got my first large taste of it in my student days back on Earth. Fly over the mesa country in southwestern North America sometime. Most of the joints there are invisible from a distance, of course; but at the edge of a butte where weathering is most prominent the blocks have frequently started to separate, and the thing looks as though it had been put together from outsize bricks."

"Hmph. Seem to remember something of the sort myself, now that you mention it. I did some digging in that area, too. I shouldn't have connected that sort of country with what we have here, though."

"Different meat; same skeleton," replied Lampert.

"But how about this volcanic ash, or mud, or whatever it is, which at least fills the joints we saw in the cliff? That's not so usual, is it?"

"Not in my experience. But granting the joints and the volcanoes, there's nothing really surprising about it. Incidentally, we don't know that this crack we're standing on has the same filling. We'd better bore down again to make sure. At least we may get some idea of the date of the volcanic action compared to that of the orogeny that tilted the block where we're camped. If there's tuff down here too, it will substantiate the idea that the vulcanism is the older."

"Why? Couldn't ash have settled down here as well as up there at substantially the same time?"

"It could. But I'd bet a fairly respectable sum that the tuff we saw in the canyon was from a mud flow, not a fall of airborne ash. That could hardly have reached the top of the cliffs—actually, the opposite slope of the mountains, where Sulewayo is working—and this area simultaneously."

"Maybe from different eruptions? I get the impression that this world has a slight tendency to produce volcanic fields rather than individual cones or flows."

"Might be. Chemistry will probably settle that question." During the latter part of this discussion Lampert had directed the mole once more downwards, and every half meter of travel another core was added to the collection. At six and a half meters below the soil the first solid specimen arrived; the others had been held together only by roots. This one, however, caused the two scientists to look at each other. Lampert nodded slowly, with a smile. Mitsuitei gave a shrug, and let an expression of resignation play over his usually impassive features.

The core was tuff, 

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