Four minutes later the tractor set out from the _Albireo_, carrying Rowson and Zaino. Six minutes after that it stopped at the base of the ash cone which formed the north side of the valley from which the lava was coming. They parked a quarter of the way around the cone's base from the emerging flood and started to climb on foot, both carrying burdens. Forty-seven minutes later they returned empty-handed to the vehicle, to find that it had been engulfed by the spreading liquid. With noticeable haste they floundered through the loose ash a few yards above the base until they had outdistanced the glowing menace, descended and started back across the plain to where they knew the ship to be, though she was invisible through the falling detritus. Once they had to detour around a crack. Once they encountered one which widened toward the chasm on their right, and they knew a detour would be impossible. Leaping it seemed impossible, too, but they did it. Thirty seconds after this, forty minutes after finding the tractor destroyed, the landscape was bathed in a magnesium-white glare as the two one-and-a-half kiloton charges planted just inside the crater rim let go. * * * * * "Should we go back and see if it worked?" asked Zaino. "What's the use? The only other charges we had were in the tractor. Thank goodness they were nuclear instead of H. E. If it didn't work we'd have more trouble to get back than we're having now." "If it didn't work, is there any point in going back?" "Stop quibbling and keep walking. Dr. Burkett, are you listening?" "Yes, Captain." "We're fresh out of tractors, but if you want to try it on foot you might start a set of flow measures on the lava. Arnie wants to know whether our landslide slid properly." However, the two were able to tell for themselves before getting back to the _Albireo_. The flow didn't stop all at once, of course; but with the valley feeding it blocked off by a pile of volcanic ash four hundred feet high on one side, nearly fifty on the other and more than a quarter of a mile long, its enthusiasm quickly subsided. It was thin, fluid stuff, as Burkett had noted; but as it spread it cooled, and as it cooled it thickened. Six hours after the blast it had stopped with its nearest lobe almost a mile from the ship, less than two feet thick at the edge. When Mardikian's tractor arrived, Burkett was happily trying to analyze samples of the flow, and less happily speculating on how long it would be before the entire area would be blown off the planet. When Marini's and Harmon's vehicles arrived, almost together, the specimens had been loaded and everything stowed for acceleration. Sixty seconds after the last person was aboard, the _Albireo_ left Mercury's surface at two gravities.