didn't spot anything that was obviously a joint. But there were several small side canyons—all narrow enough for us to wade or jump their central streams—which might have started life that way."

"Ours were quite narrow, and bore traces of volcanic ash at the bottoms."

"Eh?"

"That's right, Rob. Here's a bit of it I brought back. I thought you might want a little corroboration on that one." Krendall handed over a bit of crumbly tuff as he spoke. Lampert examined it with pursed lips.

"Maybe we'd better get back into the air, and search the neighborhood for volcanoes," he said at last. "I can't bring myself to believe in two full mountain-building cycles on this planet—and if I could, I'd have a hard time swallowing the idea of these limestone layers coming up, going down, and coming up again unaltered. How deep were these volcanic deposits?"

"Variable. Shallowest in the wider joints; in the very narrow ones, up out of sight."

"Suggesting that they've been washing out for some time since the original settling. Anything organic in them?"

"Nothing turned up yet."

"Do they extend below the present river level, or what?"

"They're at least down to it. We couldn't do any major excavating."

"If they run much below," muttered Lampert, "I'll join the roster of geophysicists who have been driven off the rails by this woozy world. Well, let's assume as a working hypothesis that the volcanic activity is relatively recent. That will at least have the advantage of keeping me sane, until something comes up to disprove it." He finished his meal in silence, while McLaughlin gave a reproving lecture on the matter of wading.

There was still a little daylight to go when all the men had eaten; and Lampert, Sulewayo and the archaeologist took the helicopter up the main canyon to check on the possibility of walking to any really new deposits.

They were sure, from changes of color already seen at various levels up the cliff face, that these existed. But it appeared that the lowest of them did not reach river level for more than a dozen miles. The distance was less mapwise, but the canyon, winding back and forth around what the geophysicist still felt must be joint-bounded blocks, went a good two miles in other 

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