

"Eh? But—but it's still ordinary soil; none of your volcanic ash."

"Tuff had been eroded out of a lot of the joints in the cliffs. There's no reason to expect it to be at the same level as the surrounding rock."

"That's true." Mitsuitei paused in thought for a moment. "If we keep on going straight down, we may just be working into a natural crack, as you say. Might it not be better to drill several holes within a few square yards here, to determine whether it is a narrow joint such as you expect or an actual edge to the rock at this level?"

"Maybe the edge of a roof, eh?" Lampert chuckled, but spoke in a manner which could give no offense. "I can do better than that. Don't need to pull up and start over; simply drill horizontally from where we are now. Shouldn't take long to get dimensions, if that's all you want." He halted the robot momentarily, and from a compartment in the drum removed something like a small theodolite mounting. This he set up on a short tripod over the point where the neck of the mole emerged from the ground, and set a pointer at right angles to the line of tall trees. Then he started the digging again.

V

Four starts in as many different directions and twenty minutes of time showed fairly conclusively that the line of vegetation which had given rise to the "street" theory was growing along a straight crack, apparently a fairly ordinary joint, in the limestone. While several more holes would have to be drilled to prove it, even Mitsuitei was willing to admit that in all probability the remaining lines would be found to be over similar cracks.

"You must admit, though, that the regularity of this joint pattern is pretty unusual," the archaeologist said at length.

"It's far from being unknown," Lampert replied. "I 

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